A New Flying Machine

jlawren3

jlawren3

The Next BIG Thing
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I have a design for an elegantly simple dual-rotor VTOL aircraft. It is a “tail-sitter” that takes off with its fuselage oriented vertically, but cruises with its fuselage oriented horizontally. It is fully supported by only its rotor blades in all its flight modes: hover, transition and cruise. The novel design offers higher cruising speeds for a given power and weight than most fixed-wing aircraft, very low noise generated when cruising, power loadings and rotor disc loadings typical of most helicopters, and excellent lateral maneuvering capability when hovering and landing.

Please see the attached 6-page White Paper which describes this new aircraft and a development proposal, less costs and team people. Building a flying model would be the next step. Meanwhile I am refining the blade element design tools developed by Ray Prouty to optimize a design to carry two people.

I have applied for a patent internationally through the PCT program; the patent application can be viewed at http://www.wipo.int/pctdb/en/wo.jsp?WO=2008140851. My email address is 2john@tampabay.rr.com.

Comments?
 

Attachments

  • New VTOL Aircraft Concept REV D.pdf
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Wasn't there already such a craft?
The rear rotor changed to a wing in-flight - or something like that.
 
Just one comment - a lot of words about the takeoff and transition from hover to forward flight, but remember that all similar designs failed because of the almost impossible LANDING. Shortly - I don't want to be a pilot of this. Triebflugel, XFV-1, XFY-1, Ryan X-13 to name a few.
 
Jemiba,

I don't have a patent yet, but I have filed a patent application. It got through the initial searches and has now been published on the web. Starting in August my patent attorneys will start the detailed filing in various countries. I think the sense is that if it has gotten this far, then with normal back and forth between my attorneys and the patent offices in various countries most if not all the claims will be allowed. Of course, that remains to be seen.

Yes, in the US in 1949 a Lloyd H. Leonard presented a paper DESIGN STUDIES OF VARIOUS TILTING FUSELAGE CONVERTIBLE AIRCRAFT at the 1st Convertible Aircraft Congress in Phiadelphia. He showed a number of possible configurations. Mine concept is shown in Figure 6.

A Ray Prouty who is a helicopter design consultant pointed this out to me after I had filed my patent application. Somewhat alarmed, I sent the information to my patent attorney. He replied (in part) "...it is possible that the Examiner could use this article as part of an obviousness rejection. However, although the Examiner may take a different position, I do not believe that the article either discloses the entire structure of your invention, or would meet the enablement requirement. Accordingly, I am not overly concerned that submitting this article would have a significant impact on the patentability of your invention."

When my attorney starts the detailed filing, which he calls the national phase, he will send a copy of that paper to the USPTO. He is required to do this as part of his relationship with the patent office.

I would say that the Leonard design is basically a concept whereas I have a design which is the enablement part that my attorney spoke about.

A key feature of my design is that I have worked out a way to transition from hover to cruise and back without climbing and diving and without using a large amount of installed power. It is so non-obvious that this is possible that I had serious difficulty in getting my aeronautical engineering cousultant to even analyze it. He wanted to make it dive to pick up speed and pull up to dissipate the speed - and back down a long way to land. Finally he agreed to analyze it and he validated my approach. I had done a lot of analysis myself, but it was a bit simplistic, so it was good to see someone with experience and well developed analysis tools have a go at it. His blade element theory method, when applied to helicopters, accurately predicts their performance because it has been developed with reference to helicopter data to do just that.

The Leonard concept does not address the transition problem; it only shows a diagram and discusses in the text the issue of directional stability. I trust that the patent office will agree that the Leonard disclosure is not enabling, that is, sufficient to permit someone to build and fly such a craft.
 
An intriguing idea. I certain see some UAV options for this. I have not done more than a glance however depending on how small you can make the vehicle and how well you can maintain a decent hover efficiency there certainly could be some applications for urban efforts (launch/recover from a roof) and an ability to hover and overlook an intersection. A cargo UAS able to quickly deliver say 500lbs of varied supplies is gaining popularity as a concept these days. Certainly might be some naval applications as well (ASW, littoral patrol come to mind.)
 
Matej said:
Just one comment - a lot of words about the takeoff and transition from hover to forward flight, but remember that all similar designs failed because of the almost impossible LANDING. Shortly - I don't want to be a pilot of this. Triebflugel, XFV-1, XFY-1, Ryan X-13 to name a few.
Click to expand...

MZ,

I wish it did not have to be a tail-sitter, but you are going to have to "pay for" an alternative flight concept in some way. Every concept that I have seen that promises vertical takeoff and high performance cruise has some kind of serious issues to deal with. In cruise my approach is marvelously clean. It has a streamlined fuselage and a smaller amount of blade area than a typical fixed-wing aircraft has wing area. It is not a tilt-rotor but a tilt craft. It should be a lot easier to tilt "pods" with people in them than to tilt rotors and engines. So I believe the tradeoffs are acceptable in exchange for the excellent performance it promises.

The pogo type aircraft was hard to land but it had a very different configuration than my craft. When hovring my craft can produce immediate lateral forces and they can be fairly large. In order to change position a helicopter has to tilt in the direction it wants to go, pick up a little speed and tilt in the opposite direction to bring the speed to zero. My craft can simpley accelerate and then decelerate without changing its attitude. This is not unlike docking a twin screw boat compared to a single screw boat. You can do amazing things with a twin screw boat if you can freely steer and reverse each prop.

The excellent maneuverabiltiy when hovering has been demonstrated by the CL-327 rotorcraft made by Bombardier which is a dual rotor craft also. It performed very well, I am told, in sea trials landing in gusty conditions on a heaving ship. If Bombardier had decided to make it nose over to horizontal when cruising, and had separated the rotors more, they would have had a great concept!

About visibility, or lack therof. For a large craft shuttling people from one vertiport to another, then landing will be very highly automated. A control system will dock it in a very precise location on top of the vertiport in order for it to mate well with extensible elevators that would emerge from below to load and unload people. The blade postioning servo motors will have a fast response and they would be controlled in a tight servo loop with inertial data and local position data enabling an accurate landing automatically, even with strong wind gusts present. I assume that there would also be a down looking TV to assure that pilot that everything is going well.

Please note that the typical flight path for a less automated craft, like the 2-place craft I envision, would be similar to the flight path of a helicopter. It would approach the landing site on a glide slope, as it were, and then settle vertically to land. In the 2-place craft I am planning I thought about using a TV camera to help with the approach. That might be OK, but I feel that it should be possible to provide windows in the craft which would be under the feet of the pilots in cruise but would provide good forward horizontal visibility when slowing up and hovering, the pilots being in a tilting cockpit.
 
It can be a tailsitter, why not? But after reading the PDF that you posted the first thing that came to my mind was the unfinished idea of a landing, especially in a bad conditions. I noticed it several times in countless aviation projects where the biggest effort was to take the vehicle to the air and authors forgot to think about how to safely and routinely land. For your craft it is even more important, because the only possibility to land (and survive) is 90 degrees descent angle. You cant glide with a relatively high forward velocity like the helicopter in an emergency can do. Your description is about the landing when everything goes perfectly, but what will happen when some error occur?

Another disadvantage, related to landing is very high center of the gravity. I know that the current description is aimed to show general philosophy of the idea, so it is not engineered with all the details, but landing gear/device seems to me as very straight, more like on some rocket. The legs should spread wider and they should have some sort of spring-back. Next step will be the internal arrangement. I don't know if you have in your mind some, so I tried to imagine some simple one. Engine is on the bottom/back part, tilting cockpit is in the middle of the propellers and probably the fuel can be on the top/front. Another fuel tank should be just above the engine and the fuel distribution can be used to balance the whole vehicle. For example during the landing the residual fuel should be only in the bottom tank, but during the forward flight the fuel should be consumed from the bottom/rear tank firstly and only after from the top/front tank to keep the center of the gravity somewhere in the middle of the propellers.

For the manned variant it is critical to have a lot of windows and at least one TV camera. Okay, once again when everything goes perfectly, this process can be heavily automated, but what during the emergency because for example the system failure when the manual landing will be needed? The pilot will be very unhappy to look through some two small windows through the rotating blades to the ground. I had the opportunity to try Mi-17 in the intermediate conditions and I must say that it is not so easy as it can look like. And the Mi-17 is perfect and stable helicopter with a very good view from he cockpit. It is very different to fly IFR in a comfortable modern airliner compared to your rocket-like rotorcraft.

That are a few points that you should think about before any demonstration. I hope that my point of view will help you to take a different look to move forward. ;)
 

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Mateg,

I agree:

"Another disadvantage, related to landing is very high center of the gravity. I know that the current description is aimed to show general philosophy of the idea, so it is not engineered with all the details, but landing gear/device seems to me as very straight, more like on some rocket."

I have been criticized about that before. In real applications it definitely would need to have a much wider stance and some kind of shock-absorbing legs. But even the prototype has shock-absorbing legs. I use a type of monofilament line with high damping to cushion the landing.

See my comment from an earlier post about approach angles; I think that it probably should approach for a landing like a helicopter. In trying to think through what would be needed for good flight efficiency I concluded that a slanted appraoch is desirable because it better uses the energy from the craft's altitude. A more straight down approach invites getting into the vortex ring regime, so that's another reason to use slanted approaches. But you are partly correct. When slowing up to land the craft is in a high drag mode as it flies very nose high so it can't be gliding. But it can still approach on a slope. As it goes down the slope it would be slowing up as it loses altitude. For the best fuel efficiency this could be done without engine power, but the engine would be standing by ready to immediately jump in if needed.

In a craft to haul people as mentioned in the PDF attachment I envision two spherical "pods" between the rotors with 8 to 10 people in each. This would be a highly automated craft. I can envision a pilot/flight attendant in the forward pod facing forward with controls at his seat. He would have the same visibility from windows that the passengers would - which in hover could be pretty good, plus TV displays. Somewhat like a bus driver he would to some extent be one of the people. During the flight when the craft is horizontal the design would permit him to walk between the pods. A bathroom would be accessible from that between-the-pods corridor. An automated regional air traffic control system would monitor all aircraft and insure that it is safe to leave the cockpit.

I think that the two engines must both be located aft of the second rotor, otherwise the exhausts will impinge on the rotor blades. I trust that fuel forward of the first rotor can counterbalance the engines and landing gear in the rear. When most of the fuel has been used up the second rotor will be carrying a disproportionate amount of the craft's weight. I will have to evaluate this case and see if any serious problems are created.

I like using two engines. Unlike a V-22, the craft would be able to operate well in hover, transition, and cruise on only one engine. For short trips especially there is a premium on getting to altitude quickly to take advantage of the low drag from the thinner air, so both engines would be used for climbout with one engine used for cruise. Gas turbines are not very efficient at part load. The craft can fly about 80% as fast on one engine as on two so it works well for it to use one engine for cruise.

Yes, the CG needs to be pretty much between the rotors, but its location is not super critical. In rotorcraft designs which can tilt to fly horizontal and which have the rotors close together it is very critical to keep the CG between the rotors. If it were shifted back to the aft rotor, for example, the aft rotor would have to carry all the craft's weight.

In my plan for hauling people, special extensible elevators would rise to the level of each pod on each side for people to quickly exit and reload. For an emergency exit I envision there being an opening axially through the craft and, of course, through the lower rotor. People would climb down ladders to the ground.

I have not been very concerned with the fuel distribution. I don't think that it has to be low when landing because we will have automated systems handling maneuvering and stability.
 
yasotay said:
An intriguing idea. I certain see some UAV options for this. I have not done more than a glance however depending on how small you can make the vehicle and how well you can maintain a decent hover efficiency there certainly could be some applications for urban efforts (launch/recover from a roof) and an ability to hover and overlook an intersection. A cargo UAS able to quickly deliver say 500lbs of varied supplies is gaining popularity as a concept these days. Certainly might be some naval applications as well (ASW, littoral patrol come to mind.)
Click to expand...

I agree that ship-to-ship and ship-to-shore would be natural applications. The craft has super good maneuverablity for docking on a ship in gusty conditions, even while the ship is heaving. The craft also has a top cruising speed and efficiency far superior to helicopters.

My patent attorney thought that it may be a while before robotic craft are accepted over cities, but he had an idea that gets around this. He said that package delivery companies find it very expensive to send a man and a truck to pick up or drop off a package, or a few packages, in outlying areas. He suggested that my craft could move packages between the roofs of outlying offices and a depot closer to an urban area.

In all sizes the hover power is governed by the rotor disc equations. If a craft is light, the disc loading can be low and the power needed to hover can be low, so small sizes can work well enough. But the drag on a rotor blades will increase as the Reynolds number goes down, so that would have to be considered when designing a small craft.
 
check this
http://www.litemachines.com/mil/mil_main.htm
http://www.litemachines.com/mil/specs.htm
http://www.litemachines.com/mil/apps.htm
 
Interesting design jlawren3, best of luck with your patent application. In your White Paper you mention an application for an unmanned version launched from a Humvee that could be used for air strikes. Would you be able to describe how such an aircraft would launch munitions without hitting the rotor blades? Would they be launched from a hover, like some of the Nutcracker designs?
 
SmithW6079 said:
Interesting design jlawren3, best of luck with your patent application. In your White Paper you mention an application for an unmanned version launched from a Humvee that could be used for air strikes. Would you be able to describe how such an aircraft would launch munitions without hitting the rotor blades? Would they be launched from a hover, like some of the Nutcracker designs?
Click to expand...

I would recommend firing weapons from the fuselage in hrorzontal flight. The craft could be built with an axial central opening, essentially a through tunnel, in which weapons could be carried. The central part of the nose could be made in 3 or 4 sections to open like a flower when the weapons are to be used. Also, if the engine which drives the rotors were equipped with a clutch, then in forward flight the rotors could be disengaged and stopped momentarilly. Then one or more Hellfire missiles mounted on the exterior of the fuselage could be fired through the stopped forward rotor blades. The rotor blades would then be given pitch angles so the rotors would quickly speed up to the speed needed to permit re-engaging the engine drive.
 
flateric said:
check this
http://www.litemachines.com/mil/mil_main.htm
http://www.litemachines.com/mil/specs.htm
http://www.litemachines.com/mil/apps.htm
Click to expand...

Wow! That is really close to what I am doing. Up until now I thought that the closest concept to mine is the Guardian CL-327 by Bombardier.

The litemachines craft clearly has co-axial counter rotating rotors with cyclic and collective pitch control like my caft. But I see this significant difference: The center of gravity is not between the rotors but is well below the rotors. It still flies helicopter like even at its top speed. I would guess that the axis is pitched forward a significant amount at its top speed, but, unfortunately, that is not a low drag orientation for the fuselage. If the CG could be between the rotors, the tail could be faired (streamlined), and the craft's fuselage could be horizontal in horizontal flight, then the top speed would be much, much higher. And the hovering maneuverability would be significantly improved.

Thanks a lot for this information.
 
Hi All,

This is an update. My patent application has been republished at: http://www.wipo.int/pctdb/en/wo.jsp?WO=2008140851. Click on the Documents tab for details.

I am also attaching an updated white paper. It is essentially a pre-proposal for funding for development work. Know anyone with $750,000 who would like to get in on the ground floor of a revolutionary development? That's what I think it will cost to get some prototypes flying, take data, update the math models, and extrapolate the results to some larger rotorcraft.

Sincerely,

John
 

Attachments

  • New VTOL Aircraft Concept.pdf
    179.4 KB · Views: 28
First allow me to clarify: I'd love to see something like this succeed, but as an engineer, I tend to point out all the bad and none of the good. Any critique is honestly meant as helpful, not venomous.

With that said, my biggest concern is in your discussion of transition. You mention a Cl of approximately .8 on the down-going rotor blade through this process. That will generate a very high induced drag on that blade (Cl^2/pi*AR*e) I assume the blade on the opposite side would see very little Cl, as to be efficient. I am, admittedly, fairly unfamiliar with rotorcraft, but this would create a large out-of-plane moment on each rotor in the opposite direction. This would in practice try to tear the aircraft apart on one side and compress it on the other.assuming that the orientation of the body is fixed (i.e. there's an up an a down in the central 'fuselage') the structure would have to be asymmetrical on the right and left sides. That, in turn, would throw (without compensation) the C.G. off of the center line. Not to say it can't be done, but consider it as a factor. This is particularly relevant if composites were to play a role in the structure as, I'm sure you know, they are much happier in tension than compression.

On a second note: the front rotor will induce a rearward velocity and a rotation to the flow. The second rotor is, because of this, operating in a different set of flow conditions. The power to both rotors would have to be equal in order to induce the same moment and keep the craft from spinning, but to accomplish that the rear rotor would have to either rotate faster or have a higher advance ratio. The rear rotor will also operate in a flow with rotation induced by the front rotor, so relative angles of attack would have to be adjusted. I'm not sure if you've looked into this, or how the effecs change as forward velocities and RPMs change.
 
AeroJadeXG,

In my attachment I didn't talk much about transition. I have attached a one-page overview which speaks more about transition.

There is a critical point during transition when only the advancing rotor blades are producing lift. For the forward rotor, for example, all the lift will be on the right side and for the aft rotor all the lift will be on the left side. Far downstream the wake should be pretty uniform with a downward induced velocity coming from the whole "span" of the craft. Since the rotors are separated axially one might wonder if some efficiency is lost with the first "half-span" producing lift well ahead of the second "half-span". Well, probably some efficiency is lost, but it probably isn't much. Conceivably there is even an advantage. It is very likely more efficient for ducks to fly in a V instead of in a straight line, side by side. Think about it.

During transition lift from each side of the craft has to be equal, otherwise the craft will roll. The fuselage has to be built to take this stress. The stress is comparable to the stress in an automobile if only the right front and left rear wheels were carrying the car's weight. This can easily be designed for.

It is the torques applied to the two rotors which must be equal, not the power. When Ray Prouty analyzed hover he came up with an interesting result which answers your concern. The top rotor generates almost exactly twice the lift as the bottom rotor while both are working at the same RPM (in opposite directions) and at the same torque. As you noted there is spin induced in the column of air by the first rotor and the second rotor sees inflow conditions than differ from those for the first rotor. In a nutshell, the air approaching the second rotor has a tilt or twist to it. If the second rotor tried to produce the same lift as the first rotor the lift vector would be tilted back a lot more than for the first rotor requiring more torque to drive the second rotor. So the second rotor must reduce its lift so the torques match. I could draw some vector diagrams, I think, and convince myself that there should be a 2:1 difference between the lift in the first rotor and the lift in the second rotor given similar torques and speed. Maybe someone else would want to do that. It is a curious situation. Was the 2:1 ratio an accident or not?

It is generally recognized that helicopters with coaxial rotors work very well. The second rotor untwists the twist in the air column induced by the first rotor. I understand that because of this a helicopter with coaxial counter-rotating rotors needs about 12% less power to hover than a comparable helicopter with a single main rotor and small tail rotor.
 

Attachments

  • A Green Machine.pdf
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mz said:
Wasn't there already such a craft?
The rear rotor changed to a wing in-flight - or something like that.
Click to expand...

MZ,

Since I started with this idea quite a few years ago I have tried to find some variations on it that might improve on some aspects. I guess there is good news and bad news here. The bad news is that I was not happy with any alternative concepts I imagined and I really did try. If the rear rotor were stopped in an X and then the X was flattened so the wings were more horizontal, then the design loses effective vertical rear fin area which is needed. If fixed wing surfaces are added, then that increases the drag at cruise. The good news is that the craft seems to an optimum of its type with no competing close relatives.

For a long time I was sure that I needed to conceive of a clever landing gear so that the carft could land in a tail-sitting mode, fold the rear rotor blades up and the forward rotor blades down and then tilt the craft 90 degrees so the fuselage was close to the ground. Then I imagined being able to taxi the craft like other aircraft. But the complexity, weight, and cost looked terrible. A better approach, I believe, is to leave it as simply a tail-sitter.

John
 
Hi jlawren3,

Firstly, thanks for introducing some original ideas to the forum and providing much food for thought.

However. when reading your paper one point intrigues me. Perhaps I might have missed it.

Wile I appreciate that the torque of each rotor is balanced and not their speed of rotation, what measures are employed to stabilise and prevent the fuselage spinning?

Vertical flight: All existing contrarotating helicopters have their C of G well below both rotors, so fuselage rotation can be controlled by differentially changing average pitch angle (therefore torque) between the two rotors. As your design has a C of G between the rotors, attempts to control Fuselage rotation this way would cause the centre of lift to change height significantly relative to the C of G and therefore stability.

Horizontal fight: As discused in previous post the rear/lower rotor provides considerably less lift/thrust than the front/upper rotor so in vertical flight the centre of lift may still be above the C of G regardless of differential pitch angles. However in horizontal flight attempts to control fuselage spin through this method might result in change of centre of lift between front and rear therefore inducing drastic pitching unless some other factor is in play to counteract this.

Transitional flight: I haven't even considered how the above would affect your concept for transitional flight.

I'm sure you must have considered this point and I will be fascinated to read your solution.

Cheers, Woody

PS. You mentioned that pitch angle (cyclic, collective etc.) will be controlled by 'fast-acting servo's. Has this ever been achieved on a conventional helicopter?, and how does it compare to a conventional swash plate in respect to: weight, cost, reliability, power consumption?, or would a swash plate simply not work for horizontal flight?
 
Woody said:
Hi jlawren3,

Firstly, thanks for introducing some original ideas to the forum and providing much food for thought.

However. when reading your paper one point intrigues me. Perhaps I might have missed it.

While I appreciate that the torque of each rotor is balanced and not their speed of rotation, what measures are employed to stabilise and prevent the fuselage spinning?

Vertical flight: All existing contrarotating helicopters have their C of G well below both rotors, so fuselage rotation can be controlled by differentially changing average pitch angle (therefore torque) between the two rotors. As your design has a C of G between the rotors, attempts to control Fuselage rotation this way would cause the centre of lift to change height significantly relative to the C of G and therefore stability.

Horizontal fight: As discused in previous post the rear/lower rotor provides considerably less lift/thrust than the front/upper rotor so in vertical flight the centre of lift may still be above the C of G regardless of differential pitch angles. However in horizontal flight attempts to control fuselage spin through this method might result in change of centre of lift between front and rear therefore inducing drastic pitching unless some other factor is in play to counteract this.

Transitional flight: I haven't even considered how the above would affect your concept for transitional flight.

I'm sure you must have considered this point and I will be fascinated to read your solution.

Cheers, Woody

PS. You mentioned that pitch angle (cyclic, collective etc.) will be controlled by 'fast-acting servo's. Has this ever been achieved on a conventional helicopter?, and how does it compare to a conventional swash plate in respect to: weight, cost, reliability, power consumption?, or would a swash plate simply not work for horizontal flight?
Click to expand...


Hi Woody,

Thanks for your interest.

I think that you are trying to think through the stability of this craft based on principles of directional stability that apply to fixed-wing aircraft. Fixed-wing aircraft are conditionally directionally stable (you are trying to get the right conditions), this craft is neutrally directionally stable, and helicopters are directionally unstable. In an attachment to an earlier post I said,

"The author flew a toy model of the craft in the 1950s as shown in Figure 1 above. When the rubber band was wound up and the craft was tossed, it flew in the direction it was pointed, horizontal, vertical or at any angle in between. As it flew it often changed its heading a small amount. Its flight path consisted of several straight-line segments with small heading changes between the straight-line segments. This behavior can be described as neutral directional stability; the direction was apparently altered by minor air turbulence and a new direction continued with no memory of the previous direction."

I did not understand what I was looking at then, but I knew it was not flying like a hand-tossed glider. About 11 years ago I spoke to Professor Steven R. Hall of the MIT Aero Department and asked him about the craft's directional stability. After reflecting on it overnight he said that it would have neutral directional stability. Sometime after our conversation I realized that this is was, in fact, what I had seen demonstrated.

Well, neutral directional stability is easier to control than unstable directional stability - so it should be easier to automatically control than a helicopter. But we live in a world where all kinds of wild things are done with intelligent controls. I understand that even a bicycle has now been steered and balanced in a close loop system. You are probably aware that the trick of riding a bicycle is initially steering in the wrong direction in order to develop tilt to enable turning in the desired direction. This behavior can be expressed in equations of motion and computer controlled.

To get to your question. I expect to have strain gages or the like at the root of each rotor blade as well blade pitch angle sensors and rotor angle and rotor torque sensors. Control circuitry will adjust each rotor blade's pitch angle continuously to give the desired blade lift as sensed by the strain gages. And rate gyros in the craft will immediately sense any tendency of the fuselage to rotate about its axis. So the roll rate of the fuselage will be under immediate, direct and continuous computer control. This will be true in hover, transition and cruise.

Such a system will also contain accelerometers and can control accelerations in all directions. It will be able to accommodate all kinds of irregular conditions such as, in horizontal flight (cruise), a CG shift so that the forward rotor is more heavily loaded than the aft rotor or, in the case of gusts from clear air turbulence, keeping the accelerations very low inside the craft - as well as all combinations of conditions.

About using electric motors to control rotor blade pitch angles. In 2003 ago I was having trouble convincing my patent attorney that I had something unique because of the existing patent HELICOPTERS WITH COAXIAL ROTORS OF CONVERTIBLE TYPE IN PARTICULAR, by Tassin de Montaigu, Patent No. 4,123,018, dated Oct. 31, 1978. He thought that this was not far from what I am doing and recommended against a patent application. Well, I then thought that driving and controlling the pitch angles of the rotor blades with servo motors might be what I could patent, so a search was done on that. Of course, it had not been done a craft like mine, but there were lots of patents on electrically controlling the pitch of rotor blades on helicopters - a well plowed field.

When I started the mechanical design of my prototype I started out planning to use a swash plate means of varying cyclic and collective pitch. I wanted something quick and dirty to demonstrate the flight principles, but I
found it very awkward to try to use a swash plate approach because the hub diameter is large and an open space is needed inside the hub. You can see that the diameter, as it were, of the swash plate would have to be very large. So, knowing the advantages of direct servo drive, I decided to go with that approach. I haven't done a cost comparison, but I imagine it is more expensive than swash plates where swash plates can be used - but my craft is probably not one of those cases. However, if this craft is ever built in quantities like city buses, the cost per blade for the servo drive will come way down.

There are neat things that can be done with a servo per rotor blade. Higher order harmonic control is possible. Suppose in transition the craft had a very high peak lift coefficient in the advancing blades and that it would be nice to be able to modify that. So rather than asking the servo for a sine wave of lift versus rotation, the control could flatten the peaks of the sine waves while keeping the same area under the sine waves - all with little or no penalties. Of course huge reductions in lift peaks can't be realized, but a small and free reduction might be very attractive. Or how about this: drive the rotor blades so as to waste power. Normally I hope to convert altitude and speed near the end of a flight to a greater distance traveled by coasting, but under some conditions it may be necessary to settle quickly as in emergency autorotation. It will be easy to overspeed the rotors by asking them to windmill and quickly absorb a lot of altitude energy. How could one dump some of this energy? By alternating the pitch of the rotor blades so that one is lifting near the maximum lift coefficient and the next one is opposing that lift, trying to sink the craft, and so forth around the rotor. (An even number of rotor blades is needed to do this trick.) I think of this as being like an egg beater, churning the air. You could never do this with a swash plate drive.

Reliability of servo drives. Most new aircraft work their control surfaces "fly by wire". Redundancies are built in. Soon even cars will be steered without a mechanical backup. So I feel that with proper design, aiming for the highest possible reliability, the servo drives for rotor blade pitch will be highly reliable, maybe better even than mechanical drives. There is also a lot that continues to be done with the software in digital controls (which these would be) to make the software ultra-reliable by self-checking, redundancies, etc.

Power consumption: modern servo drives use switching power amplifiers with this curious characteristic: most of the energy used to accelerate a load is recovered when the load is decelerated; the power supply voltage actually gets "pumped up" from the recovered energy. So accelerating and decelerating the pitch inertia of the rotor blades should not waste much energy.

Sincerely,

John Lawrence
 
Thanks John for the reply,

To get to your question. I expect to have strain gages or the like at the root of each rotor blade as well blade pitch angle sensors and rotor angle and rotor torque sensors. Control circuitry will adjust each rotor blade's pitch angle continuously to give the desired blade lift as sensed by the strain gages. And rate gyros in the craft will immediately sense any tendency of the fuselage to rotate about its axis. So the roll rate of the fuselage will be under immediate, direct and continuous computer control. This will be true in hover, transition and cruise.

Such a system will also contain accelerometers and can control accelerations in all directions. It will be able to accommodate all kinds of irregular conditions such as, in horizontal flight (cruise), a CG shift so that the forward rotor is more heavily loaded than the aft rotor or, in the case of gusts from clear air turbulence, keeping the accelerations very low inside the craft - as well as all combinations of conditions.
Click to expand...

It's all very well to say you will uses lots of sensors and technology to address the fuselage spin/stability problem but what mechanical principle do you intend to use? I for one can't understand how you can vary torque reaction (if you are using this principle) between rotors without altering their relative lift. I would understand your secrecy if this sensitive propitiatory information: perhaps you intend to use your "alternating the pitch of the rotor blades" in the same disk idea, but I can't see an easy solution.

I think that you are trying to think through the stability of this craft based on principles of directional stability that apply to fixed-wing aircraft. Fixed-wing aircraft are conditionally directionally stable (you are trying to get the right conditions), this craft is neutrally directionally stable, and helicopters are directionally unstable. In an attachment to an earlier post I said,
Click to expand...

I wasn't considering stability from a fixed-wing aircraft any more than for a cheese souffle. For your vertical flight, when you support a thing from a single point below it's C of G it's not stable, but you do have the rotor's gyroscopic effect on your side I suppose. Are you expecting to continuously vary the lift across the rotor disk to compensate for this even in vertical flight?

For horizontal flight, when you have more force applied to one side (front rotor or back rotor) of the C of G than the other you get rotation: aeroplane/helicopter/seesaw/halibut.

And I question that a helicopter is directionally unstable (unless this is some strange esoteric use of the expression) or you could put the tail boom/tail rotor/stabilising fins at the front.

Reliability of servo drives. Most new aircraft work their control surfaces "fly by wire". Redundancies are built in. Soon even cars will be steered without a mechanical backup. So I feel that with proper design, aiming for the highest possible reliability, the servo drives for rotor blade pitch will be highly reliable, maybe better even than mechanical drives. There is also a lot that continues to be done with the software in digital controls (which these would be) to make the software ultra-reliable by self-checking, redundancies, etc.
Click to expand...

I appreciate the above but I'm unaware of any current servos that have to operate with the speed/accuracy/repetition/load that you are requiring. There may be patents filed but that is very different from getting something working and working reliably (I'm currently perusing my own patent but not in engineering). The closest I can think of is hydraulic valve actuation in a car piston engine but this still doesn't have the complexity you are attempting.

Anyway it still sounds like a fascinating idea and I wish you good luck with it.

Cheers, Woody
 
Woody said:
Thanks John for the reply,

To get to your question. I expect to have strain gages or the like at the root of each rotor blade as well blade pitch angle sensors and rotor angle and rotor torque sensors. Control circuitry will adjust each rotor blade's pitch angle continuously to give the desired blade lift as sensed by the strain gages. And rate gyros in the craft will immediately sense any tendency of the fuselage to rotate about its axis. So the roll rate of the fuselage will be under immediate, direct and continuous computer control. This will be true in hover, transition and cruise.

Such a system will also contain accelerometers and can control accelerations in all directions. It will be able to accommodate all kinds of irregular conditions such as, in horizontal flight (cruise), a CG shift so that the forward rotor is more heavily loaded than the aft rotor or, in the case of gusts from clear air turbulence, keeping the accelerations very low inside the craft - as well as all combinations of conditions.
Click to expand...

It's all very well to say you will uses lots of sensors and technology to address the fuselage spin/stability problem but what mechanical principle do you intend to use? I for one can't understand how you can vary torque reaction (if you are using this principle) between rotors without altering their relative lift. I would understand your secrecy if this sensitive propitiatory information: perhaps you intend to use your "alternating the pitch of the rotor blades" in the same disk idea, but I can't see an easy solution.

"Woody,

I have been working with this concept a long time and have not asked myself that question. No, I don't have a secret proprietary solution. Actually, my consultant Ray Prouty raised it but for some reason I didn't take it seriously. He even suggested adding other fins of control surfaces which I considered an irrational suggestion. Here I have already eight active control surfaces, why are more needed, I thought. Putting this in my own words, your question is, suppose in cruise that blades on each side are producing the desired lift and have equal torques as required for no net roll moment, how can control moments for roll or pitch be developed without altering the net lift? I think that there is an even more challenging question: I say that in cruise it will be possible to accommodate a range of changes in the fore-aft location of the CG. Suppose the CG is forward a bit. Then surely more torque will be required of the forward rotor because more lift is produced. How can the aft rotor produce a corresponding equal torque without also producing more lift?

Here is my answer to my second question about compensating for a changed CG location in cruise. If the forward rotor has to produce more lift than the aft rotor and requires more torque to do so, how can the aft rotor oppose that torque without producing more lift as well? It can do so by producing torque on the aft rotor blades when they are near vertical. Is this using a non-harmonic drive to the rotor pitch angles versus rotation? Maybe, I haven't thought that through enough, but the craft can do non-harmonic pitch changes easily enough. I suspect harmonic drive alone can be used. I can test that opinion using Ray's blade element relationships which, to date, use only harmonic pitch variations in the rotor blades. I need to do that.

Actually, I think that this answer to the second question answers the first also. The rotors can easily produce torques without lift being produced when the blades are vertical. So all kinds of moments in pitch and roll are possible without affecting the net lift."



I think that you are trying to think through the stability of this craft based on principles of directional stability that apply to fixed-wing aircraft. Fixed-wing aircraft are conditionally directionally stable (you are trying to get the right conditions), this craft is neutrally directionally stable, and helicopters are directionally unstable. In an attachment to an earlier post I said,
Click to expand...

I wasn't considering stability from a fixed-wing aircraft any more than for a cheese souffle. For your vertical flight, when you support a thing from a single point below it's C of G it's not stable, but you do have the rotor's gyroscopic effect on your side I suppose. Are you expecting to continuously vary the lift across the rotor disk to compensate for this even in vertical flight?

For horizontal flight, when you have more force applied to one side (front rotor or back rotor) of the C of G than the other you get rotation: aeroplane/helicopter/seesaw/halibut.

And I question that a helicopter is directionally unstable (unless this is some strange esoteric use of the expression) or you could put the tail boom/tail rotor/stabilising fins at the front.


"Directional stability is judged like this. A craft is moving forward in equilibrium. It flies through a small wind gust. Let's say that it is a vertically upward gust. How will the craft result if someone doesn't immediately try to control the response from inside the aircraft? A properly designed fixed-wing plane will pitch up momentarily but will then return to level flight. My craft which is neutrally stable will pitch up and continue flying at a new pitched-up angle. A helicopter with respond to the vertical gust by continuing to pitch up at an increasing pitch rate. A helicopter pilot has to be ever vigilant.

In my craft the gyroscopic forces of the two rotors cancel. They do produce large moments on the structure but they do not affect the crafts handling as far as I know."

Reliability of servo drives. Most new aircraft work their control surfaces "fly by wire". Redundancies are built in. Soon even cars will be steered without a mechanical backup. So I feel that with proper design, aiming for the highest possible reliability, the servo drives for rotor blade pitch will be highly reliable, maybe better even than mechanical drives. There is also a lot that continues to be done with the software in digital controls (which these would be) to make the software ultra-reliable by self-checking, redundancies, etc.
Click to expand...

I appreciate the above but I'm unaware of any current servos that have to operate with the speed/accuracy/repetition/load that you are requiring. There may be patents filed but that is very different from getting something working and working reliably (I'm currently perusing my own patent but not in engineering). The closest I can think of is hydraulic valve actuation in a car piston engine but this still doesn't have the complexity you are attempting.

"I would offer that it should not be hard to get a servo system to duplicate the harmonic drive of cyclic and collective pitch. Now I maintain that I will also be able to make the servos respond fast enough to overcome, to a large degree, the effects of clear air turbulence. That is quite another issue. If I can change a rotor blade's pitch angle extremely quickly, how long will it be before the air flowing over the blade at its new angle actually begins to produce more lift? Actually, servos are amazingly fast now, if they need to be and if you want to pay for that performance. But maybe the speed of the servo positioning system is not the slowest thing in this control loop. A lot remains to be learned."

Anyway it still sounds like a fascinating idea and I wish you good luck with it.

Cheers, Woody
Click to expand...
 
Hi All,

This posting originally advocated large rotorcraft of this type capable of carrying a busload of people.

On Jan. 13 I said, "This "tiltplane" design concept does not lend itself to sizes that large." That may turn out to be true but there is, nevertheless, still a possibility that the usefulness can be extended to sizes that large. Studies are needed. The motivation for studying this further is the attractiveness of a VTOL aircraft that can carry a small busload of people with good fuel efficiency over distances or 20 to 100 miles.

jlawren3
 
Hi All,

This response originally espoused building these "tiltplanes" in large sizes to carry large numbers of people, but I see now that this design approach does not lend itself to doing that.


The above stated Jan. 13 may turn out to be true but there is, nevertheless, still a possibility that the usefulness can be extended to sizes that large. Studies are needed. The motivation for studying this further is the attractiveness of a VTOL aircraft that can carry a small busload of people with good fuel efficiency over distances or 20 to 100 miles.



jlawren3
 
Latest info on the "tiltplane"

Hi All,

Well, some interesting things have happened since I wrote last:

1) I will be setting up a website soon at www.tiltplane.com. It should be up in a week. (It is up now.)

2) I am presenting the paper attached on my "tiltplane" at the the American Helicopter Society Aeromechanics Specialists’ Converence, San Francisco, CA, Jan. 20-22, 2010.

3) Finally, and this is reversing a previous opinion, I still think that it is possible to build this tiltplane in moderately large sizes - up to a bus size that can carry 20 to 25 people - but it is definitely a challenge.

One thing that argues against large sizes is the bending moments at the rotor blade roots. It is no accident that the roots of the wings of airliners are very thick. That is to cope with the large bending moments from the long wings. You may have heard me say this before, but it can be shown that in transition there will be times when only two rotor blades are supporting the craft. So each of the 8 rotor blades has to be strong enough to support half of the weight of the tiltplane, which is to say that each rotor blade at its root must withstand the moment from half of the weight of the craft applied to that rotor blade plus a factor of safety.

If you were to run through some numbers you would see that it will not be possible to design large tiltplanes with long skinny rotor blades. Even for a 2-passenger size shown in the attached article, the rotor blades cannot be relatively long, thin, and narrow. The drawing in the article was made to scale. The rotor blades can be made as short as those are because there is a very powerful engine installed; when hovering the craft can be lifted by accelerating downward a relatively small column of air because of the power of the engine. A big engine also makes for a good top speed.

You may be aware that, when you scale things up and down, things usually don't behave well. As an example consider a paper airplane versus an airliner. If you were to scale a paper airplane up to even a few feet across, scaling the paper thickness up with the size, you would find that the wing area increased by the square of the size ratio but the weight increased as the cube of the size ratio. Galileo first saw this; it has been refered to this as the law of the squares and the cubes.


So that's what's happening.

Sincerely,

John Lawrence
2john@tampabay.rr.com
 

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Hi All,

I made a presentation of this "tiltplane" concept on January 20th to a meeting of the American Helicopter Society in San Francisco. It was well received. Also, I now have a website, www.tiltplane.com. On the websire you can see the paper I presented at the conference as well as a video that I had made for the conference. The video shows a two-place craft. The actions shown are: two people boarding the craft, the craft taking off, retracting its landing gear, gaining altitude and speed, retracting the extensions of its rotor blades, flying at a speed of 340 MPH at 6000 ft. altitude, coasting and losing altitude, retracting its blade extensions, slowing further, lowering its landing gear, and landing again.

The video simulation was done by www.phantommouse.com.

jlawren3
 
Great video. Good looking and quite informative.
Are retractable and quickly servo acting blades in use somewhere?
 
You mentioned the professor saying how it resembled a dragonfly, and I ran into these old German Ornithopter ideas that seem to be almost exactly similar to yours.

From Rob Arndt, this is not considered a very reliable source but it's still interesting:
http://discaircraft.greyfalcon.us/Adalbert%20Schmid%20Ornithopter.htm
 

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MZ,

First about fast DC servo motors. The motors I found at

http://trustautomation.com/Library/pdf/Datasheets/SE600%20Series%20Motors%20-%20Preliminary.pdf

are remarkable. Their model SE640-0750E04 has a mechanical time constant of 0.35 milliseconds and an electrical time constant of 6.3 milliseconds. This means that if a step change of voltage is applied to the windings, the motor speed comes up to 63% of the speed corresponding to that voltage in only (0.35 + 6.3) milliseconds. If the power amplifier could drive a command current through the motor windings immediately and and largely overcome the time lag due to the inductance, then the motor speed would respond in close to 0.35 milliseconds. There is much more I could say about inductance, power amplifiers, etc., but in any case that is one quick motor! That particular motor has a power output capability of 750 W (about 1 HP) at 5000 RPM and it weighs only 7.3 lbs. which isn't bad. I think it should make a good drive motor for changing the pitch angle of the rotor blades in the 2-man craft I described.

I am not vouching for the accuracy of this Trust Automation, Inc. datasheet, but I believe that finding a brushless DC servo motor (with suitable added gearing) to quickly position the rotor blades should not be too difficult.


About the Ornithopter link you sent - it was curious that there is some similarity to what I am proposing. However, I am separating the rotors a lot and not adding a tail for directional stability. The concept shown would have a problem with longitudinal CG variations. I think that the sketch shown could fly well only when the CG is between the rotors and that's not easy because the rotors are very close together.

In my post of March 15, 2009 I referenced a product by Lite Machines at http://www.litemachines.com/mil/specs.htm
which is somewhat like the link you sent with the rotors up front. Notice that the Lite Machines craft never noses over to fly horizontally. It can't, of course, because the CG is aft of the rotors.

jlawren3
 
Hi All,

Your comments would be appreciated.

A good application for this new UAV would be in carrying critical components between naval ships at sea or between naval ships and land bases.

This should probably be a simple implementation of the craft without retractable rotor blade extensions and without landing gear. It is proposed that conically shaped landing receptacles would be provided for the "tiltplanes" to land in. The tiltplanes would simply nest their conically shaped tails into these receptacles and the receptacles would absorb moderate vertical velocities with springs and shock absorbers. When the rotors stop turning sailors would walk up to the tiltplane, open the door in the side, and remove the items which it carried and place other items in the tiltplane's cargo space. The tiltplane's "nesting" receptacle could also contain a fuel connection for automatically refueling the tiltplane. (Small internal combustion engines which accept heavy fuel are now available, so the tiltplane could be automatically refueled with jet fuel.)

I believe that a tiltplane for transporting critical items should be designed to fly at the highest practical speed, which at sea level should be at over 300 MPH. Its fuel consumption will be a lot higher than it would be at, say, 200 MPH, but compared to the fuel consumption of jet aircraft, the amount of fuel burned will be insignificant.

Also, I think that these tiltplanes should fly only 50 feet or so above the ocean surface. This would keep them below radar detection, make them relatively hard targets for enemies in ships or in airplanes, and reduce the likelihood that they will interfere with other aircraft. But they will need to be designed to avoid ships, approaches to airports, power lines, bridges, and other obstacles.

I have some questions and would like your inputs about this possible application:

1) Is there a serious need to be able to quickly move some items between naval ships or between naval ships and shore facilities?

2) What should the cargo-carrying capability of such a tiltplane be? 50, 100, 150 or 200 pounds, or more?

3) What should be the unrefueled range of this tiltplane?

4) Do you see any possible problems with this proposed naval package-carrying application that have not been mentioned?



The above text and questions will soon be on my website at www.tiltplane.com.


jlawren3
 
Not: A New Flying Machine

Uh, did I see a design in APR of a comparable tail-sitter with a gun-turret in 'tail' ?

Think Kamal meets A-10...

Regret my copies are packed away...

Regret disastrous decline in $/£ rate meant I had to discontinue my APR subscription...
 
Hi Nik,

You probably saw something different from this "tiltplane" concept. This flight approach is brand new; no large companies have gotten involved with it yet. But I presented the concept at the January meeting of the American Helicopter Society in San Francisco. It also has the technical support of a respected consultant in the helicopter industry, Ray Prouty. Please see my website at www.tiltplane.com; there is a neat video there about this new aircraft.

Since the AHS meeting in January I have been focusing on implementing the concept as a VTOL UAV (Vertical TakeOff and Landing Unmanned Aerial Vehicle).

It is a tail-sitter which takes off with its fuselage vertical but cruises with its fuselage horizontal.

jlawren3
 
I enjoy having a new concept to advocate for. Here are some notable features of this new aircraft concept:

1) Unlike helicopters this concept offers a high-speed cruise capability of 300 or 400 MPH.

2) This concept offers a VTOL capability with an efficient cruise that compares well with fixed-wing aircraft. Helicopters have L/D ratios of only 7:1 to 9:1, but one tiltplane design I was evaluating had a L/D of 19:1 - and I was not trying to maximize that ratio.

3) Fixed-wing aircraft are subject to high on-board accelerations when flying at high speeds and low altitudes. This is related to their large wing chords and to the existence of clear air turbulence. The rotor blades of tiltplanes have much smaller chords and the angles of attack of the rotor blades can be adjusted automatically and very quickly so that when the aircraft is flying at low altitudes and high speeds through rough air it will experience relatively low accelerations.

Sincerely,

John Lawrence
 
This is the first I've seen this thread. But the thing that *LEAPS* out at me is that the vehicle design is almost *exactly* like one of Lloyd H. Leonards designs. Not just a "published in a paper" design, but a patented design. Specifically his design in patent 2387762, granted way back in 1945.
http://www.google.com/patents/about?id=dD1-AAAAEBAJ&dq=2387762
 

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Hi Orionblamblam,

If you scroll down through previous posts on this topic you will see that the Leonard patent has been discussed.

When my patent attorney first saw the Leonard patent he advised me not to file. But I pointed out that in the Leonard patent there is no mention of how that craft would transition between a vertical orientation and horizontal flight. The craft must do this and it is not easy to do. But I had worked this out to the point where I was quite comfortable that the craft could "get through transition" well.

A reputable aeronautical engineer who specializes in helicopters and helicopter-like designs, Ray Prouty, analyzed it and agreed with me.

I guess it happens every day that someone takes an idea that exists and extends it by a more intimate understanding of what is needed and how that can be accomplished.

One evidence to me that the Leonard design did not fully appreciate the potential of the concept is that the drawings also show added wings. Another is that the rotors are close together near the center of the craft which is not necessary and works against carrying cargo.

It would not be very satisfying to me to just be able to get something through the patent process; that would only be a hollow victory. I am hoping that tiltplanes can be built and that they will find many very useful applications.

jlawren3
 
jlawren3 said:
I pointed out that in the Leonard patent there is no mention of how that craft would transition between a vertical orientation and horizontal flight.
Click to expand...

That's not accurate. Figure 17 in that patent shows the process diagramatically; it is also shown in Figure 6 of another one of his (apparently many) patents #2444781, and discussed and described in the text.

L.H. Leonards patent #2446480 was just for the landing gear.

Also similar is this rather terrifying design by D. C. prince, #2622826; and a design by Joseph Stuart of General Motors, #2397632; and this much more recent design by a feller with the unlikely name of Rene C. A. Tassin de Montaigu, #4123018.




There ain't nuthin' new. :'( You've no idea how many times I've had a Brilliant Thought and then gone to Google or the USPTO and found that someone else dun thunk it up decades ago.
 
Hi Orionblamblam,

In fact, one could go as far as saying that a flying model has been demonstrated; see Figure 4 in my paper on the tiltplane.com website. (Fig. 4 shows a two-propeller toy.)

My patent attorney said that previous disclosures, as revealed by a world-wide patent search, had not reduced the concept to practice, so I am free to apply for a patent.

I personally don't feel like I am taking someone else's idea and running with it. I think of myself as taking the concept illustrated by the two-propeller toy shown in Figure 4 and reducing it to practice. I consider myself extremely fortunate in that (a) no one else has done this and (b) the concept has great promise. It is not like it has been tried and failed; it has just not been tried, at least as far as I can tell.

Fixed wing aircraft need lots of wing area to take off. That same wing area hurts their cruise efficiency at high speeds. Tiltplanes don't have this problem; they need much less rotor blade area than the wing area of fixed-wing aircraft. As a result they can cruise more efficiently than comparable fixed-wing aircraft. And, of course, they can also take off vertically. How good is that?

jlawren3
 
jlawren3 said:
My patent attorney said that previous disclosures, as revealed by a world-wide patent search, had not reduced the concept to practice, so I am free to apply for a patent.
Click to expand...

Wait. If I follow you, this means that every patent that was taken out on somethign that didn't get built... can be patented *again* by someone else?

This seems unlikely, as it would mean that people would be constantly trolling expired patents for things to re-patent themselves.
 
Orionblamblam,

I am saying that ideas or sketches similar or even equal to what I am proposing have been disclosed before, but they were never reduced to practice. The gating issue is whether or not they have been reduced to practice. If you want to apply for a patent you have to first show a useful implementation of the thing your are working on. Ideas alone cannot be patented.

Here is an interesting progression:

MILLIONS OF IDEAS
\/
SOME OF WHICH GET TRANSLATED INTO WORKING HARDWARE
\/
A SMALLER NUMBER GET PATENTED (AN OPTIONAL STEP)
\/
A STILL SMALLER NUMBER ARE USEFUL


jlawren3
 
One slight problem: IIRC, there's now a parachute system available for small aircraft. Could such a system be included ? Otherwise, there is a scary-wide 'drop-dead' envelope, below which a successful transition to 'auto-gyro' is impossible...

OT: I still can't find that 'close air support' VTOL design which had a gun-turret in its tail...
 

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