Supersonic Propeller Aircraft Design Questions

[Kryptid]

For a long time now, I've been trying to think of how a supersonic propeller-driven aircraft might be designed. If such an aircraft is unfeasible, then let's say I am speaking of a design that can come as close to the sound barrier in level flight as possible. There are some concerns about it that I'd like some input on:

(1) One way to maximize thrust while minimizing drag might be accomplished by using a "push-pull" propeller configuration akin to that used on Do 335 Pfeil. However, I've been wondering about the consequences of having a tractor propeller on an aircraft that is breaking the sound barrier. Would the shocks thrown off by the forward prop cause problems by impinging on the fuselage and/or wings of the aircraft? Could this problem be solved or would I be better off going with a pusher-only configuration?

(2) I currently intend on using one or two turboprop engines, but I have been wondering about the validity of electric motors. Can the power density of an electric motor match that of a turboprop? The reason I have considered an electric motor is because such an engine wouldn't have its power output affected by the thin air at higher altitudes. I know that batteries are nowhere near as energy dense as petroleum-based fuels, but range is not an important factor in the design.

 

[archipeppe]

http://en.wikipedia.org/wiki/Republic_XF-84H

 

[Jemiba]

The XF-84 didn't actually reached M1, but already was fitted with a very long spinner.
Maybe a layout similar to the Blohm & Voss P.192 could be used ?
( http://www.luft46.com/bv/bvp192.html )

 

[pathology_doc]

I don't think it's possible, or if it is, it's not all that feasible. Things to consider:


Remember that even a zero-bypass turbojet has to have a subsonic airflow into the compressor face.


If you're going to run your prop supersonic, it's going to need some sort of curved-tip blade section, probably broad scimitar-like paddle blades, and be made of some sort of exotic alloy if it's not going to tear itself to bits or bend/snap under high tip loads. And even if you can do it, the power required to work up to speed is going to be horrendous, which means you can forget about batteries right then and there.


The problem becomes WORSE at high altitudes because the speed of sound is LOWER in miles-per-hour terms than at sea level and your prop blades are going to be at a significantly higher Mach number for a given airspeed/rpm/advance ratio.

 

[Kryptid]

If you're going to run your prop supersonic, it's going to need some sort of curved-tip blade section, probably broad scimitar-like paddle blades, and be made of some sort of exotic alloy if it's not going to tear itself to bits or bend/snap under high tip loads.

If I recall correctly, the XF-88B (which had been equipped with a turboprop in addition to the turbojets) slightly exceeded Mach 1 in a dive with the propeller running. It didn't disintegrate, so construction of a sufficiently durable prop should be feasible (it was composed of steel). Also, the propeller had a surprising (to me at least) efficiency of 78% at Mach 1.01 (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090296.pdf).

And even if you can do it, the power required to work up to speed is going to be horrendous, which means you can forget about batteries right then and there.

I had already considered that issue, which I thought might be overcome by having a B-52 or other large aircraft carry it to altitude before releasing it for its full-powered run. The B-52 can already exceed 600 miles per hour and reach 50,000 feet, so a lot the hard work would already be done for this X-plane by the bomber itself.

The problem becomes WORSE at high altitudes because the speed of sound is LOWER in miles-per-hour terms than at sea level and your prop blades are going to be at a significantly higher Mach number for a given airspeed/rpm/advance ratio.

If one's goal is to breach the sound barrier, then that's going to happen regardless of the altitude. I considered the higher altitudes to be favorable specifically because the speed of sound is lower there (and therefore easier to reach).

 

[AeroFranz]

Kryptid,


simply put, what's happening is that your drag undergoes drastic increase due to compressibility effects. I mean A LOT. See pic below. To overcome this, you need equal amounts of thrust. In a prop-driven aircraft, thrust is defined as


Thrust = Power * prop efficiency / airspeed


unfortunately the prop efficiency falls off dramatically as compressibility effects affect the blade tips. See second picture, where D is the pro diameter, n is the RPM, V is airspeed, and beta is the prop pitch. All the curves show fall off of efficiency as the advance ratio increases.

It is a double-whammy that no reasonable amount of power can overcome. Doesn't matter if you're turboprop/battery/pedal/steam/magic powered. It's just that the open propeller does not want to operate at transonic speeds.

 

[ouroboros]

If I remember correctly, some of the more recent design attempts/research have a very streamlined bullet shape with twin contrarotating scimitar bladed propellers aft to keep the blades out of the forward shock cone from the nose. Though of that group, some may be cheating a bit as they are turboprop or UDF based engine designs with a center aft exhaust in the spinner area, so at higher altitudes and speeds, more of their thrust is derived from the turbojet portion of the powerplant. Even piston based powerplants try that trick as well, shaping and directing their exhaust to expand and jet aft.

Propellers do their best work in the lower atmosphere, but the drag is also worse there. That's why jet exhaust based powerplants do so much better at high altitudes, as the reduced pressure and drag works in their favor (at least for ground speed purposes).

 

[Kryptid]

If we are going to discount Mach 1 performance, what kind of top speed would actually be reasonable for this aircraft?

 

[AeroFranz]

You can start by looking at the current record holders. They started life as warbirds, so they were not optimized for pure speed alone. However, they were highly modified in every single part that could be reasonably altered. Because of the expense involved, they don't have custom props or newbuilt engine, although once again these were carefully selected and are not too far from the optimum.
I think it is possible to improve on current records, but as we discussed you are working in a region of diminished returns. I think the prop is the most important bottleneck here, so you'd want to check out advances in propeller aerodynamics.

 

[TomS]

The warbirds are the fastest piston aircraft, but the OP asked about turboprops.

Fastest turboprop plane that actually worked seems to be the Tu-95, which tops out somewhare above Mach 0.80 (sources differ on 0.82 or 0.87). But one gets the sense that at those speeds, a big fraction of the thrust must be coming from the turboprop's exhaust, not the props. The prop tips themselves must be fully supersonic at that point, which explains why the Bear is so incredibly (dangerously) loud. Cruise is a more sensible Mach 0.70 or thereabouts.

Whether you could achieve the same with a fighter-type aircraft is another question.

 

[malipa]

It will never work, because a propeller looses efficiency incredibly fast when it approaches the speed of sound, because the propeller tips go through the speed of sound. So this is not possible.

 

[AeroFranz]

If you read the earlier posts, you will see there was a consensus about just what you said.
Kryptid restated his question around what the achievable speed would be.


My .02 is that, as TomS mentioned, you can get your an increased percentage of residual thrust from the exhaust (which to a certain extent diminishes the whole point of establishing a prop-driven record), or you find a way to make the prop do some useful work at the high advance ratio condition. Otherwise you are stuck with the same level of speeds achieved so far.

 

[steelpillow]

As has been pointed out, the double-dose of wave drag from props and airframe makes the whole prospect inefficient and horrendously noisy.The Republic XF-84 had a supersonic prop, which earned it the name "Thunderscreech", but still only reached around 500-600 mph or Mach 0.7-0.8 (sources disagree). And it also experienced serious stability problems. If you want supercruise - supersonic flight without afterburner - you go for a bypass turbofan.

The practical top speed of a propeller is hit when the blade tips reach transonic speeds. Conventional straight-bladed planes have reached around 430 mph. They typical approach is to sweep back the blades in a sabre-like curve, raising their critical Mach no. to somewhere around 0.9. Adding blades helps, creating what is called a propfan or unducted fan. So far, these have only been fitted to large transports, but you could probably reach tip speeds around Mach 0.9+ and cruise speeds around 450 mph or Mach 0.65.

But let's throw all that to the wind and set out to show the Thunderscreech who's the Daddy. Assume the blade goes supersonic along its whole length. It becomes a (somewhat twisted) supersonic wing and much the same rules apply, save that simply sweeping back the subsonic formula of high aspect ratio is mechanically impossible due to centrifugal force. So you end up with a stubby, short but broad-chord blade which writhes in eye-numbing ways. Choose the F-104 unswept, Mirage Delta or a typical tapered-swept form to taste, but do watch those twists!

The blades will chuck out a complex web of spiralling shock waves inside the overall shock cone of the tip paths. So if you do not want to deafen your crew and mess up the wing aerodynamics, you will put the prop behind them, or out on the wings. A pusher prop would operate within the shock cone of the main airframe. If done badly this could induce unhealthy blade vibrations, but with care I don't see why it shouldn't work OK. Also, as the Thunderscreech showed, the extra blade drag magnifies its effect as a "tail at the front", making the plane aerodynamically unstable. The pusher layout should transfer the drag to the tail where it actually aids stability, in a speed regime where stability usually suffers: please ensure you include my name on the patent.

How many blades? The closer you pack them, the more their shock waves will reduce the efficiency of their neighbours. As with a subsonic prop, two will be the most efficient, add more only when you need more thrust but mechanical constraints forbid making them bigger.

 

[riggerrob]

Only a handful of (much-modified warbirds have exceeded 500 miles per hour. The most popular airframes are Grumman F8F Bearcat and NAA P-51D Mustang. Engine boost pressures were so high that it was a race to coax them through the course before their engines melted. Only the very best of race pilots could manage all those temperatures and pressures and speeds and only for a few minutes at a time ... barely enough to set FAI records on a 3 kilometer course.

Steelpillow made some good arguments in favor of pusher propellers. Mind you, they need to be as far aft as possible to improve directional stability. Think of pusher props as auxiliary tail stabilizers.
You also have to pay very careful attention to smoothing airflow into pusher props. The longer and smoother the aft fuselage the better, because any irregularity will cause localized super-sonic "spikes."
You also want to install exhausts outside the prop arc. Consider the distinctive sound made by Piaggio Avanti as pro blades "cut" exhaust gases vented forward of the prop. Ideally, you would route exhaust through the center of the (aft-pointing) spinner, but that configuration has the distinct disadvantage of increasing bearing temperatures in prop hubs that are already highly stressed.

Another factor to consider is air speeds inside the Mach cone created by the pointy nose. Unfortunately, my knowledge of super-sonic airflow can be written along the edge of a very thin wafer (Monty Python reference). Hah! Hah!

 

[riggerrob]

In 1999, David Rose bought the in-complete Mach-Buster proof-of-concept, pusher intended to race in the Unlimited class at Reno. Then Rose shifted to designing the similar Renegade with plenty of elite engineering assistance. Engineers calculated that Renegade could probably fly 495 mph, but it was never completed.

Some one else almost completed an experimental push-me-pull-you racer similar to Dornier 335 Pfeil, but struggled to find props for it..

 

[Sundog]

Just an FYI, there is a reference on this site, somewhere, regarding the supersonic props on the XF-88 and XF-84H. There are many good papers linked there and even more on the DTIC server. Some of them were quite efficient. Having said that, they're called supersonic props because they were designed to rotate at supersonic speeds. They were not designed to make aircraft go supersonic. You can think of them as early versions of unducted fans.

 

[steelpillow]

Of course, an electric motor would have no exhaust.

If you want to go piston, you will have problems providing a supersonic gas flow; you'd have to keep the exhaust temperature, and hence the speed of sound in it, high enough that its "internal" flow remained subsonic (this is true of all jet and rocket engines). It would require a classic convergent-divergent nozzle profile. You might even experiment with some kind of exhaust manifold afterburner.

While exhausting through the spinner means you will need to cool the bearings, that is probably more realistic than developing propellers to survive superhot exhaust flow from an annular nozzle. Or, you could duct the exhaust to either side and accept the duct losses.

 

[pathology_doc]

Another factor to consider is air speeds inside the Mach cone created by the pointy nose. Unfortunately, my knowledge of super-sonic airflow can be written along the edge of a very thin wafer

And then your engine does a Mr Creosote...