Design Challenge: Lightweight Multirole Fighter (LMF)

carvalho2008 said:
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Only auxiliary when dogfight....!

when BVR combat, you don't need this..
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In this case I suspect your SRAAM weapon will also be in a ventral bay as that is more or less where this size aircraft forces it. Those usually being rail launced instead of dropping away I suspect you will have an engine flameout through exhaust gas ingestion with every launch thanks to the intake being so far back.
Shockwave control when doing high subsonic, transonic or even supersonic manuvering when the ventral intake suddenly becomes the 'prime' intake will also be very interesting with all the louvres and somewhat steep intake angle required.

As VTOLicious said - more than enough companies and engineering teams have toyed with the idea of a dorsal intake for a aircraft where good high-AoA performance was one of the requirements and while enough have concluded it will work in theory, none have been convinced enough to actually build it. That says something in my view...
 
BLACK_MAMBA said:
In this case I suspect your SRAAM weapon will also be in a ventral bay as that is more or less where this size aircraft forces it. Those usually being rail launced instead of dropping away I suspect you will have an engine flameout through exhaust gas ingestion with every launch thanks to the intake being so far back.
Shockwave control when doing high subsonic, transonic or even supersonic manuvering when the ventral intake suddenly becomes the 'prime' intake will also be very interesting with all the louvres and somewhat steep intake angle required.

As VTOLicious said - more than enough companies and engineering teams have toyed with the idea of a dorsal intake for a aircraft where good high-AoA performance was one of the requirements and while enough have concluded it will work in theory, none have been convinced enough to actually build it. That says something in my view...
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Well, there are supersonic aircraft with this 100% dorsal model and they confirm that there is no impediment. It can then be argued that these are aircraft focused on bombing and that they do not engage in dogfighting to expose maneuvering angles with reduced air intake. Ok, but I demonstrated that auxiliary air intake grilles are nothing new and the concept could be applied mainly, because their function is complementary only when necessary, automatic and controlled by the FBW for a few seconds of the maneuver. It also does not harm the stelth characteristic since it is closed 99% of the time and when in use or activated, it is because the aircraft is in an approximate visual dogfight. The size is almost irrelevant, as its diameter is a tiny fraction and its length is also small compared to a normal duct from the frontal section of the aircraft. it only extends from the belly of the plane to the root of the engine on the back and can curve inside the air frame so as not to compete for space with the weapons bay. This doesn't even seem to make sense because its existence is precisely to free up space for the weapons bay or to mount a second weapons bay. Regarding the risk of ingesting gases from the missiles, as already stated, since the FBW Control System central is in charge and controlling opening and closing. Its premise is that the plane is in a maneuver of more than 6G's inclination and has to fire in exactly those seconds. The computer center only needs tenths of seconds to close the shutter and reopen it after the missile is expelled. We all know that the missiles from the bay are expelled and only after moving away do they start the engine....

The design is purely conceptual...because no one did it....obviously...no one needed it....the other stealth fighters are all classified as air superiority fighters....is this the concept in focus?? no, it's not....it's a 2nd front line stealth fighter...cheap, less demanding and simple, STOL, with interchangeable weapons bays between internal plg and play auxiliary fuel tanks, sensors and the weapons themselves at the Textron's Skorpion style....a MAKO with internal bays, an F-20 Stealth, a Manned Kizileuma and vice versa....
 
red admiral said:
Not that big actually if you take into account other variables like production numbers, rates etc.
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I mean, are we talking +-5%, 1%, 15%?


red admiral said:
You can also split into cost/mass for different aircraft bits e.g. airframe structure, vehicle systems, engine, avionics if you have enough information
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Yes, that would definitely tighten up the estimates.
 
carvalho2008 said:
Well, there are supersonic aircraft with this 100% dorsal model and they confirm that there is no impediment. It can then be argued that these are aircraft focused on bombing and that they do not engage in dogfighting to expose maneuvering angles with reduced air intake. Ok, but I demonstrated that auxiliary air intake grilles are nothing new and the concept could be applied mainly, because their function is complementary only when necessary, automatic and controlled by the FBW for a few seconds of the maneuver. It also does not harm the stelth characteristic since it is closed 99% of the time and when in use or activated, it is because the aircraft is in an approximate visual dogfight. The size is almost irrelevant, as its diameter is a tiny fraction and its length is also small compared to a normal duct from the frontal section of the aircraft. it only extends from the belly of the plane to the root of the engine on the back and can curve inside the air frame so as not to compete for space with the weapons bay. This doesn't even seem to make sense because its existence is precisely to free up space for the weapons bay or to mount a second weapons bay. Regarding the risk of ingesting gases from the missiles, as already stated, since the FBW Control System central is in charge and controlling opening and closing. Its premise is that the plane is in a maneuver of more than 6G's inclination and has to fire in exactly those seconds. The computer center only needs tenths of seconds to close the shutter and reopen it after the missile is expelled. We all know that the missiles from the bay are expelled and only after moving away do they start the engine....

The design is purely conceptual...because no one did it....obviously...no one needed it....the other stealth fighters are all classified as air superiority fighters....is this the concept in focus?? no, it's not....it's a 2nd front line stealth fighter...cheap, less demanding and simple, STOL, with interchangeable weapons bays between internal plg and play auxiliary fuel tanks, sensors and the weapons themselves at the Textron's Skorpion style....a MAKO with internal bays, an F-20 Stealth, a Manned Kizileuma and vice versa....
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The F-107 is hardly proof of the concept for a high AoA fighter. Saab and Northrop toyed with the idea but abandoned it. All the other concepts you showed were non transonic level flight aircraft - hardly proof that the dorsal intake is a viable option for high AoA roles. Louvres being opened or closed by the FBW system adds a bunch of unrequired complexity to the system. Closing them momentarily for a missile launch or similar will cause huge issues as you suddenly starve and then return airflow to the engine. Not to mention the complexity of slowing the air down and mixing the two streams when in high speed.

So far we have seen a dorsal intake is plausible when only minor AoA of a few degrees is required, and louvres work only as auxiliry vents to the main intake(s) (Mirage 3, F1 and M2000 being good examples here) in slow speed flight or when taxiing (MiG 29 and F-5). I have yet to see proof where aux intakes can be used as a primary intake like is required here. Just because both exist, doesn't imply they can work together in extreme scenarios as required here.

Designing the air intake of a transonic aircraft with high AoA capabilities is a non trivial task to begin with and with thousands of engineers having studied dorsal applications and positioning all knowing about aux intakes etc and knowing the advantages in terms of space it yields. The lack of utilisation is a sign into how sure they are it will work...
 
BLACK_MAMBA said:
The F-107 is hardly proof of the concept for a high AoA fighter. Saab and Northrop toyed with the idea but abandoned it. All the other concepts you showed were non transonic level flight aircraft - hardly proof that the dorsal intake is a viable option for high AoA roles. Louvres being opened or closed by the FBW system adds a bunch of unrequired complexity to the system. Closing them momentarily for a missile launch or similar will cause huge issues as you suddenly starve and then return airflow to the engine. Not to mention the complexity of slowing the air down and mixing the two streams when in high speed.

So far we have seen a dorsal intake is plausible when only minor AoA of a few degrees is required, and louvres work only as auxiliry vents to the main intake(s) (Mirage 3, F1 and M2000 being good examples here) in slow speed flight or when taxiing (MiG 29 and F-5). I have yet to see proof where aux intakes can be used as a primary intake like is required here. Just because both exist, doesn't imply they can work together in extreme scenarios as required here.

Designing the air intake of a transonic aircraft with high AoA capabilities is a non trivial task to begin with and with thousands of engineers having studied dorsal applications and positioning all knowing about aux intakes etc and knowing the advantages in terms of space it yields. The lack of utilisation is a sign into how sure they are it will work...
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There is no close combat dogfight of high curves at supersonic speed....the pilot cannot resist....so the high close combat air combat curves occur at much lower speeds...if it is at supersonic speed, the radius of the curve is large and not accentuated...

ok, each one in their own mission usage regime....really between high complexity, medium complexity and low complexity, it really doesn't seem to be high complexity...but we can accept the phrase...if it were good, someone I would have done it already....thank you...
 
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BLACK_MAMBA said:
So far we have seen a dorsal intake is plausible when only minor AoA of a few degrees is required, and louvres work only as auxiliry vents to the main intake(s) (Mirage 3, F1 and M2000 being good examples here) in slow speed flight or when taxiing (MiG 29 and F-5).
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HP.115 flew merrily along at 35deg AoA with a really far back dorsal intake. But then the key is designing what is in front of the intake - in this case really high sweep sharp LEs formed very strong vortices and the intake took cleaner air from between them.
 
carvalho2008 said:
There is no close combat dogfight of high curves at supersonic speed....the pilot cannot resist....so the high close combat air combat curves occur at much lower speeds...if it is at supersonic speed, the radius of the curve is large and not accentuated...
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The first turn of a dogfight is likely to be supersonic assuming a supersonic approach, and then each following turn slower and slower (and lower altitude) as the aircraft bleed off energy.

red admiral said:
HP.115 flew merrily along at 35deg AoA with a really far back dorsal intake. But then the key is designing what is in front of the intake - in this case really high sweep sharp LEs formed very strong vortices and the intake took cleaner air from between them.
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Wanting LO shaping makes getting those vortices in the correct places a challenge.
 
red admiral said:
HP.115 flew merrily along at 35deg AoA with a really far back dorsal intake. But then the key is designing what is in front of the intake - in this case really high sweep sharp LEs formed very strong vortices and the intake took cleaner air from between them.
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You are correct, but then again as you said - this was an aircraft very specifically designed around the idea and was no more than concept. In the years after with computational power improving exponentially and our ability to predict airflows improving dramatically we are yet to see another plane actually fly and explore high AoA flight with a dorsal intake. Hence the assumption the risk of it failing is not worth the expense I assume. Notably both Saab and Northrop spent plenty time evaluating the idea but never went further.
 
Scott Kenny said:
$45mil is close to the cost of Gripen C/D, isn't it?
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Maybe US$30 - 40M...20 odd years ago. It is somewhat meaningless though since they have been out of production for over 15yrs now. Even if someone were to produce a new C/D, I think you will find it is closer to the E/F price of reportedly $85M now.
 
red admiral said:
There's a lot of low fidelity US cost forecasting models that use $/lb for unit costs. KF-21 seems a pretty good near neighbour so I'd roughly expect similar $/lb for the airframe, and you've got ~half the engine costs. Looking at the masses then this would put this FAR-21 down at 65-70% of the UPC of KF-21 which is a fair saving. Sure its not as "capable" but it is lower cost. So if KF-21 is really $65mUSD upc then this would be down at around $45mUSD for equal other assumptions (like relatively small production run, low labour costs etc.) - but it could change massively with different programme assumptions.
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I think you would be very hard pressed to get a manned, 5th gen for less than US$50M. Moreover, I think the reported KF-21 US$65M is fanciful too and predict it will end up closer to US$80 - 100M a copy.
 
Perhaps the question we should be asking is,does a country that needs this kind of fighter really need full 360 stealth? Could you get away with something like the early versions of the KF-21? That would solve the internal volume issue, and the aircraft would still be useful for like 90% of the missions. You can keep a small internal bay for the remaining 10% of missions.
 
Desertfox said:
Perhaps the question we should be asking is,does a country that needs this kind of fighter really need full 360 stealth? Could you get away with something like the early versions of the KF-21? That would solve the internal volume issue, and the aircraft would still be useful for like 90% of the missions. You can keep a small internal bay for the remaining 10% of missions.
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It's not just a problem of invisibility. An underwing tank consumes between 30% and 40% of its own load to compensate for drag. Fitting everything in internal bays means increased range.
 
VTOLicious said:
To explore another option of "beast mode" I've created a fuel tank for the installation in the main weapons bay. It has a capacity of 450 gal / 1700 l (1360 kg) and a weight of 1500 kg (I accounted ~10% for the structure).

With this tank installed the internal fuel capacity is increased to 5700 l (4560 kg). That is 42,5% in addition to the normal capacity of 4000 l (3200 kg).
With the weapons load depicted the takeoff weight is 14702 kg, or 92% of the maximum.

View attachment 708855
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This plug and play stall idea is really good.
 
Desertfox said:
Perhaps the question we should be asking is,does a country that needs this kind of fighter really need full 360 stealth? Could you get away with something like the early versions of the KF-21? That would solve the internal volume issue, and the aircraft would still be useful for like 90% of the missions. You can keep a small internal bay for the remaining 10% of missions.
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No but one could save in other ways Like using cots, minimal or no RAM, low cost composites, extrem Long living airframe and open Architectur would be ways to save now and later Money while retaining most capabilitys but also WE have to consider that this probaly will be a 30-60 year live Span Fighter for small Nations who can't use Money 24/7 for those Jets.
 
GTX said:
I think you would be very hard pressed to get a manned, 5th gen for less than US$50M. Moreover, I think the reported KF-21 US$65M is fanciful too and predict it will end up closer to US$80 - 100M a copy.
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I think that's probably true but my main point is that this concept is smaller and quite a bit lower upc. Only thing is that those KF-21 costs may be significantly lower due to much lower labour costs. These are a big element of costs and there will be significant impact from say the 3x difference in labour rates between Sweden and Korea. But it depends on many things.
 
carvalho2008 said:
View: https://www.youtube.com/watch?v=b0lcla1NtK8
Note that there is no possibility of ingesting gases. The missile is expelled and the engine is activated.
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These are long range BVRAAM shots. In high AoA situations you will fire a SRAAM. Presumably or preferably carried internally to increase LO signature, and to increase range through less drag as you mentioned. I have yet to see a SRAAM that is not rail launced. I believe the weapons bay door mounted AMRAAM on F-35 will also be rail launched.
F-35 thus far cannot carry a SRAAM internally to knowledge (Brits wanted to fit ASRAAM interally but cancelled) so in a highly contested enviroment will be limited to BVR only - the prefered way to engage anyway.

Perhaps that would be the constraint placed on the fighter you prescribe. Evolve fighter around high speed, good loiter time and large internal payload with a focus on BVR kills and to avoid the merge at all costs and accept draw backs like poor high AoA performance for the BVR mission benefits. Starting to sound remarkably like the role Tornado F3 had!
 
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red admiral said:
I think that's probably true but my main point is that this concept is smaller and quite a bit lower upc. Only thing is that those KF-21 costs may be significantly lower due to much lower labour costs. These are a big element of costs and there will be significant impact from say the 3x difference in labour rates between Sweden and Korea. But it depends on many things.
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I wouldn't say that Swedish airframe builders have 3x the pay of SKorean...
 
GTX said:
Maybe US$30 - 40M...20 odd years ago. It is somewhat meaningless though since they have been out of production for over 15yrs now. Even if someone were to produce a new C/D, I think you will find it is closer to the E/F price of reportedly $85M now.
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Frack, seriously? I thought they were still in production!

No wonder they were getting hammered by F-16 sales internationally...
 
Scott Kenny said:
The first turn of a dogfight is likely to be supersonic assuming a supersonic approach, and then each following turn slower and slower (and lower altitude) as the aircraft bleed off energy.

Wanting LO shaping makes getting those vortices in the correct places a challenge.
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Exactly my thoughts. Canted fuselage side walls and corresponding sharp edges are distinctive features of LO aircraft. I have a hard time to imagine a LO shaped nose fuselage section that would foster smooth airflow at higher AoA.
img-20170216-002243-863_orig.jpg
 
VTOLicious said:
Exactly my thoughts. Canted fuselage side walls and corresponding sharp edges are distinctive features of LO aircraft. I have a hard time to imagine a LO shaped nose fuselage section that would foster smooth airflow at higher AoA.
View attachment 708993
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Only thing I can come up with is LO canards to generate and control the vortices.
 
BLACK_MAMBA said:
These are long range BVRAAM shots. In high AoA situations you will fire a SRAAM. Presumably or preferably carried internally to increase LO signature, and to increase range through less drag as you mentioned. I have yet to see a SRAAM that is not rail launced. I believe the weapons bay door mounted AMRAAM on F-35 will also be rail launched.
F-35 thus far cannot carry a SRAAM internally to knowledge (Brits wanted to fit ASRAAM interally but cancelled) so in a highly contested enviroment will be limited to BVR only - the prefered way to engage anyway.

Perhaps that would be the constraint placed on the fighter you prescribe. Evolve fighter around high speed, good loiter time and large internal payload with a focus on BVR kills and to avoid the merge at all costs and accept draw backs like poor high AoA performance for the BVR mission benefits. Starting to sound remarkably like the role Tornado F3 had!
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View: https://www.youtube.com/shorts/_Uwrn5PZh1M


Ok, but...again there is no possibility of ingesting gases....the missile is underwing...isn´t in central frame....
 
carvalho2008 said:
Ok, but...again there is no possibility of ingesting gases....the missile is underwing...isn´t in central frame....
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It actually does ingest exhaust gasses even from a wingtip launch. The moment the firing sequence is activated the aircrafts onboard computers enrich the fuel mixture to prevent a flameout. That is nothing new though and has been done on fighters for decades.

But again, this is an external carriage wingtip launch (aircraft not in VLO mode), not a weapon bay launch and more importantly (as the F-22 has proven internal rail launch SRAAM feasability), not one done directly infront of an engine air intake like would be the case for an internally carried SRAAM in a bay in front of a large aux ventral intake.
 
kqcke for you said:
No but one could save in other ways Like using cots ... extrem Long living airframe
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People offer up COTS as a solution but I am yet to see anyone seriously use such for a combat platform.

Going for an extreme long life airframe will, unless the annual hours are kept low or pilots live with higher risk actually result in higher cost.
 
GTX said:
People offer up COTS as a solution but I am yet to see anyone seriously use such for a combat platform.

Going for an extreme long life airframe will, unless the annual hours are kept low or pilots live with higher risk actually result in higher cost.
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Well with City i mean cots from Military Hardware. Like Eurofighters Radar and IRST, Gripen Computer System and and and.
One could Chose those which the Most countrys you want to Export to.
 
kqcke for you said:
Well with City i mean cots from Military Hardware. Like Eurofighters Radar and IRST, Gripen Computer System and and and.
One could Chose those which the Most countrys you want to Export to.
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Ah, so more MOTS - Modified, modifiable or military off-the-shelf
 
VTOLicious said:
Kind of "leading-edge slats", along the edge of the nose fuselage, which are automatically deployed by the FCS, could work...
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I'm starting to like this idea and made some concept sketches... The slats smoothen the airflow and guide it over the canopy towards the intake, whereas the LERX create a strong vortex lateral of the intake.
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VTOLicious said:
I'm starting to like this idea and made some concept sketches... The slats smoothen the airflow and guide it over the canopy towards the intake, whereas the LERX create a strong vortex lateral of the intake.
View attachment 709040
View attachment 709041
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A major question will be what that does to the RCS, but that's not something I have the knowledge to address in detail.
 
kqcke for you said:
How will that work? My brain for some reason Just can't understand it.
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As the nose pitches up, the plane kinda skids through the air for a bit before the relative airflow goes back to normal.

Picture a bike popping a wheelie, it's the same kind of idea. Plane is still going flat down the "road", but the air is hitting the underside.

These strakes will pop out to grab air from the underside of the plane and smooth the airflow around the fuselage to the topside and then to the inlet.
 
Scott Kenny said:
A major question will be what that does to the RCS, but that's not something I have the knowledge to address in detail.
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Large deflections of any control surface have a negative effect. It doesn't matter a lot if it's a aileron, spoiler or slat. However, RCS is no longer as important as soon as you are forced to perform maneuvers that require large deflections.
 
VTOLicious said:
Large deflections of any control surface have a negative effect. It doesn't matter a lot if it's a aileron, spoiler or slat. However, RCS is no longer as important as soon as you are forced to perform maneuvers that require large deflections.
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IIRC, leading edge control surfaces have an RCS impact, and those active strakes will add some edges to cause reflections. And that's just flying along trimmed out for minimum control surface movements.

The leading edge of the active strake can probably be a single point, while the trailing edge will probably need to be a sawtooth instead of a single point. The sawtooth will allow more of the strake to be closer to the leading edge of the wing than a single point.

The pop-out system should probably just be round rods, as that will cause the least disruption to airflow under the slats regardless of angle of attack.
 

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