Can thrust vectoring be used to enable the use of flaps on tailless delta-winged aircraft?

Question

I'm considering flying wings as well as airplanes with only vertical stabilizer for their compact size, efficiency and high speed.

As I can't find a working example of such scheme, theoretical explanation is very welcome. To compensate for rear end additional lift, thrust output will probably be placed at the nose.

Possible options by propulsion types:

Propeller

• tilted prop rotor (fixed / variable pitch)
• propwash forwarding - canard fins
• collective pitch thrust vectoring (swashplate driven)

Jet

• additional small engine at the front (electric ducted fan)
• ducts from the main engine
• fan driven by the front shaft of the engine (as in Harrier Jump Jet)
• tractor placement (as in ekranoplan)

Problems

(summary from answers and comments, thanks for valuable input)

Stability.
Change of angle of attack either intentional or by air conditions changes wing lengthwise distribution of lift. Is it manageable for human to safely maintain stability in such conditions and if not - for a computer?

Thrust availability.
Approaching landing the forward thrust is minimized, so there should be close to 90° downward thrust line deflection.

Thrust agility.
Combustion jets are sluggish. There should be a nozzle limiting the output.
Partial reverser?

Safety.
Engine out below unpowered stall-speed at landing sounds like a nose down crash.
Redundancy should help.

So, is it viable to add enough nose-up moment and stability by vectoring thrust to counter the nose-down moment produced by flap extension, thus allowing STOL capabilities with flaps at acceptable angle of attack?

Answer 1

No, for several reasons

What you want is to compensate for the additional lift from downward deflected flaps at the back of a flying wing with vectored thrust. As @Sean points out this will not bring a noticeable net benefit if the lengthwise location of both forces is similar.

But that is not all.

Besides the force equilibrium around the flying wing you also need to consider how it behaves when flight parameters change, say by gusts. Will it return to the old state? How will the dynamic behavior be?

More camber means that the zero lift angle of attack becomes more negative. A deflected flap adds camber near the trailing edge. If the wing now assumes a higher angle of attack due to a gust, the change in local lift over chord will make the wing pitch up more. The forward region of the airfoil will add more lift relative to the initial state than the rear region because the zero lift angle of attack has been shifted downwards, adding a pitch-up moment. The whole aircraft will become unstable from the added camber of the flap.

Flying wings don't need computer control. All the Hortens and Fauvels of this world flew just fine with a human pilot and mechanical controls. That is because they use reflex airfoils and wing twist so the rear part of the wing creates less lift relative to its area. This is the condition for static stability and it gets removed with a positive flap deflection.

Adding that flap will add so much instability that computer control would have a hard time to keep the aircraft stable. If the flap produces substantial additional lift, the thrust vector control would need to effect large changes in pitching moment. Now consider you fly into an airport along the glide slope, with the engine close to idle. Where would the required pitching moment come from? Thrust is insufficient and reducing your flap setting will stall the aircraft. The best solution is to not deflect the flap in the first place and to add more wing area relative to a conventional aircraft.

Answer 2

No.

Thrust vectoring produces a nose-up pitching moment by pointing the engine nozzle upwards, which pushes the aircraft's tail down and its nose up. Extended flaps increase total lift, and produce a nose-down pitching moment, by deflecting air downwards at the trailing edges of the wings, pushing the aircraft's tail up and its nose down.

As the engine nozzle(s) of most delta-winged aircraft are at about the same distance from the aircraft's center of mass as its elevons (flapevons?) are (both being either at, or very near, the rear end of the aircraft), the use of thrust vectoring to cancel out the nose-down pitching moment created by flap extension would require that the deflected engine nozzle apply a downforce (anti-lift) with approximately the same magnitude as the upforce (lift) applied by the extended flaps - completely (or almost completely) cancelling out the increase in lift produced by said flaps!¹

Aircraft with a horizontal tail (including most non-delta-winged aircraft as well as a number of delta-winged aircraft) don't have this problem, as the elevators are at the far posterior end of the aircraft, far from its center of mass, while the flaps are mounted on the wings, very close to the aircraft's center of mass; thus, a small amount of downforce from the elevators produces enough nose-up pitching moment to completely compensate for the nose-down pitching moment produced by even a very large amount of flap-extension-produced lift.² Canard aircraft (including some non-delta-winged aircraft, and also some delta-winged aircraft) don't have this problem, as the nose-down pitching moment from extending flaps on the main wing can be cancelled out by the nose-up pitching moment from extending flaps on the canard, which has the additional advantage of increasing the aircraft's lift coefficient yet further (instead of slightly decreasing it, as the nose-up elevator deflection required when the flaps are extended on most horizontal-tailed aircraft does).

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¹: If the nozzle(s) extend rearwards beyond the trailing edge of the wing, then they would have a slightly longer lever arm than the flapevons, allowing a slightly lower amount of vectored-thrust downforce and a small increase in lift with flap extension, but the additional lift gained thereby would likely not be enough to be worth the trouble.

²: In practice, this is an oversimplification; for some horizontal-tailed aircraft, extending the flaps actually produces a nose-up pitching moment. However, this is due to the interaction of the wing's downwash with the horizontal tail (and, thus, would not be the case for a tailless-delta-configuration aircraft), and the point about the elevators' much longer lever arm compared to the flaps is still valid.

Answer 3

I'm not sure why we're assuming that flap deployment will cause pitch instability. As far as I'm aware, flap deployment moves the CoP rearward, which should make the aircraft more stable. I may be missing something here, though, I'm still working my way through Perkins&Hage. I'm not quite following Peter's argument about gusts.

For now we'll work under the assumption that extended flaps will cause a rearward shift of CoP in the pitch plane and thus increased pitch stability. However, this same rearward shift will intuitively, as you noted, cause a nose down moment, which would be aggravated if you did some kind of Fowler flap setup. This is corroborated by the above linked articles and pg.29 (pdf pg.33) of this NACA data.

Regarding your ideas to cancel this moment out to allow yourself to actually use the high cL you get from these flaps:

• Ducting the jet exhaust to the front of the craft: While within the realm of physical possibility, this would likely be a highly inefficient design due to the number of sharp turns the exhaust would have to take, and the amount of heat energy it would lose while doing so. Overall, it wouldn't likely be implementable.
• Additional engine: likely weight prohibitive.
• Fan driven by shaft: The most reasonable of your concepts in my mind. As you noted, it's been done before. However, as far as I'm aware, the Harrier uses ducted jet flow rather than a fan. I think you're thinking of the F-35B.
• Tilted prop rotor: A statically tilted rotor wouldn't do a ton at low angles, and at high angles would be extremely inefficient in forward flight.
• Thrust vectoring: As others have noted, this would likely only cancel out lift gains from the flaps.
• Other options: See below

Other options:

• Dynamically tilting prop rotor: perhaps, But this would seem more mechanically complex to me (rotational drives, shaft bearings and CV joints, morphing ducts, keeping a healthy flow to turbine, etc.) than a seperate shaft driven fan and likely wouldn't end up saving you weight. Not worth pursuing in my mind.
• Aggressive leading edge slats: It seems that leading edge slats don't have too significant of an effect on the moment of an airfoil. I haven't seen anything listed in any NACA testing I've seen, it seems like they treat it as a given that there isn't too much of an effect. This design compendium seems to agree on pg.256 (pdf pg.277).

This is all, of course, addressing your interest in using flaps as a high-lift device. You might want to think about other high-lift devices, either in their place or to mitigate their use in your design so you don't have as much of a moment increase to deal with in whichever method (shaft-driven fan, please) you might decide on if you go that route.

The first, in my mind, would be blown airfoils and related high-lift devices. You're already looking at "ducts from the engine output," you could instead throttle the exhaust of the engine and route the exhaust to slots placed midway along the wing to increase boundary layer energy, thus increasing lift. If you want to get really funky, you could look into reverse flow slots(see 4th paragraph of "mechanism" section). The paper by Wake, Tillman, Ochs and Kearny on the matter is fascinating and worth getting your hands on. I'm worried what effect swept wings (inc. delta wings) or even the lack of an high aspect ratio would have on its effectiveness, though.

Alternatively, you could look into making the exhaust a venturi vacuum generator and play with boundary layer suction. This is apparently very attractive on highly swept wings, so it might be pertinent to your design.

The Shinmaywa US-1 and US-2 flying boats use a dedicated turboshaft engine inside the hull purely to provide high-pressure air to their boundary layer control system, and the resulting STOL capabilities are mind-bending, go find some videos on it. You wouldn't be able to get that much benefit from harnessed exhaust exhaust, as the Shinmaywa uses it in conjunction with some nuts flap configurations that would be far too aggressive for your planform, but it can give an idea as to the potential of boundary layer control.

Note: sorry for the lack of sources near the end. Limited on #links until I get more rep I guess.

Answer 4

One very strong reason thrust vectoring is of little or no use to offset flap-induced pitch moment: you reduce thrust to idle or near idle during landing, which would make the thrust vectoring ineffective.

The Harrier lands at higher throttle (it takes much more than 50% power just to hover with no stores aboard). If you were using fans at the nose, driven directly by the engine (like the front nozzles on a Harrier) you'd have high tailpipe thrust exactly when you want none.

Thrust vectoring from a jet is an unnecessary complication unless you need it either to hover or for extreme maneuverability.

Answer 5

Yes, but it may not be worth it.

In principle it is absolutely possible to use thrust for attitude control of a vehicle. To cite a few examples, the F-35, X-15, and Harrier all used ducted jet exhaust or high-pressure bleed air for attitude control during low dynamic pressure phases of flight.

You correctly point out in the question that the thrust would have to be at the nose (not normally where you want your engine) and inclined so the thrust line is 90 degrees downward.

So another way to ask this question would be "can you replace a canard on a delta-winged aircraft with a vertically thrusting engine"? In principle, yes. Is it a good idea? Probably not. But how good or bad of an idea it is depends on several factors. Some of the challenges have already been mentioned so I won't repeat them here, but other things to consider are:

1) How far the nose is from the C.G. Your concept makes more sense if you have a long nose on the airplane (think XB-70). For a pure flying wing you would need a lot more thrust to achieve the same pitching moment since the moment arm from the thrust location to the C.G. would be as small or smaller than the moment arm from the flap to the C.G.

2) The type of propulsion system used Tailless aircraft are very sensitive to C.G. location, so if you add a lot of weight to the nose that will make the aircraft difficult to balance. For that reason, it's more advantageous to use lightweight secondary propulsion systems (ducted exhaust from a turbine). Putting a large electric fan or secondary turbine in the nose will add weight in a difficult place to balance. There is a trade here with the benefit you get from a longer nose - you'd have to actually work it out for your design to see if there's an option that makes sense.

3) Pitching moment variation with speed Dynamic stability (include gust response, pilot handling characteristics, etc.) is going to be a challenge for this type of plane in general. But it's a solvable problem; whether its solvable with a computer in the loop or not depends on some of the other details of the design, but flying wings, canard aircraft, and delta wings have all been built which require little or no stability augmentation.

There is one big stability challenge that this concept brings with it, which is that the effective pitch-up moment you get from the thruster will change relative to the flap pitch-down moment as speed varies.

Typically you'd calculate the trim condition of the vehicle in terms of key dimensionless parameters like $C_{L_\mathrm{tail}}$ and $C_{m_\mathrm{wing}}$, and when the aircraft is in a trim state the variations in these parameters with speed are very small; so small changes in speed won't effect the trim condition of the aircraft.

If you're using a thruster to generate your pitching moment the force of the thruster is dependent almost entirely on the jet velocity of the thruster, and almost totally independent of the freestream (especially in this case where it's oriented 90 degrees). This means that, as your speed changes, you'd need to constantly be modulating the thruster input to keep the right pitch trim. Since unlike other reaction control aircraft your thruster will always be on, keeping it at the right power setting will at best add significantly to the pilot workload during the highest workload phase of flight; it's very likely you'd need a computer in the loop to make this concept work.

Bearing in mind the weight/complexity, c.g., safety, and control issues this brings up its worth thinking hard about what this buys you; adding a canard or a horizontal tail are much simpler ways of accomplishing the same thing.