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Bi-directional Flying Wing


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Flying wing designs are very good for low-drag applications, but they're pretty slow. The ways of fixing instabilities in flying wing flight result in increased drag at supersonic speeds; no supersonic flying wing has ever been built.

However, with Breaking Ground robotics, we could fix that.

One of the coolest ideas NASA has played around with is the bi-directional flying wing: a flying wing that takes off along its short axis but then can fly supersonically along its long axis:

220px-Wing_bi-directional.svg.png

Here's a rendering from a NASA study that shows it in supersonic flight:

Supersonic_Desert3_web2_reduced2.jpg

Ostensibly, the engines are mounted at or very near the vehicle center of mass and are able to rotate by up to 90 degrees to switch between the two different flight configurations. I'm guessing it also needs control surfaces that can be switched on and off.

Your challenge, should you choose to accept it, is to build and fly an aircraft which is able to take off with a prograde cockpit facing one direction, then transition to flying with a prograde cockpit at a 90 degree different direction.

Additional challenges:

  • Reuse Me! Not only can your aircraft take off and transition to stable flight at the new orientation, but it can also transition back and land safely in its original configuration.
  • Look Ma, No Wheels! Build an aircraft which doesn't use reaction wheels or RCS and relies solely on aerodynamic control through all modes of flight.
  • Double Trouble! Instead of using rotating engines, try having completely separate engines for forward and sideways flight.
  • To Infinity! Exceed the speed of sound in your "sideways" flight orientation.
  • And Beyond! Make it to low Kerbin orbit.
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24 minutes ago, vyznev said:

Not sure if this old craft qualifies, since it's uncrewed. If it does, I believe it meets the "Reuse Me", "Look Ma, No Wheels" and "Double Trouble" extra goals. :)

Oh, that's very cool!

One of the specifics about the bi-directional flying wing idea is that it's supposed to NOT be symmetric in different flight modes.

But yeah, very nice job.

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2 hours ago, sevenperforce said:

Flying wing designs are very good for low-drag applications, but they're pretty slow. The ways of fixing instabilities in flying wing flight result in increased drag at supersonic speeds; no supersonic flying wing has ever been built.

I beg to differ. ;)  Flying Wing Starboost88

nftw6DE.png

ibaq9Hp.png

zz4vBYj.png

 

It's not bidirectional, so not a valid entry here... but then it doesn't really need to be. Very stable and agile, mach 2.8 cruise, enough range to circumnavigate Kerbin twice. Why complicate things?

Moving on... I want to try my hand at making a proper bidirectional one as requested. I have a few ideas of how to achieve this. No promises though, I don't have nearly as much time for KSP lately.

Edited by swjr-swis
forgot to link the craft file
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1 hour ago, swjr-swis said:
4 hours ago, sevenperforce said:

Flying wing designs are very good for low-drag applications, but they're pretty slow. The ways of fixing instabilities in flying wing flight result in increased drag at supersonic speeds; no supersonic flying wing has ever been built.

I beg to differ. ;)  Flying Wing Starboost88

Beautiful!

Of course I mean in real life. But yes.

1 hour ago, swjr-swis said:

Moving on... I want to try my hand at making a proper bidirectional one as requested. I have a few ideas of how to achieve this. No promises though, I don't have nearly as much time for KSP lately.

I'm trying to think whether there is any better orientation to come up with in terms of engines than the one in the NASA study. Excited to see what you come up with!

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Anyway, that crazy airplane concept mentioned in your original post intrigued me enough that I wanted to try building a working replica of it on KSP and see how it flies.

Spoiler

Y5LmXUu.png

To absolute nobody's surprise, it flies like excrements. Or rather it doesn't, because it can't even take off — the elevons don't have nearly enough pitch authority to counteract the massive downward pitch moment from the top-mounted engines, and its negligible yaw stability isn't enough to keep it going straight. And the wobbly rotation servo doesn't help with that, either. About all this thing's good for really is doing donuts on the runway. :sticktongue:

Spoiler

bMZtjza.png

I could maybe fix the yaw stability by adding some kind of deployable tail surfaces, like I used on the 8-way CuBoid plane I linked above, but I see no easy way to solve the pitch problem — at least not while leaving the result looking anything like the original concept design. (A bit ironically, turning the landing gear 90 degrees and taking off narrow-side-forward might actually work better, at least if I added some extra pitch control surfaces near the nose and the tail in that orientation, since that would put them further fore and aft of the center of mass.)

Alas, all this means that I didn't even get to test how switching direction mid-flight (near Mach 1!) would've worked. I suspect it would've also ended rather poorly, though. :D

Honestly, looking at the design, I have no idea how anyone ever thought this would fly in real life, and I suspect making it flyable in KSP isn't possible without massive cheating* either.  But who knows, maybe someone better than me can pull it off. :)

*) Like adding lots and lots of reaction wheels or abusing the aero model to make parts dragless, etc.

Edited by vyznev
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…anyway, I decided to try cheating a bit by moving the engines down and out (which of course would never work in real life, but in KSP connected parts don't have to touch and same-vessel collisions are optional ;) ).

With that change, it does fly. Not well, mind you, but well enough to take off and stay in the air.

Spoiler

sS3saHX.png

VXLYQdE.png

AvzuUAt.png

Trying to rotate the engines and change flight direction went about as well as I expected, though. :P

Spoiler

Y6UPv5c.png

Mind you, that's probably in part because some of the control surfaces started acting the wrong way when I changed the control point. I think I remember having that issue with the CuBoid too, and IIRC I "solved" it there basically by having a completely separate set of control surfaces for sideways flight and locking the surfaces that were not in use based on the selected heading. That might be the way to go here, too.

Edited by vyznev
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Not a submission to the challenge but might add some inspiration. It's sort of a flying wing....why must all wings be completely flat ?

It can fly at very severe angles of yaw. The thrust is limited but increases with control input. Can fly with or without SAS. Fairly sure I left the cockpit RW off but it shouldn't be on and isn't needed. Have a go, don't worry about speed, don't worry about yaw, imagine your a seagul...steal tourist food ! I can't rememeber the custom actions, I think one is to cycle after burner, others might be gimbal on of for the engine.

https://kerbalx.com/WaveyD/ANGULL-V5

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6 hours ago, WaveyD said:

why must all wings be completely flat ?

The wing boards in KSP need to be flat so they can lego seamlessly to make the shape and size you want. If you tried them IRL then they would absolutely need to have some angle of incidence (non-zero static angle re: the forward direction of the plane during level flight) or they would produce no lift. Also, real wings will always have some amount of camber because of fluid dynamics (which isn't done in KSP).

Airfoil_terminologies.png

Not all wings in KSP are flat. All the wing pieces that aren't the boards have some camber in them. They immediately give you a hard time if you try to chain them and hope for a smooth surface. They're not meant to be chained.

If by flat you mean lack of sweep, taper or curve like in your craft file then that's another matter. A few points on why few people rarely build such wings:

  1. They just don't think of it. A flat wing usually just works for most folks and there's no amazing benefit to investing the effort into a curved wing.
  2. Many plane builders tend towards fighters and spaceplanes. Speedy planes just don't have these.
  3. Such sweep adds to roll instability which may be largely unwelcome depending on how someone has played and the design points of the particular plane.
Edited by JadeOfMaar
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14 minutes ago, JadeOfMaar said:

If you tried them IRL then they would absolutely need to have some angle of incidence (non-zero static angle re: the forward direction of the plane during level flight) or they would produce no lift.

That's true in KSP too.* KSP plane builders just have to either manually tilt their wings by a tiny amount (which is awkward, since the rotate tool will only snap to multiples of 5°) or live with the fact that their plane must keep its nose pointed slightly above the horizon in order to maintain level flight (and the slight extra drag this produces, for most conventional plane designs).

*) Easy enough to test by putting some vertical wing panels on a rocket, launching it straight up (and holding surface prograde with SAS) with aero force visualization turned on, and observing that the wings indeed produce no lift in that case.

33 minutes ago, JadeOfMaar said:

Not all wings in KSP are flat. All the wing pieces that aren't the boards have some camber in them.

Do they, tho? I was under the impression that, while e.g. the "Big-S" wing parts indeed have variable thickness, they're still symmetric and thus have zero camber (and thus work equally well even if placed upside down).

I also suspect that, even though those wing models indeed look back-to-front asymmetric (both in thickness and in overall wing shape), the stock KSP aero model probably ignores that too. That shouldn't be too hard to test: just build a plane using those wings, with nothing else attached to them, then turn them backwards (while keeping the nominal center of lift at the same place, as shown in the SPH, which may require translating the wings forward or backward) and see if the plane still flies the same way.

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39 minutes ago, vyznev said:

Do they, tho? I was under the impression that, while e.g. the "Big-S" wing parts indeed have variable thickness, they're still symmetric and thus have zero camber (and thus work equally well even if placed upside down).

Oh? I didn't see the asymmetry required for lift as a component of camber (by raw definition: having a curved surface). I acknowledge the asymmetry solely as a component of lift generation. I don't believe bi-plane wings have that asymmetry and they still work (with whatever penalty), right? For ease of placement to build in symmetry, they won't have that asymmmetry to them.

 

45 minutes ago, vyznev said:

I also suspect that, even though those wing models indeed look back-to-front asymmetric (both in thickness and in overall wing shape), the stock KSP aero model probably ignores that too. That shouldn't be too hard to test: just build a plane using those wings, with nothing else attached to them, then turn them backwards (while keeping the nominal center of lift at the same place, as shown in the SPH, which may require translating the wings forward or backward) and see if the plane still flies the same way.

Your suspicion would be correct. KSP just cares about the lift surface area number in the part and the orientation of the imaginary lift plane whose size corresponds ot that number. FAR might care about camber (due to its voxel system) but I doubt. I know it makes you care about mroe forms of drag.

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5 hours ago, vyznev said:
6 hours ago, JadeOfMaar said:

Not all wings in KSP are flat. All the wing pieces that aren't the boards have some camber in them.

Do they, tho? I was under the impression that, while e.g. the "Big-S" wing parts indeed have variable thickness, they're still symmetric and thus have zero camber (and thus work equally well even if placed upside down).

Let me do a quick test and see.

confirm.png

Yep. Those rear wings are rotated 180 degrees and they have the exact same positive or negative lift as the front ones all the way.

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17 hours ago, JadeOfMaar said:

The wing boards in KSP need to be flat so they can lego seamlessly to make the shape and size you want. If you tried them IRL then they would absolutely need to have some angle of incidence (non-zero static angle re: the forward direction of the plane during level flight) or they would produce no lift. Also, real wings will always have some amount of camber because of fluid dynamics (which isn't done in KSP).

Airfoil_terminologies.png

Not all wings in KSP are flat. All the wing pieces that aren't the boards have some camber in them. They immediately give you a hard time if you try to chain them and hope for a smooth surface. They're not meant to be chained.

If by flat you mean lack of sweep, taper or curve like in your craft file then that's another matter. A few points on why few people rarely build such wings:

  1. They just don't think of it. A flat wing usually just works for most folks and there's no amazing benefit to investing the effort into a curved wing.
  2. Many plane builders tend towards fighters and spaceplanes. Speedy planes just don't have these.
  3. Such sweep adds to roll instability which may be largely unwelcome depending on how someone has played and the design points of the particular plane.

No. I was just joking about my flying wing. Seeing as the wings on my craft are polyhedral, inverted gull wing (hence its stupid name) or just painted pieces of a cardboard box which a child sat on...it has some stability in yaw. It has a component of a vertical stab, that's the use of the curved wing, it is a vertical stab. The roll instability is balanced out between anhedral and dihedral. In kerbal, if you have the same amount of wing anhedral as dihedral, at the same angles but inversed then you don't gain roll instability. That can also be balanced such as half the area but twice the angle from horizontal. The angle of incidence of the various parts of the wings is to force the lift centre onto or near the mass. Anyway, because the inverted gull gives some stability in yaw, in the little green men classical sense, it isn't a flying wing....but I say it is a flying wing. It was just rhetorical. The design of the wing was for small ssto craft originally. Works well.

I should add that the original didn't have the million control surfaces, didn't have gimbal, just used the wheesley. The inspiration I thought it might add was in the KAL controllers. For example...the craft may have some stability in yaw but if the control surfaces initiate roll in the normal way you get a boot load of yaw, I think it was because of the cog. If you look at the KAL linked to roll you can see it changes the deploy angle of both left and right control surfaces in the same direction roll left or roll right, I think it was up. However, the normal roll authority control will move them in opposite directions as normal. The result is that one side moves down for roll and the other stays at zero (roughly speaking, depends on the amount of *hedral). This gives roll but with drag only increased on the one side....countering the 'induced' yaw. Lots of little touches can be added. The slpit flaps must also lose their normal control authority when yaw is applied otherwise any pitch or roll input will counter the split and reduce the yaw. Also when the split flaps are used they tend to produce some roll which is countered with some trim on the other surfaces. It's far from perfect but a lot can be done. It was just a challenge to make a flying wing, without gimbal, without RW and stock/breaking ground. The one I posted here is a bit overdone. If I get time tomorow I'll put the original on kerbalx....it's quite neat and clean. 

https://kerbalx.com/WaveyD/ANGULL-ONE

Here it is. The KAL roll mentioned above I think is in later versions, there is some trimming out of the roll to prevent yaw but not as extreme as I remember. Without thrust gimbal it is less forgiving if you get too much yaw but it's ok. Swap it for a panther and it's a big jump better, especially with SAS on.

Edited by WaveyD
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  • 2 weeks later...
On 10/5/2022 at 1:06 PM, sevenperforce said:

Beautiful!

Of course I mean in real life. But yes.

I'm trying to think whether there is any better orientation to come up with in terms of engines than the one in the NASA study. Excited to see what you come up with!

I'll bet you could do a supersonic flying wing in FAR even with Juno engines or AJE. 

Actually, a supersonic flying wing with a two stage sounding rocket slung underneath sounds like a pretty good first vehicle for an RP-1 career, unless you crash it. Though obviously you've gotta be careful about overheating and balsa wood wings so high altitude (but not high enough to suffocate) or rocket-boosted is probably better than trying to go mach at sea level.

(also drag rudders might cause too much drag if you just let the regular SAS fly it rather than something a little smarter).

Edited by Pds314
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