How do I make a flying wing fly knife edge? - RC Groups
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Apr 06, 2012, 04:01 PM
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Evil Twin's Avatar

How do I make a flying wing fly knife edge?

Has anyone done it with a boomer rang style flying wing? Thrust vector? Oversize wingtips? Extra fins towards the nose? I want to build a minipopwing that will knife edge.
Last edited by Evil Twin; Apr 07, 2012 at 03:19 PM.
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Apr 06, 2012, 09:28 PM
We don't need no stinkin' tail
RPM314's Avatar
maybe winglet rudders? do popwings already have those?
Apr 07, 2012, 02:03 AM
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You have to have rudder(s) to knife edge. The Popwings have fins, but no rudder. I suppose you could modify it and make some rudders, however, fins tend to be pretty small, so even if you cut rudders in the fins, it might not be enough surface to hold a knife edge.
Apr 07, 2012, 02:04 AM
Have Wings, will Fly!
dont's Avatar
Dirty great rudders on the wing tips or one on the large central fin!

Apr 07, 2012, 04:02 AM
never mind the boll⁣⁣⁣ocks
brosius85's Avatar
rudders or thrust vectoring
Apr 07, 2012, 10:08 AM
Vertical approach specialist
potshot's Avatar
Not exactly knife edge here, but his "Salmon" comes close. Check the rolling circles right on the deck. No clue how he does these things.
Apr 07, 2012, 02:50 PM
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Evil Twin's Avatar
WOW!!! That pilot has mad skeelz!!!

I started thinking about this from a statics perspective. For stable knife edge flight the sum of forces and the sum of moments have to equal zero. The rear fins and the motor thrust both have a component of up force to counteract gravity. To make the up forces equal gravity the wing will need to travel fast or have bigger fins in the rear. It seems that it would be easy to make these forces cancel out.

The hard part is making the moments cancel out. At the angle of attack needed for knife edge the up forces from the motor and especially the top rear fin cause a moment which will rotate the nose towards the ground. To counteract this moment the motor will need to thrust vector. Also the rear fins should have as much of their area as forward as possible. Perhaps the fins should be tear drop shaped. Another way to add lift and a moment ahead of the cg is to put fins ahead of the cg. Maybe fins attached by magnets on the nose, one on the top and one on the bottom? This would work similar to side force generators used on some 3d planes. I wonder how well a wing would fly with fins added in front of the cg?

I thought about adding rudders to the rear fins. It would add a moment in the correct direction but it seems like a pain and not durable. Motor thrust vectoring seems easier.
Last edited by Evil Twin; Apr 07, 2012 at 03:19 PM.
Apr 09, 2012, 01:45 PM
Takeoff=Option Landing=Must
robusp's Avatar
Originally Posted by Evil Twin
Has anyone done it with a boomer rang style flying wing? Thrust vector? Oversize wingtips? Extra fins towards the nose? I want to build a minipopwing that will knife edge.
I am flying my red eagle wing for couple of weeks now and I crashed it 4 times. Every time it was butched attempt at knife edge and being to close to the ground.
Doesn't matter haw fast its going, it will drop. Winglets are not big enough to create sufficient lift.
Its different with Scimitar,I have rudders and thrust vectoring.

I am working on a wing with rudders and thrust vectoring by using two side by side motors. It shouldn't have a problem with ke.

Apr 09, 2012, 10:35 PM
trying to miss the ground
blademaker22's Avatar
Even with thrust vectoring and wingtip rudders, a wing has a hard time doing knife edge. There isn't a fuselage to generate lift with, so you need a lot of speed and power and a very high angle of attack.
Apr 10, 2012, 01:16 AM
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Evil Twin's Avatar
I contacted the owner of a thrust vectored flying wing (see attached pic), his motor swiveled so far that he had to make cut outs for the prop. It would not knife edge and the thrust vectoring was not even very effective as a rudder. Here is his response. "It was OK, didn't quite work as planned. I don't think i had enough distance between the motor and CG, not enough moment for it to work well. It kinda worked like a rudder, but wasn't very effective sorry." I wonder what this wing would do with oversize rear fins to generate more lift and nose fin(s) to keep the nose up.

The vecjet has thrust vectoring and a rudder, together they enable very fast flat spins. I thought surely it must be able to knife edge. I asked in a vecjet thread if it could knife edge, here's the response. "I never could get mine to knife edge at all. Just not enough side area. You can fly a sort of KE for a bit but it will drop."

Based on this info I think the key to flying KE is TV along with large fins in the rear and nose fin(s), hopefully one on top and one on bottom to give equal lift. As long as the center of pressure from the side is behind the cg, the wing should still be stable. To do this any addition of nose fin area will need to be canceled out by larger fins in the rear. For my build I'm planning on trying out different nose fins and oversize rear fins along with TV. I'll experiment with cardboard fins, hopefully I'll find something that works.

I have another idea that may help KE, toeing in the fins. My theory is that the bottom fin will have an increased angle of attack and generate more lift below the cg where it is needed. The upper fin will have a lower angle of attack than the bottom fin so it will create less lift which will tend to not force the nose towards the ground. Spacers could be put on the fins to try different angles and even try toe out.
Apr 10, 2012, 08:46 AM
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fnffishcore's Avatar
My vote is for ridiculously oversized fins, mounted maybe midway into each wing half, with independently operating oversized rudders on each fin. So if you roll left into a knife edge, apply right rudder, ONLY the right fin/rudder moves; If you roll right into a knife edge, apply left rudder, ONLY the left fin/rudder moves. In my mind, the rudder would force it to pitch up some, and the opposing stationary fin/rudder would generate just enough lift (or deflection)?

I also have zero experience flying aerobatics and know nothing about aeronautics. That might be painfully evident to some of you right now
May 11, 2013, 06:13 AM
if you see a weed, pick it
malakiblunt's Avatar
Very late reply to this thread, But am tryin to do the same,
if you dont already know the parkzone stryker can knife edge, though it does have a fuse unlike a pure wing.
Parkzone F-27Q Stryker High Performance Rc Airplane (1 min 43 sec)

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