The effect of sweepback on adverse yaw on flying wings

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captarmour

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I agree, but wonder if you could ge a similar (if less effective) effect by using differential mixing, as this helps reduce adverse yaw as well, i.e. in a left turn instead of as you have described:

Port inner up, Starboard outer down, all others neutral

You would have:
Port inner up large amount, port inner down small amount, starboard outer up large amount, starboard inner down large amount.

Hope the above makes sense, I'll sketch out a diagram soon and post it here.
What about the rudder effect of the swept hinged elevon panel? Would not the others swept in opposite direction of turn cancel the rudder effect?
 

captarmour

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Sorry I crossed you. This one has all panels on TE so no thrust will be produced. On this planform, to benefit from the rudder effect of the swept hinge line elevons, in a left turn the port inner would move up and the starboard outer would move down, others neutral. The swept hinge line would would cancel adverse yaw.
 

captarmour

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View attachment 22828 View attachment 22829 My latest testbed prototype, rough and ready but just to test out the concept at this stage, originally built as a glider it turned well with little or no adverse yaw, has had its first flight under power, looked promising, needs more weight on the nose, control is via elevon mixing to two servos. Each servo is connected to an inboard and outboard elevon in such a way that the outboard elevons move up more than down and the inboard pair move down more than up to reduce adverse yaw. This is a 40% scale version of the 2.1m wingspan powered glider I intend to build eventually.
I have done a low AR version of something similar to this but the forebody was flat, 5% chord, width 30% of total wingspan, with a windward bevel on the forebody LE, which provided sufficient decalage to eliminate reflex. It had flat symmetrical wings. LE of wings were swept 33 degrees, TE was straight to form a delta.
 

John Newton

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I agree, but would this not initiate a right (starboard) turn, not left or am I getting confused? I see you were talking of all moving panels originally, sorry, missed that point with referance to induced thrust.
 

John Newton

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I have done a low AR version of something similar to this but the forebody was flat, 5% chord, width 30% of total wingspan, with a windward bevel on the forebody LE, which provided sufficient decalage to eliminate reflex. It had flat symmetrical wings. LE of wings were swept 33 degrees, TE was straight to form a delta.
Sounds intriguing, any pictures?
 

captarmour

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I agree, but would this not initiate a right (starboard) turn, not left or am I getting confused? I see you were talking of all moving panels originally, sorry, missed that point with referance to induced thrust.
no I'm confused!

The port outer needs to move up to yaw left. The starboard inner would need to move down. Thanks for the help.
 

John Newton

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What about the rudder effect of the swept hinged elevon panel? Would not the others swept in opposite direction of turn cancel the rudder effect?
They would oppose each other but not cancel the rudder effect completely as some move up/down more than others so you would still get a residual net proverse yaw effect, does this make sense?!
 

captarmour

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Understood. This caused me to think that if u have say a 30 degree sweep flying wing with wingtips swept back 60 degrees, and wings long enough that the entire wingtip is behind the cg. You mount just 1 aileron(or elevon) on each of the 60 degree swept wingtips. In a left turn the left goes up more than the right giving more rudder effect than right side. Right side working in an updraft has its lift vector tilted forward so when deflected down creates more lift and hence more thrust pulling the right wingtip forward creating proverse yaw to add to the rudder effect of the left elevon.
the problem would be too much area ahead of the hinge which may be helped by extending a reverse balance horn on the TE which would also act as an extended raked wingtip.

just a thought.
 

John Newton

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Understood. This caused me to think that if u have say a 30 degree sweep flying wing with wingtips swept back 60 degrees, and wings long enough that the entire wingtip is behind the cg. You mount just 1 aileron(or elevon) on each of the 60 degree swept wingtips. In a left turn the left goes up more than the right giving more rudder effect than right side. Right side working in an updraft has its lift vector tilted forward so when deflected down creates more lift and hence more thrust pulling the right wingtip forward creating proverse yaw to add to the rudder effect of the left elevon.
the problem would be too much area ahead of the hinge which may be helped by extending a reverse balance horn on the TE which would also act as an extended raked wingtip.

just a thought.
Sounds and interesting idea, like a "cranked" swept wing, would this use "all moving" wingtip panels or conventional trailing edge aileron/elevons? Any chance you could do a rough sketch, struggling to visualise exactly what you mean. I assume you are thinking that this would produce a roll without significant changes in the aircrafts pitch when moving the control surfaces for roll?
 

captarmour

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Look at ur pic on previous page. Draw a straight line from a point about 5 centimeters of your TE tip, to the tip of your nose. That line would be approx 60 degrees to your centerline or wingtip for that matter. That line would then be the hinge line of your ailerons. To balance so much area ahead of the hinge line you could extend the TE of the new aileron back as necessary.
Just a crazy idea.


A better idea would be to draw a line from nose to LE tip and add a triangular wingtip, that line being the new wingtip, that pivots at about midpoint of the existing unswept wingtip. Sweeping the TE of that new wingtip back about 30 degrees will give you more surface to take advantage of that up flow region. It will then be a 'raked' wingtip. If its possible to have different mounting points on its pivot shaft you could experiment with different cant angles, that is from upswept wingtips to drooping wingtips. Drooping wingtips may give more lift and better rudder effect in turns, whereas upturned tips, less drag. in a ground loop they may keep the wingtip above obstacles.
They may however add to your dihedral effect and cause Dutch roll.
U may find you may need to fly with the wingtip aileron free floating to see what angle it orients itself before u connect it. Don't be surprised if it is pointing down quite a bit.
 

John Newton

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Here's a simple idea that uses the hinge line sweep and differential to encourage proverse yaw/reduce adverse yaw, the outboard elevons go up more than down (using differential), the central straight inboard elevon always goes down a small amount when the plane is rolled left or right to counteract the pitch change caused by the upgoing elevon going up more than the down going one. The net effect would be a roll with little or no adverse yaw and little or no pitch change which is what is wanted.
straight te version-Model.jpg
 

captarmour

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Absolutely beautiful wing, elegantly simple design. With so much area moving down the deflection required is minute resulting in little drag. Much more structurely effecient than the all moving tip.
 

John Newton

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Thanks captarmour, glad u like it :), like to keep things as simple as possible without reducing performance too greatly. I wonder if there is any data about relating to how great an effect hinge line sweep has in producing a positive rudder effect, I'll have to have a look into it. I'll keep you posted how my next test flight goes with my constant chord wept back wing, when I get chance I may modify the elevon movements to the ones illustrated above for comparisson.
 

John Newton

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Nothing against all moving surfaces, have flown several models with all moving tails and they perform very effectively, just like the simplicity of a hinged trailing edge, the idea same may well work with all moving wingtip panels in place of the outboard elevons moving differentially (the centre elevon would remain).
 

captarmour

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The reason I like the outboard all moving wingtip or OHS(outboard horizontal stabilizer) is that they operate in a local up flow or upwash that tilts the lift vector forward giving thrust that helps counter induced drag. In a turn the outer tip aileron produces more thrust with increased forward tilted lift vector which not only helps with proverse yaw but helps with loss of inertia in the turn. Granted the effect may be too small to be of significant value so these controls should be as large as is practical to be of any benefit.
 

John Newton

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In a left hand turn with OHS (all moving wingtips) would not the wingtip on the outside of the turn be tilted upward therefore tilting its lift vector backward? Am I missing something?
 
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