The effect of sweepback on adverse yaw on flying wings

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John Newton

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View attachment 22754Picture an elevon straight up. It becomes more and more like a rudder as the hinge angle is increased as in the lower wing. When the hinge angle increase is combined with a tip plate or fixed rudder and a few degrees of dihedral it turns more like you're using elevator and rudder. It has a very controllable mush as well. The drawback is that it washes out the inboard section of the wing more than the outer and likely causes greater drag when the elevon is out of it's normal position. I gave up on flying wings. They look beautiful, but they're never as efficient as a conventional planform.
Thinking about this some more, if we assume that the elevons move up and down by the same amount, (i.e. no differential) to prevent the pitch altereing when rolling will not the downgoing elevon counteract the rudder effect of the upgoing one?
 

captarmour

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i was waiting to see how long someone would catch it as just recently i was thinking of the exact same design.

maybe if we have 4 elevons swept to from a M (or W) trailing edge, so that the elevons that move opposite each other are swept in the same direction.

for example in a left turn the left outer, moves up and the right inner moves down causing a roll and yaw to the left. that means though that in roll mode each elevon moves only in one direction. in this example the outer left would only move up.

in the W(in plan view the nose of the aircraft is top of the page) the downgoing elevon would be the outer one which may cause more adverse yaw.

please feel free to correct me if im wrong.
 

captarmour

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the W is probably easier as wingtip could be swept inwards to make up part of the W elevon (V or half W on each wing). very easy to visualize on a 45 degree sweep as the wingtip and TE would be at 90 degrees to each other, or maybe a 45 degree V grafted on to a smaller sweep wing.
 

PaulS

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Norman,
In your excellent dissertation on Junker flaps it spoke of airfoil flaps. If the flap is symetric in design, rather than airfoil in design does that make a difference?
 

jedi

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Swept or straight wing, stopping the ailerons short of the wing tip will smooth the lift curve and reduce adverse yaw. Big cub type wing bow is better than the sawed off Mooney tip. Mitchell B-10 polyhedral is bad for adverse yaw. N1M droop tip is good. In general large dihedral effect of swept wings make them a “rudder” airplane. Little aileron input is required.
 

John Newton

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i was waiting to see how long someone would catch it as just recently i was thinking of the exact same design.

maybe if we have 4 elevons swept to from a M (or W) trailing edge, so that the elevons that move opposite each other are swept in the same direction.

for example in a left turn the left outer, moves up and the right inner moves down causing a roll and yaw to the left. that means though that in roll mode each elevon moves only in one direction. in this example the outer left would only move up.

in the W(in plan view the nose of the aircraft is top of the page) the downgoing elevon would be the outer one which may cause more adverse yaw.

please feel free to correct me if im wrong.
This is an intriguing idea, kind of like a B2 bomber trailing edge? I wonder if you could combine it with the idea suggested in K. Nickels book of having the outboard elevons moving up more, down less and the inboard ones up less, down more to assist reducing adverse yaw (I have tried this and it did indeed reduce the adverse yaw on a 30 degree swept model)?
 

John Newton

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Swept or straight wing, stopping the ailerons short of the wing tip will smooth the lift curve and reduce adverse yaw. Big cub type wing bow is better than the sawed off Mooney tip. Mitchell B-10 polyhedral is bad for adverse yaw. N1M droop tip is good. In general large dihedral effect of swept wings make them a “rudder” airplane. Little aileron input is required.
Jedi, not sure if stopping the ailerons short makes any difference, in K.Nickels book (Tailless aircraft in theory and practice) he stated that this made no difference to adverse yaw, but it may be that more modern theory states otherwise? I agree that anhedral in the area where the elevons are positioned reduces adverse yaw.
 

John Newton

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Just test flown my latest R/C design, constant taper, 20 degrees sweep (10 degrees less than previous design), no vertical surfaces, looks promising, too tail heavy so only a brief flight so far but looked to glide well. Features split elevons that move up more on outer pair and down more on inner pair as mentioned above.
 

Aircar

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I am unsure as to whether proverse yaw is in fact a good thing --imagine a swept back wing with divebrakes extended and try to yaw it --the forward going brake presents ever more normal surface and the aft moving one ever less so it fights the yaw (which is stable) reverse the situation with spanwise airbrakes on a forward swept wing and you will have an unstable yawing co efficient --this is the extreme example to emphasize the effect of profile drag alone (and apparent wing thickness ) --the 'w' shaped (B2 like) planform has elements of both tendencies and would react differently depending on the amount of the relative spans and tapers etc --the crescent shaped wing was touted as being more ideal by some sources (lowest sweep outboard-eg Victor bomber /Vari eze ) it would be interesting to configure the same fuselage with replaceable wings and have telemetry on board to see what was what (the NASA Himat UAV did this in the 70s )-changing only one variable at a time would help isolate things. Good project for some model testing...
 

John Newton

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I agree Aircar that proverse yaw may not be a good thing, I guess it all depends on the amount and how it relates to roll control deployment. I am aiming at neutral or very little yaw with roll control application (as per conventonal aircraft), I realise this is an impossible ideal over the whole flight envelope without resorting to gyros etc. but I am hoping to reduce adverse/proverse yaw to an acceptable/controllable level.
 

John Newton

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wing2proto.jpg wing2protounderside.jpg 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.
 

captarmour

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This is an intriguing idea, kind of like a B2 bomber trailing edge? I wonder if you could combine it with the idea suggested in K. Nickels book of having the outboard elevons moving up more, down less and the inboard ones up less, down more to assist reducing adverse yaw (I have tried this and it did indeed reduce the adverse yaw on a 30 degree swept model)?
For it to work each panel in roll mode can only move in one direction always. So for example the W (V on each wing) where the wingtip is swept forward, in a left turn the port inner(TE) panel will move up and the starboard outer(wingtip) will move down while the others not move. In a right turn these would not move while the port wingtip panel will move down and the starboard TE panel will move up.
in other words the pair that are swept in the same direction will move opposite to each other but only for the same turn every time.

am I making sense? please correct me if I'm wrong.

Don't ask me about mixers, I haven't thought that far!
 

John Newton

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For it to work each panel in roll mode can only move in one direction always. So for example the W (V on each wing) where the wingtip is swept forward, in a left turn the port inner(TE) panel will move up and the starboard outer(wingtip) will move down while the others not move. In a right turn these would not move while the port wingtip panel will move down and the starboard TE panel will move up.
in other words the pair that are swept in the same direction will move opposite to each other but only for the same turn every time.

am I making sense? please correct me if I'm wrong.

Don't ask me about mixers, I haven't thought that far!
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.
 

captarmour

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I'm just thinking that the wingtip panel when moving down, because its in an 'up wash' may produce 'thrust' which will add to the rudder effect resulting in 'pro-verse' yaw.
 

John Newton

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I'm just thinking that the wingtip panel when moving down, because its in an 'up wash' may produce 'thrust' which will add to the rudder effect resulting in 'pro-verse' yaw.
Not sure, I see what you mean but I suspect the added drag of the downgoing wingtip panel will be larger than any induced thrust produced?
 

John Newton

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Here's the sketch to show what I mean, note that the connection from the control rods to the servo is not at 90 degrees but 45 degrees to give differential movements as described above. Inboard pair up less down more, outboard pair up more down less. Elevon mixing is as follows:Up: all surfaces go up.Roll Left: left hand inboard and outboard surfaces go up, right hand inboard and outboard surfaces go down.wing2b2version.jpg
 

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John Newton

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I suspect you could reduce the adverse yaw/ increase proverse yaw still further by having dihedral on the inboard portion of the wing and aheadral on the outboard portion, the dihedral break being the line between the inboard and outboard elevons, now that would make an interesting wing shape!

I wonder if the wing shown above could be twisted in such a way to have an elliptical shaped lift distribution for minimum induced drag and still have good stall characteristics?
 

John Newton

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I suspect you could reduce the adverse yaw/ increase proverse yaw still further by having dihedral on the inboard portion of the wing and aheadral on the outboard portion, the dihedral break being the line between the inboard and outboard elevons, now that would make an interesting wing shape!

You would then have anhedral/dihedral, hinge line sweep and differential surface movement all working to negate adverse yaw.

I wonder if the wing shown above could be twisted in such a way to have an elliptical shaped lift distribution for minimum induced drag and still have good stall characteristics?
 
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