Efficient Coordinated Turns

[jcpatrick]

Stuck in the house after a hospital stay, I've looked over a thread on sailplane aileron banking (http://www.rcgroups.com/forums/showthread.php?t=1056408) into turns, reverse aileron, and 'left thumb' rudder coordination into turn. That led to why we use aileron differential, and what I perceive as an inherent flaw in wing design . . . at least compared to bird flight.

This subject is about fine-tuning the utmost efficiency out of wings, so it's totally irrelevant to power planes. In a nutshell, the concept is to reduce drag and energy in every circumstance while creating conditions for coordinated turns.

As currently practiced, the inside wing on a turn is the wing that is reflexed the most, and the outside wing is more cambered. The effect is that the inside wing has less lift and the outside wing more lift; banking the whole. That is the effect we want, combined with the elevator of course. This also means the inside wing naturally travels faster and the outside wing travels slower; turning the whole plane away from the turn. Call it 'anti-coordination' if you will.

Aileron differential tries to compensate for this effect by raising the inside wing aileron more than the outside aileron goes down. This works by creating extra drag on the inside wing aileron, while the outside wing minimizes its increased drag from increased aileron camber. The net effect is to yaw the plane using drag; but it is still accomplishing this by losing energy to (deliberately induced) drag.

The 'anti-coordination' can be compensated with aileron-to-rudder coupling or manual ruddering, but both methods still lose energy by forcing the chambered wing to travel faster, increasing drag. The reflexed wing will travel slower without energy loss, but there will be some more (small) loss with the rudder itself also. Perfect rudder-to-aileron coordination minimizes these losses, but doesn't —and can't— eliminate them.

Wingerons, where each whole wing pivots from its root, overcome some of this by keeping the same chamber while changing each wing's angle of attack. But it's also true that angle of attack is coupled to drag as well. The inside wingeron wing attacks at a lower angle and has less drag, while the outside wing has a higher angle of attack and higher energy losses. The effect of drag is to rotate the ship away from the turn, not into or with the turn. So there is still some of the same 'anti-coordination', even in a wingeron setup.

Ideally, a wingeron combined with full length flaps could turn without 'anti-coordination' if these flaps used to change chamber were coupled with wingeron angles. In this scenario the inside wing would rotate to a lower angle of attack but the flaps would also increase the camber. This increases lift (overcome by the lowered angle of attack) and drag of the inside wing. Conversely the outside wing would increase angle of atack while the flaps simultaneously reflex; this decreases drag while the decreased lift is more than counteracted by the increased angle of attack.

Several changes result from this. Wingerons are notoriously sensitive or 'twitchy'; the entire wing surface is changing aspect rather than just the aileron. This chambering-wingeron should reduce that sensitivity because the flaps act counter to the overall wing change. As a wing increases angle of attack (creating a higher lift) it does so with slightly less lift due to reflex.

Another theoretical change is reduced tip-stall in tight, low-speed turns. With this concept the inside wing has higher chamber compared to the outside wing, so it has better low-speed lift; also lower stall speed due to the lower angle of attack.

Although computer radios with definable mixes are one way to accomplish all this, as a practical matter the design could be made without too much complexity. One design —good for models only— is wingerons with fixed flaps. The flaps would be fixed to the fuselage and the remainder of the wingeron pivoted at the flap hinge. This means that the wingeron incidence automatically changes chamber or reflex.

[JetPlaneFlyer]

This also means the inside wing naturally travels faster and the outside wing travels slower; turning the whole plane away from the turn.
Nope.. the inner wing describes a tighter radius than the outer wing so it must travel less distance, therefore the inner wing travels slower (not faster).. Simple geometry shows that this must be true.

In a perfectly co-ordinated steady turn some opposite aileron (i.e. inside aileron down) coupled with 'into the turn' rudder is required... Dihedral looks after this 'automatically' as with dihedral side slip increases the AoA of the inner wing which is why aileron equiped silplanes have some dihedral.

Aileron differential is not really used to increase efficiency as such; it's to reduce adverse yaw which can in some cases result in the model turning in the opposite direction to that desired, or not turning at all.

[jcpatrick]

Apologies JetPlaneFlyer, I used imprecise language.
"This also means the inside wing naturally travels faster, ie. it wants to travel faster with reflex . With camber the outside wing would naturally travel slower; turning the whole plane away from the turn." The point is that ailerons —by their nature— produce adverse yaw and therefore waste energy in the efforts to correct this and produce coordinated turns.

[JetPlaneFlyer]

Apologies JetPlaneFlyer, I used imprecise language.
The point is that ailerons —by their nature— produce adverse yaw and therefore waste energy in the efforts to correct this and produce coordinated turns.

That's true but as you pointed out yourself adverse yaw is inevitable to some extent because induced drag icreases with increasing AoA so the wing with the greatest AoA (regardless of if the increased AoA is by ailerons or wing twisting) will produce more drag.

[vintage1]

Up to about 1936 most planes were turned on rudder, with the ailerons being use to correct skidding, WWII brought in the concept of the primary control being aileron, with the rudder used to correct skidding..

[jcpatrick]

Aileron differential is not really used to increase efficiency as such; it's to reduce adverse yaw which can in some cases result in the model turning in the opposite direction to that desired, or not turning at all. I stated that aileron differential corrects yawing by "... by losing energy to (deliberately induced) drag". Do please read the post before misrepresenting it or arguing against things not said.
That's true but as you pointed out yourself adverse yaw is inevitable to some extent because induced drag icreases with increasing AoA so the wing with the greatest AoA (regardless of if the increased AoA is by ailerons or wing twisting) will produce more drag. Not true; I never claimed that either adverse yaw or induced drag is "inevitable". It may well be inevitable to those who decline to read the entire post or those who resist change because they are stuck on "we have always done it this way" type of thinking. Nor is all drag the exact same; there are known drag differences between higher-drag ailerons and wingerons but mechanics (not aerodynamics) favor a fixed wing with aileron.

Nobody is claiming frictionless flight; the concept is to minimize energy loss and to make sure any inevitable drag assists (instead of opposes) coordinated turns. The energy savings are tiny, of importance only to sailplanes and, of no significance to 3D and jet plane flyers. Also note this post is proposing something different, a design that is not-the-same-as-before. That’s why it’s posted here in Modeling Science (http://www.rcgroups.com/modeling-science-136/), not on one of the flying or forums.

Up to about 1936 most planes were turned on rudder, with the ailerons being use to correct skidding ... Yeah, but before that —way back when I was young— the Wright Flyer warped its wings. :D As close to a bird as the brothers could make it, and not a bad attempt if I say so myself.

Technically warping is better than wingeron twist; warping matches the air better, and I've been considering it. Wing warping requires a careful balance between the rigidity needed to support the plane and being flexible enough to twist. But birds also change their camber/reflex, aspect, extension, tip shape, etcetera. Of all that, I thought combining the two readily accessible and proven technologies of wingerons (implimented in models) and camber changing flaps (implimented in aircraft and models) would give the best return for the additional complexity ... the best bang for the buck in Yank-speak.

And yeah, even though birds don't have them, the Wright brothers added a rudder pretty quickly. I notice the F22 has rudders too.

[jcpatrick]

Due to (errhh) misunderstandings about the concept, I’ll try another tact.

In the first picture of conventional ailerons and aileron differential, two things stand out: One is that ailerons of any sort inherently oppose tight, low-speed turns. The inner (slower) wing is reflexed and the outer (faster) wing is cambered, completely backwards since a reflexed wing wants to (naturally) travel faster than normal and a cambered wing will (naturally) travel slower. These effects yaw the plane against the direction of a naturally banked turn.

With aileron differential the inside aileron is raised very high to reduce lift and produce extra drag. The outside aileron doesn't have to travel as far down, so there is less camber and less drag on the outside; the net effect is —by deliberately induced drag— yawing the plane in the correct direction.

Although aileron differential deliberately creates drag, this is not altogether bad. A plane that is badly yawing out of the direction of flight is less efficient, has less lift, and produces unneeded drag from the wings, fuse and tail. But aileron differential still produces more drag than is necessary, as shown below.

Another effect is that ailerons —and even more so aileron differential— opposes what is needed in tight, low-speed turns. As the turn’s radius gets smaller, the inner wing travels much more slowly, often approaching stall speeds, and therefore needs to have more lift. This is accomplished with more camber, not less.

In the second picture, wingerons accomplish rotation by differing angles of attack; but both wings keep their designed shape. There is some drag induced anti-coordination in turns with wingerons due to angle of attack related drag; but it is less than wing-and-aileron because wingerons retain their low-drag designed profiles.

To counteract this minor drag induced yaw (as much as possible) flaps can camber the inside wingeron and reflex the outside wingeron. This theoretically reduces drag to absolute minimum, while matching speed and lift requirements to the appropriate side. This reduces drag-induced yaw to a minimum, maximizes inner wing lift, and should reduce tip stalls.

These wing profiles and flaps are derived from Drela's 2-meter Aegea (http://www.charlesriverrc.org/articles/aegea2m/aegea2.pdf). Aileron deflection shown here is ±4°; aileron differential is -2° and +6°. Wingeron angle of attack shown here is ±1°; and the cambered wingerons are ±1.5° with 2° - and + flaps. The intent is to normalize amount of deflection; 25% ailerons or flaps need to deflect four times as much as the whole wing itself.

[kcaldwel]

Have you checked the drag and lift coefficients for the wings with the different AoA and reflex/camber? The increased camber on what you want to be the down going wing is going to fight the reduced AoA from the wingeron control, and the opposite on the other wing.

It would be interesting to see how the Cl and Cd of each wing works out at different starting lift coefficients.

Once the turn is established, would it be any different than holding a little opposite aileron and pro-turn rudder? The crossed controls give you camber on the inboard slow moving wing, and reflex on the faster outboard wing.

The Swedish DLG pilots have started a no-rudder trend, with the gliders using reverse aileron differential quite effectively it seems. They dial in more down going aileron to keep the nose from falling on roll initiation, and the adverse yaw doesn't seem to be a big issue.

Kevin

[MCarlton]

Looking at this, I can't see how this would really work, given that the point of aileron control is to increase camber on the "rising" wing. If you increase camber but decrease aoa, is not the net effect likely to be zero?

I can also see the model tip stalling, as the inner wing flies slowly, increasing the camber over the whole wing, will have propotionally more effect at the tip, giving, in effect, "wash in" on the inner wing.

[JetPlaneFlyer]

Sorry if I misunderstood the original post.. I think I see where you are going now.

However perhaps there is an easier and simpler solution to the 'problem'. If you remove any aileron differential and simply use rudder to deal with adverse yaw then the theoretical inefficiency of aileron differential is easily negated.

I also think that perhaps you missed the relevance of one of the points I made in my first post...If you then do as I suggested and use rudder to turn (which via dihedral increases the AoA of the outer wing and reduces the AoA of the inner) then use a little opposing aileron to increase the camber of the inner wing and reduce camber of the outer.. Then you have achieved exactly what you are aiming for but you have done it with a standard unmodified glider without the added weight and structural challenges of wingerons. All you have to do is slightly change the way you use the sticks ;)

Steve

[jcpatrick]

Kevin - I'd hoped by posting here I'd get some practical feedback, hints, advice and crtitique before I was forced to learn new aero software. :D It's been over a decade since I used any CFD, so I'd be relearning. I also see that many aero packages are ambiguous about their results. Do I input flap gaps, which in one software may negate some other factors? Etcetera.

MCarlton - the 'point' of control surfaces are to steer and guide the craft. I'm looking for a more efficient (retaining kinetic energy) method over the known aileron losses. Conventional aileron drag loss is obviously something that others also recognize, and are looking at or trying different approaches.

Steve - Rudder steering would be ideal . . . except that when a dihedral plane's rudder is turned, there is a (sometimes significant) period before the bank is initiated. The craft yaws without turning, and there is a lot of lost energy in turbulent drag before the banked turn is established. It is back to the 'anti-coordinated' turn scenario, just initiated by a different cause.

[JetPlaneFlyer]

jc,

Regarding the delay in a rudder plane's roll response.. Yes i can see that there would be a short delay while the plane actually yaws but once the yaw is established i cant see any reason why roll response should be any different from a 'wingeron' equiped model.. (providing dihedral is adequate). The air approaching the wing does not know the difference between dihedral produced AoA and wingeron AoA. You could always give a jab of pro-turn aileron to speed things up if you so desire.

There is of course a loss of efficiency involved with dihedral. For instance an 8 deg dihedral angle would be theoretically 1% less efficient than a flat wing. I'd argue that this is negligable, probably less than the losses due to added weight of 'wingerons'... In any case the lack of side slip indication on RC models (no bits of wool taped on the canopy!) means that flat wing models are always likely to be less efficient in a turn than a model that has some dihedral.

The fact that ruddder induced yaw produces a side slip angle on the fuselage also will have a small aerodynamic drag effect.. I'm not sure just how much this would be but i'd suspect on a 'skinny' fuselage sailplane it would be all but insignificant.

Steve

[kcaldwel]

Kevin - I'd hoped by posting here I'd get some practical feedback, hints, advice and crtitique before I was forced to learn new aero software. :D It's been over a decade since I used any CFD, so I'd be relearning. I also see that many aero packages are ambiguous about their results. Do I input flap gaps, which in one software may negate some other factors? Etcetera.


I think if you have some Cl and Cd vs AoA graphs for the airfoil at the range of flap deflections you envisage, you could get a pretty good idea:

Pick the graphs for appropriate flap deflection on the down going wing, and read off the Cl and Cd for the aircraft AoA minus the wingeron angle. Likewise for the up going wing. It would at least give you an idea if the roll and yaw moments will be at least in the right direction.

One thing to keep in mind is that once the roll has started, the roll movement will mean the down going will effectively appear to have wash-in in to the air I believe, proportional to the roll rate and inversely proportional to the airspeed of the plane. The up going wing should effectively be washed-out. Hmm, not easy to model....

I guess you need to break the roll into three parts: the roll initiation, when the wings develop a rolling torque to start the roll; the steady state roll, where no roll acceleration is taking place, but there is roll velocity; and roll stopping, where there has to be a roll torque to stop the roll velocity. And then there is the steady state turn situation, where you want a constant bank angle and yaw rate.

I think this would be a lot of work to figure out! Maybe quicker to build one, and see if it works! It seems beyond my CFD skills. Maybe Dr. Drela could figure it out in a spare ten minutes!

The roll-in, roll-out is the only time I could see this method saving any drag. Once the bank angle is established, conventional controls will be just as good.

I think the response might be highly dependent on the initial AoA of the wing. The flap deflection and wing combination that works at high Cl might not be right for a low Cl condition. That would be hard to program in without active feedback.

Kevin

[MCarlton]

MCarlton - the 'point' of control surfaces are to steer and guide the craft. I'm looking for a more efficient (retaining kinetic energy) method over the known aileron losses. Conventional aileron drag loss is obviously something that others also recognize, and are looking at or trying different approaches.

Yes, my bad, I misunderstood what you were getting at, I apologise

[TjW]

Stuck in the house after a hospital stay, I've looked over a thread on sailplane aileron banking (http://www.rcgroups.com/forums/showthread.php?t=1056408) into turns, reverse aileron, and 'left thumb' rudder coordination into turn. That led to why we use aileron differential, and what I perceive as an inherent flaw in wing design . . . at least compared to bird flight.

This subject is about fine-tuning the utmost efficiency out of wings, so it's totally irrelevant to power planes. In a nutshell, the concept is to reduce drag and energy in every circumstance while creating conditions for coordinated turns.

As currently practiced, the inside wing on a turn is the wing that is reflexed the most, and the outside wing is more cambered. The effect is that the inside wing has less lift and the outside wing more lift; banking the whole. That is the effect we want, combined with the elevator of course. This also means the inside wing naturally travels faster and the outside wing travels slower; turning the whole plane away from the turn. Call it 'anti-coordination' if you will.

Aileron differential tries to compensate for this effect by raising the inside wing aileron more than the outside aileron goes down. This works by creating extra drag on the inside wing aileron, while the outside wing minimizes its increased drag from increased aileron camber. The net effect is to yaw the plane using drag; but it is still accomplishing this by losing energy to (deliberately induced) drag.

The 'anti-coordination' can be compensated with aileron-to-rudder coupling or manual ruddering, but both methods still lose energy by forcing the chambered wing to travel faster, increasing drag. The reflexed wing will travel slower without energy loss, but there will be some more (small) loss with the rudder itself also. Perfect rudder-to-aileron coordination minimizes these losses, but doesn't —and can't— eliminate them.

Wingerons, where each whole wing pivots from its root, overcome some of this by keeping the same chamber while changing each wing's angle of attack. But it's also true that angle of attack is coupled to drag as well. The inside wingeron wing attacks at a lower angle and has less drag, while the outside wing has a higher angle of attack and higher energy losses. The effect of drag is to rotate the ship away from the turn, not into or with the turn. So there is still some of the same 'anti-coordination', even in a wingeron setup.

Ideally, a wingeron combined with full length flaps could turn without 'anti-coordination' if these flaps used to change chamber were coupled with wingeron angles. In this scenario the inside wing would rotate to a lower angle of attack but the flaps would also increase the camber. This increases lift (overcome by the lowered angle of attack) and drag of the inside wing. Conversely the outside wing would increase angle of atack while the flaps simultaneously reflex; this decreases drag while the decreased lift is more than counteracted by the increased angle of attack.

Several changes result from this. Wingerons are notoriously sensitive or 'twitchy'; the entire wing surface is changing aspect rather than just the aileron. This chambering-wingeron should reduce that sensitivity because the flaps act counter to the overall wing change. As a wing increases angle of attack (creating a higher lift) it does so with slightly less lift due to reflex.

Another theoretical change is reduced tip-stall in tight, low-speed turns. With this concept the inside wing has higher chamber compared to the outside wing, so it has better low-speed lift; also lower stall speed due to the lower angle of attack.

Although computer radios with definable mixes are one way to accomplish all this, as a practical matter the design could be made without too much complexity. One design —good for models only— is wingerons with fixed flaps. The flaps would be fixed to the fuselage and the remainder of the wingeron pivoted at the flap hinge. This means that the wingeron incidence automatically changes chamber or reflex.


I can't speak to how it's done in model airplanes, but in full scale, once the turn is established, steep thermaling turns are quite often done with a bit of out-of-turn aileron. If you don't apply this, the bank tends to increase.
On mine, above about 40 degrees, it tends to roll in more. Below that it tends to roll out. There's a "sweet spot" where it's pretty neutral.

I am told by Open Class pilots that this tendency is even more noticeable in the >20 meter span sailplanes, but I don't have that much money.

[Ralph Walton]

I can't speak to how it's done in model airplanes, but in full scale, once the turn is established, steep thermaling turns are quite often done with a bit of out-of-turn aileron. If you don't apply this, the bank tends to increase.
On mine, above about 40 degrees, it tends to roll in more. Below that it tends to roll out. There's a "sweet spot" where it's pretty neutral.

I am told by Open Class pilots that this tendency is even more noticeable in the >20 meter span sailplanes, but I don't have that much money.

This is the first time I've seen this mentioned at RCG, and desribes exactly what I've come across from time to time. I call this an "opposite lock" turn. It can be induced in very low speed turns with alot of elevator. With enough practice, you can repeatably come out of a left turn with full right aileron and vice versa. or back it off to find the "sweet spot".

I had slope conditions recently with just a sniff of lift, great conditions to practice minimum sink with my F3B Ellipse. I was flying at eye level and had a good horizon behind to judge sink against. If you push it too far it will result in a nasty stall, but a little but of "opposite lock" seems to result in a great turn with the least altitude lost in light conditions.

[JetPlaneFlyer]

This is the first time I've seen this mentioned at RCG, and desribes exactly what I've come across from time to time.

It's not the first time it's been mentioned because i said the same thing in post #2 of this very thread ;) .. But the original poster appeared to miss the significance of it.