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Feb 18, 2011, 06:31 PM
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ScotY's Avatar
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Aileron to flap mixing

Can anyone tell me what the best way to mix aileron to flaps for various flight modes? Specifically, is it better to have the entire trailing edge move evenly for roll input, i.e. flap and aileron surfaces move exactly together but with minimal deflection (limited by up flap movement) OR should the aileron move a "normal" amount (fairly high angular deflection) and the flap move a smaller percentage of this? I would think that if the flap and aileron surfaces are not in the same positions while banking, the trailing edge between the two surfaces might create a lot of drag. Then again, I suppose if the flap moved at least a little, this would be better than not moving at all. Hope this make sense? I suppose this question is just academic, but I am curious to hear opinions, theory, etc. There's a ton of aerodynamic discussions about CG, flying technique, etc. but I've never seen this topic discussed.

Thanks, Scot
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Feb 18, 2011, 06:51 PM
Play loud, Fly high
Gil Gauger's Avatar
"Depends", the famous answer to most questions. I set my planes up to fly with the intention of being smooth turning using only the right stick. I never use dual rates so my throws are fairly aggressive, I just choose to fly with small stick movements. Usually I set up for 15 deg up aileron at full stick, adjust the differential to address adverse yaw, then add ail-rudder mix to clean it up. If the roll rate is still soft to my taste I add ail-flap till I am happy with the feel. Depends on the plane but usually the flaps will be moving half or less than the ailerons. I put all the mixes on switches and when the sky is alive the rudder mix gets killed and the plane spends a lot of the time rolling or inverted. (Drives the power guys nuts!). Since I don't set my moldies up for dead air flying specifically as a mode, I can't offer detailed advice on changes between ail-flap mixing for various modes. The switch pilots can help you more there. I am more of a stick and trim guy. Sign of my age I guess.
Feb 19, 2011, 11:08 AM
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John Walter's Avatar
Assuming similar lengths for the ailerons and flaps, to minimize drag the flap throw should be about 1/3 of the aileron angular movement. How far you want your aileron throw depends on your flying style and the type of flying you do.
Feb 19, 2011, 02:09 PM
Registered User
Here's a post written by Darrel Perkins some time ago which I saved as the information seemed very helpful. In the post he lists data from DR. Drela.
This should help you I think

Aileron/Flap Coupling

"The original question is about aileron to flap coupling. I've been using this for years, regardless of airframe. I found it to aid in all handing characteristics. Increases roll rate while reducing total throw required in all flight modes, reduces adverse yaw tendencies requiring less aileron to rudder coupling."
From Dr. Drela:
Deflected ailerons deform the load distribution away from the ideal near-elliptical shape, and hence increase induced drag. Partially slaving the flaps to the ailerons can alleviate this load distribution deformation, and thus mitigate the ailerons' CDi penalty. The question is what's the optimum amount of ail-> flap mixing.
The lowest-drag aileron system is wing-warping as used by the Wright Brothers -- the wing is linearly twisted from tip to tip. When such a twisted wing reaches its steady roll rate, the load distribution returns to its optimum level-flight shape, and the drag penalty is zero.
With a finite number of hinged control surfaces such a linear twist cannot be achieved. But it can be approximated as close as possible if each surface's deflection is made proportional to its distance from the aircraft's centerline, measured at the surface midpoint.
If the four control surfaces have equal span, we then have:
Surface mid-span loc. deflection
L.Aile. -3/4 -100%
R.Aile. -3/4 -100%
L.Flap. -1/4 -33%
R.Flap. -1/4 -33%
So for this wing the flap motion should be 33% of the aileron motion. Using AVL I've verified that this mixing ratio produces very nearly the smallest induced drag penalty. If the flap span differs from the aileron span, the table above can be adjusted accordingly.
Longer flaps will have larger travel and vice versa.
BTW, this "distance-proportional deflection rule" strongly argues against stopping the ailerons short of the tip. The resulting unhinged tip portion should in fact have the largest deflection.
The "distance-proportional deflection rule" can be fudged if there is a tip stall problem in a sustained turn, where some opposite aileron must is held. By increasing flap travel over its "optimum" amount, the flaps can carry a greater share of the roll power, which reduces the required downward deflection of the inside aileron, and thus delays tip stall. So if your TD glider has insufficient tip stall margin, I suggest increasing the flap mixing and you should see some improvement.
The extreme case would be 100% flap mixing, which mimics full-span flaperons. Flaperons give excellent tip stall resistance, as is obvious to anyone who flies a DLG with a good 2-servo wing. A 4-servo TD wing with decent planform should not need to go to this extreme.
I have some time on a Perfect. Yes, I coupled the ailerons to flaps with good results. I probably run more than Dr Drela suggests.
Good luck with it!!! dp
Feb 20, 2011, 09:17 AM
Registered User

Aileron/Flap Mixing

The post by DP is very interesting. Question I have is does the ratio of aileron to flap mixing change considerably in the various flight modes...speed, cruise, and thermal? What seems like the best setup for today's generation of TD ships...Aspires, Supras, Explorers in the various flight mode configurations...
Feb 22, 2011, 01:41 PM
Flying What?!?
fnnwizard's Avatar
Originally Posted by beach1
The post by DP is very interesting. Question I have is does the ratio of aileron to flap mixing change considerably in the various flight modes...speed, cruise, and thermal? What seems like the best setup for today's generation of TD ships...Aspires, Supras, Explorers in the various flight mode configurations...
John, I'be been waiting to see if any of the experts respond to this, but since none yet, here's my personal settings how I do this. I am not absolutely sure it is the correct way, but base on test data, it sure seems close.

From where reflex is (or zero lift) I have ail deflecting +-15 degrees mixed with flaps anywhere from +- 5-7 degrees.
That would mark the envelope of travel. I do use slightly more than Dr. Drelas formulas also.

Now when te is in thermal mode, there may be 3 degrees of camber, so deflection of ail becomes down additional 12, and up 18 while flaps are down additional 2-4, while up 8-10.

As you can imagine, the deflections in the various flight modes are constrained within the max amount.

The max deflection amount may vary slightly from plane to plane but shouldn't by much if designed right.
Feb 22, 2011, 02:25 PM
Registered User

Check out Paul Naton's DVD's, they are very informative & helpful.

Specifically DVD's: "F3, Performance Tuning for Sailplanes, and Soaring Master Class 1, 2, & 3". Paul & Mike cover this very well.
Feb 22, 2011, 05:12 PM
Stable genius
vespa's Avatar
Basically what Drela is saying is that you should measure the distance between the fuselage and mid-flap then divide by the distance from fuse to mid-aileron. This number should be the minimum mix for all flight modes (e.g. flaps move 22% as much as ailerons). If you have yaw problems work with differential and rudder mixing, if you have tip stall problems increase the flap mixing.

So for mode-specific mixing, you might want more flap mixing in thermal and/or launch modes if needed to combat tip stalls but otherwise stick to the ideal ratio.
Feb 22, 2011, 07:06 PM
Registered User

Aileron/Flap Mixing

Tuan: Thanks as always for your insights and info.....Jon
Feb 23, 2011, 10:05 AM
Daryl Perkins's Avatar
Even before Dr Drela's info came available, I've always run about what Drela suggests because it just made sense to me on some intuitive level. It's always nice when the aero guys back up my intuition with years and years of schooling. If you use Dr D's formula, it's a way of approximating the optimum trailing edge deflection - wing warping.

If you use the formula, you'll see that you're probably not running enough aileron to flap mix. It seems everyone that looks at my set ups, is surprised at the amount of flap following I am running. Now here is the counter-intuitive part - the longer your flap in relationship to span, the MORE flap following you should run. The formula is correct.

And yes, I run it in every flight mode and have ever since I can remember. Well, except landing, it really screws with elevator compensation when you're flapped.
Feb 23, 2011, 01:09 PM
Flying What?!?
fnnwizard's Avatar
Originally Posted by beach1
Tuan: Thanks as always for your insights and info.....Jon
Jon, don't thank me. It's guys Like Daryl P, Joe W, Dr. Drela, as well as others that I get all my info and learning from. I pass it on as respect for those I receive it from.

The first couple years soaring I was getting all sorts of confusing info and most of it completely inaccurate if not outright wrong from guys I was flying with. It's after reading through many of the aforementioned guys' posts on the internet that it started clicking in my head.

And thanks DP for all your contributions to the Soaring threads, you've provided me with more info than you will ever realize .
Feb 23, 2011, 01:25 PM
Registered User
Avaldes's Avatar
The one thing that Daryl said above needs to be highlighted...turn off your flap following in landing mode!

Years ago I forgot to do this and I had a model that was just landing funny. As it turns out, when your flaps are set to large deflections, think of them as airbrakes. So when you give right aileron/rudder, your right airbrake is also retracted and the plane yaws left! The whole thing gets out of sequence and you end up fighting the thing in.

Turn off the ail-flap mixing when your stick comes down, or in landing mode and the problem is solved!
Oct 22, 2012, 12:47 PM
Proud member of LISF and ESL
I hope no one minds that I bring back older but valuable discussions. This one is full of gold and will be valuable to newer pilots entering the world of full house gliders.
Oct 23, 2012, 05:28 AM
Registered User
Originally Posted by aeajr
I hope no one minds that I bring back older but valuable discussions. This one is full of gold and will be valuable to newer pilots entering the world of full house gliders.
I guess it pays to look at every mode in regard to what you're trying to achieve and then look at secondary benefits. In any aircraft, model or real, stability and constant inertia in landing mode offers a level of predictability, and predictability allows you to judge rate of decent, time to ground and at least some level of compensation for wind conditions. Then you're able to make calculated decisions (too high/low, fast/slow, late/early) and compensate.

If you believe this philosophy, then the question is: how much Aileron-Camber-Mix adversely affects the stability and inertia of your model, with high flap angles...and is there any benefit in having it enabled in the first place?

I don't think it's a very difficult test to run at a safe height.
Last edited by Nestinator; Oct 23, 2012 at 06:04 AM.
Oct 26, 2012, 08:11 AM
Aussie F5J Team
Talking with a friend of mine John Skinner he mentioned something I have heard discussed very rarely but is very important and that is flap (acting as aileron) differential to pitch coupling.

As the differential increases there is more pitch down. This needs to be set correctly to get an axial roll.

I hope it helps,


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