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Jun 08, 2007, 12:08 PM
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F3X's Avatar

Snap Flap. How much and why?

Lets kick this forum off with a hot topic. I am looking for input from some true Aero guys out there to effectively explain how snap flap works and what is the best way to tune it for tight efficient turns.

Below is a graph from Xfoil running through Profili on a RG15 airfoil with a neutral TE and one with +2, +4 and +6 deg of snap flap.
So what should we be looking for?
Last edited by F3X; Jun 08, 2007 at 01:20 PM.
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Jun 08, 2007, 08:09 PM
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KurtMc's Avatar
My own view is that the lifting forces the cambered wing generates are best set by determining what sort of camber setting results in the best glide to climb ratio. Once you have that setting, you've determined the wing's max-cambered potential and, by mixing elevator to camber in a hard F3F turn, what you accomplish is a lifting force in the turn equal to a cambered climb-out -- and, you shorten the turn radius by 'climbing' during the turn. If you know your max lift-to-glide camber setting (before you encounter sink by overcambering and scrubbing off speed) it would seem you could pull your tightest, climbing, turns?

Last edited by KurtMc; Jun 17, 2007 at 10:08 AM.
Jun 08, 2007, 08:46 PM
Detail Freak
target's Avatar
I don't know Kurt, I believe that snap flaps certainly can be over done...
A tight turn that scrubs off speed is too much snap flap; the plane can pull the turn, but for F3F, and I assume also for the F3B speed task, that's no good.

Jun 08, 2007, 09:26 PM
Permanently Banned
pinolefm1's Avatar
I think snap flaps have to be tempered against excessive drag as target mentioned...a balance between the extra camber needed to efficiently tighten the turn w/o being so obtrusive as to increase the drag to the point of diminishing returns.
I've found that different airfoils respond differently to snap flaps ...RG15 for instance seems to like just a small amount and anything more it just slows down in the turn...
Not sure how you can optimize snap flap settings with a computer program though.I'm sure there's a sweet spot somewhere in all those squiggly lines but I never took the time to learn how to read the cd/cl charts and just adjust by feel when out flying.

Jun 08, 2007, 10:03 PM
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KurtMc's Avatar
There's a balance before drag outweighs the increased wing efficiency gained with snapflap. Can the balance be found in the setting that gives the best glide-to-climbout -- the distance the glider can travel while still maintaining climb, while cambered, before losing airspeed and dropping out (given constant windspeed, of course)? The "maximum camber efficiency" setting, my guess is, can be found in this manner. Dial that sucker into your snapflap and you're golden?

Jun 09, 2007, 12:29 AM
Registered User
timmig's Avatar
OH yeah--- I can't answer the aerofoil questions but I can tell ya what I"ve experienced with a LOT of planes. They all like different amount of snap flap to be fast and turn hard-- and accelerate out of the turn instead of "scrub" through and lose speed. It's really a balance-- one I've found by starting small and increasing until I see a "noticable" difference in speed coming out of the turn--then I back off until I recapture that speed without losing the hard turning. And sometimes--that setting changes with higher wingloadings vs light wind low ones! I've learned that sometimes, "less is more"!!!
Jun 09, 2007, 02:53 AM
Chuck it and see!
Wing-span's Avatar
Is the idea to have the whole of the trailing edge react to the snap flap function equally, or just the flaps?

SF is not a function I have used previously.
Jun 09, 2007, 07:56 AM
Registered User
F3X's Avatar
I know how I get there by trial and error but there must be a better way.
If you look at the first graph you can see that there is a drag (CD) penalty with camber at anything below a CL of .5. Above that mark adding snap flap is better. Of course this is looking at the airfoil at basically flying straight with added angle of attack (Alpha) I am sure there are many other factors going on in a turn and when the big boys post here we will learn a few tricks.

But for a simple non aero guy it looks like a multi point curve on the snap flap mix could pay off in less drag. Say something like no snap flap for the first 1/4 stick (Assuming you use full stick for the turn) and then you kick in 4-6 degrees and then from 3/4 stick to full you don't add anymore. Maybe? I use a linear mix so the more I pull the more I get. So how much extra drag am I getting by going simple on the mix?????
Jun 09, 2007, 08:22 AM
I do this for fun!
When you look at the performance of any given glider wing, there really only are 2 performance parameters - lift and drag.

So for us glider guiders who are always looking for the best performance the question is always: How do I get the lift I need right now with the minimum drag?

Snap flap is one answer to that question.

Remember, the elevator's job is to allow us to set the angle of attack of the main wing, and hence its Coefficient of lift. The elevator does not cause the aircraft to climb or descend. All it does is change the angle of attack of the main wing. This causes the main wing to make more lift and more drag (up we go!) or less lift and less drag (down we go!). The elevator itself also causes drag but let's ignore this for now.

So, what snap flap does when setup correctly is to optimise the performance of the main wing (this means: to deliver the lift that is needed right now with the minimum of drag). The result is that the aircraft performs the desired turn with the minimum loss of energy. Drag acting on the aircraft over time is what causes energy loss.

So lets say you have your aircraft in the lift band, flying straight and level and reasonably fast. Say also that both the wing camber and the elevator are at neutral to maintain this flight path. Now say that you want to fly a quick reversal - a half loop - and you want to exit the maneuvre as fast as possible. Notice that it doesn't matter whether or not you use snap flap, a certain amount of additional lift will be needed to cause the aircraft to fly the half loop at the desired radius of turn. That amount of lift is fixed and does not change as long as the entry speed and radius of turn do not change.

What snap flap does when set up correctly is change the camber of the main wing airfoil section to keep the section within its low drag 'bucket' through turns (because more lift is needed to fly the turn than to fly straight and level and that causes more drag).

Higher cambered airfoils in general produce less drag when producing high lift than lower cambered airfoils. So we use snap flap to temporarily change our slippery, low cambered airfoil (which produces minimum drag at low Coefficients of lift, ie flying fast, straight and level), to a higher cambered airfoil (which produces minimum drag at higher Coefficients of lift, ie flying a quick half loop).

It's important to remember that flying our half loop always produces more drag than flying straight and level. That's unavoidable because drag is a product of lift - more lift, more drag. Using snap flap to increase wing camber gives us a way to minimise the drag penalty of the maneuvre.

Another way of looking at it is to say that correctly set up snap flap allows us to minimise energy loss through maneuvres.

Too much snap flap is as bad as too little. In fact too much can be worse than none at all. It is one of the most difficult settings to optimise so it's probably best to start with a small amount and then increase it in even smaller increments over a number of flights until it is clearly too much, and then backing off some.

It's certainly worth the effort though, and playing with it is as good a way as any to get to know your aircraft better, so go for it and Good Luck!
Jun 11, 2007, 10:32 AM
satinet's Avatar
I've been wondering about snap flaps on a Miraj I recently bought.... Have no idea where to start to be honest!
Jun 11, 2007, 10:51 AM
Registered User
F3X's Avatar
Originally Posted by satinet
I've been wondering about snap flaps on a Miraj I recently bought.... Have no idea where to start to be honest!
For a seat of the pants set up use 8-10mm all along the TE measured at the center of the flap and mated with the ailerons at full up elevator, and set your elevator to get maximum throw at full aft stick. Max throw is the maximum usable elevator throw, add more and stall at any airspeed.
Jun 11, 2007, 11:10 AM
satinet's Avatar
thanks for the info f3x

so I should the central flap on the Miraj as well as the ailerons (it has a quirky one flap setup in the middle of the wing)??

i wasn't sure it would work.
Jun 11, 2007, 02:41 PM
AZ Outback
Robert Burson's Avatar
A few thoughts on Physics on a day that may be better without them.

The equation you are working with for the turn is F=mv2/r.

v2 is velocity squared and r is the radius of the turn. m is just the mass of the ship.

For a given turn radius and velocity you need F amount of force to the center of the turn to keep turning. This is what the wing is doing with the lift force. I think you can see if you try to double the velocity it will take four times as much force. If you try to decrease the radius by one half, then you will need twice the force from the wing.

That is the Physics, how you accomplish maximum lift force and minimum drag is the art.

The Best Soaring

PS nice addition to the Sailplane section.
Jun 12, 2007, 04:25 PM
Registered User
I use Xfoil polars and AVL to determine the best snap-flap mix as follows:

1) Using Xfoil polars with a range of flap deflection, determine the best camber deflection for a typical CL in a fast F3B or F3F turn. Typical values might be CL=0.4-0.6 . The best camber deflection is the one which gives minimum Cd at that CL. The Reynolds number must also be set appropriately for the high-speed case. For this you need the Type-1 polars, not the Type-2 polars typically used to compare airfoils.

2) In AVL, set the camber to the high-speed case (i.e. max reflex), set CL for the fast level-flight case using the C1 menu. This might be as low as CL=0.015 in F3B Speed. Also constrain elevator to seek zero pitching moment. Calculate the trimmed elevator deflection. make sure the reference X location (CG location) corresponds to the actual glider.

3) In AVL, set the optimum fast-turn camber found in step 1). In the C2 menu, set the speed equal to the high-speed case, and set the expected turn CL assumed in step 1).
Again calculate the trimmed elevator deflection.

The ratio of camber difference to elevator difference between steps 2 and 3 is the optimum camber to elevator (snap-flap) ratio. Below is the numerical example for a Supra flown in F3B Speed. The ratio is nearly 1:1, regardless of how tight the turn is. This ratio will depend somewhat on the CG position. The reason is that an aft CG requires less elevator to give a certain turn radius, and hence requires more snap-flap mix to give the same camber deflection as a forward CG.

Supra F3B trimmed-pitch elevator throws
m = 2.25 kg
V = 60 m/s
static margin = 10% MAC

CL Rturn G's Camber Alpha Elevator
----- ------ ---- ------ ----- --------
0.015 inf 1.0 -4.0 -0.98 0.59
0.400 13.8m 26.6 +1.0 -0.12 -3.84
0.600 9.2m 40.0 +3.0 +0.66 -6.16

CL=0.4: dCamber/dElevator = 5.0/4.43 = 1.13
CL=0.6: dCamber/dElevator = 7.0/6.75 = 1.04
Jun 13, 2007, 12:56 AM
Registered User
Hello Mr Mark Drela

I am very honoured to meet you on this forum.

I have 2 remarks on you proposition for setting the snapflap mix :

- instead of the "airfoil only" best L/D ratio I think that the whole wing L/D ratio must
be taken into account. The influence of induced drag is not neglectible so the optimum camber is depending of the aspect ratio and is lower than expected with an X-foil analysis only.

- I am always surprised by static margins of 10% or more because the real margin that we can measure on F3B or F3F planes are close to 0%. Our plane are very neutral.
I think that a margin of 10% is evaluated from a calculated neutral point. This computation is not accurate because the fuselage lift is not taken into account. If it is done we can compute neutral points very close from reality.

If you can read french I have just published an article on neutral point and CG calculation on Planet-Soaring and I would be very pleased to have your point of view on my approach.
For pilots who are interrested on this topic, I also began to published a serie of 3 articles on flap and snapflap settings (the first part is on line on planet soaring at that time)


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