Snap-flap in TD open class ships

[trend.ab]

I have a new Pike Sup and set it up according to Jojo's setup on the Samba page. I also had a discussion going with Jojo about a couple of points, especially the use of Snap Flap.
Jojo only uses Snap Flap in speed mode to take the reflex out when he pulls the elevator. Besides that, he flies what he calls "a clean wing".

What is your opinion with regards to this mix?
I read a post somewhere else (DLG, I guess) that basically recommends this mix to be always on.

Thanks!

[OVSS Boss]

Ditto here too. Snap flap is great in B speed, but in TD I have never had a reason for that radical of a throw to make a turn.

Marc

[Goinav8n]

I agree with Marc. Snap flaps for F3B is great but in TD I dont see an advantage

Jeff

[dephela]

I use snap flaps in B speed to add camber when the stick is pulled hard but in no other tasks.

I know there are others in B that firmly believe it should be on all the time, even a reverse that adds reflex when pushing down stick.

[trend.ab]

Seems to be an interesting topic.
Theoretically, I could imagine an argument similar to the one for use of full span ailerons, i.e. it is better to move more area (entire wing trailing edge) a little bit then less area (elevator) more. On the other hand, the elevator is on such a long lever that moving it a little bit might have much more effect...

Does anyody have an aerodynamic rationale or even wind channel data?

[ChuckA]

I know what a zap flap is but I have never heard of a snap flap. What is a snap flap and how does it differ from a regular flap.

[dephela]

Snap Flap =
Elevator to Flap coupling that usually has the Ailerons slaved to the Flaps for full span cambering. Usually it provides down flap motion as up elevator is applied ad sometimes applies reflex with down stick. Amounts and offsets also vary with users., usually "about" 1/4" "more or less".

Some use it while thermalling but I think it works best when pulling a high speed turn.

[ChuckA]

Snap Flap =
Elevator to Flap coupling that usually has the Ailerons slaved to the Flaps for full span cambering. Usually it provides down flap motion as up elevator is applied ad sometimes applies reflex with down stick. Amounts and offsets also vary with users., usually "about" 1/4" "more or less".

Some use it while thermalling but I think it works best when pulling a high speed turn.

So snap flaps is just another name for coupled flaps and elevators like I used on my control line stunt models over 50 years ago. Just never heard them called snap flaps. Incidently. zap flaps were a two position flap system used on the McDonnell F101 Voodoo fighter. Zap flaps used a simple parallel link arrangement to move the flaps down and aft without complicated tracks and linkages. Simple and light but only proveded two positions; retracted and extended.

[dephela]

Yup, just like CL stunt models have had and still use. When they start putting electronics in those planes, they'll be able to have the control over them that we do with our radios.

[Rusty P]

For flying DLG, the idea behind snap flap (a bad name for this case) is not to turn more quickly or efficiently.

The idea is that whenever you want up elevator, you also could benefit from more camber. This would be true when thermalling, or responding to any sort of bump. If you push the nose down, you generally want more speed, so reflex (on a suitable airfoil) would be a benefit.

It is arguable if that is true, but the above is the rationale for snap flaps on DLG. If you hold to this theory (and flying style), then it should also work in TD.

I figure that it is really just a programming thing. So, program in some snap flap and put it on a switch. Then try it out and see if you like it. Report the results, and we can argue with each other and you about it all :p

[OVSS Boss]

There is one thing that folks are missing here. The reason the term "snap-flap" is that iit is not just coupled elevator-flap. The set ups I have seen is a coupling that occurs after a certain amount elevator throw so you can a very immediate effect. If you see someone like Mike Smith make a turn in speed, it is so immediate it makes you gasp.

Marc

[spatial]

If you have a look at Mark Drela's supra-evo program he has a snap flap "mix" that effects flaps and ailerons in what seems to the prevalent DLG usage of the term. Elevator input has a small and progressive effect - i.e. no "snap" - on flap & alieron.

words are slippery beasts....

[trend.ab]

Spatial

It is Mark Drela's DLG setup I had in mind when I asked this question.
What is good for a DLG, should be good for an open class ship -- or am I missing something?

I would love to know how Mark Drela sets up his Supra.

Beat

[yardflier]

Spatial

It is Mark Drela's DLG setup I had in mind when I asked this question.
What is good for a DLG, should be good for an open class ship -- or am I missing something?

I would love to know how Mark Drela sets up his Supra.

Beat

Hi Beat! It's Luis from NJSS.

Mark Drela does use snap flap on his Supra. He uses 2mm up with down elevator and 3mm down with up elevator. I have my Supra set up the same way.

I'll bring a print out of his setup to the field Sunday if you are interested in seeing it. You can also look at my airplane which is setup almost exactly as Mark recommends.

Luis

[trend.ab]

Thanks, Luis

That's interesting. There seem to be conflicting opinions about this point within the top TD/F3J competitors.
Actually, there seem to be three areas that are widely discussed and where there is disagreement from one side of the spectrum to the other.

1. Snap-flap. Drela yes in all modes, others only in speed mode to take out reflex

2. Differential. Drela no, to avoid effects on pitch that differential exhibits (less down then up is a net up, with makes the plane dive), others yes depending on mode, more in thermal, less in speed

3. Aileron to rudder coupling. Drela yes, more in lauch and landing, but still 20% in normal mode. Others none at all, except in launch.

I think we just have to pick one of these setups based on gusto. I flew the pike so far with snap-flap and coupling, but I'll try without.

Looking forward to seeing you Sunday, hope weather permits flying.

Beat

[spatial]

Hi Beat,

the evo program plus throws for non-evo fliers are at:

http://www.charlesriverrc.org/articles/supra/supra.htm


Paul

[trend.ab]

thanks, Paul. Yes. Snap-flap.

[yardflier]

Thanks, Luis

That's interesting. There seem to be conflicting opinions about this point within the top TD/F3J competitors.

Beat

Yes, it is interesting. The type of flying we do involves a lot of variables: plane setup, pilot skills, pilot preference, ever-changing conditions, pilot observations, etc.. It is no surprise that any two top pilots will have different "opinions" of what is "best".

This situation is very similar to what we see in golf. Compare Arnold Palmer's swing and equipment preferences to Jack Nicklaus or Tiger Woods. These three legendary golfers obviously approach the problem of putting the club face on the ball in drastically different ways yet all are successful. I'm sure each believes his approach is "best".

We are fortunate to have Dr. Drela, an MIT professor with a Doctorate in aerodynamics, spend so much of his time researching, via software models, how our models truly perform. His expertise and objective test results give us a clear understanding, from an aerodynamics perspective, of the subtle differences that result from differential choices, aileron to flap mixing and other tweaks. Without this type of data we are left with pilot observation as our only source of information.

As an example, Dr. Drela does not use differential because aerodynamics data shows the rudder generates less drag than the upgoing aileron when countering adverse yaw. How much less? I don't know but I would bet it doesn't reduce total drag by more than a few percentage points. There is a darn good chance that the difference in lift conditions from flight to flight are greater than the drag difference between using differential or not using it.

We may not be able to discern the difference in performance caused by reducing drag slightly while turning. It may however make enough of a difference to allow you to make your time when others can not. That is, of course, as long as the pilot doesn't blow it by making the wrong decisions or manhandling the plane! :)

Beat, you may want to join Steve, Bill, Leszek, Chuck and I at the next Eastern Soaring League contest in Long Island. Mark Drela competes regularly in the ESL circuit and will be more than happy to discuss all these issues with you. He is a really nice guy and a great contributor to the ESL and our sport in general.

Luis

[trend.ab]

Thanks, Luis

Yes, I read a lot of Dr. Drela's posts, most interesting. I am looking forward to join you at the Long Island ESL contest, and to see some of the absolute best fly their planes. It is amazing, already, to see you, Lezek and Steve handle their planes. My plan is to set up the Pike once and to then get used to flying it with that setup, instead of tweaking the setup all the time. For that reason, I am wondering about those three points discussed further up.
I guess I will leave some small amount of differential in, take rudder coupling out (learn to do it by hand), and to use snap flap only for speed right now (this is the point where I am most unsure). With this, I am basically using Jojo's proven Pike setup. Looking forward to hopefully chat with you on Sunday (weather looks worse and worse...).
Beat

[jcats]

Didnt know it was called 'snap-flap'... I sometimes use it depending on the conditions especially at lower altitudes. I'm no expert but my heavy stork 2 e seems to make flatter circles. one thing's for sure- it slows the plane down so i use it carefully and sparingly.

Beat,
glad you got in contact with the NJSS folks! Great bunch o guys!

Hey Louis! I miss flying with you guys!

jun

[dephela]

There is no point to have snap flaps in "only for speed" if your intent is just to get back from a downwind position. You won't need any high speed, high G, pylon type turns for that. You shouldn't need it either for doing some zooming around on the way down after specking out. The turns you carve will be much more controllable and you'll look more the expert without that mix.

[trend.ab]

The intent to have that mix on for speed is just to return the wing to normal from reflex when turning, not to carve turns like in F5B.

Jojo is flying this way, and the rationale makes sense to me.
I am sure Mark Drela is not using this mix to make B type turns when he is thermaling, neiter.

[trend.ab]

Jun

Thanks again for putting me in contact with NJSS. I love to fly there and have a really great time with the club.

Thanks!

Beat

[dephela]

I can understand removing reflex in a turn. LOL, I'd never be in reflex and turning at a rate that would require just taking out the reflex, though you and others might.

[Chad Sullivan]

I fly a couple of Eraser Extreme's. I have snap flap mixed in on my Evo12. It is on all the time. It has really improved the low alt. handling for me. If I get a bit slow down low it seems with snap flap I can tap the nose down a bit and pick up speed much faster than without. Resulting in less alt. loss. That's nice for me I never give up and like to be able to work that low alt lift. Also with snap flap I am using much less elevator throw in general. Less elevator throw means a bit less drag. No doubt this is a personal preferance thing though. I have it mixed into the flaps surfaces only.

[djklein21]

Hey guys,

These kinds of mixing are all really plane dependent. A perfect example is any newer Drella model. These wings are optimized to significantly respond to camber changes. Their efficiency depends on the use of camber or reflex during different flight modes. These wings have a lot of engineering in them. Their hinge lines and sweep angles are carefully calculated to produce a very versatile wing, optimized for the tasks it was designed for.

The way snap flap works-

When up elevator is applied, it tilts the airfoil of the wing to a higher angle of attack. This increases the Cl of the wing (coeficient of lift) as Cl increases as angle of attack increases. Modern sailplane wings are now designed to efficiently respond to camber changes. If your wing is more effiecent at higher Cl with camber, than snap flap is good. Modern DLGs and TD ships are trending towards thinner wings that need the camber adjustment to maintain efficiency over its range of AA; reflex for speed, camber for lift, ect... These wings will benefit from elevator to camber mixing.

[trend.ab]

The Pike Sup has a wing that is optimised for camber changes. HQW, where the W stands for Woelb or camber.

Nevertheless, Jojo flies it without snap-flap. He certainly is a great pilot and one of Europe's top competitors.

There does not seem to be a rationale explanation for why some use it and some don't. Maybe that just means that the differences between snap-flap and non snap-flap are really small, as they are for differential and aileron to rudder mixing.

Beat

[yardflier]

The Pike Sup has a wing that is optimised for camber changes. HQW, where the W stands for Woelb or camber.

Nevertheless, Jojo flies it without snap-flap. He certainly is a great pilot and one of Europe's top competitors.

There does not seem to be a rationale explanation for why some use it and some don't. Maybe that just means that the differences between snap-flap and non snap-flap are really small, as they are for differential and aileron to rudder mixing.

Beat

Beat,

You hit the nail on the head! The configuration differences matter and are helpful in optimizing the sailplane but the pilot at the sticks still has a much larger effect on the performance of the airplane. Reading the air well and flying smoothly are much more critical in determining the overall efficiency of the sailplane/pilot "team". Find good air and even the worst trimmed plane goes up like the proverbial home-sick angel. Don't find the lift and the perfectly trimmed airplane will be down well before the task time is up!

Hey Jun! It is good to hear from you. You are always welcome to fly with us if you happen to be in Jersey!

Luis

[franck102]

For flying DLG, the idea behind snap flap (a bad name for this case) is not to turn more quickly or efficiently.

The idea is that whenever you want up elevator, you also could benefit from more camber. This would be true when thermalling, or responding to any sort of bump. If you push the nose down, you generally want more speed, so reflex (on a suitable airfoil) would be a benefit.

Just found this old thread while doing a search... the idea is intriguing but there is one thing I am missing here: fairly often when I apply down elevator it is to avoid a stall because my plane just hit a gust of wind or a thermal. If a mix removes some camber at the same time, thus increasing the stall speed, am I not more likely to actually stall (or won't I need more input to avoid the stall)?

[atjurhs]

franck102,

Wow, talk about ressurrecting an old thread....

I think the bulk of this thread was fairly specific to the Pike Superior which likes to be flown fast and does (in my opinion) benefit from some small amount of snap flap. But for your more generalized question, giving down elevator (to avoid a stall) would as you say also decrease the camber (if snap flaps were activated), but this decrease in camber would also speed the plane up (less lift BUT less drag, kind of like putting the plane in "speed mode"), so you'll still prevent the stall because the plane will be flying faster.

Hope this helps.

BTW, I use snap flaps on my Pike Superior and love it.

[trend.ab]

I am using snap-flap on all my planes in the meantime, quite a bit actually.
The Supra loves it, Drela himself uses 3mm in his setup.
The DLGs fly better with snap-flap, as well, in my opinion.
The reason is actually decreased drag in turns. The airfoil needs to produce more lift to fly a turn, as the plane is pulled towards the center of the turn by the wing. This increased lift is more efficaously produced with more camber, and the amount needed is proportional to how much elvator you pull.
I am not sure whether this applies to reflex with down, elevator, as well, but I am flying just 1mm or so of reflex, anyway.
My two cents.
Beat

[franck102]

franck102,
But for your more generalized question, giving down elevator (to avoid a stall) would as you say also decrease the camber (if snap flaps were activated), but this decrease in camber would also speed the plane up (less lift BUT less drag, kind of like putting the plane in "speed mode"), so you'll still prevent the stall because the plane will be flying faster.

Hope this helps.

Thanks!
As you said I should experiment with my place I guess, I think snap-flap could help with turns but I still suspect that when trying to avoid a stall the loss of lift would be immediate whereas the acceleration would take several seconds. Taking this to the extreme it's a bit like causing a stall by suddenly removing the flaps without giving the plane time to accelerate.
I guess the ideal would be snap-flap activated only when the rudder is deflected more than a certain amount... maybe coupling some camber increase to the rudder would actually work?

[djklein21]

Hey guys,

These kinds of mixing are all really plane dependent. A perfect example is any newer Drella model. These wings are optimized to significantly respond to camber changes. Their efficiency depends on the use of camber or reflex during different flight modes. These wings have a lot of engineering in them. Their hinge lines and sweep angles are carefully calculated to produce a very versatile wing, optimized for the tasks it was designed for.

The way snap flap works-

When up elevator is applied, it tilts the airfoil of the wing to a higher angle of attack. This increases the Cl of the wing (coefficient of lift) as Cl increases as angle of attack increases. Modern sailplane wings are now designed to efficiently respond to camber changes. If your wing is more efficient at higher Cl with camber, than snap flap is good. Modern DLGs and TD ships are trending towards thinner wings that need the camber adjustment to maintain efficiency over its range of AA; reflex for speed, camber for lift, ect... These wings will benefit from elevator to camber mixing.

Many here are still over thinking, or under thinking this. Snap flap is good. In fact, lets remove the snap flap term (it is confusing) and call it elevator --> camber coupling. When an up elevator command is given, the flaps and ailerons are mixed in to increase the amount of camber as up elevator is used. As you give an up elevator command, you are asking the wing to produce more lift. If you do not increase the camber, the wing makes more lift purely by increasing its angle of attack. This elevator --> camber coupling is most needed, and most apparent if you are flying fast with your wings reflexed. A reflexed wing is poor at creating high lift. So allot of camber is added with the elevator command. This produces less drag in the turn than if the wing remained reflexed. This is usually true for all of the flight envelopes. The addition of elevator --> camber coupling reduces the performance if you are already cambered to the most efficient point for Cl max. Basically only turn off the elevator --> camber coupling in launch mode. Leave it on in all other modes. Use allot in reflex mode (reflexed wing), a moderate amount in cruise mode (clean wing with no camber deflection), and less in thermal mode (slightly cambered).


This is my humble opinion but it is based on experience and science. Ask any great F3b pilot, or any great aerodynamacist. I would bet a few dollars that Dr. Drella and Andreas Herrig agree, but you will have to ask them to find out.