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Marc RC pilot

Do you use snap flaps and why?

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Marc RC pilot

Been watching a YouTube video on the use of "snap-flaps" and as far as I can work out you drop a little aileron to increase the camber/lift with the elevator kicking in a little up trim at the same time so has to minimise height loss in the turns (turning by it's nature means using various control surfaces which in turn increase drag on the glider and the snap-flap settings compensates for this by making the glider a little more "floaty" in the turns, right?).  The amount of snap-flap required will depend on model type, CG position and fight modes (?), personal preferences. 

I suppose the downside is that it slows the plane down a tad in the turn, but then again that might be acceptable in thermal/cruse mode. 

Am I on the right snap-track here? 

And do real full size gliders have this same option? ( I can't image the pilot dropping in some aileron camber on each turn not to loose any precious height) I some times wonder what is the line between fly by wire and "natural" flying being a bit of a purist myself. Or am I just in denial and need to embrace this digital programable tech and stop being set in my ways because the benefits can be worth it in flight performance?

What say you?

Cheers

Marc   

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SilentPilot

Flying full size gliders you can feel exactly what is going on so you can, indeed must, use the exact amount of elevator in the turns for maximum efficiency.
You are correct that a glider pilot will not want to lower their ailerons in a turn though. That will have a detrimental effect on the washout.

If flaps are fitted then they are used in turns but usually once the bank angle is established so as not to hinder the roll rate.


Tony 

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pete beadle

Hi all

I understand that the term "snap flap" was first used in control-line flying for C/L models that had coupled flap and elevator. It was used to make the elevator more effective and made loops tighter and faster

Using the term in R/C gliders is effectively a misuse of the term, but that's not important,  the "snap-flap" term/position/configuration is therefore, in my opinion only, a means of increasing the effectiveness of the elevator by coupling in the flap to delay the stall

I believe the term "snap-flap" is used mostly by F3F (slope racing) pilots to tighten the turns at each end of the course (lap) and to reduce the chance of "flicking out" ie inducing a high-speed stall...........does any of this help?

Regards

Pete

BARCS1702

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cirrusRC

"The reason for using it is improved efficiency/lower drag."

"To increase the lift coefficient, you either increase the angle of attack, increase the camber, just increase camber, or both.
If you just increase the angle of attack, the drag penalty is higher than if you increase camber at the same time. You don't need as much increase in angle of attack to achieve the same lift coefficient when you use camber."

You can test this in a crude way by doing the loop test.

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MikeDLG

Besides size, a big difference between our gliders and full-size is the we have fullspan flaperons, so we can change the camber of the whole wing, so pretty much maintain washout etc. 
Dave describes the the reason to use it very well in his post above.  I don’t feel it make a huge difference, so I wouldn’t stress about using it or not. I do use it because it is theoretically better and don’t detect any negative performance when using it.  It is worth trying and if you like it, continue.  

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SilentPilot

I realise that I maybe veering too far away from F3K here but some full size gliders with full span Flapperons, I’m thinking DG600 here, were reportedly a very bad design. 

Full span Aileron used as flap (camber) is seldomly a good idea in full scale terms. Quite why it seems acceptable to a DLG design escapes  me! 

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MikeDLG

Interesting. 

It is probably due to the extreme speed ranges a DLG (and other Rc gliders) operate in.  Compare the launch to gently circling in light lift.  A dlg has to transition from low drag, high speed to high lift, low speed. Camber management allows the same wing to do both pretty well. 

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Marc RC pilot

Thank you all for the very informative replies, very  interesting. 

 

4 hours ago, MikeDLG said:

Interesting. 

It is probably due to the extreme speed ranges a DLG (and other Rc gliders) operate in.  Compare the launch to gently circling in light lift.  A dlg has to transition from low drag, high speed to high lift, low speed. Camber management allows the same wing to do both pretty well. 

 

That is a factor I did not consider Mike, and quite a tall ask from our beloved DLG gliders (or any glider) when you think about it...Good point.

 Does anyone know what the sort of peak speeds are reached on a fairly competitive/computant DLG throw? I wonder what that woud equate to if scaled up to a real full size glider on a winch launch...Maybe some RX's come with velocimeter's ?   

 

14 hours ago, cirrusRC said:

You can test this in a crude way by doing the loop test.

What is the loop test cC?

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Paul Gleeson

To calculate the peak speed you need to know the launch height.

You are converting kinetic (movement) energy to potential (how high) energy. Ignoring air resistance and the drag you create when you rotate the DLG...

Mass x Acceleration due to Gravity x Height = Mass x 1/2 x (Velocity Squared ). Solving for Velocity for a given height....

V= Square root of ( 2 x 9.8 x H)

50m = 31.3 m/s or 113 km/h

75m = 38.3 m/s or 138 km/h

 

Someone check my maths..... My physics O level  was a while ago

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SilentPilot

Doesn't it depend upon the weight of the model too? 

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oipigface

I can’t claim to be a master of snap flap, but there does seem to be a missing element in this discussion. Many F3F pilots use snap-flap to reduce drag only when the plane is going fast round a sharp corner. This is achieved by having the camber change only for large movements of the elevator control. 

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Jef Ott

Tongue in cheek maths lesson...

It's easy to time the start and finish of the launch (allow a stall at the top to be sure the vertical velocity has reached zero). 

Round up the acceleration due to gravity to 10m/s (as this makes things easier and allows us to forget about drag)

For a one second launch, the initial vertical velocity is 10m/s.    22mph       Height gain is 5m (in reality it is probably not advisable to get down to zero vertical velocity at 5m).

For a two second launch the initial vertical velocity is 20m/s.     44mph       Height gain is (15m + 5m) = 20m

For a three second launch the initial vertical velocity is 30m/s.   66mph      Height gain is (25m + 15m + 5m) = 45m

For a four second launch the initial vertical velocity is 40m/s.      88mph      Height gain is (35m + 25m + 15m + 5m) = 80m 

If you put 1.21GW into the flux capacitor on a 4 second launch...

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Simon B

I use snap flap as it seems to make the model far more responsive in "cruise",  almost none in "speed" and very little in "thermal" as the wing has a lot of camber in that mode already and I'm not trying to make quick turns.

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Paul Gleeson
11 hours ago, SilentPilot said:

Doesn't it depend upon the weight of the model too? 

No, mass is on both sides of the equation so can be eliminated.

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cirrusRC

Yes, for me snap flap (negative camber) aids the push over phase of the launch, when full down elevator is deployed.      Square loops are also easier :) 

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SilentPilot

Snap Reflex :D

 

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f3fman
On 15/10/2019 at 11:28, oipigface said:

I can’t claim to be a master of snap flap, but there does seem to be a missing element in this discussion. Many F3F pilots use snap-flap to reduce drag only when the plane is going fast round a sharp corner. This is achieved by having the camber change only for large movements of the elevator control. 

I don't think we are trying to "reduce drag" when F3fing around the the turn, rather we are increasing the wing lift to get it round the corner faster, whilst accepting the fact the drag is temporarily higher than without the flap. Pulling too tight, or with too much snap-flap, is an excellent way of slowing the glider down in fact, but you do go a shorter distance, so you pay your money and take your choice  🙂

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satinet

The object of the exercise is to get the mimimum drag aka cd/coefficient of drag, for a given cl. Cl is coefficient of lift. High angle of attack is high Cl. 

Lotsbof info on wiki and rcgroups. 

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