Marc RC pilot Posted October 14, 2019 Share Posted October 14, 2019 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 Link to comment Share on other sites More sharing options...
cirrusRC Posted October 14, 2019 Share Posted October 14, 2019 https://www.rcgroups.com/forums/showthread.php?3345419-Why-use-Snap-Flaps Link to comment Share on other sites More sharing options...
SilentPilot Posted October 14, 2019 Share Posted October 14, 2019 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 Link to comment Share on other sites More sharing options...
cirrusRC Posted October 14, 2019 Share Posted October 14, 2019 "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. 2 Link to comment Share on other sites More sharing options...
MikeDLG Posted October 14, 2019 Share Posted October 14, 2019 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. Link to comment Share on other sites More sharing options...
SilentPilot Posted October 14, 2019 Share Posted October 14, 2019 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! Link to comment Share on other sites More sharing options...
MikeDLG Posted October 14, 2019 Share Posted October 14, 2019 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. Link to comment Share on other sites More sharing options...
Marc RC pilot Posted October 15, 2019 Author Share Posted October 15, 2019 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? Link to comment Share on other sites More sharing options...
Marc RC pilot Posted October 15, 2019 Author Share Posted October 15, 2019 Found info on loop test, thx cirrusRC: https://www.rcgroups.com/forums/showthread.php?1234569-loop-test-for-dialing-in-elevator-camber-mix Link to comment Share on other sites More sharing options...
Paul Gleeson Posted October 15, 2019 Share Posted October 15, 2019 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 1 Link to comment Share on other sites More sharing options...
SilentPilot Posted October 15, 2019 Share Posted October 15, 2019 Doesn't it depend upon the weight of the model too? Link to comment Share on other sites More sharing options...
oipigface Posted October 15, 2019 Share Posted October 15, 2019 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. Link to comment Share on other sites More sharing options...
Jef Ott Posted October 15, 2019 Share Posted October 15, 2019 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... Link to comment Share on other sites More sharing options...
Simon B Posted October 15, 2019 Share Posted October 15, 2019 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. Link to comment Share on other sites More sharing options...
Paul Gleeson Posted October 15, 2019 Share Posted October 15, 2019 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. 1 Link to comment Share on other sites More sharing options...
cirrusRC Posted October 15, 2019 Share Posted October 15, 2019 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 Link to comment Share on other sites More sharing options...
SilentPilot Posted October 16, 2019 Share Posted October 16, 2019 Snap Reflex 1 Link to comment Share on other sites More sharing options...
f3fman Posted October 16, 2019 Share Posted October 16, 2019 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 Link to comment Share on other sites More sharing options...
satinet Posted October 16, 2019 Share Posted October 16, 2019 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. Link to comment Share on other sites More sharing options...
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