Tim Mackey Posted August 16, 2011 Share Posted August 16, 2011 Posted by dash on 16/08/2011 21:49:55:Awwww you left out the speed of the servo If the torque goes down then the speed at which the control surface moves will increase. I dont understand that. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted August 16, 2011 Share Posted August 16, 2011 Posted by Biggles' Elder Brother - Moderator on 16/08/2011 22:08:57:Yes Dash you're quite right the speed will decrease - but its not a torque effect - its just geometry. Lets say our servo goes end to end in 0.1 secs. If its set up so that we have a torque leverage effect - so that's short servo arm long control horn we will have less control surface travel. The speed of the servo is fixed, 0.1 secs end to end, so the control surface will move a shorter distance in the same time - so yes it moves slower. In effect its like the step-down gear set again as I mentioned above. BEB But thats contradicting what Dash just said Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted August 16, 2011 Share Posted August 16, 2011 It does - but its still right! Sorry Dash I misread your post. A case of seeing what you expect to see rather than whats there I think. The control surface will move slower with the leverage effect as I stated above - assuming both motions take the servo end-to-end! BEB Quote Link to comment Share on other sites More sharing options...
Cyclicscooby Posted August 16, 2011 Author Share Posted August 16, 2011 Thanks Rich n Ian.. Both good points, especially as the ailerons are so large.. Luv Chrisie.. xx Quote Link to comment Share on other sites More sharing options...
PatMc Posted August 16, 2011 Share Posted August 16, 2011 Actually I think talk of stiffer pushrods & servo torque are red herrings. They won't cure the flutter or in fact have any effect on it. If the hinge has pulled out due to flutter it's done so because the servo to control horn linkage has stayed intact. I suspect that either (a) the ailerons are not stiff enough for the speed that the model was being flown at or (b)the hinge has already started to give way which has caused the flutter to start - which has loosened the hinge further etc...If the problem is caused by (a) then either the model needs to be flown with restraint as far as speed goes or the ailerons need to be stiffened or mass balanced. If (b) the answer's obvious. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted August 16, 2011 Share Posted August 16, 2011 Posted by Biggles' Elder Brother - Moderator on 16/08/2011 23:27:32:It does - but its still right! Sorry Dash I misread your post. A case of seeing what you expect to see rather than whats there I think. The control surface will move slower with the leverage effect as I stated above - assuming both motions take the servo end-to-end! BEB So I'm right - Dash should have said will DECREASE ? Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted August 16, 2011 Share Posted August 16, 2011 Yes! BEB Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted August 16, 2011 Share Posted August 16, 2011 Yipee - I got one right. Will you tell him or should I Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted August 16, 2011 Share Posted August 16, 2011 Posted by dash on 16/08/2011 21:49:55:Awwww you left out the speed of the servo If the torque goes down then the speed at which the control surface moves will increase. I hesitate to put another oar in but I think what Dash meant was that if the force goes down (due to mechanical (dis)advantage) then the speed at which the control surface moves will increase. I think we all agree that the torque is a constant? Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted August 17, 2011 Share Posted August 17, 2011 This is probably excessively pedantic of me, but why do servo manufacturers specify their torque using kg-cm when the correct unit is the newton-metre? Kilogrammes are a unit of mass, not of force. regardsJohn Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted August 17, 2011 Share Posted August 17, 2011 Oh John I so, so agree with you! The problem I have with this whole debate is I'm biting my tongue that the Kg is not a unit of force anyway! Its a unit of mass! What of course we are taling about is the force equivalent to the weight of a kilogram mass at standard gravity of 9.81 m/s2. I suppose the reason is that for most modellers a Kg-cm has a sort intuitive "feel" - a servo arm is about a centimetre long so if the servo is 5Kg-cm then that means with typical arm it could (in principle) lift 5Kg. And as I say folks have a feel for that. In rough terms (taking g=10.0) then 5Kg-cm is 50N-cm or 0.5N-m. BEB PS yes Martin we all agree- torque is fixed - only the force or the distance moved changes. Quote Link to comment Share on other sites More sharing options...
dash Posted August 17, 2011 Share Posted August 17, 2011 Ok now i am just confused..... or its late and im getting things wrong. Ok lower the torque of the servo servo horn bigger than control surface. This will give greater end point to end point travel so the control surface is moving a larger distance in the same amount of time. Example. ok set to more torque small servo big control surface. servo is set at .1secs and say it travels 1cm EP to EP at the TE ok same again but large servo small control surface still .1secs and say it moves 2cm EP to EP at the TE so same time frame has passed but a greater distance was traveled by the control surface so its moving faster Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted August 17, 2011 Share Posted August 17, 2011 Let us see if we can create some clarity. The servo can produce a maximum torque, say for example and going with the wrong units, 5kg-cm. It can typically move through an angle of 60 degrees or so, and can do this in 0.2 of a second. (Check the box for your own servo specs) So we put an arm on the servo that is 1cm long, and make the arm on the control surface the same length. The available maximum torque at the control surface will now be the same as at the servo, and the travel will also be the same, eg 30 degrees either side of centre. That might be a bit much for some models, so we decide to double the length of the horn on the control surface. Now, the pushrod at the servo end is 1cm from the centre, so it can exert a force of 5kg. At the other end it is exerting this force at a distance of 2cm from the control surface axis. So the torque here is doubled. But the control surface will only move through half the angle (approx) although it will take the same time end to end. So like a step down gear set, the torque has been doubled and the angular velocity halved. The total time from one extreme to the other will still be 0.2 seconds. If you want a greater movement at the control surface than the 60 degrees or so that the servo can provide, you can shorten the horn on the control surface but will have less torque to drive the surface. So for models with large control surfaces and high angular deflections you will need a more powerful servo. You also must not shorten the control surface horn too much or it will travel too far, locking up when it is in line with the rod. If the torque required to move the control surface against the aerodynamic and friction loads exceeds what is available then the control surface will fail to move. This might only occur at high speeds. Remember those tales of WWII pilots struggling to pull hard enough to come out of a dive? (Air frame deflection sometimes exacerbated their situation.) regardsJohn Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted August 17, 2011 Share Posted August 17, 2011 Blimeys, this has drifted off topic - but interesting stuff guys Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted August 17, 2011 Share Posted August 17, 2011 Correct John - I agree 100%. You said in one post what I took 3 to say! But to be fair the argument was unfolding as I was doing it! BEB Quote Link to comment Share on other sites More sharing options...
Cyclicscooby Posted August 17, 2011 Author Share Posted August 17, 2011 So.... To finish what I started.... Whilst fitting a third hinge onto all four ailerons, I noticed something else... Quite fundemental, but nothing at all to with servo arms and torque.. The wings have a 3mm CF spar running their entire length, not a 5mm like the Edge, and they were actually flexing.. !! I did notice on the second flight, that whilst less than 1st time, it was actually the WINGS fluttering..!! God knows how it didn't rip itself to bits.. I decided to fit cross-bracing using 2mm CF tube, and the third flight..... . . . WAS A DREAM... Two clicks of right aileron and she was hands free on 1/3 throttle.. Slightly tail heavy, so flight 4 had the Lipo as far forward as pos, and was much better... Only using an 850mAh 3s, so a 1300 will both add nose weight, and increase duration... YIPPEE... My first own-design plan, and it works... Thanks for all the replies, even if a bit off-topic, lots of typing was had by many and a conclusion was reached.. Luv Happy Chrisie.. xxEdited By Cyclicscooby on 17/08/2011 23:21:56 Quote Link to comment Share on other sites More sharing options...
John Olsen 1 Posted August 17, 2011 Share Posted August 17, 2011 Well, the torque of the servos is possibly relevant to the original problem. They need to have enough torque available to move the surface as required. There is a bit of a simplification in the way I've put things since of course the actual movement is non linear if the horns are not equal in length. For small angular movements we can neglect that. Getting back to the original problem:Is the linkage from the servo to the aileron sufficiently stiff? If the aileron has the horn at one end, is the aileron sufficiently stiff in torsion to prevent it twisting. (kind of hard to know this since we don't know the actual stresses applied in flight, but comparing to other similar models might help.)Is the wing itself sufficiently stiff in torsion? Remember the aileron will impose a load on the wing, tending to twist it, and this load is applied where the wing is thin, so small deflections may occur. Are the hinges providing a good positive location? (Obviously the one that pulled out is not, but we don't know for sure if that is cause or effect.)Do the ailerons hinge freely without any tendency to stick, or excessive friction? Are the holes in the horns a good fit or loose? Does the aileron have aerodynamic balance, and if so has this been overdone?In full size, mass balancers are used to help prevent flutter, I'm not sure how applicable this would be to models except maybe in the larger faster ones. All structures flex under load, the problem with aircraft is to make them stiff enough without making them too heavy. regardsJohn Quote Link to comment Share on other sites More sharing options...
Tom Wright 2 Posted August 18, 2011 Share Posted August 18, 2011 IMO a possible reason could be that flutter started due to poor hinge face to depron joint ,(some types of hinge don't like epoxy) . Pins into depron don't do much good ,so as the joints started to fail ,the flutter would start ,and on one side total detachment occurred accounting for the reduced roll control. Should by chance this be the case a pad of 1/64 ply fixed to the depron with u por and surface mounted wick hinges glued with cyno may have performed better despite the inner location of the horn ,which as Rich suggests is subjected to a greater loads due to the increased risk of minor or major flutter ,by comparison to a horn situated further out board. This seems to be a very light model not suited to high speeds could it have been a bit over powered? and so over stressing the wing and ailerons prior to the additions that added more torsional strength?Edited By Tom Wright 2 on 18/08/2011 02:08:32 Quote Link to comment Share on other sites More sharing options...
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