ron evans Posted September 13, 2016 Share Posted September 13, 2016 Hi all. Flying my floormate delta on the weekend it was quite windy and gusty and it was noticed that in a fast pass the model had a shimmy in the yaw. This was more evident flying crosswind. The model can be slowed, nose high with no yaw/roll wobble I've had with other deltas. The coffin inspired fuz has little side area, with nothing much in front of the wing so my first thoughts are that I've made the fin/rudder too big. Any input would be appreciated. Ron Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 13, 2016 Author Share Posted September 13, 2016 Here's a pic Quote Link to comment Share on other sites More sharing options...
John Tee Posted September 13, 2016 Share Posted September 13, 2016 not an earodymamist(?) but my first thought was too small. john Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted September 13, 2016 Share Posted September 13, 2016 Some stuff in here Ron Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 13, 2016 Author Share Posted September 13, 2016 Could be too small John, and probably the simplest to try out, tacking on a bit more area. I didn't mention that the shimmy didn't appear to happen in calmer conditions. Ron Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 13, 2016 Author Share Posted September 13, 2016 Thanks John S , lots of stuff on there to digest. Ron Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted September 13, 2016 Share Posted September 13, 2016 Try false cockpit shaped curves about the fuselage to break down the "flow frequency" passing about the model, full size use "fences" and "teeth", attached along areas of smooth airflow. Missiles were the same, oscillating at certain speeds, various appendages dampened these down. I myself calmed a delta by applying "reflex" This is achieved by setting the both elevators/ ailerons /elevons 1- 2mm above centre, up Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted September 13, 2016 Share Posted September 13, 2016 Ron If it really doesn't do it in calm conditions one has to start thinking about its reserve of stability. I have one plane in particular than flies nice and smooth in still air but is a bit of 'pig' in any turbulence requiring continual correction even to keep straight and level. In a delta a large part of its natural stability comes from its airflow vortex conditions. It could be that turbulence is causing these to alter and thus have a greater effect that the same turbulence would have on a normal wing. Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 13, 2016 Author Share Posted September 13, 2016 Denis, the elevons are set about 2mm up for s&l flight. Did the dive test, moving the c/g aft and trimming until I was comfortable with the amount of pitch stability. Are you suggesting a cockpit would damp the yaw oscillation, that might be easy to try. Simon, thankfully even in windy conditions the model handles well and doesn't need any more correction than when flying in calm conditions. It just does it with a yaw shimmy. An interesting point about its airflow vortex. it's got me wondering if the KF wing sect might be part of the problem. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted September 13, 2016 Share Posted September 13, 2016 We've discussed this effect a few years back - you might like to look here Ron There is an explaination of the effect there from me. There is also a very neat suggestion from Brian Cooper to combat it - which I subsequently tested on a model and found it worked well. BEB Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted September 13, 2016 Share Posted September 13, 2016 Reading through everything Ron, you do have the mentioned thin vertical stabiliser. Try temporary thickening to the surface. I believe addition of a false cockpit must have screened or softened the vertical stabiliser oscillation, but thickening the surface does sound easier. I have used the old style helicopter heading hold gyro on the rudder control of a 7lb, 56 inch wing Raptor, that smoothed out a high speed tail waggle, and these can be switched on/off via Aux 1 on your Tx Edited By Denis Watkins on 13/09/2016 18:01:45 Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 13, 2016 Author Share Posted September 13, 2016 BEB, had a look on your link, but couldn't find the fix from Brian Cooper.Did it involve thickening the TE of the fin/rudder.. Dennis, thickening of the fin seems to have worked for some people so I'll probably try for a low tech fix. A gyro on the tail would cost twice as much as the airframe..........ok, so I'm tight Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted September 13, 2016 Share Posted September 13, 2016 yes Ron - its the post just below mine! BEB Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted September 13, 2016 Share Posted September 13, 2016 Ron, go on eBay. Heli boys are using the latest all singing kit. Get the Analoge head gyro GY48V, the red one, with 3 leads, the extra lead is the on / off. Should not be more than, £5. Heli boys don't use these anymore, and may even donate one. Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 13, 2016 Author Share Posted September 13, 2016 Dennis, thanks for the tip. And thanks to everyone for all the help given. I'll try the thickened rudder TE first, just have to wait for a windy Sunday. Shouldn't have to wait too long. Ron Quote Link to comment Share on other sites More sharing options...
Former Member Posted September 13, 2016 Share Posted September 13, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 14, 2016 Author Share Posted September 14, 2016 Thanks Percy, extra fin area is on my list of fixes, but I'm going to try thickening the TE first. I'm not familiar with the Challenger, was that one of Stan Yeos flying wings. Reading the old posts, it seems quite a few models and full size have this problem. I find the whole thing fascinating. Ron Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted September 14, 2016 Share Posted September 14, 2016 When you think about it Ron, thickening surfaces is akin to "reflex", that is, using surfaces in the airflow to dampen unwanted movement. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted September 14, 2016 Share Posted September 14, 2016 Posted by Biggles' Elder Brother - Moderator on 13/09/2016 18:50:13: yes Ron - its the post just below mine! BEB I'm not seeing it either! Quote Link to comment Share on other sites More sharing options...
Dai Fledermaus Posted September 14, 2016 Share Posted September 14, 2016 Ron, Number 1 Son bought me a copy of Model Aircraft Aerodynamics by Martin Simons some time ago. There's an item on Yaw Stability which suggests that the size of the fin is a factor. You are welcome to borrow it. Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 14, 2016 Author Share Posted September 14, 2016 Thank you Mr Fledermause, weather permitting I'll be up the patch Sunday Quote Link to comment Share on other sites More sharing options...
Former Member Posted September 14, 2016 Share Posted September 14, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Dave Bran Posted September 14, 2016 Share Posted September 14, 2016 OK, what do I know, nothing, but I have designed and built a fair number of successful very fast model deltas, more than 50 over the years. Inc Slope deltas I currently have over 20 active. How effective is your rudder? It has little to push against. with no canopy plate. This also affects fin stability. You appear to have used KF section, stepped top? (picture unclear) I have used both top and bottom KF step and both at once. I have found bottom to be superior, but YMMV. I suspect that what the top step is doing is making the fin less efficient, and its already minimal. Quick test, tape on wing end fences, shaped like the fin. They will be easily removed and add side area behind CG and make the wing more efficient.. Quote Link to comment Share on other sites More sharing options...
Paul Jefferies Posted September 14, 2016 Share Posted September 14, 2016 'Fraid I haven't read all the links so forgive me if this has already been suggested but the fishtailing could be caused by rudder flutter........ I once had the pleasure of flying a very large (1/3 scale) Mustang and to cut a long story short, that initially suffered from very bad fishtailing at speed which was cured by thickening the rudder to about 3/4" at the trailing edge. Before replacing the rudder with a thicker one, you could check to see if this is your problem by simply sticking some thin strips either side of the trailing edge of your rudder and/or by sticking a small weight on it which would change it's frequency of oscillation....... Good luck! Paul Quote Link to comment Share on other sites More sharing options...
ron evans Posted September 14, 2016 Author Share Posted September 14, 2016 Dave, I thought I was a delta fan.The wing has a top step KF section, and rudder response is good, the model stall turns easily, although it looks a bit odd, and will happily fly around on R/E. A bit of a challenge for me, I'm on mode 1.Fitting a canopy, more fin area, and now wing tip fences are all on my fix list, but first I've fitted some 6mm tube over the rudder T/E, I'll try it out Sunday. Paul, the linkage to the rudder is short and straight, although there is some slop in the cheap metal servo. If flutter is part of the problem my first fix should help. Quote Link to comment Share on other sites More sharing options...
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