Geoffrey Harris Posted March 14, 2011 Share Posted March 14, 2011 My "Boomerang" trainer (through pilot error) crashed on landing and broke its back just behind the wings, Duly repaired an experienced club member carried out a test flight and it was found that the plane just wanted to climb. He put in sufficient down trim to achieve level flight but there is now about 1/4" of down elevator and this looks really strange. Obvious care was taken to keep things straight during rebuild and everything looks ok but ???????? Is it possible to adjust in any way, mechanically or otherwise so that the elevator can be neutralised to what should be its correct position. It has been suggested that CofG might be too far back but it balances exactly as it should. It is possible I suppose that I may have inadvertantly altered the tailplane incidence and if this is perhaps a factor, is there anything one can do without resorting to deliberately re-breaking and resetting???????????????? As ever I look forward to your comments and adding to my learning curve. Thanks. Quote Link to comment Share on other sites More sharing options...
Craig Carr Posted March 14, 2011 Share Posted March 14, 2011 Hello geoffrey,It may be that the repair added some extra tail weight but you say it still balances ok. My gut reaction then is the wing incidence but I'm surprised if it's out that much... You can alter the incidence by putting some thin lite ply packing strips under the main wing.. A little at a time till you get the elevator back to neutral... Once happy you can tidy them up and cyano them onto the wing seat.Craig Quote Link to comment Share on other sites More sharing options...
Richard Wood Posted March 14, 2011 Share Posted March 14, 2011 Hi Geoffrey, If possible you could try packing up the the wing at the trailing edge to alter it's incidence relative to the tailplane. 1/4" down elevator sounds quite a lot for S&L - try packing the wing T.E a little at a time - say about 1 to 2 mm & get your helper to test fly again. It might take a few flights to get it right. Craig beat me to it.Edited By Richard Wood on 14/03/2011 20:39:18 Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted March 14, 2011 Share Posted March 14, 2011 Assuming its not tail heavy the most likely explaination is that the tailplane incidence, relative to the wing incidence, is out. Normally, on most models the tailplane provides a bit of "negative lift" ie downthrust, this is to balance the natural nose down moment caused by both the wing lift and the fact that the CoG is in front of the centre of lift. It sounds to me like you have way too much downthrust from the tail plane. This would happen if you effectively "straightened" the fuselage past the straight pointwhen you repaired it - leaving the tail pointing slightly upwards IFSWIM. There are two ways of fixing this and restoring the balance: 1. you can add some negative incidence to the wing - i.e. pack up the trailing edge of the wing. 2. You can add some positive incidence to the tail plane - i.e. pack up the leading edge of the tail plane. Which one you do depends mainly on how easy they are to do. Remember, which ever you try do it slowly in stages - a 1/16th" packing is plenty to try in one go. If you try that and find you need less down elevator trim then you know you are on the right path - you then just need to fiddle with the exact amount of packing a bit to find just the right amount. Remember you'll never get it perfect - settle for making it much better! BEB Quote Link to comment Share on other sites More sharing options...
Olly P Posted March 14, 2011 Share Posted March 14, 2011 I would also check the engine thrust line, if back has been broken before wings the engine could be pointing more skyward, is there any noticeable difference in throttle to ROG?Olly Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted March 14, 2011 Share Posted March 14, 2011 Ha! Great minds think alike! Well at least you confident that we are more or less of one mind on this one Geoffrey! BEB Quote Link to comment Share on other sites More sharing options...
Geoffrey Harris Posted March 14, 2011 Author Share Posted March 14, 2011 My thanks for the prompt and informative responses and for "steering" me towards possible action. As the tailplane is well epoxied I think the first step will be to pack wing trailing edge. As an aside someone else at the club suggested adding some downthrust to the engine. ????? Appreciate all your advice, thanks again. Geoff. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted March 14, 2011 Share Posted March 14, 2011 Given that the break was behind the wing its unlikely that the problem is in the engine to wing relationship - far more likely in the tailplane to wing relationship. So I'd stick with shimming the wing for now and see how that goes. BEB Quote Link to comment Share on other sites More sharing options...
Geoffrey Harris Posted March 15, 2011 Author Share Posted March 15, 2011 Thanks for that advice. I could'nt see that the engine would need down thrust as it has always flown straight , hands off. Geoff Quote Link to comment Share on other sites More sharing options...
PatMc Posted March 15, 2011 Share Posted March 15, 2011 If the wings are shimmed at the TE to bring them back to the previous relative angle of incidence with the tailplane that will change the engine/wing angle relationship. The model now have less downthrust.Other than that it looks a little odd, is there anything wrong with leaving the down elevator trim that has been found necessary ?IMO if the tailplane can't be adjusted & the down elevator trim cures the climb the problems solved. Quote Link to comment Share on other sites More sharing options...
Former Member Posted March 15, 2011 Share Posted March 15, 2011 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted March 15, 2011 Share Posted March 15, 2011 Eric makes a good point there. The only solution that probably won't disturb the thrust line setting is to pack up the tailplane. But given that you say that's epoxied there isn't really much you can do there. You're never going to get this perfect - as I said earlier you're just going to have to settle for getting it better than it is. BEB Quote Link to comment Share on other sites More sharing options...
PatMc Posted March 15, 2011 Share Posted March 15, 2011 Is there an echo in here ? Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted March 15, 2011 Share Posted March 15, 2011 ...Is there an echo in here..... Apparently so! BEB Quote Link to comment Share on other sites More sharing options...
Geoffrey Harris Posted March 16, 2011 Author Share Posted March 16, 2011 Have digested all the advice offered but cannot "trial" aflight with raised wing t/e until next week ,weather permitting. However the comments from Eric stirred up the old grey matter.and I recalled an article on "designing your own plane", RCM@E mags of Feb, Mar, Apl. 2009 by Peter Miller, so I fixed a wing chord datum line(spruce strip) through the wing seat long enough to go over the tailplane. At the t/p leading edge the distance between them was 21/4", and at the t/e it was 17/8", a difference of 3/8". I can only conclude that in effect the tailplane needs lifting at the l/e .by this amount. Is my thinking correct?. Of course I may be tempted to leave things as they are and live with the visual aspect, on the other hand the t/p might come for a little surgery. Thanks to everyone again in furthering my knowledge and understanding. What a mine of information the forum is. Geoff Quote Link to comment Share on other sites More sharing options...
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