Roberto Alonso Posted December 19, 2009 Share Posted December 19, 2009 Hello all, I have maidened my plan-built MIG 25 today with not-so-great results. I had CG as indicated on the plan, and as with every all-moving-tail model I have, I had both 0/0 decalage and "+2 degrees up" marked on the fuse. We launched with the stab on the +2 mark, and even then I had to reeeeally pull on the stick, the model felt very nose-heavy during the whole flight, so much that I couldn't really release the stick to operate the trim. Aileron response was ok, speed was good, but it felt very nose-heavy. I would be exceedingly grateful if any builders of this model would care to share their CG point and resulting decalage with the plane already trimmed. I am a bit confused right now. Can't remember exact weight now, but I'm within spec, maybe in the upper third of the range. Any ideas? Thanks! Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted December 19, 2009 Share Posted December 19, 2009 Hi Roberto and welcome aboard.Not got this exact model, but can say that several models I have maidened recently exhibit the same properties....feeling like they are nose heavy. I put it down to the suggested COG being "too safe" and end up gradually moving it back.Usually livens up an otherwise "soggy" feeling model. Quote Link to comment Share on other sites More sharing options...
Roberto Alonso Posted December 22, 2009 Author Share Posted December 22, 2009 Hi Timbo, thanks for your reply. I am thoroughly familiar with the need to start nose heavy and scoot back to taste, and conservative CG locations on plans, but what I'm seeing here is a different animal - a CG so far forward that the model had a difficult time staying airborne without massive "up" input, or at least these were the apparent symptoms. Somehow I don't think this is Tony's choice for flight behaviour, something must be amiss. Maybe someone who's built and flown one of these will take a minute to measure their actual CG. Decalage info would be brilliant, too. Thank you so much. I am aware I am starting life in this forum with a request for someone to trouble himself for my benefit, but I am confident I will be able to give back the favour some time. Cheers! Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted December 22, 2009 Share Posted December 22, 2009 Hi Roberto - I understand what you are saying, and I am only sorry that I was unable to give you the specific info you need - Tony does visit the forum sometimes, but I will send him a PM to bring this matter to his attention, as he is most likely able to help. Cheers . Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted December 22, 2009 Share Posted December 22, 2009 Hi Roberto, I am sure Timbo is correct and Tony will be able to help, but in the meantime... I have no experience of this model, but something you wrote made my ears pick up! When you say you set the stab at "+2 degrees up" I'm a bit confused. Do you mean you set it at +2 degrees incidence? If so that, combined with a very forward CoG could explain the plane's behaviour. You will have effectively built-in a load of down-elevator on a long (because of the forward Cog) tail moment surely? "+2 degrees up" (if incidence) is of course actually 2degrees of down elevator! It might be worth retrying with the stabiliser set a 0degrees before moving the CoG. Sorry if I've misunderstood what you mean by "+2 up" But if not it might be worth a go. BEB Quote Link to comment Share on other sites More sharing options...
Roberto Alonso Posted December 22, 2009 Author Share Posted December 22, 2009 BEB, thanks for your reply. By "+2 degrees up" I meant that, if we take the wing as datum line (zero), the stab is turned 2 degrees in the direction required to command "up", that is, lower leading edge, higher trailing edge. This is a working ballpark for most models, I find. Sorry if I didn't make myself clear - I am based in Spain and English is not my mother tongue. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted December 22, 2009 Share Posted December 22, 2009 Roberto, please don't apologise - your English is excellent - better than many native speakers I know! The phrase you used is absolutely correct - I just thought I check in case you had set it the other way! Seems the answer doesn't lie in that direction! BEB Quote Link to comment Share on other sites More sharing options...
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