Danos Posted August 22, 2013 Share Posted August 22, 2013 When building scratch built planes how do define where the COG will be? Thanks any links that explain in full etcv cheers Quote Link to comment Share on other sites More sharing options...
cymaz Posted August 22, 2013 Share Posted August 22, 2013 Try This Quote Link to comment Share on other sites More sharing options...
GrahamC Posted August 22, 2013 Share Posted August 22, 2013 A couple of ways... 1] Build a very small model from balsa or depron and experiment with a glide... 2] CofG calculators - as suggested by Cymaz 3] Rule of thumb... 25% to 33% of MAC... but be careful with this 3] It looks about right [really] although be very careful with very swept wings etc because what looks right can be wrong! 4] And in my opinion, the best way.... Fly it. Get it about right before flight and then check it dynamically. A useful chart can be found in this post with a long accompanying article. Edited By GrahamC on 22/08/2013 12:04:34 Quote Link to comment Share on other sites More sharing options...
Chuck Plains Posted February 9, 2014 Share Posted February 9, 2014 Here's another super useful site. adamone.rchomepage.com I have it bookmarked. Quote Link to comment Share on other sites More sharing options...
Steve Dorling Posted December 16, 2014 Share Posted December 16, 2014 BALANCE POINT (not Cg) Just sayin' Quote Link to comment Share on other sites More sharing options...
Paul Jefferies Posted December 16, 2014 Share Posted December 16, 2014 If this is your first go at scratch building I imagine the model will be fairly conventional so as a rule of thumb, if the wing is more or less straight (ie., not swept back) and the wing section reasonably conventional, make the balance point at, or slightly forward of, the thickest point in the wing section and you won't be far wrong. You may need to adjust it on subsequent flights but it should at least fly without being uncontrollable. Paul Quote Link to comment Share on other sites More sharing options...
Matt Jones Posted December 16, 2014 Share Posted December 16, 2014 And remember, a model with a CG too far forward will fly badly, a model with a CG too far back will fly once....... Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted December 16, 2014 Share Posted December 16, 2014 Posted by Paul Jefferies on 16/12/2014 07:50:23: If this is your first go at scratch building I imagine the model will be fairly conventional so as a rule of thumb, if the wing is more or less straight (ie., not swept back) and the wing section reasonably conventional, make the balance point at, or slightly forward of, the thickest point in the wing section and you won't be far wrong. You may need to adjust it on subsequent flights but it should at least fly without being uncontrollable. Paul Yes, I'd agree. With a conventional model 30% back from the leading edge (which will more or less coincide with the thickest section point on most conventional aerofoils) will give you a nice conservative setting - if anything a little nose heavy - but that's not a bad thing for a maiden. BEB Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted December 16, 2014 Share Posted December 16, 2014 I can vouch for these, I've used them both and the resulting CG has been spot on. **LINK** Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted December 16, 2014 Share Posted December 16, 2014 Posted by Biggles' Elder Brother - Moderator on 16/12/2014 09:56:37: Posted by Paul Jefferies on 16/12/2014 07:50:23: If this is your first go at scratch building I imagine the model will be fairly conventional so as a rule of thumb, if the wing is more or less straight (ie., not swept back) and the wing section reasonably conventional, make the balance point at, or slightly forward of, the thickest point in the wing section and you won't be far wrong. You may need to adjust it on subsequent flights but it should at least fly without being uncontrollable. Paul Yes, I'd agree. With a conventional model 30% back from the leading edge (which will more or less coincide with the thickest section point on most conventional aerofoils) will give you a nice conservative setting - if anything a little nose heavy - but that's not a bad thing for a maiden. BEB what they said! Quote Link to comment Share on other sites More sharing options...
Chuck Plains Posted December 16, 2014 Share Posted December 16, 2014 Posted by Steve Dorling on 16/12/2014 07:04:18: BALANCE POINT (not Cg) Just sayin' Same thing pretty much Steve. Plus, if a builder was really fanatical, the center of gravity can be checked in several different axes. Quote Link to comment Share on other sites More sharing options...
Daithi O Buitigh Posted December 16, 2014 Share Posted December 16, 2014 As said - about 1/3 of the wing chord back from the leading edge is roughly correct, BUT, that's only an approximation. Once it's finished, test glide it to fine tune the CoG to where it's supposed to be. Too many people use the trims on the transmitter to get that, with the end result they cant really fine tune any more. If it's aerodynamically balanced in the longitudinal axis by manually adjusting weight forward or back (depending on whether the test glide gives a dive or stall) then the trim controls can be used for that final tweak Edited By Daithi O Buitigh on 16/12/2014 22:43:12 Quote Link to comment Share on other sites More sharing options...
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