60 forum posts
When building scratch built planes how do define where the COG will be? Thanks any links that explain in full etcv cheers
9175 forum posts
1240 forum posts
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
|Chuck Plains||09/02/2014 10:10:07|
1096 forum posts
Here's another super useful site. adamone.rchomepage.com I have it bookmarked.
|Steve Dorling||16/12/2014 07:04:18|
|41 forum posts|
|Paul Jefferies||16/12/2014 07:50:23|
|254 forum posts|
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.
|Matt Jones||16/12/2014 09:21:48|
|1186 forum posts|
And remember, a model with a CG too far forward will fly badly, a model with a CG too far back will fly once.......
|Biggles' Elder Brother - Moderator||16/12/2014 09:56:37|
15748 forum posts
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.
|2904 forum posts|
I can vouch for these, I've used them both and the resulting CG has been spot on.
|Bob Cotsford||16/12/2014 11:42:29|
8380 forum posts
what they said!
|Chuck Plains||16/12/2014 22:07:02|
1096 forum posts
Same thing pretty much Steve. Plus, if a builder was really fanatical, the center of gravity can be checked in several different axes.
|Daithi O Buitigh||16/12/2014 22:42:45|
1369 forum posts
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
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