Aerodynamics on oldtimer models ?

[dvint]

Flying a Jr. falcon with a Mega 16/15/8 with 7-6 prop. 23 0z. with
60 watts per lb. power. Climbs when it picks up speed even after
leading edge of stab raised 1/16. Any suggestions ?
Thanks

[Ollie]

Put the stab back where it belongs and keep adding down thrust until the problem goes away.

[BMatthews]

Sorry Ollie but this time I've got to run counter to your usually impecable advice. But since I'm in the opposite corner of the continent I guess that's all right... :D

The Falcon was originally a rudder only design. As such it has a LOT of built in decalage and a suggested balance point to go with that angular setup. It's designed to do just what you're finding dvint. But it was not designed with that powerful a motor or using elevators originally.

You can fix it by raising the stab a little more. Perhaps another 1/16 and then move the center of gravity back by 1/4 inch and perhaps as much as 3/8 inch from the position shown on the plans. If you've ever heard of using the dive test for setting the optimum balance for gliders then you could use that to set it even more critically. Try google with "glider dive test" and you'll come up with the instructions.

Ollie is right about one thing though. You're going to need some extra downthrust with that powerful a motor. I'm assuming here that you put in the stock amount to begin with?

[ChrisP]

When I got my first computer radio about 10 years ago, I totally gave up on winding clevises in and out and changing the incidence of surfaces and downthrust to trim models. I have become a totally 'fly by wire' modeller. Why have a dog and bark yourself ?
Of the 14 or so models I have in a ready to fly state, at least half of them have 10 - 15% down elevator mixed with throttle to avoid nosing up under power. And my Bloody Mary has throttle mixed with right rudder as well !
My Futaba F28 mix program is normally set up so that the down elevator is only mixed in from the half throttle point. This is very easy to do. I once had a pretty fast climbing electric sailplane which had a soft switch instead of a speed controller. Even on that I mixed throttle and elevator and was able to set the climb angle on the throttle stick without changing motor power or holding in elevator. Cooool !

OK it's not the perfect solution. If you bang open the throttle, the model noses down before returning to horizontal flight and vice versa. But I know that and therefore I don't do it. As I only fly sports models it's a very acceptable solution. Lots of my colleagues have flown my models and commented on how well they fly. I think this is one of the reasons.