_shadow_

Guys,

I have just scratched a 40 size stick, but I moved the stab higher to the base of the fin, Split the elevator to make way for the rudder. Maidened it yesterday, alls ok...but it just ballons up...I have to add down elevator to keep it flying straight at half throttle. What can I do to the design to make it fly straight without adding down trim. As I look at the ele / stab when I land, the elevator is drooping.

The wing is stick airfoil, symmetrical..saito 72 for power, 13x6 prop. 54" wing span.

Do I add negative incidence to the stab or wings?

Please help.

Regards
Marcus

Edwin

My Feedback: (2)

Put a few degrees of down trim in the engine mount first. See how that affects it. It can easily be done by putting a 2 or 3 washers between the firewall and engine mount on only the top screws. The stick is perfect for this type of tweeking with that open engine area.
Edwin

Doug D.

What Edwin suggested is a good place to start. If that doesn't help much go back and check your C/G and incidences for the wing and horzontial stab. I built one of those a few years back and if I remember they are zero and zero to the thrust line. The engine should be zero to that as well. After building a few models from plans and even a few kits it pays to have at least two incidence meters when lining up wing and tail to the trust line. Going to need another for one of my bigger projects.
The way your discribing the ballooning problem, sounds like it may be a little tail heavy. But start with engine thrust first. And if that doesn't fix it check everything else. Good luck and have fun!!! [&:]

_shadow_

Thanks for the reply guys...I'll try it out.

regards
Marcus

BMatthews

You may have your balance point too far ahead as well as a thrust problem. Do some searches here at RCU or on a general search engine such as google.com for "sailplane dive test". The instructions there will detail how to set your model for hands off flying and then do a dive and release the sticks and then watch what the model does. If it noses up quickly the balance is too far FORWARD. If it stays straight or tries to tuck under the balance is too far back. In either case move the balance a bit, retrim the elevator for hands off level flying and do another dive test. Keep going until the model is just shy of dead neutral by showing a loooooonnnnnggg nose up recovery. To avoid engine thrust line issues with this testing do all your flying at idle or just barely off idle. Using even 1/4 throttle will confuse the issue greatly.

If at that point your elevator is trimmed down either live with it or cut into the model to adjust the stab or wing to compensate. Having the elevator trimmed down is only an aesthetic issue. There's no bad aerodynamic properties to it. And next time don't build in so much decalage angle ( the angle between the wing and stab).

Hey, it's a bunch of trouble but at least you're doing your own designs. That's always a GOOD thing.

BMatthews

I know it sounds a bit odd but if the model climbs with the application of power and the thrust line isn't pointed up it's actually showing that it's NOSE heavy. With a nose heavy model you end up with more up trim (or less down trim in this case) to hold up the nose. Adding power, and thus airspeed, causes more negative lift at the tail and up she comes. Trimming the model for a more rearward CG and being able to add down trim brings the model closer to neutral and you don't get as great a speed related nose up action.

But the model becomes less pitch stable, more neutral in fact, and for some folks it'll be harder to fly. This is why trainers often have lots of downthrust because they have so much built in "up trim" so they are speed stable so the model will try to come back to level when it dives. But the downthrust helps prevent it from nosing up when the power is added.

A guy flying full sized stuff wrote up an article for an aviation magazine about a test where he C clamped the control column of a Piper Cub or similar small plane. He trimmed the plane to fly level and then added full power. He was expecting it to climb but he also expected it to speed up. Surprisingly he actually found himself and the plane climbing at close to it's best vertical climb rate at a speed JUST SHY OF THE STALL! ! !

We actually fly our models all wrong in many respects. Commercial pilots in my club and other places will tell you that in large commercial aircraft the throttle is for climbing and diving and the elevator is to speed up or slow down. Backwards? I guess it depends on how the trim is set up.