Ultra flash out of trim at full throttle.
 

Both my ultra flash and classic flash are trimmed perfectly at mid throttle but when I go full throttle, both want to go up and seem way out of trim. I sure people have been through this before, so how do you counteract that or fix it. thanks

1. Have you completely removed the seam and "build up" around the tailplane seats.
2. Balance position? Any up trim in your mid throttle position. From mid throttle if you close the throttle does the nose drop?

Dave

I need to check my stabs - I think I have a small amount of down trim in there. Mine's fitted with a P180RX so I have a wide speed range and it's pretty consistent across it trim wise.

Maybe I'm confused, but isn't this what's supposed to happen? If you increase throttle to full, you will increase the airspeed, which will increase the lift created by the wing, which will cause the aircraft to climb. A problem would be if the aircraft descended upon the application of full throttle.

What do you mean by "way out of trim" is there also a rolling or yaw component that changes?

David S

+1 David, I agree, trim is a function of airspeed.

Hmmm, I just can't remember having an out of trim flash at full throttle....


http://i384.photobucket.com/albums/o...s/P1010386.jpg

My Ultra Flash with a Rabbit 100 does not change trim between half and full, or it is so slight I dont notice it, but it is normal to have some climbling when increasing the speed if the difference is significant enough.

Well I have a rhino in it so a lot of difference between 22lbs of thrust and 36. I just flew it today and it wanted to slightly go up, but it seems everyone's pretty much does that. When I go half power and then idle, it stays level.

It stays level until the airspeed drops low enough to allow it to descend. (Which could take a while. The flash is a very clean design, so it bleeds speed quite slowly)If your jet also has down thrust in the exhaust tube, that could allow the jet to remain "in trim" or in other words not climb as much at full throttle. The increased down thrust at full throttle would/could counteract the increased lift of the wing caused by the increased airspeed.

If nothing else changes with increased throttle, I think your trim is fine. I remember the original Lightning's needed at bit of up thrust to really fly neutral. Andreas used to simply bend the bottom end of the thrust tube up at the tail and the flight effect was noticeable.

David

If the wings and stabs are symmetrical and the incidence and thrust line and CG are correct I am betting the UF is designed to not change pitch with airspeed (or throttle) changes. I am betting everything is 0/0. Just for the sake of argument lets keep the aerodynamics this simple. Say the wing and stab airfoils are sym, can't change that. And the plane comes out of a mold or jig so lets assume the incidences are correct. I haven't built mine yet but the thrust tube looks to be stuck in one location in the fuse. That leaves CG. If you are very nose heavy and trim for that then it will climb at higher airspeeds...

But a symmetrical wing section can't create lift unless you give it an angle of attack. If you have an angle of attack then as your speed changes the lift will also change. I haven't spent a long time considering the varying proportions of tailplane and wing lift and how the c.g. might affect this but I have often wondered how we can get away with such minimal trim changes over such wide speed ranges.

In full size you spend a great amount of effort to try and keep the aircraft in trim but with models, once set up properly, they rarely seem to need the same amount of constant 'fiddling'.

Fun stuff! I only really understand how full scale or "lifting" airfoils work. And yeah the trim is working big time in a real plane. I have had several gliders and slope ships over the years and they are really affected by speed changes. But most of my hobby flying has been stunt planes, pattern planes, IMAC etc. they fly straight ahead once trimmed despite the fact that there most be some angle of attack to stay airborne. It must be very small at normal flying speeds.
Mayb Dave can tell us what type of airfoil the UF has, also I would like to know what is on my CARF Hawk. Not necessarily a number, just what type.

Flash and UF are fully symmetrical. The Hawk is a semi.
The numbers are different, we don't need the same angle to fly as big, heavy (slow ratio) birds. Its all numbers! Tail, moment and and and and...way more hocus pocus that my brain knows about.

Your pattern airplanes stay trimmed by a function of incidence on the wing and stab and downthrust in the engine. As has been mentioned a symmetrical wing requires incidence to generate lift, in effect increasing the airflow over the top surface and lowering the pressure. Most aircraft have at least a couple degrees of downthrust so that as speed increases and the wing wants to continue to lift, the downthrust of the engine keeps the line straight and true. It's a balancing act between incidence and thrust angles. When I flew pattern we'd spend almost the first full season making constant adjustments to the airplanes wing and stab incidence and shimming engines for thrust until they flew virtually perfectly. Today I find a lot of trim is "mixed out" in the transmitters instead of designing an aircraft that flies neutral.
With jets the thrust tube could be adjusted to help eliminate that issue but the speed range in most all jets is much larger than any pattern airplane is. GIven that very few jets are designed to fly perfectly instead of look pretty it's no real surprise that they don't fly perfectly true all the time.

CG is right on...

Nuts, back to square one. I sure hope my UF doesn't pitch up as it accelerates under high thrust settings...
Thanks Dave, I was pretty sure my Hawk has a semi symmetrical airfoil by the way it flies. It is the first model airplane I have had (my first jet) that would really pitch up and climb from a level 1/2 throttle pass when I pushed the throttle up. And it really got my attention when I did that inverted. So I did mix in down elevator from 1/2 to WFO and it works really well.
You could try that.

I had this problem with my classic Flash. I also just mixed a little down elevator with throttle to cure it.

That's what I was thinking, but just wanted to make sure.

CG it right when the tank is empty, any up trim in elev when you flying?not much sport jet and scale jet have a fuel tank right on the CG 1/16 of an in up trim will make the plane climbing up with very high speed , just mix some down trim elev with throttle stick and fly the craft out of it :cool:
see you and Scott at scobee
Tuan

Just one thing to watch with this mix. When you open the throttle to take off, you may need to use a little more up elevator to lift off, depending on how much mix you put in.

Ok, thanks

Will do..thanks.See you at Scobee

Best is to have a little down with throttle only when the gear is up.

David

Thanks for all the replies everyone. Will try the mix and see how it goes when the wx clears :-)

That is how i almost crashed mine, i had the mixer on at take-off, full throttle and minimal airspeed, just remember, KEEP PULLING!

regards, Bart.