elevator trim changes
 

Hi all,I have a precesion aerobatic mxs that was given to me.The plane has had some repairs to the motor box.when flying at a constant speed I can trim the plane for straight and level flight,however when I increase the speed the plane dives and when slowing it climbs.The cg is per specs.Do I need upthrust or down thrust to correct this? The battery is secured and the servos and linkages are solid.

If it is diving as power is increased it has down thrust. Be sure and add up a little at a time.

David

I agree with David, it must have down thrust. Precision aerobatic planes are almost always set up 0-0-0, so put on a stand, zero out the wing incidence, and with a prop on the engine aligned vertically, check its plane of rotation. Should be 90 degrees. I expect it won't be.

^^^^^^^ Yes and I like to use a paint stick and not a prop. It's easier to put level to the stick.

David

That was it I think.Tomorrow will tell.tha repairs to the motor box caused a little down thrust.
I put a couple of washers on the bottom motor mounts will see if that works.
Thanks for the help.
Happy flying.

Agree, the first step is to know where everything is. Then you can either set to where they should be or eliminate incidence or thrust angle as the issue. This could also be CG related but measure it out is step 1.

Another thought came to mind. If the repaired motor box is not solid the "firewall" could be flexing under thrust. Once saw a DA-50 flex a firewall 1/8" during static testing. It's would not flex that much in the air but can still cause trim nightmares.

David

Incorrect wing/stab incidence can also cause speed sensitivity in pitch trim. A good test for this is to trim the plane out for level flight at full throttle. Then as the plane passes in front of you, chop the throttle to an idle. If the plane immediately pitches up or down, it's the thrust line. If the trim doesn't change until the plane slows down, then it's likely incidence.

If the plane pitches nose up when you chop the throttle, there is too much down thrust.

Since there are motor box repairs, that is likely the culprit but this is a good way to test your changes.

Dave

Thanks for all the help guys.I flew the plane today and it flys great.Amazing what alittle up thrust
can do.Im finally enjoying flying the plane.
once agin thanks
Happy flying,
John

It might be a power thrustline angle issue or it may be that the CG is not as perfect as you think and there is some elevator trim in place which is causing the issue. Or it may be a mixture of both factors.

To check the elevator trim and set it to zero stability take the model aloft and at low throttle go into a vertical dive then let go of the sticks. See if it pulls to the canopy or to the belly. Correct the elevator as required until you can perform this vertical line at low or no throttle and the model continues the dive without pulling either way. Don't forget to pull out before hitting the big "green and blue thing". :D

With the elevator set for zero pitching NOW fine tune the CG so it flies level at around 1/3 to 1/2 power. Depending on the engine or motor the Tx throttle stick might be anywhere. But you want to set it for around 1/3 to 1/2 of full power for this test.

With it flying more or less hands off with the neutralized elevator trim and adjusted CG position to work with that elevator trim NOW chop or increase power and take note of whatever it does. Use thrust line adjustments to neutralize the effects. When done you may need to tweak the CG position.

The thing is that you need to start with the PRIMARY item that generates the speed related pitch. And that is the elevator. Then you want to build on that by adjusting the CG to suit the resulting elevator trim. And only finally the thrust line issues. The reason is that if you don't start at the elevator and build from there you can end up with engine thrust line offsets that compensate for an improperly set CG and resulting elevator trim situation. And if the goal is dead nutz neutral stability so the model tracks vertical lines at any speed and power setting then you want to start out with the thrust line and CG issues separated from the elevator. Thus why you test the elevator in a vertical dive with idle or no power. Then set the CG to suit the pitch trim and only finally stack the thrust line neutralizing work on top of all that. Why this order? Power is "optional" for flying. Pitching isn't. So you start with the pitching and work up from there. That way you know one factor isn't masking some other issue.

I'm not too sure on the idea of working on speed sensitivity and CG before thrust line in this situation. Dbaque did a good explanation of how to tell the difference between a thrust line problem and an incidence or CG problem. It's pretty easy to tell once you've done it a time or two, so I don't see the point of working on CG or incidence if it's obvious there is a problem with the thrust line.

To add a bit to the discussion, getting the thrust line set right is really all about making the engine pull in exactly the direction that centers the plane's drag at its CG. So high wing planes need some down thrust and low wing planes usually need some up thrust depending on how much dihedral they have. There is more to it than that, but the basic goal is to set up the thrust line so that just the addition of power does not create any pitch or yaw effect. Speed changes often cause pitch effects with a properly set thrust line, but that's a completely different aerodynamic problem to solve. When setting up an aerobatic plane, the pilot often has to make some compromises. If you tune the thrust line for best flight at cruising speed, it may be off for vertical maneuvers and in a nose up stall with some power on. So your loops may track perfectly upright and inverted, but your hammerheads want to fall off to one side consistently. Many many thousands of hours have been spent by designers figuring out how to reduce or eliminate those kinds of conundrums, but they will always exist somewhat.