down thrust

[hillmanr2]

In an effort to understand better why things work I am posting this one.

Please correct me if I am wrong!
Down thrust is necessary in order to counteract incidence. If a plane is set to 0 degrees incidence on the wing and horizontal stab some up elevator is required in order to give the wing a positive angle of attack. Down thrust counters this so the plane flies level.

[3dubya]

That will just load the airframe having downtrust fighting lift..

[hillmanr2]

So I am incorrect. I had meant the down thrust must closely be opposite to the wings angel of attack.

[3ddd]

the wing generates lift by speed,the down thrust just counter acts the lift when flown faster than scale

[fredo]

Funny, I seem to be dealing with the same issue at the moment. My wing incidence is zero, thrust line is zero (I assume), CG is somewhere where it should be (if not a bit tail heavy) but the plane won't fly level (it's nosing down) unless I dial some up elevator. Not too sure how much is too much but I noticed that in rolls it barrells a bit. I know that also could be a differential problem but I already have some dialed up. I'm thinking it might be because the elevator already have some up trim so it can fly level. Now, is it possible that this could be simply fixed by putting the upthrust to the engine? That way I could perhaps neutralize the elevator trim which could also improve my rolls. What do you think guys? Could it still be the wing incidence?

[3dubya]

	Quote: Originally Posted by hillmanr2
	So I am incorrect. I had meant the down thrust must closely be opposite to the wings angel of attack.

Just drop the model strait down idling and trim wing of stab incidence so it goes strait down, after that go strait up and put motor in any direction it needs to go when on full throttle for a perfect up line. Than on the flats move Ur CG forward or back to control how heavy you want the back stick to feel, or forward stick for equal feeling inverted. You can move CG back far enough so there is none, but on 45 degree up lines the plane will balloon when the cg pendulums back. This is all called 0 trimming, effectively turning the plane into a neutralized state, where you control the lift when you want it which way you want it with elevator. Its so basic you need never pull out any rulers or incidence meters, flight is Ur calibration.

[Rolling Thunder]

I set everything a 0 0 0. I trim the plane to fly level. I trim it to the correct cg in flight. I trim it to fly level. I change the thrust to give me straight uplines.

Any thing more then this will just confuse me.

[hillmanr2]

	Quote: Originally Posted by 3dubya
	Just drop the model strait down idling and trim wing of stab incidence so it goes strait down, after that go strait up and put motor in any direction it needs to go when on full throttle for a perfect up line. Than on the flats move Ur CG forward or back to control how heavy you want the back stick to feel, or forward stick for equal feeling inverted. You can move CG back far enough so there is none, but on 45 degree up lines the plane will balloon when the cg pendulums back. This is all called 0 trimming, effectively turning the plane into a neutralized state, where you control the lift when you want it which way you want it with elevator. Its so basic you need never pull out any rulers or incidence meters, flight is Ur calibration.

Thanks 3dubya, I have been able to trim my plane I am just trying to figure out the explanation behind it. I asked about differential also, I can clearly see the difference but I didn't comprehend the theory. So I am trying to find the aerodynamic reasons why I do the things I do. I hope this makes sense.

[dick hanson]

Depending on the model - some downthrust is desireable -but not much
a plane with a very high thrustline and a low wing (CAP 232) can use a little downthrust - but not much -
screwing around with thrust lines is of far more value on underpowered planes - -like Piper Cub for example
Most perceived needs for changing thrust lines - are rooted in being underpowered and trying to correct for constantly changing speeds which result in the plane climbing or diving.
Full scale practice and evaluation from old aircraft is loaded with "facts' about the need for screwing with relative thrust angles
for a good aerobatic model - a setup which is pretty much 0-0-0- is a whole lot better .
get your CG right for the model -and this is NOT the same for all designs -and learn to deal with the little nuances -or wear yourself out screwing around with mixed in throttle /elevator trims etc.. A big waste of time in my book.

[westproof]

sir i have a edge 540 with dl 50 engine this is my first time to fly a big airplane i have problem when i fly if in full throttle the airplane goes up and when i goes down its like a zizaw when i full down a 1/4 throttle is going straight no problem at all any one can help me?
thanks
alvin from philippines

[Tired Old Man]

Before any other trimming is attempted the c/g has to be established. If everything is set 0-0-0 the c/g location will determine straight and level flight without the need for elevator trin in either direction. If you have elevator trim it's only good at one particular speed and in one particular attitude. Roll the plane over and it's now trimmed opposite, making your work that much harder. That c/g is also a determining factor in pitch coupling. Using elevator trim for level flight? Guess what it becomes when you fly knife edge? That trim just became a turning factor, contributing more to the coupling.

After the correct c/g has been established, as determined by straight and level flight with zero elevator trim, it's time to look at engine thrust angles. Does it zoom up when the throttle is cut? The remove some down thrust. Does it dip when the throttle is cut? Then remove some up thrust.

Vertical lines are where you will play with any side thrust items, and usually less of them than you think. Firewalls that come box stock with 2 or 3 degrees of right thrust are usually carrying way to much side thrust, making downlines a beatch. Often you'll have to input a mix for the up line to offset the excessive amount of right thrust in the firewall. You can bet the down line will require a lot more mixing because of the upline mix.... It gets even more fun when you add a small sub or ventral fn under the fuselage. Changes everything