Iv'e noticed that some of my planes look like they are "crabbing" through the air- they fly straight after they are trimmed, but seem to be skewed slightly. I was wondering if this is more a function of thrust angle, or rudder trim. The reason I ask is that on some engines, especially Evolution 2 stroke glow, the sides of the engine case are not "parallel" underneath the mounting tabs, so there is a possibility of mounting the engine slightly crooked.

Is there an easy way to tell if the engine thrust line is correct? The planes seem to fly just fine other than the issue i'm describing.

You could be slipping slightly. You can compensate for a little yaw with aileron or vice versa. The yaw could be due to rt thrust or a slight asymetrical control setup. Make sure its also not a cross wind causing it. Even if there is little or no wind on the ground there can be more wind and from a slightly different direction at altitude. Get with one of the more experienced flyers and see what they think. If you don't fly with a club then check to see if the crab is always the same direction in relation to the airplane and not the ground. If it is for sure the plane try removing crab with the rudder trim and keep it level with the aileron trim. With no wind or directly into the wind it should enter and exit a loop from straight and level on the same line. If the trim is still off it will do more of a large barrel roll.

You can tell if you have left / right or up / down thrust fairly easily. With a prop mounted (let's say a 12" prop) hold it with tips level. Measure the distance from each tip to a common point on the rear of the fuse. If the measurements are equal you have no left or right thrust - dead on the thrust line. With the tip vertical - do the same type of measurement. Equal - no up or down thrust - dead on the thrust line.

There is a rule of thumb that IF you use a 12" prop AND the tail of the fuse measurement point is 48" THEN 3/16" difference between the left / up and right / down measurements is equivalent to 1 degree off the thrust line. I THINK [:@] Hey masses - please correct the measurements if I'm wrong.

Welcome to the RCU forums, vidasj

You can verify the static test described above with a dynamic test:

Side Thrust trim:

Fly model away from you and pull up to vertical

A. Model continues straight up
B. Model veers left
C. Model veers right

A. No Adjustment
B. Increase Right thrust
C. Reduce Right thrust

(Copied from http://nsrca.us/index.php/all/flying/177-trimchart.html)

Well before you go chasing the thrustline issue, let's start with the simple stuff.

Does the yaw happen the same way every time i.e. always to the right? If not it is just normal crab due to cross winds.

Is the wing held on with rubber bands? If so is there a way to make sure the wing is in the same spot every time you put it on?

Is the rudder in line with the rest of the plane. Can you trim the yaw out with the rudder so that it doesn't happen at all? There is a chance that the plane is not "square' and will need a little rudder trim.

Tom
AMA 8026, instructor since 1995

That's exactly why my Skylark 70 flys so well. I spent a lot of time with the minor trim and mechanical adjustments to make things straight. Now, I take off, do a minor trim at a throttle setting and it will pretty much fly itself.. well, if you did all of what I did, you know exactly what I mean.

CGr.

Thanks for all the great info guys!  I will have to experiment when I can go fly again ( we just got dumped on with 12" of snow and now the temps are in the single digits here[].  This particular plane is not one that I use skis with.

The plane in question does not use rubber bands to hold on the wing, it has a slot in the fuse where a tab on the front of the wing goes, and two nylon bolts hold the rear of the wing to the bottom of the fuse.

I was particularly careful to make sure I built the tail assy. square and symmetrical to the wing, the only concern I had with this ARF build was that I had to grind little notches in the rear of the motor mount to get the rear of the engine to fit in the non-adjustable motor mount provided.  I can see that the firewall has some right thrust built in, but am concerned that maybe I mounted the engine cocked even more to the right, as the engine sides under the mounting tabs are not parallel, the rear of the crankcase is wider than the front.

I'll try to get with a more experienced pilot next time I can and ask their opinion on the yaw issue.