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How can you test yaw trim?

F1 Rocket

Well Known Member
Is there an in flight test for yaw trim? I know “center the ball” but how do I know the “ball” on my PFD is calibrated correctly?
 
Level

Is there an in flight test for yaw trim? I know “center the ball” but how do I know the “ball” on my PFD is calibrated correctly?

Put the airplane on a level surface, jacks or weight on wing tips is necessary. Pitch doesnt matter. THe ball should be in the center. If not, adjust it so it is. That's it.
Ed
 
Put an electronic level across the fuselage and let some air out of one of the main tires to level the airplane side to side. The ball should be in the middle. If it isn't shim your ADAHRS unit until it is.
 
Put an electronic level across the fuselage and let some air out of one of the main tires to level the airplane side to side. The ball should be in the middle. If it isn't shim your ADAHRS unit until it is.

That takes care of the roll axis alignment but what about the yaw axis?
 
The yaw axis is taken care of when you orient your ADAHRS so the arrow points forward. But this does raise an interesting question: As most already know, the conventional "ball" shows the zero slip condition only within the validity of an approximation: the torque exerted by the rudder matters, but the sideways force can be ignored. This is usually a pretty good approximation, except on twins with an engine out, where the large rudder forces required cause the approximation to fail - and twin pilots learn to fly on one engine with the ball slightly displaced, and one wing raised ('raise the dead'), to keep drag at a minimum. So the question: Does anyone know if modern EFIS units are able to calculate the true zero slip condition, or do they just electronically mimic the conventional ball?
 
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That takes care of the roll axis alignment but what about the yaw axis?

It's not only roll. I'll try to explain.
In static conditions the ball will go out to whichever side is lower. Think of a real slip ball, or a regular bubble level. Yaw or (uncoordinated flight) will cause a sideways acceleration which will push the ball out to one side. You can only replicate that on the ground by tilting to one side.
 
Thanks for the replies but, is there an inflight test for yaw trim that does not rely on a slip/skid ball?
 
Yaw String

The T33 had a string mounted on the nose of the aircraft that the front pilot could see while flying.

It was used to correct yaw during flight by adjusting rudder trim.

Carr
 
The T33 had a string mounted on the nose of the aircraft that the front pilot could see while flying.

It was used to correct yaw during flight by adjusting rudder trim.

Carr

Yes, we used this method on sailplanes. But I think the propeller slipstream will affect the string.
 
So If i'm reading this correctly, are you thinking that the airplane is flying in a crab in unaccelerated wings level flight? Not with respect to the ground, but genuinely flying in a crab through the air mass with the ball centered?

If that's the case, Then about the only thing I can think of is to tuft someplace out of the prop wash and maybe take a peek with a go-pro.

In a practical sense, if the ball is erroneously centered, it also seems like it would always break toward the "upwind" wing in a stall.

Did I win?
 
So If i'm reading this correctly, are you thinking that the airplane is flying in a crab in unaccelerated wings level flight? Not with respect to the ground, but genuinely flying in a crab through the air mass with the ball centered?

If that's the case, Then about the only thing I can think of is to tuft someplace out of the prop wash and maybe take a peek with a go-pro.

In a practical sense, if the ball is erroneously centered, it also seems like it would always break toward the "upwind" wing in a stall.

Did I win?

Yes...not by much, I think it’s half a ball out. I’m looking for a method to determine if it really is half a ball out.
 
In principle flying uncoordinated adds drag, so it should show up as a loss of airspeed. Not sure if common ASI can detect the change from a half ball out. Probably best tested with a two axis autopilot engaged in heading and altitude hold, on a very smooth-air day. Use rudder to slowly sweep ball from left to right, look for max airspeed. Or, see if the first noticeable drop in speed happens symmetrically, ball-left vs ball-right.
 
If You’re Asking HowMuch of a Trim Tab is Needed..

Trial and error. You can mount a wedge made of balsa, plastic, or aluminum using double sided tape. progressively cut it down until the aircraft is in proper trim at cruise. Afterwards, you can mount something permanently. Of course, I’d first make sure the ball is centered when the plane is on the ground and level.
Terry, CFI
RV9A N323TP
 
Perhaps the good old Turn coordinator would be useful as it shows yaw and roll, if the airplane is not rolling it will indicate yaw.
But I think the AHRS does the same thing only using electronic gyros to show yaw with the 'Ball'.
If you install a yaw damp sys the AP will center the ball base on yaw forces.
But as my good friend Matt likes to say, I went to public school so what do I know...
 
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Slip Indicator

Yes...not by much, I think it’s half a ball out. I’m looking for a method to determine if it really is half a ball out.

I think you are over-thinking this. Why don’t you just temporarily put a real “slip/skid indicator” ball on your panel or glare shield to test this? Look for “Lev-O-Gauge” at a marine store or aircraft spruce.

A plane that is out of yaw trim will fly in 1 of 2 conditions: #1 is wings level, skidding turn. In this case, when the wings are level with the horizon, the plane will fly a skidding turn and will not hold a heading. Here, the slip/skid indicator (the ball) simply measures the lateral acceleration produced by the uncoordinated turn. #2 is steady heading, wing low, i.e. a forward slip. In this second case, the out of trim plane is flown with a wing slightly low to offset the yaw tendency. Here the slip/skid indicator simply behaves as a level to indicate that the gravity vector is no longer perpendicular to the aircraft’s lateral axis.

Skylor

P.S. Those of you that are multi-engine pilots know that the procedure to “trim” the plane with an engine out is to “center the ball” with the wings level using the rudder, then “raise the dead engine” with aileron until the ball is slightly off-center. This is done so that a bit of lift vector is used to offset the lateral force of the rudder which is being used to control the asymmetric thrust. Without “raising the dead engine” the plane will actually fly in a true crab. The plane will hold heading with the wings level and ball centered but I will drift sideways off-track. A similar condition could exist on a centerline thrust aircraft if there were a large drag difference between the two wings, but I don’t think this scenario is very likely unless something is grossly out of rig.
 
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