[Jamie]

As many of you know, my rudder was crushed in an unfortunate hangar incident. A friend of mine allowed me to borrow his rudder (his RV is still the basement) and I flew it for a while. I ended up building a new rudder and flew the new rudder for the first time today. This means that I have now flown three rudders on my airplane.

The first rudder was pretty darn near perfect -- inclinometer ball was exactly centered during flight and no rudder correction was needed. The second (borrowed rudder) needed about 1/4 ball left rudder to get the ball perfectly centered.

That brings me to the new rudder. It's 1/2 ball out to the right!

So basically I've flown three rudders and each one has done something a little different with the inclinometer.

I inspected my rudder vs. the borrowed rudder and there is virtually no visual difference at all. I was extra careful with the new rudder to build it with the trailing edge as absolutely straight as possible. It's certainly straighter in that respect than the other rudders.

So if your newly minted RV flies a little out of center, it doesn't necessarily mean that the v-stab offset is wrong or that the fuselage is twisted. There's a lot of variability thrown into the equation with the construction of the rudder.

Of course, now the question for me is how to determine what the real culprit is. Was the first rudder really the straight one or is it the newest one? Hmm....

Not that it really matters in the long run. I'll just throw a balsa wood wedge on the rudder.

 

[Rick S.]

Sorry, shoot me now, but this sounds like Goldilocks and the three rudders...cept the first one was jussssstttttttt right. Consider not the straightness of the TE, consider the spar in relationship to the skins. Since the rudder horns are mounted direct to the spar, the spar may not be symetrically aligned with the skins or I mean the angle of the skins measured from the spar may differ from left to right. It's late, maybe I should just go to bed.

 

[plehrke]

Of course, now the question for me is how to determine what the real culprit is. Was the first rudder really the straight one or is it the newest one? Hmm....

The answer is that the combination of all the variability in the fuselage, fin, and rudder was aerodyamaically straight with your first rudder. The rudder itself may have been slightly crooked. If you think the new rudder is straighter then what your plane needs is the origninal crooked rudder. You can make the new one crooked by added the rudder tab you see on a lot of the RVs.

 

[Geico266]

It is amazing what a little change makes to the flight characteristics. I flew my 9A for 6 months with no paint. After painting is was a different airplane. Added weight for sure, but it just flew different.

 

[Kevin Horton]

My first guess would be small variations in the skin profile at the trailing edge. Two things come to mind:

1. Differences in trailing edge radius - Perhaps, because the prop wash puts the vertical tail and rudder in asymmetric flow, the rudder trailing edge radius will affect the rudder like it does the ailerons. I think the prop wash would put higher pressure on the left side of the vertical tail than on the right. So, if the trailing edge radius is increased, that would cause the air on the high pressure side to follow the radius a bit as it leaves the rudder, which would deflect the air a bit to the right, which would push the rudder trailing edge to the left, which would put the ball out to the right.

If you want to risk an experiment, try a very minor squeeze on your rudder trailing edge, and report back. I predict this will move the ball towards the left (assuming you have an engine that rotates CW, as viewed from the cockpit).

2. Side to side differences in trailing edge radius - Put a ruler against the skin, and see how the trailing edge radius compares on both sides of each rudder. Specifically, check the distance from the trailing edge to the most aft point where the skin touches the ruler. If this distance varies from one side of the rudder to the other, that could lead to a bit of deflection of the air as it passes by the trailing edge.

 

[hecilopter]

Is this the original -7 rudder

with the radius on the trailing edge, or the new -9 rudder that tapers to a thin edge?

One thing my buddy with a -7A and the newer -9 rudder noticed is that the older -7 rudder on my airplane is more nimble on the rudder pedals. The -9 rudder on his airplane is very stiff to push on the rudder pedals in flight.