RV-6A tail to RV-7?

[N184DA]

A local RV6A owner asked me to help him swap out the Vertical Stabilizer & Rudder on his 6A, with new RV7A components.

I have 500+ hours on my RV4 which I built, and I am in the process of completing an RV9A. Unfortunately, I have no real experience with the handling qualities of the RV6/RV7.

I believe the owners intent is to improve stability, and attach the new vertical with a built-in offset so he can remove his somewhat large rudder trim tab.

Questions:
- What are the advantages/disadvantages to this modification?
- If he gives me the green light, are there any caveats that I should know prior to ordering replacement parts?

Thanks in advance,

 

[GrayHawk]

I won't attempt to describe the handling changes one might expect. Will leave that to Mel and others who have spun the RV-6. But here is my understanding of some of what your friend is asking for.

The late RV-6 kits (~ year 2000) were delivered with the larger VS and counterbalanced rudder of the RV-8. This combination was tested in the RV-7 but was marginal so the RV-7 used the RV-9 components.

I have been working to improve my older bought-flying RV-6A. One area of improvement has been to go through all of the control surface alignments towards the goal of most efficient flight. A part of this led to discovery of misalignment of the whole empennage relative to the fuselage. The VS on the RV-6 was spec'd at zero offset. Mine may have been zero relative to the HS, but the whole empennage was skewed to the right giving the VS a built in right offset. Needless to say there was a large rudder trim tab. I built new attachment parts to correct the HS alignment and then went a step farther, and added approx. 1/4" left offset to the VS, removed the rudder trim tab, and the plane flies like a new plane, straight, faster, and more efficient.

However, this is not the end. I have the thinner skin elevators and the lower surface of the right elevator has a crack that was stop drilled but now seems to be expanding. Along with this the trailing edge of the right elevator is about 3/4" higher than the left (control horn drilling). So I have been toying with the idea of using a late RV-6 empennage (RV-8 counterbalanced) I picked up as a total replacement to fix these problems.

Later Edit: I also have the option of either repairing or replacing the elevators only, thus keeping the original RV-6 empennage as several have suggested.

 

[gvgoff99]

SB on VAN's site

Derrick, You may want to read:
http://www.vansaircraft.com/pdf/sb02-6-1.pdf
if you have not done so already. It sounds like a lot of work for minimal gain from what is written in the SB. Others have put the 7 empennage on their 6's during the build stage. It may be worth doing a search to find them and then emailing them or call (if they are on the white page list http://www.vansairforce.net/rvwp.htm) to find out what their experience revealed.
I have an early 6-A and "small" rudder. When it starts to Dutch roll (infrequently and only in turbulent air) I find that firmly planting my feet on the rudder pedals dampens the effect significantly.
That being said they are experimental and if he wants a larger rudder ................go for it.

 

[Mel]

Unless you intend to do a lot of spins, you won't notice much improvement with the larger tail. Yes the larger tail gives you more authority, but it also weather vanes worse. So that pretty much cancels out.
Unless you have a big motor and/or constant speed prop, the -6s tend to have a slightly aft C/G. The bigger tail makes this worse.
Bottom line; I've been flying with the small tail for 15 years and see no reason to change. IMHO, the smaller tail looks better too.

 

[Norman CYYJ]

I agree with Mel. You don't want to change to the larger tail on the 6. It works very well they way Van designed it. As Mel said the CG will be farther aft which is a bad thing and also the nice ground handling will disappear.

 

[N184DA]

I agree with Mel. You don't want to change to the larger tail on the 6. It works very well they way Van designed it. As Mel said the CG will be farther aft which is a bad thing and also the nice ground handling will disappear.

Well, as I mentioned,, the decision is not mine to make as it's not my airplane.
I am just the "grunt" here

But thanks to all who responded, I will package all of these responses and present my "case" to the owners and see where they wish to take this.

 

[JonJay]

in regards to skin cracking

You might want to check to insure the trailing edges are bent completely. I believe an improperly bent trailing edge can lead to cracking of the first rivet due to the induced stress. You can check this by laying a straight edge on the skin. The skin should be straight all the way back to within 1/2" or so of the trailing edge. If it is not, the trailing edge was not "squished" enough. If done correctly, it is very difficult to keep the underlying stiffener from broadcasting through the skin as it will make contact with the skin prior to completing the bend. In my opinion, it is ok to see a little evidence of that stiffner as you know you have the bend done well.

90 hours, no cracks, but still waiting to see and crossing my fingers. Regardless, no reason to switch to a new tail just to go to .020 skins. Build new elevators if you must.

 

[GrayHawk]

< xxx snip xxx>
90 hours, no cracks, but still waiting to see and crossing my fingers. Regardless, no reason to switch to a new tail just to go to .020 skins. Build new elevators if you must.

Agreed and a good suggestion as it looks best to stay with the original tail. I'll look into a new set of elevators which would solve both the thin skin and alignment problems.