Vertical Stab offset

[AJ85WA]

Hi guys

I'm in the process of mounting the vertical stabilizer on my Rv6. I built a rv7 tail for my six and was reading the plans which states to mount the VS at a slight offset with a washer at the back.

My RV6 plans say not to worry about it and just mount a trim tab later.

I just wanted to check seeing that I could go either way at this stage.

(need to order a new attach plate if I put it offset but no biggie)

AJ

[L.Adamson]

Originally Posted by AJ85WA

Hi guys

I'm in the process of mounting the vertical stabilizer on my Rv6. I built a rv7 tail for my six and was reading the plans which states to mount the VS at a slight offset with a washer at the back.

My RV6 plans say not to worry about it and just mount a trim tab later.

I just wanted to check seeing that I could go either way at this stage.

(need to order a new attach plate if I put it offset but no biggie)

You can do it either way. My short tailed 6 required a tab. Used a taped on piece of wood wedge for a while. Personally, I don't mind the look of tabs at all.

[GAHco]

I fit mine with a 3/16"-1/4" offset but its not flying yet.
I will report back, but I don't know when

[Mel]

Either way works fine. Remember if you offset the vertical, you will need to rework the fairing.
On the other hand, the fairing will probably need rework anyway.
Just something to think about.

[AltonD]

1/4" on an RV7A, rudder is feet-off, no trim required. But then again, wheel pants will throw off the yaw trim.

[rvmills]

Here's a curve ball question with respect to offset...

Last summer I built and installed an 8 VS and Rudder for my 6 (to add the counterbalance for some added safety margin for racing). When I installed the VS, I placed it with the leading edge in the same spot as the previous VS (minimal offset, if any).

Prior to the mod, coordinated flight required right rudder in climb and at slower speeds, no rudder in normal cruise, and a tad of left rudder at race speeds or relatively high speed descents...what I would consider pretty normal (or the porridge being just about right).

After the mod, I find I need more right rudder than before...in fact, it needs a bit of right rudder even at high speeds now. A bit surprising, given the taller tail and higher aspect ratio rudder (versus the 6 tail). Then again...what do I know?

I figure a trim tab would alleviate this, but I'm considering adding a touch of offset (and suffering the slings and arrows of fairing re-work). That is if its worth the effort to keep drag low (and salvage speed).

Double-checking here, but the direction of offset required is leading edge left (opposite the required rudder input)...correct?

Also, has it ever been shown that offset is less drag than a tab? Just wonderin'!

Currently being baffled by baffles (making an RV-10 baffle kit fit an S-6...good fun!), and will be researching the tail data after that job is done...but thought I'd take the shot and ask the question when I saw this thread! Thanks for any input!

Cheers,
Bob

[Bob Axsom]

Our RV-6A is a short tail version with no offset. Without a tab of some sort it is a right rudder all the time airplane similar to Bob Mills' description of his super 6 after the RV-8 vertical stabilizer and rudder installation. My original solution was the after market plastic wedge. That worked fine but I didn't like the looks of it because of a flaw I induced by filleting the edges with microbaloon and epoxy. It looked great initially but after painting the epoxy seperated from the thin rudder skin around the entire perimeter. I fixed it with a bead of clear silicone sealing the resulting crack in the paint. As I modified the airplane for speed I observed that the wedge was no longer "perfect" for cruise and I had to start having a little right rudder pressure. I often thought about offsetting the leading edge of the vertical stabilizer to the left to build in a little right rudder. Because of some minor damage to the rudder in a ramp incident I had the opportunity to reskin the lower portion of the rudder and add a removable metal tab. Since that mod the ball is once again centered in cruise and it is ground adjustable if needed in the future to compensate for additional speed mods. With that tail configuration with my mod to cover all the access holes in the rudder with small metal plates I suspect the offsetting the leading edge of the vertical stabilizer left to induce a right yaw force might be a little less "draggy" than flying through the air with left tab to force the rudder right to force the tail of the airplane left and produce a feet off the pedals centered ball. It still crosses my mind from time to time like when I see you posting about it. Since I am a little in the "don't fix it if it ain't broke" camp (AKA lazy) I haven't even come close to starting that mod.

You may have read the thread with a similar theme concerning the horizontal stabilizer incidence change - raising the the leading edge to eliminate the cruise flight down elevator trim and its perceived drag effect on the RV-4's cruise speed. The trusted VAF aeronautical observation in that case stated that there would be essentially no effect on speed except some slight drag on the leading edge of the counterbalance horns that could be minimized by rounding them (which also minimizes the dynamic protion of the counterbalance). In a conflict of theory and experimental observation, our friend and current SARL RV Red speed record holder told me a few years ago when I told him I was considering that on our RV-6A, "that was the most effective speed mod I made on my RV-4."

My thought is the reduction in the need for offsetting the rudder by building in an offset of the vertical stabilizer to the left is more efficient in getting the required yaw force than flying with the rudder offset to the right and the drag in the case of a aerodynamic counterbalanced rudder off set for yaw is higher than a nonaerodynamic counterbalanced rudder off set to achieve the same yaw force. Maybe the fellow in texas that just bought a wind tunnel could run a test on that although at the scale he would have to run the test you might not see the difference.

Bob Axsom

[bpattonsoa]

I finished my -6A in 1999 and was mounting the fin in about 96. The user group at that time had a big discussion on offsetting the fin. I did a lot of thought about the issue and decided to do an offset. I took the number from the four or five persons who seemed to be the most knowledgeable (almost all of them had a different opinion) and averaged them. Unfortunately I didn't record the number, but is was something like 3/8 - 1/2".

Since Vans fiberglass parts delivered at that time were not of the best quality (understatement), I already had to rebuild the fairing.

What is amazing is that the ship flies straight with out a trim tab and the ball is dead centered at all airspeeds! However .....

After flying a couple of years, I started looking for more speed. I put a 1/8" shim under the front spar mount on the stab since I was flying with a fair amount of nose down trim at cruise speed with 2 persons and fuel . That gave me about 2-3 knots, an almost neutral trim and no noticeable change in flight characteristic.

I put a piece of tape on the rudder cable next to the pilots seat and measured the position of the rudder in cruise. I was flying with about 7 degrees of rudder. Essentially the rudder was deflecting due to the fin offset and the net result was a no yaw, but a bunch of drag.

I purchased a new fairing in preparation of removing the fin offset. Unfortunately, the story ends there because I got involved as the Crew Chief for Super Sports Air Racer, Bad Intentions. After that was over, we moved and built a new house at Independence Airpark (not completed yet, next month) and purchased a RV-10 kit.

So I can't say what the result would be of going from an offset fin to straight (yet)

[Bob Axsom]

I assume the vertical stabilizer (fin) is offset 3/8"-1/2" to the left (please confirm). Which way was the 7 degree rudder offset? I really want to get my head around your initial test results. When you are done we may have the only real comparable data. With your reference tape in place did you return the rudder to the straight alignment with the centerline of the vertical stabilizer? and if so what were the results? Do you have comparable speeds under those two conditions? From how I read your description I understand that the rudder 7 degrees out of alignment with the vertical stabilizer is the natural cruise flight position with no input from you - is that correct? If that is not correct can you give a little more description just to get me on the right track. THIS IS GOOD STUFF!!! One more question, is this a short tail 6 with no counterbalance (like mine - I assume it is because of the build date)?

2-3 kts sigh...

Bob Axsom

[C-FAH Q]

I installed my 7 vertical stab with no offset, I was looking for more speed and thought this may help. What the offset does, is lets the rudder trail straight behind the stab,if you have no offset, you need to counter act that with more rudder deflection= more rudder to keep ball centered during flight which in turn equals large rudder tab. I am planning to reset my stab with offset and remove the large rudder tab needed to correct this.