What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

RV-3 Tail Tale

Ironflight

VAF Moderator / Line Boy
Mentor
Well once again, we have a good story of how much ?fun? it is to build an RV-3. As much as an art as it is a science, our latest ?tale? involves a mystery of a mismatch between the vertical and horizontal stabilizers?..

We have been attaching the Vertical Stab this weekend, and ran into this interesting problem ? once we had bolted the rear spar to the back of the fuselage, the forward spar is about 3/8? back from the tab that sticks up from the Horizontal Stabilizer forward spar to join it. We traded some notes with the engineering guys at Vans and they are OK with a thick spacer and four AN3 bolts, so the problem is solved ? but the mystery remains. {BTW ? when I call Vans guys on the RV-3, I generally am not looking for a ?factory position? on anything ? the truth is, no one there but Van has built one, and that was a LONG time ago. Rather, I call them for an independent sounding board for engineers familiar with the general design. It relieves them from feeling that I am depending on their answers to be ?official??}

The Problem:
IMG_4123.JPG


Here?s the thing ? we didn?t build our tail feathers, so we have been backtracking and measuring through the drawings to see if there is an error. The placement of the horizontal stabilizer on the fuselage is dependent on where the vertical attachment bars (for the rear spar) are located ? they are located on the F-310 bulkhead. That bulkhead is in turn set relative to the aft bulkhead (F-311) by the tail wheel mount weldment. The Vertical Stab aft spar is bolted to that aft bulkhead, so that that point can be considered as a ?zero point? for measuring the assembly. In our case, the distance between the F-311 and F-310 is exactly per drawings, and the distance between the HS spars is perfect (10.5 inches). So we know that the HS forward spar (and its tab) is where it should be relative to the aft spar of the VS. So the question then was the distance between the VS aft and forward spars. What we found is that on the Vertical tail drawing, there is a measurement that basically sets the distance from the aft spar to the bottom of the forward spar ? it is given as 14-11/16?. There is a change note bubble by it ? and when you check the change notes, it says that it was previously 15?. That is almost the exact distance of our discrepancy (5/16 - approximately 3/8?). The change to the drawing was done in 2001.

Now the fellow that built our tail (how many folks are lucky enough to get a QB tail on a -3?!) was on his fifth RV. Everything he has built has been to about 1/64? tolerance to the drawings. He obviously built this exactly to the drawings, but it doesn?t fit the HS, which is built to the drawings. So it?s our current theory that the Horizontal Stabilizer and Vertical Stabilizer drawings don?t currently match. The question we have, of course, is if any of the other current builders out there have run in to this mismatch. Since most tails are built long before the fuselage ? and you need the fuselage to know how these are going to fit together, it is a good thing to know when you are building. As I've stated many times, building a -3 is an adventure - you have to enjoy solvign little puzzles like this!

Oh - Our fix was to make a spacer that measured 3/8? (built from two pieces of 3/16?x1 - ?? bar), measured carefully for edge distance for four AN3 bolts, and match drilled to join the two spars. I then drilled some lightening holes that fit with proper edge distances to keep from getting the CG too far aft. Not a hard fix.

The Fix:
IMG_4141.JPG


The Spacer:
IMG_4147.JPG


Paul
 
good reading paul, after building a 4,a6,and a7A I still am learning. remember guys, any bum can drive a nail, but it takes a craftsman to build a house.
 
Nice spacer!

{BTW ? when I call Vans guys on the RV-3, I generally am not looking for a ?factory position? on anything ? the truth is, no one there but Van has built one, and that was a LONG time ago.

Paul

Greeaaaat.... so does this mean I can just call you instead? :D

The unbuilt tail kit that came with my project is from spring of 2008, and ironically I just started working on the HS tonight. I'll check my drawings tomorrow and look for the descrepancy. This will be something to keep in mind while building the ancient fuselage, I am sure.

Thanks for being the trail blazer, Paul!!
Katie
 
My 6A (1996) with an empennage kit ( 1995) had the same deal. There was suggestions of spacers at that time.

L.Adamson
 
Just a thought.....

I've seen several early kits built where the aft horizontal stabilizer ribs were riveted directly to the spar stiffeners rather than notched and riveted to the spar web. This effectively moves the spar forward. Not 3/8", but in the same direction.
 
Greeaaaat.... so does this mean I can just call you instead? :D

The "RV-3 Builders Corps" is a small but hardy (and crafty) band of brothers and sisters. Share your isues and solutions here whenever you have them and you'll get support! :D
 
Last edited by a moderator:
same problem

There must be one in every class, I had the same problem on my -4. had to put a spacer in, however, on my -3, I didn't. Go figure.:confused:
 
My dwgs specified 1/8" max thickness for this spacer
I moved the entire H-Stab forward
Mike
 
My dwgs specified 1/8" max thickness for this spacer

Yup, our instructions said the same thing, which is why I chatted with the engineering types to see if thicker was a problem.

It was also pointed out that the forward VS spar of the -10 is held on with a SINGLE AN4 bolt....so 4 AN3's should be sufficient.....;)
 
It would be interesting to find out if Randy Levold had the same problem Paul. I don't think I have seen any reference to this problem on his site. This is also a candidate for one of Andy's gotchas.
 
Paul, Are those shims under the HS attachment angle? I hope I didnt mess something up cause I dont have them. I had to use a .063 shim between the VS and HS. I also put a doubler on the VS as well as 2 additional AN3 bolts after I botched one of the original bolt holes. It looks like you're about to experience one of the true joys of building the 3 - fabricating the emp fairing...hehehe.
 
Stabilizer Shims

Paul, Are those shims under the HS attachment angle? .

An RV-3 with the stabilizer mounted flat to the longerons will require a sizable Elevator TE-Down deflection at cruise. Sometime in the early 80's a recommendation evolved (from the Bakersfield Bunch, maybe?) that the placement of a shim under the fwd stabilizer spar would reduce this effect without having too much of a deleterious effect on elevator effectiveness in the landing flare. As I recall, the most common suggestion was 1/8" for a light pilot and 3/16" for Bubbas, but I have seen one RV-3 that had a 1/4" shim.

My 1984 Plans show no shims.

One factor complicates this discussion somewhat and should at least be considered. A dimensional error on the 1984 plans and instructions (which most RV-3s were built from prior to the RV-3B) placed the top of the F-303 bulkhead slightly further aft than it should have been to produce the desired 1degree wing incidence (top .5" further aft rather than .42" desired). It may explain why some airplanes require a slight upward push on the wing trailing edge to align the rear spar with the fuselage attachment stub, but the effect on wing incidence is probably very small.
 
It would be interesting to find out if Randy Levold had the same problem Paul. I don't think I have seen any reference to this problem on his site. This is also a candidate for one of Andy's gotchas.

I wrote to Randy to see what he did, but haven't heard back yet. Imagine! Some folks place a holiday weekend over the RV Forums.....:p

Paul, Are those shims under the HS attachment angle? I hope I didnt mess something up cause I dont have them. I had to use a .063 shim between the VS and HS.

Yes, we shimmed the leading edge to drawing specs. These call for a 1/4" spacer on the rear deck and under the rear spar (putting the rear spar up 1/4"). You then do the "spacer on top of the HS" thing to make your level parallel to the cord line, and set it to "zero". It took about 3/8" of shims to do this for us - and this is dependent upon where, exactly you attach the horizontal angle to the forward spar of the HS. (If you don't have the spacer under the rear spar, then you will be limited to ONLY adding positive incidence if you need to make a change...)

An RV-3 with the stabilizer mounted flat to the longerons will require a sizable Elevator TE-Down deflection at cruise. Sometime in the early 80's a recommendation evolved (from the Bakersfield Bunch, maybe?) that the placement of a shim under the fwd stabilizer spar would reduce this effect without having too much of a deleterious effect on elevator effectiveness in the landing flare. As I recall, the most common suggestion was 1/8" for a light pilot and 3/16" for Bubbas, but I have seen one RV-3 that had a 1/4" shim.

My 1984 Plans show no shims.

One factor complicates this discussion somewhat and should at least be considered. A dimensional error on the 1984 plans and instructions (which most RV-3s were built from prior to the RV-3B) placed the top of the F-303 bulkhead slightly further aft than it should have been to produce the desired 1degree wing incidence (top .5" further aft rather than .42" desired). It may explain why some airplanes require a slight upward push on the wing trailing edge to align the rear spar with the fuselage attachment stub, but the effect on wing incidence is probably very small.

While I have lots of respect for, and actively seek the experience of, those who have built the -3's before, the real problem is that there are so many variations (in plans, in model numbers, in engines, avionics, and battery placement), that recommendations for specific incidence settings almost need to be accompanied by this kind of configuration information to be relevant. I have also heard that positive incidence is a good thing - but for what engine and prop? I am going to go the route of setting it per the latest plans (ca. 2001), and see what I get. I don't plan to do a paint-ready empennage fairing until after we have flown - making it much easier to adjust later. I know it will fly with the Van's number - we can adjust from there.

Great information capture and discussion guys - which is what I hope we can do and continue with the RV-3 forums!

Paul
 
Last edited:
the plans are correct

Paul,

With the work required to scratch-build an empennage fairing, I wanted to get the HS incidence right on the first try. Read everything I could find on the old RV-3 Yahoo and Matronics lists. Recommendations were all over the chart. In the end, I went with what was on the plans. At 170 knots, the elevator trim tab is dead-nuts streamlined.

Tony Spicer
RV-3B 11395
310 hours
 
Paul,

With the work required to scratch-build an empennage fairing, I wanted to get the HS incidence right on the first try. Read everything I could find on the old RV-3 Yahoo and Matronics lists. Recommendations were all over the chart. In the end, I went with what was on the plans. At 170 knots, the elevator trim tab is dead-nuts streamlined.

That's great to know Tony!

I might still leave that dang fairing unfinished until it is time to paint though....I might build something else crooked...;)

Paul
 
Trim Range

Paul,

At 170 knots, the elevator trim tab is dead-nuts streamlined.

Tony Spicer
RV-3B 11395
310 hours

That's very interesting, Tony. Did the RV-3B plans enlarge the area of the trim tab? The original -3 tab looks to be about 1/2 the area of the RV-4 tab, and even with big throws it was not unusual to run out of trim at both ends of the speed range in both my -3's.

What is the slowest speed that you can achieve trim (zero stick force) in clean and full flap configurations power-off in your aircraft?
 
What is the slowest speed that you can achieve trim (zero stick force) in clean and full flap configurations power-off in your aircraft?

If I gave you a number, it would be a guess, and most likely not a very accurate one. I never use anything but full flaps in the pattern. When the flaps come down, the trim comes full back. That's usually at 80-90 knots. With full up trim a slight amount of back-pressure is required all the way to the flare. Hardly noticeable when you get used to it. Flaps up between 90-180 knots the trim lever never moves more than 1/4" in either direction. At the higher speeds, you don't move it enough to feel it. Touch it and apply just a little pressure is all it takes. Until you get used to it, it's a bit touchy. Don't think I would like electric trim.

Tony
 
RV3B trim

Tony,
We have approximately the very same trim issue as you describe.
#1 you need to trim full nose up, (landing configuration) #2 at cruise small trim movement gets, (quick reaction)
 
Iron,

We had the same issue (horizontal tail spar forward of the fin front spar)
We also saw a picture of an RV10 and made a spacer to fill between the fin spar and the horizontal tail spar just like you did. Works great! You have to screw around with the fin/tail fiberglass fairing anyway, so no big deal, "except thinking about it":)
 
Yes, we shimmed the leading edge to drawing specs. These call for a 1/4" spacer on the rear deck and under the rear spar (putting the rear spar up 1/4"). You then do the "spacer on top of the HS" thing to make your level parallel to the cord line, and set it to "zero". It took about 3/8" of shims to do this for us - and this is dependent upon where, exactly you attach the horizontal angle to the forward spar of the HS. (If you don't have the spacer under the rear spar, then you will be limited to ONLY adding positive incidence if you need to make a change...)
Here's what I did. I used a larger angle for the front spar (1x1?). Then instead of shimming up the front spar I dropped the rear spar a fraction under the 1/4" and raised the front spar relative to the angle; being sure to maintain bolting edge distance. The result is a HS with the same incidence only slightly lower relative to the longeron. Its true when they say no two 3's are built the same.
 
Now Im curious. For those of you that have flying 3's, what is your HS incidence? Did you set it per plans (0.0) or did you raise the leading edge up a bit? Mine is set at 0.0 and I really hope I dont have to raise it up.
 
That's a good question Rob, but we also need to know which model of -3 (straight, A, B), and which engine/prop as well, since CG is going to be a big player.
 
Stab Mounting

That's a good question Rob, but we also need to know which model of -3 (straight, A, B), and which engine/prop as well, since CG is going to be a big player.

I took a closer look at your stabilizer mounting picture and now realize where some of the confusion concerning shims is coming from. You mounted the stab attachment angle higher on the fwd stab spar face than shown on the drawings. The drawings show the lower surface of the mounting angle to be slightly below the lower edge of the spar (approx 1/8" below the spar bottom edge along the center doubler.)

Your fwd 'shim' looks like it results in placing the stab in about the same position as it you had mounted it "flat" with the attachment angle mounted lower on the spar face as shown on the plans. The lower edge of the spar doubler is not visible in your picture, but I would guess that it is abour 1/8" above the fuselage deck plate. Your 0-degree incidence is the same as what most of us think of as 'unshimmed' when compared to other aircraft.

Your choice of angle placement is beneficial from the standpoint of edge distance for the bolt holes in the spar, and I think I will use your method when I get to the point of attaching my stab.
 
Your choice of angle placement is beneficial from the standpoint of edge distance for the bolt holes in the spar, and I think I will use your method when I get to the point of attaching my stab.

Bingo - I wanted to make sure I had plenty of edge distance on the (thinner) spar material), so mounted the angle a bit higher - that meant the shims needed to be a bit thicker. Every time I pull or push the stick in this airplane, I want to have confidence that the tail is firmly attached....;)

Paul
 
one other (different) thing....

Paul,
a possible heads up - you didn't build the tail feathers - I notice on the picture showing the vertical stabilizer rear spar and skin that there is no overhang of the skin beyond the rear spar flange. On other RV's there is a 3/4" skin overhang that partially covers the gap between the stabilizer and the rudder. I don't know whether the RV-3 is different, and it may just be cosmetic in any event, but you might get a surprise when you are ready to mount the rudder.
Bill Brooks
Ottawa, Canada
RV-6A finishing kit I
 
Bill... good point.

RV-3 DWG 7 shows the details. The actual drawing of the rudder shows no overlap, but some diagrams top of LHS do. The skin extends past the spar 1/8" at the bottom of the rudder skin and 1/4" at the top... and glad it does becuase that's what our rudder has :)

Not sure it has much of a significance if it was missing? Doubt it'll cost more than about 10K on the top speed Paul :D

Andy
 
Paul,
a possible heads up - you didn't build the tail feathers - I notice on the picture showing the vertical stabilizer rear spar and skin that there is no overhang of the skin beyond the rear spar flange. On other RV's there is a 3/4" skin overhang that partially covers the gap between the stabilizer and the rudder. I don't know whether the RV-3 is different, and it may just be cosmetic in any event, but you might get a surprise when you are ready to mount the rudder.
Bill Brooks

We hung the rudder the other day Bill, and it fit nice - as a matter of fact, we had to back it out just a bit because it was slightly contacting the edge of the skin at the top at full deflection, so I think I'm satisfied with it. Good head's up though - there are so many details on the -3 that we all do differently - hence the point of the detailed posts when I come up with something. As Rob pointed out, there are probably no two RV-3 tails exactly alike!

Paul
 
We had that problem too... on my 6A

Paul,

I ran across the same problem. Fortunately, I met another builder who also had the same problem and shared with me the solution.

IMG_1935.JPG
 
It looks like your offset is in the opposite direction though John! What vintage kit was that?
 
me too

I moved the horizontal stabilizer foreward 3/16" by placing a 3/16" spacer between the front face of the horizontal stabilizer rear spar and the fuselage attaching bars.

Robert "Earl" Young
RV3B
 
Paul,

I had the same problem on my 8 when I fitted the empenage. Would post a picture but never learned how to post pictures.If you will go to my builders page in kitlog click on all and go back to 8/07/07 and 8/16/07 there is some pictures of the spacer I made. Never really figured out why I needed the spacer but suspect my jig position on the rear bulkheads three other 8's where built on this jig don't know if they had the problem but I made the spacer and built on. The picture you posted the spacer is on the same side of the spar as mine was but in the picture posted by someone else spacer was placed on the opposite side of the spar. There is a link to my web page on bottom of post.
 
I moved the horizontal stabilizer aft [edit] 3/16" by placing a 3/16" spacer between the front face of the horizontal stabilizer rear spar and the fuselage attaching bars.

Robert "Earl" Young
RV3B

That would have been the easiest thing to do - if we hadn't already drilled the holes in the longerons for the forward attachment bolts when we found the mismatch in the vertical spars....:(
 
one change leads to more changes...

Moving the H Stab aft might limit the 'up' angle of the elevs, as the aft bulkhead is (or is supposed to be) the 'up' stop.

Carry on!
Mark
 
stability...

How much can you move a horizontal stab (aft or forward, mostly forward) before you affect flight stability? (Just the old modeler in me asking questions again...)
 
Paul,

Do the RV-3 plans show 4 AN3 bolts in that pattern or 3 AN3 bolts straight across?

I need to put 3/16 or so shim in my retrofit tail and was wondering if the box bolt pattern would be stronger if going 1/8 or so over Van's recommendation.
 
Back
Top