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View Full Version : RV-8 TAIL ASSY ON A ROCKET


RV-4
03-04-2017, 09:06 PM
Hello Gents

Any harm in installing a RV-8 Tail assy on a Rocket?? I know of one and was wondering if there might be any do's & don'ts..

Thanks

Bruno

N395V
03-05-2017, 07:55 AM
Which Rocket?........................................... ............................................

flynwest
03-05-2017, 09:03 AM
Bob Mills put one on his Rocket 6

Toobuilder
03-05-2017, 09:04 AM
I didn't build it, but my Rocket has an -8 tail.

I understand from Mr. Mills that the taller tail is a speed penalty. Out of concerns over the higher speed of the Rocket and potential susceptibility to flutter, I also removed the rudder trim tab the builder installed. It was quite sloppy and I have not missed it one bit.

RV-4
03-05-2017, 09:49 AM
Milt

It is a HR 2, it looks good on the airplane but was wondering if it could affect the aircraft in any way...

Bruno

Steve Barnes
03-05-2017, 10:52 AM
When I built my Super Six (sister ship to bob Mills) I had an unused RV 8 empennage and installed it during the build. Has worked perfect for 1250 hours. I not sure if Bob has an RV 7 or 8 empennage on his 6.

Steve

Tom Martin
03-05-2017, 11:32 AM
I built two HRIIs using the RV8 tail. They fit fine and I used the 8 tail for two reasons. It was easier to build then the 4 tail and also it had a counterbalanced fin. I have not flown a HRII with a RV4 tail but my RV4 had, at times, a tail wags the dog kind of yaw situation. My rockets with the 8 tail did not have this syndrome. That is the good news.
On the negative side:
The 8 tail is heavier and we really do not want that extra weight back there.
Also the elevators on the 8 are adequate but a wider chord would be better. I could run out of trim in some loading conditions and I wonder how well the plane would fly in an engine out scenario.
Although I never made any measurements I wonder if the RV7 elevators would work on the 8 stab?
While building the 8 tail I would recommend reinforcing the forward VS attach point as well as the forward HS stab spar. My HRIIs had stock engines and were fine with the stock tail, however if you plan on higher speeds be aware that you may be in uncharted territory.

It is great to see some renewed interest in Rockets. They are a joy to fly and I can hardly wait for spring weather!

jrs14855
03-05-2017, 12:21 PM
That is normally an issue with the folded trailing edge skins. I had a hangar neighbor years ago with a then brand new RV4 and it was a pretty significant issue based on his description. I looked at it and the skin at the trailing edge was bulged out significantly. I told him to squeeze the trailing edges and he said the problem went away. This discussion seems to come up repeatedly.
Once again I recommend Barnaby Wainfans excellent article on this subject a few years ago in Kitplanes.

F1Boss
03-05-2017, 03:32 PM
I would suggest a few beef-ups to stiffen the structure for Rocket speeds - make your own .040 spars for example. Also the elevs are a bit on the small side, as obvious on a 200HP/metal prop RV8. It won't 3-point solo without some ballast (I suggest a 120lb PAX in the back seat - possibly one on the female side of things) or 1/2 flap, but to some that characteristic is not a problem.

Other than that, it fits up fine! Use 1"X1" angle at the H Stab fwd spar attach in the fuselage at the #10 bulkhead (under the aft deck plate). There is more to the aft fuselage story...

Remember the airframe Vne is a TAS number.

Carry on!
Mark

scsmith
03-05-2017, 03:43 PM
That is normally an issue with the folded trailing edge skins. I had a hangar neighbor years ago with a then brand new RV4 and it was a pretty significant issue based on his description. I looked at it and the skin at the trailing edge was bulged out significantly. I told him to squeeze the trailing edges and he said the problem went away. This discussion seems to come up repeatedly.
Once again I recommend Barnaby Wainfans excellent article on this subject a few years ago in Kitplanes.

Wait! Don't confuse a dutch-roll yaw oscillation with a vague or bi-stable rudder issue. Two very different things. A poorly formed trailing edge will cause the latter, but not the former.

jrs14855
03-05-2017, 06:10 PM
Wait! Don't confuse a dutch-roll yaw oscillation with a vague or bi-stable rudder issue. Two very different things. A poorly formed trailing edge will cause the latter, but not the former.
No confusion, This was described to me as uncommanded rudder movement. It went away after squeezing the trailing edge. I did not fly the airplane.
I have experienced a similar issue on Pitts ailerons. The Pitts aileron trailing edge is .020 x 2" wide strip folded into a V and then further formed to the proper angle. Underbent trailing edges on the Pitts remove any centering force. The ailerons will move at least half travel if you let go of the stick. Squeezing the trailing edges eliminated this and made it like most Pitts. I did fly that airplane both before and after and with a pervious set of wings.
All of this and much more is in Barnabys article in Kitplanes.

RV Jerry
03-06-2017, 11:17 AM
I have built several Rockets both with RV8 tails and with RVV 4 tails RV8 woks ok but my preference is the 4 tail ,it's lighter easier to taxi in x- wind and has a little more elevator area and suits my eye. The reason I originally used the 8 tail was a concern for flutter but it seems to be a non issue.
Jerry

RV-4
03-06-2017, 09:44 PM
Thank you guys for the good info.

Cheers

Bruno

131RB
03-06-2017, 11:02 PM
I have no experience with the 8 tail. I have the 4 tail on mine and most likely would do it that way again. It flys great and I prefer the look.
One thing I didn't see anyone else mention to keep in mind. Last time I checked their were no service bulletins on a 4 horizontal stab.
Ryan

vfrazier
03-07-2017, 12:54 PM
Just a data point: I had an RV-4 tail on my Rocket and am installing the same on my F4.

The RV-4 tail on the Rocket did not "wag" that I ever noticed.

Jim Winings has some interesting stories about flying RVs that have improperly formed trailing edges. It's an easily overlooked problem, but usually easily corrected too.

F1 Rocket
03-20-2017, 09:37 PM
N14ZM has the RV-8 tail. Flys great although it could use a bit more authority during the flare when I'm flying solo with a lot of fuel. Since I hate flying by myself it's not a serious issue with me. Never seem to have trouble finding someone to act as ballast in the rear seat :)

autoglide
03-22-2017, 03:02 AM
My 22 yr old Rocket (Production #1) has absolutely no bad habits at all....orig built w a 4 tail.....a great bird indeed!

Marv Bein
N54HR

rvmills
03-23-2017, 02:12 AM
My Super Six was originally built with a stock 4 tail. As I began to race, and started to do speed mods that increased the top speed, I found myself cornered...actually squeezed between, Dave Anders and John Huft (modifiers and racers of considerable note) during dinner at AirVenture Cup. They said in chorus, "when ya gonna put a counterbalanced rudder on that beast?" So I built a slightly modified and strengthened 8 V-Stab and Rudder, and it has flow well in this configuration. The mods included pinning the stiffeners together to make them more similar to ribs, adding a doubler to the aft V-stab spar (to absorb the inputs from the stiffer rudder and avoid just moving a potential point of failure), adding a small, calculated amount to counterbalance weight, and doubling the connection between the counterbalance cavity and the main structure of the rudder. All done with some consultation with an appropriately knowledgeable aircraft engineer. The added height did cost a bit of speed (2-3 knots).

I've flown HRIIs with a 4 rudder, and they all flew great...none seemed to exhibit any tail-wag. Elevator authority was always good too, but when 2 up, the bigger bloke has always been in the front (I was instructing the owner then).

An 8 tail works well, but as Mark said, flown conservatively in descents, the 4 tail may be just fine. If you want to go race-fast, or are concerned about overspeeding in descents, the ultimate win-win here may be a modified 4 tail, built with a counterbalance. Greg Nelson built and raced a beautiful F1 Rocket with such a tail, and the tail is a work of art...the plane even has 3-axis electric trim. The plane is very fast as well. I believe Dave Anders and Robby Attaway built similar tails for there 4 and 6, respectively. Were I to do it over again, I would go that route. You may look into those gents' accomplishments in this area.

Another mod that Steve Smith and I have talked about is using pressed-fit bearings instead of rod-end bearings for the elevator and rudder connections, where possible. Steve may be able to add some more data points on this as well.

Cheers,
Bob