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-8 Tail on my Fastback -4?

Pittsbuilder

I'm New Here
Hey guys, I'm thinking about putting the RV8 tail on my RV4. I have Several reasons for wanting to do this. I inherited the project partly complete, and am converting to Fastback (mod mostly complete.) The horizontal stab was built with several degrees twist at each tip. If the HS is inline with the axis of the aircraft, each tip is twisted down. Also, my 4 is an old kit, which has the thinner skins, and I fear that I will eventually have issues with cracking anyway. A friend of mine has an -8 tail mostly built that I can pick up for a decent price, thinking about pulling the trigger.

I know several members here have done this swap, and I would love to get some info from them. How hard was the conversion? Exactly what is involved to make it happen? Difference in flying qualities? I tried to message one of the aforementioned members, but to no avail. I also tried the search, and found a couple posts with some mention of it, but not really much helpful info. Any help/suggestions would be greatly appreciated.
 
I am re doing a 89 -4 into a fast back and changed out the tail. i thought about using the -8 tail and it can be done pretty easily from what i understand but the determining factor for me was the -8 tail weighs more. About 10-15 pounds more if I recall. I got a second hand -4 tail section and was able to use just the feathers with my existing vert and horizontal stabs. The -8 rudder has a larger surface area too.
 
10-15 lbs? Wow, that's a lot more than I would have thought. I might guess 5lbs more, but I wouldn't have thought 10+. I am not super concerned with tail weight as I am going to have an IO360 and CS prop up front that will help in that department.
 
The -4 tail is ez to build. The elevator & rudder skins are what are thin, and can be swapped out for thicker skin. I would do that and save the weight, especially if it is 10-15 pounds more(thats a whole lot of moment arm wayyy back there).
 
Digging yourself a hole?

Hi Michael,

I think you are digging yourself a hole. I love my 4 but if I am objective about it I would have built it lighter. In fact I am in weight saving mode at the moment.

The 'problem' with the 4 is that unless you build really light you are eating into both aerobatic and max weight limits. Also they tend to be tail heavy.

There is nothing wrong with the standard 4 tail, not all of them have cracking on the elevators and rudder. I suspect the larger 8 tail would leave it over finned and ruddered.

What would I do if I built another 4?

1. Fastback...... with you on this.
2. IO 340 or tuned 0-320 to keep the weight down.
3. Catto or light VP prop. ( they tend to need a heavy prop or weight at the front to balance the aeroplane. ( Another reason to avoid weight on the tail.)
4. Light weight battery.

Minimal panel weight, no strobes landing lights.

The 4 is a sports car, not a tourer!!!

Just my opinion!!
 
I built a RV4 with the standard tail. Then I built two HRIIs and used the RV8 tail rather then the RV4 tail.
The RV8 tail will bolt right on, the only modification is the need to raise the bottom forward tip rib of the VS. Depending on how high your fast back is you might not even have to do this.
In terms of flight quality, the larger counterbalanced rudder works very well with a HRII. It stopped that little tail wags the dog thing that sometimes happens with a RV4; YMMV. The elevators on the RV8 tail work on the HRII but would be better if they were a bit larger. I am sure that they would work just fine on a RV4.
The bad news, as others have mentioned, is added weight. OK for a HRII but no so good for a RV4 regardless of how big an engine you put on it. Weigh your RV4 tail and then weigh your buddies RV8 tail. Borrow some weight and balance records and run the numbers.
My advice would be to fix or replace your RV4 HS, fly with the control surfaces that came with it and if need be to build new control surfaces later with heavier skins.
If the HS is sound, and the "twist" is the same on both sides you could just fly with it and see how it behaves. Another complete tail can be built after you are flying your airplane and replace the original parts before final paint.

To answer your original question again. Yes, the RV8 tail will fit. It is your plane, recognize the options, and built it as you wish.
 
Gain v loss

Hi

Another way of looking at this is gain v loss.

Purely in terms of aircraft performance you gain nothing by fitting an 8 tail but you loose useful load, aerobatic load.

Ok you will not have to build a new stab but if you search these threads a lot of people are struggling with the weight limits on a 4 trying to push a quart into a pint pot!!

The closer to 900lbs you can get your 4 the better, 1000 is acceptable but will put a 100lb reduction on what you can carry. I am lucky I am not a heavy guy 165lbs and my wife is light too. I can carry a 200lb person in the back seat and with full fuel I am at gross 1550 lbs in the UK.

I took a friend up for a flight yesterday and he summed it up just like me....a 4 is a sports car, build it and fly it that way.
 
Don't do it.

RV-8 tail
Rudder = 8.5 lbs
V-stab = 8.5 lbs
H-stab = 17.5 lbs
L elevator = 8.5 lbs
R elevator = 8.5 lbs
Total = 51.5 lbs

RV-4
Rudder = 4.5 lbs
V-stab = 7 lbs
H-stab = 19 lbs
L elevator = 4.5 lbs
R elevator = 4.5 lbs
Total = 39.5 lbs

Difference of 12 lbs (the wrong direction). Part were not painted however, primed inside.
 
I did it and it worked great!

Seems every few years this topic comes up.

I converted to the RV-8 tail about 4 years ago. Net gain on the T/W 3#.

After seeing the post from Axel I decided to confirm what I remembered from the conversion. So I dug out of storage my old RV-4 tail (built in 1995) weighed in at 44#, .016 skins on control surfaces, zinc chromate primer - no paint.

The 8 tail weighed 50#, epoxy primer! elevators balanced! farings on.

I used one of the pads from the digital scales that we use to weigh airplanes. They round to the nearest #. I came up with a difference of 6#, 7# worst scenario.

My 4 is equipped with an O/360 & Hartzell CS.

A friend of mine converted the same time. His 4 has an IO 320 with wood prop. He likes the conversion.

The most significant difference is: much more stable flight. I don't need any more down trim with a backseat passenger. I do 3 point landings - the flare is more like the 8. No loss in top speed.

The advantage of the 8 tail is easy to build compared to the 4 & it's going to be straighter than the 4.

Axel, it is a mystery to me if the elevators you weighed were balanced how they can weigh the same. Both the 4 & 8 elevators that I have weighed, there is a noticeable difference in weight between the right & left elevators. I used a bathroom scale to weigh the elevators individually.

Would I do it again. Yes.

We have 2 RV-4's in the shop being built with RV-8 tails.

Loyd
 
I also replaced the empenage on my RV4, with an 8 emp. due to the tornado at Sun&Fun. I used the 8 because I just didn't want to build another tail jig. I am very pleased with the 8 emp.
No loss in top speed. Seems more stable in cruise flight (195 to 200) and remains effective throughout the landing sequence.
I also went to an electric trim, cutting a pound from the mechanical system.
My tail wheel weight changed 2.3 pounds.

I also like the looks of the larger tail.
Great "WOW" factor.
Loyd Remus and I did the mod at about the same time. We are bith very pleased.

I will be at Sun&Fun, parked at the far West GA camping area.
Come on by and take a look at N85007.

Joel Sidell
 
Axel, it is a mystery to me if the elevators you weighed were balanced how they can weigh the same. Both the 4 & 8 elevators that I have weighed, there is a noticeable difference in weight between the right & left elevators. I used a bathroom scale to weigh the elevators individually.

Loyd

Those are my numbers from two uninstalled (and near balanced) tails. They were all individually put on the scale and rounded to the nearest .5 lbs.

Don't know what to tell you :confused:

I will pass. Had to put an 18 lb weight in front of my prop on my flying rv because weight and balance issues.
 
Thanks A lot guys for all the great responses!! I was looking specifically for information on the total installed weight change, and had expected it to be in the 3-6# range. After reading a bit, I think I'm going to go with the 8 tail, in spite of the somewhat heavier tail. I know that building light is usually the best thing to do, But for me, I feel like if I can keep the total weight increase on the tw to less than 8lbs, I'll be fine. She is going to be swinging a big heavy engine and prop way out front after all. I'm going to weigh my existing tail surfaces, and compare them to the 8 tail. I will give everyone here a breakdown of my findings, as well as exactly how it impacts my tw weight when I get to weighing.

Thanks again everyone for the great input and info!!
 
I did it and it worked great!

Seems every few years this topic comes up.

I converted to the RV-8 tail about 4 years ago. Net gain on the T/W 3#.

After seeing the post from Axel I decided to confirm what I remembered from the conversion. So I dug out of storage my old RV-4 tail (built in 1995) weighed in at 44#, .016 skins on control surfaces, zinc chromate primer - no paint.

The 8 tail weighed 50#, epoxy primer! elevators balanced! farings on.

I used one of the pads from the digital scales that we use to weigh airplanes. They round to the nearest #. I came up with a difference of 6#, 7# worst scenario.

My 4 is equipped with an O/360 & Hartzell CS.

A friend of mine converted the same time. His 4 has an IO 320 with wood prop. He likes the conversion.

The most significant difference is: much more stable flight. I don't need any more down trim with a backseat passenger. I do 3 point landings - the flare is more like the 8. No loss in top speed.

The advantage of the 8 tail is easy to build compared to the 4 & it's going to be straighter than the 4.

Axel, it is a mystery to me if the elevators you weighed were balanced how they can weigh the same. Both the 4 & 8 elevators that I have weighed, there is a noticeable difference in weight between the right & left elevators. I used a bathroom scale to weigh the elevators individually.

Would I do it again. Yes.

We have 2 RV-4's in the shop being built with RV-8 tails.

Loyd

Good report sir, thanks for the info. Does the RV-8 tail section increase your RV-4 VNE from the normal RV-4, 182 KIAS to over 200 since the RV-8 has VNE over 200 KIAS?

Has your crosswind capability improved?

Are most guys buying a quick build RV-8 tail section for the job and are the RV-8 skins .020 like the newer 4's?
 
Also, I've noticed the tail wag phenomenon mentioned earlier in my RV-4 and wonder if you ever experienced that in turbulent air and if that improved as well.
 
I am repairing a damaged -4. I bought a -8 tail that was already built and primed.
Here are the weights of the un-installed parts.
rudder - 8.2 lbs
V-stab 8.8
R-elev 7.6
L-elev 7.3 (no trim tab)
H-stab 18.0 (different scaled rounded to .5)

I studied the differences of the two before I bought the -8 tail.
The -8 V-stab is about 5.5 inches taller than the -4.
The main difference in weight comes from the counter-weights
-8 rudder has one, -4 does not
-8 elevator tips do not extend as far forward as the -4 from the hinge line so the -8 elevator weights are a little bigger.

Another interesting note. if you look at the whole series of RVs.
The 3,4 and 6 do not have a rudder counter weight, the rest do. I did some research and never found a good explanation why Van went to a counter-weighted rudder after the -6. I have not heard of any flutter problems with the 4 or the 6, so I have considered removing the counter weight on -8 rudder when I install it, then it would be like a -4 and 6 tail.

PS: I seem to remember a post where Smokey said he had a friend that had an -8 tail installed on a -4. Maybe he can chime in about how it flew. compared to all the stock -4s he has flown.
 
Counter Weight Culture...

PS: I seem to remember a post where Smokey said he had a friend that had an -8 tail installed on a -4. Maybe he can chime in about how it flew. compared to all the stock -4s he has flown.

Tiki,
I helped a friend install an 8 tail on a 4 and it worked fine. However comma, as an RV4 builder/enthusiast it wasn't aesthetically pleasing IMHO, simply too big. I feel the same way about all the post RV6 RV's. My HR2 has a stock RV4 tail installed and I believe very pleasing lines and the rudder isn't counter-weighted. Even at the highest indicated airspeed I ever encountered (250 KIAS at Sea Level) It was never an issue. My current RVX is a stock RV6 fuselage with an RV4 tail modified to fit the aft fuselage. It is the best control balance of any RV I have flown yet.

My own opinions don't matter but I believe Van counter-weighted the later model RV's rudders as a liability issue. Since their VNE is posted higher than previous RV's it only makes sense that all tail surfaces being counter balanced gives them greater margins on paper. With the original RV8 prototype shedding a wing inflight with 2 fatalities, I'm sure flutter of any type is a concern, whether realistic or not.

My good friend Hollywood built a counter-balanced rudder for his RV4 retaining the same size vertical fin. It required a bit of cutting and engineering but looks great and works well.

Personally, I like the stock tail...

V/R
Smokey
 
...Personally, I like the stock tail...

I'm with you on that.

I have an -8 tail on my Rocket and am considering going back to a -4 vertical or cutting down the existing unit. I will retain the counterbalance however.

Some people have claimed no speed loss with the -8 tail, but I suspect that's at RV speeds, not Rocket speeds. I know of a Rocket that lost a good chunk of speed with the move to a -8 tail.

...and yes, the short tail looks much better.
 
Wondering if the RV-4 tail weight was with the .016 or .020 skins?

This thread and weight were long ago but wondering if this weight on the -4 control surfaces was .016 or .020 skins. I've got a couple cracks on trailing edge rivets I found during condition inspection so new surfaces or a -8 tail might be in order in a while.

RV-8 tail
Rudder = 8.5 lbs
V-stab = 8.5 lbs
H-stab = 17.5 lbs
L elevator = 8.5 lbs
R elevator = 8.5 lbs
Total = 51.5 lbs

RV-4
Rudder = 4.5 lbs
V-stab = 7 lbs
H-stab = 19 lbs
L elevator = 4.5 lbs
R elevator = 4.5 lbs
Total = 39.5 lbs

Difference of 12 lbs (the wrong direction). Part were not painted however, primed inside.
 
This thread and weight were long ago but wondering if this weight on the -4 control surfaces was .016 or .020 skins. I've got a couple cracks on trailing edge rivets I found during condition inspection so new surfaces or a -8 tail might be in order in a while.


.016 skin.
 
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