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7 VS. 8 rudder...for the 7

Mark33

Well Known Member
Please excuse my ignorance but would someone please describe clear and concise way the differences in the 7 and 8 rudder? There?s been some discussion on a different thread regarding some possible safety concerns regarding the 7 rudder and how an 8 rudder may be a better choice for the 7. It would also be helpful if someone could maybe post a picture or two of the differences and/or a link that details the differences. I'm about to start building the rudder for my -7 and if there's really a benefit to going with the 8 rudder, then now's the time for me to make that change.

Thanks,
Mark
 
Rudders

Mark,
First, there are a lot of -7/-9 rudders flying around that have not departed the vertical. So the design isn?t defective, just maybe less tolerant to over speed events such as dog fighting, blotched acrobatics, and now a bird strike. The -8 rudder appears to maybe have a little bit more margin in it. The -7/-9 rudder is larger than the -8. The -7/-9 rudder has a rivited trailing edge, the -8, -6, -4,-3 all have a folded edge. That?s the primary differences. Maybe the -8 skin is thicker. The folded trailing edge sometimes had the problem of developing cracks unless the builder applied generous amounts of proseal to the inside when riviting the skin. The rivited trailing edge was supposed to be an improved way of building the rudder.

Someone else can post the pictures to show the differences as I don?t have a hosting site for pictures.
 
Thanks Carl, I?ll read through the post in the link that you provided and study up on it. Thanks again!!

Mark
 
Hi Mark,

I don't know how long you've been around RV's, but there was a similar panic a number of years ago about the RV-8 wing (which happens to be the RV-7 wing, too, but no one ever seemed worried about the -7 wing in the ensuing years).

The factory demo -8 had a catastrophic wing failure, killing the demo pilot and the passenger. It was almost certainly simple over-stress of the wing, flown way outside the spec'd limits. But it still took a couple of years for all the hand-wringing to die down. There are still individuals that think the -6 wing is stronger, based on counting the bolts. (?)

Similar stuff happened much earlier with the -3; guys flying them like they were unlimited acro mounts, and pulling the wings off.

There's still speculation about the -7's tail, but it isn't the rudder at issue; it's the vertical stab (which, if I'm not mistaken, is the same vertical stab as the -8, but no one's....etc etc).

I think if you look at pure data, it's hard to see a real issue with the rudder itself. I'd have to think that when the factory decided to go the -9 rudder on the -7 to improve spin recovery performance, they actually tested its performance at the speeds of the -7 prior to offering the change to their customers.

If you can't get comfortable with the plane, then maybe a different choice is in order. But I think it's worthwhile to have data and logic drive my emotion, rather than the other way around.

FWIW,

Charlie
 
There's still speculation about the -7's tail, but it isn't the rudder at issue; it's the vertical stab (which, if I'm not mistaken, is the same vertical stab as the -8, but no one's....etc etc).


FWIW,

Charlie


So what is unique about the -7 vertical stab?
 
Hi Mark,

I don't know how long you've been around RV's, but there was a similar panic a number of years ago about the RV-8 wing (which happens to be the RV-7 wing, too, but no one ever seemed worried about the -7 wing in the ensuing years).

The factory demo -8 had a catastrophic wing failure, killing the demo pilot and the passenger. It was almost certainly simple over-stress of the wing, flown way outside the spec'd limits. But it still took a couple of years for all the hand-wringing to die down. There are still individuals that think the -6 wing is stronger, based on counting the bolts. (?)

Similar stuff happened much earlier with the -3; guys flying them like they were unlimited acro mounts, and pulling the wings off.

Thanks again,
Mark

There's still speculation about the -7's tail, but it isn't the rudder at issue; it's the vertical stab (which, if I'm not mistaken, is the same vertical stab as the -8, but no one's....etc etc).

I think if you look at pure data, it's hard to see a real issue with the rudder itself. I'd have to think that when the factory decided to go the -9 rudder on the -7 to improve spin recovery performance, they actually tested its performance at the speeds of the -7 prior to offering the change to their customers.

If you can't get comfortable with the plane, then maybe a different choice is in order. But I think it's worthwhile to have data and logic drive my emotion, rather than the other way around.

FWIW,

Charlie

Hi Charlie, thanks for the thoughtful insight. I'm fairly familiar with RV's...I've had two -4's and a -7 and I'm currently flying a -4, but I'm always trying to learn more from those that's blazed the trail before me. When I read about possible safety issues I try to take extra notes. As with this "possible" rudder issue, there seems to be mixed facts (or feelings) regarding a particular subject. I try to ask pertinent questions and hopefully get good input form more experienced builders and then make an informed decision based on the information I receive along with my own mechanical knowledge. I guess when it comes to something like this rudder issue, I tend to lean toward the side of caution...especially if there's really no downside to it. Just the fact that the skins are thicker on the 8 rudder I would think would be a bonus. So, I guess long story short, I'm still on the fence as to which way to proceed. Once again, if there's really no downside to making the change to the 8 rudder, and even if there's only a small margin of safety gained, I think it may be a smart move. If my airplane were finished and flying, then I don't think I'd even think about swapping out the rudder. I agree, there's lots and lots of -7's flying around with the 9 rudder and I really don't think they're in any danger of falling out of the sky.

Thanks again for your input and knowledge on the subject.
Mark
 
Oh, I agree about cautious & thoughtful examination of any new data that can affect safety. Having said that, remember that Van's didn't switch rudders on a whim; they saw less than optimal spin recovery characteristics in the -7 when flying with the -8's rudder. So... if you'll never spin a -7, and you're happy with the rudder authority using the smaller rudder, then yeah, go for it & avoid any possibility of issues with the -9 rudder.

But I learned to fly in a Luscombe, and while I freely admit that my -4 has adequate rudder authority, it still, after almost 25 years of flying -4s, feels like it could use more. I don't want to feel really inadequate when I finish the -7. :)

Have you verified that the -9 skin is really .016? I don't remember, and mine's stored away at the moment, so I can't measure. I was surprised to hear that it is .016, since IIRC, control surfaces used to be that thin on the earlier models but they replaced them with .020 in later kits due to the regularity of cracking.
 
I have flown my 7A with both the 7 and 8 rudders. I decided to try the 8 rudder because the 7 rudder felt too heavy in comparison to my plane's light elevator and aileron forces. (My plane is 1,062 lb, and a little tail heavy with P-valve, Catto, and EarthX up front)

Differences include:
The 8 rudder feels lighter, which makes it nicer for mild acro and cross wind landings.
Both rudders handle stiff sock crosswinds well, but theoretically, the 7 rudder might handle more.
The 7 rudder recovers from a spin faster, but the 8 gets the job done. My 7 rudder would initiate an opposite spin if full travel was applied. I spin regularly in both directions, but limit myself to two turns, so I can't comment on fully developed spin recovery.
The 7 rudder taxis better, less brake required.
The 7 rudder has better yaw stability in turbulence, better for cross country flying.
I am considering returning to the 7 rudder. For me, the fun factor of lighter, more harmonized controls doesn't quite offset the reduction in yaw stability.
Can't comment on flutter, I stay well below VNE, especially during acro.

Jay
 
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