Mikeyb
Well Known Member
Vans comment on rudder balancing
https://vansairforce.net/community/showthread.php?t=204594&highlight=Balance+rudder
https://vansairforce.net/community/showthread.php?t=204594&highlight=Balance+rudder
At the end of the day I don't think it's been shown that an RV-7/7A operated within the published design/performance envelope is at any risk.
If there was a systemic fault here….
BTW, I met a fella at OSH with an RV-7 rudder installed on his beautiful new RV-8. He said he thought it would give him more control authority.
No one disagrees.
Trouble is, despite good intentions, sometimes they get outside the envelope. The plain truth is everyone makes a mistake from time to time.
So if you "beef up" the design, however you do it, and enlarge the envelope, won't you still have the problem of people making mistakes and exceeding it?
How far do you go to ensure that no pilot ever violates a design constraint?
This is distressing information. I hope that individual reads this thread and retrofits an RV-8 rudder to his RV-8.
So if you "beef up" the design, however you do it, and enlarge the envelope, won't you still have the problem of people making mistakes and exceeding it?
How far do you go to ensure that no pilot ever violates a design constraint?
Sharp guy, just didn't know. He said he would change it when he returned home.
You can't expect to do stupid things in airplanes, even if by mistake and survive all of them.
The key point here is to increase the margins and then DON'T enlarge the envelope. Enjoy the existing envelope with increased margins.
So are you saying RV-7 pilots are the only ones that are doing stupid things and making mistakes?
As I said earlier, I don't think RV-8 pilots are any better or more careful than RV-7 pilots, and RV-8s are not coming apart in-flight due to the failure of the rudder.
So are you saying RV-7 pilots are the only ones that are doing stupid things and making mistakes?
As I said earlier, I don't think RV-8 pilots are any better or more careful than RV-7 pilots, and RV-8s are not coming apart in-flight due to the failure of the rudder.
Not all all, just where do you draw the line at Vne over margin, flutter and structural strength on any aircraft, not just RV7s?
The demo RV-8 lost a wing due to overstress. Do we condemn the design because of that one accident saying it should be stronger?
Van's can't be expected to make their designs fool proof in every situation and still meet the design mission. It looks like Van's has done substantial testing with other rudders and published the results. For anyone who is worried about this, they can change the rudder and maybe widen the margins. Kudos to Van's for doing the test flying and putting the results out.
The key point here is to increase the margins and then DON'T enlarge the envelope. Enjoy the existing envelope with increased margins.
But in the case of the RV-7s, Van's took the unmodified RV-9 rudder and put it on the RV-7 later in the history of the RV-7. The RV-9 rudder is designed for the -9 which has lower Va and Vne speeds than the -7 does, thus reducing structural margins when installed on the -7s. The -9/-7 rudder is weaker and less stiff than the -8 rudder, which was designed for the same Va and Vne speeds that the RV-7 airplanes are designed for, since the RV-8 and the RV-7 have the same structural design speeds.
It's hard to get by the fact that there have been approximately 8 RV-7/7A accidents involving the rudder, with 11 fatalities. And they continue to happen with some regularity (see Post #1). There have been none for the RV-8/8A that I know of.
The simple retrofit of the -8 rudder to the -7 gains a significant amount of strength and flutter safety margin (similar to what the RV-8s have), with no appreciable change in spin recovery characteristics.
Two things:
1. Do we *know* the root causes of all of those accidents? Not hearsay, not some of them, actual causes of every one? Are they ALL the same failure modes? All the same flight regimes? Absent some clear engineering evidentiary trail that points to a common cause, that being a rudder that is underdesigned, this is all just conjecture.
2. It doesn't matter much if the "margin" you're talking about is 10 knots or 100 knots...the *pilot in command* is not supposed to be messing around in the margin area. That it appears that the margin is somewhere around 40 knots or so, anecdotally, it seems like Van's has done their part, now we should do ours by staying within the envelope.
…. I was at Anti Splat recently getting the bearing upgrade. He has some fixes for the 7/9 to strengthen the forward vertical spar attach point. He feels that is the failure point in a rudder event. BTW, I have toyed with being a novice machinist for the past 20 years and Allan is really good. He also gave me some #9 lead shot to put in my steps.
I’m building a 9A and intend to put shot or sand in the steps. How will you seal the drilled hole?
I think this has been a educational thread, both about the importance of airplane limits and what can be done should you wish to increase margins on a -7.
While the vertical stabs have failed in these accidents, it's happened after other items failed first (ie. the rudder). The forward spar on the vertical stab is a red herring. Reinforcing the attach point of the vertical stab as AntiSplat suggests just creates stress concentrations elsewhere.He has some fixes for the 7/9 to strengthen the forward vertical spar attach point. He feels that is the failure point in a rudder event.
In my humble opinion, having to use TAS when flying a RV coupled with a narrow margin rudder is a recipe for more inflight failures in future. Data is not emotional and it’s going to happen. That’s just the way it is. Education to individuals may slow the occurrences but relying on human memory is just not good mistake proofing. How far does one go? Well, if a rudder change will bring the inflight failure rate of the -7 down to the -8, then that’s all one can expect. Trying to educated all the pilots about watching TAS and not exceeding it has not been effective.
Looks like the 2 high profile -7 breakups were caused by pilots hooning around with nary a thought of any airframe limits- 234 and 244+ knots respectively- WAY over Va and Vne.
While the vertical stabs have failed in these accidents, it's happened after other items failed first (ie. the rudder). The forward spar on the vertical stab is a red herring. Reinforcing the attach point of the vertical stab as AntiSplat suggests just creates stress concentrations elsewhere.
Did the pre 2002 -7 come standard with -8 rudder?
Yes, it came with the 8 rudder.
Thanks for verifying what I thought I read sometime ago.
I am in the middle of building the new rudder when I noticed this thread. I will finish the larger rudder for the practice and to hone my skills but install the original “8” rudder.
Although the RV-10 and -14 rudders still have riveted trailing edges, the internal structure is different from that in the-7/9. Stiffeners are tied to the rudder spar with shear clips, and the stiffeners themselves are much more robust - the horizontal flanges are joined to their mates on the opposite side with a rivet about halfway down the rudder chord. Also, the counterweight skin is integral to the rudder skin and not riveted on as with the -7/9.
My professional flight test work has only dealt with structures and flutter at a conversational level, but my sense is that these changes make the rudder quite a bit stiffer. Only Van's can say whether those changes were remedies for a weakness in the -7/9 rudder design, or if they were done for another reason.
14 pages of comments.
Numerous posts about changing Vans design or components.
Did I miss a Vans comment?
Seems like they would have something to add.
<snip>
Did I miss something?
If there were something noteworthy to add, yes.
So far this is nothing but conjecture that "something is wrong" - but it's all based on pilots exceeding the published limits of the airplane. Vans will most likely not make any comment at all on accidents where the pilot exceeded the limits of the airplane - the cause of the problem there is obvious. Don't do that.
Is there any evidence at all that shows a failure while WITHIN the published limits of the airplane? Did I miss something?
Fair question in the experiment world but as stated before several times in this thread just be aware you are now a test pilot. And for sure you need to go back to phase 1.“Don’t do that” is very simplistic. I’m sure those that have oversped their aircraft had not done it intentionally and may have some inexperience or had made a critical error. It is true these aircraft are slippery and point the nose downhill for an aircraft cruising reasonably close to VNE puts it in a “be very careful camp”…..so the question remains why have 7 rudders fallen apart in high speed events due flutter but not on 8’s as I am sure many 8’s have experienced overspeeding but none have crashed? Are we ok to ask the question does the 9 rudder actually suit the 7 given the different speeds these aircraft fly at? Is the 8 rudder stronger….putting aside the spin characteristics……it’s a fair question.
“Don’t do that” is very simplistic. I’m sure those that have oversped their aircraft had not done it intentionally and may have some inexperience or had made a critical error. It is true these aircraft are slippery and point the nose downhill for an aircraft cruising reasonably close to VNE puts it in a “be very careful camp”…..so the question remains why have 7 rudders fallen apart in high speed events due flutter but not on 8’s as I am sure many 8’s have experienced overspeeding but none have crashed? Are we ok to ask the question does the 9 rudder actually suit the 7 given the different speeds these aircraft fly at? Is the 8 rudder stronger….putting aside the spin characteristics……it’s a fair question.
If there were something noteworthy to add, yes.
So far this is nothing but conjecture that "something is wrong" - but it's all based on pilots exceeding the published limits of the airplane. Vans will most likely not make any comment at all on accidents where the pilot exceeded the limits of the airplane - the cause of the problem there is obvious. Don't do that.
Is there any evidence at all that shows a failure while WITHIN the published limits of the airplane? Did I miss something?
...so the question remains why have 7 rudders fallen apart in high speed events due flutter but not on 8’s as I am sure many 8’s have experienced overspeeding but none have crashed? ... Is the 8 rudder stronger….putting aside the spin characteristics……it’s a fair question.
So here what we have is a product with the same specifications but which stands apart from it's brethren in a particular failure. It is used, and respected equally by the user population but has a higher failure rate. This is not conjecture it is a statistical fact.
“Don’t do that” is very simplistic. I’m sure those that have oversped their aircraft had not done it intentionally and may have some inexperience or had made a critical error.