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  #151  
Old 08-25-2022, 11:00 AM
PhatRV PhatRV is offline
 
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Quote:
Originally Posted by scsmith View Post
... In Rockets, and several Reno-racing RVs, the fwd fin spar gets a doubler that runs part way up (to the middle rib IIRC) which stiffens the fin in torsion (and also somewhat in bending). With this added thickness from the standard fin spar plus the doubler, a thicker attach bracket should also be used. The rear fin spar also gets additional strengthening/stiffening.
I wonder if any of the Reno-racing RV modified the vertical stabilizer by making it shorter or smaller. It seems the large standard kit size is for aerobatic flying and spin recovery but I suspect it works against the high top speeds that are seen at Reno. I've seen some of the Reno specific racers (non-RV) that have very diminutive tails.
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  #152  
Old 08-25-2022, 11:12 AM
scsmith scsmith is online now
 
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Quote:
Originally Posted by rv6ejguy View Post
Critical errors are often fatal in aviation.

Testing to 10% over Vne is pretty standard in the industry and Van's has done that.
FAR Part 23 requires 15%.
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  #153  
Old 08-25-2022, 11:20 AM
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Tandem46 Tandem46 is offline
 
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Quote:
Originally Posted by airguy View Post
This is not a Vans problem. This is a piloting problemmÖ
Yeah, EXCEPT this is not happening to the RV-3, RV-4, RV-6, RV-8, or even RV-7s equipped with the -8 rudder. Only RV-7ís with the -9 rudder.

Quote:
Originally Posted by airguy View Post
If the 7 scares you, don't fly it.
Yeah, I donít think thatís the issue here.
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  #154  
Old 08-25-2022, 11:30 AM
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RV8JD RV8JD is offline
 
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Quote:
Originally Posted by rv6ejguy View Post
Critical errors are often fatal in aviation.

Testing to 10% over Vne is pretty standard in the industry and Van's has done that.
Quote:
Originally Posted by scsmith View Post
FAR Part 23 requires 15%.
Actually, for Part 23 airplanes, the airplane is to to flutter free to 1.2 Vd by analysis (20% above Vd). Part 25 requires only 15%.

Flight flutter testing is done to Vd, which is 10% above Vne (1.1 Vne), to demonstrate that adequate modal damping margins exist at Vd, and that there is no large and rapid reduction in damping as Vd is approached.

Of course, Experimental airplanes are not governed by Part 23, but Van's has stated they test to Vd.
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Last edited by RV8JD : 08-25-2022 at 12:11 PM.
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  #155  
Old 08-25-2022, 12:08 PM
jrs14855 jrs14855 is offline
 
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Default Dive Testing

There is of course no requirement for EAB to test to part 23 or part 25 requirements.
The Wittman Tailwind in 1953 was the first EAB to be allowed to carry passengers. The CAA(FAA) requirements were a dive test to 110% of Vne and a 4G test while loaded to gross weight.
The first five Tailwinds were tested to this standard.
On one of those tests a sandbag slipped out of place and pushed against the stick. The resulting dive was near 300 statute indicated and the G meter showed plus 8.3 and minus 3.8. Minor bend in windshield support, no other structural damage.
The elephant in the room is the question: was the RV7 ever dive tested with the RV9 rudder???
Why has Van's and or the FAA not canceled the aerobatic certification for the RV7 with the RV9 rudder?? Why have they not reduced the Vne of the 7 with 9 rudder as a safety precaution?
RV4 and 6 zero structural breakups. RV8 one wing failure caused by G load far in excess of design limits.
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  #156  
Old 08-25-2022, 12:21 PM
Desert Rat Desert Rat is offline
 
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After watching this thread grow to 16 pages with great interest, I've decided to chime in with a few thoughts, in no particular order;

if you beef up one area, you are frequently just transferring the initial failure to an adjacent area

slippery airplanes can reach Vne in a heartbeat if you're inattentive, especially when you are coming out of something that can't reach Vne unless you point it at the center of the earth.

flutter margins aren't something to be trifled with, nor are they something to be improved upon unless you have a big brain plus the ground and flight test equipment to back up your proposed fix.

A few of the modifications that are being talked about here aren't something that we would do in the certified world without a ton of modeling and static test articles, followed up by flight testing with an emergency egress system, pilots with chutes, and sometimes spin chutes on the flight test plane.

Yeah, I get that with a big enough statistical sample, you can predict X number of events over time, and this statistic seems to say that if you overspeed an RV7 you can rip the rudder off before the rest of the tail comes apart.

This set of statistical data would also seem to indicate that the builder has a choice; he can either install a big rudder and have smaller flutter margins (maybe) or install a small rudder and have poorer spin recovery characteristics. Pick one-live within the constraints of your choice. It's obvious where Van stands on this because back in the day they ponied up free rudders for everybody.

I'm going to take the stance that there isn't anything inherently unsafe about a well proven design, and be thankful that it's been in service long enough to identify potential danger areas to stay away from. Is it as forgiving as a 172? In this particular area, probably not. But you know what, it's not as forgiving of being left out in the rain either. That doesn't make it a bad airplane, just different considerations.

I could look at any two airplanes and pick them apart as to which one is better and which one is worse. A lot of you guys are old enough to remember when the first Citation came out and the Lear guys started saying that it was so slow that it was going take bird strikes from the rear. It wasn't very long before the Citation guys started saying that a Citation was doing everything it could to keep you alive and a Lear was doing everything it could to kill you.

My point is that like most things in life, you have to weigh the pro's and cons, mitigate the cons while being confident that the cure isn't worse than the disease, then live with the result.

This is an interesting discussion for sure, but as for me, I'm going to accept that my airframe has some well documented areas that need to be respected, with different considerations that a Cessna (or whatever). I'm building the airframe to it's well proven design, with the big rudder that Van's recommends/supplies and will try to live within those constraints.

edit- I have one final thought here; I've seen some comments about modifying stuff and the rebuttal has been "you're going into test pilot mode" followed by somebody else chiming in; "you're in test pilot mode when you're in phase 1" or something to that effect, implying that these two things are somehow equivalent.

I'll respectfully opine that no, in phase 1 you're in CONFORMITY pilot mode. You're confirming that your particular airplane behaves the way everybody is expecting it to, because that's the way it was originally designed and built to behave. If it's out of conformity with the rest of the fleet, then you try to figure out why and fix it, but it's extremely unlikely if you build to plan that something is going the be catastrophically out of whack.

In this case, it would seem to hold true whether you use the small rudder or big one you're in safe, well documented territory. They are both known quantities with known characteristics. But, If you go off the reservation with extra stiffeners or balance weights or otherwise start monkeying around with your own "improvements" without knowing what you're doing, you're truly in test pilot mode, which isn't where most of us want to be without significant training and safeguards in place.
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Last edited by Desert Rat : 08-25-2022 at 12:33 PM.
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  #157  
Old 08-25-2022, 12:25 PM
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rv6ejguy rv6ejguy is offline
 
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Quote:
Originally Posted by scsmith View Post
FAR Part 23 requires 15%.
Thanks Steve for that info. I think Van's mentioned testing to 220 KTAS AFAIK so that's a bit short of the FAR 23 requirement.
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  #158  
Old 08-25-2022, 12:32 PM
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RV7A Flyer RV7A Flyer is offline
 
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There's an awful lot of talk about statistics and failure rates and whatnot, but you know what I haven't seen?

Actual statistics and failure rates and other numerical data.

How many RV-7s are there? How many with each rudder option? How many RV-8s?

Now, the claim is that there are 0 RV-8 rudder flutter-induced fatalities. Maybe, but let's accept that for now.

How many RV-7 flutter-induced fatalities? Below Vne? Above Vne? With which rudder?

I've seen nothing to convince me that there's any problem with the larger rudder other than when a pilot grossly exceeds Vne there's an increased risk of flutter-induced failure. No s**t, Sherlock, as we used to say.

I'm with the poster who asked to see any evidence of rudder issues on any aircraft flown within the envelope. Haven't seen it yet.
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  #159  
Old 08-25-2022, 12:58 PM
jask jask is offline
 
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Quote:
The kit manufacturer had conducted flight testing of the RV-7A prototype. No indications of flutter were encountered at a speed of 217 knots. Additional theoretical flutter analysis was done where the flutter speed was calculated to be 300 knots for the baseline design


This is from a previous thread. It looks like the initial testing with the 8 rudder looked pretty good.
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  #160  
Old 08-25-2022, 05:27 PM
jrs14855 jrs14855 is offline
 
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Default Arlington, AZ RV7

Some interesting highlights from the NTSB Probable Cause:
"cockpit canopy, vertical stabilizer and rudder were found about 1 mile from the main wreckage".
Probable Cause:
Inflight overstress separation of vertical stabilizer and rudder during flight-----Contributing-----was an inflight collision with a bird. (end)
"The RV7 is a two place, tandem seat,------(can't they do any better than this??)
Comments: No discussion of why canopy and vertical tail were all found together. No discussion of how bird feathers might have gotten under the passenger seat. No discussion of size/weight of bird which was identified as Rock Pigeon which is under one pound. No discussion of aircraft speed but a detailed discussion of ground track.
They do mention that the overstress could have been a startle reaction to the bird(s)
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