VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.






VAF on Twitter:
@VansAirForceNet


Go Back   VAF Forums > Main > Safety
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #51  
Old 08-18-2022, 03:25 PM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Southwest, USA
Posts: 2,519
Default Vans problem

Quote:
Originally Posted by jrs14855 View Post
The Polen Special is derived form the Midget Mustang I which was designed by a Pier Engineer in the 40's. The Polen flew successfully for many years and then developed rudder flutter. The fix was a carbon fiber rudder.
Years ago I met a Harmon Rocket builder/pilot who bragged about flying at 300 indicated. It would seem that if the -7 does have a rudder problem the first thing to consider would be the construction and balance of the rocket rudder.
Again apples to oranges but since the starting point for the Harmon Rocket is RV4 parts, maybe John Harmon and other HR builders know things that would be applicable to the RV7.
So this is Van's issue, if there is an issue. When it comes to flutter and adding rudder mass, I can see where this gets complicated real fast. I don't want to be a test pilot for someone else's TLAR (That Looks About Right) engineering. JMHO

Although the RV 7/7A/9/9A are as easy to fly as a Cessna 152 or 172, they are not the same; they are one level up from the trainer market and their limits must be respected. For example, I don't think anyone can ever exceed Vne in a 152, even point straight down at Max RPM. But Van's RV's, Piper's, Cirrius, and more advance Cessna's will.

When someone finds an RV accident where the operator did not exceed the recommended operating limitations, then I will get a little excited. YMMV
__________________
John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Dues paid 2022, worth every penny

RV9A- Status:
98% done, 2% left to go
To Go: wing mounting, engine baffles, wing tips, move to airport
www.pilotjohnsrv9.blogspot.com

Last edited by PilotjohnS : 08-18-2022 at 03:31 PM.
Reply With Quote
  #52  
Old 08-18-2022, 03:39 PM
Z-EDD's Avatar
Z-EDD Z-EDD is offline
 
Join Date: Oct 2013
Location: Harare, Zimbabwe
Posts: 241
Default Better the devil you know.

Quote:
Originally Posted by BillL View Post
Could you help the flutter-ignorant engineers with some understanding please. I thought that decreasing mass, moment, and increasing stiffness of a flight control would always extend the flutter speed. Given fixed design otherwise.

I do understand the aft fuse, HS, VS structures are all involved the dynamics, and one time at low altitude I hit a burble at 130kts (straight and level) and the tail seemed to shake like a wet dog. Nothing like this has happened in 250 hrs since. It would make me happy if there was a full ground vibration test performed, but that is out of my control.
Ok, so I'm not a learned engineer, but I have been involved in aviation for several decades and thousands of hours. My understanding is that it is not as simple as this. If it were, it would be easy for designers to avoid flutter, but that does not appear to be the case.

IMHO the structural design has little to do with the problem, other than stiffness having some effect on when flutter occurs. Talk of 'unzipping"rudders is misguiding. Flutter causes destructive loads, if this does not occur on the trailing edge, then it will someplace else.

What we have here with the RV-7 is a mature design with significant service experience that shows that the rudder flutters at a fairly consistent speed significantly above VNE. Owners fitting anything other than the standard rudder built to plans are IMHO going into test pilot territory. You may get an improvement in flutter margin, but you just as easily may make it worse. If there is an improvement, then who is to say that something else will not fail shortly after? Most airplanes see downward failure of the horizontal stab as the first failure above VNE.

Training and superior judgement to avoid the situation in the first place is the answer IMHO.
__________________
Ed Fleming
RV-7- Flying as of 12/23/20
Harare, Zimbabwe
Reply With Quote
  #53  
Old 08-18-2022, 03:48 PM
Z-EDD's Avatar
Z-EDD Z-EDD is offline
 
Join Date: Oct 2013
Location: Harare, Zimbabwe
Posts: 241
Default

Quote:
Originally Posted by Snowflake View Post
Posts like this worry me. The point of the counterbalance is to balance the rudder, just like the elevators.


Nor is mine. But it's a different design, with different size and shape and different speed limitations on the airframe. Van did switch to a counterbalanced rudder on the -6 before they stopped selling the kit, as part of the development of the -7. A number of -6's have been upgraded to that tail, or the -8 tail, to get increased rudder authority.
Please don't be worried Rob. The -7 rudder has a counterbalance weight, but it is NOT designed to be fully balanced. It would take a significant amount of additional lead to do so. Would adding more lead improve the flutter margins? I don't know, but I'm not going to go into test pilot mode to find out.
__________________
Ed Fleming
RV-7- Flying as of 12/23/20
Harare, Zimbabwe
Reply With Quote
  #54  
Old 08-18-2022, 04:55 PM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Southwest, USA
Posts: 2,519
Default difference?

So does flying with your feet on the rudder pedals vs with your feet off the pedals, make a difference in rudder flutter margin?

Do the flutter test pilots do the testing with their feet on the pedals, and do we all fly around with our feet off the pedals???

Seems like feet on the pedals would be more effective than more counterbalance weight.
__________________
John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Dues paid 2022, worth every penny

RV9A- Status:
98% done, 2% left to go
To Go: wing mounting, engine baffles, wing tips, move to airport
www.pilotjohnsrv9.blogspot.com
Reply With Quote
  #55  
Old 08-18-2022, 05:36 PM
RV8JD RV8JD is offline
 
Join Date: Jan 2005
Posts: 619
Default

Quote:
Originally Posted by PilotjohnS View Post
So does flying with your feet on the rudder pedals vs with your feet off the pedals, make a difference in rudder flutter margin?

Do the flutter test pilots do the testing with their feet on the pedals, and do we all fly around with our feet off the pedals???

Seems like feet on the pedals would be more effective than more counterbalance weight.
Adequate fin/rudder flutter margins should not depend on whether the rudder pedals are restrained or not. That is where proper flutter prevention design comes in, e.g., adequate torsional stiffness of the surfaces, proper amount of mass balance weight & location, etc.

Flight flutter testing is usually conducted "stick-fixed" and "stick-free" (including the rudder), but mostly "stick-free", to demonstrate that proper flutter margins exist.

Re: "Seems like feet on the pedals would be more effective than more counterbalance weight." It's not!
__________________
Carl N.
RV-8
KAWO

Last edited by RV8JD : 08-18-2022 at 06:04 PM.
Reply With Quote
  #56  
Old 08-18-2022, 06:11 PM
BillL's Avatar
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 6,838
Default

Quote:
Originally Posted by rockwoodrv9 View Post
Would it be as simple as producing a skin that folds at the edge rather than being riveted? How about a piece of .062 or even thinner bent to scab on over the trailing edge?
Don't know about an aluminum design Rock, and I don't know that a carbon part could even be made that would be 1. equal aerodynamically, 2. lower in mass, 3. stiffer in every mode, and 4. lighter in overall weight and 5. shorter CG with the same balance mass of the 6/7/8/9. Pretty tall order.

Based on Lancair rudder construction, it appears possible, but by an expert.

The Vne for the 7 is quite a bit higher than the 9 already, so I think you have a better margin staying within the 9 limits. With the 7 the only model having so many inflight structural failures (what ever the reason) and the 9 having none, you should sleep well.
__________________
Bill
RV-7
Reply With Quote
  #57  
Old 08-18-2022, 06:49 PM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 11,054
Default

Quote:
Originally Posted by rockwoodrv9 View Post
I'm looking for a way to strengthen the zipper. If I took a 4" piece of .062, bent it to about 160 degrees (not sure if that is the correct bend) and then epoxy or rivet to the trailing edge?
It would move the rudder CG rearward, which is probably a very bad idea.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #58  
Old 08-18-2022, 07:36 PM
rockwoodrv9 rockwoodrv9 is offline
 
Join Date: Dec 2011
Location: Meridian ID, Aspen CO, Okemos MI
Posts: 3,203
Default

Quote:
Originally Posted by DanH View Post
It would move the rudder CG rearward, which is probably a very bad idea.
That is a very good point that makes sense now that you mentioned it. Thanks
__________________
rockwoodrv9a
Williamston MI
O-320 D2A
Flying N376E
Reply With Quote
  #59  
Old 08-18-2022, 07:46 PM
PaulvS's Avatar
PaulvS PaulvS is offline
 
Join Date: Aug 2019
Location: Western Australia
Posts: 1,277
Default Pilots

Isn't the simplest solution for pilots to respect Vne and Vno (and a couple of other design limit numbers)?
__________________
Paul vS (yes I'm also a Van)
Building RV-6A #22320 O-320 FP. Wings and tail complete, fuselage almost done, working on canopy.
Flying my Aeroprakt A-22 STOL and the aero club's RV-9A while I build
Reply With Quote
  #60  
Old 08-18-2022, 09:05 PM
sailsunfurled sailsunfurled is offline
 
Join Date: Oct 2021
Location: Gulf Breeze,FL
Posts: 30
Default

Why not use a different rivet on the trailing edge? Un-zipping may be only on the shop end? That the side of the rivet process that depends on the builder to "get it right" with mushroom the shop end? Do not know?

Quote:
Originally Posted by rockwoodrv9 View Post
I understand the reason. Shipping for all the tail kits that go out is big too. Im looking for a way to strengthen the zipper. If I took a 4" piece of .062, bent it to about 160 degrees (not sure if that is the correct bend) and then epoxy or rivet to the trailing edge? That would strengthen it considerably and maybe help with AV type wind damage too.

I flew today and on my walk around, I tried flexing the rudder like it would in the wind. It feels pretty solid, but I can see how a wind gust could weaken it and after several smacks, I could see it weakening. I imagine flutter do the same thing in seconds.
__________________
RV-7A in progress
2022 Dues Paid
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 09:08 PM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.