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  #11  
Old 11-29-2006, 11:51 AM
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gmcjetpilot gmcjetpilot is offline
 
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Default Odd

Quote:
Originally Posted by MikeJ 7A
Are there different limits based on rolling G's for RV's? MikeJ
No, I think Van calculated the envelope for worse case, which may or may not include aileron hinge moment.

There are many overlapping flt conditions. So the typical simplifying method engineers use is just calculate the worse case or case's and superimpose them and design for that. This gives you extra margin since many conditions may not happen exactly at the same time. These "artificial" design loads keeps from having to calculate stress and strain for every different conditon, just a few critical ones which covers all cases. In the day before computers this was common. Still is. The down side is you may overbuild some parts and add extra weight. Most parts on planes are designed to the critical load (which already has a 1.5 safety factor on it) and still have extra margin. Some parts may be way stronger than needed just for practical reasons. On the other hand some parts may be just strong enough with a zero margin. We have that 1.5 factor. In our case 9 G's verses 6 G's. You are not to exceed 6g's even though the plane is designed for 9g's. The extra margin is to cover every ones back sides, manufacture and material variations and pilot mess ups. Its not their to use intentionally.

With that said it's unusual to have rolling G limit and "symmetrical G limit" in civilian planes. There is a difference but usually its rolled into one conservative limit. I suspect its possible they messed up and "found" a load case they forgot to include that was slightly more critical. Many times a new plane design is being built and tooled while the final analysis is being done and changing the structure is difficult, expensive or impossible. It's possible they made a decision to make this distinction and save weight? Its also possible pilots where finding new ways to break the wing. May be North American was designing to a fine degree? I know everyone that flew the T-38 remembers them fondly.
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Last edited by gmcjetpilot : 11-29-2006 at 12:09 PM.
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  #12  
Old 11-29-2006, 12:06 PM
amilder amilder is offline
 
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Quote:
Originally Posted by JonathanCook
...I know that this goes against what Van's says, but this kind of failure is probably due to flutter...
I would think that if properly-designed wings were fluttering to the point of destruction, all the control surfaces should have long since parted ways with the airframe. If the RV-8 wing is so poorly designed as to destructively flutter before the control surfaces, I would think there would be a lot more wrecked RVs littering the countryside
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Last edited by amilder : 11-29-2006 at 12:14 PM.
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  #13  
Old 11-29-2006, 12:23 PM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by amilder
I thought the term "flutter" was reserved for control surfaces? I would think that if properly-designed wings were resonanting to the point of destruction, all the control surfaces should have long since parted ways with the airframe. If the RV-8 wing is so poorly designed as to destructively resonate before the control surfaces, I would think there would be a lot more wrecked RVs littering the countryside
I understand what Mr. Cook is saying but on a RV the wings are so stiff they are not going to flutter.

Wing flutter is a real deal but on those big floppy swept jet wings with engines hanging off them. I have seen air tunnel video of a B-747 wing and it was amazing and scary. I fly a large twin (B757) but sitting in the back of a B747 in turbulence you can see the two engines on one wing getting into weird synchronized motion or going opposite directions. Its fascination and OK as long as the deflections do not get divergent.

Flutter is not just a natural frequency issue its also an aerodynamic issue, this science is called "aeroelasticity". It's both harmonics, structural natural frequency and how it reacts works aerodynamically. There was the infamous case of the Lockheed Electra, that got whirl flutter. The engine and engine mount set up harmonics that got the wing oscillating and with the aero loads went divergent, wing gone. Just simple stiffening of the engine mount solved the problem. The wing was fine.

A friend does this "aeroelasticity analysis" and they actually add weight in the wing structure, not for strength but to tune flutter out. It was frustrating when the aero guys said add more material when it was not structurally needed.

However the RV wing is so rigid the natural frequency so high and well damped its not and issue in the flight region we fly in. Our balanced flight controls are also fairly immune if flown with in limits. Flown outside limits you are a test pilot.

(Trivia: some flight controls in Boeing jets use depleted uranium for flight controls to get the mass needed in a small space.)
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Last edited by gmcjetpilot : 11-29-2006 at 12:28 PM.
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  #14  
Old 11-29-2006, 12:24 PM
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rv6ejguy rv6ejguy is offline
 
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Straight and rolling G loadings are way different. Many older military aircraft have specific limits for both. Northrop built the T-38 by the way.
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  #15  
Old 11-29-2006, 12:32 PM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by rv6ejguy
Straight and rolling G loadings are way different. Many older military aircraft have specific limits for both. Northrop built the T-38 by the way.
What does that mean. You mean the stress is different in the structure. G loading is G loading at the CG of the plane.

Aileron's cause more (or less) camber which increase lift. They also cause wing twist and torsion from their hinge loads. All this adds and superimposes with the G load, however most planes wrap all their worst load cases together for one load limit. A plane with large span or high rolling inertia can add loads to the wing by "pulling" and than throwing full aileron into it, GA planes not so much for many reasons. Different planes have different critical conditions.

The Boeing 767 has 4 ailerons. The out board ailerons LOCK out at higher speeds to reduce loads. Other jets limit control surface deflection at high speed. On new jets, F16 the computer limits the loads not the pilot.

Northrop made the T-38? Doha! I did not know about the older military planes had split G limits. I know some GA planes have a flap down flap up G limit.
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Last edited by gmcjetpilot : 11-29-2006 at 12:49 PM.
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  #16  
Old 11-29-2006, 01:07 PM
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billdianne billdianne is offline
 
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I may be dreaming , but I thought I read somewhere that Vans modified the spar after the accident by extending the reinforcing bars past midspan because of the weak point caused the outboard fuel tank and wing panels converging there.
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  #17  
Old 11-29-2006, 02:46 PM
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Default Spar change

The spar was modified, although I'm not sure when. We were told one was no better than the other (Really, then why was it changed???) The earlier spars have an aerobatic limit of 1550#, the new ones 1600#. Going by memory, I think these numbers are right.
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  #18  
Old 11-29-2006, 02:57 PM
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Quote:
Originally Posted by billdianne
I may be dreaming , but I thought I read somewhere that Vans modified the spar after the accident by extending the reinforcing bars past midspan because of the weak point caused the outboard fuel tank and wing panels converging there.

I'm not sure I'd call it a "weak point." Maybe a better way to say it is an "area of four high stress locations."

Check this thread

http://www.vansairforce.com/communit...ead.php?t=9774

I assume the post by Charles Kuss is an accurate description of the RV8 wing.

Notice Van made this change to provide more margin even though the original RV-8 wing design was tested and shown to successfully meet design limits. That gives me a lot of confidence in Van's abilities as a design engineer.

(Update: This thread has educated me...the wing spar design tested in the NTSB investigation was apparently not the same wing spar design as used in the ill-fated RV-8, N58RV. However, my faith in Richard VanGrunsven as an aircraft design engineer remains unshaken. I originally chimed in to reply to a post erroneously stating flutter was the cause of the N58RV crash.)
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Last edited by rv7boy : 11-30-2006 at 10:35 AM. Reason: Added update note in regard to tested wing configuration
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  #19  
Old 11-29-2006, 06:54 PM
cruzer cruzer is offline
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Default overbuilt ?

I have been inspecting aircraft since 1978 and have found only 2 small cracks in certified aircraft engine mounts. The 12 years I worked at Vans there were 10 times plus that many accounts of cracks and damaged mounts on the factory demo aircraft. I also personally have welded 7 customer mounts for RV aircraft that had fatigue cracks in the dyna-cup rings and supporting tubes. It is dangerous for anyone to think any aircraft or part is overbuilt and therfore routine inspection of it isn't needed. Also for the record on the RV8 wing. The flutter test that was conducted at Vans was on the RV6A which had a different spar in it than the RV8 that crashed. The load test after the crash was done on a pair of customer wings that the builder abandoned, they also had a different spar than the crash aircraft. The crash aircraft had a different aileron on the right wing, it had a different D section shape. No one knows FOR SURE what happened. The speculation on this subject is just that.
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  #20  
Old 11-29-2006, 07:47 PM
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Sam Buchanan Sam Buchanan is offline
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Quote:
I have been inspecting aircraft since 1978 and have found only 2 small cracks in certified aircraft engine mounts. The 12 years I worked at Vans there were 10 times plus that many accounts of cracks and damaged mounts on the factory demo aircraft. I also personally have welded 7 customer mounts for RV aircraft that had fatigue cracks in the dyna-cup rings and supporting tubes. It is dangerous for anyone to think any aircraft or part is overbuilt and therfore routine inspection of it isn't needed. Also for the record on the RV8 wing. The flutter test that was conducted at Vans was on the RV6A which had a different spar in it than the RV8 that crashed. The load test after the crash was done on a pair of customer wings that the builder abandoned, they also had a different spar than the crash aircraft. The crash aircraft had a different aileron on the right wing, it had a different D section shape. No one knows FOR SURE what happened. The speculation on this subject is just that.

(This message is from a forum moderator)

Cruzer, please attach a signature to your posts.

Thanks in advance,
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Last edited by Sam Buchanan : 11-29-2006 at 07:58 PM.
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