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Exactly how strong are RV's?

N941WR

Legacy Member
This past weekend my father-in-law and I took a walk down the flight line at his local airport (Collagedale, TN) and we came upon a man doing an annual on O-300 powered C-172. The cowling was off so I spent some time looking at how Cessna installed that engine. What struck me was how small the engine mount tubes were compared to what is on my -9. They looked to be about the same size as the RV's aileron push tubes.

This really had me thinking about how over built / engineered our planes are. Don't get me wrong, I think this is a good thing because Van has no idea who and how these things are really put together.
 
I'll venture in here...

I don't know if Van redesigned and made a specific engine mount for your rv9, since they share a lot of components with the RV7, but on the aerobatic RV versions you have a huge heavy engine way out front that needs to be supported by those bars when you pull 6G's without deforming. OK up to 9Gs before breaking.

It is a little different in a Cessna, I think I remember reading 3.8G's max somewhere..

So a long story short, just by visualising the thickness, don't assume it is overbuild. Only a stress analysis will really tell you what is going on, especially on the engine mount supporting a big weight at one end.

Secondly the engine mount comes already assembled, so I doubt that any home builder error can creep in there, so there is no need to over design it.

Just my 2c

Regards
Rudi
 
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Very strong

Bill,
I read the report about the -8 that lost a wing. Van collected the pieces and took them to a business in California that scientifically analyses these things with all sorts of gauges and gadgets that I've never heard of....specialists in their field.

The wing broke where the computer said it would, near the outboard end of the wing tank, not inboard, and the conclusion was that the metal revealed at least a 9G pull, or greater!! :eek:

The grapevine story goes something like..."He fell out of a maneuver, split essing out the bottom and used the wing removal tool in his right hand to do just that."

Sadly, the pilot asked more of the airplane than the 9G's it had been designed to. Yes, they're very sturdy little rocketships.

No, they're not indestructible.
 
Wings breaking after fuel tank?

What happened to the 8 wing that broke after the fuel tank does not happen from pulling too many Gs. I know that this goes against what Van's says, but this kind of failure is probably due to flutter. Any airplane designer will tell you that the maximum stress in a cantilever airplane wing is located at the root of the wing. This is where the wing and the fuselage meet. The wing itself is equally loaded along the length of the wing and therefore the bending load (this is the load that fails the wing) becomes less as the distance from the fuselage gets greater.

Flutter happens when vibration is equal to the natural frequency of airplane or any part of the airplane. When a part or assembly is vibrated at its natural frequency then the part vibrates in such a way that the amplitude of the vibration theoretically go to infinity. Therefore the vibrations induced at flutter destroy the part. If you have seen the movie where the Tacoma Narrows Bridge vibrates to destruction, then you have seen flutter. The wind does not move the bridge up and down, but the frequency of the wind passing over the bridge is the bridge?s flutter frequency. At this frequency the vibrations go to infinity (in theory) and the bridge self destructs. Here is the link to the bridge vibrating. http://www.curee.org/projects/woodframe/element5/modules/seismic/images/tacnb.mpg

Complex assemblies have many flutter frequencies, but most or all of these frequencies are above the maximum speed useful to the assembly. In our case most or all of the flutter frequencies are above top speed of the airplane. In some Lycoming engines you can not run the engine at specific speed ranges. This is because one of the flutter or natural frequencies of the engine is at this speed. This is an example where the first natural frequency is below the operating speed, but safe operation is between flutter frequencies. At Pratt our engines had many speed ranges where operation was not allowed.

The question people ask is how do you fix flutter problems or stop flutter from happening. To stop flutter you must change the natural frequency of the part. In this case the natural frequency of the wing. This is not accomplished by making the wing stronger. To change the natural frequency the stiffness of the wing must be changed. Again stiffness is not strength. Changing the frequency can be accomplished by changing existing parts in the current wing. It could be changing the location of ribs or adding parts to change the stiffness of the wing. The wing does not need to become stronger just the stiffness has to be changed. If any ones says that there wing is strong enough so that flutter is not a concern then they do not understand the cause of flutter, because any wing weather on the RV7,8 or the SR71 will fail at the wings flutter frequency.

In the case of a wing there are many frequencies. It appears that the RV8 wing broke mid span therefore this appears that the wing broke at the first flutter or natural frequency. If the wing had broken at 1/3 or 2/3 the span the flutter or natural frequency would have been the second natural frequency.

I am I concerned about flutter on my 7? The answer is no, because I am not going to be flying my 7 above top speed which is most likely below the speed necessary to cause flutter.

Any more questions can be discussed here or my email jonathan.cook (AT) symech (dot) com.
 
JonathanCook said:
Any airplane designer will tell you that the maximum stress in a cantilever airplane wing is located at the root of the wing. This is where the wing and the fuselage meet. The wing itself is equally loaded along the length of the wing and therefore the bending load (this is the load that fails the wing) becomes less as the distance from the fuselage gets greater.
And, Van, like any aircraft designer, knows that for the lightest weight, you should reduce the amount of material in the wing spar as you go further outboard, as the loads are lower. Ideally, the wing spar material would be tapered, gradually reducing in strength as you go out the span, so every part of the spar was equally stressed. In this case, you cannot predict exactly where along the spar it will fail.

Take a look at an RV spar. You will see that it is beefiest near the root, and the amount of material is reduced in several steps as you go outboard.
 
It's not what Van says....it what the NTSB said.

Jonathan,

Perhaps you have a better insight than the NTSB, but in their investigation of the subject accident, they reported the following:

"...The main spar had evidence of a ductile fracture due to a positive overload. The spar material met design specifications for metal composition and hardness. There was no evidence of fatigue or corrosion. The outboard section of the left wing did not exhibit any evidence of aeroelastic divergence. A flutter test showed the aircraft design was free from flutter to speeds above its design envelope. Wing load testing showed the wing design was able to support a limit load, +6 g's. The wing also supported an ultimate load, +9 g's, for 3 seconds without failure..."

They also estimated the airplane to have been loaded at 89 pounds OVER the MAGW for aerobatic performance for the RV-8 at the time of the failure.

They also determined from the nonvolatile memory of the engine management system that the engine rpm and manifold pressure were equivalent to the power required for level flight at 191 mph as opposed to maximum maneuvering speed of 142 mph.

There were a lot of us at the time who were concerned that Van may have unknowingly "cut a corner" in the RV-8 wing design. When the NTSB published the probable cause, it was very obvious that one of the two pilots, whether intentionally or accidentally, yanked the wing off that plane.

Your conjecture of flutter doesn't support the physical evidence of the RV-8 wing failure let alone the fact that thousands of RV-7 and -8 wings have flown thousands and thousands of hours without occurrence of flutter.

According to the NTSB the probable cause(s) was:

"the intentional or unintentional sudden application of aft elevator control by an undetermined aircraft occupant that exceeded the design stress limits of the aircraft. The aircraft gross weight, which exceeded the maximum allowable for aerobatics, and airspeed, which exceeded the maximum maneuvering speed for the weight, were factors in this accident. "

I feel confident the NTSB report provides a thorough and accurate portrayal of the failure mode. I further feel confident in Van's expertise as an aircraft design engineer. There was a lot of discussion in the RV Yahoo Group(s) at that time (1998) and the archives may still be available. I haven't checked but I do remember the discussions.

I do agree with your position that flutter is a phenomenon that is to be respected, but I don't agree that the subject RV-8 wing failure was due to flutter.

Perhaps we are talking about two different RV-8 accidents. The NTSB report I refer to can be found at:

http://www.ntsb.gov/NTSB/brief.asp?ev_id=20001211X10121&key=1

and is NTSB report number LAX98FA171. It's the only report of an RV-8 wing failure I'm aware of.
 
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I am not aware of any root wing failures on the RV6 through RV10 which were built as per the plans. Any of the overload accidents that I have read about have resulted in stab or outer wing panel failures. The short winged RVs have an aileron pushrod hole in the rear spar at the junction of the flap and aileron plus the tanks end at about the same span position. The spars become thinner just outboard also. This seems to be the point of failure when overstressed. I understand in the RV8 accident that it was over the allowed aerobatic weight and above maneuvering speed but well below Vne. Full stick deflection under these conditions will result in a structural failure.
 
No worries, well not completely, there's a pilot involved

The most dangerous part of the RV is between your ears. :D

ENGINE MOUNT, MINES BIGGER THAN YOURS
If Cessna wants to use bigger tubes for an engine mount, fine, but what is the wall thickness? Was it a Continental engine cradle mount? Engineering is science, art and commerce. There is no telling why you had the impression Cessna's engine mount was more beefy? I can tell you the engine mount on a RV is built like a brick outhouse. I know of no real issues with the RV engine mounts. There have been cracks however, but that is not common with any engine mount. There's a FAA publication for mechanics of critical service reports from other mechanics submit on all kinds of aircraft all over the country and world. In some cases RV's show up, but it's mostly your typical Beech, Cessna, Piper and larger commercial aircraft. Many reports involve potentially serious cracks in control systems and even things like engine mounts. I can tell you all planes have issues from time to time with their engine mounts, but by all means during annual and every time you have your RV cowl off, look at the engine mount. If you don't know how to do inspections or look for cracks, ask a A&P to show you. Pay special attention to the welds and use magnification and good light. Just so you know there is lots of redundancy on the engine mount and one crack will not cause you to lose the whole engine. If you look at more engine mounts on other aircraft, they can look real weak. The RV engine mount looks very strong to me.

I've been messing around with RV's for 18 years and have a back ground in aerospace structure. RV's are some what overbuilt. Certainly the engine mount is not an issue. I've also had the pleasure of being a CFI in my early flying career, flying probably +20 different models of GA aircraft from 6 or more manufactures. I can say the RV strength is not in question.

Let me tell you a story. Ladies have that sixth sense, God bless them. This one lady flew a lot in different GA planes, like Cessna's with her husband, albeit as a white knuckled passenger. When she transitioned to the RV her husband built, she loved it! :D It just felt more solid, especially in turbulence, and she could see more. From a pilot stand point sloppy cable controls, sounds of airframe oil canning (clunk-clunk) and doors popping open from airframe flexing does not build confidence. I have experienced all those scary things in factory planes, but not RV's.


LIGHT DUTY?
There are parts on the RV that are not as robust as they could be, but they where designed that way intentionally for performance. The gear comes to mind. It's designed for the purpose landing and taking off, not smashing. It works perfectly but not OVER BUILT. A C-152 has stronger gear. Still the RV when abused do their job and may have an advantage of not causing greater damage to the plane. I've heard about bent RV gear from landing accidents that are replaced successfully without other airframe damage. A hard landing in a C-152 or C-172 tends to bend and wrinkle firewalls; They are not immune from a ham fisted pilot.

Another light duty items may be the canopy. The tip up is there to get caught in the breeze when open. The slider canopy is more resistant to wind when open on the ground, but its not like the door on your car. It's a design trade off for the beauty of the bubble canopy view.

Regarding the canopy, RV's can dig-in a flip, especially trikes (sorry that's my opinion) when landed off-field on soft unprepared surfaces. The bubble canopy we love, makes egress harder if you flip upside down. Every RV'er should have safety or emergency canopy "tools" to cut, break and dig their way out if they do flip. I don't worry about it, but the type of canopy we have has pros and cons. I have seen my share of Cessna's on their backs as well, but the doors should be easier to get out.


FAILURES?
Have there been inflight airframe failures? Yes, a few RV-3 wings have come off, most happened in the early 80's. There where almost no failures for a long time, than the last two I know of where in 1995/98. All of the accidents where either from builder construction flaws (mostly with early RV-3's) and/or pilot error exceeding G limitations. The RV-3 did go through several wing mods early on. which of course improved the bred and made it easier for builders to construct properly. The original design was adequate but Van basically add even more margin. Think about this, many factory planes rely on one bolt to hold the wing on. I've heard of RV's flying WITHOUT THE REAR SPAR BOLT INSTALLED. That's no rumor.

Of course there was the tragic prototype RV-8 wing failure in California. That was also determined to be an overload beyond the max allowed. The RV-4 by the way gets even lighter stick forces in pitch with a rear passenger. I assume the RV-8 is the same. This is what probably contributed to doom the RV-8 prototype: two big guys (over gross), friends who knew each other, one never flew a RV, the other pilots guard was down due to familiarity with the co-pilot and lighter controls with a rear passengers. Both had ag-crop dusting experience in heavy planes which have much higher control forces.


PILOT RESPONSIBILITY
With a fast high powered plane with light controls pulling wings off is possible. Just dive near Vne, pull as hard as you can, the wings will come off. I have seen airshow videos of serious aerobatic planes folding wings. Not pretty but it happens. Keep in mind a RV has an ultimate Pos G of 9, limit or operating G of 6.0........ 99.9% of normal "Gentleman's Aerobatics" can be done at or around 3 G. At 3 G's you have a factor of 3 to failure. Ultimate means the plane will not fail but may suffer permanent deformation. Limit load (6G's) can be experienced with out permanent damage. There have been more than one RV that went past 9 G's and lived, albeit in some cases with airframe wrinkles! :eek: You can pull the wings off a RV or any plane, Yep.

Cessna's benifit from being slow. The dynamic forces at 120 mph are much lower than 200 mph by near a factor of 3 times. Point a RV nose down and watch the airspeed wind up; low drag, high power, light controls is fun but carries responsibility. Considering the excellent safety record of the RV, its clear that that average pilots can fly RV's with little problems. Like I first said, most the danger is between the ears.

How many factory airframes have broke-up in flight? Beech Bonanzas, especially V-tails had a bad reputation for coming down in pieces. Most of it was pilots flying too fast or losing control in IMC conditions. Like the RV-3 wing, Beech re-re-designed the V-tail attachment to make it even stronger. Also like the Bonanza it's a high performance plane with low drag.


SUPER PILOTS NOT NEEDED, JUST SMART ONES
A Cessna is a truck and a RV is a racing sports car. It easy to get in trouble with a hot overpowered sports car, but trucks also crash. However in the right hands a sports car handles better, stops better and accelerates better and can be safer than a big truck, which can't turn or stop easily. I'd rather fly a RV than a little Cessna or Piper any day. One reason is greater performance, which if used properly gives greater safety margin. The ability to use less runway and climb faster to a safe altitude is a real plus.

Yes, you have to be careful not to pull the wings off any plane, especially high performance planes like RV's with lighter controls, but than again RV's have aerobatic strength airframes, verses a normal and utility category Cessna's.
 
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Not quite

Hi Jonathan,
The Bearcat had wings designed to break off at about 2/3rds span, four feet from the wingtip. The controls were very light and the engineers feared that a low time pilot in an air combat situation may overstress the wings so they built in a fail safe feature. This is the most convenient place that the racers use to shorten the wings. When they broke, the remaining wing would be spared a big amount of bending load...so yes, they can break far away from the root.

The Bear could then be flown home and landed safely with either one or both wingtips missing...so the flight manual states. ;)

The RV wings may not have a linear spar strength design as they move outward, the reason the -8 failed about half way out.

Regards,
 
Another thing to consider when loading up the aircraft is the difference between symmetrical loads and unsymmetrical loads ("rolling G's").

An aircraft pulling 6 G's with ailerons deflected will not distribute that load evenly between both wings, right?

As I recall from my T-38 IP days, we had lower G limits for unsymmetrical loads.
For lighter weights the limitis were -2.9 to +7.2 for symmetrical and 0 to 5.1 for unsymmetrical.

Are there different limits based on rolling G's for RV's?

MikeJ
 
Odd

MikeJ 7A said:
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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JonathanCook said:
...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 :eek:
 
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amilder said:
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 :eek:
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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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.
 
Thanks

rv6ejguy said:
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! :D 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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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.
 
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.
 
billdianne said:
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/community/showthread.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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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.
 
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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Rolling G

Conventional wisdom, which comes from FAR Pt23 I believe, is 2/3 maximum symmetrical g with full aileron deflection at Va.

re. earlier discussion, I have a finite element analysis of my G-200 wing which shows the highest stresses at about 40% span.
 
Incidentally, if you want to speculate on ANYTHING it's interesting to note that the NTSB found a broken elevator trim clevis. I think they also found the trim motor at full extension (i.e. full up tab...full DOWN trim). It's not a stretch to think that plane was trimmed full down for whatever reason, the clevis broke and suddenly all that stick force that went into maintiaining level flight caused a massive pitch up and subsequent wing failure.

I don't know...I haven't read the NTSB report in almost 2 years and I'm much too lazy to look it up now, so maybe I have my accidents confused. Look it up and decide for yourself.

Anyhow, my guess is the answer to the question asked is "as light as possible and just barely strong enough to keep most pilots from killing themselves".

Remember....RV's are overengineered. There's a big difference between that and overbuilt. Overengineered implies redundancy in the design and tolerance for some sloppy work. That doesn't mean it's way stronger than it needs to be....it means you can be a little off and still make it just as strong as it was designed to be. Big difference....
 
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10! Ten

cruzer said:
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 therefore 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.
Interesting statistics 10 cracks! :eek: May be it's the way the plane is flown?

Well in 20 years of being around 30 RV's there has been few cracks. I flew at a field with at least 15-20 RV's flying at any one time. Over many years I don't recall any one with engine mount complaints, except one older RV-4, whose firewall fittings cracked. It had older style firewall engine mount that where upgraded many years ago. The engine mount itself was OK.

While I am thinking about the older RV's like the RV-4 used very thin skin for the elevator (0.016). They would crack near the trim tab cutout typically. IT seemed it was fine with a little 150HP engine and wood prop, but when more people started using bigger engines and constant speed props the problem was more prevalent. Van LONG ago went to (0.20) thick elevator.
 
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Loss of control?

P, what clevis, the trim tab? Yea I don't like that plastic trim tab clevis either.

You mention uncontrollable plane? :eek: Is there an accident report or what. There are like 5,000 built and flown and we know they really fly wonderfully. Very few if any RV's have crashed due to the airframe failure. Some engine issues, sure, but as far as airframe structure and controls, I can't think of any. I have over 1000 hour in RV's. What RV are you flying P.

(I did a quick check and there have been a few control jams, this is one that is scary, ball point pen and jar under floor boards caused greif to this RV).

RV-8, June 05, 2005, NY, N61TW
The pilot/owner of the homebuilt airplane began the takeoff roll. At rotation, the airplane pulled ?hard? to the left, and pitched up ?more aggressively? than a standard takeoff. The pilot applied full down elevator, ?but the nose would not come down.? The airplane then pitched nose down, the pilot applied full up elevator, and the airplane attained a level pitch attitude prior to ground contact. When asked about the performance and handling of the airplane, the pilot/owner said, ?Everything was perfect with the engine.? He added that about the time of rotation, he felt a bump, and surmised that he had struck a runway light or that a wheel brake had locked. Examination of the airplane revealed an ink pen lodged beneath the rudder bar. As a result, more force was required for a right rudder input than a left rudder input. The pilot/owner said he routinely stored pens, unsecured, on the ledge next to his right knee. Further examination revealed a 50-ounce glass jar beneath the front seat, in close proximity to the forward control stick. The jar?s lid displayed indentations that the pilot said had not been there prior to the accident. He said the jar was kept in the airplane as a relief container, and that it was placed on a ledge, unsecured, prior to takeoff. report: http://www.ntsb.gov/ntsb/brief.asp?ev_id=20050609X00746&key=1
 
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wing failure

JonathanCook said:
What happened to the 8 wing that broke after the fuel tank does not happen from pulling too many Gs. I know that this goes against what Van's says, but this kind of failure is probably due to flutter.

This may not be the same RV-8 accident, but the one I remember the wing failed in straight and level flight. So, this was very probably due to overstressing the airframe and not doing a through inspection afterwards and at frequent intervals to see stress fractures develop. If you go back to the structural breakup of TWA Flt 800, '96' I believe when the nose came off the rest of the aircraft pitched up rapidly. This caused the wings to fail at some point outboard of the wing root. I believe at some point outboard of the flap line. The wing destruct in this case was not flutter. Not saying you are wrong in your analysis, but over stress does cause premature failure.
Former USAF Pilot, Retired TWA Captain. No engineer just a flyer.
Regards,
John S.
 
I think apples oranges

John Stiegelmeyer: Yea I get you but of course TWA 800 was a totally different deal, and doubt any RV nose fell off causing a pitch iup. As far as level flight, unlike the big gets the little plane can change directions very fast. In level flight, if you are going fast enough, and pull hard enough the wing will fail. From a ground observer it may seem like the plane was going level at the time.

Here is the RV-8 prototype accident (notice one eye witness says plane was level):

Summary
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001211X10121&key=1

Factual detail
http://www.ntsb.gov/ntsb/GenPDF.asp?id=LAX98FA171&rpt=fa

Prob cause:
http://www.ntsb.gov/ntsb/GenPDF.asp?id=LAX98FA171&rpt=fi

Airframe deficincy was not found to be a probelm. This was following metilurgical analysis of the wing and Van's static load test subsiquent to the accident with a pair of RV-8 wings built (badly) by a customer, representaive of the wing on the prototype.
 
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I wonder if-------

This is all conjecture, as I haven't yet built a RV wing, but I am wondering------

Is the fuel tank a load carrying member of the total wing structure????

The tank just bolts on to the spar, and doesn't appear to help carry the full span flight loads. Therefore, the inner part of the wing must carry the flight loads with the spars and rear skin only, while the outboard section of the wing utilizes the "D" tube of the front ribs/skin also---------and in an area of lower stress to boot.

So, I would expect the spar to be reinforced in the inboard end------------and yes, in fact it appears to be.

Now, if the inner portion reinforcement was terminated incorrectly (too soon, or too abruptly) just where the flight loads would transition from the entire wing structure to just the spar rearward, at the outboard end of the tank. Seems to be an ideal failure point.

Now throw in a few violent maneuvers-----------

Any of you engineer types have input??

Mike
 
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Has any one replaced the plastic clevis on the trim with metal clevis and where did you purchase them at :)
 
I don't think a clevis has ever broken. Even if it was a broken clevis on that one aircraft, the report states that the clevis was modified with a notch (definate no-no).

I'm sorry I even started the thread down this path. The only point I was trying to make is that for whatever reason, someone yanked on that elevator to break that wing...simple as that. The structure performed exactly as intended. The whole clevis thing was more of a mental exercise and I regret even mentioning it at this point.

The stock clevis is perfectly OK :D
 
Reply to Mike S

Mike S: The tank is part of the wing D section and carries torsion loads. The front spar carries almost all of the bending loads; the tank and outboard leading edge probably none. The tank also strengthens the front spar against buckling. Your conjecture about load paths is wrong. The front spar gets thicker as it nears the wing root because bending forces increase dramatically at the root. You are correct, if the spar is not properly sized or if the increase in thickness of the spar bars is not done smoothly at the correct places, then a stress riser at that location can cause in an unanticipated weakness. Many thousands of us have bet our lives, and continue to be our lives, that Van designed a strong, safe wing. Based on its service history, it is a very safe design. Now, why did the 8 wing break midspan as opposed to some other location? You'd have to ask the man who designed the wing. I will speculate that it has something to do with the outboard front spar being stronger than required for just the flight loads. This is often the case in airplanes because the wing has to be stiff as well as strong, and the requirements for stiffness in the outboard section (and the requirement for ruggedness for ground handling) often results in an outboard spar that is stronger than needed than to just carry the bending forces. I'm not an engineer but I am interested in these types of questions. A good primer is "Design for Flying" by David B. Thurston, McGraw-Hill, 1978.
 
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Thanks

Steve, thanks for the reply----------you are right, of course, concerning the torsion loads, I was only addressing the bending loads.

As far as the design engineering, I have no doubts there, just wondering if a builder took "short cuts" with the spar reinforcements (I.E. just cut them off at the end of the tank instead of a long taper or whatever the design is) could that then have contributed to the failure at that point.

Mike
 
Concerning an RV-8 crash several years ago:

As far as the design engineering, I have no doubts there, just wondering if a builder took "short cuts" with the spar reinforcements (I.E. just cut them off at the end of the tank instead of a long taper or whatever the design is) could that then have contributed to the failure at that point.

The RV-8 (and I believe all RV-3, 7, 9, and 10) main spars are delivered from the factory fully assembled. They have machined spar caps instead of the doublers used on the RV-4 and -6. This has removed the possibility of the builder mis-assembling the spars.
 
Thanks

O.K., got it.

As I said in my first post in this thread I havent yet built a RV wing, I was just speculating what may have led to this particular failure of this one aircraft.

Seems like the culprit lies elsewhere.

Thanks for the info.

Mike
 
gmcjetpilot said:
Other jets limit control surface deflection at high speed. On new jets, F16 the computer limits the loads not the pilot.

True. The Viper will let us only pull 9.0g's, however, g overshoots in the realm of 9.2-3 g's are possible. We also have rolling (asymmetric) g limits that are lower than 9g's. But the computer does not factor in the rolling g's. If the rolling g limit is 6g's the jet will let you roll while pulling 9g's which would technically be an over-g.

When we carry bombs we have g-limits as low as 5.5g's (symmetric). The jet will still let you pull 9g's. I doubt the wing would fail, however, it would likely cause quite some damage. I have only seen a 7g over-g in this configuration which caused no structural problems.

Just thought I would add something to this discussion - as it is well above my cranium.

My question to this discussion would be...I thought that the horizontal stab would typically fail before the wing?

Mark
 
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RV Strengths and Weakness

You may or may not have heard about my experience with a pre 1984 RV4, an article was published in "Sport Aerobatics". My airplane was built by two very experienced builders but I damnd near pilled the engine off of it. We found all 4 weldments were broken on inspection and had to replace them with the post 1984 redesign. This aircraft was never overstressed on landing or airborne. This is a weakness of the RV design in the RV4 of pre 1984 manufacture. Roger Moore
 
Regarding airliner wing design

?The Boeing 767 has 4 ailerons. The out board ailerons LOCK out at higher speeds to reduce loads?.

I think this statement requires some clarification. It is true that, depending upon the design, the outboard ailerons are prevented from deflecting either when the flaps are up or above a certain speed.
My understanding of this feature is to prevent aileron reversal at higher speeds, wherein the aileron deflects but has so much effect that it twists the outer wing in the opposite direction, the net result being the aircraft rolling in a direction opposite to that commanded. The B-47 comes particularly to mind. In its handling notes it describes-at increasing speeds-the ailerons having less and less effect, no effect at all then having the reverse effect.
One way to prevent this would be to make the wing more resistant to the twisting force realized from the aileron deflection. The downside of this solution is greatly-increased structural weight as well as impacting all the other design compromises in the wing.
Above certain speeds, an inner aileron( with a shorter moment arm to the longitudinal axis than an outer aileron) can generate the desired rolling forces and therefore it?s a better design decision to build a wing with two, smaller ailerons on each wing. With the outer unable to move above certain speeds, the outer wing can be built lighter, making the overall airframe lighter.
 
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