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-   -   Exactly how strong are RV's? (http://www.vansairforce.com/community/showthread.php?t=13030)

N941WR 11-28-2006 09:29 PM

Exactly how strong are RV's?
 
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.

RudiGreyling 11-29-2006 12:09 AM

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

pierre smith 11-29-2006 07:20 AM

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.

JonathanCook 11-29-2006 10:10 AM

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/woodfr...ages/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.

Kevin Horton 11-29-2006 10:44 AM

Quote:

Originally Posted by JonathanCook
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.

rv7boy 11-29-2006 10:56 AM

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?e...11X10121&key=1

and is NTSB report number LAX98FA171. It's the only report of an RV-8 wing failure I'm aware of.

rv6ejguy 11-29-2006 11:31 AM

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.

gmcjetpilot 11-29-2006 12:06 PM

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.

pierre smith 11-29-2006 12:28 PM

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,

MikeJ 7A 11-29-2006 12:37 PM

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


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