Van's Air Force
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- Thread starter SquawkVFR
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As you noted, you’ll need to go back into phase 1 (contact fsdo) and demonstrate each maneuver. Not required but I’d suggest a parachute.
I believe Vans has done exactly what you’ve suggested, e.g., turned the airplane upside down and loaded the wings, with a calculated distribution of sandbags, to simulate the positive g loading. Perhaps they would share details with you.
I believe Vans has done exactly what you’ve suggested, e.g., turned the airplane upside down and loaded the wings, with a calculated distribution of sandbags, to simulate the positive g loading. Perhaps they would share details with you.
Jonnyb
Well Known Member
I think you have that the wrong way round. A 6G load test would have a load applied to the top of the wing, bags of sand or cement. A negative G load test would mean flipping the aircraft and placing a load on the underside of the wing. Also the load isn't uniformly distributed along the wing. The wing root takes much more of the load than the tip. On your RV-7 the wing tip itself is part of the lift producing structure, but its unable to carry much loading. If you start applying loads to this area then you will likely break something.
As said above this has already been done for you by Vans. So I'm wondering what it is that you want to achieve by doing this to your aircraft? I think your time would be much better spent ensuring that the aircraft was built to plans in the main load bearing areas. Center section to wing spar join, and making sure the horizontal stabiliser is attached to the main upper longerons with the correct edge distance.
Kit Planes did a series of articles on testing aircraft structure, worth a read.
Jonnyb
Well Known Member
No, you're right! I miss-read your post, and managed to confuse myself (which isn't hard!) my apologies.
You would have to flip the aircraft or come up with a rig to apply loads across the wing structure.
Im still interested in why you would want to do this. The chances of it going wrong are quite high, and the testing has been done for you by Vans already. The vast majority of builders and buyers don't undertake this sort of testing. So while I would agree that inflight is not the best time to find yours wasn't built correctly, you will be will with in the ultimate limit load (9G) at 6G.
If it were me I would be more interested in making sure the thing was built correctly and then trusting Vans testing than trying to come up with my own tests outside of a lab equipped with proper test equipment.
Jonnyb
Well Known Member
There's a good thread on RV wings here:
vansairforce.net
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...
vansairforce.net
First off, I've seen several RV aircraft, when checking the Ops Limits during a pre-buy, that are not approved for aerobatic flight. In the cases I've seen, it wasn't that the airplane had, in any way, deviated from the build plans. Rather, it was a mistake on the DARs part in simply not changing the document in Word before issuing the document or the owner stated he/she didn't intend to do aerobatics. I almost every case, the seller wasn't even aware that the restriction was in the ops limits! So, I absolutely agree, have someone that knows these airplanes do a good inspection of the wings, center section and empennage to ensure they are plans built, then coordinate with the local FSDO to get the aerobatic restriction removed.
To start loading up structure, in an unknown process, is to invite disaster.
To start loading up structure, in an unknown process, is to invite disaster.
David Paule
Well Known Member
The OP has the load direction correct.
In the test envisioned, there's a probability that the landing gear or its attachments, would be considerably overloaded.
In any event, given both the history of the airplane model and its design and testing by Van's, this test is not necessary. Instead, get a competent aerobatic pilot to demonstrate that aerobatics are safe in your airplane, if done consistent with Van's speed and load requirements. Get suitable training, parachutes for you and your passenger, if you have one, and enjoy your airplane.
Dave
In the test envisioned, there's a probability that the landing gear or its attachments, would be considerably overloaded.
In any event, given both the history of the airplane model and its design and testing by Van's, this test is not necessary. Instead, get a competent aerobatic pilot to demonstrate that aerobatics are safe in your airplane, if done consistent with Van's speed and load requirements. Get suitable training, parachutes for you and your passenger, if you have one, and enjoy your airplane.
Dave
Majorpayne317641
Well Known Member
You’ll have to work with FSDO or DAR to get a a new practice area if you don’t live in the same area as the original which means new AW and Ops Limits. Then you make log book entry to place back into phase 1. Notify FSDO you are placing back into phase 1. Then you fly your manuevers you desire and fly a minimum of 5 hours in your practice area before placing back into phase 2. Make a log book entry of phase 2 again and the manuevers tested and approved for your plane. All of it is a formality. The aircraft can handle it so long as you stay within its design limits it was engineered for by Vans.
I understand the desire to test something you didn't build to verify it's built as designed. There have been many examples of build errors shown by inspectors on this site that could have made aerobatic flight hazardous. I'd think a more efficient method of verification would be a deep inspection comparing the plane to the drawings and construction manuals. Experienced builders and inspectors can also provide guidance as to key items to look for.
If testing is the only way to satisfy I'd think you'd need to build a couple fixtures. Remove the wings and attach to a fixture that mimics the fuselage spar attachment. Then load as needed. The second fixture would need to mimic the wing spar. Hang the fuse from it and distribute weight from spinner to tail according to cg calcs. It would be a lot of work but a fun experiment.
If testing is the only way to satisfy I'd think you'd need to build a couple fixtures. Remove the wings and attach to a fixture that mimics the fuselage spar attachment. Then load as needed. The second fixture would need to mimic the wing spar. Hang the fuse from it and distribute weight from spinner to tail according to cg calcs. It would be a lot of work but a fun experiment.
wawrzynskivp
Well Known Member
Some assumptions can be insidious harbingers of unintended consequences...
Jjackh10 is right in that we shouldn't assume the landing gear and the way they attach are capable of managing loads opposite to how they are designed to work. Maybe yes, maybe no, but not a good place to assume. I once redesigned how a mast stepped into a large sailboat which is a fairly simple compression load. When I got around to testing that structure it became clear that I couldn't be sure my tests were going to prove my design apart from the actual in-service evaluation.
It's a noble effort to be sure, just really hard to do in a way that is going to give you meaningful metrics. For example: if done properly you will observe X amount of elastic deformation in the applied force vector and Y amount of ancillary deformation in downstream skins as a result of unmanaged compression loads. So how much is too much?
Best case is maybe to repeat exactly what Van's did and see if your displacements are close to what they observed?
Jjackh10 is right in that we shouldn't assume the landing gear and the way they attach are capable of managing loads opposite to how they are designed to work. Maybe yes, maybe no, but not a good place to assume. I once redesigned how a mast stepped into a large sailboat which is a fairly simple compression load. When I got around to testing that structure it became clear that I couldn't be sure my tests were going to prove my design apart from the actual in-service evaluation.
It's a noble effort to be sure, just really hard to do in a way that is going to give you meaningful metrics. For example: if done properly you will observe X amount of elastic deformation in the applied force vector and Y amount of ancillary deformation in downstream skins as a result of unmanaged compression loads. So how much is too much?
Best case is maybe to repeat exactly what Van's did and see if your displacements are close to what they observed?
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Desert Rat
Well Known Member
Yeah, I get that it was built in somebodies garage, but realistically, this idea is no different than saying that you want to do compression load testing on a 172 or Bonanza wing to confirm what Cessna/Beechcraft is telling you.
Production airplanes load test with test articles, then build real airplanes to match. It's not like Cessna is piling sandbags on the wing of every 172 that comes out the door.
If it was built to plan, it will do what it' supposed to do. if it wasn't built to plan, it may or may not. Far better to confirm the build quality than potentially wreck it by load testing.
Production airplanes load test with test articles, then build real airplanes to match. It's not like Cessna is piling sandbags on the wing of every 172 that comes out the door.
If it was built to plan, it will do what it' supposed to do. if it wasn't built to plan, it may or may not. Far better to confirm the build quality than potentially wreck it by load testing.
Majorpayne317641
Well Known Member
It’s even harder if you are testing without any instrumentation like applied strain gauges in strategic points based on your analysis of where elastic/plastic deformation can/could occur and what you will do with said data once collected. It’s not necessarily as easy as load the wings and see if they hold. That’s half the way to the actual answer. Hence why engineering has already been done. What is needed is verification of the build to plans. Everything else is retesting something that’s already a proven design within the performance limits of the aircraft.
Majorpayne317641
Well Known Member
Another good point. If you plastically deform the aircraft when testing, your plane is done until you do major repairs to fix the damage.
wawrzynskivp
Well Known Member
This is a really strong point. It sparks the question whether Van's considered the material that underwent their testing to still be airworthy. That said when load testing structures to validate design stiffness we don't need to go to the limits. I would only go to a limit on the way to an intentional discovery of ultimate load (breaking it which you want to avoid). Since our artificial loads won't match actual use loading it wouldn't be prudent to apply a full design load. Better to validate at some fraction but again you have to have the expected metrics in hand before you get underway. If Van's had been using strain gauges, then that would be a part of finding that sought after repeatability or something that shows you a deviation (weakness).
It's good food for thought. There has to be a Non-Destructive testing scheme to scratch this itch. Probably a lot more work to get there properly than it initially seems. But here in our Experimental Category there's goodness in trying our own things in a safe manner. I personally would follow the crowd here and inspect for plans compliance and signs of problems and call that done. One could go as far as pulling and inspecting some bolts, even magnaflux them and do some die testing of the aluminum attach points to check that block but for an airframe that hasn't been pulling the aerobatic Gs or showing signs of corrosion or poor craftsmanship in those areas that's probably in the overkill dept.
Depends on the desired level of testing. Testing to yield or failure is destructive so yea, don't fly it. The OP seemed to just want to validate design limits. So properly loading the airframe with weights to +6g and -3g on the ground would be no different than doing it with flight testing. It would only be destructive if the assembly was incorrect, thus failing the testing. Certainly a safer way to test than in the sky.
I'm certain many RV builders set their OP limits as Vans recommends without full testing. Actually pulling +6 -3, gross weight testing at cg extremes, fly up to VNE, etc. That seems a bit careless or maybe arrogant to me. They're assuming they assembled the airplane perfectly according to design, despite the many examples of mistakes posted here and other places. I know I screw up occasionally, so if/ when I build I intend to test fully. I enjoy jumping out of airplanes so I will do it in the air hahaha.
Majorpayne317641
Well Known Member
Going up to the design limit is not being careless it is max performing the plane. It has a safety factor built in as with any engineering design. Should you go above the limit? No. Could you accidentally over G or over speed? Yes. There are consequences if you exceed the limit of course. Either way going up to but not exceeding a limit is completely safe if done intentionally.
I'm sorry, maybe I didn't word that right. I agree, I was suggesting NOT testing to design limits could be viewed as careless. Careless maybe isn't really the right word either. Maybe just less than through? My view is that testing to those limits would validate that the builder hasn't made a mistake or deviation that erodes an engineered safety factor.
Sam Buchanan
been here awhile
How so? What if a builder error had cut the margin to 10%, or 5%, or 2%............what happens then to the engineered safety margin?
Any aircraft structure that is known to have been exposed to loads higher than limit load (whether during testing or inadvertently in flight) should not be used for flight without some very detailed inspection, and even then I would be very suspicious regarding its suitability for flight, so the answer to your question is an emphatic no.
The primary reason for this is that even though testing standards would allow a new set of wings (for example) to be used for each test scenario that is done, just one set of wings was used for all tests at Van's in the interest of saving time, $$$, and to be very conservative regarding the test results.
Because of this, a single set of wings was often loaded to limit load (6 G's for the aerobatic models) and ultimate load (9 G's for the aerobatic models, and the load which must be attained without catastrophic failure, but permanent deformation damage is allowed) multiple times.
Anyone visiting Van's for a tour can see one of the actual test wings on display in the demo aircraft hangar.
It is an RV-7 wing with cut away sections that allow for inspecting structure inside. If I remember correctly, it was statically loaded to 9 G's positive, at least once, but I think probably a minimum of twice.
There is no discernible damage but I would never fly it (well, probably would if an enemy army was coming over the distant hills, and there was a perfectly good RV-7 siting there minus a left wing
)Right, 2% is good enough to fly. So the options are to assume the build is good enough to have some margin and risk finding your wrong during an inadvertent excursion? Or test to verify in a controlled situation?
Majorpayne317641
Well Known Member
Generally speaking engineering safety factors are usually 1.5 which is why I think the 6G and 9G testing was done for the aerobatic model. Every design is engineered to certain limits, sometimes engineers use a safety factor of 2.0 or 1.25 depending on the application.
First, your plane, your choices.
(Also, this is NOT a commentary on what you are capable of doing as I have no idea.)
But to add to the conversation ...
I know MANY people who have flown their RV's in competition aerobatics as well as airshows with aerobatics. Just about EVERY maneuver one would think of doing in these planes. And probably right near the limits.
Many of these people were the builders, but some were not.
To my knowledge, NONE of them tried to do additional validation of the design by ground loading the wings as an example. On the other hand there was always a lot of inspection with several sets of knowledgeable eyes placed upon the aircraft to make SURE that the construction was up to par and REMAINED up to par.
If it were me and I was concerned, I would get the BEST "Pre-buy Inspector" that I could hire and the BEST aerobatic pilot that was willing and have them inspect the plane for quality of construction and test fly the plane through all maneuvers desired.
As implied in prior posts, things beyond that run the risk of "unintended consequences".
But ...your plane, your choices.
(Also, this is NOT a commentary on what you are capable of doing as I have no idea.)
But to add to the conversation ...
I know MANY people who have flown their RV's in competition aerobatics as well as airshows with aerobatics. Just about EVERY maneuver one would think of doing in these planes. And probably right near the limits.
Many of these people were the builders, but some were not.
To my knowledge, NONE of them tried to do additional validation of the design by ground loading the wings as an example. On the other hand there was always a lot of inspection with several sets of knowledgeable eyes placed upon the aircraft to make SURE that the construction was up to par and REMAINED up to par.
If it were me and I was concerned, I would get the BEST "Pre-buy Inspector" that I could hire and the BEST aerobatic pilot that was willing and have them inspect the plane for quality of construction and test fly the plane through all maneuvers desired.
As implied in prior posts, things beyond that run the risk of "unintended consequences".
But ...your plane, your choices.
Majorpayne317641
Well Known Member
You should also know with your background that eventually the design and validation of materials is “good enough.” Even a certified Extra 300 can’t go through all of these tests on every single iteration built or it would be unuptanium at a gold plated cost.
It might be worth looking at the accident data to get an understanding of where the real risks are. Offhand I would say that probably more RV-7s doing aerobatics have come to grief from exceeding speed limits or just plain hitting the ground, rather than from pulling the wings off.
Jonnyb
Well Known Member
Some parts of the world require the aircraft to be taken to its limits before it’s cleared for aerobatics.
I seem to remember that Flightchops had to take his RV-14 to the limits. And taking an aircraft to Vne is something that’s done in flight tests quite regularly
Seems to me the issue is over speeding and the tail coming off.
The only accident I can remember where the wings were pulled off the plane was the early prototype RV-8. And then Vans bought out a new wing spar. But even then I think they pulled more than 9G. Thats from memory, so stand to be corrected on that.
I'm not an engineer but I've fixed enough of their errors in my career to know how their taught to think hahaha. Mburch makes a great point, but idk if statistical analysis is an engineers favorite math. The most efficient way to perform the testing would be to put a parachute on and fly it to the desired limits. That's what test pilots do. But I also understand efficiency and engineers don't go together
. (To be clear I'm just teasing with the engineer comments. I have had to repair their mistakes in my career, But I've also literally had their designs save my life more than once when I've made mistakes. Much respect brother!)If your determined, performing static testing on the fuse and empennage wouldn't be difficult just time consuming. Basically a sawhorse style fixture in the spar carry through. and a steel deck bolted to the engine mount. Then just add sandbags in a balanced manner. 6x the engine weight on the deck, 6x pilot & pax in the seats, The remainder stacked in the horizontal and sprinkled through the rest of the fuse.
That's exactly what I was suggesting and what I intend to do if I ever build one.
Talk to Van's and have a good inspection about the build quality and adherence to plans on your aircraft.
As quoted in the old RV builder newsletters put out by Van's, they purposely built their load test wings in a sloppy fashion, trying to replicate a lower standard build quality. They don't expect every builder to be an experienced A&P with the highest quality standards.
