Van's Air Force
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Main Wing Spar - Design
- Thread starter ryykblum
- Start date
scsmith
Well Known Member
No need
Hi,
The RV-8 spar is probably among the most analyzed and tested spars in any experimental aircraft. After the in-flight failure of the factory demonstrator, the factory undertook an extensive test and evaluation of the wing design, including static loading to failure. I believe the results substantiated that the wing and spar met the design limits, but like anything, if you overload it, you can break it. I believe that as a result of that testing and lessons learned there was some modification made to the spar design -- although there is nothing wrong with the original, they still found areas to improve.
This is just my opinion based on recollections, to take it for what you paid for it.
I can also tell you that I have personally done some reverse-engineering on the RV-8 spar as part of the design of a wing for a different project. I have found no faults with it.
Hi,
The RV-8 spar is probably among the most analyzed and tested spars in any experimental aircraft. After the in-flight failure of the factory demonstrator, the factory undertook an extensive test and evaluation of the wing design, including static loading to failure. I believe the results substantiated that the wing and spar met the design limits, but like anything, if you overload it, you can break it. I believe that as a result of that testing and lessons learned there was some modification made to the spar design -- although there is nothing wrong with the original, they still found areas to improve.
This is just my opinion based on recollections, to take it for what you paid for it.
I can also tell you that I have personally done some reverse-engineering on the RV-8 spar as part of the design of a wing for a different project. I have found no faults with it.
Mike S
Senior Curmudgeon
Veetail88
Well Known Member
Spar
Here's how I analyzed it.
After the prototype broke, real live aerospace engineers analyzed the heck out of the design, made some improvements on it and with hundreds flying, some that really push them with acro, none have broken.
Not being an aerospace engineer myself, I rely on their expertise and imperical data. I worry more about fault I might bring to the table.
Course, I do that every time I get in a wide body as well, or my car for that matter.
Here's how I analyzed it.
After the prototype broke, real live aerospace engineers analyzed the heck out of the design, made some improvements on it and with hundreds flying, some that really push them with acro, none have broken.
Not being an aerospace engineer myself, I rely on their expertise and imperical data. I worry more about fault I might bring to the table.
Course, I do that every time I get in a wide body as well, or my car for that matter.
Sid Lambert
Well Known Member
RV-8 Wing Spar Design
Thanks all for your replies.
I have been looking at the RV family for years as a way to get into aerobatics, combined with having a plane with a little extra speed for long distance trips. The 8 has always come out on top when compared against other options (most of my time is in small Cessnas). Having recently sold my plane, I am getting antsy to start on a project, and am still convinced the 8 is the plane.
My concern stems from having very little aerobatic experience, and so when I start, mistakes will be inevitable. I don't yet know the full story of the factory 8 mishap, but I have heard it involved the demonstration pilot giving the stick to the prospective buyer in the back, who then proceeded to overload the wings. Mistake.
To Sid Lambert's question... "You think it just 'looks' weak?" Simple answer is yes. I looked at one under construction last week, while hearing the story of the factory mishap from the individual showing me the spar. When I compare the wing load ratings of the RV-4 against the those of the 8, it prompts me to ask why the big difference?
Regards,
Rick
Thanks all for your replies.
I have been looking at the RV family for years as a way to get into aerobatics, combined with having a plane with a little extra speed for long distance trips. The 8 has always come out on top when compared against other options (most of my time is in small Cessnas). Having recently sold my plane, I am getting antsy to start on a project, and am still convinced the 8 is the plane.
My concern stems from having very little aerobatic experience, and so when I start, mistakes will be inevitable. I don't yet know the full story of the factory 8 mishap, but I have heard it involved the demonstration pilot giving the stick to the prospective buyer in the back, who then proceeded to overload the wings. Mistake.
To Sid Lambert's question... "You think it just 'looks' weak?" Simple answer is yes. I looked at one under construction last week, while hearing the story of the factory mishap from the individual showing me the spar. When I compare the wing load ratings of the RV-4 against the those of the 8, it prompts me to ask why the big difference?
Regards,
Rick
Mike S
Senior Curmudgeon
Bob Kuykendall
Well Known Member
I guess it kind of depends on which end you looked at. Like all well-designed structures, the RV-8 wing spar is optimized for the bending moments that prevail at each spanwise location.
The bending loads are greatest at the wing root and drop away very quickly as you go outboard. So when you look at the outboard end of the spar, it looks like there's nothing there, and there pretty much isn't, nor does there need to be.
When you look at the wing root, you see massive slugs of aluminum. And when you look at the mid-span, you see a lot less than half of the metal at the root, because the bending loads drop off a lot more steeply than linear away from the root.
By the way, here's a picture of Steve Smith (scsmith, who posted above) helping to test the wing spar on my kit glider project. That's Steve on the left:
Personally, I think that there are a lot more important things to worry about when building an RV-8. Things like the integrity of the fuel plumbing and the electrical system probably probably have a much greater impact on the overall safety of the aircraft.
Thanks, Bob K.
PaulRomano
Member
Rick,
To your specific question addressing the differences in the -4 and the -8:
The aerobatic gross weight of the -4 is 1375lbs (+6/-3 g), and the aerobatic gross weight of the -8 is 1600 lbs with the -1 wing and 1550 lbs without the -1 wing (+6/-3 g). The details can be found here:http://www.vansaircraft.com/public/downloads.htm#wb and here:http://www.vansaircraft.com/public/flyrvs.htm#AERO.
To your specific question addressing the differences in the -4 and the -8:
The aerobatic gross weight of the -4 is 1375lbs (+6/-3 g), and the aerobatic gross weight of the -8 is 1600 lbs with the -1 wing and 1550 lbs without the -1 wing (+6/-3 g). The details can be found here:http://www.vansaircraft.com/public/downloads.htm#wb and here:http://www.vansaircraft.com/public/flyrvs.htm#AERO.
jjconstant
Well Known Member
There was a guy in my hangar row who, when I was building, would always come up, lean over the open fuselage, point to the spars and say "that just doesn't look strong enough!"...every time he came in. It really started to bug me. He built a very nice Glasair II and witnessed an RV 6 being built so had loads more experience than I had, but it really felt like he was trying to undermine my confidence it my choice to build a 7A. Whenever it would get to the inevitable acknowledgement that unless you're mining your own bauxite and smelting it, there are some things you're going to have to take on faith. He would sneer "oh, you're one of those 'in Vans we trust' guys" and I would say yes I trust in the design and in the evidence of thousands of examples flying. I finally shut him up by telling him that if he was going to use the "it doesn't LOOK strong enough" school of engineering he shouldn't be flying anything without king posts and flying wires or at the very least wing struts, because any cantilevered structure couldn't possibly LOOK strong enough!
It's possible to tie yourself in knots over these kind of doubts but ultimately you have to answer for yourself the question "do I trust the designer, materials and the manufacturer (you)"? For me, the biggest act of faith involved the third one
All best
Jeremy Constant
It's possible to tie yourself in knots over these kind of doubts but ultimately you have to answer for yourself the question "do I trust the designer, materials and the manufacturer (you)"? For me, the biggest act of faith involved the third one
All best
Jeremy Constant
Shockwave
Well Known Member
Here's the NTSB report on the factory demonstrator:
http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20001211X10121&key=1
Towards the bottom of the full narrative you'll see they tested another wing to +6g with no deformation occurring, and then to +9g for 3 seconds without failure but with some deformation.
The wing is more than strong enough for what it's designed for, but that doesn't mean it's indestructable.
Keep in mind 6g is quite a bit, and while I can't find a definitive source at the moment, I think a typical human without a g-suit will blackout around 6g's.
http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20001211X10121&key=1
Towards the bottom of the full narrative you'll see they tested another wing to +6g with no deformation occurring, and then to +9g for 3 seconds without failure but with some deformation.
The wing is more than strong enough for what it's designed for, but that doesn't mean it's indestructable.
Keep in mind 6g is quite a bit, and while I can't find a definitive source at the moment, I think a typical human without a g-suit will blackout around 6g's.
Andy Hill
Well Known Member
Rick...
AFAIK, the -1 RV-8 wing is essentially the same as the RV-7 wing.
Please understand here the concern is not the "wing spar", but the CG range / stick force per 'g' and low VA of the RV range. So re:
and the RV range, please do not "learn" aerobatics in an RV. it is a "poor" aerobatic trainer for a number of reasons, which end with the fact it is easy to overstress.
All (aerobatic) RVs are designed essentially to the same principle - design load +6/-3g, ultimate load + 50% on that e.g. +9g, at aerobatic gross weight (for the benefit of doubt, IMHO that includes wing fuel). If you pull more g than that, or pull that g at more weight, the wing will fail - as it is designed to do. It is not (nor should it be) "over designed".
AFAIK, the -1 RV-8 wing is essentially the same as the RV-7 wing.
Mike,
Thanks, I appreciate this link! If it was a tail dragger I would be on the phone. All just part of the dream. Center-line seats, bubble canopy, fast roll rate, sustained inverted flight and tail dragger.
Rick
Bob Kuykendall
Well Known Member
I think Andy means that the light controls of the RV (low stick force per g) make it easy to exceed the +6g limit load because it doesn't take that much stick force to crank on that g. Those light stick forces are a large part of what makes the RVs very sweet flying airplanes, but the limits must be respected.
Also alluded to in this thread is the fact that in the RV-8 the stick force per g is noticeably different between solo flying and flying with a passenger. A passenger moves the CG aft and makes the stick force lighter than when solo, for positive pitch maneuvers at least. But again, the behavior is predictable and the limits are well known, you just have to respect them.
Thanks, Bob K.
Bob, Good information. The only part I did look at was the wing root. And as I have noted, my engineering experience includes very little mechanical or aeronautical. So my conclusions at the time were simply those of an uninformed person. I will be doing much to change that in the coming months. Comments like the one you made on pluming and wiring are always interesting, and worth paying attention to.
Thanks,
Rick
Hi Chanler. Thanks for these comments. Just for fun, I looked around and found this link that discusses effects of G on human body in great detail. Not definitive, but informative, and very entertaining. By the way, it generally concurs with your statement about G limits.
http://csel.eng.ohio-state.edu/voshell/gforce.pdf
regards,
Rick
N355DW
Well Known Member
That's true, but as a general rule you would have to hold the six G's continuously for a lot longer than you would ever do it in most aerobatic maneuvers or sequences. Only if you are in a dog fight would you be likely to do this. Most of the time in aerobatics you will not be pulling hard G's for more than about three or four seconds at a time, that's about how long it takes to go 90 degrees, like from horizontal to vertical. If you tense your leg and stomach muscles before doing it you can pull far more than 6 G's for several seconds without greying out. I have pulled as much as 11 G's when my adrenaline got pumping at a contest now and then. I don't like pulling that much, the wing is at too high an angle of attack at that hard a pull, creating more drag, and you will scrub off too much energy, losing vertical performance or speed for the next maneuver. In the S2-B the best performance seemed to be around 5-6 G, and the Extra 230 liked about 6-7 G. The reason being the judges will grade you better if the corners are sharp and the vertical lines are long. Not supposed to be a grading criteria I know, but this is true nevertheless.
G effects on your body also can change based on things like how acclimated you are to G forces, whether you are dehydrated, tired, etc. It can also depend on how the maneuvers are strung together. One of the easiest ways to get G-loc is to fly inverted (-1G) for longer than 8-10 seconds or so, and then pull 4 or more G's suddenly. The way this was explained to me is when all the blood is going to your head for a bit while you are inverted, your autonomic nervous center lowers your blood pressure thinking it's too high, and then when you suddenly pull positive G's, your brain can't raise the pressure soon enough to compensate. A square loop is a good place for this to happen. Ask me how I know (I quit pulling hard when I started to grey out).
I'm going to try very hard not to pull more than 4 G's in my -7 when it's done, and will keep the maneuvers very smooth. Metal has a way of fatiguing that wood and carbon fiber do not, and while I trust Van's engineering, I know that gentle and smooth is probably a good idea for a metal spar.
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N941WR
Legacy Member
If you ever looked at the inside of a C-172's wing, you would never ride in one again!
flyenforfun
Well Known Member
Nevermind, already answered...
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