Putting a tapered wing on an RV

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Fenix

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No, I did not understand the RV-6 wing attach. I have seen several RV-8 and then a few sailplane style install. That the RV-6 is quite different from the RV-8 is a bit of a surprise. I have only seen a couple pictures of RV-6 wings and the spars looked long enough to cross the whole airplane...The butted spars with some steel straps bolted through the caps always bothered me - the caps loses section area and straps have to carry all of the moment across the gap. That makes it essentially a one piece wing with the bolts near the wing roots carrying all of the lift but not bending, and the straps carrying all of the bending.

In this case, total lift reacted at the root fittings has to be adequate for carrying total lift, which should be OK. With a one piece wing or a two piece wing with overlapping spars (sailplane style) the bending moment at the root is the bending moment between the lift bolts, and the strap system must carry the bending moment at the root. More span is more moment is maybe the stock straps and bolts are OK, and maybe they need beefup for longer wings. Either way, you have to do the check ...
It is my understanding (subject to rebuttal from those in the know) that the RV3 and 4 spar design are the same with the 4 being stretched "just a bit" due to "market requests" to shove a second person in there if they are not too big. Then "market pressure" to have everyone in the front seat, or "my wife does not want to sit in back" or I want a two place plane that is actually more capable of two people than the RV4 which is sort of a one and a half person plane brought about the side by side RV6 but except for the width of the fuselage (and some changes to the stick/control arrangement) the design remained virtually the same.

But there are still some who want centerline seating/tandem design and a "real 2 place plane" so it's time to revisit the RV4. While the Harmon Rocket had already done this to some extent (can really carry two adult men) it was a lot different with a 6 cyl engine, clipped wings, etc. It was/is a 2 place plane but for a different role than the RV4. So Van's looks at the RV4 and how to make it a real 2 place plane without the very limited aft passenger size limit due to aft CG problems (solved in the Rocket by putting a big heavy engine up front). OK now I'm speaking for Van's so let me re-state this is my "take" on the development of the line and I am not nor have ever been a Vans employee so I am not qualified to speak for them, yet I will continue..... (opinion follows):
By time they were developing what would become the tandem RV8 they are in the "hey we're now selling a lot of planes" group. Also they probably realize that there are not many plans/scratch builders out there because most of what we sell are kits. Given the advances in manufacturing, CNC controls, the size of our operation and facilities, we can approach this a bit different. Homebuilt planes have always been made up of parts that you could make in your garage with a few tools. The RV3 and its derivatives (4 and 6) are like that. Afterall the RV3 (or maybe it was the RV1 or maybe even an RV nothing before the 1) came to be by Van (Richard VanGrunsven) building a metal wing for a Stits Playboy, and then apparently also "metalizing" the fuselage (or something like that).

Anyway the RV8 is a significant departure from its predecessors (but not the aileron) using quite a few parts which are not practical to make in your garage. The spar caps that are milled instead of "stacked up" are one example. Soon followed manufacturing accuracy that allowed for pre-drilled parts that you can just line up and bolt or rivet with no need to build jigs/fixtures. Solving the "aft CG problem" without putting a 6 cyl up front was done by moving the 4 cylinder further forward and that is how the forward baggage compartment came to be. Moving the landing gear (which had always attached to the engine mount) that far fwd was not so good so the RV8 became the first RV to have its landing gear mounted to the fuselage and not to the engine mount and thus the RV4 is flown "legs apart" straddling the center section box while the RV8 is flown "legs together" to stay between the structure inside the fuselage that supports the landing gear. Notably, for this discussion, the 8 began to use the "3 piece wing" with the separate center section to which the wings attach near the fuselage sides. Perhaps this change was due to some of the "concerns" that Billski mentioned about the "center splice". Billski, I will add that, even not being an engineer, the center splicing assy just "looks" like maybe there could be a better idea....

The manufacturing advances and ease of kit construction/assembly that were offered in the RV 8, as well as a number of other refinements, were then incorporated in the side by side RV6 and re named the RV7. And this is where the RV7 has the "three piece" wing spar and the RV6 remains as described of the RV4. The 7 was basically the 6 modernized so there was no reason to still offer the RV6 so it was no longer offered (but is still of course supported). The RV4 was enough different than the 8 that it was/is still offered (though a year or so ago Vans announced longer lead times for RV4 products) but it was not "modernized". Ditto for the RV3, still offered, but not brought up to a pre drilled easy to finish kit.

After the release of the RV8 and 7 it is clear the market wants easy to assemble kits with predictable results and a high completion rate which also leads to good resale values. Vans is an unparalleled success in the homebuilt kit industry and their product lines explodes to encompass slower planes for the "timid" (RV-9) bigger planes for those who want 4 seats (RV10) and a variety of other derivatives and roles are filled. Along the way the trend continues toward planes that are made of parts not practical to build in your garage and the plans change from large blueprints that give the required info to make the individual parts to a binder that shows how to assemble the parts produced in their factory, bringing into aviation a large group of people that would not previously have considered getting into building a plane in their garage. Finally there are enough RV's out there that the insurance company recognizes them and they are easily insurable and even ATC lets you call it "RV whatever" instead of "experimental". The RV line is now a significant contributor (and maybe the savior) of GA. Thanks Van!
Gosh, not sure how I got into all that...... sorry

Anyway, Billski, in regard to "....maybe the stock straps and bolts are OK, and maybe they need beefup for longer wings. Either way, you have to do the check"

I think I have done that check in the engineering report for my wing. Not asking you to do any work for me for free, but if you'd like to see the report for your own benefit, curiosity, interest, or whatever I'd make it available to you for all the effort you've put into helping others here at HBA, but I can't do so for a couple months until I am re-united with the report.
 

Fenix

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Not sure how many I can attach at once. Here are a few more.

One is the comparison of the wingtip mold in progress to the original wingtip just to show the size comparison.
 

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Fenix

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One question. Is your shop in the kitchen or do you have a kitchen in the shop. Either way its a nice setup you have.
Ahh the eternal question. My wife and I differed on the answer and after the first plane she left. Now the answer is settled, I have a kitchen in my shop. Good observation Jedi.
 

User27

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RV-6 wing root design.
The RV-6 wing root design is very different to the RV-8. It is similar to the RV-4 with a built up laminated spar (from 0.24" x 1.5" 2024-T3 strip), but the SBS fuselage means the bolted portion is longer. The central part of the spar has 4 x 0.25" strips plus one 0.125" strip and two 0.040" shear webs, the cap strips taper out with the shortest being 36" long. The RV-6 has the same span as the -4, as the fuselage is (around) 20" wider the sub spars (for the want of a better term) are each 10" longer. A large frame is built into the fuselage that the stub spars push into (usually requires some lubrication). The same joining straps as used with the RV-4 are employed, with close tolerance bolts, but actually do very little. There are 18 bolts holding each top and bottom spar flange to the fuselage frame with the inner 5 being close tolerance the remaining 13 are AN3, plus 2 each side in the spar web close to the fuselage skin in a stiffener block. 76 bolts in all. Most of the loads are taken by the first few AN3 bolts on each side. There is little history of failures in this part of the RV-6 wing. Van's have historically not been forthcoming about structural failures (not surprising, really) but I believe the few failures that have occurred have been at around the flap/aileron junction, and usually due to pulling too hard on the wing removal lever.
 

Fenix

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How did you roll the leading edges?
I vacuum bagged them down around a piece of PVC pipe. I had to experiment with a couple different sizes of pipe to "suck them down around" until I got a radius that worked out pretty well. This process does not make them fit precisely around the nose of the ribs (the same I believe is true of a Vans kit, at least for the RV4) but puts a roll in them that allows them to contour nicely to the ribs when pulled around them. Trying to wrap an "untreated" piece of aluminum around the "nose" is NOT something I was able to make work. Since the wing is tapered and the radius changes it turned out that I was able to find a size of pipe (I don't recall what that was right now but it is in my notes) that worked for the main tank. For the aux tank I had to use two sizes of pipe and "step" them as the same size from root to tip did not allow for a nice wrap around the leading edge at both ends. I was able to do this with a simple standard shop vac. The entire tank skin is enveloped in a plastic (painters drop cloth) envelope that uses a bit of duct tape, and then the vacuum is attached to the end of the pipe to suck all the air out through a hole drilled in the PVC pipe which it is all being sucked down around. If this is not clear I can likely find some pics of the process.
 

wsimpso1

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It is my understanding (subject to rebuttal from those in the know)
...
Anyway, Billski, in regard to "....maybe the stock straps and bolts are OK, and maybe they need beefup for longer wings. Either way, you have to do the check"

I think I have done that check in the engineering report for my wing. Not asking you to do any work for me for free, but if you'd like to see the report for your own benefit, curiosity, interest, or whatever I'd make it available to you for all the effort you've put into helping others here at HBA, but I can't do so for a couple months until I am re-united with the report.
All those comments on Van's history are well and good, but do seem thread drift. The rest of the comments support the notion that a tapered wing of more span could be a serious add to the Van's airplanes. I would like to have seen flying reports on davefried's plane, and look forward to them on Fenix's plane.

The concern that I had raised had to do with the OP concluding that the basic structure was OK because his MAC was in the same place as the stock bird. I have pointed out that the real max bending moment does increase significantly when span is increased even while tapering the wing. Now maybe the existing structure had enough excess in it that this does not seriously compromise strength, but communicating that a proper check should be performed was my point.

Since the "ship has sailed" on davefried's bird, and that sailing appears imminent for Fenix, I was primarily addressing all of the other folks that might read this and think that increasing span without doing all of the detail design calcs is an OK thing to do... It is possible to increase span and still have good margins IFF excess margins were present in the first place. However, if the original design had been optimized, even at just one spot, the wiggle room for more span is lost and some level of revision becomes necessary. I have personally witnessed builds where span was added and may not have included sufficient revision to handle the increased span... I would like to not see that level of meticulous building wasted again.

That Fenix has done what he feels is due diligence is terrific. I have no desire to "bless" his mods and design calcs - his mods are his responsibility, and he seems to be taking them seriously. I do think his "report" would make interesting reading.

Billski
 

Toobuilder

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I vacuum bagged them down around a piece of PVC pipe...

... If this is not clear I can likely find some pics of the process.
This is how I plan to do the big fuel tanks (75 inch skins) on the Rocket, but I have never done it before. I'd sure like to shortcut the learning process and see pictures. Do you clamp the pipe to the aluminum to keep it in position on the LE, or something else?
 

Fenix

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All those comments on Van's history are well and good, but do seem thread drift. The rest of the comments support the notion that a tapered wing of more span could be a serious add to the Van's airplanes. I would like to have seen flying reports on davefried's plane, and look forward to them on Fenix's plane.
Billski,

Message received and understood, including the thread drift....

I applaud your cautions about making changes "out of school". The more I learn about aircraft structures (which is a slow process from messing with things and asking questions) the thing that stands out the most is how much there is to know and how what seems like a simple change which really should not affect much, affects A LOT. Many times an idea I once had that I thought was oh so minor I have since learned along that way would not have been a good idea.
I agree with your statement to "other folk that might read this" to be diligent in researching any mods they are considering. Often the outcome is quite a surprise. Researching mods can be hard to do, engineers are expensive and usually don't want to fuss with the guy who calls them up and "has a great idea....."
This forum and the willingness of those who have this expertise to assist is fantastic - I wish I'd discovered it years ago.

For the reasons stated above I would make the engineering report available to you (when I am back home). It would be solely for whatever curiosity or benefit it was to you with "no strings attached" and no request or requirement for review or feedback.
 

Fenix

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This is how I plan to do the big fuel tanks (75 inch skins) on the Rocket, but I have never done it before. I'd sure like to shortcut the learning process and see pictures. Do you clamp the pipe to the aluminum to keep it in position on the LE, or something else?
There are quite few kinks to learn about this process and you will learn them all on your first attempt. After that it is quite easy. I'd be happy to discuss all of this with you in advance and you could probably save yourself ruining the first piece of tank skin (like I did). Not sure how the PM message works here but if you sent me your email that way and I found it I'd be happy to send you a degree of detail that most here don't want to read.

But in short, I have found it best to duct tape the pipe across the middle of the sheet you wish to form by running a strip of duct tape on both sides of the pipe the full length of the sheet. After the pipe is fixed in place to the sheet I folded the entire skin into sort of an envelope, though a bulging one, and put a strip of duct tape the full length of the raw edges where they come together. Because you want to close this envelope up you should put the pipe in the middle of the sheet, even though the portion which wraps over the top of the tank will ultimately be longer than the one which wraps around the bottom of the tank. After it is formed you can cut the bottom side shorter but it is much easier to vac down if the two raw edges meet up to be taped together. Then I wrapped the aluminum envelope or "taco" (but it is a "closed taco") in plastic, use a heavy drop cloth or something heavier. I don't remember the mil thickness I used but I used two layers of the heaviest painters drop cloth at Lowe's. There is other plastic that is even thicker but I used the drop cloths. Also I padded the sharp corners with more duct tape and/or cardboard to keep them from punching through. One 3/16 hole in the PVC pipe to evacuate air will get the job done. The more you drill the faster it will collapse or vacuum down. Faster is not really better because as it collapses and wraps the aluminum around the pipe it tends to push the pipe back away from the fold and you have to do the best you can to hold it tightly in the fold. One person on each end of the alumium panel is highly recommended. Once it vacs down I hold it there about 20 or 30 seconds then release the vacuum and cut the plastic open and take all the tape off. Once it is free you will find the sheet has bent 90 degrees with a blunt rolled radius.

I actually have done this in the past to make a "standard" RV tank and just looked in my notes to see what size pipe I used for that operation. Using a too small pipe can put a tight radius in it that you can't open back up. Guess how I know that. Ok so I threw away my first two attempts. On the successful forming of the standard RV4 wing tank (which I believe would be the same as for your Rocket) I used a 2" Schedule 40 PVC pipe. I found that this was not tight enough of a radius and then repeated the procedure with a 1.5" sch 40 pipe and had what I wanted. In making the skin for the other wing I was reluctant to just use a 1.5" pvc pipe in the first place because I did not know if it would actually yield the same result as the two step process on the other side. I opted to spend an extra 30 minutes and a few dollars of plastic to just repeat the process first with a 2" and then with a 1.5". It might work just fine with a 1.5" from the start and only doing the shrink down once, but I was not willing to take that chance having already ruined two pieces....

I'll try to find some pics of the process to attach, maybe tomorrow.

Again, if you want more boring detail we can talk privately. Good Luck with your tanks.
 

Toobuilder

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Thanks much. Pretty much as I planned, but keeping the pipe from walking back from the LE was my worry. I suppose you really dont need to be concerned about the possibility of the PVC bowing in the middle, because the aluminum will not allow that when the "taco" tightens up...

Good stuff. I'll shoot you an IM so we dont bore the rest of the forum.
 

bifft

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The concern that I had raised had to do with the OP concluding that the basic structure was OK because his MAC was in the same place as the stock bird. I have pointed out that the real max bending moment does increase significantly when span is increased even while tapering the wing. Now maybe the existing structure had enough excess in it that this does not seriously compromise strength, but communicating that a proper check should be performed was my point.
I've also done the math on this (link to the spreadsheet I used in this post from a former taperwing RV thread). I have a -8 not a -4, so aimed on having the same bending moment at the attachment to the center section. Using Shrenk's approximation to calculate that moment. What I find is that with taper you can get more span. But not very much. By my math I can increase the span from 24ft to just over 25ft with a taper ratio of 54% or so.

Not directly applicable to a RV-4 due to the significant difference in how the wing attaches to the fuselage. I'm also using the aerobatic weight and 6g instead of gross and 4.4. But his numbers do pass the smell test for me.
 

rv7charlie

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To stay in the drift for a moment...

My understanding, from what Van 'his own self' has said, is that while the -4 *looks* like a stretched -3, it's a completely separate design. Interesting tidbit; I believe that the 'prototype' -6 (the Chard 6) actually came before the -4; Art was Van's prototype builder, and basically built one for himself because he couldn't get Van to do a SBS. Van finally relented after persistent demand for SBS seating.

The -8 came before the -7. The -8 came about due to the growing Bubba Factor plus demand for ease of installation of the angle valve IO360s, and opened the window into matched hole tooling.

The -7 uses some components common to the -6 (basically most of the fuselage raw components), but was the 1st to be all pre-punched structure & skin, including the wings, fuselage and tail, and uses virtually all the wing & tail components from the -8, to reduce inventory requirements. The -8 became all pre-punched after the -7's introduction.
 

TFF

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I think the tail on the 7 was more than just reduced inventory. The 6”s vertical has always not been not as effective as the same on the 4. The wider fuselage needed more vertical. That the 6 exploded the RV world was the magic combination. They were not going to not miss out and not have an improved side by side. The 4 and 8 are the best dream makers; the 6 and 7 are the best reality makers.
 

davefried

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Wing loads

I had a long think on what I had done to prepare for the new wing design thirty odd years ago. There are some gaps in my records as well as my memory. That comment I made about the spanwise location of the Mean Aero Chord is not related to the bending moment of the wing.

I did use Schrenk to understand the spanwise distribution of load on the wing and recognize that as an approximation there are issues. It carries load at the tip where it might expect to be reduced. In this case the loading would be further out than expected giving an increased bending moment at the root. Using it is conservative.

Schrenk is an average of a linear lift distribution and an elliptic one. The design has a manoeuvring speed of 115 kt and is limited to 6g at 1375 lb. This gives a panel load (we are just talking wing loads in this example) of 4125 lb at Y = 63.8 in for the original and 66.5 for the tapered wing. The tapered wing root bending moment is 4.2% higher than the original wing.

The RV-6 wing is two parts joined in the centre with steel straps and many bolts. The spar caps are laminated and the spanwise extent of these was extended as required. Since the spar tapers there was an issue of the taper diminishing the height of the caps moving outboard. I solved this by reducing the vertical spacing between them. The inner edges remain parallel while the outer taper. This required making the caps deeper and as a result I am carrying around more material than I might need. The design is not optimized and I am not concerned about the strength of the spar caps.

Using Schrenk for stall analysis is a bit of a problem as it underpredicts CLmax. At some point on the span the local Cl will reach the section Clmax. This doesn’t mean the wing stalls there only that the separation begins. For the modest taper that I chose, this begins at 35 to 40 % of span. For this reason, I chose not to twist the wing. I haven’t felt the need to address tip stall further as the flying qualities are excellent.
 
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