Putting a tapered wing on an RV

[Toobuilder]

The "performance increase" of the tapered wing is as easy to quantify as the difference between the hershey bar "sport" wing on the Rocket, and the tapered "Evo" wing on the F-1 Rocket. Down low, no real difference, but up high, the Evo wing really comes alive. My Rocket really falls on its face above 10k, but the Evo is in its element.

 

[rv7charlie]

Y'all need a subscription to Kitplanes. Barnaby Wainfan has recently spent months explaining the ups, downs, ins, outs, etc of various wing planforms.

 

[Fenix]

Pardon my lack of education, but I thought the whole idea of a tapered wing (versus a Hershey bar) was so the maximum G loading could be increased without additional spar weight? If people are building tapered wings and the result is 4.4G (from a post above)..... what did you get for the extra work?

Also, perhaps the engineers here can have pity on a poor uneducated simpleton like me. Wouldn't the fuel in the inboard tanks be what you had to get rid of to allow aerobatic loads on the spars?

My goal was not to increase G loading without increasing spar weight, however the allowable load factor of my tapered wing is in excess of that of the original. Part of this came from the need to make it withstand a bad landing with full outboard fuel tanks (this would actually be a neg G load) as I have no way to dump fuel mass (except very slowly through the exhaust pipes) in the event of landing much sooner than planned.
The 4.4 G I designed for is at 1868 pounds (more on that below) whereas IIRC my standard wing is 4.4 at 1500 LBS and 6 G (Aerobatic limit) of 1375 pounds. A 6G limit could be calculated for my tapered wing and would probably be higher than 1375 pounds but I did not request that computation. Of course such a limit would depend on where the mass was located in the plane. Very low fuel with a lot of fuselage weight (passengers) is not the same as a small solo occupant with a lot of fuel on board. It is my assumption that when Vans set the limit at 1375 they were assuming a worse case loading scenario (the one that puts max bending load on the spar).

The 1868 pounds gross weight exceeds the published limits of the RV4 and likely most specifically its landing gear. However I had the wing engineered from the beginning to be used on a custom built fuselage similar to the RV4 but capable of more weight. In the meantime the F4 Raider has been developed which is very close to what I had in mind for my custom fuselage. Flying this wing first on my RV4 is sort of a proof of concept without having to build an entire airplane and a good opportunity for a performance check of the taper wing vs the rectangular wing.

As stated above by another poster, the concern with the fuel in the outboard tanks for aerobatics is the spin recovery and also the simple fact that the roll response and stick forces with fuel mass near the wingtips is going to be poor and not fun as aerobatics should be.

What did I get for the extra work? I like to monkey with things and solve challenges. I like to create and just assembling a kit, whether it is a BBQ grill or an airplane, does not give me the sensation I crave. The other thing I stand to gain is an airplane that is "prettier", at least to my eye, which is the eye I seek to satisfy. There is probably no other pursuit in which giving a nod to subjective beauty costs so much in terms of other areas of sacrifice as it does in aircraft design. I think Van must have understood this very well. He also seems to have understood how to run a manufacturing and marketing balance with very narrow tolerances for error. He succeeded in an industry where almost all (except the big guys - Cessna, Boeing, etc.) have failed. Along the way he did more for GA and tens of thousands of people that wanted to fly than probably anyone else ever has except for Orville and Wilbur.
But I (along with many others I suspect) make a lot of decisions based on how things look. These include the model of vehicle I drive, certainly the color, the interior of my home, and to lesser extent the exterior, and to a very large extent the girls I choose to talk to and aspire to date as well as countless other decisions that make no logical sense.
I read the forum debates of how much better a tapered wing would look (to some) and the concern that it would affect stall speed and characteristics too much. I too wondered what the effects would be, so the other thing I will get is the answer (which yes some engineers and computer simulations could also have provided). I could have built a wing that would have maximized certain "goals" such as a different airfoil or MUCH higher aspect ratio since I was going through all the effort anyway. But this would not have really answered the question of simply the effect of tapering because I'd have changed so many other things.

Obviously I did opt to extend the range somewhat by increasing fuel capacity. Some suggested a change in the airfoil to a "faster" one to increase the range, but when I looked at these it was apparent that most faster airfoils (laminar flow for example) have a much smaller area forward of the spar, where the fuel tank is. This reduced the fuel quantity so much that the range actually was reduced, even with higher TAS. Yes, the fuel tanks could have been moved, but then I have totally undone the goal of just changing from rectangular to tapered.

 

[wsimpso1]

Lets recall that in these tapered wing RV's all the rest of the airframe including the attachments and center section stay the same, so you can not stretch the g's nor can you extend the span. The lift distribution will remain pretty close to elliptical.

If you hold span and root chord, but taper, you should make modest drag reduction from reduced wing area, modest induced drag reduction at the tail from lowered trimming loads, some modest weight reduction from skin area, and maybe improve roll rate.

If you hold root chord and wing area, span goes up, you reduce span loading which reduces induced drag everywhere, but now you have to increase spar weight to carry the increased bending.

These are standard design tradeoffs with the constraint that you really can not increase loads anywhere in the rest of the airplane... the wing mountings being my concern.

Billski

 

[Fenix]

Billski,

You speak sagacious truths. The interactions you point out are quite clear when stated as clearly as you did.

However, it is perhaps worthy of mention that in the RV7 and 8 there are wing mounting points to be concerned with, which you stated. These designs have a spar carry through section to which the wings attach on each side near the fuselage skin.

In the RV-4 (and I believe also the RV-6) the situation is not the same. What you have is a spar box or opening through the fuselage through which the wing spars pass. They are joined at the middle but this method of joining could be modified to increase "integrity". Then there are additional provisions (4 AN4 bolts in the RV4) that are tasked with attaching the fuselage to the wing structure. This also lends itself to modification to increase the wing to fuselage integrity.
The spar box is of a given size that limits spar overall dimension but there is no "spar carry through" that must not be overloaded in the RV4 (or the F1 Rocket or F4 Raider except in the case of the F4 there is a provision to use RV8 wings and therefor a "pre-engineered" spar carry through.)

In regards to the rear spar, all designs (RV4, 6, 7, and 8) have a spar carry through that must either be changed or "complied with". I think (subject to correction) however that the rear spar loads are minimal given this is a metal skinned (not fabric) wing.

In my case I did not attempt to engineer more "spar center section integrity" but I think it is quite possible if that was in the design goals of someone modifying an RV4 or RV6. In the case of an RV 7 or 8 I believe it is exactly as you described. All of the above is subject to correction as I have no credentials (not an engineer) to make any such assertions.

 

[davefried]

Lets recall that in these tapered wing RV's all the test of the airframe stays the same including wing attachment points, so you can not stretch the g's much nor can you extend the span very much. The lift distribution will remain pretty close to elliptical.

If you hold span and root chord, but taper, you should make modest drag reduction from reduced wing area, modest induced drag reduction at the tail from lowered trimming loads, some modest weight reduction from skin area, and maybe improve roll rate.

...

Billski

I held the root chord while changing the taper, span and area. What I was after is as you stated, as well as improved lift and drag from the more efficient wing. The latter compensates for the reduction in wing area. The load capacity hasn't changed so there is no concern about the structure connecting the wing to the fuselage. I just took the design that extra step, it is still the same mission. Much more than that and it becomes a new airplane.

 

[davefried]

For Fenix:

You mentioned that you started with a flyable RV-4 and are preparing a new tapered wing to replace the original. What a great opportunity to compare the two.

The predrilled spars were not available when I built my wing and I cannot simply plug a new wing into the existing match drilled centre-section. I assume that you have solved the problem of transferring the bolt hole patterns to the new spar. As I recall they were drilled undersized and then reamed to fit.

What was the gross weight of your RV and with all of the new fuel available, what weight are you planning for the new configuration. You mentioned the capacity earlier but perhaps you plan to fly long distance solo.

Edit:

I just re-read post 23. This new wing is something you are trying out with an eye to a custom fuselage. The original gross weight for the RV-4 was 1500 lb., increasing that to 1868 lb. is rather ambitious. The original fuel capacity was 32 gallons, the additional 40 gallons that you are planning amount to 240 lb.. You seem to be allowing for additional weight growth. Do you have a weight comparison for the old and new wing panels?

For the sake of flying qualities, the two wings can be flight tested at the same flying weight, lots to learn there.

 

[Lendo]

I agree , a Tapered wing does look nicer, for the same area the Span will be longer and the Tip will be smaller with lower Induced Drag. One might think teh smaller the tip the better but Texts suggest no smaller than 50% Root, but I have seen 40 and 45% mentioned, but concern over Aileron Reversal is also mentioned due to the Tip being too flexible.
Also Texts suggest that tapering thickness 15% to 12% does tend to increase the Stall at the Tips, where washout helps in this area it also reduces Lift efficiency.

Ever wonder what some design with a straight leading edge, well sweep also reduces efficiency by distorting the Airfoil shape as the incoming air sees it. I didn' think about the fuel tank issue ( good one) However sweep to 20° measured at 25% is small enough not to be a great concern, however after that the inefficiency increases exponentially requiring more wing area to off set the losses. Note for instance all he RV Vertical tails.

A straight Trailing Edge keeps the Trailing vortices even. I guess this helps in a number of ways, so too does Elliptical Lift Distribution which the Tapered Wing is the closest shape, other than an actual ellipse.

One would then think a Rectangle wing would be best - except for the Induced Tip Vortex Drag.
Everything's a trade-off.
George

 

[Fenix]

For Fenix:

You mentioned that you started with a flyable RV-4 and are preparing a new tapered wing to replace the original. What a great opportunity to compare the two.

The predrilled spars were not available when I built my wing and I cannot simply plug a new wing into the existing match drilled centre-section. I assume that you have solved the problem of transferring the bolt hole patterns to the new spar. As I recall they were drilled undersized and then reamed to fit.

What was the gross weight of your RV and with all of the new fuel available, what weight are you planning for the new configuration. You mentioned the capacity earlier but perhaps you plan to fly long distance solo.

Edit:

I just re-read post 23. This new wing is something you are trying out with an eye to a custom fuselage. The original gross weight for the RV-4 was 1500 lb., increasing that to 1868 lb. is rather ambitious. The original fuel capacity was 32 gallons, the additional 40 gallons that you are planning amount to 240 lb.. You seem to be allowing for additional weight growth. Do you have a weight comparison for the old and new wing panels?

For the sake of flying qualities, the two wings can be flight tested at the same flying weight, lots to learn there.

Yes it was the opportunity for a "head to head" comparison that really piqued my interest. The engineer who worked with me on this was also interested in the ability to make this direct comparison. If it had not been for the opportunity to compare the two I don't think the engineer would have been curious enough to take on this job.

Yes the issue of building a new wing that would fit in place of the original one was a point of real concern given the "hand made" nature of most homebuilts, to include the RV4. And yes, they are all joined with close tolerance bolts as you described. I spoke several times with Wayne of Phlogiston products (who made a lot of the spar components and a lot of completed spars for the RV4 specifically) about this challenge and how it could be tackled. He was very helpful, an all out "good guy" and gave me a lot of tips, techniques and advice for how to make an accurate reproduction of the original spar.

Of course it involved taking the wings off of my plane to make patterns, not only of the main spars but also of anything else I thought might be relevant, most notably the relative positions of the main and rear spar as these also change a bit from plane to plane. Taking out all those (I think there are 28) close tolerance bolts in a somewhat hard to reach place, and putting them back in again was not much fun. But in about 2 weekends work I had them off and back on again with a nice set of patterns. Soon I get to do it all over again, and then I will know how well the new wings fit. I may be unpleasantly surprised, but I'm pretty confident they will line up well after taking all of Wayne's advice.

In regard to your question about the weight limits:
As I recall the gross weight for the RV4 is 1500 LBS. My original empty weight as I recall is 1050. Yes this wing is engineered for considerably more weight than the 1500 Lbs of the RV4. Some of the RV4's limits are due to its wing, which in my case will be changed. Of course there are also limits of the fuselage (of which I have not analyzed) and most notably the landing gear. My original plan was to fly this wing on my RV-4 and, if I liked the way it behaved, to then build a new airplane for this wing and return the RV4 to its original condition. The additional weight capability of the tapered wing is intended to be used on the "next plane". I don't intend to fly the RV4 beyond its stated design limits. Yes, as you stated, you can fly further solo because you can put more fuel.

I think I stated above (or maybe only intended to) that the plane I had in mind for this wing was something very similar to what is now the F4 Raider. So instead of designing a new fuselage for this wing I may just build the F4 design and put a tapered wing on it. If I do that this wing will not work because the F4 is wider than an RV4 (the plane I would have scratch built for it would not have been wider). So this wing may end up staying on my RV4 forever, yes with more weight capacity than can be used on that fuselage. But now that I have all the fixtures and techniques sorted out I believe I can build another set of wings quicker than I can design and scratch build a fuselage. I began to take notes along the way of things I learned in the building of the first wing so I would not forget them before building the second wing. Having all the fixtures and techniques and tricks sorted out I built the second wing from scratch in about 225 hours. I have had the analysis done for extending the center portion of this wing by 4" for use on the F4 so that is likely what will follow if I like the characteristics of this wing. If I go this route I will have to comply with the limitations of the F4 of course, but IIRC the gross weight limit of the F4 is 1850, very close to the 1868 I designed for.

The final decision of whether I use the F4 design or complete a design of my own fuselage will not be made until flight testing is done. Due to the washout I put in the wing the "average angle of incidence" is a bit less as it relates to the fuselage than it is on the original RV4. This may result in the plane flying somewhat "nose up" to get the required "average angle of attack" for level flight. The engineer was particularly curious about the outcome of this condition. We don't think it will make much difference (but again this design was subject only to a structural analysis, not a performance analysis or stability analysis) but if it turns out the plane does fly "nose up" with this wing compared to the original wing the fix will be to change the angle of incidence when mounting the wing to the fuselage. If this turns out to be needed I will not build the tapered wing F4 but instead do a new fuselage design as I believe the incidence on the F4 is that same as on an RV4.

As far as a weight comparison of the rectangular vs tapered wing panels: No actually I have not weighed the new wing panels yet. They have not yet been fully assembled with flap, aileron, and both tanks all on the wing at the same time so I never took occasion to weigh an assembly. Yes, I could simply weigh the components individually and arrive at the unpainted wing weight but it never dawned on me to do that and now I am not on the same continent as my project so sadly cannot provide a wing weight. However I will post that info when it is available and then later the performance changes I measure from flight testing.

I hope that addresses your questions in a meaningful fashion, but if you have more, feel free to ask, I'll do my best with the info I currently have access to.

 

[BJC]

Very interesting project. I hope that you will accurately document all performance parameters in each configuration. Too many modifications are described as “better” without actual data.

Please post photos when you can.

BJC

 

[davefried]

Yes it was the opportunity for a "head to head" comparison that really piqued my interest. The engineer who worked with me on this was also interested in the ability to make this direct comparison. If it had not been for the opportunity to compare the two I don't think the engineer would have been curious enough to take on this job.

Yes the issue of building a new wing that would fit in place of the original one was a point of real concern given the "hand made" nature of most homebuilts, to include the RV4. And yes, they are all joined with close tolerance bolts as you described. I spoke several times with Wayne of Phlogiston products (who made a lot of the spar components and a lot of completed spars for the RV4 specifically) about this challenge and how it could be tackled. He was very helpful, an all out "good guy" and gave me a lot of tips, techniques and advice for how to make an accurate reproduction of the original spar.

Of course it involved taking the wings off of my plane to make patterns, not only of the main spars but also of anything else I thought might be relevant, most notably the relative positions of the main and rear spar as these also change a bit from plane to plane. Taking out all those (I think there are 28) close tolerance bolts in a somewhat hard to reach place, and putting them back in again was not much fun. But in about 2 weekends work I had them off and back on again with a nice set of patterns. Soon I get to do it all over again, and then I will know how well the new wings fit. I may be unpleasantly surprised, but I'm pretty confident they will line up well after taking all of Wayne's advice.

In regard to your question about the weight limits:
As I recall the gross weight for the RV4 is 1500 LBS. My original empty weight as I recall is 1050. Yes this wing is engineered for considerably more weight than the 1500 Lbs of the RV4. Some of the RV4's limits are due to its wing, which in my case will be changed. Of course there are also limits of the fuselage (of which I have not analyzed) and most notably the landing gear. My original plan was to fly this wing on my RV-4 and, if I liked the way it behaved, to then build a new airplane for this wing and return the RV4 to its original condition. The additional weight capability of the tapered wing is intended to be used on the "next plane". I don't intend to fly the RV4 beyond its stated design limits. Yes, as you stated, you can fly further solo because you can put more fuel.

I think I stated above (or maybe only intended to) that the plane I had in mind for this wing was something very similar to what is now the F4 Raider. So instead of designing a new fuselage for this wing I may just build the F4 design and put a tapered wing on it. If I do that this wing will not work because the F4 is wider than an RV4 (the plane I would have scratch built for it would not have been wider). So this wing may end up staying on my RV4 forever, yes with more weight capacity than can be used on that fuselage. But now that I have all the fixtures and techniques sorted out I believe I can build another set of wings quicker than I can design and scratch build a fuselage. I began to take notes along the way of things I learned in the building of the first wing so I would not forget them before building the second wing. Having all the fixtures and techniques and tricks sorted out I built the second wing from scratch in about 225 hours. I have had the analysis done for extending the center portion of this wing by 4" for use on the F4 so that is likely what will follow if I like the characteristics of this wing. If I go this route I will have to comply with the limitations of the F4 of course, but IIRC the gross weight limit of the F4 is 1850, very close to the 1868 I designed for.

The final decision of whether I use the F4 design or complete a design of my own fuselage will not be made until flight testing is done. Due to the washout I put in the wing the "average angle of incidence" is a bit less as it relates to the fuselage than it is on the original RV4. This may result in the plane flying somewhat "nose up" to get the required "average angle of attack" for level flight. The engineer was particularly curious about the outcome of this condition. We don't think it will make much difference (but again this design was subject only to a structural analysis, not a performance analysis or stability analysis) but if it turns out the plane does fly "nose up" with this wing compared to the original wing the fix will be to change the angle of incidence when mounting the wing to the fuselage. If this turns out to be needed I will not build the tapered wing F4 but instead do a new fuselage design as I believe the incidence on the F4 is that same as on an RV4.

As far as a weight comparison of the rectangular vs tapered wing panels: No actually I have not weighed the new wing panels yet. They have not yet been fully assembled with flap, aileron, and both tanks all on the wing at the same time so I never took occasion to weigh an assembly. Yes, I could simply weigh the components individually and arrive at the unpainted wing weight but it never dawned on me to do that and now I am not on the same continent as my project so sadly cannot provide a wing weight. However I will post that info when it is available and then later the performance changes I measure from flight testing.

I hope that addresses your questions in a meaningful fashion, but if you have more, feel free to ask, I'll do my best with the info I currently have access to.

Thanks for taking the time to explain the goals of your project. It is well considered and will be interesting to hear of your progress.

 

[wsimpso1]

I held the root chord while changing the taper, span and area... The load capacity hasn't changed so there is no concern about the structure connecting the wing to the fuselage.

Increase the span and maintain same g-load is same lift but centered further out on the wing, and MUST EQUAL increased bending moment in the attachments and center section. The center section is NOT automatically fine just because the lift is unchanged.

Did you do any checks of the center section design under the increased bending moment at the root? Has anyone else done this? Maybe the RV6 has excess capacity here, but you had better know prior to pulling g's.

Billski

 

[wsimpso1]

In the RV-4 (and I believe also the RV-6) the situation is not the same. What you have is a spar box or opening through the fuselage through which the wing spars pass.

Where the wing spars overlap each other spanwise and are pinned to the fuselage, it is true that the fuselage sees no bending moment, and the mounting points see that same lifting forces. But if you have same g load, same airplane weight, and more span, the spars and the attaching bolts connecting the spars together MUST see more load scaling with the increased bending moments in the wing.

Reduce aerobatic weight and/or max g accordingly and you are home. Leave aerobatic weight and max g alone, and you had better have checked your margins on the pins connecting the two wings together, as their loads do increase.

Billski

 

[davefried]

Increase the span and maintain same g-load is same lift but centered further out on the wing, and MUST EQUAL increased bending moment in the attachments and center section. The center section is NOT automatically fine just because the lift is unchanged.

Did you do any checks of the center section design under the increased bending moment at the root? Has anyone else done this? Maybe the RV6 has excess capacity here, but you had better know prior to pulling g's.

Billski

I increased the span and reduced the chord moving outboard (taper). The spanwise location of the mean aerodynamic chord is within a fraction of an inch of the original wing. At the design weight there should not be a change in root bending moment.

 

[Fenix]

Very interesting project. I hope that you will accurately document all performance parameters in each configuration. Too many modifications are described as “better” without actual data.

Please post photos when you can.

I intend to document performance parameters in as many aspects as I could think of - and do so numerically! I did some data collection flights with the standard wing at a recorded weight and CG that I would be able to duplicate with the tapered wing regardless of whether it weighs more or less and has some moment shift. I collected a variety of data to include stall speed with flaps up and down, TAS, roll rate (I timed an aileron roll at full deflection, not sure that is the scientific standard but it is numeric data), rate of climb, rate of descent at engine idle, etc.
I will do the same during the testing of the new wing and report. It will be subject to the accuracy of my instruments and consistency of piloting skill, or lack.

I will try to figure out the photo posting procedure and put a few up. Maybe tomorrow.

FYI in regard to timing, I had intended to do the "wing swap" and begin flight testing in summer 2020. Covid and the travel mess prevented me from re-uniting with my plane/project. So now I hope to do that in summer 2021.

 

[Fenix]

Where the wing spars overlap each other spanwise and are pinned to the fuselage, it is true that the fuselage sees no bending moment, and the mounting points see that same lifting forces. But if you have same g load, same airplane weight, and more span, the spars and the attaching bolts connecting the spars together MUST see more load scaling with the increased bending moments in the wing.

Reduce aerobatic weight and/or max g accordingly and you are home. Leave aerobatic weight and max g alone, and you had better have checked your margins on the pins connecting the two wings together, as their loads do increase.

Billski

Billski,

I believe you have the "correct mental picture" of the arrangement. Perhaps you have even seen RV4 blueprints and understand it exactly. But in the interest of clarity: The wings are joined with a steel strap on the fwd and aft sides of the upper and lower spar caps. (So 4 steel straps). The spar caps themselves butt up against each other at the Aircraft C/L. The steel straps and spar caps are through bolted with the close tolerance bolts. (I think it was a total of 24 AN5 bolts and 4 AN4 bolts). Then near the fuselage left and right skin there are a total of 4 AN4 bolts (these are not close tolerance bolts) that transfer loads between the wing assy and fuselage. These 4 bolts do NOT also attach the left and right wing panels together. So there is one assy and bolts which join the two wing panels together and another assy/bolts which attach the wing assy to the fuselage. I think you already understand the above, and don't think it makes any changes to your logic.

Yes I understand your description how these loads will be higher than they were before, even at the same gross weight and G limits. The loads on these wing joining straps and bolts were analyzed at 1868 LBS and 4.4 G's (assuming that the wing was fully fueled. If the fuel were very low and fuselage were loaded up with cargo and heavy occupants it would be a difference scenario - so there is a "zero fuel weight limit" also)

The results were that it meets the "150% rule" of A/C design. However it is not much beyond that and, while Vans calls out that oversized NAS bolts can be used in this assy if needed, I was advised in the engineering report that 4 of the bolts could not be oversized, due to edge margins. Some of the edge margin reduction is caused because I used the steel straps from Vans that are heat treated to a specific specification. I did not want to attempt to reproduce this procedure. Because Vans straps are angled for the "standard dihedral" and because of taper my upper spar cap is not the same dihedral as the lower spar cap (which is standard RV) the strap on the upper cap begins to run closer to the edges of the spar cap, reducing edge margin.
So the math has been done to check the above described condition at 4.4 G's but I did not ask or determine at what load condition it was OK to 6G's. Aerobatics are not my intention with this design. Thanks for your input and alerts of possible hazards.

 

[Fenix]

ncrease the span and maintain same g-load is same lift but centered further out on the wing, and MUST EQUAL increased bending moment in the attachments and center section. The center section is NOT automatically fine just because the lift is unchanged.

Did you do any checks of the center section design under the increased bending moment at the root? Has anyone else done this? Maybe the RV6 has excess capacity here, but you had better know prior to pulling g's.

I stand to be corrected but I think the construction of the RV6 center section is the same as I described above of my RV4. That being that there is no separate fixed center section that remains a part of the fuselage when the wings are removed. In the case of the RV6 of course (being side by side instead of tandem and so wider) the bolts that attach the wing assy to the fuselage are further from the A/C Centerling and thus further from the "wing splice" than they are in the narrower RV4. I'm not sure how this plays into the equations.

So in regard to the question of doing checks of the center section under increased bending and the matter of the RV6 possibly having excess capacity:
As I described in the case of my plane (above) these checks were done at the wingspan of 25' which I understand to be the same as Dave's wingspan. So it appears that, yes, there is "extra capacity". Here I should repeat that my checks were done with a narrower fuselage which would change the point at which the fuselage loads are imposed on the wing spars. This may be relevant! I don't know. I also don't know if Dave built his wing spar according to the standard RV4 and 6 design except for taper. But if he did: The wing spar in my tapered wing is of the same basic design as the standard RV4, that being of stacked up and riveted together spar caps and an aluminum web. The spar caps all end at different points so the spar gets progressively less substantial as you move outboard. While this is similar to the standard RV4 spar in design, the actual point at which these spar caps terminate is a lot different. Mine extend substantially further outboard than in the standard wing, this is in part due to the potential of landing with full fuel in the outboard fuel tanks. While the spar caps in the outer wing portions are "thicker" than in the standard wing, the area near the center section is identical in amount of material. It really can be made no bigger and still fit in the "spar box". While the order of the "spar stack" is a bit different the amount of material remains the same. For example I think the standard spar IIRC is two .040 aluminum webs separated by one of the 1/4" thick spar caps and then has more 1/4" spar caps stacked on until the spar pocket is full. My tapered spar has, in the center region, one .050 and one .032 (so it is actually .002 thicker but still is expected to be able to be shoved into the spar box) web sandwiched against each other and then the 1/4" thick caps are stacked up in the same number. The spar caps are the same width also (I think 1.5") as the standard ones. So in short my "center region" is nearly identical to the standard RV4 in this region. If Dave's is also "per standard RV4/6" I suspect the analysis of my spar should largely carry over to his - subject again to any effect caused by the wider fuselage.

Good points and thanks for the cautions Billski. Also, when I return to the US I can provide my engineering report (it is an inch thick in printed form and not available for email) to you for a more thorough analysis to determine if in fact it "carries over" to your (Dave's) wing. Perhaps you've been pulling 6 G's for years now and have already determined its suitability.

 

[wsimpso1]

I increased the span and reduced the chord moving outboard (taper). The spanwise location of the mean aerodynamic chord is within a fraction of an inch of the original wing. At the design weight there should not be a change in root bending moment.

I urge you to revisit your calculations. Since you are talking about the location of MAC, it sounds like you are using a lift distribution based upon plan form area. This raises calculated moments over the entire span that we then feel compelled to design for.

Reality is that lift is pretty much elliptically applied to the span regardless of planform and wing taper. Want some evidence? There are lots of photos of tactical jets pulling g's on a humid day. The standing cloud above the wing looks elliptical, even with sharply tapered wings. This is because pressures on both the top and bottom of the wing unavoidably become equal at the tip. There is a huge field of air around the plane making the leakage of air around the tip a big smoothly changing thing. So it looks elliptical and falls to zero at the tip.

The argument remains on a couple points:

You can either assume lift is spanwise elliptically distributed (ignoring effect of chord) or assume pressures are spanwise elliptically distributed (including effect of chord). Either way, if you preserve wing area and lift the same load, the bending moment goes up when you increase span... You have to lose a lot of wing area to only have the same bending moment when span goes up.

A lot of folks like Shrenk's Approximation, where you split the difference between uniform pressure and elliptical pressure. Hmmm. I do not favor its use - real lift must fall to zero at the tip, and Shrenk's Approximation gives us higher load out to the tip, giving us higher bending moments that we then design in extra structure that adds to excess weight. Better to play with lift and pressure distributed elliptically and use closer approximations. Want to put in a deflected aileron? You just multiply lift (or pressure) by a factor through the part of the span with the aileron.

PM me if you want to discuss this privately...

Billski

 

[wsimpso1]

But in the interest of clarity: The wings are joined with a steel strap on the fwd and aft sides of the upper and lower spar caps. (So 4 steel straps). The spar caps themselves butt up against each other at the Aircraft C/L. The steel straps and spar caps are through bolted with the close tolerance bolts. (I think it was a total of 24 AN5 bolts and 4 AN4 bolts). Then near the fuselage left and right skin there are a total of 4 AN4 bolts (these are not close tolerance bolts) that transfer loads between the wing assy and fuselage. These 4 bolts do NOT also attach the left and right wing panels together. So there is one assy and bolts which join the two wing panels together and another assy/bolts which attach the wing assy to the fuselage. I think you already understand the above, and don't think it makes any changes to your logic.

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, maybe the stock straps and bolts are OK, and maybe they need beefup for longer wings. Either way, you have to do the check to know that it is safe to fly.

Billski

 

[Toobuilder]

FWIW, the general consensus is that the RV-6 laminated spar is massively overbuilt and essentially unbreakable.