Putting a tapered wing on an RV

[davefried]

That seems correct. Since the "Schrenk line" is an average of elliptical lift and planform lift, if the planform were elliptical both the planform line and elliptical line would be, well, elliptical. Therefore the average of the two identical lines would be the same identical line. The only way the schrenk line will go to zero at the tip is if the planform goes to zero at the tip, such as the elliptical planform or the unlikely case of a wing that tapered to a point at the tip.

Here is a wing planform where Schrenk will predict zero lift at the tip.



Not the best way to increase effective aspect ratio as the tip vortex is looking for an edge out at the tip to attach to. This one will attach somewhere between the tip and the aileron edge.

 

[cluttonfred]

As I have said to myself many times on various projects, "You're not building the Space Shuttle."

The heart of practical engineering judgment is discerning between zero and near zero (to three or four decimal places). Since your margin of safety in aircraft design will be 50% or greater, you can decide whether those thousandths will matter.

(Please note this is not an endorsement of sloppy engineering, only a perspective on the usefulness of the precision of our calculations. Run your wing bending loads with purely trapezoidal lift distribution, then by Shrenk, then by lifting line theory, and see how much practical differenece it makes in your spar moment of inertia requirement, and decide for yourself.)

 

[davefried]

Have you ever spun it? I would expect that at least to be a little different.

No spins thanks. Remember the posts about the change in fin size going from RV-6 to 7?

 

[BoKu]

...This one will attach somewhere between the tip and the aileron edge.

...And all the wing outboard of that is less than useless--basically all drag and no lift.

 

[rv7charlie]

RE: my lost in build quality noise comment...
This was not intended to be a comment on sloppy workmanship, but a quick & dirty way of saying details/fit/finish/etc will typically make more difference than the wing planform, on performance. Example:
The *claimed* top speed of a Lancair 360 (200 HP) is 260 mph, though I haven't seen many owners claim that high a speed. The fastest Hershey Bar wing RV6 (180 HP) that I'm personally aware of, built by Tracy Saylor, topped out at north of 235, in closed course races. The fastest RV4 (200 HP), built by Dave Anders, was tested by the CAFE Foundation at 250 mph. IIRC, he's gone significantly faster than that in races; he's constantly changing stuff on the airframe.

Now, speaking as a 3-time RV purchaser and current builder, I think the Lancair is a truly stunning airplane; far better looking than any RV. To get top speed roughly equal to a well-executed RV, it gives up a *claimed* 8 mph in stall speed, at 63 mph. But with 73 sq ft of wing vs 110 sq ft for the RV, I kinda suspect the numbers are fudged a bit on the low end, as well (likely closer to 75 than 63). So if you give the Lancair enough wing to meet the RV's stall speed, the top end for a really clean RV will be as high (possibly higher) than a typical Lancair, which should be, out of the box, near-perfect aerodynamically, with that tiny wing and perfect looking fuselage.

So....performance based on wing planform choice can easily get lost in build quality noise. ;-)

On the RV7 rudder issue: Some -7 builders have opted to go back to the smaller -8 rudder, preferring its control harmony over the larger -9 rudder. Van never said that the -8 rudder was unsafe on the -7; just that spin recovery was not quite as quick they wanted for customer-flown a/c. It's worth mentioning that the factory cautions about playing with spins in all their models. They don't tell you not to do it, but to expect...exciting... spin rates.

edit: Not much to show yet on the rotary (Mazda Renesis) front. The FWF is 'done', but the plane hasn't flown. So as they say about how long battle plans survive...

Charlie

 

[Fenix]

RE: my lost in build quality noise comment...
This was not intended to be a comment on sloppy workmanship, but a quick & dirty way of saying details/fit/finish/etc will typically make more difference than the wing planform, on performance.

Charlie,
In thinking further about your build noise comment I later came to the conclusion that what you meant by "noise" was as you have above clarified. Basically that the difference in a rectangular vs tapered planform will be smaller than the differences in performance from one "identical" copy to the next, thus difficult to measure among the "noise". I suspect you are correct, not to mention the "noise" of the margin of error of measurements and repeating the exact same condition (weather, weight, CG, etc.)
However even this determination is of some value as some express that a materially higher stall speed will be part of the "price" of a tapered planform. Yes the engineers can answer this question, and some have provided some of that information above. For some reason I just chose to get this answer "in aluminum" instead of on paper. Plus I was ready for a new project and wanted to learn something about design.....

 

[rv7charlie]

I'm thinking something like a "wankel/mazda" not something like a Gnome. What stage is that conversion in? I'd like to see a pic of the engine cowl if it is built.

Should have answered this earlier. My cowl is the standard James 'Holey Cowl' that he made for 180 HP Lycs. I find myself always trying to keep my options open, which is usually worse than just fully committing to something. But James also made a cowl for Mazda rotaries. Below is one of the nicer examples. I tried to find a better shot of just the cowl, but all my pics are either with people standing in front, or of the actual engine installations with the cowl removed.

Charlie

 

[davefried]

The comment about details/fit/finish/etc. typically making more of difference to performance than wing planform is a bit of a stretch. I am sure that there is some variation between different versions of the same aircraft due to fit, finish and other installation choices, that's typical. Changing the planform is unusual and unless that is the only thing being changed, very difficult to discount on the basis of build quality.

It is possible to predict the differences due to planform analytically. In the case of my wing, I have posted about stall speeds that were not significantly different from the original and that was by design. Any changes that result from the planform modification are likely to be modest, I didn't set out to revolutionize the design.

Someone once put a different wing on a Stitts Playboy and learned something. For me, the exercise was worth the effort, I learned a lot. Others will have a very fine result if they build it to plan and simply fill the predrilled holes with rivets.

 

[rv7charlie]

Hi Dave,

My sincere apology if my comment was offensive. It wasn't intended to be, as I wrote in later posts. I tried to clarify with what I wrote comparing a Lancair360 to a couple of Van's designs, where the performance differences really are more dependent on attention to detail than wing planform.

As you know, the Playboy wing change was much more than a planform change, and the real benefits weren't seen until a completely new airplane was designed. The RV1 was restored a few years ago & IIRC, donated to EAA. The wing change was certainly a significant improvement (in performance) over the original Playboy, but gave up the wing fold mechanism and didn't come close to what we think of as RV performance today.

I applaud your willingness to do something different. It will almost certainly *look* better than my straight wing RV, and it would likely outperform mine, if you could get me up to nosebleed levels (you can't). My point was that on a pure performance basis, one could almost certainly make the plane perform at or better than a tapered wing, with simple aero cleanup processes.

Again, it was intended to be a light-hearted comment about aero realities, and I'm sorry if it was offensive.

Charlie

 

[davefried]

Hi Dave,

My sincere apology if my comment was offensive. It wasn't intended to be, as I wrote in later posts. I tried to clarify with what I wrote comparing a Lancair360 to a couple of Van's designs, where the performance differences really are more dependent on attention to detail than wing planform.

As you know, the Playboy wing change was much more than a planform change, and the real benefits weren't seen until a completely new airplane was designed. The RV1 was restored a few years ago & IIRC, donated to EAA. The wing change was certainly a significant improvement (in performance) over the original Playboy, but gave up the wing fold mechanism and didn't come close to what we think of as RV performance today.

I applaud your willingness to do something different. It will almost certainly *look* better than my straight wing RV, and it would likely outperform mine, if you could get me up to nosebleed levels (you can't). My point was that on a pure performance basis, one could almost certainly make the plane perform at or better than a tapered wing, with simple aero cleanup processes.

Again, it was intended to be a light-hearted comment about aero realities, and I'm sorry if it was offensive.

Charlie

I wasn't offended only surprised that you maintain a simple aero cleanup would produce performance gains in excess of a planform change. Do they have to compete?

I would love to see an RV pulled from a close tolerance mold with all of the tweaks to the surfaces, gaps and cooling flow that you could imagine. I don't believe it would be simple but it would be one slick little airplane. Then I would put a laminar flow tapered wing on it making it that much better (and looking like a Lancair 360 with fixed gear).

The planform change was part of an aero cleanup as was the free castering tailwheel (no chains), the retractable tiedown rings, radiused wing root fairings and hidden rudder cables. I don't know which contributed the most as I just built it that way rather than one change at a time.

I have made performance estimates due to the planform, I will try to post a little more on that subject.

Not offended,
Dave

 

[rv7charlie]

I'm looking forward to seeing your test results. I'd love to be proven wrong!

On the aero cleanups issue, both the RV6 and RV4 I mentioned are well documented as to their speeds, in competition with other RVs (and Lancairs, Glassairs, etc). They both have the original hershey bar wing planform, but there are various 'details' that you might only notice if they're pointed out. I've seen both planes, and talked to the builder of the -6 and the -4's builder's wife, who knew more about the plane than most builders.

Some of the stuff is really non-intuitive. For instance, you mentioned radiused root fairings. I wouldn't pretend to know how yours will perform on the tapered wing, but on the straight wing RVs, it's been repeatedly demonstrated (before/after on the same a/c) that the 'stock' flat wrap sheet metal fairing Van supplies is measurably faster than typical radiused 'glass root farings.

Have you looked at the F1 Rocket Evo numbers? It's really hard to compare, because we're going from a stock RV wing to the 'clipped' Rocket wing, to the tapered, increased span Evo wing. Here's the writeup from Mark, the Team Rocket principal, back in 2005:
Final performance Assessment | Team Rocket Aircraft
Unfortunately, after mentioning the importance of specifying altitude (even more critical when wingspan changes are in play), he omits it in his numbers. He does 'estimate' about a 10 kt increase for a pure wing change, but that's from the original clipped Rocket wing (not the wider span RV wing), and again, altitude isn't specified. For reference, a bone-stock 200 HP RV8 is listed at ~190 kts top speed. Here, we're talking about a 290 HP tapered wing Rocket. For a 'sanity check', he's also claiming a 4-5 mph reduction in stall speed vs an RV, with a heavy 6 cyl engine instead of a 4 (300 lbs higher gross weight), and with 102 sq ft of wing vs an RV's 116 sq ft. In classic internet fashion, 'Amazing (If True)'. ;-)

A better comparison might be to one of the 'Super 8s' that are flying with 6 cyl engines. This EAA article makes interesting reading. The numbers in that article do at least pass an initial 'smell test'.

Charlie

 

[Lendo]

rv7charlie, I guess your not using a PP in the Rotary, if you did I had a suggestion port design.
George

 

[rv7charlie]

Hi George,

I'm using a Renesis. I've considered purchasing a 2nd engine & P-porting the intake, but right now I'm just trying to finish assembly and get the plane flying. I'm always open to new ideas, so fire away. We might want to take it to the rotary section, though; it doesn't have much to do with tapered wings. ;-)

Charlie

 

[btravis]

Just to throw in a data point, I have been racing my O-360 carbureted RV4 on and off for over 10 years. When I first started racing, I was turning in top TAS at 8000 ft of about 208 mph. I have consistently improved the aerodynamic drag of the airplane though out the years, and my airplane will now do about 227 at 8000 ft, and 231 with some clipped tips that I use for racing. Some of the speed increase is also due to more horsepower due to faster cruise speeds. I have a fixed pitch prop, and when your airplane goes faster due to less drag, it also increases HP due to increase RPM using the same prop. Somewhere along the way, I also put 10:1 pistons in my engine. I have always wanted to put a tapered wing on it, so following this thread with a lot of interest.

 

[bifft]

When I've done the math on tapered, longer wings I get better climb and glide, but not top speed. That is at the same altitude, but it would mean better speed up high where you are at a higher angle of attack (lower indicated but higher true speed). My math showed about 6-7% better glide with 25 vs 23ft span, it would take some careful flying to measure the difference. Matches up with davefried's repot of not having noticeably different flying qualities.

 

[Fenix]

When I've done the math on tapered, longer wings I get better climb and glide, but not top speed. That is at the same altitude, but it would mean better speed up high where you are at a higher angle of attack (lower indicated but higher true speed). My math showed about 6-7% better glide with 25 vs 23ft span, it would take some careful flying to measure the difference. Matches up with davefried's repot of not having noticeably different flying qualities.

I will soon be replacing the rectangular wing on my RV-4 with a tapered wing I built of the same basic design, but incorporating a 64% taper. Also the span has been changed from 23' to 25' to reduce the wing area change. I think the wing area on the rectangular wing was 111 sq ft and on the tapered version it is 100 sq ft - IIRC

Prior to taking the plane out of service for the wing exchange I did several test flight to gather data on a variety of things to include stall speeds, cruise speed (at altitudes up to 17,500' IIRC), rates of climb and descent, roll rate, etc.
Once flown I will report the findings here. However, as you stated above, the comparison will take some careful flying to measure the differences. The ones that will be particularly affected by poor pilot technique will be rates of climb and descent. Additionally these numbers can be affected by a change in atmospheric conditions (vertical stability of air mostly) from one test to the next. I did not painstakingly gather this data on numerous occasions after having checked the atmospheric lapse rate to ascertain vertical stability. So very probably the change in these particular performance factors will be more accurately determined by your math than my piloting.

 

[davefried]

When I've done the math on tapered, longer wings I get better climb and glide, but not top speed. That is at the same altitude, but it would mean better speed up high where you are at a higher angle of attack (lower indicated but higher true speed). My math showed about 6-7% better glide with 25 vs 23ft span, it would take some careful flying to measure the difference. Matches up with davefried's repot of not having noticeably different flying qualities.

I'm sure that there are differences but since all I have is the modified condition there is nothing to compare. Subjectively it is an RV-6, I can use the math to talk about the differences. If I had a before and after I would need some carefully controlled conditions to be objective as you said.

 

[davefried]

Something to compare to...

The CAFE Foundation did an APR on an RV-6A. The aircraft was instrumented and carefully tested. Having nothing else, and for the sake of the discussion I will use it to illustrate what I did.

http://cafe.foundation/v2/pdf_cafe_apr/RV-6A%20Final%20APR.pdf

The Foundation is gone from the net but somehow the link works YMMV

I did a component drag analysis for the original design and then added a healthy factor to account for excrescences and interference. The plot below compares my result with the APR polars. That they are so close to a flying aircraft is encouraging, this is my baseline model for further discussion.

 

[davefried]

This is the polar comparing the original wing and the tapered.

Original wing: Min Drag 136.6 lb @ 92.4 ktcas L/D max = 12.08 Flat plate area 2.359 ft^2

Tapered wing: Min Drag 123.9 lb @ 89.7 ktcas L/D max = 13.31 Flat plate area 2.273 ft^2

View attachment 108915

 

[davefried]

oops, here is the polar...