Fun Wing Tip Experiment (uploaded for Skywagon)

skywagon8a · Jul 22, 2020

As I mentioned in post #52, previously I had seen a minimum GPS speed of 28 mph. I can't explain that number as today I proved it wrong. Today is high overcast with zero to 1 mph of wind. Glassy water everywhere and smooth air. I was able to determine that minimum speed in ground/water effect was 30 mph IAS and 41 mph on the GPS. Flown in opposite directions with the numbers being the same in both directions. This was with power enough to keep it in the air. The elevator at minimum speeds is very touchy. With proper control inputs I was able to lift off the water at 30 mph IAS with 56 degrees of flap. When I attempted to pull it off at a slower speed, the water drag increased enough to extend the run holding the Cub on the water until 33 or 34 mph IAS. The next time I get this same weather opportunity, I'll try lifting out one float to reduce the take off speed. I didn't think of that today.

Another thing I noticed during test #6 was there was just a touch of right wing heaviness. I attributed that to not having full fuel. Today with full fuel, it was still there but only at the new higher cruise speed. The wing now flies very slightly nose down in cruise, perhaps only 1 degree difference than before. Perhaps there is a slight shape difference somewhere which causes this? The ailerons were displaced by about 1/16" to 3/32" to correct the heaviness. When I reduce the power to 2300 rpm the heaviness became minimized and the ailerons were streamlined.

BC12D-4-85 · Jul 22, 2020

You know this >

But if the floats are limiting the takeoff speed via AOA try increasing the relative angle between them and the wing. Sometimes 3/4" is all it takes. The price for that can be altered cruise speeds. I've fiddled with adding and subtracting float angles using older struts with bolts attaching the end fittings. Inter strut to fuselage blocks of varying size also works.

Assuming the wings have equal incidence at the fuselage and spanwise, having right wing heaviness can result from propeller slipstream effects. Slightly higher AOA on the left wing vs right creating more lift on the left. See if it varies by rpm; as you noted 2300 reduces the effect. See FAA sourced figure below.

The nose down (lower lift needed to maintain the same level flight) is expected as wing loading decreased.

Gary

skywagon8a · Jul 22, 2020

Gary, These little discrepancies only showed their faces after lengthening the wings. I was very fussy about making certain the extensions did not produce any unwanted wash in/out. The floats are at just the right compromise for take off and cruise. These are 7 GCBC struts with an extension on the rear strut. It will easily clear trees at the end of a 1000 foot long pond. That's not bad for any float plane. Prior to increasing the cruise speed by extending the wings, it could easily out run two different local PA-12s on wheels without pushing the power up. There is also a local 7GCBC on floats which is a lot slower. Usually -18s and 7GCBCs on floats are very similar in speed. Of course 150 vs 180 hp makes a big difference too.

BC12D-4-85 · Jul 22, 2020

Pete....Ahh the fun of an Experimental. Do you have an AOA sensor or top panel/heads-up display? It would have been interesting to follow that aspect through your tests.

Gary

skywagon8a · Jul 23, 2020

None of that fancy electronic stuff Gary, only a lot of yarn under the left wing tip and a sensitive rear end. Early on (a few years ago) I made a boom mounted airspeed indicator with a flexible vane controlled pitot tube. It needed a more rigid pole and some calibration. You should have seen that flutter at a low speed. It's a wonder it didn't break off. I should blow the dust off that.

The yarns at the trailing edge of the extension flow straight back until just at the stall, when they lift up 45-60* or so. Initial stall is straight forward wings level which progressively becomes a bucking wing walking ride if the stick is held back. Nothing unusual there.

A camera mounted on a pole above the cabin looking out at a yarn covered wing would be educational. Actually a second camera below the wing would also be helpful. One could get carried away with lots of spendy gadgets. As it is now, I have $0.00 invested.

skywagon8a · Aug 16, 2020

Test #7
A fence between the aileron and the extended wing. Does this effect the span wise flow enough to increase the authority of the aileron to dump lift thus improving roll control? Result...maybe, maybe not. It was very difficult to determine whether there was a difference or not.
It did improve the lift at stall speeds on the new extended portion of the wing. The only observed difference to be able to say this was the action of the yarns. During the stall with the fence, the yarns on the extension flowed smoothly straight aft. The yarns on the other wing without the fence lifted smoothly about 15 degrees. Neither wing extension's yarns indicated disturbed flow when the rest of the wing stalled.

Conclusion.....not worth the effort.

It does jump out of the water when light on fuel with the prop blades set at a cruising angle.

skywagon8a · Aug 16, 2020

For a pictorial of the building process, look here:
https://www.supercub.org/forum/show...ons-for-squared-wing-tips&p=780433#post780433

skywagon8a · Sep 4, 2020

On the Square wing? thread mike mcs suggested a round extended tip in aluminum. That would involve compound forming of the skins which could be rather labor intensive. Especially without the use of an English wheel. Mike, I am not a pop rivet fan and will not use them. They do have their place, just not on my planes.

I found this report by Sighard Hoerner to be interesting: https://apps.dtic.mil/dtic/tr/fulltext/u2/a800374.pdf

A rather simple trapezoidal shaped tip would closely resemble a round tip aerodynamically. With that thought, a simply constructed aluminum trapezoid could be easily built. My current fiberglass square tips have 756 square inches of area. A trapezoid with a 6" chord tip rib would increase the span of each tip by 10" without changing the total wing area.

Hoerner suggests a 50 degree sweep back at the 1/4 chord. This would resemble a rounded tip aerodynamically in my opinion.

The upper camber would remain in line with the wing's upper surface with all of the taper being in the lower camber.

What say you engineering types? Are there any aerodynamics specialists on board here? All opinions considered and welcomed. This, if done would be a winter project.

BC12D-4-85 · Sep 4, 2020

A previous discussion: https://www.supercub.org/forum/showthread.php?41894-Wing-Tip-Shape-Summary&highlight=PA-12+wingtip

See Post #96 and 108. If I had a choice that would be it for a mod to a round tip wing.

Gary

skywagon8a · Sep 5, 2020

This is sort of what I am thinking.

This one is more like Dave's picture in the other thread.

BC12D-4-85 · Sep 5, 2020

Surely the benefits of both wing designs above have been discussed. Aspect ratio may have been their goal. Is there a link to Wittman's or other builder's basis for each?

Gary

skywagon8a · Sep 5, 2020

Those are just internet pictures. I don't know whether Steve Whitman was involved with the tip design or not. He was good getting efficiency out of a plane.

Since a Cub doesn't have the elliptical shaped wing of a Spitfire to begin with, I figured this would be close or as close as possible under the circumstances.

The Hoerner report in post #68 seems to point in this direction. I readily admit I don't understand much of his engineering data.

skywagon8a · May 17, 2021

My interpretation of Hoerner's report in post #68.
First pic of the new tip uncovered. Second pic of it mounted on the extension.

New tip on the wing replacing the original square tip.

Original tip.

Test #8

69 more square inches of area than the old squared tips, less than 1/2 a square foot. 10" more span, but tapered so the effective span increase is difficult to compare. One new tip with one old tip on the other wing, the difference in lift was about 1/2" +/- of aileron deflection to counteract the extra lift created by only 69 square inches of area. The old tip wing stalled before the new tip wing.

Conclusion: New tapered tip provides more lift with no other observed differences.

Oops!
Edit: Clearly I must have had a brain interruption. The above dimension numbers are wrong. They should be for one new tip:
271.5 more square inches or 1.9 square feet with an increase of 15" of span per tip over the original square tip.

BC12D-4-85 · May 17, 2021

Nice fun after #68. Only 69 insq but longer span/aspect ratio. How about a view of the tip from the wing's end? Now tuft test lower surface of tip to note airflow (angled towards outer tip?). Are those tufts on the outer aileron?

Gary

skywagon8a · May 17, 2021

Test #9 This is the final configuration. (for now :smile

New tips installed on both wings with the wing extensions. Total wing span = 41' 8.5", Aspect ratio = 8.38:1, Wing loading @ 2000# gross = 9.64#/square foot.
Original PA-18 span = 35.3', Aspect ratio = 6.9:1, Wing loading @ 1750# gross = 9.8#/square foot.

Net empty weight increase 21.8# providing 115.2# net increase of lift. I could have reduced the weight of the extensions a bit by using .020" skin instead of the .025" which is what I had in stock. Empty CG moved 0.3" aft.

Flew off the glassy water indicating 37 mph. Initial climb rate 1100 fpm. Cruise speed 108 mph with the nose slightly nose down. Slightly less drag than the original configuration.

Stall was very gentle, normal with no surprises. Power idle glide, no flaps 500 fpm. No noticeable change in stability. When increasing the span one would expect less directional stability, there was none noticed.

Making the tips and extensions was a lot of satisfying work.
Just for the fun of it I top coated the Stewarts system with

from Lowes.

skywagon8a · May 17, 2021

Gary,
Placing tufts on the new lower tip surface will be a waste of time, since it is impossible to see any of it from the cockpit. I do not have camera equipment to attach to the wing. It would be interesting to see the flow.

Those tufts are on the outer portion of the aileron. They always flowed smoothly aft, even during stalls.

skywagon8a · May 17, 2021

More tip pictures.

BC12D-4-85 · May 17, 2021

Hoerner's lateral "vortex core displacement" must be helping. What's the wing wash now? Great test. I still wonder what would happen if just the flared portion were attached to the wing w/o the short extension? That portion could be made from a form mold eventually.

Edit: I see the difference now between Test#8 (tip w/no extension) vs Test#9 (tip w/extension added). Did you note a change with the extensions?

Gary

CharlieN · May 17, 2021

Ohh, I am waiting for next spring when the wing extension gets trimmed, ailerons moved out with the flaps gaining even more span than they have now.

skywagon8a · May 18, 2021

BC12D-4-85 said:
Hoerner's lateral "vortex core displacement" must be helping. What's the wing wash now? Great test. I still wonder what would happen if just the flared portion were attached to the wing w/o the short extension? That portion could be made from a form mold eventually.

Edit: I see the difference now between Test#8 (tip w/no extension) vs Test#9 (tip w/extension added). Did you note a change with the extensions?

Gary

The washout has not changed from the original setting. It appears that the amount of improvements have become smaller with each change. Unless I find something else, it appears any further improvement to the original clean Cub wing without changing flaps and leading edge devices would require an airfoil change. The Piper washout is important for aileron response during minimum speed flight. I do understand that some in Alaska change theirs to wash-in for added lift at take off speeds. This sacrifices aileron control at minimum speeds which is not good.

CharlieN said:
Ohh, I am waiting for next spring when the wing extension gets trimmed, ailerons moved out with the flaps gaining even more span than they have now.

That would require major changes to the wing which I am not willing to accomplish at this point in my life.

It appears that Dave Calkins' wing tips here: https://www.supercub.org/forum/show...p-Shape-Summary/page3&highlight=PA-12+wingtip in post #108 closely match the information from Hoerner's report which I utilized for my tips. That is, a sweep back of 50 degrees at the 1/4 chord location provides minimum drag. Look at his tip, it has a curved leading edge so the 1/4 chord location at each station would have approximately averaged a 50 degree sweep. Based upon all of my above tests, it appears the simplest tip change for a stock Cub would be Dave's tip. The wing area is increased without sacrificing the sweep back. The advantages of my modifications appears to be the increased wing area and aspect ratio. The actual tip shape makes very little difference.

Oops! I had to correct some numbers in post #73.

skywagon8a · May 20, 2021

I neglected to do a full power level flight test in the Test #9 configuration. This is it, 128-129 IAS (mph). That is a considerable improvement over the previous 121 IAS.

Normal cruise power 108 IAS.

Also did a minimum speed test over the water in ground effect. 25 IAS with 35 on the GPS. The difference is likely the angle of attack errors. While the elevators were sensitive, they were not as sensitive as they were prior to the beginning of this wing tip experiment.

skywagon8a · May 21, 2021

BC12D-4-85 said:
Now lower wing loading (good goal) = lower AOA for level flight at a given weight, airspeed, and density? The airfoil has the lowest Cd at around -4* alpha; best Cl/Cd around 0*. Approaching that lower angle through reduced wing loading should help.

Gary

Hmmm? I didn't think to look at the bottom of the wing while I was doing the full power test. It may have been close to the -4* alpha which would have explained the 8-9 mph increase in the top speed. The speed did appear to increase more rapidly as it went above 121+. This would indicate a reduction in airfoil drag. This would have been enabled by the increase in aspect ratio. This could also help explain why an Aztec (another great airplane) does so well with all it's bulk using the same airfoil. I don't have one available to test this theory.

This is the cruise alpha, maybe -1*?:

The full power alpha would have been more negative.

CharlieN · May 21, 2021

skywagon8a said:
Hmmm? This could also help explain why an Aztec (another great airplane) does so well with all it's bulk using the same airfoil. I don't have one available to test this theory.

Humm, we got one here, wonder if the owner wants to play? He is a half owner is the flight school here and my lead guy for my chapter's IMC and VMC programs.

skywagon8a · May 21, 2021

Please do Charlie. First using a precision level find a location above the wing which matches the wing lower surface. Perhaps a tape on the door or pilot's side window? This so that you can get a reference to the horizon.

Then take note of the angle of attack and speed at different power settings. See if it will pick up a fair amount of speed for little increase in power once the angle of attack goes negative.

This may explain why I have no difficulty in diving to my 150 mph red line.

CharlieN · May 21, 2021

I will look into that and see if he is interested. I will be at RUT overseeing a few private Young Eagles flights tomorrow. Not sure if he well be there but can send him a note.

CharlieN · May 21, 2021

skywagon8a said:
Perhaps a tape on the door or pilot's side window? This so that you can get a reference to the horizon.

But we don't have a a true horizon up here, ground is to dang lumpy.

A simple construction protractor or even one of my digital ones. Humm they are magnetic base, curious to see if the compass likes it.

Seaworthy · May 21, 2021

" it could easily out run two different local PA-12s on wheels without pushing the power up. " Hey----you never said we were having a race!

BC12D-4-85 · May 21, 2021

For some fun keep an eye on the fabric between the ribs especially with washout root to tip......how much of the wing's fabric is concave ^ (more lift per section) or neutral - (less lift per section). As we add power and speed up don't we lower the nose to reduce AOA by unloading the horizontal tail (sheds drag) to maintain level flight? The question here might be how much does Piper's washout affect cruise speed for better or worse? Champs think it's worth having none. Piper and many others disagree for various reasons including control and stall behavior.

Gary

skywagon8a · May 21, 2021

The amount of washout as prescribed by Piper likely has little effect upon the cruise speeds since there are very few of the various models which are able to cruise at anywhere close to the -4* alpha. The fastest one is the PA-22, which with it's short wing doesn't go fast enough to approach the -4*. It's been a long time since I've flown a Tri-Pacer. As I recall they were a 120 mph airplane. It wasn't until my long winged, low wing loading Cub exceeded 120 did it begin to realize the effects of the -4* alpha. I would expect the PA-22 would need to exceed the 135 mph range before it would be approaching the -4*. Consider if the washout is 2.5*, the only portion of the wing which is at 2.5* is the tip rib. All else is less. I dare say that none of the planes which have the washout removed have the ability to cruise anywhere near the speeds necessary to approach the -4* alpha. Most of the reports which I've seen here indicate top cruise speeds of less than 100 mph. Not all, but most.

The trimmable stabilizer does have a great deal of effect since the tail does unload as the speed increases and the forces are trimmed. The Champs with their fixed, leading edge down stabilizers would run up against a drag wall since the more nose down the wing becomes the more the stab is angled into the relative wind.

I haven't noticed the flexing of the fabric between the ribs on the bottom of the wing. I have noticed it on the top of the wings. The fabric between the ribs is extremely taught. I doubt the 9.6 lbs/square foot would deflect it much.

wireweinie · May 21, 2021

You guys make electricity sound easy.

Web (life coach)

Fun Wing Tip Experiment (uploaded for Skywagon)

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