Fun Wing Tip Experiment (uploaded for Skywagon)

CharlieN said:

I have wanted to stay out of this, but I guess I am not.
For a straight wing aircraft the wingtip for the most part does not matter.
If you read allot of Hoerner's design theory, and follow this with the work of Mike Arnold and Burt Rutan. Keep in mind Burt has had many very able minded people in his employ.

Look at Arnold's AR5 and the AR6, look at the tips used on two of the lowest drag planes ever made. Simple rounded tips.
Arnold's AR5 did not even run a spinner. It has been proven the stagnant air around the prop hub has little to no effect on drag. They are pretty and they can make a good profit.
Even Hoerner, late in life published papers that once there was true data on his various tip designs, they made little to no difference.

Where a wingtip can make a difference is on a swept wing. Not only do the sell seats, but they save a few pounds of fuel, less than the few hundred pounds of seats sold. Freighters have little need for real fancy tips. A simple upturn will do. Freight dosen't care.

Just wait till you see what my plane will have

The wing tip essentially does not matter. Companies offering droop or upswept tips make their best money if they hire very good marketing people.

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How about the tufted tip pictures Jerry posted years ago? Looked like from that the the droop tip wing had the air flowing over the aileron and the tip bow wing had it spilling off the end of the tip.

Steve Pierce said:

Raspet was killed in a Super Cub that had a vacuum cleaner motor run off the engine and the suction was run through a double windshield and a series of precision holes drilled in the leading edges. They had a problem with these holes getting clogged. alked to an engineer that worked with him and another person that was a builder of his designs. Unfortunatley all but one have passed on and he is in his 90s.

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Some of the PA-18's boundary layer suction experiment (and other STOL test summaries) is also available here: https://www.sae.org/publications/technical-papers/content/670245/ I have a copy and it's a good read especially the bibliography for further links.

Wonder if reversing the air flow through the small suction holes would have unclogged them?

Gary

Speaking of blown wings. With the advent of modern compact and lightweight leaf blowers, maybe old ideas will now work?

A friend posted this as a joke, but if used differently....




Transmitted from my FlightPhone on fingers...

Seal the Cub upper flap fairing to wing and outboard exit. Drill holes along the rear edge or under the rear lip. Pressurize the inboard fairing entry with electric blower to energize flow over deflected flap. Drink more coffee then go test.

Gary

supercrow said:

.... then added some very shallow droop tips. The ONLY difference that was obvious to me was that the tips lightened the aileron pressures slightly. Have always been very happy with the leverage of the aileron being outboard for lifting a float out of the water sooner that you might be able to do otherwise. I never could tell if the droop tip offered a little more cushion in ground effect. Maybe? Or maybe it just looked a little more finished?

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When the droop tips were installed on this airplane, initially I could find absolutely no difference in the flight characteristics. This was a case of the original rounded tips being removed and the droop tips being screwed on. After going through all of the usual flying antics trying to find something, I did. Power idle glide, full flaps, gear up, descending steep banked turn to a landing on the water. During the flare to arrest the sink rate while still banked I noticed a bit more cushion than with the original tips. A definite improvement in this condition. Normally this plane would not have a cushion in that condition so would just settle through the descent making a solid landing. This was only noticeable in a banked descending turn. During a power idle wings level landing there was no noticed difference.

This airplane has 68" longer wing span than the original Seabee as well as 8 feet more total flap length. These droop tips on a stock Seabee may have further improvements, I have not had the opportunity to fly the single engine Bee with droop tips. I have flown them with no alterations, tip plates, 12" and 24" extensions. All three of those were an improvement. As those of you who have flown the Seabee know, it glides like a streamlined anvil. Any extra on the tips helps.

Test #2
The other wing with it's extended tip. The two tips extend the span by 3" over the original square tip. The extra 3" provides 15.3# of lift enough to support the weight of the new extensions. The wingspan in this condition is 37'3".


What are your thoughts as to any differences between this configuration and two tips like the one below?





These two pictures are to show the flap and aileron configuration and how they are different from a stock PA-18. Maximum flap deflection is 56 degrees.



Finally manged to post a picture.

How does it fly with the change?

Gary

BC12D-4-85 said:

How does it fly with the change?

Gary

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I still have two more alterations and tests in mind before I comment. Though last evening I thought of a third. That third one will take some research and construction time. Still thinking on yes or no on that one.

Test #3



Open wing tip covered with fabric.

Test #4



Added fence below wing tip ending at trailing edge. This is a water spray rail for the EDO 3500 floats.

Test #5



Moved fence in Test #4 aft 4".

Final configuration remaining.

Any pictures of what the yarn was doing in flight?


Sent from my iPhone using SuperCub.Org

Interesting developments in your experiment. We await any results. Here's a speculative question, but in addition to being attached as shown the EDO spray rail was angled out from the tip bottom 45* more, or then even 90* (or parallel to the wing bottom) what might the result be in cruise and low speed?

Gary

mike mcs repair said:

Any pictures of what the yarn was doing in flight?

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No pics of the yarn in flight as the angle from the cockpit is too shallow and I do not have a camera to mount on the struts. I'll talk about the yarn later.

BC12D-4-85 said:

Interesting developments in your experiment. We await any results. Here's a speculative question, but in addition to being attached as shown the EDO spray rail was angled out from the tip bottom 45* more, or then even 90* (or parallel to the wing bottom) what might the result be in cruise and low speed?

Gary

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Hmmm? An idea I hadn't thought of. After yesterday's tests that might be worth trying. That would have the effect of increasing the wing area and aspect ratio so would not address the wing tip shapes other than the spillage of air. I think I shall forgo that idea as the bending moments on the extensions which I built could cause damage. This based on my observations of the yarn and what happened to the fabric.

Look at what happened to the fabric. This was prior to the fence being installed. The disturbance worked from the trailing edge forward.

View attachment 50032

Pete, that attachment is broken.

Charlie, It works when I click on it. I also do not know why it showed up as an attachment rather than just a picture? I'll attempt to post again.

How's this?

Cool, that works.
It might have been linked internally to your hard drive, something I have seen with my own postings here and on other sites.

A comment of what Gary is suggesting, the 45° out angle would simulate a drooped tip. The flat extension would as you state be a wingspan increase. One issue with either of these is that being a flat plate it would take vary little angle from straight into the airflow to develop allot of turbulence. Something that would do no good but also due to the location and size might not be of much arm. The flat extension would be the most sensitive to AOA.

I am sure that with this possibility of span increase your mind is working towards moving the ailerons out with a resulting increase of flaps over what you have already done.

I'm not considering making any changes to the ailerons as they now are moved outboard to the extended tips. The current inboard end/hinge location is the same as the original second hinge/bellcrank location. So far I've found no lack of aileron response, so no need for any changes.

Any impressions yet on how it flys vs the original tips? That torn fabric might mean a better method of attachment is needed.

Arctic Tern aircraft went through a similar tip evolution. From the original Interstate's Cadet's rounded bow tip, to a squared wing extension with an added Hoerner-type up flare, to a plain boxed end like your current design (most common I've seen), to finally another up flared tip after Bill Diehl sold the design.

I had the original Arctic Tern squared off plus Hoerner tips on my first Taylorcraft in the early '70's but there was no way of determining if they were more effective than the factory design before or after.

Gary

Test #6
This was the final objective.
Total wing span 39' 1/2" Area 206.1 square feet, Wing loading @2000 lbs 9.7/sq ft.

As Jerry correctly noted in post #7, symmetry is important. I'll give you an example. Long long ago I owned a Lake LA-4-180. When taking off from a friend's grass field I hit a small bird putting a dent in one of the wing mounted sponsons. If you are familiar, you will know that these sponsons are very aerodynamically clean. I took the damaged sponson off the wing to repair it. While it was off I decided to go flying. The airplane was five mph slower with one sponson off and one on. So, I removed the other one. The cruise speed went back up to the original speed as it was with both sponsons installed. Why the 5 mph difference? The small amount of drag produced by one sponson well outboard on the wing needed to be counteracted by using opposite rudder. The Lake is a rudder airplane which can be flown trimmed by using the rudder alone. The speed loss was caused by the drag of the offset rudder.

With this knowledge, I did these tests one wing at a time. I felt that I would more easily be able to determine small differences in flight characteristics. It worked, as all of my changes made just a little bit of difference. Had both wings been the same on each test, the differences would have canceled each other out.

Total drag is difficult to measure without sophisticated equipment and/or analysis. So for this purpose I used full throttle level flight speed vs rpm. All things being equal an engine/prop/airplane will only turn a certain rpm at a certain speed. If the drag is reduced, the engine doesn't need to work as hard so the top speed and rpm will increase. During Test #2 maximum speed was 119 mph @2660 rpm. During Test #6 maximum speed was 120-121 mph @ 2710 rpm. Final determination was, a small reduction in drag with the increased span and closed tips.

Test #3 One wing enclosed with fabric. The drag shifted to the opposite wing, so closing the rib reduces drag. The yarns still showing stable flow except for the outboard 2" during stall when the flow curled up and out. Also the fabric was pulled away from the rib from the trailing edge forward to about 1/2 chord as you can see in post #47. Also the fabric was cut with overhang on the bottom to observe the air flow. In all flight modes it trailed aft and flush so was no help in learning anything. The fabric was attached to the edge of the rib/skin total thickness of .057" with Stewarts glue. Pulling off from the trailing edge seems to indicate more outward pulling force aft than forward. Post #47. It pulled this far during it's test, no further with the flat plate tests.

Test #4 A 4" high fence under the wing tip flush with the trailing edge. Top speed rpm was 2690 - 2700 indicating less drag with the fence installed. Nothing else noted.

Test #5 Moved the fence aft of the trailing edge 4". Result a bit more lift at low speeds than with the fence terminating at the trailing edge and a bit less drag.

Test #6 Installed the original Backcountry tips to the new extensions for a total span increase of 2' 1/2".
New top speed of 120-121 @2710 rpm. An increase of 1 or 2 mph.

New stall speed of 32 mph IAS down 2 mph. Lift off the water indicating 32/33 mph. I was unable to get any GPS minimum speed comparisons due to the lack of calm winds. Previously I was able to find a minimum calm wind GPS speed in ground/water effect of 28 mph.

New cruise speed of 105-108 mph IAS @2400 rpm/8 gallons per hour. Prior to the extensions the cruise at the same power setting was 103-105 mph IAS.

Rate of climb indicated no noticeable difference. However, the angle of climb was considerably improved.

After the installation of the original Backcountry tips to the extensions, the aileron response became less crisp though still adequate. Actually became more like an unmodified Cub. The stability became a bit less due to the increased span, as expected. Glad I have the ventral fin installed. Cubs really could use a bit more tail area.

I increased the span on my 185 by the same amount years ago. The 185 showed a greater improvement than the Cub. Perhaps it was because the 185 seems to fly more behind the power curve than the Cub? When loaded a stock 185 on big amphibs flies tail low/high drag. The Cub tail high. Finding the best combination of power/lift/speed/performance is a never ending quest.

Based upon these tests, my opinion is that the wing tip shape of a squared wing tip is not very important. Perhaps a bit of a flare outward near the trailing edge would help in reducing that outward pull? The shape which seems to indicate the best improvement however slight is the flat plate below the tip extended aft of the wing trailing edge a bit. In this case 4" was tested. This is not a comparison with the stock Piper rounded tip. We all know that the elliptical wing on a Spitfire is reported to be the most efficient shape. This makes me believe that the original rounded tip which Piper used, may actually be the best shape.
The increase in wing area/span was the greatest improvement however slight.

This was fun. I used up some spare materials which I've had taking up space. I had fun making the tip extensions. Other than the entertainment factor, was it worth it? Perhaps, perhaps not. Will I remove the extensions? No. I operate my Cub at 1999 lbs gross. Any higher gross and the State charges more than double for the annual registration, so 1999 it is. This Cub has the 2300 lb wings. I suspect the extensions would prove more valuable operating at the higher gross weight.

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. What washout if any does your wing have? It affects. Another mod option might be to add a fillet in front of the vertical stab like Cessna did on some models to increase tail area and improve stability. My long wing Cubs could have used more vert tail area. The PA-12 had a forward fillet (example below; the 6" gear removed). On conventional gear it flew well; on floats it needed more via a ventral fin.

Gary

Wash out is 1.2 degrees.

I need to fly it more to evaluate the new aileron response. If I feel it needs some attention, I came up with a simple idea for adding Helio like interceptors. That will require some research into just what percentage of chord and height at full aileron deflection would be the most effective before I cut any metal. This can be accomplished by just adding an attachment to the aileron tip rib. I seem to recall that Dave Calkins did something along these lines a few years ago?

Cub Crafter's approach to flap and aileron effectiveness - revised form and cove: http://cubcrafters.com/c/2017/02/g-series-flaps-and-ailerons-faqs/
Cessna Caravan's approach to aileron effectiveness - a spoiler and upper fin: https://www.flyhpa.com/2016/11/a-pilots-review-of-the-cessna-caravan-ce-208/ -and- http://twinandturbine.com/article/flying-grand-caravan-ex/

Gary

Gary,
I am not going to make any alterations which can not be reversed by just removing a few screws in a short amount of time. That alone cancels either of these suggestions. The Caravan aileron appears to have a servo tab in order to lighten control forces. I do see the Caravan spoiler just ahead of the flap. That apparently is to dump the lift from that portion of the flap. If I do the spoiler thing, it will be mounted in the new extension someplace to dump some of that lift.

Ok that would help and be easier than messing with the flapped area. Kill the tip lift. Can the aileron horn geometry be changed? Like shorter on top to raise the up aileron more or on the bottom to increase deflection? Not sure about all that just wondering.

Edit: Top connection is short already. Maybe the geometry can't be messed with to any benefit.

Gary

BC12D-4-85 said:

Ok that would help and be easier than messing with the flapped area. Kill the tip lift. Can the aileron horn geometry be changed? Like shorter on top to raise the up aileron more or on the bottom to increase deflection? Not sure about all that just wondering.

Edit: Top connection is short already. Maybe the geometry can't be messed with to any benefit.

Gary

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I looked at that when I was building this Cub. It would have required too much in the way of alterations defeating my purpose of getting the project flyable sooner rather than later.
I have an experiment in mind to determine where and how much would be appropriate for a spoiler. First a bit more flying analysis with it as is.

How about a short Gurney flap on the outboard aileron to improve its lift when deflected (like the Caravan's flaps above have)? Drag and control forces may increase some.
Gary

I've already played with a Gurney flap. While it does do what they say, it is minimal. Not suitable for this application.