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07-04-2020, 09:38 AM
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Join Date: Jun 2020
Location: Byron Center, MI
Posts: 40
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Surface Tension - Aero Drag
Back in the day, I raced RC boats. We used Imron paint over carbon/kevlar laminates to finish the boat. The first time I took my bright glossy boat to the lake, it would not get up on plane. This was a proven design that would go 100 mph. I was stumped and started looking at engine and drive train. Then someone came up to me and said "Your boat will never get up on plane until you scuffed the bottom of the boat". I said "I just spent days prepping and painting this thing to get it perfect." He replied "Well, it will make a nice looking bouy until you scuff the bottom of the boat.
Eventually, I gave in. The boat immediately took off on plane. I went home and did some research. He was absolutely correct, the shiny paint finish on the bottom of the boat was allowing the water to just grap the boat with surface tension.
My point to all this is, does this apply to air over a surface?
Eventually, I scuffed the top of the boat as well to reduce aero drag and picked up 8 more MPH.
Interest to hear if anyone has experimented with this? The closest thing might be a painted plain vs an unpainted plane. As most do not paint until the 40 hours are flown.
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07-04-2020, 10:07 AM
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Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,189
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Data
I would be interested in seeing data on the extra 8 mph by scuffing the upper surface of a model boat.
The glass smooth bottom, however, is definitely an issue. I had the same experience...
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Bob
Aerospace Engineer '88
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07-04-2020, 10:38 AM
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Join Date: Feb 2015
Location: Locust Grove, GA
Posts: 621
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3M Scotchcal drag reduction film
Quite a few studies performed back in the 80?s with ?riblets?. Worked on high speed aircraft as well as low speed watercraft. Similar Reynold?s numbers possibly?
https://apnews.com/7593cedd8df4e6d218f3aae9bfdacb14
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Krea Ellis
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07-04-2020, 10:43 AM
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Join Date: Mar 2018
Location: Buena Park, California
Posts: 279
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The aerodynamic for aircraft is different because of the surface tension you mention.
Here a Youtube link that explain why the AR5, the world speed record holder aircraft, flew so fast. The link explains the important of getting the aircraft skin surface smooth and uniform.
https://youtu.be/rxvoDbZpoY8?t=1983
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07-04-2020, 10:55 AM
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Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 1,005
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Reminds me of a buddy that refused to get his Bonanza repaired after it got pelted with hail. He claimed it was faster with the golf ball effect applied.
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Ralph
built a few RVs, rebuilt a few more, hot rodded some, & maintained/updated a bunch more
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07-04-2020, 10:57 AM
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Join Date: Dec 2014
Location: Just Minutes from KBVI!
Posts: 1,039
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Quote:
Originally Posted by KatanaPilot
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At what airspeeds is this effective? I found a reference to Cathay Pacific operating A340s with riblet film (not clear how much of the airframe has it applied) with a demonstrated reduction of skin friction drag of 5%-8%. I would imagine that the drag reduction at lower airspeeds would be somewhat less. I further assume that it's proportional to TAS, not IAS, but I could be wildly wrong. Any insights from the aero engineering crowd?
https://www.sciencedirect.com/topics...eering/riblets
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07-04-2020, 11:04 AM
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Join Date: Mar 2014
Location: Prescott, AZ
Posts: 739
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Quote:
Originally Posted by Ralph Inkster
Reminds me of a buddy that refused to get his Bonanza repaired after it got pelted with hail. He claimed it was faster with the golf ball effect applied.
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I had a Piper Turbo Arrow III many years ago that had hail damage all over it. The plane was faster than any other Turbo Arrow I've flown. We figured it might be the golf ball effect.
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07-04-2020, 12:59 PM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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Quote:
Originally Posted by FasGlas
I had a Piper Turbo Arrow III many years ago that had hail damage all over it. The plane was faster than any other Turbo Arrow I've flown. We figured it might be the golf ball effect.
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OK, just who is going to be the first to scuff their paint job and add the golf ball dimples to test the speed? If roughness increases speed why don't universal rivets go faster than countersunk?
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Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
Last edited by BillL : 07-04-2020 at 01:09 PM.
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07-04-2020, 01:03 PM
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Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,573
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To the OP and others, gasses do not have surface tension like liquids do. So in air, there is no surface tension problem.
Riblets work in the micro sublayer of the turbulent boundary layer. NOT for laminar flow. So where you put them really matters. Orientation of the riblets is crucial. You can not reproduce the riblet effect by sanding - the profile shape of the scratches from sanding is not the right shape.
Flight tests on jet liners have confirmed the benefit of riblets, but operational problems negate the benefit. In particular, the plastic film is not porous enough to breath out the minute leak at each rivet of a pressurized fuselage, so the film blisters at each rivet. Cleaning and maintaining the riblet film is also difficult to do properly.
One of the most significant uses of riblets was on the hull of the America's Cup boat Stars and Stripes, which raced against Australia's Kookaburra III in Freemantle and reclaimed the cup.
For general aviation airplanes, you want the surface as smooth as you can where ever there is laminar flow (first 15--20% chord on RV 3,4,6,7,8, back to the spar on the RV-10 and RV-14 if the riveting is done well, and a good fraction of the lower surface of the RV-9 if the riveting is done well) I have not been able to measure any increase in top speed on my RV-8 with wings clean compared to covered with bugs. I imagine that the -10 and -14 should demonstrate a measurable benefit to clean smooth wings fwd of the spar.
Once you have turbulent boundary layers, it really doesn't matter much, as long as the surface is not outright "rough".
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Last edited by scsmith : 07-04-2020 at 01:08 PM.
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07-04-2020, 01:25 PM
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Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 1,005
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So in actual fact my buddy may have been wishful he was faster with the pitted surface? He was known to exaggerate his golf scores too.
Steve, thanks for your insights.
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Ralph
built a few RVs, rebuilt a few more, hot rodded some, & maintained/updated a bunch more
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