Author Topic: Foil Physics Question RE: AR and drag  (Read 17140 times)

JohnnyTsunami

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Foil Physics Question RE: AR and drag
« on: March 09, 2022, 02:32:30 PM »
There is a non-intuitive aerodynamic fact where drag at low speeds favors high-aspect wings, and drag for high speeds favors low-aspect wings. Folks have been pushing the HA foils to the max, the only limit being too wide to turn, or too small to lift in lighter wind. I'm wondering if low AR wings have their place - not in terms of being forgiving and friendlier, but in terms of actually performing better from a physics standpoint at high speeds.



Above is a general chart showing total drag on the top line. The chart shows that lift-induced drag is the main source of drag at low speeds, and parasitic drag is the main source of drag at higher speeds.

Lift-Induced Drag is lowered by a HA wing (somehow...)
Parasitic Drag is caused by more leading edge interacting with the air/water - a shorter fatter wing is better for reducing this.

At low speeds, the high AR wing has LOWER lift-induced drag, and the low-AR wing has HIGHER lift-induced drag. Since HA wings have lower lift-induced drag, they perform better in terms of efficiency at lower speeds - the total drag is lower on the left hand side of the graph.

At high speeds, the lift-inducing drag is minimal, and the parasitic drag becomes the major source of the drag - this is the right hand side of the graph. A lower AR wing (larger chord), has HIGHER lift-induced drag, but a LOWER parasitic drag than a high-AR wing. So, once you have the power to get to the high speed, the lower AR wing is superior in terms of top speed and low drag.

This is why fighter jets have short fat wings and gliders have crazy high AR wings (+30AR?)/


Is anyone an engineer/physicist and know how this works in water? Is the crossover in terms of what the main source of the drag the foil is experiencing a thing that is relevant to foiling?
     i.e. is the crossover on the graph above at 20mph, or is it something like 200mph in water - and therefore not relevant to foiling?   

MikeLima

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Re: Foil Physics Question RE: AR and drag
« Reply #1 on: March 09, 2022, 05:10:39 PM »
I have no idea, but nerding out is one of the reasons I love foiling.

Btw, what is the source for that graph?
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JohnnyTsunami

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Re: Foil Physics Question RE: AR and drag
« Reply #2 on: March 09, 2022, 09:12:37 PM »
Just the wiki entry for parasitic drag.

But lots of much better info in this glider handbook by the FAA, "Aerodynamics of Flight." https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/glider_handbook/media/gfh_ch03.pdf

"The greater the aspect ratio of the wing is, the lower the induced drag is."

"The elliptical wing is more efficient in terms of LD, but the wing’s uniform lift distribution causes the entire span of the wing to stall simultaneously, potentially causing loss of control with little warning"

I think we can relate to that. Plenty to nerd out on that especially the talk about wing shapes on 3-7. All the stuff we talk about is there.


I guess I just had a narrow question about the rough speed when the lower AR wings are better at high speeds in the water.

surfcowboy

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Re: Foil Physics Question RE: AR and drag
« Reply #3 on: March 09, 2022, 10:01:17 PM »
Water. It's the source of all the uncertainty in foiling.

It breaks all the aerodynamics info, or at least most of it. Hydrodynamics are a whole other bag.

sflinux

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Re: Foil Physics Question RE: AR and drag
« Reply #4 on: March 09, 2022, 11:08:42 PM »
This is a good read by Theodore A. Talay:
https://practicalaero.com/wp-content/uploads/2010/04/NASA-SP-367.pdf
Quote from: JohnnyTsunami
"So, once you have the power to get to the high speed, the lower AR wing is superior in terms of top speed and low drag”
You lost me here.  A low aspect ratio has benefits at low speed.  At low speeds all forces are low.  At low speeds you can adjust the angle of attack to increase lift with a low stall speed.  At high speeds, drag forces become important.  Low aspect wings have excessive drag with change of angle of attack.

Quote from: JohnnyTsunami
“This is why fighter jets have short fat wings and gliders have crazy high AR wings”.
Fighter jets have shorter wings to be agile and for the wings to be structurally sound to support the g forces.

Lower aspect ratio has benefits at certain speeds:
“Substantial increases in the critical Mach number occur when using an aspect ratio less than about four. However from previous discussions, low-aspect-ratio wings are at a disadvantage at subsonic speeds because of the higher induced drag.”

It would be interesting to see how mach speeds translate to hydrodynamic speeds, as water is 1000 times more dense than air.
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Wingfoil2001

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Re: Foil Physics Question RE: AR and drag
« Reply #5 on: March 09, 2022, 11:45:52 PM »
So rather than calculate it, I tried it. Almost back to back on the Moses 633 (modified to fit Naish fuselage)and the Naish 1040HA. I had to change down to a 4mtr wing as the wing picked up for the 1040 session, was on a 5.5 mtr for the 633.
Here is the speed graph, I was surprised the thick low aspect 633 was so quick, could almost say quicker that the 1040HA.
The 1040HA is my normal ride so a bit smoother.

surfcowboy

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Re: Foil Physics Question RE: AR and drag
« Reply #6 on: March 10, 2022, 07:31:31 AM »
Guys! Pro kite racers are doing it wrong!

I have Maliko, Iwa, and Kai "pro race" wings for sale. $2,000 each.

Respectfully, the above has not been my experience. Nor anyone else that we've seen on these forums, the internet, or the water. Following this with interest to see why that is.

Not saying this is wrong, but saying there's very little research or real world data to support this. I hope you guys change the industry. But if you want to borrow my thick wing and test, buzz me. I get 10-20% boost without trying.

JohnnyTsunami

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Re: Foil Physics Question RE: AR and drag
« Reply #7 on: March 10, 2022, 10:23:00 AM »
Based on what I’ve read, this is all the same in the water, just the speeds are different.

Yes, low aspect foils have advantages at low speeds such as turning and a more predictable stall.

The lower AR airplane wing is better at high speeds, is what I meant.

I’m aware of what kite foil racers use, I was winging on one yesterday afternoon and I haven’t come close to maxing it out. Something that was confusing is axis having tow wings that are super low AR.

The big differences in the water are the idea of ventilation and super cavitation that happens around 60+ knots (the most efficient foil at those speeds is actually a backwards triangle, see SP80 and sirocco and the current world record speed champ for sailboats).  http://www.sailrocket.com/node/288

Great little vid on super ventilation v. cavitation and the triangle ventilating foil. https://m.facebook.com/SP80.record/videos/sp80-explained-episode-4-what-is-going-on-beneath-the-surface/754052565518895/


That may answer my own question, that HA foils are more low drag and efficient at all speeds below super cavitation/ventilation at 60 knots, and those are the only relevant speeds to us mere mortals.
« Last Edit: March 10, 2022, 10:29:09 AM by JohnnyTsunami »

Dontsink

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Re: Foil Physics Question RE: AR and drag
« Reply #8 on: March 10, 2022, 03:38:01 PM »
High speed airplanes (jets) use wings with lots of sweep to delay the formation of shockwaves when close to Mach 1.
Fighters use short span wings for maneuvrabilty,high load factors and very high speeds.They usually have huge thrust to weight ratios so takeoff perfo is not a big problem.

But assuming that low AR ,delta shapedfoils are going to be faster for us in the range of speeds we use is a big leap...and i think it is wrong.You get more lift from the same surface with high AR (less loss at the tips),so you can ride smaller,so you get less parasitic drag...


If you want a fast foil make it very small,thin and high aspect.It will be a b... to get up on foil though.
Look at IKA and race windfoil gear.

KdMaui said that in the Tarifa event the top racers used 500-600cm2 foils (probably kitefoils) and big boards and wings to get up quickly.Like 90l and 6m2 in up to 20kt.

surfcowboy

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Re: Foil Physics Question RE: AR and drag
« Reply #9 on: March 10, 2022, 06:59:28 PM »
I thought Kane's observations were wild. Giant boards and wings, tiny foils. But I get it. Get up fast, hang on, go fast.

radair

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Re: Foil Physics Question RE: AR and drag
« Reply #10 on: March 11, 2022, 07:07:55 AM »
So rather than calculate it, I tried it. Almost back to back on the Moses 633 (modified to fit Naish fuselage)and the Naish 1040HA. I had to change down to a 4mtr wing as the wing picked up for the 1040 session, was on a 5.5 mtr for the 633.
Here is the speed graph, I was surprised the thick low aspect 633 was so quick, could almost say quicker that the 1040HA.
The 1040HA is my normal ride so a bit smoother.
With all due respect, your graph is meaningless unless you were using identical wind wing size in identical wind speeds. What you're showing is apples and oranges.

"... lower AR wings are better at high speeds in the water" and "So, once you have the power to get to the high speed, the lower AR wing is superior in terms of top speed and low drag” <- this is not even remotely true, in fact just the opposite is true. Also, there are a lot more factors than AR; foil section and planform play a big role. Especially foil thickness.

jondrums

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Re: Foil Physics Question RE: AR and drag
« Reply #11 on: March 11, 2022, 03:19:09 PM »
I think you are missing that for our sport - everything we do would be considered "low speed" as far as physics is concerned.  So I really don't think we'll see speeds such that drag is reduced by going to low-aspect foils.

Wingfoil2001

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Re: Foil Physics Question RE: AR and drag
« Reply #12 on: March 12, 2022, 11:31:04 AM »
So rather than calculate it, I tried it. Almost back to back on the Moses 633 (modified to fit Naish fuselage)and the Naish 1040HA. I had to change down to a 4mtr wing as the wing picked up for the 1040 session, was on a 5.5 mtr for the 633.
Here is the speed graph, I was surprised the thick low aspect 633 was so quick, could almost say quicker that the 1040HA.
The 1040HA is my normal ride so a bit smoother.
With all due respect, your graph is meaningless unless you were using identical wind wing size in identical wind speeds. What you're showing is apples and oranges.

"... lower AR wings are better at high speeds in the water" and "So, once you have the power to get to the high speed, the lower AR wing is superior in terms of top speed and low drag” <- this is not even remotely true, in fact just the opposite is true. Also, there are a lot more factors than AR; foil section and planform play a big role. Especially foil thickness.
Yes, it was a pity the wind picked up a bit when I got on the 1040HA, but I was still surprised a 30 mm thick LA wing was as quick as a 16mm thick HA wing, even in lighter wind.
Will do the exercise again.

Califoilia

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Re: Foil Physics Question RE: AR and drag
« Reply #13 on: March 12, 2022, 12:12:00 PM »
This is why fighter jets have short fat wings and gliders have crazy high AR wings (+30AR?)
Not very scientific here, but the above pretty much sums up my view of the subject, and I've used the analogy in the past when talking about AR on the beach with friends.

Fighter jets are very maneuverable ("surfy"), but have little glide when not being push by their strong engines (wave). Whereas the glider with its super high AR does as its name says and glides incredibly well with no power (wave) once its been pulled into the sky with some other powered vehicle (tow plane or feet running off a cliff) to get it to speed to create its lift; but the glider's not going to be maneuvering or turning (surfing) anywhere near that of the jet fighter.

So it becomes the "right tool for the right job," when deciding what you're planning to do with your wing to find the right AR for it (surfing or pumping). As I tell guys looking to demo different gear thinking it's going to be the holy grail to find what they're currently searching for in a foil..."When you get something, you give something up."

IOWs, if they really love how their current setup is surfing but come looking for a HA wing so they can pump all day and connect more waves, I make sure to tell them not to expect it to slice and dice as well as their current setup will..."get something, give something up."

Yes, there are newer HA wings that are getting shorter and shorter (AXIS 899 and 799) that are turning better than their much longer brothers (999 and 1099) but still have enough lift to get them off the water...if the rider and board are small enough, young enough, and talented enough with the right surf to make them off the water to make them work. It's another story when old geezers like myself on sup foils throw them under our board and expect to go out and do what the young guns are doing on their potato chip boards and tiny HA wings...man's got to know his limitations. :( :-[ ;D
« Last Edit: March 12, 2022, 12:16:33 PM by Califoilia »
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MikeLima

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Re: Foil Physics Question RE: AR and drag
« Reply #14 on: March 12, 2022, 03:10:11 PM »
I think you are missing that for our sport - everything we do would be considered "low speed" as far as physics is concerned.  So I really don't think we'll see speeds such that drag is reduced by going to low-aspect foils.

I’m reading the Hail Mary Project, and in it relativistic speeds are discussed, including it’s effects on time, but what was new to me was that as you decelerate, objects in front of you get farther away, despite the fact that you are approaching them at unfathomable speed. But I think this is faster than most wing foilers!
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