Author Topic: Wing shaping and wing washout  (Read 1340 times)

SUPeter

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Wing shaping and wing washout
« on: February 20, 2019, 12:34:01 PM »
Yes, I know, What the hell does this mean?
I am making a new wing(41"W x 9" cord) in hopes of downwinding and if I get good enough, some paddle assisted flatwater takeoffs.  As I sit here and ponder the wing segments glued to my anhedral form I can not help but wonder if I would benefit from having some washout at the wing tips.

What is washout, you ask?  Washout is when the wing tips are slightly twisted down to decrease the angle of incidence  when compared to the root of the wing.  If all wing segments are in the same angle of incidence, there is no washout. 
Washout is used in airplanes to decrease sudden stall in low speed situations. Though in the world of hydroplanes, there may be no benefit. 

I am wondering if anyone has ever taken the time to notice whether or not the angle of incidence of their wings root is identical to the angle of incidence of the wing tip.  To do this one would have to use a level and place it on the middle of an upturned wing exactly for and aft. Make it so the level reads level by altering the boards position.  The more difficult part is to place the level in an absolutely parallel position to the middle line of the wing, but this time near the wings tip(3"from tip)?)  Are the measurements level or does the level now angle up ever so slightly(down when surfing)?

Just curious, as I have always made my wings with no washout.   I would love to hear from those more knowledgable in aerodynamics and hydrodynamics. My guess is most wings have no washout and I will just have to experiment or, more probably, stick with what I know.


PonoBill

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Re: Wing shaping and wing washout
« Reply #1 on: February 20, 2019, 02:48:50 PM »
I've thought about it a bit too. From what I've seen there isn't much in the way of modeling being done in the human-powered hydrofoil world. Most stuff is cut and try at best, so taking a cue from other designs is probably not a great idea. Looking at the dynamics of pumping with and without washout would be a worthy effort since the AOA changes very quickly over a substantial range. Adding enough washout to lag the AOA change seem like it would reduce the drag at the wingtips during the pump upwards, and increase lift on the pump down. That seems like a potential benefit. I'd try it with very small increments since, like everything else with hydrofoils, the effect is going to be much more than expected.
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SUPeter

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Re: Wing shaping and wing washout
« Reply #2 on: February 20, 2019, 04:48:12 PM »
Thanks PonoBill!  I may try a 1 degree washout on this new wing, 2 degrees at most.  It may be very noticeable when turning when the outside wing is going much faster than the inside of turning wing.  Also, during the turn the downturned/anhedral wing tip of the higher outside of curve wing is in a greater position to create lift. Decreasing this lift at the wing tip may stabilize turning.  Won't know till we go?
« Last Edit: February 20, 2019, 05:05:25 PM by SUPeter »

PonoBill

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Re: Wing shaping and wing washout
« Reply #3 on: February 20, 2019, 09:38:52 PM »
I'm going to try a high-aspect wing for downwind with variable camber. I'll make the trailing edge out of fairly soft urethane with a bit of camber in it. At slow speed the camber should boost lift, at higher speeds it will flatten. At least that's my theory.
Ponohouse is for sale: http://www.ponohouse.com
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

SUPeter

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Re: Wing shaping and wing washout
« Reply #4 on: February 21, 2019, 05:18:16 AM »
Yes on the high aspect wing! I am also going that route. Be careful with flexible wing portions.  Depending upon where the center of lift of that portion is located, the portion could actually twist upward creating more lift and lift induced drag.  I believe foil shapes such as Eppler 817(H105?) or similar have a more rearward center of lift and would twist down at higher speeds.  Using an internal skeleton/external skeleton may help to alleviate unwanted twist at the expense of too much stiffness.  Best of luck!

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Re: Wing shaping and wing washout
« Reply #5 on: February 22, 2019, 04:29:04 PM »
if youre going for a flexible wing would you not build it with wash in so when you unload it in a pump the AoA increases but when your weight comes back on it flattens out again? i try to always have a bit of washout in my wings, the current one has a little less then desired but still noticeable. go foil tail wings have a nice amount of washout (last pic)

im a big advocate of tip fences after seeing the results on my foil, incredible how much more speed and 'drive' it has while pumping so you could probably get away with no washout and 1 or 2 sets of fences along the wing to achieve the same result
you can see in pic. 3 the angle of the tell tales in relation to the centre line of the foil while in level flight- angled back in towards the fuselage by a good 10 degrees

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Re: Wing shaping and wing washout
« Reply #6 on: February 22, 2019, 04:35:39 PM »
and it performs extremely well for 5kg of low quality laminated plywood

surfcowboy

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Re: Wing shaping and wing washout
« Reply #7 on: February 22, 2019, 06:36:10 PM »
Ah, you guys both inspire me. My rig is really done, I just need a test pilot. (Need to advertise that a bit when it stops raining here.)

Love seeing that cutoff board and plywood perform like a commercial foil.

Peter, we need some footage of you too.

fly2surf

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Re: Wing shaping and wing washout
« Reply #8 on: February 22, 2019, 06:45:19 PM »

What is washout, you ask?  Washout is when the wing tips are slightly twisted down to decrease the angle of incidence  when compared to the root of the wing.  If all wing segments are in the same angle of incidence, there is no washout. 
Washout is used in airplanes to decrease sudden stall in low speed situations. Though in the world of hydroplanes, there may be no benefit. 


Twisting the wingtips down is also done on airplanes so that as they enter a stall you still have roll control and can stop any roll while simultaneously pitching down to break the stall.

On traditionally configured aircraft roll control is achieved through the ailerons, which are out towards the tips of the wings (on the trailing edge), thus the desire to have a lower AOA on the outer portion of the wing than at the root.

This doesnít really apply to foils since they donít have ailerons.

Iím not saying donít try it, but you shouldnít try it just because airplanes have downwashed wingtips.  It might be beneficial so that your entire wing doesnít stall at once, but is stalling wings a problem when foiling?

Any washout you design in will result in less total lift for your wing area.

As to the thread drift about high aspect ratio wings, I would suspect they would work well in downwind situations right up until people push the wings longer and thinner and then they break (if constucted with traditional foil wing layups).

SUPeter

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Re: Wing shaping and wing washout
« Reply #9 on: February 25, 2019, 04:48:38 AM »
F--ckin awesome!  A big thank you to everyone who shared their outlook on this often times confusing and developing topic.   I love seeing those pics container.  Your wing is beautiful and the fact that you got it to lift off in flatwater is impressive on all counts. If you don't mind me asking,, what is the projected surface area of your wing?  Mine will be somewhere between 1800-1900 sq cm.  I'm hoping its enough.  41" wide, 9.5" cord.  If not, Ill just build a bigger one.  number 8?   I hope not.

PonoBill

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Re: Wing shaping and wing washout
« Reply #10 on: February 25, 2019, 09:09:10 AM »
Actually, the benefits (and disadvantages) of washout apply to wings with no ailerons. The ultralight I used to fly has washout to improve stability and assist with stall recovery. I learned firsthand what happens when the washout is eliminated because the cables stretched and we didn't pay attention to wing profile. My shins still have the notches where the landing gear crossbar hit them.
Ponohouse is for sale: http://www.ponohouse.com
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

fly2surf

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Re: Wing shaping and wing washout
« Reply #11 on: March 01, 2019, 05:15:03 AM »
Thatís too bad you crashed your ultralight.

Generally itís dihedral that is used to improve stability, maybe you lost some of that too?

Having the wingtips washout tends to make stalls more gentle, especially if it is progressive as it goes outboard.  First, just the center of the wing stalls, then the stalled area moves farther out, then finally the whole wing is in a stall.

In aircraft this can allow for more reaction time which is nice for entry level flying.

So if the OP is suffering from sudden full foil stalls, then washout could be beneficial as I stated.  But he will need a bigger wing to produce the same lift as a non washed out wing. The bigger wing will have more drag.

PonoBill

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Re: Wing shaping and wing washout
« Reply #12 on: March 02, 2019, 01:50:40 PM »
The crash occurred after classic porpoising during takeoff==five or six climb and stall cycles followed by a splat as I cut the throttle. It wasn't too bad, I basically just parked it a little too high. Properly adjusted, taking off was as you describe, with the wing stalling a little at the center but not further outboard. Losing the washout eliminated that little automatic crappy pilot compensation.
Ponohouse is for sale: http://www.ponohouse.com
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

SUPeter

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Re: Wing shaping and wing washout
« Reply #13 on: April 16, 2019, 09:11:34 AM »
With regards to washout again.  A major wing manufacturer puts 1-2 degrees of washout into their wings.  It just seems to make sense to me.