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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Wed Jan 15, 2014 10:54 pm 
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edt wrote:
Arcsrule wrote:
the foil must generate lift. a doubt 'flat' will generate enough lift to make you smile. i'm not an engineer but i have slept in a Holiday Inn Express.

This is not true. Check out any angle of attack / lift coefficient diagram for instance here's one with a cesna and british jet

edt, please pardon any ignorance on my part, but can you clarify how your example illustrates that a the foil need not generate lift?


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 12:06 am 
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edt wrote:
Tiago1973 wrote:
i wonder if in practical terms there is tons of a difference between a crude airfoil shape and a flat plate


Not "tons". But enough that the only airplanes that have symmetric airfoils are supersonic airplanes and stunt planes. Then there are boat props, airplane props, helicopter propellers, some are symmetric some not, depending.


here we go, it appears to be > than ~4x more



guess it may pay going through the trouble of "air foiling" that front wing..


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 12:16 am 
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Tiago1973 wrote:
edt wrote:
Tiago1973 wrote:
i wonder if in practical terms there is tons of a difference between a crude airfoil shape and a flat plate


Not "tons". But enough that the only airplanes that have symmetric airfoils are supersonic airplanes and stunt planes. Then there are boat props, airplane props, helicopter propellers, some are symmetric some not, depending.


here we go, it appears to be > than ~4x more



guess it may pay going through the trouble of "air foiling" that front wing..

Excellent video


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 1:53 am 
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jimovo wrote:
edt wrote:
Arcsrule wrote:
the foil must generate lift. a doubt 'flat' will generate enough lift to make you smile. i'm not an engineer but i have slept in a Holiday Inn Express.

This is not true. Check out any angle of attack / lift coefficient diagram for instance here's one with a cesna and british jet

edt, please pardon any ignorance on my part, but can you clarify how your example illustrates that a the foil need not generate lift?



some people think you have to have a curved upper surface of a wing for it to generate lift. This is obviously untrue.

Stick your hand out the window while you are driving the car and tilt it up. Your hand will be pushed up. That is lift. It is caused by the angle of attack of your hand to the wind.

The purpose of the curve in the wing is to generate lift with a lower angle of attack so that less drag is generated.

Hydrofoils do not need to be shaped like airplane wings in order to generate lift. But they do need to be designed so that the drag is very low that is the most important thing.


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 3:22 am 
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Thanks for posting the video Tiago1973.. it is a great primer on foil shape.

:wink: CG


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 5:42 am 
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edt wrote:
some people think you have to have a curved upper surface of a wing for it to generate lift. This is obviously untrue.
Stick your hand out the window while you are driving the car and tilt it up. Your hand will be pushed up. That is lift. It is caused by the angle of attack of your hand to the wind.
The purpose of the curve in the wing is to generate lift with a lower angle of attack so that less drag is generated.
Hydrofoils do not need to be shaped like airplane wings in order to generate lift. But they do need to be designed so that the drag is very low that is the most important thing.

Ah - so you were not disputing:
Quote:
the foil must generate lift
but rather disputing that he:
Quote:
doubt 'flat' will generate enough lift to make you smile

I'm not sure how useful it is to dispute something as subjective as doubt... it is even less useful to dispute someone else's doubt regarding their speculation as to what would make some third party 'smile'...
You do provide useful information however ;)


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 6:32 am 
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jimovo wrote:

I'm not sure how useful it is to dispute something as subjective as doubt... it is even less useful to dispute someone else's doubt regarding their speculation as to what would make some third party 'smile'... You do provide useful information however ;)



Jimovo I have seen plenty of educated people tell me that an airplane gains its lift because the top of the airfoil is curved. And if an airplane was built with flat wings, it could not go up because it had no lift. I believe that is because in school they teach this because they want to illustrate the Bernoulli principle, but the problem is they make a mess of aerodynamics by doing so. Planes will fly perfectly fine and have plenty of lift with flat wings. They will have more drag because they will have to fly at a higher angle of attack. I know it must seem weird to have to state something so obvious but I have heard it many times, and it is in fact the principle question behind the original post. Do the foils have to be curved like this? The answer is no, they will fly fine without them being curved on the top. They will still as much lift as a cambered wing, but at the cost of additional drag.

Sorry to belabor the obvious but sometimes it has to be said.


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 10:26 am 
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Thanks edt for your info and Tiago for that vid. What a great and simple way to demonstrate the affects of shape and drag.

I remember as a kid in the car, going down a motorway with a flat hand out the window. Flying it up and down just by adjusting the angle of attack, to find the sweet spot. to be able to hold the full weight of my arm in the middle of the window, happy days lol.

So basically the answer is yes you can use a flat wing for a hydrofoil and they are being used. But a foil shape reduces drag, which helps in gaining speed and thus enables a lesser angle of attack. This is more efficient and water is 784 times denser than air. Our speeds required for lift are a lot lower than an aeroplanes wing, also hydrofoils are a lot smaller. Being more efficient must make it nicer to use as well!

So it depends if you wanna cruise or rip?

It's all interesting stuff :D


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Thu Jan 16, 2014 10:00 pm 
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pretty sure that flatness of the foil will need more power to overcome the in-efficiency. so flat might work very well at 15 knots but not very well at 10kts--as an example. my only reason for the foil is for light winds--6-8knts....

then again, I'm not an engineer.


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 Post subject: Re: Do the wings even need to be aerofoil shaped?
PostPosted: Fri Jan 17, 2014 2:42 am 
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For the small size of hydrofoils, flat vs curved would have negligible difference.


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