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foil design

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Guttorm
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Re: foil design

Postby Guttorm » Wed Jun 18, 2014 8:08 am

When all the lift is created by the frontwing and the rearwing is there mostly for traction/stability why are most of the rearwings curved down or up in the tips? 
Its logical in the frontwing to reduce wing tip wortices, but for the traction rear wing the pressure should be 50/50% up/down so a straigt foil should create less drag, so why?

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Hawaiis
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Re: foil design

Postby Hawaiis » Wed Jun 18, 2014 8:24 am

Guttorm wrote:When all the lift is created by the frontwing and the rearwing is there mostly for traction/stability why are most of the rearwings curved down or up in the tips? 
Its logical in the frontwing to reduce wing tip wortices, but for the traction rear wing the pressure should be 50/50% up/down so a straigt foil should create less drag, so why?

According to the aerospace industry, the wingtips will create 10% or so of better efficiency.
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james
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Re: foil design

Postby james » Wed Jun 18, 2014 9:12 am

its called end plate effect, fooling the flow into thinking that the surface is larger than it really is and setting up a tip vortex.

its great on planes and stuff that moves at 500mph in AIR

not sure exactly how well it all applies to water at 25mph..due to completely different densitiy (air vs water) rapid changes in angle of attack and being so slow its not even funny

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Peter_Frank
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Re: foil design

Postby Peter_Frank » Wed Jun 18, 2014 10:55 am

Guttorm wrote:When all the lift is created by the frontwing and the rearwing is there mostly for traction/stability why are most of the rearwings curved down or up in the tips? 
Its logical in the frontwing to reduce wing tip wortices, but for the traction rear wing the pressure should be 50/50% up/down so a straigt foil should create less drag, so why?


Correct !

Well, almost, as I think there could be a slight downward lift on some, in order to get a neutral trim at different speeds, or the opposite for beginner foils, but that is an extremely small contribution, close to and sometimes zero I think.
That is another issue, not important here...


So you are right, the stabilizer could just be flat/straight for minimum drag.
I also believe many, actually most foil stabilizer wings ARE flat, right ?

But curving the stabilizer can be used instead of having a rudder (yaw stability).
Some also make the stabilizer in a "V" to avoid having a rudder.

This way is also used in planes, to avoid drag and mechanical nuissance :D
Although, it is a bit of a religous item also, like many design features unfortunately still is :roll:

Apart from drag and yaw stabilizing effects, some also want to avoid having the stabilizer in the direct wake turbulence of the main wing at the same time, so having a different curve or V than the main wing, means the stabilizer is never fully in the turbulence shade, but only part of it :thumb:

8) PF

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Hawaiis
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Re: foil design

Postby Hawaiis » Wed Jun 18, 2014 6:00 pm

Water density is 2000 times more dense than air at crusing altitude, so even at 20 times slower speed than airplanes. The effects of the wing tips are more profound on the hydrofoil.

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Re: foil design

Postby james » Wed Jun 18, 2014 6:16 pm

i give up...

all hydrofoils should be flat have no tail stabiliser have a pair of twin tip fins bolted to them and ideally not achieve a speed greater than 15 kts, the use of carbon or any composite should be severely limited wherever possible a heaven help you if you produce a wing without Anhedral (contradicting the earlier bit about flat wings and twin tip fins) but you conveniently ignore that bit...

your plane wing scenario has a surface area how big compared to a hydrofoil? hydrofoils have an idiot stood on top of it giving the most basic of trim commands and has non movable surfaces (currently)

and lastly for your plane wing picture to be 'right' you have put your twin tip fins on the wrong side of your front wing...

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Hawaiis
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Re: foil design

Postby Hawaiis » Wed Jun 18, 2014 6:37 pm

The wingtips are more effective on high AR wings like passenger planes, less effective on low AR wings like jet fighters. The gliders were among the first to adopt the wingtips. They were first pointed down like a hydrofoil, but passenger planes pointed up due to landing clearance.

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Re: foil design

Postby james » Wed Jun 18, 2014 6:47 pm

hahaha!

they pointed up due to clearance issues?

they point up due to pressure migration around the tip of the wing from the bottom to top surface that creates a tip vortex that if left unchecked adds drag, by adding the tip section it helps keep the airflow attached for longer and lends the airflow around the wing a smoother release and less drag and therefore better fuel economy.

landing clearance issues? seriously where do you get this stuff?

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Hawaiis
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Re: foil design

Postby Hawaiis » Wed Jun 18, 2014 7:26 pm

Point up or down or both ways, they all do the same thing. It just make more sense to point up on the airplane and point down on the hydrofoil.
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Tiago1973
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Re: foil design

Postby Tiago1973 » Thu Jun 19, 2014 12:19 am

Hawaiis wrote:
Guttorm wrote:When all the lift is created by the frontwing and the rearwing is there mostly for traction/stability why are most of the rearwings curved down or up in the tips? 
Its logical in the frontwing to reduce wing tip wortices, but for the traction rear wing the pressure should be 50/50% up/down so a straigt foil should create less drag, so why?

According to the aerospace industry, the wingtips will create 10% or so of better efficiency.


from memory: induced drag is generated by lift, i.e., no lift -> no induced drag (aka wing tip vortices)

so if the rearwing is there without generating lift then those tips do not make much sense, not for the purpose of reduce drag / increase efficiency

also to note that they may reduce induced drag, but always increase surface drag. you need to add them both to see if the whole picture make sense

... now: back to the game of thrones


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