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Wing loading as a metric

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gbrungra
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Re: Wing loading as a metric

Postby gbrungra » Sun Jun 09, 2019 8:32 am

Peter_Frank wrote:
Sat Jun 08, 2019 10:54 am
RalfsB wrote:
Sat Jun 08, 2019 10:45 am
Peter_Frank wrote:
Thu Jun 06, 2019 5:38 pm
It is a lot smaller, so not much influence.

And the back wing reduces overall lift, as it pulls a bit down, negative lift usually.

8) Peter
Sometimes it is not a lot smaller. When I use the speed wing in the front, the back wing looks about 2/3 the size of the front wing.
And regarding the negative lift: front and back wings work together as a system, the negative lift of the back wing translates into an increased positive lift of the front part.

No, it does not.

Sorry to say, but you can not increase the lift of the front wing, so any downwards lift from the stab is total loss in overall lift for your foil :(

8) PF
Do you think RalfsB meant that the flow off the front wing can change the effective angle of attack of the rear wing/stabilizer? I agree that each wing/stabilizer produces its own lift (positive or negative), and that the total lift is a sum of the 2.

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Peter_Frank
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Re: Wing loading as a metric

Postby Peter_Frank » Sun Jun 09, 2019 12:20 pm

gbrungra wrote:
Sun Jun 09, 2019 8:32 am
Do you think RalfsB meant that the flow off the front wing can change the effective angle of attack of the rear wing/stabilizer? I agree that each wing/stabilizer produces its own lift (positive or negative), and that the total lift is a sum of the 2.

You are right, the flow changes the flow aoa of the rear wing indeed.

At high lift especially, there is a significant "downwash" from the main wing, so the flow points down after the main wing (and up in front of the main wing), in aerodynamics.
This changes the rear wing aoa so the rear wing lifts even more down, meaning even when set at a geometrical neutral aoa at the given max main wing lift, it still pulls a tad down thus reduces overall lift from the system.

How big this effect is in water, and close to the water surface, I dont know exactly - but the point is this effect changes the rear wing lift so it is even more negative.

It is possible to make planes/gliders with lifting rear wings, apparently normal looking gliders, but you move the COG rear to get balance, and have the rear wing flat on the bottom side so you can use the lift more efficient, but you lose overall efficiency and stability so not used anymore.
On some hydrofoils it is reversed, the top side is flat, or at least less curved than the bottom side of the rear wing, because it pulls slightly down usually, as if not the hydrofoil becomes unstable (less stable).

On canards you have both wings lifting, correct.
Downside is the main wing is in the wake turbulence from the front wing, thus not as efficient because of disturbed flow.
Apart from dynamic issues in aviation, but these are not present the same way on hydrofoils, luckily.

Short answer for normal hydrofoils is, the main wing will change the flow of the rear wing, so it lifts even more down to some degree (or less up if set to lift, but this trim leads to instability and you would eventually be much better riding stabless then) :D

8) Peter


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