# Load & Bending Moment on a wingsail flap

Discussion in 'Hydrodynamics and Aerodynamics' started by Erwan, Aug 20, 2015.

1. Joined: Oct 2005
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### ErwanSenior Member

Hi Everybody,

As presented in the title, I wonder how to calculate the loads on the third element of a wing sail.

I could find some info in the Abott, but it's not enough for me as I 'm still rookie with CFD.

If the flap is a NACA 12, I wonder if just considering max Cl could be enought for the upper limit of aero loads (static loads of course) ?

According to the distribution pressure that have been posted on some threads, regarding a 2/3 elements wingsail, it seems that the aero loads on the third elements are not that big ?

But even for a basic Cl calculation, it's better to have an AoA, and I have no idea what could be the relevant AoA for the 3rd element.

Best regards

EK

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### PetrosSenior Member

You do not need CFD to accurately estimate loading on a wing surface, for the first sixty years of aircraft construction is was determined reasonably accurately with slide rules. To determine max bending moment on the main spar (or mast in this case) you would take the max loading, which would be:0.5 x Vmax^2 x mass(air) x Cl x area.

You assume this force acts through the aerodynamic center of the wing-sail (any aircraft design book can tell you how to find the AC for any wing shape). Vmax is the max wind speed you expect to be operating in, and the Cl is the max for that profile (no reason to assume an AOA since you are designing for max loading). Typically for strutrual design you would use a safety factor of 1.5. If you are building super light for a racing design you can go as low as 1.1 but likely you will suffer failures during unplanned for maneuvers or sailing conditions.

to design individual elements on the surface (ribs, etc) you would have to use the lift distribution to determine what the max force is on that element.

Typically for preliminary design for a sail surface you can use 1 lb per SF of sail area, which works okay for typical soft sails, but is not accurate for a foil shaped wing sail.

good luck.

3. Joined: Oct 2005
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Location: France

### ErwanSenior Member

Thank You Petros,

I have been a bit confused by the Abott

I realize how my question was candid if not stupid just after posting it.

According to the light aero load on the flap, I wonder if inflatable ribs could be enough for stuctural rquirements, and it would provide a crash/capzise-resistant flap, which is the weakest part of a 2/3 elements wing.

That's the idea behind the stupid question

Cheers

Erwan

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