Derek70x
Topic Author
Posts: 2
Joined: Mon Dec 02, 2019 3:17 pm

"Center of Lift"

Tue Dec 03, 2019 3:14 am

Hello, forum,

I'm a civilian, and I have a question that requires knowledge of Aeronautical Engineering.

Re: almost all airplane wings having the thickest part of the wing camber at the leading edge:
[*]my understanding is the wing's "center of lift" also cheats toward the front of the wing
[*]is this because the center of lift and thickest camber are correlated?

[*]Is there any way to design a wing where the center of lift (fore to aft) is in the center of the wing fore to aft?
[*]What are the disadvantages?
[*]Is lift affected? How? How much?
[*]Is drag/resistance affected? How? How much?

I'm talking about the "camber" lift effect of a wing,
and not the "angle of attack" lift effect of a wing, I can figure that part out myself.

Anything else big I should know?

Application:
I am designing a trimaran (a kind of boat with a center hull and one stabilizing hull to each side)
and I'd like to design a side hull where pushing FORWARD through the water automatically creates "lift" to push that hull sideways, always toward the main hull, so always in the same direction, and each side would be a mirror of the other, both sides pushing at least somewhat into the center.

Both sides would not be pushing against each other and cancelling each other out, only the hull away from the wind is in the water 95% of the time.

This would solve a problem that is normally solved by other methods, for which I don't like the trade-offs.

This idea has been tried before with the original Hobie catamarans. I believe it was discontinued for two reasons:
[*]too much drag
[*]normal wing shape with the center of lift shifted toward the leading edge like a normal wing created the need to also shift the boat's sail forward for complicated reasons related to good boat engineering, which can sometimes cause the catamaran to tip forward end over end in certain situations.

If I can create a "wing" with the center of lift in the center of the wing, and it doesn't have significantly more drag than other ways this is resolved, I'd like to try it for the boat I'm building because it's simpler and more elegant way to solve the problem.

Truly, thank you - Google is a very messy place to study Aeronautical Engineering, I hope this works better.

Derek
 
Derek70x
Topic Author
Posts: 2
Joined: Mon Dec 02, 2019 3:17 pm

Re: "Center of Lift"

Tue Dec 03, 2019 3:59 pm

Let me simplify the question:

Is there a way to create a wing with the center of lift near the fore to aft center of the wing?

(I've already accounted for "angle of attack", so just via other factors besides that.)

What would be the downsides of any such methods, and how much impact would it have?

Thanks!!
 
LH707330
Posts: 2248
Joined: Fri Jun 15, 2012 11:27 pm

Re: "Center of Lift"

Tue Dec 03, 2019 5:42 pm

Sailor and PPL here...

I'm sure there's a way to get the CL further aft on a wing, supercritical foils have aft CLs relative to more conventional shapes. At the speeds a boat sails though, it's not practical to design for that, as the flow conditions are different. Assuming you're building a boat with a cloth sail of some sort, as opposed to a solid wing, you're going to have a hard time moving the CL back that far without losing lift from the luff and potentially stalling it out if the camber on the front half is too low. If you're going for a rigid wingsail, then you could move it aft to match the CL with the spar/mast, or have a slight offset to make it easier to trim and to have a bit of helm force.

Regarding the toed-in amas, those will create a bunch of induced drag, because the draft is low for the chord and you're asking them to create lift. You may also suffer from insufficient outboard buoyancy, which could lead to diagonal pitchpoles off the breeze. Is there a reason you don't want to use a daggerboard?

It would probably help if you explained more of your reasons and design constraints, that would help get you better answers.

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