# A sail is not a wing

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Feb 7, 2021.

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### Will GilmoreSenior Member

I'm sure I'm wrong, but from what I've been reading, there is little or no forward component to lift, only the lift relative to angle of attack. A sailboat can adjust angle of attack independent of direction of movement. An airplane, to adjust angle of attack, changes the entire plane's direction.

A sail plane moves because it has vane like wings that act as an incline plane in the air. Lift, as it falls along its incline might have a forward component, but that isn't why they go forward. That is more a consequence of their inclined pitch forward as they glide downward.

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### tlouth7Senior Member

It sort of depends on your frame of reference. Lift and drag are normally defined perpendicular and parallel to the free stream (air) respectively. Drag will be backwards with respect to the direction of travel of the glider, and lift perpendicular to it (by definition). Because the nose of the glider is pitched down there is a forwards component of gravity in this reference frame, which provides the thrust. Given that lift is defined as perpendicular to the direction of travel edit: with respect to the air we can see that it provides no thrust, because it has no forward component.

If instead you consider a reference frame perpendicular and parallel to the ground then clearly gravity has no horizontal component. Instead the sum of lift and drag (the aerodynamic force on the wings) will have a forwards horizontal component. In this reference frame it would be easy to believe that the glider could keep travelling horizontally forwards forever, but of course we know this is not possible.

In the image below you can see the two possible coordinate systems, horizontal and vertical, or rotated to be parallel and perpendicular to the "Flight Path" line. Ignore the maths.

I'm afraid this simply isn't true. If boats were limited to moving down an air pressure gradient then they could only ever sail towards a depression (on port tack in the northern hemisphere); clearly this is not what we experience. Gliders must always move downwards through the gravitational field, but boats do not have to move downwards through the air pressure field.

I think you have followed a red herring by thinking in terms of pressure gradients. A kite behaves exactly the same regardless of whether it is flying in a gradient wind or being towed in still air behind a car. A boat behaves exactly the same regardless of whether it is sailing on still water in a 5kt gradient wind, or in still air (wrt the ground) in a 5kt current.

A mention a kite specifically because it illustrates the key difference between a boat and a glider: in constant wind a kite can stay in the air indefinitely whereas a glider cannot.

Last edited: Feb 23, 2021
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### Will GilmoreSenior Member

Yes!

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### DCockeySenior Member

I assume the readers of this thread are aware that gliders say climb, cruise at constant altitude and stay aloft for long periods of time by taking advantage of updrafts (upward flow of air relative to the earth's surface).

That is the conventional definition.
The direction of travel of the glider is not always the same of the freestream direction of the air in a reference frame fixed to the glider if the glider, in particular if there is an updraft or a down draft. Per the previous statment Lift and drag are normally defined perpendicular and parallel to the free stream (air) respectively. Switching to defining lift and drag relative to the direction of travel of the glider is consistent with the mistake which has been made consistently in this thread.
A glider in an updraft does not have to have it's nose pitched downward to have a forward component of thrust. In many situations it will have the nose pitched upward so that the wing is at angle of attack relative to the freestream direction to generate sufficient lift.
It is possible if there is an updraft. Gliders have stayed aloft for very long periods (many hours) using updrafts.
That image is only valid if there is not an updraft.

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### gonzoSenior Member

Gliders normally have a negative angle of attack in the wings, while powered planes have neutral to positive angle of attack. The lift component has to have a forward component for a glider to go forward, hence the negative angle of attack. A powered plane gets forward thrust from the propeller or jet engine. That is not to say that the glider can't go forward with a positive angle of attack. However, like a sailboat tacking, it can do that by using its momentum.

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### DCockeySenior Member

Gonzo, how are defining angle of attack? Are you using the angle between the lift vector and something? Or are you using the angle between the freestream and an refernence direction on the wing or glider?

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### gonzoSenior Member

On a plane the reference is always the horizontal.

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### GlueandcoffeeJunior Member

For a sailboat. You can change the angle of the sail in relation to the centerline. But for a given heading there is a correct setting . Oversheet or undersheet and the leeway angle will change regardless of the centerline. Direction of travel , angle of attack and sheet setting are all directly related.
An aircraft can also change angle of attack without changing the pitch of the plane. They do this with flaps. For any given pitch there is a correct flap setting. Too much flap at too high a pitch you will stall. Like beam reaching with the sheet fully in.or not enough flap while in a dive can lead to . ... well , not a very good glide. And over speed and lack of control. Like a boat head to wind sheeted out.the difference is 90°.
A boat moves because it has wing like sails inclined to produce lift with a forward component. Lift, which is inclined with relation to horizontal ground and/or vertical gravity, on a glider in trim flight, definitely does have a forward component in relation to the horizontal ground and or the vertical gravity. That is precisely why they go forward. If lift has no forward component it is rock. If they have some other force pushing forward it is no longer an unpowered aircraft. If you increase the pitch or flaps or angle of attack there is less forward component of lift. If pitch goes above 0° horizontal then lift inherently goes past 90°vertical then inherently there is no forward component of lift. Lift is always perpendicular to airflow in relation to the vehicle. Up,down, foreward and backwards are in relation to horizontal ground and vertical gravity. Angle of attack or leeway angle is related between fluidflow direction and chord/boom or centerline.

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### DCockeySenior Member

That may be the Gonzo definition of angle of attack, but it is not the conventional definition that has been used for over a century. The conventional definition of angle of attack is the angle between a reference line or plane and the direction of the freestream relative to the airfoil or aircraft. Angle of attack is independent of the horizontal direction.

Deploying flaps does not change the angle of attack unless the pitch angle of the plane also changes. Angle of attack is the angle between a reference line or plane and the direction of the freestream relative to the airfoil or aircraft. The reference line or plane does not change when flaps are deployed. The lift and moment will change when flaps are deployed, which may result in the plane pitching or climbing.

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### GlueandcoffeeJunior Member

But I didn't say they could only move towards a depression. Similarly a glider does not always need to move towards gravity. This is not what we experience. There are a whole host of conditions that each vehicle experience. It is the sum total of all forces in combination that make each respective vehicle work. Upwind , downwind , high, low , fast current, slow current , thermals, ridge lift, sink ,flaps, sheets, glider weight, keel size, gravitational potential energy , positional potential energy, depressions , anti cyclones etc etc etc. The list goes on. All these things make the vehicles work. But the physics is the same. Every single thing you can say about a plane has an equal physical counterpart on a boat but turned 90°. A glider can not fly down wards forever. It will git the ground. A boat can not sail dead down wind forever . It will get to the centre of the depression. A glider cannot continuously gain altitude. It will run out of air pressure to support it at high altitude . A boat cannot beat upwind forever. it will get to the centre of a high pressure area.

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### gonzoSenior Member

OK, I have no argument with that. Definitions are the best way to avoid confusion. Are we both referring to the angle to the chord line?

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### gonzoSenior Member

I thought the flaps would change the chord since they are changing the foil.

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### GlueandcoffeeJunior Member

Tie a rope to a glider in enough wind and you can. It's the same as putting an engine on. It doesn't depend on wind. You are replacing thrust Do the same towing a boat or glider you can go anywhere. Null argument in Context.

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### gonzoSenior Member

I have kites shaped like a glider.

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### GlueandcoffeeJunior Member

Spot on with almost everything. But a glider must maintain some pitch below the horizon or it will lose air speed. No matter if in lift or sink the the angle of attack and climb rate will compensate to accommodate lift. The angle of attack will always be positive in relation to the aircraft accept in the case of negative g loads. Like flying inverted or doing outside/down loops or heavy turbulence.

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