A sail is not a wing

I said:

To which you replied:

But previously you had said:

The only way to gain energy from a pressure gradient is to move down it. Just like how the only way to gain energy from a gravitational field is to move downward through it. So you did imply that a boat has to always move towards the area of low pressure, even if you didn't mean to. Additionally, a glider does always have to move downwards (if it wants to maintain speed), with respect to the air mass it is in. It can move upwards relative to the ground if in an updraught, but it will still be moving downwards through the air. This is an important distinction.

Indeed, and I have tried to be consistent in adding the caveat of a glider operating in constant (in both time and space) wind.

The contrast I have been trying to make throughout this thread is that a glider in constant wind cannot stay airborne indefinitely because it must trade gravitational potential energy for work against drag. In contrast a boat in constant wind and/or current can travel indefinitely in any direction it likes, including a net movement upwind. This is a direct result of the boat operating in two media simultaneously.

I have added an edit to my previous comment to make clear that I meant the direction of travel of the glider with respect to the air.

 

Tie a rope to a glider and you have a kite. That is precisely my point. The only difference between a glider and a kite is that the kite interacts with two media moving at different speeds: the ground and the air. Because it does so it can stay aloft indefinitely.

 

A glider can remain airborne at constant or increasing altitude as long as there is an updraft of sufficient magnitude. In that situation it does not need to trade gravitational potential energy (with respect to altitude relative to the earth) for work against drag.

There is subtlety to the mechanics of gliders. To fly forward with constant velocity in an updraft the force of gravity is needed, and the vertical speed of the glider flying at constant velocity will always be less than the vertical speed of the air. In other words the glider with constant velocity will be decending relative to the air.

Gliders have made flights of very long distances by staying in the updraft on the windward side of a mountain chain. In two different locations I have watched hang gliders circling indefinately in steady updrafts. To descend they flew out of the steady updrafts.

 

Yes there is indeed a subtlety here, that an object can extract work from gravity despite moving upwards relative to the ground, because it is still moving down relative to the air. This works because of Newtonian relativity, basically the equivalence of non-accelerating reference frames.

It's rather a thread drift, but if you really want to bend your mind about this, ask yourself: why is running uphill on a treadmill harder than running flat? You aren't moving up relative to the Earth!

 

From the moment it sets sailors losing positional potential energy against the atmosphere around it.
When I said this I miss spoke

The 2 driving forces acting in opposition on a boat are wind and water. Relative to wind, water or land one might be moving in a different direction or speed to one another. Arbitrary to define a single one to use but useful to understand the forces relative to one another.
The 2 driving forces on a glider are in opposition on a glider are gravity and upward motion of air relative to the ground.relative to Air, ground and (I'll get flack for this) gravity , one will be moving in a different direction or speed to one another. Arbitrary to define one to use but useful to understand the forces in relation to one another.

Your assumption of the second medium in opposition allowing for infinite travel is nullified when you assume that a gliders medium is not moving relative to something else.
A boat extracts energy from its media moving horizontally in relation to ground. A boat has no use for vertical moving air.
The same argument now with all parameters turned 90 degrees.
A glider extracts energy from its medium moving vertically in relation to the ground. A glider has no use for horizontally moving air.

 

The frame of reference is normally the centerline and the waterline plane.

 

Gliders can operate in air that doesn't have upward motion relative to ground.

 

For the sail, the frame of reference is boom and air. The waterline plane is horizontal but so it the plane in which wind blows therefore waterline and wind dont interact with one another.

And boats can operate in air that has no motion in relation to the ground.

Like I said in earlier replies. I know gliders are limited by altitude. A 90 degree argument for that is boats are limited by the coast or their proximity to the centre or a depression or anti cyclone. Both vehicles are limited by position. Both are powered by relative motion of media. The fact that in the real world a glider cant fly uninterrupted in a straight line forever is a bad argument because boats can't either.

 

Yes, but without an updraft the glider will always be losing altitude and/or speed. Without an updraft the energy of the glider will always be decreasing: kinetic energy + gravitational potential energy = (1/2 * mass * velocity^2) + (weight * altitude).

 

Right but similarly a boat without opposition from the water will also be losing energy. Only going downwind. Eventually going to the centre of a depression.

 

It is not valid to compare a glider to a sailboat. The first operates completely submerged in a compressible fluid. The second operates at the interface of a compressible gas and an incompressible liquid.

 

What did the Wright Brothers think of a sails energy. Seems like they got off the ground with some battened sails.

 

The compressiblity of the fluid has no bearing on the argument. The type media has no bearing on the argument. How we use the forces applied by the media is what is being argued. And every force between boat, plane,glider, kite, land yacht , ice boat ,rail kart with sail/wing, air, water , gravity or atmospheric pressure all have a direct analog in the locomotion of a vehicle from A to B , upwind , downwind , high or low whether it is rotated 90° or not.

 

Both air and water are compressible though for a given pressure change air changes density by a much larger amount. Compressibility effects are completely and totally irrelevant to the the entire discussion in this thread.

At the wind and water speeds sailboats operate at both the air and water are essentially incompressible. At the speeds almost all gliders operate the compressibility effects of air can be neglected for almost all purposes. (Change of air density with altitude, atmospheric pressure and temperature may be significant.)

 

If you mirrored sailrocket along the waterline , hung a weight in place of the foil and made the rudder (also mirrored on waterline)big enough to control air then you would have a very odd looking but somewhat functional version of a cross between a canard plane and a hang glider.