# Physics of Flight - From where does lift come?

Gold Member
Consider a simple prop plane like a Cessna. I'm a little confused about the origin of the energy for lift. We know the engine accounts for all the thrust and the wings have drag that varies by angle of attack, which also changes lift. But does the engine account for all the energy in lift? If not from where does the extra energy come? It seems there's some basic concept in Bernoulli's Principle that I may be missing here.

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Draw a diagram of forces for the aeroplane in steady horizontal flight. (lift, drag, weight, and thrust) Are all four forces consumers/producers of energy?

HallsofIvy
Homework Helper
There are only two possible sources of energy for an airplane- the engine and gravitationl potential energy due to the airplane's height. In particular, "Bernouli's principle cannot add energy."

Gold Member
There are only two possible sources of energy for an airplane- the engine and gravitationl potential energy due to the airplane's height. In particular, "Bernouli's principle cannot add energy."
So if the airplane rises it must expend at least the same amount of energy from the engine (assuming no losses) as the change in gravitational potential (plus whatever is needed to overcome drag/friction)?

mfb
Mentor
Right. A real airplane will need a lot more energy, but the change in potential energy is the absolute minimum.

Gold Member
Right. A real airplane will need a lot more energy, but the change in potential energy is the absolute minimum.
Got it. I think that set me on the right course, so to speak. :)

I was getting messed up with Bernoulli and flight but thinking about it in light of HallsofIvy's hint Bernoulli's principle seems more like buoyancy, or at least very similar.

Right, the engine does work against the drag force and in rising flight against gravity. But where does that leave lift: does the lift force do work in flight?

[Bernouilli's principle is an application of conservation of energy.]

Gold Member
Right, the engine does work against the drag force and in rising flight against gravity. But where does that leave lift: does the lift force do work in flight?

[Bernouilli's principle is an application of conservation of energy.]
Work requires a displacement. Since there is no displacement (change in up/down) in level flight there is no work being done in that direction.

correct, and as lift is always drawn at right angles to the direction of travel there is never any work done by the lift force even when the plane is changing altitude.

Gold Member
correct, and as lift is always drawn at right angles to the direction of travel there is never any work done by the lift force even when the plane is changing altitude.
??? I always thought lift was "up" - but it seems to me you're saying lift has a horizontal component when the airplane is changing altitude???

drag is always opposing the direction of travel and lift is always at right angles to drag. If this was not the case then an aeroplane could never pull out of a vertical dive.

rcgldr
Homework Helper
Right, the engine does work against the drag force and in rising flight against gravity. But where does that leave lift: does the lift force do work in flight? [Bernouilli's principle is an application of conservation of energy.]
In level flight, the lift force the air exerts on the wing doesn't do any work, but the equal and opposing downwards force that the wing exerts on the air causes downwards acceleration of air, and there is a resulting downwards veloctiy at the moment the affected air's pressure returns to ambient. So there is work being performed on the air (so part of the process of generating lift and drag violates Bernoulli).

Note that all that energy generated by the engine is being transferred to the air, changing it's kinetic energy (and somewhat it's temperature). In the case of a glider in a steady descent (fixed speed and glide ratio), the decrease in gravitational potential energy corresponds to an increase in energy of the air.

The change in momentum of a volume of air as a wing passes through would correspond to the impulse (force x time) generated by the wing. I'm not sure of the net change in pressure of the air just after the wing passes by.

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I think I can manage one more statement before I'm out of my depth. The lift and drag forces so far discussed aren't real in the sense that you can point to a few molecules of air and say that is where the lift is or that is where the drag is. On a real wing the lift is produced over a large area and can be more accurately represented by a lot of small lift forces of various sizes pointing in many different directions. This lot can be summed and then resolved into the familiar lift and drag. This neglects that there is also rotation of the air molecules around the wing and this also consumes energy . Where I'm on shakey ground is I believe that this loss due to rotation is just combined with the drag to give the airfoils lift drag curve.

And completely out of my depth with the following because I thought that Bernouilli's principle can be applied to a wing whose airstream is attached and that was in practice how the wing pressure curves were actually calculated by computer simulation.

Gold Member
drag is always opposing the direction of travel and lift is always at right angles to drag. If this was not the case then an aeroplane could never pull out of a vertical dive.
When someone earlier mentioned weight, as opposed to load, it threw me off since weight is strictly a downward (toward gravity) force.

If lift is always perpendicular to thrust and thrust is always in the direction of travel, with load and drag, respectively, being opposite those forces, I can live with that. I've spent my whole life translating from one coordinate system to another so whatever convention you want to use is fine with me. :)

rcgldr
Homework Helper
And completely out of my depth with the following because I thought that Bernouilli's principle can be applied to a wing whose airstream is attached and that was in practice how the wing pressure curves were actually calculated by computer simulation.
But then you'd need a mathematical model to predict the local airstreams speeds, and such a model could also predict the pressure differentials as well, since the changes in air speed coexist with changes in pressure, although not quite, since some work is down on the air, resulting in a net wash that is mostly downwards (lift) and somewhat forwards (drag). Usually a somewhat simplified form of Navier Stokes Equations is used to model airfoils.

But then you'd need a mathematical model to predict the local airstreams speeds, and such a model could also predict the pressure differentials as well, since the changes in air speed coexist with changes in pressure, although not quite, since some work is down on the air, resulting in a net wash that is mostly downwards (lift) and somewhat forwards (drag). Usually a somewhat simplified form of Navier Stokes Equations is used to model airfoils.
So correct me if I have this wrong: at the point in the model where you have enough information to successfully apply Bernouilli's principle you no longer need to apply Bernouilli's principle?

rcgldr
Homework Helper
At the point in the model where you have enough information to successfully apply Bernouilli's principle you no longer need to apply Bernouilli's principle?
My understanding is that a model has to deal with co-dependencies between pressures and speeds, so it can't just calculate one of the parameters and then use Bernoulli to calculate the other. Also Bernoulli is violated by the amount of work done by a wing onto the affected air, but I don't know by how much.

CWatters
Homework Helper
Gold Member
If lift is always perpendicular to thrust and thrust is always in the direction of travel, with load and drag, respectively, being opposite those forces
At the risk of confusing people...

A body moving through the air experiences forces distributed all over it's surface. You can resolve these into orthogonal lift and drag components if you want (eg for convenience) but it's not true to say that they exist in that form.

Consider an aircraft in a vertical dive. The horizontal forces must sum to zero because it's not accelerating horizontally. Weight is acting in the same plane as thrust so the lift force is zero (or close to zero).

When you get onto looking at stability it's necessary to resolve these forces into one that includes a torque or pitching moment but I've long forgotton all the details. Just to say that this is one reason why the lift force on the tail of most conventional aircraft is downwards in level flight.

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rcgldr
Homework Helper
When you get onto looking at stability it's necessary to resolve these forces into one that includes a torque or pitching moment but I've long forgotton all the details. Just to say that this is one reason why the lift force on the tail of most conventional aircraft is downwards in level flight.
Cambered air foils generate a downwards pitching torque. For stability, most aircraft have the center of mass in front of the center of lift, which also produces a downwards pitching torque, countered by a downwards lifting force at the tail that produces an upwards pitching torque, sensitive to the air speed. Once trimmed, if the aircraft pitches up, it loses speed, the tail generates less torque, so the nose drops and the aircraft gains speed again. If the aircraft pitches down, it gains speed, the tail generates more torque, so the nose rises and the aircraft loses speed. The idea is that the tail is trimmed so that the aircraft tends to remain in level flight at a specific speed without control inputs, and it's called positive pitch stability.