Energy balance in fixed wing flight, action/reaction

Question

When an airplane is in flight the engine of the plane overcomes drag until the plane is moving forward fast enough to balance drag. In a stable configuration the air moving over the wings creates lift that balances its weight and the plane maintains altitude. I’m assuming that the wings displace enough air downward to counteract the effect of gravity. But by far most planes do not have a power to weight ratio anywhere close to 1:1, especially not at cruising speed. How do these forces balance out? If weight require lift and produces drag and drag requires thrust, by what mechanism does the engine get its force multiplied?

Answer 1

Remember lift and weight are vertical, and thrust and drag are horizontal. They don't (directly) influence each other.

Your first sentence is incorrect: at takeoff, the engines thrust accelerations the plane to takeoff speed which establishes proper airflow for lift, and the control surfaces (slats, flaps...) tilt to direct the air downwards. It is this lift (which greatly exceeds the thrust) that counteracts the weight of the plane. The thrust never has to fully overcome the weight, except in a rocket, which is very inefficient.

In steady flight, the thrust is continuously balanced by drag (as it must, since drag is a reaction force. It can never exceed the thrust) and the plane's forward speed remains constant. Separately, the lift is a function of that speed and balances the weight.

One of the main reasons the lift forces is greater than the drag forces is that the plane's cross sectional area if you are beneath it looking up, is much greater than the area while you are on front of it looking towards it. That is why planes are designed the way they are. Think of a paper airplane: you want it to catch as much air as possible from below, while being literally paper thin from the front.

In response to your comment: in steady flight (constant speed, not ascending or descending) the requirement is that the vertical forces be equal. Not mass of air or anything else. The lift force is proportional to the lift area $A_L$ and the square of the speed $v$, i.e

$$F_{Lift} = \mathrm{Weight} = kA_Lv^2$$

So a plane with a very large area $A_L$ needs a smaller speed $v$ to produce the same force of lift, compared to a plane with smaller area. That is one reason fighter jets can have short, stubby wings compared to passenger jets, as the fighters have much more power per lb of weight, and so can generate higher speeds.

Another way to think of it is think of the plane as stationary and the air coming in at speed. The plane's wing deflects the air down and forward from its original path, which means the plane is directed up and backwards. The engine thrust cancels out the backwards force (drag) leaving only the upward force. In this reference frame the plane is little different than a helicopter hovering, by forcing air downward to balance its weight.