I’m not great at physics and have no knowledge of aeronautics, so this whole chain of reasoning might be wrong.
A plane stays in the air because air is moving over the wings, which generates lift. However, that air is moving because the engine is moving the plane forward. There is no other source of energy. Therefore, some of the engine’s energy is going into keeping the plane in the air, and some is going into accelerating it forwards, or keeping it at the same speed (fighting air resistance).
Therefore, if the plane points straight up, the engine should be able to support it hovering in the air. If it didn’t have enough power to fight gravity when pointing straight up, it wouldn’t have enough power to fight gravity when moving horizontally, either.
(Okay, some older engines only worked in certain orientations, but I don’t think that’s a problem for jet aircraft, or any aircraft built after WWII.)
So why can only certain planes fly vertically?
I think the question is more about, “Why is it that a jet pushing a wing horizontally such that the wing deflects air downwards is so much more efficient than cutting out the middleman and simply having the jet push downwards.”, because it seems at first like the wing is magically creating energy out of nowhere.
The answer might be easier to understand in terms of leverage. A wing acts kindof like a lever, it converts a small amount of force applied at one point & direction (drag), into a larger amount of force in a different point&direction (lift).
The wing, because it is wide, is able to gently redirect a LOT of air downwards at a low speed. In this way, a small amount of fast air (high energy, low momentum) is able to cause a large amount of slow air (slightly lower energy, much higher momentum) to move.
Thank you. This is the answer I was looking for. I understood wings as a means to convert forward airspeed into vertical force, and they are, but I didn’t consider that there could be mechanical advantage. (and, of course, I didn’t realize that’s what I was confused about.)
Other folks had covered the wing aspect, I wanted to discuss the engine portion. Both are cogent.