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?
In military jets that can fly vertically, it’s because the engineers prioritize performance and capability over efficiency. Passenger planes prioritize efficiency over acrobatic ability.
This part is half-right: the work of the engine is going to accelerating the plane forward, or (when thrust and drag are in equilibrium) maintaining the current velocity. But in level flight, the engine is not “keeping the plane in the air” - it is impossible for it to contribute to lift directly because it’s force vector is 90 degrees from the lift vector.
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).
This is where you make an unsupported leap:
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.
A car can accelerate horizontally because its engine can rotate its tires to apply horizontal force due to friction and mechanical advantage; does that mean it can drive straight up a wall? Of course not (outside of some specialized bouldering vehicles). The engine lacks the power to lift the car straight up, and the tires lack the grip to hold on to a vertical surface. The drivetrain is designed for efficient road cruising, not high power and grip
It’s the same for aircraft, generally: a given engine usually has enough power to accelerate the aircraft horizontally, and applies this through some kind of prop or jet rotor. But this combination is tuned for efficient cruising, not vertical climbing. The engine won’t provide enough power, and the prop can’t move enough air, to sustain vertical flight indefinitely.
“But Sleet01,” you cry, “helicopters exist!” Just so! They trade cruise efficiency for vertical thrust by greatly increasing the size of the prop, increasing the mechanical advantage so that less engine power is needed to hover or climb vertically. That’s like putting 4" wheels covered in suction cups on your car - now it can go straight up, but you can’t go very far or very fast!
“But Sleet01,” you exclaim, “F-15s exist and can fly vertically almost to space!” Indeed they do, but in order to fly an F-15 vertically you need to burn immense amounts of fuel, almost 400 gallons per minute. That’s like putting two turbo V8s in your Jeep - now you have the power to go vertical, but only for a couple minutes!
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.)
It’s because of the “lift to drag ratio”. Airplanes in level flight at ordinary speeds generate about 15x as much lift as drag meaning if the engine spends 1 unit of work moving the plan forward, the wings give 15 units of work* upwards. So flying level needs about 1/15th the engine power of going straight up. (I’m using “work” very sloppily here, not in a precise physics sense.)
You can see this in sailboats too, which can travel faster than the wind when they’re sailing at an angle to the wind. Efficient boats travel faster when they’re going almost perpendicular to the wind, not straight downwind! This is because the “lift” of the sail pulling the boat forward even more strongly than the push of the wind in the downwind direction.
While I can’t give an intuitive explanation for why this is, there’s a very easy demonstration that it’s true: kites. If a kite had a lift-to-drag ratio of 1, then it would fly at 45° up. It would fly 50 meters downwind of you when it’s 50 meters up. But any decent kite can fly at a much steeper angle than that; sometimes they look like they’re right over your head! That’s because with a lift to drag ratio of e.g. 10, the 1 unit of drag gives 10 units of lift; if it’s 10 meters downwind it will be 100 meters high.
The plane staying in the air is due to lift. Lift is generated by the wings in a traditional aircraft. To generate that lift, air has to be moving over the wings at a particular rate.
Now consider this: in a car, train, etc., you can slowly accelerate up to a given speed. The smaller the engine, the longer it takes to accelerate up to speed X. The same is true in aircraft. Once the aircraft moves fast enough, the wings will produce enough lift to raise it.
For an aircraft engine to produce enough lift to lift the plane vertically is a different matter entirely. Because the wings are no longer producing lift, the propellers (assuming not a jet, but principles are the same) have to be the source of lift. Note that propellers themselves are little wings that produce lift from air flowing over them, but not the same amount of lift as larger wings.
tl;dr: an aircraft engine generally only needs to produce thrust to move the aircraft laterally quick enough so that the wings produce the lift. Without the wings, the math changes drastically.
Here is an easier example.
Think of a truck that sits on a street that is going slightly uphill. There is no way that you and your buddy could lift that truck straight up in the air. But you can relativily easy push it uphill. In the end the truck is going up.
Now, what’s the physics behind that and how does it relate to planes? Well, you don’t have to lift the entire car when pushing. Most of the force from gravity is resisted by the ground. You only have to push against the much smaller horizontal component that tries to push the car downhill.
With planes you basically just replace “ground” by “lift”. So instead of tires pressing against the ground, you have wings pressing against the air. And a jet engine instead of two guys pushing.
Basically a plane’s engines are pushing the plane up a hill made of air.
