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Why can't all planes fly straight up?
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this post was submitted on 05 Sep 2023
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No Stupid Questions
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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.
This is where you make an unsupported leap:
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.)
Other folks had covered the wing aspect, I wanted to discuss the engine portion. Both are cogent.