4 ms·

Imilcin is precisely right. Rockets are an exception, in that they work perfectly well in a vacuum, where a propellar would do nothing. Rockets are easiest und

Imilcin is precisely right. Rockets are an exception, in that they work perfectly well in a vacuum, where a propellar would do nothing. Rockets are easiest understood by concervation of momentum. In the case of a rocket, this is for the combined momentum of the rocket and the spent fuel/exhaust. The rocket "trows" the exhaust backward with a momentum exactly opposite to the momentum gained by the rocket. A wing or the propellar. They accelerate the aircraft by "throwing" air in some direction, with a momentum per unit of timemass equal to the acceleration they cause. (In order to maintain their altitude, this acceleration must counteract the "gravitational acceleration", g of 9.81m/s^2) This all follows from Newton's laws, where "Force" is the time derivative of momentum F=(dp/dt)=ma => a=(dp/dt)/m. With conservation of momentum for the aircraft + air, one gets: (dp_aircraft/dt) + (dp_air/dt) = m_aircraft * -9.81 m/s^2 If dp_aircraft/dt = 0 (level flight), then: dp_air/dt = -m_aircraft * 9.81 m/s^2 In other words, lift is generated by accelerating air in the opposite direction of the lift. Airodynamic theory, including Bernoulli and Navier-Stokes equations only serve to explain _how_ the wings accelerate the air down.

lordnacho 5y ago
> Rockets are an exception, in that they work perfectly well in a vacuum, where a propellar would do nothing. A sideways rocket would... fall to the ground. Just like anything that doesn't have a way to oppose gravity. > Airodynamic theory, including Bernoulli and Navier-Stokes equations only serve to explain _how_ the wings accelerate the air down. But those are the very point of this discussion. Does the force, enough to hold up the entire aircraft, come from essentially pinging tiny tennis balls off the wing, or does it perhaps come from a complex interaction of the air with the wing, involving ideas of boundary layer separation?
- trashtester 5y ago
  > A sideways rocket would... fall to the ground. Just like anything that doesn't have a way to oppose gravity. Well, a helicopter pointing it's rotor horizontally also falls down. Why do you assume that the rocket should point horizontally? (Technically, of course, rockets _can_ and _do_ point horizontally, after going fast enough to maintain orbit.) > But those are the very point of this discussion. Does the force, enough to hold up the entire aircraft, come from essentially pinging tiny tennis balls off the wing, or does it perhaps come from a complex interaction of the air with the wing, involving ideas of boundary layer separation? There are multiple factors at play. Deflection of air downwards is a side effect of those factors that creates lift. From your previous post: > It is indeed the air that pushes the wing up, but you have to ask yourself why the wing would need to be shaped in a certain way if it's merely deflection. If it was that, you could just have flat boards at an angle and be done with it. Creating lift can be done perfectly well by a flat board. The wings are shaped as they are to minimize the drag/lift ratio at the speeds and angles of attack the wings will operate under. And for that all the complexity of the airodynamics and air/wing interactions come into play.

lordnacho 5y ago

> Rockets are an exception, in that they work perfectly well in a vacuum, where a propellar would do nothing. A sideways rocket would... fall to the ground. Just like anything that doesn't have a way to oppose gravity. > Airodynamic theory, including Bernoulli and Navier-Stokes equations only serve to explain _how_ the wings accelerate the air down. But those are the very point of this discussion. Does the force, enough to hold up the entire aircraft, come from essentially pinging tiny tennis balls off the wing, or does it perhaps come from a complex interaction of the air with the wing, involving ideas of boundary layer separation?

trashtester 5y ago
> A sideways rocket would... fall to the ground. Just like anything that doesn't have a way to oppose gravity. Well, a helicopter pointing it's rotor horizontally also falls down. Why do you assume that the rocket should point horizontally? (Technically, of course, rockets _can_ and _do_ point horizontally, after going fast enough to maintain orbit.) > But those are the very point of this discussion. Does the force, enough to hold up the entire aircraft, come from essentially pinging tiny tennis balls off the wing, or does it perhaps come from a complex interaction of the air with the wing, involving ideas of boundary layer separation? There are multiple factors at play. Deflection of air downwards is a side effect of those factors that creates lift. From your previous post: > It is indeed the air that pushes the wing up, but you have to ask yourself why the wing would need to be shaped in a certain way if it's merely deflection. If it was that, you could just have flat boards at an angle and be done with it. Creating lift can be done perfectly well by a flat board. The wings are shaped as they are to minimize the drag/lift ratio at the speeds and angles of attack the wings will operate under. And for that all the complexity of the airodynamics and air/wing interactions come into play.