Why must VTOL engines be larger than normal engines?

[David Lewis]

E'lir Kramer wrote: "Could an aircraft achieve lift by blowing a fast stream of air over an internal fixed wing?
I.e., by having an onboard wind tunnel with a wing suspended inside?"

David Lewis wrote: The basic idea is valid but the wind tunnel/internal wing is unnecessary.
Propwash blowing over a conventional wing, arranged properly, can reduce stall speed and increase rate of climb.

 

[sophiecentaur]

Could an aircraft achieve lift by blowing a fast stream of air over an internal fixed wing? I.e., by having an onboard wind tunnel with a wing suspended inside?

Conservation of momentum tells us that what happens on board, stays on board. If the result of the internal 'wind tunnel' is not a net injection of downward momentum to some air then the lift can't be improved. We discussed the Bernoulli vs Newton III many times (I thought we had general agreement in the end) and, whatever the (important and highly relevant) local pressure situation happens to be, if there's no net down draught then there's no lift.
For 'wind tunnel' , substitute ' large box of flying canaries' and (like the wind tunnel) the idea doesn't hold weight.
An internal duct, which vents downward could provide more lift, of course, but that isn't the same as a totally internal system.

 

[David Lewis]

A plane goes up when the sum of upward forces exceeds the sum of the downward forces.

This is true when the climb is initiated. When an airplane is in an ongoing climb, however, the sum of upward forces can be equal and opposite to the sum of downward forces.

 

[A.T.]

This would violate Newton's First Law. When an airplane is climbing, the sum of upward forces is equal and opposite to the sum of downward forces. The net force on a free-body is zero unless it's accelerating.

To start climbing it must accelerate upwards.

 

[David Lewis]

Point well taken.

 

[A.T.]

You are 100% correct that the aiplane will speed up, but this acceleration will be brief and transient, and mostly horizontal.

Only the vertical component of acceleration is relevant to climbing. How long the acceleration period lasts depends entirely on the situation.

 

[David Lewis]

You're right. Only the vertical component of acceleration is relevant when the plane initially begins to climb. However, the airplane can continue to go up when sum of all forces is zero.

 

[davenn]

When an airplane is in an ongoing climb, however, the sum of upward forces can be equal and opposite to the sum of downward forces.

You're right. Only the vertical component of acceleration is relevant when the plane initially begins to climb. However, the airplane can continue to go up when sum of all forces is zero.

then it will hover, it won't climb

 

[cjl]

then it will hover, it won't climb

No - when the airplane is climbing, steady state, it is not undergoing any acceleration, so the forces can indeed sum to zero. To initiate the climb, the vertical net force must exceed zero temporarily, but this is just a transient state.

 

[davenn]

No - when the airplane is climbing, steady state, it is not undergoing any acceleration, so the forces can indeed sum to zero. To initiate the climb, the vertical net force must exceed zero temporarily, but this is just a transient state.

doesn't sound right
need to see proof of that

 

[cjl]

It's simple physics. F = ma. In a steady climb, acceleration = 0 so F_net = 0.

 

[davenn]

It's simple physics. F = ma. In a steady climb, acceleration = 0 so F_net = 0.

if you were climbing in zero gravity ?

you are going to have to do a better job of explaining

 

[A.T.]

if you were climbing in zero gravity ?

No, gravity is part of the F in F=ma.

you are going to have to do a better job of explaining

How about you explain what is wrong about it?

 

[David Lewis]

Davenn, I understand your skepticism. According to everyday experience, it may appear that a force needs to be applied to an object in order to make it move -- and that's partly correct. In reality, however, a force only needs to be applied temporarily in order to get it moving. After that, you take away the force and it keeps on moving by itself -- indefinitely. The reason this might be counter-intuitive is that friction and air resistance tend to obscure the underlying law.