Question posed on Facebook

[Chas]

Now I was going to use an aircraft carrier as an example Chas. I discounted it because of the steam (yes) catapult which is an additional thrust, necessary because of the short runway, and as such adds another dimension to the conundrum.

Take away the catapult and assume planes can get going without and it’s the perfect analogy.

The catapult is just to increase the effort supplied from the engines, the same effect as adding extra engines to the aircraft

 

[StarCruiser]

The catapult is just to increase the effort supplied from the engines, the same effect as adding extra engines to the aircraft

Ah, but the catapult pushes against the ship whereas engines push against the air. Otherwise what you say is true. In other words, the planes need extra thrust to account for the shorter runway which only needs to be long to get up to take off speed. So, get to take off speed quicker and a shorter runway suffices.

 

[clivehorridge]

Well that was an enjoyable interlude...

Cheers Chas...

 

[clivehorridge]

Yay. Winner winner chicken dinner.

Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.

See, that's what I said. It was in my book of physics for Boy Scouts.

Is that Boy Scouts on roller skates on conveyor belts though Chris?

 

[Chris]

Actually it was taken from Arthur Mee's Blackout book. Fun things to do with dead relatives in an air raid.

 

[clivehorridge]

Actually it was taken from Arthur Mee's Blackout book. Fun things to do with dead relatives in an air raid.

 

[clivehorridge]

And what’s made my suspension turn red?

 

[moggy1968]

And what’s made my suspension turn red?

Running over pedestrians

 

[chapel gate]

Wtf!

 

[Chas]

Actually it was taken from Arthur Mee's Blackout book. Fun things to do with dead relatives in an air raid.

Oh Chris, you are a wag.

 

[fbnss]

I think post no.2 in this thread was the first correct answer Chas, it didn't say it needed an explanation

 

[chapel gate]

And post number seven the first to give an explanation..

 

[Chris]

Hey I'll split the prize with you guys if it means that much to you

 

[frank rabbets]

Can a helicopter take off from a rapidly descending lift ? I doubt it due to the vacuum created above the lift.

 

[Chris]

I doubt it if the inertia of the helicopter is greater then the maximum payload it can lift under normal take off conditions.

 

[Towpack]

Possibly. It lifts off by pushing against the ground (the lift) but as they're both descending at speed the copter would have to move forward to avoid being caught in it's own downwash.

 

[fbnss]

Airwolf could do it

 

[Gary820]

Need more info, what in flight movie is playing

 

[frank rabbets]

The rotor won't create "lift" in a vacuum though so the rotor will spin but the helicopter will be stuck to the lift roof.

 

[clivehorridge]

How can a lift possibly create a vacuum above it?

But even if it could, nothing with wings (and a ‘copter’s rotor is its wings) can fly in a vacuum, they need air displacement to create lift.

Without the vacuum red herring, the ‘copter and the plane scenarios relate to the same principle. The rotors act against the air around them, not against the ground or the floor of a lift.