Best Tech. Debate: Groundspeed vs. airspeed

[lbridges 10+ year member]

The correct answer, like most things in life, is it depends.

If a) the aircraft is stationary to a fixed ground point, and b) there is no airflow over the wings because of how the propulsion system is mounted, then regardless of the relative speed of belt to aircraft, the aircraft does not take off (assumes belt induced air currents generating lift are essentially zero).

 

[SSS 18734 5,000+ posts]

assuming the conveyor belt and the airplane are moving against each other at the same speed, there would be absolutely no airflow over the wings, and therefore no lift.

now if we replaced the conveyor belt with wind going at that same speed, then the plan would take off without gaining any horizontal distance.

 

[jgarcia9102 10+ year member]

OMG, i just thought of something.....

And yes this has been in my mind ever since it was posted...

At first i thought of this as in, the faster the engines go, the faster the belt turns against it, but the belt isnt turning the air, only the wheels. The engines are pushing the air, not the ground. Whats the wheels do dosent matter to the plane. the fact that its on a treadmill has no effect on the plane at all.

if the plane was driven by its wheels, then no it wouldnt fly, but planes arent driven by wheels, they are driven by forcing the air behind them in order to push themselves forward.

i get it. wow, that took me a while......

i feel better now

 

[JAZN 5,000+ posts]

OMG, i just thought of something.....
And yes this has been in my mind ever since it was posted...

At first i thought of this as in, the faster the engines go, the faster the belt turns against it, but the belt isnt turning the air, only the wheels. The engines are pushing the air, not the ground. Whats the wheels do dosent matter to the plane. the fact that its on a treadmill has no effect on the plane at all.

if the plane was driven by its wheels, then no it wouldnt fly, but planes arent driven by wheels, they are driven by forcing the air behind them in order to push themselves forward.

i get it. wow, that took me a while......

i feel better now

//content.invisioncic.com/y282845/emoticons/veryhappy.gif.fec4fed33b4a1279cf10bdd45a039dae.gif

 

[jgarcia9102 10+ year member]

well rather than just know your right, you should have said that to begin with. wouldda saved me a headache...

 

[68rustbucket 10+ year member]

can i change my vote, im an idiot

 

[twoohfour 10+ year member]

The correct answer, like most things in life, is it depends.
If a) the aircraft is stationary to a fixed ground point, and b) there is no airflow over the wings because of how the propulsion system is mounted, then regardless of the relative speed of belt to aircraft, the aircraft does not take off (assumes belt induced air currents generating lift are essentially zero).

This guy has 9 posts and has been a member for almost 2 whole years?

Wow. Efficient guy.

 

[AllStar1500bd 5,000+ posts]

no way it takes off, you would need a resistant force in order for it to take off, which would be the air flowing by the wings.

however, sounds like a task for mythbusters

 

[JimJ 5,000+ posts]

It's not really that complicated...

The only reason the plane flies is the pressure differential between the top and bottom surfaces of the wing. The plane is literally sucked upwards by the lower pressure area that's created on the upper surface of the wing. If you don't have any airflow over the wing, you don't have a difference in pressure; no lift is generated.

 

[Chevyaudio 5,000+ posts]

Well im confused by the question posed...If the conveyor is traveling at 150 and the plane is traveling at 150 then the thing is sitting still, even if the wheels are moving that has nothing to do with the plane lifting off the ground. Lift is the pressure diference between the top and bottom of the wing. Note I only read the first post and then posted here, therefore I didnt read anyone elses answer.

 

[JimJ 5,000+ posts]

Well im confused by the question posed...If the conveyor is traveling at 150 and the plane is traveling at 150 then the thing is sitting still, even if the wheels are moving that has nothing to do with the plane lifting off the ground. Lift is the pressure diference between the top and bottom of the wing. Note I only read the first post and then posted here, therefore I didnt read anyone elses answer.

The situation, if I understood it correctly, was the plane accelerating under its own power, but for every MPH of ground speed gained, the conveyor belt would counteract it in the opposite direction, cancelling out the plane's movement and making the ground speed equal zero under full thrust.

The air movement over the wings is really what matters here.

 

[Beat_Dominator 5,000+ posts]

No, for every 150mph of the wheels, you get 150 from the belt, so the wheels are going 300mph.... if anything, the slight friction in the wheels berings will require the plane to reach a higher speed to take off..... the plane will still move down the runway though....

 

[PhatTonyDeMarco 5,000+ posts]

No, it seems many of you are not realizing what is being asked... The plane itself is on a conveyer (or treadmill) and the wheels are only being effected by this conveyer. The plane would be remaining in the same position and only its wheels would be turning (hence the reference to a human on a treadmill, legs moving and body stationary.) The propulsion system is moving air across the wings, and because the plane is not actually moving 150mph in the opposite direction and only its wheels are, the plane achieves life.

 

[Beat_Dominator 5,000+ posts]

What? Like Pinocchio?

 

[bonesninja 5,000+ posts]

agreed, the engine will pull the plane along because it acts on the air, the conveyor belt only affects the wheels

not exactly true...a prop plane sitting on a runway can redline its engine but not move at all if the brakes are on. the prop is for forward momentum so the air may reach high enough speeds to actually create the lift. In this situation the treadmill and the prop are basically counter-acting each other. Just like stated by someone else--if you are running on a treadmill you do not increase the flow of air past your body and the props are not designed to actually create the air flow, they are designed to create forward momentum til you reach a point where the flow of air across the wings/foils creates more upforce than gravity creates downpull