will it take off?

[shvl73]

How come when you put a plane at the end of the runway with the engine at idle and you take the brakes off, it still doesn't go anywhere? The propeller is turning. It is forcing air behind it. The brakes aren't on. But it won't move. Something is holding it in place even though air is moving past the propeller towards the rear of the plane. You have to power up the engine to get it to force enough air through the prop to overcome the resistance to the forces that are holding it in place. If there was no resistance to the forces holding it in place, as soon as you turned the propeller, the plane would vanish into the distance with a blinding flash, probably similar to the Enterprise jumping to warp speed. Wooosh BANG! We'll find it circling that planet next to Neptune searching for Klingons.

There just isn't enough power being applied to move it at idle. A car with auto transmission won't alway proceed from a stop light at idle if there is any small resistance (such as a small incline) although force to move it forward is still being put through the transmission. Add power, it moves.
Put a more powerful engine in the car, it'll move at idle.
Put a more powerful engine in a small airplane, it'll move at idle.

 

[Ray2210]

I've seen this on other forums too and it's always worded a little differently. Which speed does the conveyor track and match? The ground speed in relation to the conveyor or the planes air speed which is in relation to the ground the conveyor is sitting on? Are all of the laws of physics applicable because in order for the conveyor to match the planes wheel speed in feet per minute it would have to be able to accelerate almost to infinity infinitely fast. If we assume that all of the laws of physics are not applicable and the conveyor can in deed match the wheel speed then we might also assume that the wheel bearings will have no friction and the tires will have no rpm limit at which they will explode. If we assume that the planes motivational force is via thrust as us usually going to be the case then yes it must be able to fly. If the conveyor is matching the planes air speed then whether the laws of physics are assumed to apply or not then yes the plane must fly. If this were an acutal engineering problem alot of givens are missing. The planes motivation force is generally going to be assumed to be thrust which is applied to the air around the aircraft. The only forces able to hold the plane back would be a head wind ( none stated), drag ( nothing out of the ordinary stated), friction from the wheels due to rolling resistance due to the tires, wheel bearings or from applied braking force (none of which are given) or that the plane is still tied down (also not stated).

No I'm not a pilot but I'm an engineering technologist. My brother is however a flying instructor and I have piloted a plane with him. If you make some unorthadox assumptions the plane can be made not to fly. If you assume the laws of physics applicable and that the plane normal and typical as in the real world with a pilot, not tied down, the wheels turn like they are supposed to, etc, etc then nothing can keep it from being able to reach an air speed at which it can fly.

 

[zeuspaul]

If the conveyor belt is going 100 mph backwards relative to the ground how fast is the plane going forward relative to the ground? Hint...They are moving the same speed but in opposite directions. The plane thus is traveling 100 mph forward. It flies!

Same as above..the belt is going 100 mph backward relative to the ground and the plane is going forward 100 mph relative to the ground. How fast is the belt traveling relative to the plane? Hint..they are going at the same speed but in opposite directions. The belt is going 200 mph backward relative to the plane and the plane is going 200 mph forward relative to the belt. It flies!!

 

[NewToy]

How come when you put a plane at the end of the runway with the engine at idle and you take the brakes off, it still doesn't go anywhere? The propeller is turning. It is forcing air behind it. The brakes aren't on. But it won't move. Something is holding it in place even though air is moving past the propeller towards the rear of the plane. You have to power up the engine to get it to force enough air through the prop to overcome the resistance to the forces that are holding it in place. If there was no resistance to the forces holding it in place, as soon as you turned the propeller, the plane would vanish into the distance with a blinding flash, probably similar to the Enterprise jumping to warp speed. Wooosh BANG! We'll find it circling that planet next to Neptune searching for Klingons.

The mass of the plane itself holds it in place. You have to exert enough thrust to move the mass plus the inherent friction/rolling resistance in the landing gear. Back to the old "equal & opposite REaction" deal again. Take your little ball bearing toy and replace one of the end ball bearings with a ping pong ball. Pull it back ALL the way, let 'er go. What happens? Nothing, not enough force exerted by the ping pong ball to move the heavy steel ball on the opposite side. Same thing with the plane in your scenario, you have to exert enough thrust to overcome the mass of the plane. That's why they don't put a giant GE jet engine on a Cessna. They don't want it to vanish as soon as the brakes are released. It all goes back to the laws of physics, no magic involved.

 

[daTeacha]

Moss Road -- A stationary object such as a plane with an idling engine still needs a force large enough to overcome its inertia to get it started moving. Once moving, it takes less force to maintain a steady forward speed since all you need to overcome is friction. The idling engine/prop does not generate enough force to overcome the resting inertia of the plane. If you push it by hand to a certain calculable speed, the speed will be maintained by the engine but only if the forward acting force from the prop is sufficient to overcome the friction between the plane and the ground (and air). Once moving, the plane's momentum will help it keep moving, but you need to overcome inertia to get it started.

Putting it in automotive terms, it takes more force and energy to accelerate your car from 0 to 70 mph in 1/4 mile than it does to drive that same 1/4 mile at 70.

 

[Egon]

After all this time I would suspect the plane has crashed due to an overheated tire blowout.

Anyone patent the "New Runway System " yet ??

Gotta be a fortune in it somewhere's if it works. I'd suggest substituting rollers like a dyno rather than a conveyor belt. Rollers are easier to make and should last longer.

 

[BillyP]

Egon, I don't have a clue of what you're talking about

 

[MossRoad]

He's talking about coffee! Will it have enough force to make him fly?

 

[BillyP]

Of course he'll fly! He'll fly to the bathroom. Gosh, I hope he makes it all the way

 

[RobS]

If you make some unorthadox assumptions the plane can be made not to fly...

I think you've hit on the crux of this problem. It's intentionally vague enough that everyone gets caught up in the various assumptions that need to be made. Of course, that's also what makes it fun, like any good debate.

I'm going to assume the plane has run out of gas and the winds are calm...