Airplane Riddle

[BigAlTheBird]

Hmph. Well I won't admit defeat, and I won't admit that I don't know anything.

[garublador]

It's OK, you just made an incorrect assumption about how planes work. You're a physics major, not an aerospace engineer.

[LesStrat]

Maybe not, but James Young of Styx is.

[Stratman]

Not if your flying (Grounded) on TED - United Express.

It seldom leaves on time.

[Zerberus]

Yes, because it really has nothing to do with ground speed - it's all about air speed, which gives it lift.

Exactly.

The engines push away air towards the back, the wheels are free to rotate however they please (within the limits of their design i.e. bearing wear and such). As a result, the conveyor belt has zero effect on the plane except that the ground speed guage (if even available, many smaller planes don´t) will be showing abnormally high values for a takeoff (namely 2x the airspeed).

To match the plane´s airspeed the plane has to have airspeed, which would mean it´s moving relative to the air around it. A conveyor belt cannot reduce the airspeed unless it´s so huge that it´s creating a huge cushion of air above it (which ironically may in itself lift the plane and foil the plot).

When a plane is at the end of a runway, right beofre takeoff, the engines are at full speed, but the plane can stay both:
A. Stopped on the runway
and
B. On the ground.
...

This is because all planes have wheel brakes for takeoff, landing, taxiing and ground emergencies. Being able to control the plane on the ground (esp during landing) makes for a lower body count

The drivetrain argument some usually post is irelevant because larger aircraft that sometimes have one disengage it to allow the speed necessary to be achieved for takeoff, and smaller aircraft do not have one at all (note that nowhere is the size or type of the plane specified. It could also be a small radio controlled plane for all we know)

I have flown with an airspeed of 100 knots and been moving BACKWARDS relative to the ground. I have also seen planes take off with a ground speed slower than I can run. Planes only really care about the ground when they´re refueling or parked. Or in the dismal case that they may be headed towards it for whatever reason.

Sometimes it helps to ask a pilot

[BigAlTheBird]

I just got owned by a German. Insert any number of jokes right there.

[LesStrat]

He ain't really German. He's a military brat.

[UfoPilot]

the airplane engines push against the AIR . They do not drive along the ground. How fast or slow the wheeles spin has nothing to do with how much forward movement the plane generates.

[BigAlTheBird]

He ain't really German. He's a military brat.

I figured, but he's over there.

[GuitarStv]

Maybe I'm being stupid here . . . but I'm not coming to the same conclusion as everyone else here.

1. The wings need air to flow around them quickly to generate lift.
2. The engines do not generate lift for an airplane. They generate thrust, which normally moves the plane forward. The forward motion of the aircraft generates lift as the wind passes the wings.
3. Because the planes wheels are on a conveyer belt, the plane itself is stationary with respect to the Earth. (Sum the vectors - you have a positive velocity from the engines of the plane and an equal and opposite negative velocity from the belt.)
4. Assuming there is no wind on this day, nothing is causing the air to push past the wings of the plane.
5. Without motion with respect to the Earth (and without motion with respect to the air around the wings) the plane cannot take off.

AIR (at rest)
WING (not moving relative to the ground or air - only to the belt)
AIR (at rest)
Wheels (spinning but not changing position horizontally)
Conveyer Belt (Moving)
GROUND

Now I'm not an aerospace engineer. . . but I am an engineer working on aircraft design at an aerospace company. I may be wrong here, but I'd appreciate it if someone can tell me which point that I'm making a bad assumption on. Thanks!

[firebirdV]

Maybe I'm being stupid here . . . but I'm not coming to the same conclusion as everyone else here.

1. The wings need air to flow around them quickly to generate lift.
2. The engines do not generate lift for an airplane. They generate thrust, which normally moves the plane forward. The forward motion of the aircraft generates lift as the wind passes the wings.
3. Because the planes wheels are on a conveyer belt, the plane itself is stationary with respect to the Earth. (Sum the vectors - you have a positive velocity from the engines of the plane and an equal and opposite negative velocity from the belt.)
4. Assuming there is no wind on this day, nothing is causing the air to push past the wings of the plane.
5. Without motion with respect to the Earth (and without motion with respect to the air around the wings) the plane cannot take off.

AIR (at rest)
WING (not moving relative to the ground or air - only to the belt)
AIR (at rest)
Wheels (spinning but not changing position horizontally)
Conveyer Belt (Moving)
GROUND

Now I'm not an aerospace engineer. . . but I am an engineer working on aircraft design at an aerospace company. I may be wrong here, but I'd appreciate it if someone can tell me which point that I'm making a bad assumption on. Thanks!

The wheels don't propel the plane forward, the turbines/props do that. Therefore, even if there is a conveyor belt going in the opposite direction, it will only be putting some friction on the wheels and the airplane will still go forward. I guess the wheels might start to smoke or something, I don't know about that part, but the plain would probably still take off.

I still think it's a trick question.

[firebirdV]

I'm trying to think of how the wheels would react, but it's tough. They would be going in the same direction as the conveyor belt but because the wheel and the belt will have different angular velocities (I think you can say that...), wouldn't there be a force somewhere?

I don't want to think about physics right now, but that's my half assed attempt at an explanation, and I take no responsibility if it's wrong or if I misspelled anything.

*Edit*
After thinking about it for a second longer, I've decided that a belt can't have angular velocity. I think you could use a velocity tangential to the wheel though.

[JB_From_Hell]

I agree with GuitarStv that it seems the wheels have more to do with it than some are saying....

[firebirdV]

I agree with GuitarStv that it seems the wheels have more to do with it than some are saying....

Even if the wheels provide resistance, if you assume that they don't break, they'll just slow down the plane. The friction is little copmared to jet engines. The wheel problem in and of it's self is interesting though.

[JB_From_Hell]

I'm trying to imagine something else to compare it to, like running on a treadmill holding a kite, but can't think of anything comparable.

[Kiobe]

If the speed of the belt exactly matches the speed of the plane as it trys to move forward, the belt going backward, the plane won't ever get the airspeed it needs to get airborn. Like running as fast as you can on a tredmill you feel no wind and get nowhere. T

[Fusion1]

What if the conveyor belt was going 25 mph faster than the plane's thrust was propelling it forward? Wouldn't the plane be moving backwards? The odd part to speculate on is whether a conveyor belt moving in the opposite direction of a plane going at the rate of speed a plane needs to take off is approaching 300mph. Is there even a conveyor belt that travels that speed? And if it could travel that speed wouldnt the weight of the plane cause the conveyor belt to malfunction? If there are conveyors with that kind of speed, then we should forget this riddle and start implementing 300mph moving sidewalks. Forget the bullet train and step onto the super highway!

[Fusion1]

Also when I first saw the title in here, what came to my mind was the riddle on the episode of Monk where he goes on his first flight. Some annoying girl in front of him stumps him with this riddle:

ANNOYING GIRL:"Pete and repeat are on a boat, and Pete fell out. Who was left?"

MONK: "Repeat"

ANNOYING GIRL:"Pete and repeat are on a boat, and Pete fell out. Who was left?"

MONK: "Repeat"

ANNOYING GIRL:"Pete and repeat are on a boat, and Pete fell out. Who was left?"

MONK: "Repeat"

[GuitarStv]

The wheels don't propel the plane forward, the turbines/props do that. Therefore, even if there is a conveyor belt going in the opposite direction, it will only be putting some friction on the wheels and the airplane will still go forward. I guess the wheels might start to smoke or something, I don't know about that part, but the plain would probably still take off.

I still think it's a trick question.

It doesn't matter what propels the plane forward. Moving at 100 miles per hour by way of propeller has the same effect as moving at 100 miles per hour by wheel, which has the same effect as moving 100 miles per hour by jet turbine. Untill the plane has enough air moving over (and under) the wings, the plane can't achieve lift. If there's no wind, then the plane has to achieve a ground speed fast enough for take off. Since the plane isn't moving relative to the ground, this never happens.


As far as the friction from the wheel goes . . . I think that we can assume that the plane wheel operates like a standard wheel. There is some friction caused by the weight of the plane pushing down on the wheel. This is the friction in the vertical axis of the question that keeps the plane attached to the runway (in this case the runway is a conveyer belt). When the conveyer belt moves backwards in the horizontal axis it doesn't effect the friction in the vertical axis. The engines of the plane don't have any effect on this friction since the thrust is working in the horizontal axis only (assuming we have a standard plane and not a helicopter or VTOL capable plane). The friction on the bottom of the tires ensures that the plane will move backwards because of the conveyer belt and forwards because of the thrust from the engines - however, it doesn't change with application of additional thrust or conveyer belt speed (they are in the wrong axis).

|
V Fg = Acceleration due to gravity - keeps plane on ground

-> Fc = Acceleration of the conveyer belt (same as plane)

<- Fp = Acceleration of the plane

When you sum these forces, Fc and Fp cancel out. The only force left is the force of gravity (which keeps the plane grounded).

[GuitarStv]

What if the conveyor belt was going 25 mph faster than the plane's thrust was propelling it forward? Wouldn't the plane be moving backwards? The odd part to speculate on is whether a conveyor belt moving in the opposite direction of a plane going at the rate of speed a plane needs to take off is approaching 300mph. Is there even a conveyor belt that travels that speed? And if it could travel that speed wouldnt the weight of the plane cause the conveyor belt to malfunction? If there are conveyors with that kind of speed, then we should forget this riddle and start implementing 300mph moving sidewalks. Forget the bullet train and step onto the super highway!

Actually, the conveyer belt thing is not that far fetched . . . a small planes (Cessna, Twin Otter) takes of at close to 100 mph. I'm sure that you could design a treadmill to go that speed. The only difficulty would be syncing the acceleration of the plane to that of the conveyer belt. As far as weight is concerned, if we can move the plane (using a jet engine /turbo prop/ whatever) then we have already shown that we can design an engine capable of handling the weight. A conveyer belt is basically an engine and a belt. I don't think it would be too hard to develop some kind of chain meshed belt to take the weight of a plane.