Airplane Riddle

[garublador]

All that said, I still think you’re best bet is a harrier.

No belt movement at all. I like it!

[nepalnt21]

plane's can't fly. they are too heavy.

[garublador]

Not an intuitive problem.

I have one that's harder, but possibly more well known, too.

[JacksonMIA]

What if it was a pig? :6:

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

Good job, Frenchie.

[Markk]

Whats the answer???

OMG, 5 pages and still people dont' get it?

Of course it'll take off. Jet's or propellers do the work, not the wheels. The conveyor belt affects the wheels, and further to that, it doesn't "push" the plane back, it just spins the wheels if the propellers or jets are doing their thing.


Think of it like a bird. If a bird was on this conveyor belt and started flapping it's wings it would move forward because it could just lift it's legs. The legs here are like the wheels on the plane.

[ratherdashing]

The friction from the wheels is only the friction between the wheels and the plane. The only other force pushing backwards is wind resistance.

Another analogy that might help some people is imagine there's a roller skate on a moving treadmill. If you put your hand behind the skate you will be able to easily hold it there. With just a little force you can push the skate forward on the belt. Now instead of a roller skate, imagine a plane and instead of your hand pusing forward, imagine the engines or propellor providing the force.

Put a roller skate on a moving treadmill without propping it up, and it will roll off the back. Your hand on the skate to keep it stationary is an application of force: you are essentially applying the same amount of force as the treadmill, but in the opposite direction. The reason you don't have to use much force is because the skate is on wheels and thus has very low friction. Note: LOW friction does NOT equal ZERO friction: that's why you have to put your hand there in the first place.

The original question said that the belt matched the plane's speed, which to me suggests that the plane's engines are not able to generate enough thrust to overcome the friction.

Unless the surface of the belt has less friction than the surface of the runway, the plane will not take off.

[ratherdashing]

OMG, 5 pages and still people dont' get it?

Of course it'll take off. Jet's or propellers do the work, not the wheels. The conveyor belt affects the wheels, and further to that, it doesn't "push" the plane back, it just spins the wheels if the propellers or jets are doing their thing.


Think of it like a bird. If a bird was on this conveyor belt and started flapping it's wings it would move forward because it could just lift it's legs. The legs here are like the wheels on the plane.

Birds and planes do not generate lift in the same way. Planes accelerate along the ground - birds accelerate vertically, like a helicopter or a Harrier jet.

[B2D]

Was it an African plane or a European plane?

[Robert Delahunt]

Why are we still discussing this? It's a question that is meant to trip people up in their answer. I doubt anyone could set up a large enough conveyor belt to do that job, much less also handle the weight of the aircraft, whatever it may be. As for the lift comments, that don't matter. Lift is generated by an airfoil going through the air: air speed across the wing gives lift. Thrust is generated by an engine of some sort, and in the case of the conveyor belt, I doubt it would matter.

As for landing, most if not all standard aircraft land at barely over their stall speed, so that they can gently glide down to contact with the runway. For fighters, this speed can end up being faster, but the principle is the same, for a standard landing. At first touchdown, the wheels go from zero to whatever the aircraft's speed is. This doesn't wear them out (except the tires), in terms of their bearings, etc.

Lift is also not always generated the same way. Most airliners use "standard" or "Bernouli" lift, in that the top of the wing/airfoil is more curved than the bottom, hence lift in the way we were taught in elementary school. Fighters and other aircraft generate lift this way, but also as a principle of the air deflecting off their underside. Fighters essentially can fly like an aicraft, or like a missile, because they are weapons platforms with huge engines. The thrust to weight ratio of an F-16, for example, is greater than 1:1, meaning they have more thrust than weight, so they can fly somewhat like a missile. There are aerobatic teams that do all kinds of nifty non-standard-aircraft things, and we call them stunts.

Right now, I work on the CV-22. Before this, the F-117 and then the F-16. The F-117, due to shape, generates most, if not all, its lift by deflection.

Anyways, to make a long story short, the aircraft's wheels are free-spinning. Until you move the conveyor belt fast enough to make friction along the wheel great enough to hurt forward motion, it don't matter. With as little friction as those wheels have, I would think you'd have to move the conveyor belt a LOT faster than the aircraft's takeoff speed to keep it from taking off.

The conveyor belt versus aircraft thing is only a mythological trick question. Seriously, this thread has gone on long enough. I doubt the original poster wanted it to turn into this deeply scientific / engineering question. He just wanted to have fun is what I'm thinking.

[ratherdashing]

Now that I think about this a bit more, I think the plane will take off. The force of the engines is on the air around them, not on the ground. The belt provides resistance on the wheels, but probably not enough to cause the plane to get stuck.

That said, the question wasn't worded very well.

http://www.straightdope.com/columns/060203.html

[Markk]

Birds and planes do not generate lift in the same way. Planes accelerate along the ground - birds accelerate vertically, like a helicopter or a Harrier jet.

Think of it like a bird. That's the key word, like. Why didn't you attack on me on comparing legs to wheels, that's also obviously wrong. Birds generate thrust/lift at angle in order to fly, they can't hover (except for a few). :P


I just had finished writing this awesome reply when I thought, maybe I should just check the link....and after that I deleted my awesome reply.

[garublador]

Now that I think about this a bit more, I think the plane will take off. The force of the engines is on the air around them, not on the ground. The belt provides resistance on the wheels, but probably not enough to cause the plane to get stuck.

That said, the question wasn't worded very well.

http://www.straightdope.com/columns/060203.html

The question was worded fine. The fact that the conveyor maches the planes forward speed is supposed to trip you up. If it were a car, then the wording wouldn't make as much sense. You'd want the belt to match the speed of the wheels in that case.

This is a small point, but the belt doesn't actually impart any force on the plane. The friction between the wheels and the plane is the only frictional force pushing the plane backwards. The belt makes the wheels move faster, which probably causes there to be more friction, but the extra friction is not from the belt itself.

I actually knew this would end up like this. One of the funny things about this riddle is how hard it is to convince some (sometimes really smart) people that the plane will take off.

[Zerberus]

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.

How about running on a treadmill with a jet engine strapped to your back?

The engine will propel you instead of your legs, which will drag for half a second on the treadmill before you leave the treadmill headed for the kitchen table and your dumbfounded wife

[tone4days]

i am trying to think about what the ride would be like for a passenger on that plane ... i suppose that it would be about like any other takeoff ... but i keep having images like an old time bugs bunny cartoon .. where the plane is revving and revving but seeming to remain 'still' over the same point on the earth .. then at the moment it achieves sufficient lift, it slingshots forward as it 'catches up' to its airspeed

[Markk]

How about running on a treadmill with a jet engine strapped to your back?

The engine will propel you instead of your legs, which will drag for half a second on the treadmill before you leave the treadmill headed for the kitchen table and your dumbfounded wife

It's answers like these that put a smile on my face

[The Golden Boy]

What's moving the air over/under the wings? If it's a propeller in front of the plane, can the propeller force enough air over/under the wings to generate enough lift to free itself from the ground? If nothing is pushing or pulling air over/under the wings, the vehicle cannot take off.

It's not how much air the engine is capable of moving, it's about how much air the engine can get over/under the wings to create lift. Otherwise the conveyor belt can go fast enough to counter the forward thrust of the engine that is not going over/under the wings.

[ratherdashing]

What's moving the air over/under the wings? If it's a propeller in front of the plane, can the propeller force enough air over/under the wings to generate enough lift to free itself from the ground? If nothing is pushing or pulling air over/under the wings, the vehicle cannot take off.

It's not how much air the engine is capable of moving, it's about how much air the engine can get over/under the wings to create lift. Otherwise the conveyor belt can go fast enough to counter the forward thrust of the engine that is not going over/under the wings.

This is what I thought too, but it's kind of missing the point. A car accelerates by applying force to the road, which would mean the conveyor would hold it in place. A plane, however, moves by applying force to the air around it. The wheels are really only there to reduce friction caused by the runway. The belt may make it a bit harder to take off (due to a slight increase in friction), but it doesn't affect the plane's ability to thrust against the air.

[tone4days]

What's moving the air over/under the wings? If it's a propeller in front of the plane, can the propeller force enough air over/under the wings to generate enough lift to free itself from the ground? If nothing is pushing or pulling air over/under the wings, the vehicle cannot take off.

It's not how much air the engine is capable of moving, it's about how much air the engine can get over/under the wings to create lift. Otherwise the conveyor belt can go fast enough to counter the forward thrust of the engine that is not going over/under the wings.

nah, man .... that's not it at all ... the straight dope article linked above is a good read to clarify it .. no matter how you slice it, the conveyor belt can only increase (in fact, double) the friction of the wheel ... but the wheel isnt part of the propulsion system .. in this example, the only thing involved in getting the plane to move forward fast enough to create sufficient lift for takeoff is the engine .. the engine is unaffected by the conveyer belt

t4d

[garublador]

the conveyor belt can only increase (in fact, double) the friction of the wheel ...

I haven't looked at the specs for airplane wheels, but are you sure that's correct? Doubling the fricion seems a bit far fetched to me. Is the friction between the wheels and the plane a linear function of speed?

[Robert Delahunt]

I haven't looked at the specs for airplane wheels, but are you sure that's correct? Doubling the fricion seems a bit far fetched to me. Is the friction between the wheels and the plane a linear function of speed?

I would think that it becomes logarhythmic when the wheels reach their maximum design speed. I'd have to research typical max wheel speeds, but I'm thinking since they're free-spinning, it's not an issue. Landing gear issues I've seen never arise from the speed at which the wheels are turning but instead from how hard a landing it was. According to this it seems they're designed for 160 upward to 230 (mph?), but one lists as 217K (217,000 mph?!). However, I'm not sure if it's load-carrying speed and/or if in this scenario (conveyor belt) if merely because they're spinning 2x faster if they would shred out. I mean, if your tires are meant for 140mph, and you used a car in the scenario at 280mph, would merely them spinning at that speed cause shell-out or would it require speed plus road vibration encountered plus vehicle weight? I.e. if they're on the tire balancer and you spin them 280mph, do they shell out simply from moving that fast? Around the time you get to the aircraft's stall speed (i.e. above this speed, wings generate lift), you should be losing weight on the tires due to lift on the wings. Since airliners tend to have larger, more "lifting" wings, they don't require as much speed as, say, an F-16, to take off, hence less speed required to get lift and take off. If it's 160mph for an F-16 (according to unreliable sources), probably my guess is 120-ish for airliners (due to simply seeing them take off next door when I was at Kirtland).

Beyond this, I've never heard of tires shelling out just because the aircraft landed at 280ish mph, assuming the plane touched down very gently. I've also seen tires blow at normal landing speed because the pilot landed way too hard. Landing is much harder on tires, so I'm thinking this "can tires handle speed" isn't really that much an issue. The only issues I've seen from faster-than-normal landing speeds is due to brake overheat, which can actually cause tires to explode (we're talking titanium rims red hot). However, just having the brakes at red hot can also not cause tires to explode if noticed quickly enough and the aircraft stopped and the brakes and rims allowed to cool down. The only reason it blows tires is because it super-heats the tires (300psi if i remember correctly) and this causes the air inside (nitrogen) to expand past the tire's ability to keep the air in.

Still, this conversation is really going past "beat a dead horse" into corpse mutilation. I'm surprised the metaphorical horse even exists any more.