Airplane physics question

Am I reading this correctly? You're assume having the wheels locked (and thus a non-moving treadmill) will be equivlent frictional-wise to having a moving treadmill and unlocked wheels? If so, that's a wrong assumption because the coefficient of static friction (i.e. friction of a moving wheel) is different that the coefficient of kinetic friction (i.e. dragging a locked wheel). The coefficient of kinetic fricition is less than the coefficient of static friction and it would be easier to take off if the wheels were locked and a non-moving runway.

Edit: An example of this is in an F1 race. You don't want the brakes to lock up because the car slides (kinetic friction). You want anti-locking brakes so you always have the most fricition possible (static friction).

 

i dunno

i give up banging head on wall, it doesn't matter one iota where the propulsion is coming from, if the ground's being taken away at the same rate that the object would move forward, then it remains stationary.

simple to me!!!

 

No. The coefficient of friction depends on the relative motion of the tire to the runway or belt. When the wheel is rolling the coefficient of friction is the static coefficient because the surface of the tire is stationary relative to the runway/belt. Only when the tire starts to slide relative to the belt/runway does the coefficient of kinetic friction apply.

All I'm saying is that at some point (if the plane has enough power) the belt cannot generate enough friction to counteract the force of the airplane engine, no mate rhow fast it goes "backwards." At that point the tire will slide on the belt.

Hope this helps. I am VERY confident of my analysis, and if I haven't explained it well enough I am only too happy to try again

 

So we know know that having locked wheels would make it easier for the plane to move forward. Now it's a question its ability based on wing design and power. Both of these factors were not given by the poster so technically we cannot answer this question. I would say that something like a fighter jet would easily be able to do it, but not sure of anything else (I would not be surprised if a 747 did it, though)

 

That's all you had to say. All I needed to know is if you were assuming something.

I've already said this in the quote you quoted of me.

 

close but no cigar there buddy. Your talking about the maximum traction (friction) which is not the case here. Lock brakes on your car and try to push it... not gonna go anywhere is it, take the brakes off and one guy can push just about any average car. Free rolling friction is FAR less than kinetic, you guys are taking this all way too far. There is effectively very little load on the wheels, it's not turning, accelerating, or stoping by changing the force at the wheels. It's just rolling friction, same as rolling a tire down the street.

To say that a car would effectively go faster with the wheels locked than with them rolling is absolutely obsurd.

 

I was comparing a kinetic friction on non-moving road to static friction on moving road that matches the speed (it looks stationary, just like a car sitting still with the engine off). I know that it's eaiser to move a car with wheels turning instead of dragging FROM A STAND STILL.

 

An F1 car in a wind tunnel. The car rolls on the "treadmill", which matches it's speed and the car does not move forward or backward (with some assist of the car being anchored). Downforce, the opposite of lift, is not applied or does not occur until the wind is turned on.

The plane will not take off because no lift is generated (still air). If it could, the wind would not be neccessary in a wind tunnel, just the treadmill. Lift and downforce are created by moving through the air, not by moving against an opposite moving surface.

A plane in a wind tunnel would not "take off" until the wind was turned on.

 

LMAO, that is the WORST analogy ever. yes planes need air speed to create lift, thats a given. But being that they are not propelled BY THE WHEELS ON THE ROAD, wheel speed is 100000% irrevelant. The prop/turbine is what powers the plane, throttle that ***** up and it doesn't care how fast the wheels are moving at all. ALL it would have to overcome is the slight rolling friction of the tires.

If the track is moving in the opposite direction of travel at the same speed it makes no difference what so ever, the plane will still travel forward based on the thrust of the engines which have nothing to do with the wheels.

 

Before we all get this wrapped around our ankles, let's all take a deep breath.

It's a fun, interesting, and tricky little question. And as I said, I enjoyed figuring it out. But when you set it up, it really is freshman-year engineering stuff. Maybe some of you will now appreciate what engineers do, because this the sort of thing that convinces folks they should go into something easier. This is STATICS! not just first year, but first SEMESTER engineering.

Sorry to be arrogant about it, and, again, if I didn't explain it well I am happy to try again. I don't know contract law and I am a lousy manager, but this stuff I am good at: trust me, the answer is: if the plane can drag its locked wheels down the runway it will take off.

 

So even if the wheel speed is 100mph, hit the clutch and let the car coast to a stop... have another equal vehicle at the same speed hit the brakes hard enought to lock up all 4 wheels. which car is going to travel farther?

For the one millionth time, the wheels are totally irrevelant here. All they do is physically support the plane off the tarmac, have NOTHING to do with propelling it down the runway. It's not turning, stoping, etc. The only acceleration force is the slight bit to turn the wheels. To say an airplane capable of flight could not take off because the road was moving at an equal speed in the opposite direction is just silly.

 

I agree, stuff like this is very elementary. But my point is that it does not even need the wheels to be locked. The rolling friction is FAR less than draging them, and yes it will easily achieve flight.

 

The wheels can roll all they want, but the constraint of the problem says that any wheel motion is counteracted by the movement of the conveyor.

 

I see what you guys meant in your last posts. I assumed 2 fricitional forces only and didn't realized there are 4 total (2 for each scenario, rolling friction for the moving one). So yes, rolling friction is less than the kinetic one from draggig locked wheels and it will be easier to take flight while moving on the treadmill. I'm done with this thread, lol, I put way too much though into this to get a wrong answer (use dragging) and also a right answer (it will take off).

Edit: I still think a fighter jet could take off with locked wheels. They're too bad@ss not, too, lol.

 

I meant to say wheel rotation. Sorry.

 

This is true, but the AIR PLANE is not powered by drive wheels now is it? All you would be doing is slightly raising the very miniscule amount of rolling friction. Quit thinking of it like a car that is driven by wheels, thrust is completely independant of wheel movement all together.

sheesh, it's not rocket science people.... hehe well in one way it kinda could be, but thats another discussion alltogether.

 

Any forward movement to achieve a take off speed is negated by the treadmill, so it can not get through that transition from non-moving to take off speed.

If I am in a float plane (we remove wheels from the question) on a river that is flowing opposite of me, I need to over come the river's speed before I can achieve forward movement on my way to lift off speed.

If there is a river that is flowing 100 mph, and my lift off speed is 100 mph at 100% throttle, I am applying enough throttle to lift off, but I am not moving, just couteracting the river and burning gas.

 

OK, I've done all I can here. I'll check in to see if anyone has any questions specific to my analysis, but ONE MORE TIME!

the answer is: it depends. Does the plane have enough power to overcome the maximum possible friction that the belt can generate? IF not, then the plane can't fly. if so, then it will. One more time with the picture thing:
If the engine is powerful enough, the plane will take off, dragging the wheels on the belt no matter how fast it goes backwards. The friction generated by the belt has an upper limit, the engine power does not.
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Forward MOVEMENT is NOT negated by the treadmill. Revolution of the WHEELS is negated by the treadmill. Don't confuse the two because a plane does not require wheels for forward movement (some use skis for snow planes other pontoons for water). Do you think a fighter jet can take off with locked wheels and therefore a non-moving treadmill?

 

Nope your completely wrong. You would need to overcome the friction of the water ONLY. You are NOT being propelled BY THE WATER. IT's not a flipping boat with a prop in the water is it? NO, it would take a bit more power to achieve the required air speed to overcome the FRICTION of the water but thats it.

Boy am I glad most of you guys are not engineers, society as a whole would be doomed.

 

I am in 100% agreement with Dubai in his analysis of the situation. The "how is the plane powered" argument only comes into effect if you're saying that the thrust can overcome the friction between the wheels and the conveyor, and the plane can drag itself to takeoff speed.

The thrust is 100% independent of the wheel movement, true. But the forward motion of the aircraft is 100% dependant on the relationship between the rotation and sliding of the wheels against the conveyor.

And its not rocket science. Its statics, and a hint of dynamics if the plane actually starts moving forward.

 

WHO SAYS the wheels have to be DRAGING? They roll just fine, and the friction of a rolling wheel is VERY VERY low. Most any plane would easily overcome this rolling friction. Wheels don't have to slide, skid, or drag, they are built to ROLL. To say that the rolling friction would be so high that a plane could not achieve takeoff speed is downright silly.

 

Thank you. And Merry Christmas everyone!

 

The wheels aren't necessarily dragging. They're actually spinning quite fast. But, as fast as they're spinning, the conveyor is moving backwards at the same "speed". If the aircraft is moving forward whatsoever, it is because the thrust is overcoming the static friction between the wheels and the conveyor.

 

This is true, but you guys are making it out like this friction is some horendous force. If a plane has the power to propell it's self to takeoff speed, and then to cruising speed of a few hundread mph, the rolling wheels are going to have very little effect on it's takeoff performance what so ever.

At any rate I think we've established that it would most definitely achieve flight with very little trouble.