Imagine a plane is sitting on a treadmill

Have you realized the majority of people posting in this thread are males?

Note- Ive screwed the quote up, but my bit is in bold.

@ the non-believers:

Imagine a short take-off plane. These take off at an airspeed of 25mph. So to take off the wheels would be rotating at 50 mph. Still think there's be enough resistance to stop the take-off?

These planes have 100bhp engines and weigh <300kg. I could probably push one at 10 or 15 mph from a standstill. I don't think I could stop one taking off if it was at full power though. Assuming the engine and prop run at 10% efficiency, that's 10 bhp vs. me. No contest - I'd get dragged down the runway until I let go and the plane would take off.

There's a thread about whether or not a kitten can take off under full power if he's stood on a treadmill in the ladies' section.

@ wheel

I base my opinion on the question. I haven't considered, nor care to, the possibility of a treadmill moving that fast. I agree it would be going very very fast. But based on the question's premise if a tread mill can do it, then the plane wont take off.

It's obvious that the power from a jet engine can overcome the friction in the wheels. A plane wouldn't be able to take of *normally* if it couldn't.

And even if you assume the conveyor moves fast enough to keep the plane dead still, it would need to accelerate at about 40G to do this. After like 5 seconds the conveyor would be spinning at like 5000 mph and there would be enough wind generated by the moving conveyor to lift the plane.


So how exactly is a conveyor going 4000 MPH "matching the speed of the plane"? It doesn't make sense, planes don't even go that fast.

Is this endless thread from hell??? Best yet I mentioned it to some colleagues here and it is now the topic of conversation in the office. Arrrrrgh no escape!!!

The question says the conveyor moves at the same speed as the plane - not 50 times quicker. Yes, theoretically a conveyour spinning at Mach 10 would stop the plane taking off - only by virtue of the fact that it would break the plane into tiny pieces, moments before it broke itself into tiny pieces.

No - if the plane moves at 25mph forwards, the treadmill moves backwards at 25mph. The wheels spin at 50 mph. Not very fast really. Not a great deal of resistance.

Oh and I just realised the conveyor could NEVER spin the wheels up that fast, because there isn't enough friction between the conveyor and tyres. So assuming the plane has normal tyres, it is IMPOSSIBLE to keep the plane still, even if the conveyor can spin at MACH 1 million instantly.

You know, the original post never specifically says that this plane has wheels. So lets imagine it doesn't- instead the plane has waterplane style skis, which have the best lubricant on earth applied to them- will the plane take off then? yes.

Ever watched professional bowling? Ever notice how the ball can be spinning backwards and yet moving forwards down the lane? Same bit of "magical" and "super complicated" physics involved in that as well.

The trick is this: if the plane is not moving, how fast is the treadmill turning? Answer: 0mph.

Therefore if the treadmill is turning the plane must be moving.

It's all in the wording.

ok folks, basic laws of physics...airplanes fly beacuase of lift...lift is caused by the the air traveling faster over the top of the wings than the bottom, in simplistic terms you get lower air pressure on the underside than the top, so the plane moves upwards...ie flies, the only factor involved is the wind speed on the wings, this is how a helicopter flies, the aircraft stays still and the "wings" move and the helicopter takes off, if there is enough air speed it all works

Jesus, I'm just a stay at home mom, and even I get this!

"A headwind will reduce the ground speed needed for takeoff," a quote from this site

http://en.wikipedia.org/wiki/Take_off

so before take off an aircraft has to reach a specified groundspeed (speed relative to the ground) this speed can be reduced if by a headwind becuse the headwind increases the amount of airflow over the wings.

If an Aircraft cannot move relative to the ground (and there is no headwind) it cannot take off.

The skateboard and pushing against a wall analogy doesn't work because jat engines don't push against air. They operate on the same principles of a rocket engine(only a rocket has to take it's air source into space with it)...Thrust and Newtons principle of action/reaction.

It is going the same speed as the wheels touching it. Im not arguing the possibility of the mechanical structures, simply assuming the premises in the question occur, then the plane is motionless.

Your a few hundred posts behind us mate ;-)

soory, iws busy, had to go the airport

It's a question of mental flexibility I think. You've got to be able to dump your instinctive answer, which for most people is to equate a plane and a car. Some people don't seem to be able to do this.

Umm, yeah. But noone is saying that the plane won't reach it's specified ground speed- it just doesn't need the wheels to achieve it. It's moving because the turbine engines are blowing at such a force that it pushes the plane forward. Don't believe it? Try going to the gym and wear your roller skates on the treadmill (at 0% incline) turn on the treadmill. What happens? You stay in one spot. What happens when you have a turbine engine attached to either arm? You move. Simple.

and if you can attach a turbine engine to your back, then the tread mill wil increase in speed as you do. And since you have a engine on your back it is possible to have a tread mill move that fast.

Exactly, that was my initial knee-jerk response as well. Thankfully, I've moved from the dark side of the force. No pun intended!

Ah right, so the question should say "the speed of the planes wheels", not "the speed of the plane"?

So, what you are basically saying is, if you assume the plane has really sticky tyres (so it can grip the conveyor accelerating at 40G), and the conveyor is perfectly frictionless with the air above it, the plane won't take off.

I agree with you on that.

@ kitty-kat
Excuse me - since I'm not a stay at home Mom but u can carry a turbine engine in your arms ? Everytime I try this when I'm wearing some roller skates I fall down so I could use some tips. kthx.

It will? Why is that? My wheels are just rolling. Are you then suggesting that (without hypothetical turbines on arms) if I turn the incline up to 12% that I won't still roll backwards?

The answer is right there in the question people: if the plane is not moving, how fast is the treadmill turning? Answer (again) 0 mph.

The treadmill can only turn if the plane is moving.

If the plane is moving it has airspeed, and when it gets enough it will take off.

when is this plane taking off by the way. if i was sat on it i'd be pretty knarked by now.

I've gotten mighty strong from carrying these little monsters around!

At what stage does the force from the change in angular momentum induced by the ever accelerating wheels on the ever accelerating conveyor belt force the plane to veer off to the left and crash and burn?

Mine took off at 15:30 yesterday, along with a few other people shortly afterwards. Some people followed later in the evening, others are still stranded on the conveyor belt.

Exactly, there are so many things that don't make sense if you assume they meant the wheel speed, that the only sensible understanding is that they meant the *air* speed of the plane is matched by the conveyor. A plane with magically sticky tyres, a conveyor accelerating at 40G, a pilot able to steer a plan on this conveyor, and conveyor that doesn't generate any wind, it's just, well, wrong.

Excellent, thats one for the no fly guys!! Wait, sgb, that was sarcasm huh?? shit!

Yes I am making the basic assumption that the plane is not moving, because the question says that the plane and conveyer move at the same speed. So I am assuming that the conveyer is going balls fast to keep up with the thrust off the jets. Also though let us remember that we are assuming that there is a runway size conveyer belt that can withstand the weight of a jet plane, in that light,not a stretch.

Nominated for Best Post of the Thread Award!

No I wasn't being sarcastic. If the belt can accelerate at ~40G and the plane tyres can take it and no wind is generated by the belt, then the plane won't move or take off. They are very big assumptions though that are not physically possible, which is why I think they must mean the conveyor matches "air speed" and not "wheel speed".

Edit: And don't tell me to "imagine" that these things are possible because they are not physcially possible, and if you are going to start imagining then I am going to start imagining Angel Gabriel coming down with an f-off big winch to lift the plane up and make it fly

Exactly. If so if the plane doesn't move, the treadmill must be moving at the same speed, which is 0 mph. Capisce?

I completely agree that if the question is a plane is going downa runway at x mph and it has a conveyer belt under it going x in the opposite direction, can the plane take off. Answer, sure if the runway is long enough. Not much of a challenge. I read the question to ask, if a plane is perfectly motionless, but has its engines pumping at full blast, can it take off based solely on the engines. Answer no, unless its a prop that has generated enough wind to give it sufficient lift.

@sgb, again we are imaging a runway size conveyer that holds a plane. We aren't really working the realm of reality are we??

Not the same thing though, is it. According to the question, if the plane is not moving, neither is the treadmill, since they must move at the same speed.

Not at all , it depends on the reference frame. Jeez.

Therefore: plane moves at 10 mph, treadmill moves at 10 mph. Plane moves at 20 mph, treadmill moves at 20 mph.

It is impossible, according to the wording of the q., for the plane to be at a speed of 0 mph and the treadmill to be at a speed of 50 mph.

Yes we are, buy a model plane and take it to a car wind tunnel that has a belt rolling road. There's no reason why you couldn't build one the size of a runway, just a bit pointless.

Maybe it's a tiny plane similar to this helicopter I bought for Xmas...

http://www.highlights.com/jump.jsp?itemID=...814&sccat=E0902

@ wheel
Of course it's not - the plane could be at a speed of 0 mph relative to the air and the treadmill could be at a speed of 50 bazillion mph relative to the same.

You are right. But you've got the wrong frame of reference. A plane's engines do not cause it to move by acting on the ground, but by acting on the air. Provided the plane is free to move, because it's got wheels, or skis, or floats, or some other low-resistance supports, it will move forward, regardless of which way the wheels are turning.

@ Wheel - I DONT/COULDNT HAVE THE WRONG FRAME OF REFERENCE COS I DIDNT SPECIFY ONE IN MY POST!

I will bet the resistance in a plane of similiar quality to that helicopter will be so great that the conveyer throws it right off the back of the conveyer.

Like this plane you mean? I'm getting this one for xmas

For this to be correct, the treadmill would need to be moving at 50 bazillion mph relative to the air. It's not, it's at rest on the earth and relative to the air, but it is rotating.

@ Wheel
Hello ? You seem not to be familiar with the idea of a conveyor belt - the rotating surface could be travelling at whatever speed (relative to the air) whilst the supports could be at rest.

Dude, that's a sweet little plane. I've been flying RC for >20 years, including big and expensive stuff, but I still have a blast with the little cheapo foamies. I'm picking up one of these guys today. They're only 26 Euros!

Oooooh where from? In Munich?

Oh, I've done loads with cars, but never planes/helicopters. I would be too scared of landing the first time and smashing it up, hence why I'm getting a cheapo small one.

One question, the little helicopters and planes all say 2 channel, surely that isn't enough to control it? I mean with a helicopter it goes up and down, forward and backwards, and surely needs to go left and right as well??? confused...