Will the plane take off?

[Guest airsick]

I just got an interesting question emailed to me that might interest some of you.

 

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"

 

 

[Guest Redair]

Sounds like any ground speed the plane may have will be cancelled by the opposing motion of the belt, therefore it should remain in one place. This being so there will be no airspeed and therefore no lift produced and therefore no flight and therefore a very long and boring journey into nowhere!

 

I think.:confused:

 

I can feel another lie down coming on!

 

Redair.

 

 

[slartibartfast]

I'm not biting. Last time we discussed this on here I got it so wrong.

 

 

[Deskpilot]

This has been discussed previously, and the answer is, yes the plane will take off normally except that it's wheels will revolve twice as fast as normal. Think about it, I took some time to be persuaded.

 

 

[Guest Ken deVos]

For those interested in the arguments for and against, here is the thread:

 

http://www.recreationalflying.com/forum/showthread.php?t=1602

 

Enjoy .... :big_grin:

 

 

[Admin]

I think these types of questions/puzzles are fantastic - makes us all think

 

 

[Captain]

Hey Sickie

 

Need some more info.

 

1 Is the conveyor thingy wide enough to create a "ground effect" over the wingspan or is it just the width of the undercarriage?

 

2 Is the conveyor sufficiently wide and is its surface rough enough to induce a draft?

 

3 What is the maximum speed of the conveyor?

 

4 What colour is the conveyor?

 

Regards Geoff

 

 

[Guest airsick]

1. As wide as the runway so yes.

 

2. No draft.

 

3. Quicker than you.

 

4. Depends. Is it a grass strip or sealed? You choose.

 

 

[Guest Redair]

OK, to further dig myself into a hole, or prove that I am right.... if a hang glider pilot runs far enough and fast enough, (forget the hill) the airspeed increases enough to lift said craft into the air. So, by thinking sideways, we can assume that if the same hang glider pilot was to attempt his take-off run whislt on a tread mill, (with the similar properties to those of the original question) then he would surely be running on the spot, therefore, again, no airspeed, no lift, no flight!

 

Redair.

 

 

[Yenn]

Correct Redair. The propulsion method depends upon the ground, not the air. Maybe if he had a treadmill connected to a propellor it would fly.

 

 

[Ultralights]

the hang glider pilot will not move as his feat are providing his motion against the ground, and therefor airspeed, whereas the aircraft is using a propellor or jet engine pushing against the air to gain forward speed, so it will fly no matter how fast the conveyor is going.

 

 

[Flyer]

ooowwww...my head hurts again..:yuk:

 

A car dyno is a great way of explaining motion through the wheels. For evey action there is an equal and opposite reaction but as there is no action from the wheels, just a reaction from the propellor thrust the conveyor belt will make no difference. I agree with deskpilot.

 

The plane will fly...;)

 

regards

 

Phil

 

 

[Guest Redair]

That's what I said all along!

 

Redair.

 

 

[Captain]

Yes. I agree with Doug.

 

And my simpleton's explanation is that all the wings and the air will care about is that they have the correct speed to do their "lift" thing .... and the question confirms that they will be allowed to develop speed.

 

So those little wheels would be spinning a 2 X normal rpm but the wings and the lift don't give a rats about what is happening down at the wheels .... unless it has high drag.

 

But I was never real good at sighance.

 

 

[Ben Longden]

Sorry, my brain is in the fridge.... ;)

 

But...

 

If the forward motion of the aircraft is countered by the opposite motion of the conveyor belt runway, then the aircraft will remain stationary relative to the ground away from the conveyor belt.

 

So, this means that the forward motion of the aircraft is not moving through the parcel of air relative to it.

 

So, this would mean no lift, therefore no flight... I think... ;)

 

UNLESS

 

you are a Gerry Anderson THUNDERBIRDS fan, where he used this technique to film model aircraft takeoffs, flight and landing. (with a moving rear screen as well)

 

Ben

 

 

[flie43]

Yeh, but Ben the forward motion of the airctaft is not countered by the conveyer only the wheels are. So the prop will propell the A/C forward until flying speed

 

I think

 

 

[BigPete]

The wing is the thing! ;)

 

If it is not moving fast enough thru the air then there is not enough lift.

 

So if it is stationary (on the treadmill or whatever), in other words not moving forward thru the air, there is no lift and no flight. ;)

 

Otherwise I could lock on my brakes, rev the dickens out of my motor and take off with no ground roll. :big_grin:

 

regards

 

 

[eyecast]

Yes.

 

Air movement over the wing caused lift.

 

The air is the parcel not the ground.

 

The prop causes air movement over the wing giving lift.

 

The position of the prop/jet/s in relation to the flying surface/s vary the lift.

 

:;)3::;)3:eyecast:radioactive:

 

 

[markendee]

Fantastic question.

 

Makes one think - real hard.

 

Of course it will fly, the prop and wheels are not geared together.

 

Remember your rotary wing theory!

 

 

[Guest brentc]

YES YES YES IT WILL FLY!

 

Hopefully you have good tires and wheel bearings as they will be working overtime.

 

 

[Case]

It's quite simple. The aircraft is not anchored to the runway. The air that it is in is stationary (assuming nil wind). The propeller will pull the aircraft forward through the air. The aircraft will move forward as usual but the wheels will be turning twice as fast as normal because the runway is moving backwards.

 

The runway could be going backwards at light speed and assuming the wheels have frictionless bearing the plane will still fly.

 

 

[Howie]

I think I must of seen this question posted on every aero forum in the last couple of years.

 

The point is it is a riddle. It is worded in such a way that there are several possible interpretations. I witnessed a couple of mates get so worked up over this they came to blows. It was so funny and so sad at the same time.

 

Short answer is there is not enough information in the question. (But I think the plane will take off!!)

 

 

[Ben Longden]

With the propwash theory providing airflow over the wings.... correct me if I am wrong, but the prop is a bit smaller than the wingspan, and the air forced by the prop over the fuselage does not venture forth over much of the wing area..

 

What everyone knows about propwash is that as the throttle is advanced, more rudder is needed, as the parcel of air moved by the prop is forced back over the fuse and empenage in a spiral motion.

 

As the prop pulls the AC through the parcel of air, the forward motion of that AC in that very parcel of air would then see a movement of air over the wings, causing lift.

 

But if the AC remains stationary relative to that parcel of air, so there is no movement of air over the wings, there will be no lift, therefore no flight.

 

I think...;)

 

Ben

 

 

[Case]

If it were a car on the runway it would not move. If it were someone running (like on a treadmil) they would not move either. The reason is that the source of forward drive is in contact with the runway (feet, car wheels). The wheels of an aircraft are not the driving force so don't have to overcome the backward motion.

 

 

[Ross]

I am assuming zero air velocity relative to the ground at the location of the attempted takeoff.

 

The conditions at the start state that the conveyor moves at exactly the same speed as the aeroplane but in the opposite direction.

 

So if the planes speed is measured in relation to the surface of the conveyor belt the conveyor will match it and the ground speed of the plane cannot get to take off speed relative to the ground because it will not move relative to the ground and therefore have no air speed.

 

So it is an artificial constraint that the conveyor speed match the speed of the plane relative to the conveyor.

 

If the plane speed is measured relative to the ground then the conveyor speed will match it and the wheels will be rotating fast enough to cope with double whatever the plane's air (ground) speed is and the plane could take off satisfying the condition that the plane speed and the conveyor speed are equal but in opposite directions.