Will the plane take off?

[motzartmerv]

Ive been watching this brainteaser for a while now without biting...but i can't help myself..

 

Th only question really is will the aircraft move?? If the ground underneath it moves in dirct opposite to the foward motion (potential) of the plane then the answer is no.. The propwash is no where near enough to produce enough lift to overcome the weight of the aircraft, and anyone that thinks it is needs a lesson in basic physics, if it was then why would aircraft need runways at all, just add full power and lift of the ground???

 

Its not even a brainteaser, its a no brainer.. if the wing doesn't get propelled through the air then the science doesn't start to work...she just sits there and revs her little engine ...

 

The real brainteaser for me is how 'pilots' can think a wing will work without moving??

 

 

[hihosland]

The only question really is will the aircraft move?? .....The propwash is no where near enough to produce enough lift to overcome the weight of the aircraft, and anyone that thinks it is needs a lesson in basic physics, if it was then why would aircraft need runways at all, just add full power and lift of the ground???

Its not even a brainteaser, its a no brainer..

True the prop wash won't produce enough lift. But it will accelerate the aircraft, moving runway or no moving runway.

 

If the aircraft was on on ice skates on wet ice it would take off just as it would on the moving runway.

 

Dat's my view

 

Davidh

 

 

[Guest brentc]

Nice one Biggles, but unfortunately totally irrelevant!

 

The aircraft is not a car and it's wheels aren't driven by anything, they are free-wheeling and the forward thrust has nothing to do with the conveyor at all.

 

If you were going to try to use physics to explain this, try explaining the friction on the wheels and how an increase in speed doesn't increase the friction. That might help.

 

Either way, with or without physics, the plane WILL take off.

 

I'm at the stage of offering money now if I'm wrong. I'm thinking $100 if someone can prove otherwise.

 

 

[Guest airsick]

I agree with Brent but I really wish I could prove him worng, I could do with $100. :)

 

 

[Captain]

I agree with Brent but I really wish I could prove him worng, I could do with $100. :)

I have a plan. Just start a 2nd Poll that asks the question "Is Brent wrong?".

 

We'll all vote "Yes" and the $100 is yours ...................... to split with us all.

 

BC won't mind as he has heaps to spare.

 

Hope this helps

 

Geoff

 

 

[slartibartfast]

The real brainteaser for me is how 'pilots' can think a wing will work without moving??

If you re-read the thread I think you'll find that not a single person suggested that prop-wash would be enough to create the required lift. Taking off normally would do that.

 

While you're there, watch the video of the actual experiment. The plane easily overcame the conveyor speed (with virtually the same amount of throttle no matter what the conveyor speed was) and when given full throttle had plenty of speed to take off.

 

Empiricism is much more valuable than theory, supposition, speculation and hypothesis combined.

 

<moderator mode>

 

Replies like this are dismissive of other people's opinions and therefore show a lack of respect. Please don't.

 

</moderator mode>

 

 

[motzartmerv]

Ok..the original question says "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).Nowhere does it say it cancells only the speed of the wheels, but the speed of the plane.. ....Therefore the plane goes nowhere...if this is wrong, someone tell me how, if the runway cancells the forward speed EXACTLY, then the plane just sits there does it not??

 

Oh, and starti, mabye ur reading a different thread, here's a quote from someone on the first page.

 

"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"

 

 

[slartibartfast]

No MM, because the conveyor has nothing to cancel the plane's movement with. It can go backwards all it wants, but the wheels just turn. Because the thrust is coming from the prop via the air it acts on, and not via the wheels, the conveyor has no affect on the plane's ability to move forward.

 

Watch the video (not the MythBusters one - I didn't even bother to watch that) - it really does show exactly what would happen.

 

BTW - the experiment in the video doesn't link the conveyor to the plane's speed, but it doesn't need to. If the conveyor is linked like that, the maximum speed the conveyor will move is the maximum speed the plane will go. They ran the conveyor backwards much faster than the plane needed to go to take off, so they exceeded the requirement IMO.

 

Cheers,

 

Ross

 

 

[Guest Baphomet]

brentc and Starti, the plane will take off, but only if you ignore the condition set by the question, which implies the conveyor can add friction to the equation (which can't be achieved). You must have missed my post about this. go to the link and read about coefficients of friction, etc.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#plo

 

The movie with the model plane is largly irrelevent, it simply demonstrates that the condition set by question, can't be achieved in real life.

 

I'm concerned that people think that because thrust is provided by a propellor, it somehow makes a difference. IT DOESN'T MATTER if thrust is applied by a prop, a jet, driven wheels or a winch down the end of the runway the thrust (motive force) in each case is transferred to the runway surface via tyre rubber, and the force resisting movement in each case is friction, the amount of which is detirmined by the coefficient of friction which is dependent upon the type of surface, and the mass of the aircfaft. The only time its relevent is when the aircraft lifts off, at which point driven wheels won't keep you flying. If this is difficult to grasp you probably need to revisit basic physics. Do some searches on Momentum (a body at rest, etc) Conservation of Energy, Work, Force and Friction

 

Cheers

 

Ian

 

 

[slartibartfast]

The condition imposed by the question i.e This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction) cannot be achieved, therefore it will fly.

Ian, I presume you mean this condition?

 

I don't understand why this condition cannot be met. This doesn't imply the conveyor can add friction to the system, it simply states that the conveyor will move backwards at the same speed it detects the plane moving forward.

 

So, if the plane moves forwards at 10 knots, the conveyor moves backwards at 10 knots. As you say, the wheels are already turning, so the initial friction (the object at rest) has been overcome. When the plane continues to accelerate and reaches 20 knots, the conveyor is now doing 20 knots backwards. This has no real affect on the plane, the wheels are simply doing 40 knots.

 

Is this not so?

 

Unless you assume the conveyor is matching the wheel speed and not the plane speed. But this does not make sense and is not what the question states. It explicitly states that the conveyor will match the plane speed.

 

Is the argument coming down to whether the wheel speed or the plane speed is being matched?

 

BTW - the motive force is not transferred to the runway surface via tyre rubber. The undercarriage is simply there to reduce friction. Scraping the fuselage along the runway would not be a healthy way to go, but even then a plane could take off with enough thrust. At least once.

 

Ross

 

 

[motzartmerv]

When i was at uni we would often be posed curley questions like this and we were taught to STICK TO THE QUESTION CRITERIA, no matter how bizzare it may seem, the object of the exercise was to not allow conventional thinking to overwrite the conditions set down in the question..

 

The Video proves nothing becasue the conveyor wasn't tuned to match the speed of the plane...Look dudes, its simple, i'll use your numbers,

 

foward velocity =10 kts

 

opposite velocity =10kts

 

overall velocity (in relation the still air) =0

 

When i run on a treadmill i may be running at 20 km/h, but the treadmill is cancellin my forward speed exactly so i go nowhere...

 

The original question said that the treadmill is tuned to match the speed....THE SPEED..i'll say it again...THE SPEEEEEED;) Thats the key word, or the significant factor...

 

cheers...

 

 

[motzartmerv]

PS..I have a way of solving the problem, my father is a maths and science teache at a prestigeos private school, he specialises in physics, so i'll put the question to him thisafternoon and get him to give us the maths...would that serfice as an answer??...

 

;) I will even post the answer if im wrong...

 

 

[hihosland]

Ian did say

 

"""I'm concerned that people think that because thrust is provided by a propellor, it somehow makes a difference. IT DOESN'T MATTER if thrust is applied by a prop, a jet, driven wheels or a winch down the end of the runway the thrust (motive force) in each case is transferred to the runway surface via tyre rubber, and the force resisting movement in each case is friction, the amount of which is detirmined by the coefficient of friction which is dependent upon the type of surface, and the mass of the aircfaft. """"

 

Surely thrust is is applied to the airframe and reacts against the air. I cannot see that any force is applied to the runway surface, in the hypothetical example or in any other situation. Other than the downward force supplied by gravity that is.

 

When a flying aircraft accelerates from x knots to x+y knots where is the reaction on or from the runway?.

 

Aircraft fly in air, air that is moving over the wings. Air moves over the wings by propwash which will accelerate the aircraft regardless of the speed or otherwise of the runway. Eventually that acceleration results in sufficient air flow over the lifting surfaces for the plane to lift off.

 

davidh

 

 

[motzartmerv]

How is it accelerating?? how does it gain speed when the ground beneath it cancells out the forward motion?

 

 

[hihosland]

MM Did say

 

"" foward velocity =10 kts

 

opposite velocity =10kts

 

overall velocity (in relation the still air) =0

 

""

 

Which would be true if the opposite velocity was being supplied by a propeller on the tail pushing in a rearwards direction, but it is not. It is merely spinning the wheels.

 

HOWEVER put a ski plane on the moving runway and have the runway apply a rearward force equal to the forward thrust then the aircraft will not fly.

 

and

 

""When i run on a treadmill i may be running at 20 km/h, but the treadmill is cancellin my forward speed exactly so i go nowhere...

 

The original question said that the treadmill is tuned to match the speed....THE SPEED..i'll say it again...THE SPEEEEEED;) Thats the key word, or the significant factor...""

 

True and a car on a dnyometer also goes nowhere because both the runner and the car rely on reaction against the road for their forward motion. But not the aircraft.

 

The moving runway could go at the exact speed, twice the speed or half the speed of the aircraft and the plane on wheels would still fly.

 

cheers...

 

 

[Guest Flyer40]

I haven't cheated to see how previous discussions of this question have panned out. But I think the question is either very dumb and cannot be resolved or very clever and cannot be resolved.

 

I think you're correct Slarti when you ask " Is the argument coming down to whether the wheel speed or the plane speed is being matched?".

 

The flaw in the question is about speed. As pilots we know there is more than one speed involved in flying, but which speed is not defined in the question. It would be unscientific to assume the question related to air speed.

 

As motza says "STICK TO THE QUESTION CRITERIA".

 

But if you do that, and don't make the assumption the question is referring to airspeed, then you can't conclude that the aircraft will fly. Nor can you conclude that it wont fly because the question is flawed.

 

It's still fun tho.

 

 

[Guest Baphomet]

Hihosland, you answered your own question, it's the weight of the aircraft acting on the surface of the runway via the undercarriage that resists the movement of the aircraft. The heavier the aircraft the more friction and the more thrust required to overcome it. The key to the real-world situation, is that it only takes a fraction of the force required to start it moving, to keep it moving.

 

Slarti, you are correct re transfer of thrust to runway only in the case of driven wheels is that the case. The point I was trying to make however is that it is the friction between runway and undercarriage that is resisting the force propelling it forward. "for every action....." the prop/jet moves air backwards, so the airframe wants to move forward, but is prevented from doing so by friction.

 

This might be coming down to how we read the question. When I first read it, I envisioned a scenario where aircraft speed was taken from the ASI and that the conveyor control added increasing amounts of drag (friction) so that despite the increasing amount of power delivered by the engine, the airspeed remained zero and hence the plane wouldn't fly. In other words, it's all about net thrust, if there is no positive balance, the aircraft won't accelerate and will never reach flying speed.

 

If it were possible for propwash to make it fly, then we would all levitate whenever we did a run-up check.

 

I'm voting for a ruling from Motzartmerv's dad :-)

 

 

[w3stie]

How is it accelerating?? how does it gain speed when the ground beneath it cancells out the forward motion?

MM I think this is the crux of the matter. I have now seen the light. The conveyor cannot slow the acceleration, it just spins the wheels faster. If the thrust was from driven wheels as per a car, then yes, acceleration cancelled out neatly. BUT, the thrust is against the air not the conveyor and the air is standing still (wrt the scenery).

 

So, the a/c will eventually see take off speed (that's airspeed), although it's ground speed (if you put a speedo on the wheels) would be twice the airspeed.

 

NOTE this assumes that the conveyor is only matching the plane's airspeed and *not* set up to infinitely increase in speed to keep the plane in the same place wrt the scenery.

 

 

[Yenn]

Bigglesworth. Your post looks a sif it was written by John Brandon. No I am not accusing you of stealing it, but point out that common sense can be hidden by long windedness, but I believe that was your aim.

 

Keep up the good work.

 

By the way John Brandon can be read in the current RAAus mag on Page33

 

 

[Bigglesworth]

In slow motion for the non-physicists amongst us. And to claim the $100 off brentc:laugh:

 

BTW, frame of reference problems can look very hard until you see the point and then they are obvious. I love them.

 

Firstly we have to work out the speed of the conveyor belt.

 

It moves backwards at the same speed as the plane but in reverse.

 

What speed is the plane?

 

Past the scenery it will go at the airspeed.

 

Over the conveyor belt it goes at a "groundspeed".

 

Think about this.

 

If the belt tries to match the airspeed, it is pointless. it can't do any good to any form of vehicle, be it plane, car, boat, or UFO, other than make its wheels turn at double time.

 

If it tries to match the groundspeed and keep the plane stationary, it is doomed to failure (as shown in videos etc). BUT, on the way to failure, it will speed up until it reaches breaking point. OR until the wheel bearings seize from overheating and the plane is gounded.

 

So, brentc, if I can build a conveyor belt fast enough to seize your wheels up, that $100 is mine.

 

A side note: A wind can stop a plane, but not a car, until it reaches breaking point; a conveyor can stop a car, but not a plane, until it reaches breaking point.

 

PS. John's section is cool. if only it was about something better like performance tweaks:)

 

 

[Guest brentc]

For you Biggles because you're slow to agree with me, it's now $150. I'm considering offering you money if you'll finally realise that the aircraft WILL take off. I'm fresh out of ways to explain it, other than to get you to watch the youtube video posted earlier which explains it all.

 

Try thinking about my earlier posts. You have a treadmill in a house with an engine stand on it and a 6 Cylinder Jabiru engine with prop. Do you REALLY think that the engine and prop won't move if they are at full power and the beefed-up treadmill is running flat out?

 

 

[Timm427]

She'll Fly

 

Unless of course its one of those new fangle models with the motorised wheels and no prop,fan or jet.

 

 

[Guest Macnoz]

I had to jump in at this stage if for no other reason than to get this topic back up the top off of general discussion page!

 

Apart from all the empirical evidence available and some other references on this site which I think includes

 

http://mythbustersresults.com/episode97

 

I look at it this way;

 

Lets imagine the plane sitting on the conveyor tethered forward securely and squarely. The wheels and bearings are excellent and friction is negligible etc.

 

Start up the conveyor at speed x. The wheels of the plane revolve at speed x. The plane sits there going nowhere – maybe snaking over and back a little which you correct for with nose wheel steering or even tail wheel steering!

 

Run the conveyor at whatever you want or it can do up to infinity!

 

Plane stays in one spot held by rope and wheels obligingly turning at infinity.

 

Clear prop!

 

Complete engine run ups and all check list of course because you are going someplace. The only thing you must do different from normal check list is do not apply brake at any stage.

 

To do so would be a bad bad thing!

 

Now you can visualize that as you increase thrust all other things being equal the plane will move forward and up – and away you go.

 

You at some stage need to release that rope that had you tethered!

 

So again, This is one last time. The Conveyor belt DOES NOT stop the plane from moving forward. It cannot. The only affect it has is the speed the wheels spin at.

 

 

[w3stie]

I don't reckon you will get most planes' wheels to turn at infinity. 200 knots max before they disintegrate. 50 knots for the Arena Aviation Tomahawk :)

 

 

[eastmeg2]

What has suprised me most about this thread is that nobody has refered to the vector diagram we were probably all shown when we started our training for a pilots license/certificate. That's the one with a side-on picture of a plane with 4 arrows around it to represent the forces at play.

 

Namely:

 

1. Lift pointing upwards

 

2. Weight pointing downwards

 

3. Thrust pointing forwards

 

4. Drag (Induced + parasitic) pointing backwards.

 

Think about those and the answeer should be clear. Rolling restistance will add very little to #4.

 

Cheers,

 

Glen