Will the plane take off?

[youngmic]

The plane takes off backward!

 

This is entirely a good thing, as planes which appear to fly backward typically go faster and burn less fuel than those which appear to fly conventionally forward.

 

Mick

 

 

[Guest brentc]

Ben, I can call you to explain if you wish! The plane WILL take off!

 

There is some resistance from the wheels. Eg. If a plane is stationary (engine off) and the conveyor starts, then of course the plane will move backwards slowly, therefore it will take some force to make it stay stationary, but not all that much, very like a car on a dyno. That will translate into a little bit of extra power being needed to make the aircraft move forward and take off.

 

Next time you are at Tullamarine on the coveyor out to the Qantas gates, walk then run backwards against the flow of the conveyor and try to imagine you are a plane. Put your arms out horizontal and make buzzing noises with your lips to simulate a takeoff.

 

 

[Deskpilot]

Ben, and anyone else that thinks the plane will stand still. Forget all the theory of propwash and wing size and width of the 'conveyor' ,what ever. You all accept that when the prop rotates it creates a negative pressure (lift) on its leading face and thus pulls the aircraft forward and through the air.ie, a wing in motion but stood upright, and rotating. Right? OK, now, the aircraft moves forward, RELATIVE to the AIR, forget the ground. It will keep moving forward and accelerating until it is going so fast that the rotational speed (lift) cannot overcome the drag of the aircraft. Yes? By this time the aircraft is well airborne and we haven't even thought about the ground what-so -ever. That merely moves beneath the aircraft. Initially the wheels start to rotate but the conveyor matches that rotation speed in the opposite direction so the wheels have to turn twice as fast. Does the wheel speed have any effect on the speed of the aircraft through the air, NO, they're independent of each other. The prop doesn't drive the wheels as per a car. Think of the wheels as a barrier of non adhesion grease between the aircraft and the conveyor. There is no mechanical link between the air (in which the aircraft is moving) and the conveyor which is going a zillion miles an hour in the opposite direction. The wheels are just an interface twix the two and their only function it to freely rotate.

 

I hope that makes it clear, the aircraft will fly off the ground just as it would a fixed runway. If still not clear, try this. Would a recreational aircraft be able to take off from an aircraft carrier doing 25knots but do it over the stern (assume no wind). Similar scenario.

 

 

[Guest RogerRammedJet]

Nope sorry! The aircraft will just sit on the conveyor belt with its wheels spinning like crazy!

 

To create the lift required for the aircraft to fly, there must be airflow over the wings (not just prop wash as some have alluded to). If the force driving the aircraft forward (thrust from props, jets, rockets) is countered from the outset by a force in the opposite direction (the culmination of frictional forces between the tyres and the runway/conveyor belt), the end result will be nil forward motion of the aircraft - therefore nil airflow over the wings - therefore the aircraft will never generate sufficient lift to become airborne!

 

Physics:101

 

Rog

 

PS: If you don't believe me - go buy a small electric aeroplane and try it out on a treadmill!

 

 

[Ben Longden]

PS: If you don't believe me - go buy a small electric aeroplane and try it out on a treadmill!

I watched it happen as a kid... on the telly... glued to THUNDERBIRDS.

 

Funny, That was what I used as a 'visual' when thinking about the problem.

 

Also have a look at the BAK... it clearly shows that lift over the wing is responsible for lift...

 

Ben

 

 

[Guest Redair]

Missed the point

 

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

Mmmm, thinks, doesn't this mean that if the runway is moving backwards at the speed of light, your aircraft will take off before you even get to the airfield, and will then return so you can wave to your non-existant self in the future, after you've gone home because your aircraft wasn't there when you arrived.

 

I hope that is clear.

 

Redair.

 

 

[Guest airsick]

Mmmm, thinks, doesn't this mean that if the runway is moving backwards at the speed of light, your aircraft will take off before you even get to the airfield, and will then return so you can wave to your non-existant self in the future, after you've gone home because your aircraft wasn't there when you arrived.I hope that is clear.

 

Redair.

I have a couple of questions.

 

Firstly, the aircraft has taken off before you even got there so how can you wave to your non-existant self in the future if you never got to the airport in time to get in the plane before it took off?

 

Secondly, if the aircraft wasn't there when you arrived but returns shortly thereafter and is empty (as established in question 1 because you never got into it in the first place) then wouldn't you just stick around and hope the pilotless plane lands safely so you can go for a fly?

 

;)

 

 

[Guest Redair]

A brief answer to your question Airsick, and I hope it will explain the main thrust of my hypothesis...... No.

 

Redair

 

 

[Deskpilot]

Guys, remember that the conveyor does not move until the plane moves. Therefore, when the plane reaches 5knots, the runway is only moving a -5knots and the wheels are turning at a speed equal to 10 knots. The plane however IS moving forward. It has to to maintain the initial criteria of the question. The rearward motion of the runway has no effect on the planes speed (assuming frictionless bearings). As the plane accelerates, the runway also accelerates and the wheels rotate 2 X planes speed.

 

The analogy of an electric plane and treadmill cannot work as the relationship between plane speed and mill speed cannot be mimicked easily.

 

Rog, If the planes just sits there with no forward speed, the runway will also have no speed and some invisible arm reaching from the heaven would be needed to hold the plane still at full throttle. Also, the power of the engine, jet or rocket has no input other than to move the plane. The opposition is not a power as such, merely the runway moving at negative rate equal to plane speed.

 

Fun isn't it!

 

 

[Guest Redair]

By George I think I have it! Is there a doctor in the house who can get rid of it for me?

 

But seriously, the treadmill experiment could be proved by attaching a piece of string to the front of a model aircraft and holding the other end, whilst standing in front of the treadmill. no matter how fast the treadmill goes, (except for light speed which would cause a hole pile of other problems) the aircraft would remain in a fixed position on the treadmill. If however the string were pulled by the person holding it, the aircraft would indeed move forwards along the treadmill belt. Therefore proving that the real aircraft could never take-off, because real planes don't have bits of string tied to the front of them. And even if they did, you would never be able to pull it fast enough to create the lift to make it fly. The plane still won't take off!

 

Redair.

 

I bet you'll all be pleased when I get better and go back to work?

 

 

[Guest airsick]

The answer is quite simple really, the plane will take off.

 

Think about it this way. The aircraft is not 'pushing off' by exerting force on the ground like you do when you walk. Rather it is 'pulling' on the air in front of it. You can picture this as follows.

 

If you stand on a treadmill with roller skates on and the treadmill is moving but you are remaining stationary relative to the ground you are not going anywhere. Now however, if you pull on a rope that is anchored to the wall in front of you will you move relative to the ground? Yes. Once you have overcome the initial friction of the wheels you will begin to move. The interesting thing is what happens to the wheels of the aircraft which Deskpilot touched on.

 

Assume that the conveyor assesses the aircraft speed by sensing the rotation of the wheels. As the wheels begin to roll the conveyor will speed up to counteract the speed increase of the wheels. As Deskpilot suggested though this then causes the wheels to spin faster (10 knots in his example). The conveyor then thinks that the aircraft has accelerated from 5 knots to 10 knots and so further compensates resulting in the wheels spinning at 20 knots. And so it goes until the conveyor and wheels are travelling at an infinite speed.

 

This of course will not hold if the conveyor senses the movement of the aircraft relative to the ground around it. If the aircraft moves forward at 5 knots relative to the ground then the conveyor will move backwards at 5 knots meaning the wheels are spinning at 10 knots. Just as Deskpilot said. As long as the aircraft maintains a steady state then so will the conveyor.

 

(Another interesting point - if this results in the aircraft remaining still relative to the ground (which it won't but let's assume it does for a moment) then the conveyor will stop. The plane then accelerates back to 5 knots and the conveyor starts again. again, round and round we go...)

 

But all of this is a moot point. The aircraft will take off. This video demonstrates it -

 

 

The aircraft will maintain a constant airspeed once it has overcome the initial friction of the wheels. This is illustrated in the video by the guy holding the throttle constant but speeding up the conveyor yet the aircraft stays still (0 airspeed). It shows that the speed of the aircraft and the rate at which the wheels are spinning are independent.

 

 

[w3stie]

NO. Imagine you have taped wool tell-tales over the wings and at the tail. You rev it up to maximum thrust, and the conveyor speeds up to keep you in the same spot. You would observe the following;

 

The engine is running full blast.

 

The scenery is not moving.

 

The undercarriage sounds like it's about to come apart.

 

The tail telltales are streaming out behind in the prop wash.

 

The wing telltales (and this is important) are strung out close to the fuse, but outside the propwash, they're hanging limply.

 

The only airstream over the wing is from the propwash. There is no airflow over the wings proper. So the question is can this a/c fly in it's own propwash?

 

I think not. You could find out by doing a full power run-up :)

 

 

[Guest airsick]

W3stie,

 

What caused the wheels to turn in the first place? They turned because the plane moved relative to the scenery. Therefore the plane must have some forward movement which, if allowed to accelerate by holding the throttle flat out, will eventually cause enough airflow over the wings to take off. The ability to takeoff has nothing to do with the propwash, just like normal.

 

 

[Guest Baphomet]

Roger is correct, the plane will NOT take off. I'm assuming the conveyor is being driven, and imparting sufficient friction to the wheels to exactly counter the thrust provided by the propellor or jet engine (acting through the wheels). The net effect is no movement of the wing through the air mass, and therefore no lift/flight. The more thrust that is applied the faster the conveyor moves and the faster the wheels turn, but they never turn fast enough to overcome the ever increasing friction imparted to the wheels by the moving conveyor to propell the aircraft forward relative to a fixed point

 

The only reason a plane can take off normally, is because there is sufficient net thrust available, after friction has been overcome, to accelerate the aircraft to flying speed. Thats why we never take off uphill. (less net thrust available) In this case there is never enough thrust available because it's being consumed by the increasing friction imparted by the conveyor.

 

Whew, surely it's not that hard :-)

 

 

[Guest Redair]

OK, I watched the video, but the only thing I can see is that there is no accurate speed measurement of the aircraft, either ground, air or for that matter wheel speed. So, the fact that the model can accelerate along the belt is most likely due to the belt not matching the speed increase of the model.

 

Redair.

 

 

[w3stie]

Airsick

 

"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). "

 

The thrust from the prop moves the plane forward *relative* to the belt surface, so as far as the wheels are concerned, it's moving. But according to your description, the conveyor increases it's backwards motion to keep the a/c stationary *relative to the scenery*. Assuming the air is attached to the scenery, not the conveyor, then the wings feel no airflow (except propwash). No airflow=nolift=noflight.

 

For anyone claiming it can fly, can you please explain where the airflow is coming from over the wings??:)

 

 

[crashley]

Of course the plane will takeoff because the prop is pulling the plane forward through the air which makes it create lift the wheels have nothing to do with it

 

they just stop the plane scraping along the runway

 

 

[Guest brentc]

How about this?

 

Let's place our plane on that treadmill and see what happens. If the wheels were perfect - that is, there is no friction in the bearings (and no deformation of the wheels as they spin) - then something interesting happens. When we turn on the treadmill, the plane stays stationary on its own. The wheels simply spin along the track, and impart no force to the plane. If you had a car with frictionless axles, and you disconnected the whole drive train, the same thing would happen to your car.

 

The only reason that a plane or a car moves backwards on a treadmill is that the wheels are somehow partially locked to the axles. In a plane, this is because of minor friction in the bearings. In a car, it's because of the drive train. If you want the car to stay still, you have to turn the drive train at the proper speed. If you want the plane to stay still, you have to overcome the minor bearing friction. And again, since friction does not change with speed, you don't have to exert any more force at higher speeds. If you run the treadmill at 5mph and turn on the plane's engines just slightly, they will provide enough thrust, pushing against the air, to keep the plane still. If you then increase the treadmill speed to 500 mph, you won't need to adjust the throttle on the airplane - it will remain stationary. That's because it's seeing the same frictional force that it was at 5mph. Thus, it doesn't matter how fast the treadmill is moving - if the pilot does not want to remain stationary, then he won't. It only uses the very first bit of power from the engines to keep the plane stationary. As the throttle is increased from that point, it moves forward just as it would on any other runway. It's pushing against the stationary air!

 

If you don't believe me, imagine this (or even try it at home): you're standing on a skateboard on a treadmill. You hold onto the handrails of the treadmill and turn it on. Of course, you'll remain stationary (relative to the ground). In fact, you only need to use a very light touch to stay stationary - perhaps a few fingers pressed against the handrails. Crank up the treadmill speed as high as you like. You'll still only need the same light touch to remain stationary. At any time you like, you can move forward - closer to the treadmill console - by simple pulling on the handrails. If you had a jet engine, or super-strong hairdryer, you could use this to propel yourself forward instead of holding onto the handrails. In fact, if you're really careful, you might be able to do this at home with a skateboard and a leafbower, but I doubt you'll have a sensitive enough control of your leafblower thrust to get yourself to remain stationary.

 

So you see (oh please tell me you see), the conveyor operator cannot force the plane to remain stationary. And if the plane isn't stationary, it can take off.

 

There's a dedicated website for this!

 

http://www.airplaneonatreadmill.com/

 

 

[Guest kuzzy]

No.

 

The question of whether the aircraft will fly may be considered the same as "will the aircraft accelerate and achieve groundspeed"

 

The scenario:

 

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)..

 

suggests that this is not possible, because the 'ground' (i.e. the conveyer belt) will always be moving at the exact opposite velocity. The mode of power is irrelevant because the the very ground over which the aircraft is moving will always be moving in such a way as to counter act the aircraft's movement, and thus no relative motion will be achieved.

 

It seems that the only way it would work would be if there was a lag in the system that meant that the acceleration of the conveyerbelt was discernably delayed to that of the aircraft which it was trying to match. If the lag was long enough (or the aircraft's acceleration great enough) then the aircraft would achieve relative motion (and thus airspeed) and may be able to take off. That however was no the question posed.

 

I think the pilot's best bet is to use the cross strip.

 

 

[Admin]

I think the pilot's best bet is to use the cross strip.

 

 

[Guest Redair]

Kuzzy, my thoughts exactly.

 

Redair.

 

 

[w3stie]

brentc said "So you see (oh please tell me you see), the conveyor operator cannot force the plane to remain stationary. And if the plane isn't stationary, it can take off."

 

The penny drops. The conveyor cannot keep the plane stationary wrt the scenery, so the a/c must accelerate through the air .

 

Whew :)

 

 

[Guest brentc]

The key to the solution (that it can fly) is in the friction of the wheels. This remains constant say perhaps at 10 knots and above because the wheels are free-spinning and not attached to a drive-train of any kind, therefore, once a small amount of power has been applied, any excess power becomes airspeed and groundspeed.

 

Imagine this....... you have a treadmill, an engine a propellor and a vertical wall.

 

The engine sits on wheels that are free spinning and the prop is attached as per normal. It's sitting on a treadmill and there's a wall directly behind it.

 

You accelerate the engine flat out.

 

Do you really believe that the engine won't move forward along the treadmill ? It will literally launch itself off the treadmill and smack into the end of it!

 

The answer is in the friction, or lack thereof.

 

If that doesn't make sense..... I'll try it in aircraft terms.

 

You are in your lounge room and have a treadmill. You have a 120 Hp Jabiru (or a 914 turbo if you are that way inclined). You have a Jabiru timber prop or a Carbon bolly propellor (3 blade if you wish). The engine is on an engine stand on wheels. The engine stand is sitting on a treadmill and it has bearings in it's wheels.

 

You power up the engine and the treadmill simultaneously.

 

The wheels will spin very fast as the treadmill and the engine shoots across the room!

 

Doesn't it?

 

 

[Flyer]

And your missus is about to send your bonce bouncing across the room with the rolling pin for mucking around with engines in the lounge room and buggering up her treadmill again. ;)

 

Regards

 

Phil

 

 

[w3stie]

The whole argument revolves (no pun) around whether you believe it is *possible* to build a treadmill that can prevent an a/c gaining airspeed (forward speed relative to an observer) by increasing the rolling friction against it. If it is a perfect machine then yes, it can accelerate the treadmill to a million km/h, bursting the tyres and wrecking the a/c. Of course you can argue that the a/c is also a perfect frictionless machine, so is not affected.

 

It's probably a good argument to have after a few drinks. Actually I think I need a few drinks after all this mental effort