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Author Topic: Aeroplane Conundrum  (Read 22982 times)

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Offline Barman

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Re: Aeroplane Conundrum
« Reply #90 on: May 02, 2007, 02:37:35 PM »
and the aeroplane?  whip:

.... could not take off due to lack of forward momentum generating lift.

OK?


rolleyes:

So the two cancelled each other out?  confused1:
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Misunderstood

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Re: Aeroplane Conundrum
« Reply #91 on: May 02, 2007, 04:01:45 PM »
and the aeroplane?  whip:

.... could not take off due to lack of forward momentum generating lift.

OK?



So the two cancelled each other out?  confused1:

I believe so.   

But the barman spoke too quietly to make out what he said.     confused:

Offline Barman

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Re: Aeroplane Conundrum
« Reply #92 on: May 02, 2007, 04:04:10 PM »
and the aeroplane?  whip:

.... could not take off due to lack of forward momentum generating lift.

OK?



So the two cancelled each other out?  confused1:

I believe so.   

But the barman spoke too quietly to make out what he said.     confused:
I said..

Oh, I see...  redface:
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Misunderstood

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Re: Aeroplane Conundrum
« Reply #93 on: May 12, 2007, 04:40:28 PM »
I'm not so sure now  clock:  I am becoming persuaded by the argument of the motive force pushing against the air and not the moving runway.

It could - may - possibly - arguably be that the 'plane would acheive forward momentum independently of the wheel speed.

There again, if the wheels were rotating twice as fast then the runway would accelerate to twice the speed to match it. and it would - might - counter the additional effort.

Einstein was very clear about equal and opposite reactions, but he never mentioned wheels...  rubschin:

Two asprins please Barman, and a bowl of milk.

Offline Barman

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Re: Aeroplane Conundrum
« Reply #94 on: May 12, 2007, 08:12:19 PM »
Just imagine for a moment that instead of the conveyor control being automatic, a little man adjusts it at will to different speeds.

At the start, both the aircraft and conveyor are stationary.

Then, the pilot starts the engines and begins to taxi forward on the conveyor at 5mph. His little speedometer in the cockpit shows 5mph while the airspeed indicator also shows 5mph.

Spotting this, the man turns on the conveyor in the opposite direction at 5mph (shrewd guy).

High up in the cockpit, the pilots speedometer still shows 5mph but as the ?ground? is moving backwards at the same speed, the air speed indicator shows zero ? he is stationary with respect to the ground.

Frustrated, the pilot cranks open the throttles further and accelerates to 15mph. Now his little speedometer shows 15mph as that is the speed that he is travelling along the conveyor. Unfortunately, the conveyor is travelling at 5mph is the opposite direction so his air speed indicator only shows 10mph.

Seeing the aircraft get away from him down towards the end of the conveyor, the little man cranks his speed wheel up to 15mph.

Back in the cockpit, the speedometer still shows that the ?plane is travelling at 15mph down the conveyor, but as the conveyor is now going at 15mph in the opposite direction, the air speed indicator drops to zero again.

I can continue, but you get the idea?
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Offline Nick

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Re: Aeroplane Conundrum
« Reply #95 on: May 12, 2007, 08:55:04 PM »
I wouldn't fly with them. They should all get off and go to the EasyJet desk.

Problem solved drumroll:
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Misunderstood

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Re: Aeroplane Conundrum
« Reply #96 on: May 14, 2007, 12:48:36 AM »
Just imagine for a moment that instead of the conveyor control being automatic, a little man adjusts it at will to different speeds.

At the start, both the aircraft and conveyor are stationary.

Then, the pilot starts the engines and begins to taxi forward on the conveyor at 5mph. His little speedometer in the cockpit shows 5mph while the airspeed indicator also shows 5mph.

Spotting this, the man turns on the conveyor in the opposite direction at 5mph (shrewd guy).

High up in the cockpit, the pilots speedometer still shows 5mph but as the ?ground? is moving backwards at the same speed, the air speed indicator shows zero ? he is stationary with respect to the ground.

Frustrated, the pilot cranks open the throttles further and accelerates to 15mph. Now his little speedometer shows 15mph as that is the speed that he is travelling along the conveyor. Unfortunately, the conveyor is travelling at 5mph is the opposite direction so his air speed indicator only shows 10mph.

Seeing the aircraft get away from him down towards the end of the conveyor, the little man cranks his speed wheel up to 15mph.

Back in the cockpit, the speedometer still shows that the ?plane is travelling at 15mph down the conveyor, but as the conveyor is now going at 15mph in the opposite direction, the air speed indicator drops to zero again.

I can continue, but you get the idea?

Yeah Yeah! I get it!  And for a while I accepted it. But then got to an advanced stage of thought.... hence the headache....

The argument you put is compelling - for a car.  And by that, I mean a vehicle that achieves it's propulsion through its wheels and therefore relies totally on its adhesion to the surface for motion.  Thus +10 and - 10 = zero.

However, an aircraft does not rely on ground adhesion for its propulsion, indeed if it did then as soon as left contact with the runway it would stop - or at least begin to slow down as it drive train is broken.

So, to fly the drive has to be independant of the ground. And if it is independant of the ground then it matters not what its ground speed is.   Imagine an aircraft flying at an airspeed of 100mph WITH a wind of 30mph then its groundspeed would be 130mph, however if it was flying INTO the same wind its airspeed would still be 100mph but its groundspeed would by only 70mph (less the 30mph blowing the other way) so the difference would be 60mph relative to the ground.  Now here's the interesting bit......

Travelling at 100mph into a 30mph headwind leaves it flying at 70mph ground speed means that if the plane reduced its forward speed to 30mph then it would simply drop onto the runway because its groundspeed is 0.

All this means that the speed relationship between the aircraft and the runway is dynamic and not mechanical therefore the takeoff speed is measured in the forward air pressure exerted and not to its relationship to the ground.

If it can land stationary at 30mph then it can take off stationary by the same relative air forces at work and the ground speed counts for nothing, it's a distraction.

Offline Barman

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Re: Aeroplane Conundrum
« Reply #97 on: May 14, 2007, 04:53:05 AM »
Oh dear oh dear, I shall be smoted for this for sure...  noooo:

It matters not if the propulsion system is through the wheels or via jet engines mounted on the wings? The fact remains that for the aircraft to lift off (assuming conventional flight and not rocket flight, VTO, etc.) it has to achieve air flow over the wings ? a 747-400 needs 180mph before the wings generate enough lift its maximum take off weight of 875,000lbs off the ground.

In the scenario described the treadmill exactly matches the speed of the ?plane so it will never achieve this. However fast the wheels are travelling in relation to the treadmill the aircraft no forward speed in relation to the earth and therefore no air flow over the wings.

The only way that an aircraft in this situation could take off is if a sudden storm blew up and provided a ?wind? over the wings equivalent to the take off speed required.

Quote from: Bouncer
Travelling at 100mph into a 30mph headwind leaves it flying at 70mph ground speed means that if the plane reduced its forward speed to 30mph then it would simply drop onto the runway because its groundspeed is 0.

No. It depends on the ?stall speed? of the aircraft. If it is 30mph or lower it would continue to fly even though the ground speed had reduced to zero. Indeed it is even possible for an aircraft to fly backwards with respect to the ground into a strong wind.

Quote from: Bouncer
All this means that the speed relationship between the aircraft and the runway is dynamic and not mechanical therefore the takeoff speed is measured in the forward air pressure exerted and not to its relationship to the ground.

No. In flight the speed over the ground is totally irrelevant and can even be negative as I described above. To achieve flight however the aircraft needs to gain forward momentum with respect to the ground and the treadmill cancels this out. The only way to overcome this would be to add so much power to the aircraft that the friction between the wheels and the conveyor was overcome and the aircraft slid down the conveyor to take off. However, the amount of power required would be so vast that a conventional aircraft would simply be destroyed by it.

Quote from: Bouncer
If it can land stationary at 30mph then it can take off stationary by the same relative air forces at work and the ground speed counts for nothing, it's a distraction.

No. The thrust available to a conventional (non military) aircraft is insufficient to overcome its mass in that way. A 747-400 for instance has a MTO weight of 875,000lbs but only 253,200lbs of thrust available. The engines overcome the frictional forces and move the aircraft forward along the ground ? thus achieving air flow over the wings and eventual take off.

The Harrier has a maximum take of weight for vertical take off (VTO) of 18,950lb and a maximum thrust from the Pegasus engine of 23,800lb. I.e. the thrust is grater than the weight and it can lift it straight off the ground. In a rolling take off (where the wings are used to generate lift as the aircraft travels forwards) the maximum take off weight is increased to 31,000lb.
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grumpyoldsoldier

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Re: Aeroplane Conundrum
« Reply #98 on: May 14, 2007, 07:14:04 AM »
Contrary to popular belief, aircraft are sustained in the air by faith alone. One determined cynic can, in theory, ground an entire 747. In practice, the professional commitment of aircrew to the continued flight of the plane keeps it in the air. A moment of self-examination on the part of the pilot, however, can be disastrous. I thought I would share that with you because your debate is very very silly, and you should all go and stand in the corner whip:

Offline Barman

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Re: Aeroplane Conundrum
« Reply #99 on: May 14, 2007, 07:23:16 AM »
Contrary to popular belief, aircraft are sustained in the air by faith alone. One determined cynic can, in theory, ground an entire 747. In practice, the professional commitment of aircrew to the continued flight of the plane keeps it in the air. A moment of self-examination on the part of the pilot, however, can be disastrous. I thought I would share that with you because your debate is very very silly, and you should all go and stand in the corner whip:
lol: lol: hehehehe...

I shall go and post that on all the sites debating it.  whistle:

I trust there is no copyright fee involved?  noooo:
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grumpyoldsoldier

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Re: Aeroplane Conundrum
« Reply #100 on: May 14, 2007, 08:10:06 AM »
Go for it big boy!

Offline Barman

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Re: Aeroplane Conundrum
« Reply #101 on: May 14, 2007, 08:14:21 AM »
Go for it big boy!
I applaud your positive attitude?
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Re: Aeroplane Conundrum
« Reply #102 on: May 14, 2007, 12:12:32 PM »
Stop complicating the issue with characteristics of hugely differing aircraft where the speed and mass vary enormously but the basic principle remains the same, namely the airflow across the wing surface causes the lift to overcome gravitational forces.

Most of your argument agrees with mine especially with consideration to the airspeed compared to groundspeed. They have no common factor.

A stationary aircraft facing a headwind speed exceeding its lift factor will take off. Conversely an aircraft facing downwind would never take off.   Tat is the reason why aircraft always face into the wind to take off, thus reducing the effort required.

It is the mind boggling aspect of Einstein?s equal and opposite reaction that decides the issue.

All agreed so far?

Despite all of the variables, the issue is; can the aircraft achieve the airflow needed to generate the lift required?

Reason against:  As the wheel rotation is always countered there will be no forward motion therefore no lift will occur.
Reason for: As the thrust is exerted onto air and not the ground the wheel speed is immaterial.

Damn you!      You are definitely going to be smitten.

The equation stands in my mind as follows:

If the wheels are in a permanent state of equal speed with the runway then the physical state is exactly the same as if the brakes were applied to the wheels.

As an ordinary aircraft does not have sufficient power to overcome the braking effort then it would indeed seem that motion would be impossible, and as such, air flow would not occur. (Excepting wind speed) and so, lift is not attainable.

I must therefore reconsider my position ? at least until I think of something else.   evil:

Offline Barman

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Re: Aeroplane Conundrum
« Reply #103 on: May 14, 2007, 12:45:59 PM »
Quote from: Bouncer
A stationary aircraft facing a headwind speed exceeding its lift factor will take off. Conversely an aircraft facing downwind would never take off.   Tat is the reason why aircraft always face into the wind to take off, thus reducing the effort required.
No. If you think about it? Once the airspeed over the wing (from the headwind) is sufficient to overcome its mass it would indeed momentarily lift off the ground but then be blown backwards?

Only if the aircraft were tethered would it be able to lift upwards in a strong wind.

Irrelevant I know but I thought I?d mention it?
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Re: Aeroplane Conundrum
« Reply #104 on: May 14, 2007, 01:37:47 PM »
Quote from: Bouncer
A stationary aircraft facing a headwind speed exceeding its lift factor will take off. Conversely an aircraft facing downwind would never take off.   That is the reason why aircraft always face into the wind to take off, thus reducing the effort required.

No. If you think about it? Once the airspeed over the wing (from the headwind) is sufficient to overcome its mass it would indeed momentarily lift off the ground but then be blown backwards?

Only if the aircraft were tethered would it be able to lift upwards in a strong wind.

Irrelevant I know but I thought I?d mention it?


Not if it were under full power and only restrained by the wheel resistance. As soon as the craft leaves the ground the wheel effect will cease and normal forward momentum - relational to the air flow - would commence.  You said yourself that an aircraft can fly 'backwards' in relation to the ground. so as soon as the ground effect is removed it leaves the equation.  as long as lift is maintained it will fly.  Lift is defined by adequate airflow not forward momentum.

Using your model a glider would not work.

Seeing as you want to argue about something.............  :)