The "Aeroplane on treadmill" conundrum...
[B]dublinpilot[B]you're talking shoite me auld son. 
If there was the relationship between wheel bearing friction and speed that you describe then your car would never move away from the kerb as friction would increase in proportion to the speed! 
Fortunately, after overcoming the initial "sticktion" of a stationary bearing, bearing friction remains realtively constant as long as the bearing in question is properly lubricated appropriate to the speed it is rotating / sliding at.
If there was the relationship between wheel bearing friction and speed that you describe then your car would never move away from the kerb as friction would increase in proportion to the speed! Fortunately, after overcoming the initial "sticktion" of a stationary bearing, bearing friction remains realtively constant as long as the bearing in question is properly lubricated appropriate to the speed it is rotating / sliding at.
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You have made an incorrect assumption that invalidates your case.
Friction of movement between two solid objects does not have a simple relationship with speed. Friction is greatest when they are stationary in relation to each other, but drops immediately when they start to move and thereafter remains low - it doesn't rise quickly with speed.
Friction of movement between two solid objects does not have a simple relationship with speed. Friction is greatest when they are stationary in relation to each other, but drops immediately when they start to move and thereafter remains low - it doesn't rise quickly with speed.
If I remember my leaving certificate physics correctly the law was "The co-efficient of static friction is greater than the co-efficient of dynamic friction."
Once the aircraft starts to move forwards relative to the conveyor belt, the level of friction immediately drops. However it does not disappear. If I remeber correctly, it increases thereafter with speed, albeit slowly.
I'll readily admit that the velocity that the converyor belt would have to be traveling would likely rip the wheels off before it gave enough friction to hold up the engine, but none the less, in theory it seems possible.
[b]dublinpilot[b]you're talking shoite me auld son. If there was the relationship between wheel bearing friction and speed that you describe then your car would never move away from the kerb as friction would increase in proportion to the speed!
dp
dp
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Penguina
They already have this and have it in production.
The belt is running at about 15-20 knts and the fan is anywhere between 0 and 50knts of wind.
Its called an aircraft carrier
Actually it doesn't matter what speed the threadmill is doing the only thing the aircraft carrier is doing is steaming into wind to get the intial airspeed up. The relative speed of the ground under the aircraft wheels will have no relation at all on take off distance presuming no friction.
PS it really is quite amusing seeing how none engineers think the world works. How effects which engineers instictively ignore are given huge weighting and how complicated very simple engineering systems become.
They already have this and have it in production.
The belt is running at about 15-20 knts and the fan is anywhere between 0 and 50knts of wind.
Its called an aircraft carrier
Actually it doesn't matter what speed the threadmill is doing the only thing the aircraft carrier is doing is steaming into wind to get the intial airspeed up. The relative speed of the ground under the aircraft wheels will have no relation at all on take off distance presuming no friction.
PS it really is quite amusing seeing how none engineers think the world works. How effects which engineers instictively ignore are given huge weighting and how complicated very simple engineering systems become.
I am a bear of little brain so please only use short words when trying to explain to me how stupid I am; surely if every time the prop bites at more air to move the aeroplane forwards along our moving runway the runway moves backwards then the aircraft never moves in space, therefore has no airspeed, therefore sits on the conveyor belt revving itself to death.
I know some of you will find it difficult to believe that someone this stupid has access to a computer but it makes my head hurt so please explain.
I know some of you will find it difficult to believe that someone this stupid has access to a computer but it makes my head hurt so please explain.
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Jafo,
Ignore my arguments above, because they are really more theory than practice.
Try to think of it the other way around; an airplane landing on a conveyor belt.
The airplane is in a 60kt headwind, and the pilot leaves some power on......enought to maintain level flight with a nose up attitude.
Just as the airplane is about to touch down the pilot flares a little more so that the wheels barlely touch down, and the weight of the aircraft is partly supported by the wheels and partly by the conveyor belt.
All that happens is that the wheels are spun by the conveyor belt, but the engine still pulls the plane forwards though the air, and the airflow over the wings support it weight.
The pilot then lets the angle of attack fall so that all the weight is now supported by the wheels, but leaves the engine running. The airplane still stays stationary on the conveyor belt.
If the pilot now applies full power, the aircraft accellerates forward and climbs away.
All the conveyor belt is dowing is spinning the wheels. The airplane is pulling against the air, not the belt (ignoreing friction).
I hope that helps!
dp
Ignore my arguments above, because they are really more theory than practice.
Try to think of it the other way around; an airplane landing on a conveyor belt.
The airplane is in a 60kt headwind, and the pilot leaves some power on......enought to maintain level flight with a nose up attitude.
Just as the airplane is about to touch down the pilot flares a little more so that the wheels barlely touch down, and the weight of the aircraft is partly supported by the wheels and partly by the conveyor belt.
All that happens is that the wheels are spun by the conveyor belt, but the engine still pulls the plane forwards though the air, and the airflow over the wings support it weight.
The pilot then lets the angle of attack fall so that all the weight is now supported by the wheels, but leaves the engine running. The airplane still stays stationary on the conveyor belt.
If the pilot now applies full power, the aircraft accellerates forward and climbs away.
All the conveyor belt is dowing is spinning the wheels. The airplane is pulling against the air, not the belt (ignoreing friction).
I hope that helps!
dp
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The air isn't as such stuck to the moving ground.
the confusion here is because you making it very complicated.
The propellers job is to transfer power to the air which results in a force towards the rear of the aircraft. The air is not attached to the belt so it is free to be accelerated by the prop producing the force.
The wheels on the ground are just there to stop the prop hitting the ground.
While taking off they have minimal resistance to forward motion but in the engineers brain all it is, is a force resisting the plane going forward in the oppersite direction to the force produced by the prop.
Now the wheel/ belt is treated as a system in it self. The sum total of all the proccesses going on will produce a resisting force. And usually in the grand scale of things it will be forgotten about by engineers because its less than 1% of all the other forces.
Now you add all the forces up resisting the forward motion of the aircraft so thats the forces due to the wheels, forces due to aero drag and you subtract them from the force produced by the propeller. If the resultant force is positive the plane will accelerate forward relative to the airflow if it is negative it will go backwards if it is zero it will stay stationary.
Its a bit like all the stuff to do with the difference between Ground speed and airspeed when your doing nav.
With any system engineers are taught to simplify first down to just the basic forces involved and not really care how they are produced. Then form whats called a free body diagram which graphically shows you whats going on. Some get by with just sticking numbers next to the arrows and some preffer to draw the arrows to scale. In this case you would have. Where x is the plane.
T<---------------------------------X->D
Now the problem is that experence allows the engineer to forget about some things because they know that it won't matter for that problem. We tend to work to 3 significant figures any thing less than that just gets forgotten about because of all the other presumtions and arse covering with materials means you will be very lucky to get an answer that will be correct to anything more than that anyway. You can see from the diagram that the force forward is way more than the force the other way. This would be at the start while the aircraft isn't moving relative to the air flow.
As it speeds up the aero drag increases so it becomes.
T<---------------------------------X------------->D
Then once it becomes airborne the drag produced by the wheels is removed. Thats why some times you feel a little bust of acceleration as the wheels lift.
And the plane continues to accelerate until.
T<---------------------------------X--------------------------------->D
Then the plane is at a constant speed relative to the airflow.
So whats happening at the ground is really irrelavent to whats happening to the airflow around the wings and prop. The wheels ground produce a force and the propeller produces a force. If the force from the propeller is greater than the force from the drag from the ground it will move forward. Which will then make airflow over the wings which will produce lift for flight.
MJ
the confusion here is because you making it very complicated.
The propellers job is to transfer power to the air which results in a force towards the rear of the aircraft. The air is not attached to the belt so it is free to be accelerated by the prop producing the force.
The wheels on the ground are just there to stop the prop hitting the ground.
While taking off they have minimal resistance to forward motion but in the engineers brain all it is, is a force resisting the plane going forward in the oppersite direction to the force produced by the prop.
Now the wheel/ belt is treated as a system in it self. The sum total of all the proccesses going on will produce a resisting force. And usually in the grand scale of things it will be forgotten about by engineers because its less than 1% of all the other forces.
Now you add all the forces up resisting the forward motion of the aircraft so thats the forces due to the wheels, forces due to aero drag and you subtract them from the force produced by the propeller. If the resultant force is positive the plane will accelerate forward relative to the airflow if it is negative it will go backwards if it is zero it will stay stationary.
Its a bit like all the stuff to do with the difference between Ground speed and airspeed when your doing nav.
With any system engineers are taught to simplify first down to just the basic forces involved and not really care how they are produced. Then form whats called a free body diagram which graphically shows you whats going on. Some get by with just sticking numbers next to the arrows and some preffer to draw the arrows to scale. In this case you would have. Where x is the plane.
T<---------------------------------X->D
Now the problem is that experence allows the engineer to forget about some things because they know that it won't matter for that problem. We tend to work to 3 significant figures any thing less than that just gets forgotten about because of all the other presumtions and arse covering with materials means you will be very lucky to get an answer that will be correct to anything more than that anyway. You can see from the diagram that the force forward is way more than the force the other way. This would be at the start while the aircraft isn't moving relative to the air flow.
As it speeds up the aero drag increases so it becomes.
T<---------------------------------X------------->D
Then once it becomes airborne the drag produced by the wheels is removed. Thats why some times you feel a little bust of acceleration as the wheels lift.
And the plane continues to accelerate until.
T<---------------------------------X--------------------------------->D
Then the plane is at a constant speed relative to the airflow.
So whats happening at the ground is really irrelavent to whats happening to the airflow around the wings and prop. The wheels ground produce a force and the propeller produces a force. If the force from the propeller is greater than the force from the drag from the ground it will move forward. Which will then make airflow over the wings which will produce lift for flight.
MJ
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Mad Jock:
Oh yeah!
Damn this thread. I took ages getting to sleep last night because I just couldn't stop my brain from drafting a checklist for short-field-conveyor-belt take off technique to best use the residual airspeed in the initial stages of the climb-out before drag decayed it back to what your prop could supply!
Oh yeah!
Damn this thread. I took ages getting to sleep last night because I just couldn't stop my brain from drafting a checklist for short-field-conveyor-belt take off technique to best use the residual airspeed in the initial stages of the climb-out before drag decayed it back to what your prop could supply!
Well if this were true (which it is not!)
we could solve the need for huge big aerodromes all over the country (for take off at least)- no need to demolish ancient woodland or entire villages to site a new runway - just install a small treadmill under the wheels of an airliner - whizz it up and away you go, according to those without a grasp of how an aircraft flies. - AIRSPEED
We could then sell of half of Heathrow for housing (to accommodate treadmill technicians?), since you no longer need the take off runway!
we could solve the need for huge big aerodromes all over the country (for take off at least)- no need to demolish ancient woodland or entire villages to site a new runway - just install a small treadmill under the wheels of an airliner - whizz it up and away you go, according to those without a grasp of how an aircraft flies. - AIRSPEED
We could then sell of half of Heathrow for housing (to accommodate treadmill technicians?), since you no longer need the take off runway!
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Originally Posted by mad_jock
They already have this and have it in production.
The belt is running at about 15-20 knts and the fan is anywhere between 0 and 50knts of wind. Its called an aircraft carrier
Actually it doesn't matter what speed the threadmill is doing the only thing the aircraft carrier is doing is steaming into wind to get the intial airspeed up. The relative speed of the ground under the aircraft wheels will have no relation at all on take off distance presuming no friction.
The belt is running at about 15-20 knts and the fan is anywhere between 0 and 50knts of wind. Its called an aircraft carrier
Actually it doesn't matter what speed the threadmill is doing the only thing the aircraft carrier is doing is steaming into wind to get the intial airspeed up. The relative speed of the ground under the aircraft wheels will have no relation at all on take off distance presuming no friction.
This is all tooooo much for my poor little brain - I am planning to fly a REAL aircraft off a REAL stationary runway, tomorrow!
Well, thank you all for trying, anyway.
I think I'll stick to real aeroplanes and static runways but, before I do, I need a very long lie down in a darkened room after reading all the posts.
I think I'll stick to real aeroplanes and static runways but, before I do, I need a very long lie down in a darkened room after reading all the posts.
Why do it if it's not fun?
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A man who has had occasion to post on pprune 256 times is on very shaky ground, very shaky indeed!
Penguina
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Join Date: Dec 2001
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Hmm, I'll get my hat now I think......
FFF
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Right Kiddies - today I took stue's advice to "GET OUT MORE" and spent the day flying, but I still can't get this puzzle out of my mind!
.
I am no physicist, but I have to agree with J.A.F.O. above. My very simple thinking is as follows:-
Flying an SEP off a short runway today:-
I lined up, held it on the toe brakes, and applied full power - RPM read 2800, ASI nil and the aircraft remained stationary.
Only after a roll of about 200m, with the ASI increasing towards 65kts, did it show any inclination to fly.
.
If I then put the same aircraft on your mythical conveyor belt:-
I use no brakes and apply full power - RPM reads 2800, ASI nil and the aircraft remains stationary, albeit with madly spinning wheels.
.
So, the only differences between the two scenarios are:-
- Stationary wheels -v- madly spinning wheels
- Travelling through a 200m long parcel of air -v- staying in the same bit of air
.
So, I still do not understand what forces there are, acting on the aircraft, to make it fly off the conveyor belt, that are not present at the moment just after the brakes were released on the runway.
.
PLEASE, PLEASE would someone explain this to me in very short simple words, otherwise I will spend another sleepless night trying to understand it!
.
I am no physicist, but I have to agree with J.A.F.O. above. My very simple thinking is as follows:-
Flying an SEP off a short runway today:-
I lined up, held it on the toe brakes, and applied full power - RPM read 2800, ASI nil and the aircraft remained stationary.
Only after a roll of about 200m, with the ASI increasing towards 65kts, did it show any inclination to fly.
.
If I then put the same aircraft on your mythical conveyor belt:-
I use no brakes and apply full power - RPM reads 2800, ASI nil and the aircraft remains stationary, albeit with madly spinning wheels.
.
So, the only differences between the two scenarios are:-
- Stationary wheels -v- madly spinning wheels
- Travelling through a 200m long parcel of air -v- staying in the same bit of air
.
So, I still do not understand what forces there are, acting on the aircraft, to make it fly off the conveyor belt, that are not present at the moment just after the brakes were released on the runway.
.
PLEASE, PLEASE would someone explain this to me in very short simple words, otherwise I will spend another sleepless night trying to understand it!
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I've not read the last few pages of this thread, but I still don't believe the aeroplane would fly!
IF it was in a wind tunnel in this scenario, yes, but just in a gym or whatever, it wouldn't.
Imagine that (god forbid) you're at the gym running on the treadmill - you have no "relative" wind, therefore if you were a wing, no angle of a attack, therefore no lift!
tKF
IF it was in a wind tunnel in this scenario, yes, but just in a gym or whatever, it wouldn't.
Imagine that (god forbid
) you're at the gym running on the treadmill - you have no "relative" wind, therefore if you were a wing, no angle of a attack, therefore no lift!tKF
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Have I missed something here?
Unless I have completely missed the point here (which is not impossible due to the amount of posts) then the said aircraft would be effectively stationary with respect to both the ground and the air as it is actually not moving - just overcoming the speed of the conveyor and the drag of the gear?
I thought that one of the basic principles we all knew was that for an aircraft to fly we need lift, which is created by airflow moving over wings i.e you need airspeed.
No movement through the air = no airspeed = no air moving over wings = NO FLIGHT
I'll grab another beer!
I thought that one of the basic principles we all knew was that for an aircraft to fly we need lift, which is created by airflow moving over wings i.e you need airspeed.
No movement through the air = no airspeed = no air moving over wings = NO FLIGHT
I'll grab another beer!
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Stue:
A man who has had occasion to post on pprune 256 times is on very shaky ground, very shaky indeed!
A man who has had occasion to post on pprune 256 times is on very shaky ground, very shaky indeed!
Good call, i think ill go flying today then! and as for how many times you have posted FFF........??
