Theoretical question about airplanes

[fuji]

The plane has to be moving forward through the air in absolute terms or it won't get any lift under its wings and it won't take off.

The only way it could take off is if it moved forwad faster than the treadmill. Potentially since the wheels are free the treadmill is actually irrelevant--the plane would move forward at the same speed as it always does, the wheels would just spin twice as fast as the plane is going. In that case it would lift off.

However if it's just sitting there stationary on the treadmill with the wheels spinning it's not going anywhere.

 

[KBear]

It is the air speed that counts for take off, the ground speed, or conveyor speed, does not really matter.


Sort of like the question, if a man is standing on the moon, where there is no air, and releases a pen he is holding, will the pen.

A) Float away
B) Float where it is
C) Fall to the surface of the moon

 

[Moraff]

Now I've gotta watch the episode to see how it turns out. How much ya wanna bet that Adam manages to get himself hurt by the toy plane somehow?

 

[The Fruity Hare]

But wouldn't this be like taking a model putting it on a treadmill and pushing it? If the force exerted either by my hand or by the aircraft's engines is counterbalanced by the opposite speed of the treadmill wouldn't the airspeed be zero? I agree that if the aircraft had a thrust ratio greater than the opposite force exerted by the treadmill so that it had forward airspeed and could accelerate to v1 than the above would be more or less correct.

The tires are free wheeling and not a factor in the driving force of the aircraft. If you are standing on a treadmill on a pair of roller blades and holding on to the railings, you stand still while the wheels spin. You don't have to exert much energy to remain upright because the wheels are counteracting the motion of the treadmill. In essence, once the wheels start spinning, it should require almost no effort to remain stationary until the thrust forces the craft forward. I think most people are having a difficult time visualizing this because they are thinking in relation to a car which is powered through the wheels.

If you have a friend standing beside the treadmill pulling a rope tied around your waist, he is like the thrust of the engines. Once he starts moving you will move forward at the same speed as him.

 

[hairyfucker]

Reload said that the treadmill keeps speed with the aircraft. This is one key statement.

There are a few different thoughts going on here. Bottom line. If the wings are not moving with relation to the air surrounding them it will not take off based on aerodynamic lifting forces (normally perpendicular to the fluid flow - i.e. up). Reload said that the runway moves in the equal and opposite direction of the aircraft thus it is standing in place and burning fuel.

The only way for this aircraft to take off would be to have an additional lifting or thrusting element applied to remove the treadmill element from the equation (at least for a split second).

BTW Moraff, you are correct vectored thrust is nothing more than aimable thrust and as implemented on the AV-8B allows the craft to take off vertically, land vertically, go backwards, hover, and spin.

 

[Moraff]

Reload said that the treadmill keeps speed with the aircraft. This is one key statement.

There are a few different thoughts going on here. Bottom line. If the wings are not moving with relation to the air surrounding them it will not take off based on aerodynamic lifting forces (normally perpendicular to the fluid flow - i.e. up).

True

Reload said that the runway moves in the equal and opposite direction of the aircraft thus it is standing in place and burning fuel.

False, the runway is not providing thrust against the plane since the wheels can spin freely. If the wheels provided the plane's means of acceleration (as in a car) you would be correct.

BTW Moraff, you are correct vectored thrust is nothing more than aimable thrust and as implemented on the AV-8B allows the craft to take off vertically, land vertically, go backwards, hover, and spin.

Yes the Harrier is a neat plane fer' sure.

 

[papasmerf]

here is the way it works
lift is determined by wind moving under the wings in conventional aircraft.

http://library.thinkquest.org/3142/lift.htm

 

[RoadwarriorII]

It is the air speed that counts for take off, the ground speed, or conveyor speed, does not really matter.

The ground speed is completely irrelavant. Taking off from a runway into a strong headwind means that the aircraft only needs to accelerate to it's takeoff speed relative to the airspeed. If the wind is 20 mph right down the runway and your plane lifts at 60 with the flaps lowered, you only need to accelerate to 40 MPH relative to the ground. Used to love flying on windy winter days...the plane would lift like a homesick angel.

Theoretically, with enough headwind virtually any aircraft could land at zero groundspeed...ignoring the limitation of actually trying to manoever the plane on the ground in that kind of wind.

 

[tboy]

Breaking my oath one more time lol I just had to chime in to this:

1) Roller blade scenario: there will always be resistance to you standing still due to the friction of the wheels onto the surface of the treadmill, the resistance of the bearings inside the wheels plus that of your shifting weight patterns.
2) The turbo prop scenario: the blades of the propellors are only moving air over a small portion of the wings so you wouldn't achieve lift off. The props pull the airframe through the air allowing the total surface of the airfoil to provide lift. The only way you could get this plane off the ground with no forward momentum would be if you mounted two huge fans in front of the plane (like a wind tunnel).
3) The motorized treadmill (which is really a converyor belt) would have to exert the same force on the landing gear as the engines are providing thrust in order to keep the plane stationary. As stated previously, the bearings and wheels would not be able to withstand these forces for long since they are designed to handle take off and landing speeds (typically around 160 mph).
4) newtons law applies here: for every action there is an equal and opposite reaction. As stated, the only way you could counteract the forward thrust is to exert that same thrust onto the landing gear and at max thrust, 4 engines x 27,000 lbs of thrust each, the undercarriage would not survive long.

The scnario described here is basically the reason NASA uses rockets instead of wings for lift. The OP has basically negated the lift provided by the wings by preventing the plane from travelling through the air.

Again, there is always resistance when you have two surface contacts, think of those indoor treadmill ski hills where you stand with your skis on a rolling inclined matt. The skis are slippery, the matt is slippery, but you stay in one spot. For that matter there is even resistance or friction of the air on the wings and fuselage. There is a critical point where a wing can provide enough lift to carry the load but not impede the forward momentum too much as to make it inefficient.

 

[johnhenrygalt]

If the aircraft accelerated via the wheels (like a car) you would be 100% correct. However an aircraft accelerates by taking the air in front of it and pushing it out behind. The wheels only serve to reduce the friction effect of the ground.

So the plane would accelerate relatively normally while the wheels spun faster and faster until the plane left the ground.

Of course one could argue about whether the wheels would survive a rotation speed of 2x what they normally experience on takeoff.

We have a winner. Moraff got the answer some 20 posts ago. Just read his post and stop trying to figure out a problem that was already solved.

 

[johnhenrygalt]

So as the plane accerates forward, the treadmill runway matches the same speed in the opposite direction.

Of course this part of the question is misleading. The treadmill runway cannot "match the same speed in the opposite direction" of the aircraft to keep the aircraft stationary with respect to the air.

 

[plunker]

The more important question is if the pilot was running out of runway for takeoff and asked everyone to simultaneously fart directly down into their seat, would the plane takeoff quicker?

 

[fuji]

The more important question is if the pilot was running out of runway for takeoff and asked everyone to simultaneously fart directly down into their seat, would the plane takeoff quicker?

No. Nor would asking everyone to jump help. Moving everyone to the back of the plane might alter the angle of attack and help a little bit.

 

[pineappleguy]

So if the plane is taxiing south on the runway, and the pilot sees a bear on the runway and turns to the west, and the plane taxis some more and the pilot sees another bear on the runway and turns to the north, and the plane ends up back where it started... What color are the bears?

 

[Duckman]

The more important question is if the pilot was running out of runway for takeoff and asked everyone to simultaneously fart directly down into their seat, would the plane takeoff quicker?

First of all, asking everyone to fart is not the same as getting everyone to fart. Then, you would need to have holes in the bottom of all the seats leading to the underside of the plane to vent the exhaust gas.

Next question: how much fuel could be saved on a 4 hour flight with, say 200 passengers, if the airline made everyone take a dump before boarding?

 

[tboy]

tboy my friend, you should ahve stuck to our oath!.

The treadmill/conveyor belt spinning the freewheeling landing gear wheels has almost NO effect on the airspeed seen by the wings. I'd say NONE but that would not be precisely correct.

The hard thing for folks to get their heads around is their car oriented idea that the plane moves forward as a result of wheels imparting the forward motion.

Airplanes are moved forward by thrust provided by the propellors/jet engine. The propellor nor the wings don't know or care if the plane is taking off on ice, a flowing river, or a conveyor belt.


ice

Ah, but the difference between those surfaces you mentioned and a treadmill is the fact that those surfaces are not exerting a backward force on the airframe. They are stationary.

As the example stated the treadmill/converyor belt would exert an equal and opposite action to counteract the thrust of the engines in order to keep the plane stationary.

If it was unpowered and freewheeling then the plane would take off as normal and may even get airborne quicker due to the reduced drag of the landing gear, bearings, tire friction etc.

Sorry rub, you know I respect you and all but the drag of the landing gear has a tremendous effect on how quickly or how much thrust it takes a plane to achieve take off (or air) speed. You see, there is resistance in the bearings, and especially the tire contact with the runway. Hence why the rolling resistance of freight cars on railway track is so little. The contact patch is about the size of a dime and you have two extremely hard surfaces.

Ever watch a seaplane take off from water? It takes forever for it to achieve sufficient speed to achieve lift that is due to the tremendous resistance of the water. Put that same plane on skiis and it takes off in half the distance......

BTW: a great example of the resistance of the landing gear is demonstrated everytime a plane lands: you can tell by the smoke coming off the tires as it touches down. if there was no resistance the tires would immediately match the speed of the passing runway and there'd be no smoke....(think of a kick turn while rollerblading, there is little mass in the wheels so they can pretty much immediately change directions with little effort).

I just remembered something else: I watched a lot of the shows on the development of the new airbus and if memory serves, it took a 30,000 hp motor, 5 minutes to get the wheels up to speed during the landing gear test (where they slammed the gear down on a stationary wheel after achieve max rotation to test the durability of the tires, struts, components). Of it took that long to spin 2 of the 20 odd tires, that's a lot of resistance no?

 

[lickrolaine]

if it worked like this why would we need runways?Every airport would sell off their vast amount of land for billions,and keep just enough for the treadmill.

 

[DGrohl]

the answer is no

Airplanes take off b/c of the differences in air pressure flowing of the over and underside of a plane's wings. The treadmill that matches the speed of plane would simply make the plane stay stationary - thus there would be no lift = no fligh

*Edit* Actually, just thinking about it - if the plane is moving forward, the wheels are actually moving backwards. If there's a treadmill like runway moving in the opposite direction, it would only increase the speed of the plane. *I think* So yes, the plane would lift off.

 

[tboy]

......

And planes landing "smoke" their tires because the tires take a bit of time to get rotating as fast as the plane is going. They have to accelerate rather quickly. .....

Yes, and the reason they take time to accelerate up to speed? INERTIA and the mass of the wheels and their assemblies increases that inertia.....

For the record, it would be near impossible to create a treadmill/converyor belt that could counteract the thrust of the engines. I think the average engine puts out about 27K pounds of thrust, x 4 engines = 108K pounds of thrust. So the treadmill would have to be powered by the same 4 engines.

As for anchoring the plane (also known as tethering) that was what I was thinking would be a lot simpler than the treadmill scenario. Now no airliner would be able to achieve lift this way but there are some fighters that could. The F15 for example held (still holds?) the record for acceration of zero to x thousand feet and I believe the air force did an experiment where they placed an F15 in a vertical cage and launched it....and it did in fact take off. (btw this is also known as not using the wings for lift which is how rockets and the space shuttle fly)

Again, for the record, the engines of a typical airliner do not provide enough air movement over the wings to provide lift without it actually moving through the air....

 

[KBear]

...if memory serves, it took a 30,000 hp motor, 5 minutes to get the wheels up to speed during the landing gear test (where they slammed the gear down on a stationary wheel after achieve max rotation to test the durability of the tires, struts, components). Of it took that long to spin 2 of the 20 odd tires, that's a lot of resistance no?

Dont think your memory is serving you well. A 30,000 hp motor would be the size of the plane. 1 - 2 hp would likely be enough if you have a couple of minutes to get the wheels up to speed. the power would be required to accelerate the wheels up to speed, keeping the wheels at a speed would take little power.