will it take off?

[silverdollar6]

If it could fly, why waste all that money on runways?

 

[shvl73]

If it could fly, why waste all that money on runways?

I'll bet landing without a runway would be interesting.

 

[turnkey4099]

I have carefully noted the posts with bogus ideas so I can qualify future advice given by those authors.

Listen to everyone but qualify your sources.

I didn't see what the dificulty is. The original post clearly stated the givens and the outcome should be obvious to the most casual observer.

Since the conveyer belt runs as fast as is required to keep the plane standing still with respect to "the world" there is no wind over the wings except as provided by local weather and I suggest there isn't enough wind to equal or exceed the speed needed to let the craft lift off. Actually, unless the person stating the problem allows for wind, there isn't any.

The air is standing still with respect to the world, i.e. no wind. The aircraft is standing still with respect to the world due to the clever control of the conveyor belt. So the aircraft is standing still with respect to the air. No air flow over trhe wings eguals no lift.

If we temporarily lift restrictions of maximum thrust of the aircraft and make it super powerful, eventually, at enourmous speeds (hypersonic), the drag of the surface of the conveyor belt will propel enough air down the "runway" to provide enough lift such that the aircraft will lift of the runway with no apparent forward motion with respect to the world.

Once free of the conveyor belt and airborn the aircraft will be propelled forward but if the conveyor belt/runway does not extend forward of the aircraft a sufficient distance or if the conveyor belt is slowed to a stop the plane will return to the ground (landing or crash? can't say, not enough information)

If the aircraft doesn't develop enough forward speed with respect to the world and its zero wind before leaving the region of conveyor developed wind the pland will lose lift and fall out of the air. Not enough information is given to know if the aircraft will developp flying speed before abruptly leaving the region of conveyor accelerated air.

Ignoring the super powerful propulsion idea for the plane and a hyperfast conveyor belt, the problem is quite trivial. Aircraft like sailboats are governed by relative wind. The only relative wind (above zero knots) in the original problem statement is that blown over the wings by the prop if there is one. Jets would have much less. Developing enough lift to take off using the prop blast is possible but not achievable to the vast majority of aircraft.

If we can use an F-35 or a Harrier jump jet or an Osprey then the problem as stated is meaningless so it must be assumed these special aircraft were not to be used.

A practical concern would be overheating a piston engine. You would be at max throttle for an extended period and only have the limited airflow created by the prop with no cooling assist from "ram air" due to motion through the air.

Oh by the way, the pitostatic system would indicate zero or near zero airspeed and that is what counts for wings generating lift.

Pat

There is one wee, minor thing you are missing. Physics. As soon as the motor on the plane is reved up, there will be air movement and the airplane will be pulled through the stationary air you have postulated. It will also be pulled through the air even if there is a wind. The only way the plane will remain stationary in relation to the air around it is for the motor not to be running. For it not to take-off you somehow have figure a way to do away with the force of the propellor (or jet) on its surroundings.

Harry K

 

[Tom_Veatch]

Tom,

That's a generous statement. But... I feel the problem solver can safely assume the reference for speed is the same for both the plane and convayor. It would be very odd for two objects speed referenced from diff ref points with out coment.
...

There are so many folks who insist that the aircraft would not move (relative to the world) and hence would never achieve flying speed, I tried to envision some way that might be true. The only way I could do it was to assume the airplane speed in the problem statement had to be the speed relative to the conveyor surface and the the speed of the conveyor was relative to the world. That's the only way I can imagine the airplane would remain stationary (no airspeed) relative to the world.

That is a somewhat bizarre way to interpret the problem but I can find no other assumptions that lead to a no-fly solution. As mentioned earlier, it could be a difficult, if not impossible, situation to initiate and would require that the pilot use only enough thrust at any given time to overcome the rolling resistance of the wheels and bearings.

Well, it's been fun, but after over 330 posts, I think this thread has about run it's course.

 

[the_sandman_454]

The only ways the plane will remain stationary is for the engines to be off, or the parking brakes to be locked so they resist the thrust produced by the engines.

In any other situation the thrust from the engines will push the plane forward relative to the conveyor belt, because the wheels are on nearly friction free bearings, and should not have sufficient resistance without the brakes locked to prevent forward movement of the aircraft regardless of what the conveyor does.

 

[Egon]

Anyone here know how a treadmill works?

 

[daTeacha]

You could tie a real plane in place on a real moving conveyor belt, run the belt up to any speed, and the wheels would match it without the plane going anywhere. The wheels are not driven by the engine in any airplane I know of.

On stationary pavement the engine does drive the wheels. What else drives the wheels? Engine creats thrust and moves the plane forward driving the wheels. What you likely ment to state is the wheels don't drive the plane.

What actually makes the wheels spin is friction between the wheels and the surface on which they rest. The engine makes the plane move through the air. If it's in contact with the ground, or water, or snow or ice, those parts of the plane touching the supporting surface will experience a certain amount of friction. If those parts are wheels, they will turn. If they are skis or pontoons they will slide along in the direction which produces the least friction -- usually forward and parallel to the long axis.

Which brings us back to the conveyor -- the lift of the wings is not dependent on the motion of the wheels or the conveyor. When the plane moves through the air, it will fly. The conveyor may cause the wheels to spin at a speed inconsistent with the velocity of the plane through the air, but when the airflow over the wings is fast enough the plane will lose contact with the conveyor and fly.

If you want to get really technical about what does what, try to figure out why a physicist will say the ground exerts the force against the tires of your car which makes the car move forward. The rotation of the tires is actually trying to move the ground backward relative to the car, but the road surface exerts a reaction force on the tires, causing them to move forward. It's the same line of thought that says your chair is pushing up on you as you sit on it. You aren't holding the chair down, but it's holding you up. Your tires don't make the road move, but it makes the car move.

Bringing this around to tractors again, the type of tires you choose -- Ag, R4, or Turf -- dictates how strongly the turning force of the tractor is translated into forward push on the tractor exerted by the ground. If the shear strength of the area of the soil in contact with the tires is insufficient, then the tires do indeed make the ground move instead of the other way around. In effect, the tractor induces something like a laminar flow of the upper surface of your lawn against the underlying strata by grabbing hold of it and pushing it backwards harder than the roots or soil molecules can push forward because they are not attached to other soil particles well enough.

 

[the_sandman_454]

Anyone here know how a treadmill works?

Yes, I am very familiar with a treadmill. Try this: Get a treadmill with hand rails all along it. Put rollerblades on and start at the back. Using your hands on the aforementioned rails, propel yourself forward relative to the treadmill. Your rollerblades on the treadmill are nearly irrelevant to your ability to pull yourself forward. In this situation, your arms are acting as the plane's engines. Therefore yes, if the pilot applies enough thrust from the engines, he can most assuredly take off.

 

[GIJOE]

Yes it will take off!!!

Planes takeoff and land, with, against, perpendicular and any angle in between for that matter, from a naturally occurring conveyor every day and it doesn't matter what the wind speed or wind direction is. It's called the rotation of the earth. At the equator, the earth rotates just over 1000 mph and decreases to zero at the north and south poles. Now, lets take a 747-8 jumbo jet and use as an example. The 747-8 is not THE largest, but one of the largest aircrafts in the world. It has a takeoff speed of just under 200 mph, with a max level speed of 600 mph. It has a max takeoff weight of 960,000 lbs. and four GE turbo fan engines producing 266,000 lbs. of thrust (which will skid the tires if the brakes are locked upon take off). A plane taking off or landing in northern Greenland or the southern tip of South America (which is about as far north and south as you can get without landing on solid ice) doesn't have an easier or harder time than a plane taking off or landing in Kenya which is divided by the equator and moving just over 1000 mph. In Kenya, the conveyor has 5 times exceeded the planes takeoff speed and the engines are not even reved up yet, and the plane still takes off at just under 200 mph. The water may go straight down the drain but that is another topic

 

[daTeacha]

But the air moves with the earth, so the analogy doesn't work.