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Posted: Fri Mar 17, 2006 10:59 am
by josh
I doubt that the thrust coming out of the jet engines would generate enough overall airflow directly over the wings. the reason being is the jets are concentrated air flow and not always directly over the wings. Generally the airflow comes from the plane moving through the air and in this case the air is not moving relative to the plane.

I wonder if anyone has actually tried this?

Posted: Fri Mar 17, 2006 11:05 am
by Grim...
And oddly simular (but far more amusing) explanation from another forum:
But you seem to be thinking that the conveyor build is actually conveying the airplane backwards at the same speed as the engines are moving it forward. But that shouldn't be the case unless the pilot has the brakes on. Otherwise, it's fairly easy to move the airplane against the force of the treadmill.

A thought experiment. Suppose you're on the treadmill at the gym and someone sets the speed to "WHOAH!". You're going to go flying backwards. Now, suppose you have rollerblades on. With just one finger wrapped around the handbar, you'd be able to resist the force of the treadmill no matter what the speed was set to (up until the bearings on the rollerblades started smoking.)

Now, imagine someone gave you a Wile E. Coyote-style Acme rocketpack and lit it off. What direction do you think you'd go?

Posted: Fri Mar 17, 2006 11:09 am
by Benjamin
OK. I have a degreee in Aeronautical Engineering (BS, Miami University, '79). I worked at GE at the Aircraft Engine facility in Evendale. I helped design and test the CF6-80C engines used on the Boeing 767. I also have a Single Engine (Land) pilot's license since 1975. But enough about me.

The plane will accelerate and take off, assumig the fuselage or landing gear are not locked onto or attached to the conveyor. What matters is AIRSPEED, not ground speed. Now at takeoff, or 'rotational speed', when the airspeed over the wings start producing lift and the nose rotates up, the airspeed and groundspeed are very close if not identical (in a calm or no wind situation). That is why aircraft LAND and TAKE OFF into the wind, whenever possible. The added knots of a headwind (or headwind component in the event of a crosswind conditon) increase airspeed, and thus lift, and reduce the groundspeed for takeoff or landing.

I have on several occasions flown a Cessna or a Piper Cub in a 'slow flight' situation where the plane is flying nose up (and flaps down) into a headwind that is more than the forward (airpseed) of the aircraft. So the plane will have a negative groundspeed and appear to 'fly backwards' or back up.

Simply put: The engine (or prop) produces THRUST. This is one of the fundamentals of flight (thrust, lift, weight, and drag). Once the inertia of the plane is overcome by thrust and the plane ACCELERATES, the airflow over the wings will reach the speed needed to produce LIFT. Thrust continues to overcome DRAG and the aircraft will 'fly'. Actually, the curvature of the wing causes the air to have to move faster over the high or upper side of the wing, and subsequently have LOWER pressure while the underside of the wing has higher pressure, and the wing gets 'blown' up into the air or 'sucked' up into the air (take your pick). It will be independent of the speed of the conveyor, as described in a previous message, because the THRUST is against the air, not against wheels on the ground or conveyor.

Or let's use this. I'm sitting in a Cessna on a very long flatbed truck. The truck driver gets his truck movig down the freeway at 100 mph. The wings of my Cessna are producing LIFT and the plane would have a very short take off run on the back of that truck. From an observer off to the side, it would appear that the Cessna just 'rises' off the truck bed.

Now if I have the same Cessna on a conveyor (brakes on) and the conveyer starts to move, I release the brakes of the Cessna and start givin' her the throtttle. The plane WILL start to move forward as the prop creates thrust. We have several speeds here, though. We have the speed of conveyor relative to a fixed point on the ground OFF the conveyor. We have the speed of the conveyor relative to the speed of the airplane. According to the description of the problem, that speed will always try to be 0 as the conveyor will speed up or slow down to match the GROUND SPEED of the aircraft. ANd we have, as I mentioned above, AIRSPEED, which is the speed of the air moving past the aircraft fuselage and over wings, as measured by a pitot tube (air pressure relative to the static port on the side of the fuselage).

The wheels are not locked to the conveyor, nor will friction of the wheel bearings be an issue at all. The prop (or fan) will produce the necessary thrust to overcome the inertia or mass of the aircraft and drag of the wings and empennage. When that occurs the aircraft WILL move forward RELATIVE TO THE AIR it is thrusting against. The conveyor is irrelevant. Period.

It's like an aircraft carrier that is in motion and moves to allow the planes to have a headwind to take off.
http://forum.physorg.com/index.php?show ... entry42748

Posted: Fri Mar 17, 2006 2:01 pm
by josh
But what if the treadmill continues to accelerate? Different story. In principle it's possible to accelerate the treadmill at a rate that will exactly counteract any forward force you care to apply. (This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.) The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off.
http://www.straightdope.com/columns/060303.html

He points out that there are two possible answers, it depends if the treadmill / conveyor is accelerating to compensate for speed, or for force. So both answers are valid? (well depending on the interpretation of the original question)

Posted: Fri Mar 17, 2006 2:54 pm
by Christopher
jshpro2 wrote:what would happen if you tried to take an airplane off on a giant treadmill moving in the opposite direction of the planes movement?
My already very high opinion of Feyd has only increased by the fact he has elected not to step on this giant treadmill (or should I say, "step in this giant turdmill" ;)).