01 February 2008

Airplane on a Treadmill

Mythbusters did a little spot about an airplane on a treadmill the other night. The question that they were trying to answer was, if the treadmill were to move opposite the plane's direction at it's takeoff speed would it be able to take off?

The answer is yes, because the airplane cares not a bit about what speed it's wheels are moving.

Airplanes function by way of airspeed, not ground speed. The difference is the velocity of the air relative to the wings of the aircraft, not the velocity of the ground relative to the aircraft. To generate lift, air must be moving over the wing surfaces, the more the air moves the greater the lift is generated. You can take off in a small light plane in a strong headwind and not move an inch relative to the surface of the earth. Helicopters move their own wings in a rotary motion to achieve that velocity, which is how they can generate the "airspeed" needed to lift at zero ground speed. This is also why helicopters are called rotary wing aircraft, as opposed to fixed wing (regular) aircraft. This is also how kites work, by the way.

In fact, the US Navy takes full advantage of this fact when launching carrier aircraft, they juice up the speed of the ship and head it into the wind to create wind over the deck for both takeoff and landing. This makes the takeoff and landing distances far smaller, and with the aid of catapults and arresting gear heavily laden (swallows - NO) high performance aircraft can and do take off and land from teeny tiny airfields built on top of ship's hulls. Been doing it for a while now.

In fact, the DoD managed to launch and recover a Marine KC-130 from the deck of USS Forrestal in 1963 without the use of catapults or arresting gear, taking full advantage of the aircraft's phenomenal short-field capability and the speed of the carrier.

But I digress. The entire reason I'm bringing this up (or, as comedian Ron White says, I told you all that so I could tell you this) is that according to Neal Boortz, some people are claiming foul on the whole Mythbusters scenario.

As Boortz puts it, "They're saying that the Mythbusters guys cheated because they allowed the airplane to move relative to the earth. Yeah ... as hard as it is to accept, there are actually people out there who are so dismayingly stupid that they think that the rules of the game were that the airplane would not be allowed to roll forward when the propeller started turning."

For those of you not able to figure this out, there is absolutely no way on Earth anyone could have STOPPED the aircraft from moving, relative to the Earth, once the brakes were released and the throttle advanced. Yes, the aircraft would have been dragged backwards on the tarp and the motor would have had to throttle up slightly to maintain position relative to the earth (or zero ground speed) while the tarp was moving, but that's because the aircraft's propeller would have had to overcome the force of the friction from it's wheels and bearings. Once that friction was overcome, the wheels would have had absolutely no further effect on aircraft speed.

During the Mythbuster's testing, they ran a control test of the aircraft's normal airspeed and takeoff distance. The aircraft being used was a form of ultralight that only required an airspeed of 25 MPH to generate sufficient lift to take off. The testing was held in early morning when the relative winds were non-existent to keep the tarp from blowing all over the field, as had happened the evening previous. With no relative winds (that means the air moving relative to the earth's surface), the airspeed and ground speed of the aircraft would have been equal.

At the time of rotation (that's when the nose comes up, the tail goes down, and the wheels leave the ground) the aircraft was traveling 25 MPH relative to the ground (ground speed). Again, with no relative winds this means the aircraft's airspeed was also 25 MPH. This was done before the aircraft was put on the treadmill, by the way. The airspeed and takeoff distance was recorded, I don't remember the takeoff distance but I thought the 25 MPH airspeed was remarkable so I remember that. The aircraft landed and was then put on the tarp that would serve as a treadmill. The treadmill was accelerated (by way of a towing truck) to 25 MPH relative to the ground at the same instance the aircraft's pilot unlocked it's brakes and pushed the throttle forward.

The tarp was moving in an opposite direction to the aircraft. When the aircraft achieved sufficient airspeed, corresponding to 25 MPH ground speed, it rotated and lifted from the tarp. The ground speed and takeoff distance were COMPLETELY UNAFFECTED by the tarp's speed and direction. The aircraft's wheels at the time of takeoff were spinning at an estimated 50 MPH, and the aircraft was not bothered a bit by that fact. The aircraft was concentrating on gaining airspeed, not ground speed, driven by it's propeller, not it's wheels.

All of this, for what it's worth, was fully explained on the show.

If, for instance, there had been a headwind for the aircraft, it's ground speed and takeoff distances would have been lower and shorter respectively. The aircraft is not concerned by the speed of the ground under it's wheels, it is only concerned with the speed of the air over it's wings. As I said, you could have taken off in that aircraft with a 25 MPH headwind without moving relative to the surface of the Earth.

For what it's worth, the pilot of the aircraft bought into the farcical notion that ground speed has something to do with airspeed, and didn't believe the test would be successful. He was proven wrong, and admitted to his amazement. This tells me he needs to go back to Aviation 101 to learn how an aircraft actually works.

And for the record, I are not a pilot, I are just a knuckle-dragging ex-Navy aircraft electronics tech. Rudimentary flight physics was taught to me in a class called Enlisted Basic Aviation Training, which was over 25 years ago. We didn't put an aircraft on a treadmill, but we did discuss the effects of airspeed and groundspeed on fixed and rotary wing aircraft.

And now you know.

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