Limitations on the Use of Automation at Your Company

[sayrejm]

you would need mobile wind tunnels that can match the acceleration of the aircraft until the aircraft can depart the effects of the wind tunnel without experiencing a significant amount of wind shear

 

[CX880]

good thinking that maybe possible but then you'd have to design other tunnels that "catch" the airplane on landing.

 

[centralpilot]

What if you put an airplane on a giant treadmill designed to match the speed of the plane and hold it stationary as the airplane accelerated.

Would it ever take off?

Good god. Are we starting this debate again?

http://www.youtube.com/watch?v=YORCk1BN7QY

http://forums.flightinfo.com/showthread.php?t=107382&highlight=airplane+treadmill

 

[sayrejm]

That show is stupid and usually wrong. First and the only point needed is that the treadmill was NOT matching the speed of the aircraft accelerating in the other direction. By definition, this would only have occurred if the velocity of the aircraft relative to the surface of the earth remained at zero thus setting the free-stream velocity to zero thus setting Lift equal to ZERO. anything else is only a result of a poorly controlled experiment. It is like seeing what lifts more weight, a helium balloon or a hydrogen balloon. Only you cant find two balloons the same size so you settle for one that is twice as big. Then, despite any mathematical proof that says otherwise. you come to a conclusion in your back yard and claim it is true because you are on the discovery channel....

It's crap...

But what do I know. I just have a Master's in Aerospace Engineering. Not like I know what I am talking about or anything.

 

[CX880]

An airplane doesn't know how fast it goes over on the ground. All they did with that experiment was speed up the rotation of the wheels but not the airplane. In order for an aircraft to fly there has to be airflow over the wing. That experiment was a total failure and it was obviously made for people that don't know anything about the principles of flight.

 

[sayrejm]

Exactly... and they didnt start by pulling fast enough allowing the airplane to accelerate forward.

Now... the tricky part is uncoupling that concept of pulling backwards as fast as the airplane would accelerate forward on solid ground. You have to think of the axial force exerted on the landing gear as the treadmill moves backwards and see how that force increases as the velocity of the treadmill increases.

the propeller at take off power will exert the same force for the most part at all times in the take off run.

now there is a certain velocity of the treadmill that will exert an equal and opposite force on the gear. I personally believe that speed would be much larger than that of the liftoff speed of the airplane due to the rotation of the tires (slippage) but that would have to be determined by empirical evidence gathered by actual testing.

So, as the car pulls the mat in one direction there are three possibilities.

One: he is pulling at a speed slower than that required to exert a force on the gear struts equal to the force of thrust created by the prop and the aircraft will accelerate forward in air and lift off. (this is what happened in the example as the car accelerated from rest. rest = speed less than that required)

Two: he is pulling at a CONSTANT speed required to create a backwards force on gear strut which will equal the force created by the prop at take off power and the airplane will remain stationary with zero airspeed.

three: he is pulling faster than that required thus causing a greater force on the gear strut than that force by the propeller and the aircraft will get pulled backwards.

All these assume a constant takeoff power setting creating a relatively constant force (thrust vector)

Again, the trick is not to visualize this as a car pulls in one direction accelerating at the same rate the airplane normally accelerates. This caused a smaller drag force than that by the prop and the airplane took off.
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A better way to visualize this is to start off with the airplane tied down at the wings and an endless treadmill.

apply take off power and the airplane will strain forward against the ropes.

move the treadmill at a speed which generates a force great enough to relax the tension in the ropes.

That is your equivalent treadmill speed. (notice this is about force and not speed. It would take an experiment to determine what speed this would be and it would be different for every airplane based on rated takeoff power)

Now, any speed less than this will always allow the plane to accelerate forward as thrust is greater than drag and the airplane will take off.

Any speed less than this and the airplane will be drug backwards as thrust is less than drag.

Again, if you simply accelerate (starting at rest) the treadmill backwards like the car did on mythbusters the plane will always accelerate regardless of wheel speed as the takeoff thrust is greater than the drag created by the treadmill.

It is NOT about aircraft speed. It is about the force created by the propeller and its relation to the force created by the drag of the treadmill.

=======================
By the way... the more I think about this the more I wonder if it is even possible to pull the treadmill under the wheels fast enough to create a force large enough to equal the force created by the propeller. I wonder if the dang tires wouldn't fail first.

This post evolved after thinking through the free body diagram of the forces acting on this problem. they supersede my previous post where I used the words "matching the speed of the airplane."

After further thought.... I discovered that in my last post I didn't properly explain what I was thinking and how it actually was saying something the wrong way.

 

[sayrejm]

I think I effectively killed this thread. I am so proud of myself!

 

[ASA_Aviator]

Wow. This discussion. Again.

The treadmill will not hold the plane stationary because they are free rolling. Therefore the plane will take off. The wheel speed may be double, but the plane will fly.

Every.

Single.

Time.

This isn't rocket science folks, and those of you with degrees in engineering or whatever should be ashamed of yourselves.

 

[sayrejm]

That is exactly what I just said... However when you pull backwards it will cause the wheels to spin faster but there will be an increase in drag as a result of friction. Now it might be realistically impossible to run a tread mill that fast but it is mathematically possible to show that a fast enough treadmill will create enough of a force to counter the force of the prop. However, I think that it would have to be so fast that the tires would fail from spinning too fast.

 

[glasspilot]

I love how you start with, "I have a Masters in Aero Space engineering whatever" and said the plane won't fly.

Then you post a novel and talk yourself into the plane will fly as the tread mill would have to go soooo fast as to cause axle or tire failure to hold it still.

Glad to see your super duper masters degree in aero lift theory flux design didn't give you the education that you just gave yourself in just one post on FlightInfo.