Mythbusters....

[ajstoner21]

I just watched the episode and the 'Mythbusters' didn't conduct the experiment according to the myth that's being discussed here. They simply simulated a tail wind. If the plane is moving forward through the air (in the episode it was) then of course it'll will take off when it reaches Vr. The wheels mean nothing.

If the conveyor is moving in the OPPOSITE direction of the plane at the exact speed of the plane then the plane will never see any forward ground speed or airspeed other than what the prop generates - and the headwind. The prop alone isn't going to generate enough wind (volume or velocity) for the wings to see Vr airspeed therefore forward motion will be required.

Myth confirmed.

Are you a licensed pilot?

You are correct in that, the prop wash alone wouldnt generate enough airflow for the wings to support the plane. The prop will however, generate a huge amount of force required to get the plane rolling, and overcome drag. This force is what pulls the plane forward through the air. Power is NOT delivered to the wheels. Let me say that again. Power is delivered to the propeller, not the wheels. If power WAS delivered to the wheels, like a car, you are right, in that the wing wouldnt see any forward speed. The only thing the conveyor will do, is make the wheels spin that much faster as the plane is taking off. You can argue there is additional force to overcome due to rolling friction in the wheels/bearings, but that is probably neglible.

By the way, the prop? it really isnt there just to keep the pilot cool. (I know... stop it in flight and the pilot will break out in sweats).

How do you propose they test the "myth being discussed here"?

 

[Palmpilot]

I thought trolling was prohibited.

It worked, though!

 

[Bob Noel]

It worked, though!

It was pretty feeble.

 

[MadseasoN]

Really?

Tell me, when you line up on the runway centerline, and apply full throttle - why does your airplane move?

Tim, today when I did that (short field practice) it didn't move until I released the brakes. I understand what the prop does and why the plane moves forward.

The plane needs forward movement to build up enough airspeed to fly, that is unless you fly in hurricanes or tornados. A magic/opposite running/speed matching treadmill would inhibit forward movement the same as my brakes did this morning. I think that is what the myth is about.

Tie down the rear of your plane with a cable and run the engine up to full throttle. Does it lift off the ground?

 

[ajstoner21]

Tim, today when I did that (short field practice) it didn't move until I released the brakes. I understand what the prop does and why the plane moves forward.

The plane needs forward movement to build up enough airspeed to fly, that is unless you fly in hurricanes or tornados. A magic/opposite running/speed matching treadmill would inhibit forward movement the same as my brakes did this morning. I think that is what the myth is about.

Tie down the rear of your plane with a cable and run the engine up to full throttle. Does it lift off the ground?

The plane DOES NOT need forward movement relative to the ground to fly but it does need relative wind. Think about why we land into the wind? We have maybe 70 knots indicated airspeed, but maybe 50 knots relative to the ground...

Try flying in a 50 knot headwind.... you might not be physically moving forward if your airspeed is low enough, but your wings think they are. If your plane is not tied down, your plane would still fly with the brakes locked, if you had a strong enough wind.

The treadmill does nothing to inhibit forward movement if the prop is what drives the plane through the air. you obviously do not understand what a prop does and what makes an airplane fly.


Note to self... stop feeding the troll

 

[MadseasoN]

The plane DOES NOT need forward movement relative to the ground to fly but it does need relative wind. Try flying in a 50 knot headwind.... you will not be physically moving forward, but your wings think they are. If your plane is not tied down, your plane would still fly with the brakes locked, if you had a strong enough wind.

Correct, an airplane does not need forward movement relative to the ground to fly. It needs forward movement relative to the air. You need airspeed, not groundspeed. That's established.

The treadmill does nothing to inhibit forward movement if the prop is what drives the plane through the air. you obviously do not understand what a prop does and what makes an airplane fly.

Um, yes a prop drives the plane through the air. This forward motion results in airspeed, the airspeed it needs to fly. But without the forward motion you won't have that airspeed. If this weren't the case then we could put on the brakes and run the engine up to full speed then leap into the air. I'm not sure why I'm having to explain this.

 

[Greg Bockelman]

MadseasoN, in the real world, would the treadmill in any way shape or form be capable of preventing the propeller from pulling the airplane through the air?

 

[ajstoner21]

What would you have done differently, if you were the mythbuster?

 

[MadseasoN]

MadseasoN, in the real world, would the treadmill in any way shape or form be capable of preventing the propeller from pulling the airplane through the air?

No, it wouldn't.

I guess my theory gives too much credit to the treadmill. My understanding of the myth is that the treadmill could [magically] compensate for the forward movement of the plane.

EG: I can 'ski' on a treadmill with a skateboard but eventually I won't have enough strength to overcome the friction of the wheels on the treadmill and won't be able to pull myself forward. So I guess if the plane had enough power it could overcome the effect of the treadmill and move forward.

And that guess is a compromise, because the magic treadmill would always win out.

 

[Greg Bockelman]

No, it wouldn't.

Kind of makes the whole argument moot, doesn't it? Since there is no way the treadmill can prevent the propeller from pulling the airplane through the air, there is no way the treadmill can prevent the airplane from taking off. In "Realville", that is.

I guess my theory gives too much credit to the treadmill. My understanding of the myth is that the treadmill could [magically] compensate for the forward movement of the plane.

Key work is "magically". There is no way the treadmill can offset the prop pulling the airplane through the air.

EG: I can 'ski' on a treadmill with a skateboard but eventually I won't have enough strength to overcome the friction of the wheels on the treadmill and won't be able to move forward. So I guess if the plane had enough power it could overcome the effect of the treadmill and move forward.

At what point would that be? The friction of the wheels is statistically insignificant in this exercise.

 

[Skip Miller]

MadseasoN, forget for a moment the difficulty of taxiing to the runway in this condition: If a plane were lined up and waiting on a runway of glare ice, as friction free as you can imagine it, could the plane take off from that position?

If not, why not?

Since the surface is close to friction free, could you take off with the parking brake firmly applied?

Now consider the treadmill. The wheels have very little traction (in the fore and aft direction) thanks to the roller bearings in the wheels. If the plane can take off on the ice, why can't it take off on the treadmill?

The thesis here is that it doesn't matter that the wheels are rolling or even in which direction. The only thing that matters is that the engine produces thrust and with the brakes off, the plane will roll down the treadmill. F=MA. When Vr airspeed is reached, pull back and flight happens!

-Skip

 

[Bobcat1]

So whaddya suppose this guy's relative wind is?

http://www.youtube.com/watch?v=jpep88CaXLw

 

[MadseasoN]

Kind of makes the whole argument moot, doesn't it? Since there is no way the treadmill can prevent the propeller from pulling the airplane through the air, there is no way the treadmill can prevent the airplane from taking off. In "Realville", that is.


Key work is "magically". There is no way the treadmill can offset the prop pulling the airplane through the air.


At what point would that be? The friction of the wheels is statistically insignificant in this exercise.

If you ever water ski or get pulled behind a car on a skateboard (yeah that happens a lot) you'll know that friction matters and eventually you would not be able to pull yourself forward to overcome that friction. Wheels help but they have limits.

 

[MadseasoN]

So whaddya suppose this guy's relative wind is?

http://www.youtube.com/watch?v=jpep88CaXLw

He had a tailwind there at the end.

 

[Cap'n Jack]

Tim, today when I did that (short field practice) it didn't move until I released the brakes. I understand what the prop does and why the plane moves forward.

The plane needs forward movement to build up enough airspeed to fly, that is unless you fly in hurricanes or tornados. A magic/opposite running/speed matching treadmill would inhibit forward movement the same as my brakes did this morning. I think that is what the myth is about.

Tie down the rear of your plane with a cable and run the engine up to full throttle. Does it lift off the ground?

Tying down a plane is different than placing it on a threadmill.

<SNIP>
Um, yes a prop drives the plane through the air. This forward motion results in airspeed, the airspeed it needs to fly. But without the forward motion you won't have that airspeed.

Nope- don't need motion relative to the ground at all. I remember as a student doing slow flight over Flemington, NJ as a primary student into the wind and hovering over the traffic circle.

Also- observe this:

If this weren't the case then we could put on the brakes and run the engine up to full speed then leap into the air. I'm not sure why I'm having to explain this.

I think you are neglecting that the effects of brakes on wheels are different than of a threadmill.

 

[Greg Bockelman]

Methinks some people can't be convinced.

 

[MadseasoN]

Methinks some people can't be convinced.

Can't be 'convinced' or can't be 'persuaded'?

Since discussing this here I do have a new outlook on the 'myth' but I still think it would not be possible if you strictly follow what the myth suggests.

 

[Greg Bockelman]

Since discussing this here I do have a new outlook on the 'myth' but I still think it would not be possible if you strictly follow what the myth suggests.

Some people tend to get wrapped up in the minutia of what they interpret the myth to suggest.

Forgetting all of that for a moment, in the real world, if you were to put an airplane on a treadmill that can match the airplane's forward acceleration in the opposite direction, will the airplane take off?

Why or why not.

 

[TMetzinger]

Can't be 'convinced' or can't be 'persuaded'?

Since discussing this here I do have a new outlook on the 'myth' but I still think it would not be possible if you strictly follow what the myth suggests.

There is one fallacy in the the way the myth is presented sometimes.

"The treadmill moves at the speed the airplane is moving in the opposite direction".

Well, at first, the plane isn't moving AT ALL. Then, because the treadmill and wheels are in direct contact, the speed of the treadmill is ALWAYS equal to the wheel speed.

Try this thought experiment:

Airplane on treadmill, brakes off, treadmill off.

Tie rope to nose strut and other end to tree off treadmill ahead of nose.
Turn on treadmill.

What happens? Airplane stays in one spot relative to the earth, treadmill and wheels are rolling.

Now, pull on the rope. Airplane starts moving closer (forward)relative to the tree. Wheels are now turning faster but you still have direct contact with the treadmill. Speed the treadmill up. What happens? The airplane keeps moving forward until-unless you get the treadmill and wheels moving so fast that bearings seize. And it will be a crapshoot as to whether the wheels or the treadmill seizes first. If the treadmill siezes, the airplane will not suddenly shoot forward.

If you're with me on the above, substitute "propeller thrust" for "pulling on the rope", and you should see the light.

If you're not, please take your airman certificate to the nearest FSDO and tell them you don't need it any more.

 

[wsuffa]

Amazing. 59 posts in THIS thread, and we still don't have agreement.

 

[TripleZ]

Amazing. 59 posts in THIS thread, and we still don't have agreement.

We could discuss upwind to downwind turn.

 

[wsuffa]

We could discuss upwind to downwind turn.

Or radio announcements: "Any traffic in the pattern, please advise".

 

[Aztec Driver]

There is one fallacy in the the way the myth is presented sometimes.

"The treadmill moves at the speed the airplane is moving in the opposite direction".

Well, at first, the plane isn't moving AT ALL. Then, because the treadmill and wheels are in direct contact, the speed of the treadmill is ALWAYS equal to the wheel speed.

Try this thought experiment:

Airplane on treadmill, brakes off, treadmill off.

Tie rope to nose strut and other end to tree off treadmill ahead of nose.
Turn on treadmill.

What happens? Airplane stays in one spot relative to the earth, treadmill and wheels are rolling.

Now, pull on the rope. Airplane starts moving closer (forward)relative to the tree. Wheels are now turning faster but you still have direct contact with the treadmill. Speed the treadmill up. What happens? The airplane keeps moving forward until-unless you get the treadmill and wheels moving so fast that bearings seize. And it will be a crapshoot as to whether the wheels or the treadmill seizes first. If the treadmill siezes, the airplane will not suddenly shoot forward.

If you're with me on the above, substitute "propeller thrust" for "pulling on the rope", and you should see the light.

If you're not, please take your airman certificate to the nearest FSDO and tell them you don't need it any more.

This is probably the best "easy" answer there is to this. And I agree with the last part.

And to the point of the wheel bearings seizing, the wheel speed will never be more than twice the rotation wheel speed, so the friction at those speeds are really insignificant.

 

[gkainz]

here's a wet treadmill ...

no takeoff ...

 

[Palmpilot]

My understanding of the myth is that the treadmill could [magically] compensate for the forward movement of the plane.

And there you have it.

 

[PilotAlan]

And there you have it.

It's the difference between magical thinking and a basic understanding of physics.

 

[Palmpilot]

It's the difference between magical thinking and a basic understanding of physics.

As soon as magic is introduced into the discussion, the answer becomes arbitrary. No wonder the argument never gets settled.

 

[MadseasoN]

Alright forget the magic.

Here's another flawed version of the experiment. As you can see at 2:28 he has to add additional power [thrust] to overcome the force of the treadmill pulling the plane backwards. THIS IS THE KEY.

His experiment is flawed because he didn't gradually increase the speed of the treadmill as the myth suggests. My point is IF you increase the speed of the treadmill AND the thrust of the plane simultaneously then eventually the plane would not have enough power to overcome this "initial friction" (as the video calls it). Run that treadmill up to 200mph gradually and see if the plane can keep up. No way.

Plus the power to weight ratio is unreaslitic for those little model airplanes.

http://www.youtube.com/watch?v=4owlyCOzDiE

 

[ajstoner21]

Alright forget the magic.

Here's another flawed version of the experiment. As you can see at 2:28 he has to add additional power [thrust] to overcome the force of the treadmill pulling the plane backwards. THIS IS THE KEY.

His experiment is flawed because he didn't gradually increase the speed of the treadmill as the myth suggests. My point is IF you increase the speed of the treadmill AND the thrust of the plane simultaneously then eventually the plane would not have enough power to overcome this "initial friction" (as the video calls it). Run that treadmill up to 200mph gradually and see if the plane can keep up. No way.

Plus the power to weight ratio is unreaslitic for those little model airplanes.

http://www.youtube.com/watch?v=4owlyCOzDiE

How do you propose this experiment be done, to test the "myth being discussed here"?



How much force due to the resistance of rolling do you think a Cessna wheel will generate?

 

[MadseasoN]

How do you propose this experiment be done, to test the "myth being discussed here"?

The guy in the video was close except he increased the speed of the treadmill in steps rather than gradually. If you increase the speed in steps and allow the plane to 'catch up' then it's just like starting over from 0 mph. We already know the plane has enough power to move forward from 0 mph.

You know, now that I think about it you would need a treadmill that could infinately accelerate and have no top speed. Back to the magic!

 

[Palmpilot]

The guy in the video was close except he increased the speed of the treadmill in steps rather than gradually. If you increase the speed in steps and allow the plane to 'catch up' then it's just like starting over from 0 mph. We already know the plane has enough power to move forward from 0 mph.

You know, now that I think about it you would need a treadmill that could infinately accelerate and have no top speed. Back to the magic!

I'm not clear on what you're matching the speed of the treadmill to. Can you give a clear and unambiguous description?

 

[tyndall]

I think I see where MadseasoN is going with this. I believe Mystbusters calls this "ramping it up", by using unrealistic parameters to get results that are otherwise impossible to achieve.

Spinning the treadmill fast enough could, in theory, cause enough friction to prevent takeoff due to lack of power. However, that speed would be near impossible to achieve. Before you'd run out of power, the tires would probably overspeed and explode or you'd have a bearing failure.

If a 172 can lift off at 70kts, you'd have to suck out enough power with the treadmill to stop it from accelerating to prevent lift off. As we are only dealing with wheel friction, what treadmill speed is necessary to suck that much power? 300mph? 400? more? What are the odds of the wheels staying together to find out?

At what point does a myth become so unrealistic that its pointless to prove? Try this myth and get back to us, "if you drive your completely stock Ford pickup at 500 mph, uphill, at night during a solar eclipse, with an Elvis and Justin Beiber duet playing live on the radio, the ashtray will implode.

 

[TMetzinger]

Let's do this.... take a 100 mile treadmill. Put a 172 on one end of it, with the brakes set. Start the treadmill moving at 200 MPH (which is much faster than a 172 ever goes).

So your 172 is now moving backwards with the treadmill and will fall off it in 30 min.

Now in the 172, release the brakes. What happens?
Start the engine. What happens?
Increase to takeoff thrust. What happens?


hint: THE EARTH DOES NOT MATTER. Only the treadmill surface and the air matter.

 

[jesse]

Let's do this.... take a 100 mile treadmill. Put a 172 on one end of it, with the brakes set. Start the treadmill moving at 200 MPH (which is much faster than a 172 ever goes).

So your 172 is now moving backwards with the treadmill and will fall off it in 30 min.

Now in the 172, release the brakes. What happens?
Start the engine. What happens?
Increase to takeoff thrust. What happens?


hint: THE EARTH DOES NOT MATTER. Only the treadmill surface and the air matter.

You'd never get there. The airplane would stay in one place on the treadmill, but the aircraft would experience a 200 mph wind (headwind or tailwind) and quickly depart the treadmill.

 

[TMetzinger]

You'd never get there. The airplane would stay in one place on the treadmill, but the aircraft would experience a 200 mph wind (headwind or tailwind) and quickly depart the treadmill.

RATS, you're right.

 

[mikea]

We could discuss upwind to downwind turn.

I know! I know! I know! Richard Collins says you always add airspeed when landing with a tailwind!

 

[Palmpilot]

I know! I know! I know! Richard Collins says you always add airspeed when landing with a tailwind!

Yikes! Does he really say that?

 

[timwinters]

And to the point of the wheel bearings seizing, the wheel speed will never be more than twice the rotation wheel speed, so the friction at those speeds are really insignificant.

If you're referring to my post then please recall that I was commenting on the cartoon that had been previously posted. In the cartoon's problem (reposted below for easy reference), the treadmill would travel as fast as the wheels, not as fast as the airplane. It's impossible for the treadmill to match the speed of the wheels if the airplane is moving so, by attempting to do so it would accelerate to infinity...or until the bearings seized...or until the tires were spinning fast enough to generate enough friction to hold the plane stationary...which I don't think would occur until the bearings seized.

Regardless, this version of the problem (with the treadmill matching the speed of the wheels instead of the airplane) would be virtually impossible to test. And I think this is where madseason is getting confused. He thinks the treadmill is supposed to match the wheel speed...not simply the airplane speed. It would indeed be impossible for the plane to take off in that scenario. But that's not the scenario being contemplated here...matching the AIRPLANE speed is.

 

[Palmpilot]

It's impossible for the treadmill to match the speed of the wheels if the airplane is moving...

Why? If the plane was moving forward at 50 knots, wouldn't the wheels also be moving forward at 50 knots? Why couldn't the treadmill then be moving backwards at 50 knots (it it were expensive enough)?

 

[PilotAlan]

Why? If the plane was moving forward at 50 knots, wouldn't the wheels also be moving forward at 50 knots? Why couldn't the treadmill then be moving backwards at 50 knots (it it were expensive enough)?

No.
The wheel movement is based on airspeed. The whole thing of the treadmill matching wheel speed is a physical and mathematical impossibility.

Think of this:
Treadmill speed = wheel speed (your hypothetical)
However, wheel speed only equals treadmill speed at zero airspeed.
If you accelerate the treadmill to 10 knots and hold the plane still, the wheels are spinning at 10 knots.

Now, if I accelerate the plane to 1 knot airspeed, then:
Treadmill speed = 10 knots, and wheel speed now = 11 knots.
If you accelerate the treadmills to 11 knots, the plane is still moving forward at one knot, so wheel speed = 12 knots.
With any forward speed, the treadmill would instantly have to accelerate to infinity.
Therefore, the hypothetical of treadmill = wheel speed is not possible.

The myth is that the treadmill matches the forward speed of the airplane. IOW, if the plane is moving forward at 10 knots, the treadmill is moving backward at 10 knots. Wheel speed equals 20 knots.
Making that assumption, what happens?