Plane and a Treadmill - DPChallenge Forums

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

12/06/2006 05:10:44 PM · #179

Originally posted by BrennanOB:

Originally posted by Gordon:
No fundamental physical principles are broken here. The plane takes off.

Gordon, you have bee assumig a jet all along. Does a prop plane take off?

Yup. No difference in the argument. Treadmill matches speed, not force.

F=ma forward from the prop.

any backward force is a small component of the rotation applied to the wheels, due to the matching speed.

The treadmill doesn't accelerate to counteract the forward force on the plane, so it doesn't accelerate anywhere near quickly enough to create a Fbackwards to balance Fforwards

DjFenzl

12/06/2006 05:11:55 PM · #180

ok, so the treadmill is running 10,000 miles an hour, the plane suddenly slams on the breaks and the sudden back pressure hurls the plane into the air!!! (or disintegrates the plane)

did I get it right?

BrennanOB

Canon EOS-1D Mark II

12/06/2006 05:15:31 PM · #181

I agree that the treadmill isn't the issue, I just have trouble picturing a little Cessna prop pulling in enough air to create the needed lift all along the lenght of the wing given the static air it is in, and the ground friction ( not the wheels here, just the air mass being pull into the rotors )

Message edited by author 2006-12-06 17:16:55.

Canon EF 24-105mm f/4.0L IS

12/06/2006 05:18:12 PM · #182

Originally posted by BrennanOB:

I agree that the treadmill isn't the issue, I just have trouble picturing a little Cessna prop pulling in enough air to create the needed lift all along the lenght of the wing given the static air it is in and the groung friction ( not the wheels here, just the air mass being pull into the rotors )

You still seem to assume prop planes pull air in to cause lift. That seems to be the issue. They push forward against the air with the propeller, which creates forward thrust. That causes air to move over the wings. Because of the shape of the wings, lift is generated (by the difference in pressure in the gases traveling at different speeds)

Lower pressure above the wing, higher pressure below (because of the longer curved path on the top of the wing. The wing is pushed up, hence lift.

All the prop does is move the plane forward, it doesn't push air over the wings, other than indirectly.

Patents4u

Canon EOS-40D

12/06/2006 05:18:52 PM · #183

Originally posted by BrennanOB:

I agree that the treadmill isn't the issue, I just have trouble picturing a little Cessna prop pulling in enough air to create the needed lift all along the lenght of the wing given the static air it is in and the groung friction ( not the wheels here, just the air mass being pull into the rotors )

You seem to incorrectly assume the plane does not move relative to the ground, that the treadmill cancels the forward motion of the plane. We are not talking VTOL (vertical takeoff and landing) here, we are talking normal flight. To someone on the ground (not the treadmill), the plane moves right on by them and takes off.

Canon EF 100mm f/2.8 USM Macro

12/06/2006 05:27:37 PM · #184

I didn't read all the replies, so I'm sure this has been said.

The plane can only fly if it has a fast enough air speed to create enough lift under the wings to overcome gravity. It can only have enough air speed if it has enough thrust to overcome drag. It has nothing to do with how fast the wheels are spinning. I'm not sure I understood the original scenerio. If the runway was spinning in the same direction as the plane was going, then yes it would take off once it had enough speed to create the lift under the wings. If the treadmill was running opposite the plane and in essence the plane was sitting still even though the wheels were spinning, then no the plane will not fly. Good question though. ;)

Spork99

OM System OM-1

12/06/2006 05:28:47 PM · #185

Originally posted by jahoward:

. If the treadmill was running opposite the plane and in essence the plane was sitting still even though the wheels were spinning, then no the plane will not fly. Good question though. ;)

Read the thread, learn why you're wrong.

BrennanOB

Canon EOS-1D Mark II

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

12/06/2006 05:29:28 PM · #186

Originally posted by Gordon:

All the prop does is move the plane forward, it doesn't push air over the wings, other than indirectly.

I hold that the propeller pulls the plane forward, to created air speed, and that the wings use that air speed to generate the lift. In a situation where the air mass is moving with the plane, such as a sudden tail wind, the prop engine cannot create enough pull to maintain that lift.
On our treadmill, the air mass is static, and the only air speed the plane can use for lift is that which is generated by the plane itself. If there isn't enough air speed, the positive and negative pressures above and below the wing equalise and there is no lift.

mist

Canon EOS-60D

Canon EF 28-135mm f/3.5-5.6 IS USM

12/06/2006 05:30:37 PM · #187

You can figure this out without a magic treadmill, all you need is a wind tunnel.

Assuming that the treadmill cancels out any forward motion of the plane, as was assumed in the question, then if you just stick a plane in a really powerful wind tunnel, you have duplicated the scenerio.

Start the wind speed at zero and increase it. If the plane takes off, then that's the value of wind movement over the wings that you need to achieve take off. If it doesn't, then the plane would never take off. If the plane takes off, then it could take off from the treadmill provided there was an actual wind. In the right direction.

12/06/2006 05:34:36 PM · #188

Originally posted by Spazmo99:

Read the thread, learn why you're wrong.

I'd be interested to read the whole thing. Maybe tonight, but all I can say is the plane will not fly without air speed. If it is sitting still, the only airspeed it has is a head wind. If the headwind is strong enough, the plane might fly. But maybe I'm misunderstanding the scenerio. It the plane on a treadmill much like when I'm on a treadmill? I'm running as fast as I can, but not going anywhere.

Sigma 24-70mm f/2.8 EX Aspherical DF for Canon

12/06/2006 05:37:11 PM · #189

Originally posted by jahoward:

Originally posted by Spazmo99:
Read the thread, learn why you're wrong.

I'd be interested to read the whole thing. Maybe tonight, but all I can say is the plane will not fly without air speed. If it is sitting still, the only airspeed it has is a head wind. If the headwind is strong enough, the plane might fly. But maybe I'm misunderstanding the scenerio. It the plane on a treadmill much like when I'm on a treadmill? I'm running as fast as I can, but not going anywhere.

It is exactly not like that. Which is why the plane takes off.

hankk

Canon EOS-5D

12/06/2006 05:39:12 PM · #190

Originally posted by jhonan:

Originally posted by hankk:

Originally posted by jhonan:

Originally posted by hankk:
What is the speed of the plane relative to? What do we consider to have a speed of 0 MPH?

An observer at the side of the runway.

Assuming the observer is not on the treadmill-runway (I think that's what you meant) if the plane had a speed of 600 MPH going north, the treadmill would have a speed of 600 MPH going South.

Yes, sorry, that's what I meant. I assume the original vaguely-worded question implies that the treadmill's speed can adjust to make the plane appear stationary to an onlooker.

That's probably the trick part of the question. But if the plane is stationary with respect to the observer, it has a speed of 0 MPH. The treadmill can adjust to this speed :-)

12/06/2006 05:39:21 PM · #191

Originally posted by jhonan:

Of course, the beauty of a thread like this is that every time someone new joins the argument starts all over again... :)

Sorry about that. :)

Canon EF 100mm f/2.8 USM Macro

12/06/2006 05:40:47 PM · #192

Originally posted by BrennanOB:

On our treadmill, the air mass is static

No it isn't. The treadmill moves to match speed with the plane. This does not mean that the plane then stays still. That's the logical mistake that all of the people still on the ground are making.

The treadmill accelerates. This imparts rotational forces on the wheel. These forces mostly do nothing. A very small fraction of that force moves the plane backwards.

The plane accelerates forward. The treadmill speeds up. The wheels spin faster. The backward force on the plane increases another small amount. The plane still accelerates.

and so on and so on.

The treadmill matches speed. It does not match force (which would mean matching F=ma. This is much greater acceleration in the treadmill than that required to match speed with the plane - which is what the original question said. It matches speed. Which will in no circumstances, stop the plane from moving. Prop or jet.

FireBird

12/06/2006 05:41:12 PM · #193

Originally posted by Southern Gentleman:

and landings with the wind.

Sure hope I don't fly with you anytime soon. :)

Spork99

OM System OM-1

12/06/2006 05:41:16 PM · #194

Originally posted by jahoward:

When you run, your foot pushes on the ground to propel you forward. If you are on a treadmill, that same push only keeps you from going backward.

The plane does not push on the ground. It pushes the air behind the plane, which is not affected by the treadmill and generates thrust. The treadmill only serves to make the wheels on the plane turn faster, but does not retard the forward motion of the plane.

FireBird

12/06/2006 05:47:00 PM · #195

Originally posted by legalbeagle:

An interesting problem:

Or how to keep a DPC'er busy for a couple of
hours.