A jet plane on a large treadmill

**Mustonen** · 10-20-2016, 04:27 AM

The wheels operate under the same mass*acceleration paradigm. Less mass, so they'll have to make up for it with acceleration.

The friction thing really isn't all that interesting to the discussion at this point.

At least read the last page or two of the discussion before launching into your lecture, professor.

**jono** · 10-20-2016, 07:05 AM

As has been noted, even "speed of the wheels" is ambiguous; I suppose most of the recent discussion could have clarified that we were taking it to mean rotational velocity rather than translational as billyk takes it. Both are valid assumptions, which just goes back to interpreting the question.

**billyk** · 10-20-2016, 07:19 AM

The wheels are part of the airplane and accelerate at the same rate as the airplane does. So, if the airplane (and its wheels) are moving at 200 mph with respect to its inertial frame of reference (which is the earth, not the runway) it will take off because it has wind moving over its wings. It doesn't matter that the conveyor runway is moving at 200 mph in the opposite direction, unless that causes the wheel bearings to seize. OK done here.

**jono** · 10-20-2016, 07:49 AM

The answer to the question assuming speed means translational velocity was well and truly hashed out years ago. In the case of the question you addressed that is still an assumption, however. And as you know from basic physics, unless a question is unambiguous you state your assumptions and solve accordingly. I think it's clear that really wasn't the intent of this question.

**Brock Landers** · 10-20-2016, 08:48 AM

Originally Posted by billyk

OK done here.

promise?

p.s. I think you guys are overlooking the role friction plays

**jono** · 10-20-2016, 09:17 AM

Originally Posted by Brock Landers

p.s. I think you guys are overlooking the role friction plays

Yes and no; I don't see friction changing the answer from"yes" to "no," so in that sense it can be neglected. But if you want to build the impossible treadmill, yeah, it needs to be considered both at the tires and bearings and most importantly in the treadmill itself. Course in that case you have to look at response times for power sources and inertia of the treadmill system, so that problem has a couple more levels.

Hey east or bust, if you find some willing students, there really should be a kickstarter or something so all the internet can participate. Bet putting this out of its misery would be worth millions. I'd give a buck to see a PID controlled treadmill at, say, 1:10 scale get the belt speed up to 80 mph while keeping a model plane within 6" of its starting position. (As long as the only sensed variable is wheel speed, no cheating.)

**Brock Landers** · 10-20-2016, 11:23 AM

Look harder at the friction component. The answer is right in front of your nose.

**TNKen** · 10-20-2016, 01:41 PM

It could never happen. The wings will get hung on the hand-grips.

**Cross** · 10-20-2016, 04:54 PM

If any part of the friction in the wheels was to keep the planes engine from overpowering the treadmill and moving forward landing would be a serious issue for a plane. The friction needed in the wheels to keep the plane from moving on the treadmill would require the wheels to be spun up to speed prior to a plane landing on a normal runway.

The friction of a plane using floats or skis is far greater then that of wheels. Yet those planes are able to over come that and take off. A float plane going upstream or downstream on a river will still take off at the exact same airspeed. Now the speed of the floats moving through the water will vary by the speed of the waterflow.

**1080Rider** · 10-20-2016, 05:09 PM

Originally Posted by TNKen

It could never happen. The wings will get hung on the hand-grips.

FTW!!!

**abraham** · 10-20-2016, 06:05 PM

Originally Posted by jono

Hey east or bust, if you find some willing students, there really should be a kickstarter or something so all the internet can participate. Bet putting this out of its misery would be worth millions. I'd give a buck to see a PID controlled treadmill at, say, 1:10 scale get the belt speed up to 80 mph while keeping a model plane within 6" of its starting position. (As long as the only sensed variable is wheel speed, no cheating.)

In a wind tunnel or not?
And how do you want the thrust scaled? You can do crazy things with RC planes because the thrust to weight is so damn high

**Jong Lafitte** · 10-20-2016, 06:47 PM

Originally Posted by TNKen

It could never happen. The wings will get hung on the hand-grips.

LOL! That's it! I think a B1-B or an F-14 could do it, but not a 747. As the wings approach the hand-grips, you sweep the wings backwards and fly through it Cole!

**jono** · 10-20-2016, 07:40 PM

Originally Posted by abraham

In a wind tunnel or not?
And how do you want the thrust scaled? You can do crazy things with RC planes because the thrust to weight is so damn high

No wind tunnel. Everything to scale, thrust, wheel inertia, mass; ducted fans would be ideal, preferably with electric power that's tuned to mimic throttle response of the turbines. Maybe just two engines at 2x power for simplicity. It would be like getting the real thing to spin its wheels up to 800 mph without moving more than 5'. That would be impressive. And at scale there's no worries about shock waves and the tires could be strong enough not to end in a fiery crash. Not that that wouldn't be good too.

**Adolf Allerbush** · 10-20-2016, 10:44 PM

This thread always cracks me up. Ever notice that everyone assumes that the 747 is sitting on the treadmill with its landing gear down? That's not stated in the original problem tho.

**flyman683** · 10-20-2016, 11:20 PM

What if the plane is on skis?!!?

MIND BLOWN.

**Adolf Allerbush** · 10-20-2016, 11:22 PM

Originally Posted by flyman683

What if the plane is on skis?!!?

MIND BLOWN.

What if it's on its tail?

Point is, just about everyone assumes a lot more than is provided in the original problem to come to their conclusions. Its fucking hilarious to observe.

**Core Shot** · 04-13-2018, 08:47 PM

34 pages debating a poorly worded question

The answer depends on the assumptions you make.
You can’t keep the plane stationary without applying brakes.

Frictionless brakeless wheels make the treadmill irrelavent. In such a case, with the engines off, the treadmill can run forward or reverse 10,000mph and the plane does not move. It also does not take off since the engines are off, it’s not moving, and hence no lift.

When you add the condition that the treadmill keeps the plane from moving through space, you necessarily assume brakes are being applied to counteract the engine thrust. Whether the treadmill moves or not, the brakes keep the plane from taking off.

ASS U ME

**jono** · 04-13-2018, 09:03 PM

It's not the brakes, it's the hand grips. You just gotta skip to the last page sometimes.

**billyk** · 04-13-2018, 09:33 PM

You guys trapped me once. Not again.

**iceman** · 04-13-2018, 09:36 PM

It seems like it would be hard to get a jet plane up on a treadmill in the first place.

**Viva** · 04-13-2018, 10:13 PM

Originally Posted by iceman

It seems like it would be hard to get a jet plane up on a treadmill in the first place.

And why would the plane even want to go on the fucking treadmill in the first place? Isn't Crossfit all the rage, now?

**DJSapp** · 04-13-2018, 10:47 PM

But how do you get a 747 to do burpees?

**Art Shirk** · 04-13-2018, 11:10 PM

20 more pages here we come.

Sent from my iPhone using TGR Forums

**Gepeto** · 04-14-2018, 12:11 AM

Hey, threads with cliff notes - sweet - never had the time to read the good mixed with the dribble.

All but your last assumption is true: Whether the treadmill moves or not, the brakes keep the plane from taking off.

*given the frictionless contact patch, the 747 is inert with the treadmill @ 10,000mph.

no brakes necessary