Riddle me this batman

[DVINNY]

Time to write to mythbusters.

That's the best idea that I've heard yet.

And I'm serious. we should.

 

[civengPE]

DVINNY,

I went to mythbusters site and they have a 9 page forum discussion on the exact same topic.

It looks like this debate has made the rounds before.

 

[civengPE]

Ok. This is my last attempt to convince the NON-BELIEVERS! rayers: :brick:

Assume that the wheel hubs are frictionless just for argument sake.

When you turn on the conveyor, the plane, with the engine off, does not move.

now provide thrust from the engine. Viola! The damn thing starts to move.

since the wheel hubs have very little friction compared to the thrust the plane will take off!!!

Just draw a Free Body Diagram.

Rant Over!!!

 

[civengPE]

Check it out.

http://community.discovery.com/eve/forums/...76/m/5471975538

 

[MA_PE]

I'll try it a again. Planes are not rockets. they don't fly from thrust alone.

They NEED the lift from the air traveling over/under the wings. Whena a plane is stationary the whhels support it. At the end of the runway thrust is applied against the static air mass behind them. Note the thrust nneds to react against something because in a vaccuum you wouldn't get any tendency for forward motion. As the plane starts to move forward it pushes through the atmosphere causing lift. The amount of lift is proportional to the velocity of the plane relative to the atmosphere. In the subject problem you don't allow the plane to got forward. Instead, the ground moves back so the plane stays in its initial location. However, the air mass moved by the turbines needs to go somewhere right? Air is compressible so it can displace rather readily allowing the plane to slip back with the conveyor. At the initial start up, I believe this scenario will happen because the thrust is still ramping up. Each time the plane tries to move forward the conveyor restores it ot the initial location. Work by the turbine is used to move air from in fornt of the plane to the rear. This will continue until one of the following conditions controls:

1) Resistance to the thrust behind the plane (I believe this is controlled by air density which is a function of temperature and velocity) remains sufficient to increase the velocity of the moving air mass around the plane to a critical velocity where the plane will rise due to the lift provided by the air traversing the wings.

2) There is not sufficient resistance to the thrust to increase velocity and there is not enough lift to raise the plane off of the conveyor so the plane continues to "roll" in place ad infinitum.

3) the air mass moved by the jets is sufficient to saturate the area behind the jet such that air behind the jet is no longer compressible, a net forward thrust develops which overcomes the normal force of the plane, the jet breaks free and launches up or forward off of the conveyor with the wheels spinning at inifinite RPM's and crashes to a violent death because ti still hasn't reached critical velocity relative to the local ambient air to fly.

 

[civengPE]

"In the subject problem you don't allow the plane to got forward. "

Thats it! The plane does go forward. There is nothing holding it back!

 

[EdinNO]

MA'a theory sounds the most correct to me. It MAY rise into the air theoretically, but I ain't buying no ticket for that flight!

Ed

 

[civengPE]

Ed, It won't rise into the air. It will take off normally.

There is no force to counteract the thrust. Therfore the plane accelerates.

 

[EdinNO]

Again, I'm still not 100% sure on this one. MA's latest 3 scenarios seem the most logical to me. Maybe no one else has broken it down explicitly enough for me to see it any other way.

Theoretically, MA's post covers three possibilities. Perhaps the jets do induce movement of the overall surrounding air mass across the wings. That could, possibly happen I suppose. I'm not arguing that EVENTUALLY, this could occur. Would it be enough movement, enough velocity to lift the plane? I don't know. I wouldn't want to be on it.

I would like to see a concise, scientific arguement on both sides so I can make a better juudgement.

Either way, great discussion from both camps.

Ed

 

[EdinNO]

I think that in MA's first scenario, he is agreeing that it will fly and take off.

Not sure though.

 

[MA_PE]

If a plane required the ground to be stationary in order to fly, sea planes wouldn't be able to take off.

OK we'll use your example. the sea plane is on a river the river is running wildy. The seaplane goes to take off by running up river. If the wind over the river and the river flow are traveling at a fixed velocity wich is equal to his critical velocity, I predict the following will happen. As he points up river and throttles up the relative poisiotn of the plane will be moving downstream. The rate he moves downstream will decrease as he throttles higher because his upstream velocity is increasing. (Remember kids, change in velocity with time is acceleration). He continues to accelerate until he reaches critcial velocity at which point he is now stationary with respect to his position over the river and he is airborne because he has enough lift to raise the plane.

The key is that the air around the plane needs to move at the critical velocity to lift the plane.

I don't believe that a jet, allowed to slip downstream would be able to take off because I don't think thrust alone is sufficient to over come the loss of ground during the start up.

Come to think of it. Are there any jet sea-planes? That might answer the question.

FWIW: I'm having a good time.

 

[petergibbons]

The plane takes off. The belt can't counteract the acceleration of the plane. The belt REACTS to the plane so the belt acceleration would always occur after the acceleration of the plane thus the plane moves forward. It might take a long time for the plane to reach the required speed for lift-off though. But what do I know?

BTW: My head hurts!

 

[civengPE]

NEED BEER HEAD HURT.

Beer was always one of my best design aids in college. We had a fridge in the concrete lab that was supposed to be for samples etc. We kept it full of beer.

Our profs would come down and have a few with us. it was always a good time.

 

[MA_PE]

MA_PE, you are thinking about a sail boat, which requires the wind to move. A plane does not need moving air, it needs to move against air by way of thrust. The air could be moving or stationary and a plane would fly from solid ground or a river. I am having a good time as well. However, the truth of the matter is that as a damn civil engineer, I would just design a better runway, and not fool around with the damn thrust and all that freaking mechacal BS.

sapper:

do you read my posts at all? FWIW: I'm an ME with a structural PE.

Planes don't need thrust to fly they need a relative velocity between the plane and atmosphere sufficient to produce enough lift under the wings to make the plane fly (ever heard of a glider?)

It has nothing to do with sailboat. Of course that powered by the realtive velocity between the air and the ground.

Using the same example if you had the wind blowing upstream at the same speed that the river is flowing down stream, you could theoretically have a sailboat that is filled with wind but making no headway upstream.

Just like uncles and cousins, it's all relative.

 

[DVINNY]

I'll try it a again. Planes are not rockets. they don't fly from thrust alone.

No,

They don't fly from thrust alone. BUT THEY DO MOVE FROM THRUST ALONE.

they fly because the wind gets under the wings

 

[EdinNO]

Peter's theory makes sense. The conveyor would react to the palne's motion and eentually, before the conveyor could react and "catch up" to the mothion of the plane, the plane could pop up into the air from that second or two of forward motion relative to the air. It would be sketchy in my mind, but I suppose it could happen. Its all theoretical and I woudl still think the plane could crash pretty quickly if things weren't just right.

But then again, what if we took the original statement to mean that the conveyor was perfect and there were no lag in the conveyor's speed to the plane's speed.

I still see the best scenarios so far as being the three by MA just a few posts up.

Ed

 

[DVINNY]

You guys are thinking WAY too much into this.

The plane would not lag or anything. The wheels would be spinning twice as fast, but the plane would take off just as it would on an asphalt runway.

 

[civengPE]

Or concrete. Don't forget about concrete. everyone always forgets about concrete.

I think I'm losing it.

Is it just me or does it annoy you when peole refer to concrete as cement? :brick:

 

[EdinNO]

What do you mean by the wheels would spin TWICE as fast? Not following you on this one?

Ed

 

[DVINNY]

Is it just me or does it annoy you when peole refer to concrete as cement? :brick:

My wife was doing it last night.

Said her parents need to cement their whole walk.

Go figure.