Physics Question for you.....

[Alex Franke]

I thought that Bernulli's principle (that's the air pressure above/below, right?) was only a minor part of the equation -- that the primary factor was in fact the air bouncing off the bottom of the wing.

It doesn't matter if the conveyor is at a constant speed or accelerating. It's impossible for the conveyor to accelerate enough to cancel out the forward movement of the airplane unless the wheels stop functioning properly.

Hmmm... Stop the plane, release the brakes, turn off the engines, throw the conveyor into reverse and crank it up as fast as it will go. Once the wheels start rolling backwards, it'll accelerate very slowly at a rate related to the friction in the wheels, right? So then at some point it'll reach a terminal velocity and you still won't be able to fling the plane into the air, so long as the friction in the wheels remains the same -- even if the conveyor is going a zillion miles a hour in a seemingly favorable direction. Is that right?

[JimD]

While corrected (correctly) once already (ideal gas law is not very applicable, Bernoulli's principle is the most typical explanation of lift but is only a partial explanation) I will still attempt another input.

It seems to me that the key confusing thing about the question is whether there is motion of air over the wings of the plane. If the air motion is higher than the take off velocity for the airplane, it will fly. If it is slower, it will not.

With respect to the conveyor belt questions, I think the key thing is to note is that the conveyor belt cannot be moving at a constant velocity, as they normally do, and keep the plane stationary. The engines supply thrust which results in acceleration - increasing velocity. A constant velocity conveyor belt would initially move the airplance backward and if the engines were powerful enough and/or the frictional forces low enough, the airplane would ultimately overpower the constant speed conveyor belt. For the plane to remain stationary and lift to not be generated, the conveyor needs to accelerate at the same rate in the opposite direction as the airplane. Moving at the same rate could mean it accelerates at the same rate but I think it is more clear to say it will accelerate at the same rate but opposite direction than to say it moves at the same rate - constant rate movement is what we assume for a conveyor which would allow the airplane to accelerate and overcome the conveyor (possibly what happened on mythbusters).

Jim

[leehljp]

Well, it does matter in some ways... How do you develop acceleration if your thrust is being delivered through a drivetrain to the wheels? In that case you need to have a high degree of friction between the wheels and the conveyor (so the engine can turn the wheels which in turn push *relative* to the conveyor not the ground).

This follows the same 2x wheel speed issue as before, but now the engine is coupled to the wheels instead of letting them spin freely. Unlike the propeller engine which only needs to overcome a little extra wheel speed friction the drivetrain engine needs to drive the wheels twice as fast. If you can normally go 60mph in your car at 4000 RPM you would now need to go 8000 RPM to maintain a 60mph ground speed (120mph relative to the conveyor).
Kristofor.

I FINALLY SEE CLEARLY what "Take Off" people are talking about. Yes it will.

In my profession and with a few knot headed people, I have to split hairs so much that I am just used to it. My NEXT illustration was to place an elevated "stop" a few inches over the rolling runway in front of the wheels to guarantee that the speed of the wheels matched the speed of the rolling runway WITHOUT FAIL, in which case - NO it will not.

THANKS KRIS, your explanation triggered that "Ah Ha"!

[Wood_workur]

even if the airspeed were precisely matched to conveyor it would take off. thats the speed in the cockpit. if it was the ground speed then it would not take off.

and I love those dovetails. I might just have to use those in the blanket chest I'm working on.... thankfully I have a lot of scraps to practice.

[Kristofor]

P.S. I have to add: All the arguments about the plane's thrust acting on the air were also misleading. It doesn't matter where the plane is getting the force from. You can tie a rope on the plane's nose with the plane's engine off, apply enough force, whether by several guys pulling or with a truck or whatever, and the plane will still move. Move fast enough, and the plane will take off. Of course, with the engines off, bets are off on how long it stays afloat. But it will take off.

Well, it does matter in some ways... How do you develop acceleration if your thrust is being delivered through a drivetrain to the wheels? In that case you need to have a high degree of friction between the wheels and the conveyor (so the engine can turn the wheels which in turn push *relative* to the conveyor not the ground).

This follows the same 2x wheel speed issue as before, but now the engine is coupled to the wheels instead of letting them spin freely. Unlike the propeller engine which only needs to overcome a little extra wheel speed friction the drivetrain engine needs to drive the wheels twice as fast. If you can normally go 60mph in your car at 4000 RPM you would now need to go 8000 RPM to maintain a 60mph ground speed (120mph relative to the conveyor).

I don't know if average plane engines have enough beef to provide double the needed RPM for takeoff without blowing up, if not they could never takeoff if they were driven by their wheels. Of course, if they did take off that way I know they wouldn't get very far since there's not a lot of thrust generated by spinning your tires in the air.


Kristofor.

[Russianwolf]

A recent episode of MythBusters proved that the plane will take off.

They put a Piper Cub on a very large conveyor belt. The motion of the conveyor belt pulled enough air under the plane to give it lift.

Gene

Hi Gene,
The conveyor has little to no effect on the air. It's just that the conveyor also has little effect on the plane's ability to propel itself throught the air to achieve lift.

[gfunderb]

The Plane Will Take Off!!

A recent episode of MythBusters proved that the plane will take off.

They put a Piper Cub on a very large conveyor belt. The motion of the conveyor belt pulled enough air under the plane to give it lift.

Gene

[Russianwolf]

Here's the opened box. (Again, this isn't mine) It was cut using an Incra Jig from what I understand.


Although this isn't the case in this one, I've seen others that have two parrallel dovetails so that each of the two hidden faces of the box would also have a dovetail in them. All of them are great work IMO.

[Alex Franke]

As an example. Below is a picture of a box (not mine). How do you open it?

I could see that working if the joint formed an arc of a circle...

An that's the craziest dovetail I've ever seen. It must go together at 45 degrees, right? Did you cut that?

[leehljp]

Going with dovetails.

Just how are these put together or taken apart?

[Russianwolf]

How does answering BT3Central's physics questions make you a better woodworker?

The best woodworkers I've ever seen have a knack for thinking through the problems that the wood is throwing at them.

Anybody can slap a couple pieces of wood together and call it a box.

The better woodworkers take into account the grain orientation and how the joinery is going to be affected over the life of the box.

So thinking through things and thinking outside the box can indeed help you be a better woodworker.

As an example. Below is a picture of a box (not mine). How do you open it?

[Anna]

Yeah, I think I get the whole friction part. Regardless of how that plane is connected to the ground, there's some about of friction, no matter how trifling. Once the engines overcome this friction, it'll take off -- unless the conveyor can somehow add more friction to compensate (e.g. plane on skids and conveyor getting more and more rubbery).

But once the initial friction is overcome by thrust, you can't add more drag just by moving the conveyor faster.

Is that right?

So it's physically impossible for any conveyor (unless it can dynamically increase the friction) to make a plane appear stationary to the tower. Right?

Yep, that's pretty much it. The part about the plane's speed is also moot because the treadmill's speed, whether it matches the air speed or ground speed, is immaterial.

I made the same assumptions with respect to being stationary as seen from the tower because my point of reference are cars and people on treadmills. Now, I think I now just a little bit more about planes.

P.S. I know this is making me a better woodworker at some level. I just know it!

I sure hope it's making me one. Lord knows I spent enough time on this riddle over the weekend. Maybe that should be the next "physics" question: How does answering BT3Central's physics questions make you a better woodworker?

[Russianwolf]

So it's how you read the question. All technicalities aside, if you read it as "The plane will somehow be kept stationary as viewed from the tower", then it would be correct to conclude that the plane won't take off. After all, you can make leaps like this in thought experiments, can't you?

P.S. I know this is making me a better woodworker at some level. I just know it!

Nope, it's not how you read the question, it's your knowledge of all the parts that are factors and your assumptions from them.

Most people don't realize that the wheels are freespinning and assume that the tire rotation is what moves the plane. The reality is the plane's movement and the wheels; contact with the ground is what rotates the wheels.

The first assumption will lead you down the wrong path and the second assumption send you along the correct one.

The question is worded in a way that MIGHT encourage the wrong assumptions, but all the information you need is there.

[Alex Franke]

The original question simply states that the conveyor matches the PLANE'S speed. Airplane's speed is measured as air speed, not wheel speed like in a car. . . .The speed of the plane can indeed be matched by the conveyor if that speed is the air speed.

Hmm. This is why I'm not a physicist. To me, the original question suggests that the plane would somehow appear stationary to the tower.

Alex, I think the "trick" to the question is that the plane's wheels are practically frictionless. I mentioned in one of my posts that if the system is frictionless, the plane will takeoff. I didn't realize that in the real world case, the system does approximate a frictionless system

Yeah, I think I get the whole friction part. Regardless of how that plane is connected to the ground, there's some about of friction, no matter how trifling. Once the engines overcome this friction, it'll take off -- unless the conveyor can somehow add more friction to compensate (e.g. plane on skids and conveyor getting more and more rubbery).

But once the initial friction is overcome by thrust, you can't add more drag just by moving the conveyor faster.

Is that right?

So it's physically impossible for any conveyor (unless it can dynamically increase the friction) to make a plane appear stationary to the tower. Right?

So it's how you read the question. All technicalities aside, if you read it as "The plane will somehow be kept stationary as viewed from the tower", then it would be correct to conclude that the plane won't take off. After all, you can make leaps like this in thought experiments, can't you?

P.S. I know this is making me a better woodworker at some level. I just know it!

[JeffG78]

19 pages........ Do I hear 20??

This is nothing Mike. Over at the Discovery Channel's site, they were nearing 100 pages just a few days after Mythbusters aired. They have probably doubled that by now.