will it take off?

[Wayne County Hose]

This thinking stuff is too much like work for me. I'm still trying to figure out if the horse pulls the cart or pushes it. You guys are up to airplanes? And Egon claims to be behind the times.

 

[ray66v]

["I'm still trying to figure out if the horse pulls the cart or pushes it".]
LOL, How many posts will there be over this?

 

[David Cockey]

Welcome to the thread David. Nothing wrong with what youve deduced. However, there is another way of looking at this which makes more complete use of the physics. It is essentially an extrapolation of the conveyors effect on the situation. I believe it came up to counter the false claim that the conveyor could have no inhibiting effect on the thrust of the plane. The physics of this counter thrust via rotational acceleration of the wheels is described pretty well starting about 1/3 of the way in the thread in the 300's. I dont believe many understand it.
larry

The other way of looking at is wrong. The conveyor does not inhibit the thrust of the plane. The wheels and tires spinning at twice the normal speed due to the conveyor moving will slightly slow the acceleration of the plane, but the plane will still accelerate to take off speed. The inertia of the wheels and tires is effectively doubled compared to a takeoff on a normal runway, but the effect on acceleration of the rotational inertia is small compared to the inerita due to the mass of the airplane.

It seems that many don't understand a couple of things. The first is that the wheels on airplanes do not provide thrust. The thrust which moves the airplane comes from the propellers and engine exhaust. Also obviously misunderstood are basic physics, Newton's laws of motion in particular.

It is amazing how this thing hangs around.

 

[Wayne County Hose]

This whole argument is just inconceivable to me. This is the type of discussion that happens after a few beers at a bar, not on a forum full of intelligent (?) men. Mythbusters is a decent half hour show that drags on to an hour. I have some show suggestions for them.

1. I think they should unzip their flies and walk thru a chicken coop. Let's see if the early bird really does gets the worm.
2. See if the dog didn't stop to take a poop if he would have caught the rabbit anyway.
3. When we were kids and we said/did something dumb, my grandma would tell us to "go poop(edited) in our hat and pull it over our ears." Let them test if this would make them smarter.
4. They should set up microphones in the woods to listen for a tree falling. If nobody is around, will it make a sound?
5. Test if the amount of beer in a man's system is proportional to the vision impairment of beer goggles. On a side note, this will also determine whether a 2 at 10:00 is really a 10 at 2:00.
6. They should see if the following phrase is true: A skunk sat on a stump, the skunk thunk the stump stunk, the stump thunk the stump stunk.
7. Staying on their airplane theme, they could place a camera in the cockpit of their plane and see if he who fly upside down really do have crack-up.
8. Last but not least, they should check all the forms of capitol punishment, on themselves, to see if any of them are really "cruel and unusual."

 

[patrick_g]

Just to throw a stick in the ocean of theoretics:

Replace the wheels with skis (or ice skates) and ice the belt. There just went all the energy being applied to spinning up wheels, etc.

Harry K

Very good Harry, then the magic conveyor will really have to rev up so the friction of the skis will be sufficient to equal the prop thrust and hold the planes forward speed to zero and eliminate lift.

Pat

 

[SPYDERLK]

The other way of looking at is wrong. The conveyor does not inhibit the thrust of the plane. The wheels and tires spinning at twice the normal speed due to the conveyor moving will slightly slow the acceleration of the plane, but the plane will still accelerate to take off speed. The inertia of the wheels and tires is effectively doubled compared to a takeoff on a normal runway, but the effect on acceleration of the rotational inertia is small compared to the inerita due to the mass of the airplane.

It seems that many don't understand a couple of things. The first is that the wheels on airplanes do not provide thrust. The thrust which moves the airplane comes from the propellers and engine exhaust. Also obviously misunderstood are basic physics, Newton's laws of motion in particular.

It is amazing how this thing hangs around.

Your argument for your statement contradicts the statement. I agree with your argument. The wheels provide counter thrust.
larry

 

[CDsdad]

I've never been on a plane, but plenty of helicopters. The question of if it will take off is not important. The important question to ask if it takes off is: Will it safely land? If it won't take off, oh well, moot point.

 

[patrick_g]

patrick_g:

You said "Again, depending on the definitions of terms and ignoring the current state of the art in building conveyors and airgraft wheels/tires/axles, you picks your definitions and you gets your answer. Either contention can be supported, depending on your assumptions." I am very curious what the assumptions you see that would prevent takeoff.

The original post conditions are a plane on top of a conveyer like device that rolls its surface backward at exactly the speed the plane is moving forward. This is not a complicated device, certainly not magic.

By the way, my favorite teaching bowline is collapsing a slip not (simple noose) made back from the working end after putting the working end through the loop of the noose.

Steve

Steve, Attending to the high priority issue first and then getting to the magic conveyor belt:

That method of tying a bowline is a very good teaching technique as well as having practical importance. In the case of the former it lets you build on the simple familiar (overhand knot) and turn it into the desired new knot (bowline) and in the latter case addressing practicality it allows you to pre-tie part of the knot so yo can capture something and finish the knot before conditions interfere. For example you want to secure a flailing boom or tie to a bollard and conditions are just too wild to allow you to fully construct the bowline in place.

Now for the magic conveyor... IF you strictly define the conveyor's role to be ONLY to match the plane's forward speed but in opposite direction the plane (no magic involved), if it has just a little excess thrust above and beyond that required to take off with no conveyor it will indeed take off after just a little more take off roll. The wheels will be rotating at twice liftoff speed and we don't approach relativistic speeds moving us from Newtonian toward Einsteinian physics.

Much more fun was the version much discussed here where the conveyor's role was to speed up as required to prevent forward motion of the aircraft. This would work using either the rotational inertia of the wheels or friction of skis or whatever.

If you accelerate the conveyor sufficiently (non linear "jerk" where jerk is a defined physics term, the third derivative of distance with respect to time or the acceleration of acceleration if you will. If you do not limit the speed and the change of speed of the conveyor it can prevent the plane from taking off. If the conveyor is allowed to be "magic" and not self destruct then the plane's wheels will self destruct and long before relativistic speeds are achieved from plain old centrifugal force. If the plane's wheels are allowed to be magic and handle any RPM then the plane will run out of gas and not move forward.

Oh, bye the way, I think debate of this sort to be fun, and instructional and just plain good fun so long as we avoid the ad hominem approach. If we don't get too emotionally charged in favor of or opposing an idea, who knows someone might even learn something.

Unfortunately, much of what is posted seems to be in line with the story of the three blind men and the elephant, each touching a different part and disagreeing on what the elephant is like.

In the main we discussed a limited number of variants:

1. Conveyor moves to EQUAL the forward speed of the plane.

2. Conveyor moves to CANCEL the forward speed of the plane.

These are entirely different propositions!

In the first (1) the plane probably takes off with just a little extra roll and its wheels spin at twice its forward speed until after lift off.

In the second (2), The case of the "magic conveyor" the plane does not take off.

Pat

 

[SPYDERLK]

Very good Harry, then the magic conveyor will really have to rev up so the friction of the skis will be sufficient to equal the prop thrust and hold the planes forward speed to zero and eliminate lift.

Pat

Energy to energy: Conveyor to Ski #FxD/sec =~< Plane HPx33000/sec...Conveyor Distance[feet] /sec is very high.........Still, it seems to me that if #F is less than thrust the plane would forever move. Im getting a logic disconnect on this one. I believe its a mix of reference frames - the work done on the ski is mostly radiated; not mechanically transmitted from the conveyor to the airframe.
larry

 

[patrick_g]

Energy to energy: Conveyor to Ski #FxD/sec =~< Plane HPx33000/sec...Conveyor Distance[feet] /sec is very high.........Still, it seems to me that if #F is less than thrust the plane would forever move. Im getting a logic disconnect on this one.
larry

Yes, obviously, if the friction between the skis and conveyor produce less force at the conveyor's speed than the planes thrust then the plane will accelerate.

However IF the conveyor runs fast enough the counter force can equal thrust (or exceed it.)

Again, choose which propositioin you want to discuss:

1. Conveyor moves to equal the plane's forward speed or
2. Conveyor moves to stop the plane's forward speed.

These are vastly different propositions. Either can be achieved if in #2 we use a "magic conveyor" capable of unlimited acceleration and speed whether on skis or wheels or a channel filled with moving water and use a float plane.

I don't think we are in disagreement over physics, just maybe in precisely clarifying the initial conditions and givens.

Forget the tree. What if Helen Keller fell down in the woods. Would she make a sound?

Pat