Member

Joined: 18 Jan 2008
Posts: 106
Location: Indianapolis

Anybody seen the preview for next weeks Mythbusters episode? Airplane on a conveyor belt. How do you think it will come out?

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

Airplane on the conveyor belt? Haven't heard about that one. What is the idea behind it?

Member

Joined: 18 Jan 2008
Posts: 106
Location: Indianapolis

Never heard the argument on it? If it is about what i think it is. Can an airplane take off of a conveyor belt that is running the opposite direction of the plane at the same speed. I think it would.

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

No it won't take off. You need the air flow across the wings. However the plane could take off while standing still if pointed nose first into a 170kt wind. But it doesn't matter how much power you give your engines if there is no air movement across the wings.

Member

Joined: 18 Jan 2008
Posts: 106
Location: Indianapolis

I can see it both ways. Ever flown an R/C plane? Think about this, start the engine and hold it in your hands and you can feel it pulling, right? Then imagine setting it down on a treadmill, will the plane stop trying to pull against you? I don't think so.

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

I can guarantee you with 100% certainty that it will not get up in the air. You need a set number of kts of air speed across the wing to generate lift. You could be standing still and facing a 160kt head wind and lift up off the ground. Or you could be going forward at 60kts into a 100kt head wind which will generate 160kts of air flow across the wing. As long as the air flow is a certain amount over the wing thats all that matters. It can be any combination of head wind and the plane going forward.

Why do you think planes take off into the wind instead of with the wind at the tail?

Member

Joined: 18 Jan 2008
Posts: 106
Location: Indianapolis

The thrust comes from the engines pushing against the air, not the ground. The engines are still going to go forward and with it will go the plane. Its not like a car where the propulsion is coming from the wheels.
Stand on a skate board on a treadmill and have a buddy push you forward while increasing the speed of the treadmill, will you move forward or would you buddy feel like he is pushing on a wall, your buddy will be able to move you forward.

Member

Joined: 15 May 2006
Posts: 728
Location: West Lafayette, IN

As soon as it comes off the ground its going to lose all of the ground speed anyway. My guess is it won't work, but who knows.

Member

Joined: 18 Jan 2008
Posts: 106
Location: Indianapolis

The belt is running in the opposite direction of the plane, but in the same direction as the thrust of the engines.

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

ATAIndy wrote:

As soon as it comes off the ground its going to lose all of the ground speed anyway. My guess is it won't work, but who knows.

Consider this... why is it that when jets run up their engines with the brakes on that they don't come up off the ground? Even when a jet takes off the engines are at full throttle long before the jet comes off the ground. It has to generate that forward speed to get the air moving across the wings. Otherwise they'd just take off while standing still.


Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.


http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

Member

Joined: 18 Jan 2008
Posts: 106
Location: Indianapolis

Why do you feel that there will not be any forward movement of the aircraft. The wheels have nothing to do with how fast the aircraft moves, the engines push against the air not the ground. The tires would just be spinning much faster than on a normal take off, which might be a problem if they can not withstand the speed.

I could be completely off but i truly believe it would still work, because the thrust comes from moving air not the ground. Search in Tech/ops of A.net for "if a plane took off a conveyor belt"

Many many many arguments there.

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

Reason I think it won't go anywhere is this....

If the plane was sitting on the conveyor belt and the belt was moving from front to back the plane would roll backwards. Firing up the engines would generate thrust which would offset the movement of the belt.

If you were in a boat and you were floating down street at 5mph and fired up your engine and added 5 additional mph you would be moving 5mph faster than the water. You'd be moving 10mph faster than the surrounding land. If without adjusting engine speed you were to turn your boat around and go upstream you'd end up standing in place. When your turn hit 90 degrees your down stream progress would have been reduced to the speed of the water which is 5mph. Once you completed the 180 degree turn you forward progress would have ended.

Its like walking on a treadmill.

Member

Joined: 18 Jan 2008
Posts: 106
Location: Indianapolis

Okay, however, the boat is just like a car on a street in that the the water is what the motor is pushing against counter acting on each other. Strap a jet engine to the boat and what would it do?

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

It honestly doesn't matter what the conveyor belt speed is. Unless that plane gets enough air flow around the wing it isn't coming up off the ground. Its going to need a strong enough head wind, enough forward speed, or a combination of the two to get enough air flow to create the lift. Without that it won't work. If its on a conveyor belt the belt better be long enough that the plane can generate enough forward speed. Forward speed isn't relative to the conveyor belt but rather the air around the plane.

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

I spoke with my father-in-law who is a mechanical engineer and worked in the aerospace industry. It was kind of as i thought. If the plane isn't moving it will not get up off the ground. Now you can generate the power needed to get off the conveyor belt but you'll need enough forward movement to get lift. Well forward movement or headwind. The only thing that matters is the movement of air in relation to the wing. The ground movement (in this case the conveyor belt) is irrelevant.

So assuming your jet needs 160kts of air flow over the wing and 7500 feet of runway to take off you'll still need 160kts of air flow over the wing and you'd need a conveyor 7500 long plus extra distance for the jet to overcome the wheel friction on the belt. The only thing a conveyor belt will do is increase take off power requirements and takeoff length.

Member

Joined: 15 May 2006
Posts: 728
Location: West Lafayette, IN

Indy wrote:


Consider this... why is it that when jets run up their engines with the brakes on that they don't come up off the ground? Even when a jet takes off the engines are at full throttle long before the jet comes off the ground. It has to generate that forward speed to get the air moving across the wings. Otherwise they'd just take off while standing still.


Well, my point was since the airplane is getting all its momentum from the conveyor and not the engines, as soon as the plane leaves the conveyor its going to lose all its momentum. Engines are attached to the plane and therefore provide thrust after the plane leaves the ground, conveyor, not so much.

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

Oh so the conveyor would be taking the plane forward and not trying to roll it backwards against the force of the engines? So not like a treadmill but rather one of those moving walkways at the airports where you walk on them and you are hauling ass compared to the people around you?

Member

Joined: 15 May 2006
Posts: 728
Location: West Lafayette, IN

Yeah, thats what I had imagined. Aren't we supposed to have those ass haulers in the new terminal? lol

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

Yes there will be ass haulers in the new terminal. In fact in one of the aerial photos you can see where they go. And you can see them actually being installed in either a news photo or video. I don't remember which. But you can clearly see the ass hauler.

Site Admin

Joined: 15 Jun 2005
Posts: 2316
Location: Indianapolis, IN

BTW if the conveyor belt is used to move the plane forward you could actually get the plane off the ground with the engines turned off if the belt was moving fast enough. I think its like the sling shot concept they use on aircraft carriers.