Plane on a treadmill

[miguel]

So, to use your example, you would need a windtunnel with enough airflow to balance the amount of thrust from the engines to get the plane airborne yet remain stationary.  In this case, you are only moving the ground, so no force is cancelled out and the plane takes off as normal.

it's not a matter of air balancing out the amount of thrust....it's a matter of airflow at the wings.  think of gliders...they have no thrust...they merely have enough forward momentum to get air flowing around the wings.

[Dean]

A treadmill only matters for things that depend on transfering energy to the ground for forward motion.  

A plane's engines do not do this.  The air around the plane will still be standing still when the treadmill starts, so it can still push against it and move through it.

Have any of you ever seen a model plane or bird trying to fly in a strong headwind?  They are working, producing thrust, and flying, yet make little or no head way.  That's because the AIR is the medium to which they impart thrust.  It is the speed through the air which matters to an aircraft.

[Dean]

....it's a matter of airflow at the wings.

Right!  As soon as the engines begin producing thrust, the plane will move forward through the air.  Because the treadmill is moving the ground, not the air.

[darkrose]

Ok.  Here's my next crack at it.  

The engines are producing thrust, which is meant to push the plane forward, which in turn will create the necessary airflow under the wings for takeoff.

The engines do not produce enough airflow when the plane is standing still (via the treadmill cancelling out the forward motion) to create lift under the wings.  They are not meant to.

I know what some people are thinking....what about when the plane is in the air?  Well, once a body is in motion, it tends to stay in motion.  The engines only need to produce enough thrust to counter gravity and drag and keep the correct amount of negative pressure under the wings to keep the plane in the air (i.e. minimum windspeed).

Since the force (thrust and windspeed) it takes to get the plane airborne is greater than the force (thrust) needed to keep the plane in the air......

No the plane will not take off.

Is that a better explanation?   ](*,)

[miguel]

A treadmill only matters for things that depend on transfering energy to the ground for forward motion.  

A plane's engines do not do this.  

a plane's engines do indeed do this on the ground.  a plane's wheels do not have a drive mechanism...they "freeweel," for lack of a better term.  they do have brakes, and on airliners, the front gear can steer.  but for forward movement, the engines blow air aft, creating forward movement on the ground.  

i'm sure you've seen tugs tow planes in reverse when they back out of terminal gates.  that's because the wheels have no drive mechanism...and because the engines don't produce enough fwd thrust to move in reverse (except for the C17 cargo plane and some other specialized aircraft).

[miguel]

The engines do not produce enough airflow when the plane is standing still (via the treadmill cancelling out the forward motion) to create lift under the wings.  They are not meant to.

I know what some people are thinking....what about when the plane is in the air?  Well, once a body is in motion, it tends to stay in motion.  The engines only need to produce enough thrust to counter gravity and drag and keep the correct amount of negative pressure under the wings to keep the plane in the air (i.e. minimum windspeed).
 

that's a PERFECT explanation!

[Dean]

That's not how planes fly, though.  

The design of the wing of an airplane is such that when air moves over it, higher pressure is produced below the wing and lower pressure is produced above the wing.  This is called lift.  A plane MUST have lift to fly.  A plane can take off when the lift produced by the wings is greater than the weight of the plane.  The minimum air speed to keep a plane in the air is it's stall speed.  Notice ground speed is irrelevant.

If you could point a giant fan at an airplane and generate enough air speed to just exceed the stall speed of a given airplane, it could lift off and fly without moving forward.  Now, the air would also push the plane backwards, since it generates friction as it moves over the airframe. Some thrust is required to overcome the friction, or eventually the backward motion of the airplane would begin to match the speed of the air.

Ground speed doesn't figure into an airplane's ability to fly., therefore, the treadmill doesn't matter.

[miguel]

That's not how planes fly, though.  

The design of the wing of an airplane is such that when air moves over it, higher pressure is produced below the wing and lower pressure is produced above the wing.  This is called lift.  A plane MUST have lift to fly.  A plane can take off when the lift produced by the wings is greater than the weight of the plane.  The minimum air speed to keep a plane in the air is it's stall speed.  Notice ground speed is irrelevant.

If you could point a giant fan at an airplane and generate enough air speed to just exceed the stall speed of a given airplane, it could lift off and fly without moving forward.  Now, the air would also push the plane backwards, since it generates friction as it moves over the airframe. Some thrust is required to overcome the friction, or eventually the backward motion of the airplane would begin to match the speed of the air.

Ground speed doesn't figure into an airplane's ability to fly., therefore, the treadmill doesn't matter.

now i'm confused, because it sounds like you made a 180 in your argument.  i need a drink!!!!!  c'mon over to SoCal for lunch so we could have a beer!  

[djrussell]

i'm sure you've seen tugs tow planes in reverse when they back out of terminal gates.  that's because the wheels have no drive mechanism...and because the engines don't produce enough fwd thrust to move in reverse (except for the C17 cargo plane and some other specialized aircraft).

actually most planes have thrust reversers. a reverse gear if you will. i don't know if they use them to scoot around the runways though. i imagine they could but it's probably a lot cheaper to operate those little tug cars than to spool up the engines. i think they're mainly used during emergency landings with brake failure.

[djrussell]

c'mon over to SoCal for lunch so we could have a beer!   :wink:

[Dean]

When I get there, it won't be lunchtime anymore!  

[miguel]

i'm sure you've seen tugs tow planes in reverse when they back out of terminal gates.  that's because the wheels have no drive mechanism...and because the engines don't produce enough fwd thrust to move in reverse (except for the C17 cargo plane and some other specialized aircraft).

actually most planes have thrust reversers. a reverse gear if you will. i don't know if they use them to scoot around the runways though. i imagine they could but it's probably a lot cheaper to operate those little tug cars than to spool up the engines. i think they're mainly used during emergency landings with brake failure.

thust reversers are always used during landings.  they function as brakes, and without them a jetliner would overshoot the end of the runway.  they are doors on the engine cowls that open and divert engine thrust fwd.

have you ever been a a plane when it touches down and all of the sudden you hear what sounds like an extrememly loud rushing of air for about 45 seconds?  that's the TRs being deployed.

they're great brakes, but they don't produce enough thrust to move a plane in reverse.

[miguel]

When I get there, it won't be lunchtime anymore!  

seriously, we should meet sometime halfway between me and you.  we could pick up other Z riders along the way.

where on the forum would be the best place to post this idea?

[Dean]

Just in the General Discussion forum, I 'spose.  I could try to get ahold of the other four Z750 riders from around here and see if they could come along.  

Vanashke could maybe join us from Utah...  

Would be cool, for sure!

[pauly75]

Maybe this will answer the question.  

Look at a Harrier Jet, the thrust it requires to lift is forced to the ground, providing lift.  It may be stationary at start, then tilts the engine(air flow) for forward movement.  No need for the treadmill to have no ground speed.  It is only able to lift off re-directing the thrust towards the ground.