Will it take off?

I guess I still don't understand.

What about a glider on a conveyor, held in place by a rope.
Here's an example:



By making the conveyor move fast enough will the glider start to rise? Even without a fan or strong wind blowing towards it?

Dont know how this discussion could go on for 7 whole pages on ATS !!
Anyway the Bernoullis principle should be satisfied . i.e :
Pressure head + Velocity head + datum head are equall above and below the airfoil section.
The only thing that matters is the flow of fluid below the airfoil member should be less than the flow over the airfoil . So as the velocity below is lesser the pressure is greater and thus the pressure on the upper face is lesser and the airfoil moves upwards.
Here is a nice tool by NASA:FoilSim
This is otherwise a mad program to write on ones own, trust me !! Obviously the flow model is not that which you would get through the Reynolds stress eqaution(Turbulence Modelling) and you might not get to choose the type of discretization scheme you want to apply but for the basics it is pretty good. I suggest you ppl use the probe function and study to pressure distribution over the leading edge and also the pressure-velocity variation over both faces.

Originally posted by missed_gear
This site's section has a nice, full, lengthy explanation of this myth that has apparently been circulating for some time:

The Pilot's Lounge


mg

This is just this guys opinion. It is not scientifically proving that the plane would take off.

look at the treadmill, free rolling but a bicycle on it and ride it, do you move forward .. no

Why because the treadmill is rotating under you, thus your location is the same and no airspeed no ground speed.

Now get on the bike and dont pedal and power the treadmill it you dont pedal do you stay in the same spot..... no

since there is no forward force or rotation then the treadmill pulls you and the bike backwards, because you and the bike are mass X due to gravity.

Now if the treadmill is traveling at 3 meters per second, you most pedal at the rate where the wheels on your bike are also traveling at 3 meters per second in order to not be carried off the treadmill by the belt.

IT DOES NOT MATTER if you pedal the wheels or use a rocket or a propellar strapped to your back. It does not matter how you create the forward momentum as YOU the BIKE and THE WHEELS are all part of the MASS that is one unit upon the treadmill. The faster the treadmill goes the faster "forward" motion/energy you must have to stay in one place. You could have giant wings strapped onto your bike but without greater positive forward motion you will not be able to move down the treadmill without imparting greater force/ velocity then the opposite velocity generated by the conveyor belt.

so 3 ms back 4 ms forward (positive velocity is then 1 ms) With out positive velocity you can not have forward positional motion, therefor no positive airspeed.

Kruel the only thing that will attempt to lift the glider is the "buffeting" caused by the turbulent airflow generated by the conveyor moving at a ground speed of 60mph.

Its like placing a balsa wood toy plane over a giant belt sander. Sure the wing will generate some "lift" but without positve forward motion it will bounce up and then down on the cushion of air as it travels away from the turbulent air into the smooth air away from the surface of the belt the wings will lose "lift" and drop back down.

And potentionally if the plane has enough specific mass to overcome the amount of "lift" acting on the planes wings by the buffeting of the turbulent air then the plane could pass through the turbulent air due to Mass+ Gravity and come crashing right into the surface of the sander and or conveyor belt.

A regular airplane though with a much higher specific mass therefore would need a much higher amount of turbulant boundary air capable of acting on the wing to create "lift" to get the plane into the air, again without postive forward positional movement there would be nothing to keep the plane in the air and the plane would slam back into the ground.

More then likely this would require the surface area equal to 4 times the surface area of the lifting surface of the plane plus twice the velocity relative to the normal airspeed required to take the plane into the air under normal conditions.

And again this is only to crreate enough surface generated turbulance to get the plane to lift off the ground but the plane would not be able to stay in the air.


The example that the pilots lounge use as the basis to debunk the idea that the plane would not fly they sighted an experiment with a rubber band wind up plane on a treadmill. The trust to weight ratio of a rubber band plane is not the same as a regular airplane as you can hold it in your hand motionless and let go and it will fly away from you.

Try holding your J3 propeller plane 20 feet off the ground at full throttle and tell me if it "flys into the air" and you will be comparing apples to apples.

In fact point that rubber band plane straight up and I bet you it goes straight into the air, lets put the J3 on its tail pointing straight up at full throttle, does it leap into the air just like the rubber band plane...

Nope.

So sighting it as a basis that a real plane would take off is irrelevant. Because I doubt you will find a plane with such power to weight ratio. Maybe a German Comet but thats nothing less then a rocket with wings and a pilot

[edit on 17-2-2006 by robertfenix]

Right, so the basic idea is it needs some sort of airflow to get any lift. That's the point I'm trying to make here... which is why it confuses me that some are saying the treadmill idea would work. Unless the conveyor creates the wind speed, (or in the case of a plane, the propellor creates the wind speed) I don't see how it could work.

Originally posted by robertfenix

Originally posted by missed_gear
This site's section has a nice, full, lengthy explanation of this myth that has apparently been circulating for some time:

The Pilot's Lounge


mg

This is just this guys opinion. It is not scientifically proving that the plane would take off.

look at the treadmill, free rolling but a bicycle on it and ride it, do you move forward .. no

Why because the treadmill is rotating under you, thus your location is the same and no airspeed no ground speed.

Now get on the bike and dont pedal and power the treadmill it you dont pedal do you stay in the same spot..... no

since there is no forward force or rotation then the treadmill pulls you and the bike backwards, because you and the bike are mass X due to gravity.

Now if the treadmill is traveling at 3 meters per second, you most pedal at the rate where the wheels on your bike are also traveling at 3 meters per second in order to not be carried off the treadmill by the belt.

IT DOES NOT MATTER if you pedal the wheels or use a rocket or a propellar strapped to your back. It does not matter how you create the forward momentum as YOU the BIKE and THE WHEELS are all part of the MASS that is one unit upon the treadmill. The faster the treadmill goes the faster "forward" motion/energy you must have to stay in one place. You could have giant wings strapped onto your bike but without greater positive forward motion you will not be able to move down the treadmill without imparting greater force/ velocity then the opposite velocity generated by the conveyor belt.

so 3 ms back 4 ms forward (positive velocity is then 1 ms) With out positive velocity you can not have forward positional motion, therefor no positive airspeed.

Since you like to talk about the bicycle, I'll explain it to you with a bicycle.

Imagine your sitting on a treadmill on a bicycle with no pedals (magically balancing yourself of course) and the treadmill starts moving backwards. Initially the whells will spin in place but soon you will be moving backwards with the treadmill right?

Now what if someone in front of the treadmill threw you a rope and tied the other end around a tree, then although the treadmill kept moving back you would just stay in one place while the wheels spun under you.

Now what if its a magic tree and started walking away. Then the tree would pull you forward regardless of what the treadmill is doing while the wheels simply spin faster, reflecting the speed of the treadmill + the speed of the magic walking tree.

This is because the force pulling you forward is more than enough to over come the little bit of friction that helped the treadmill pull you backwards before you had the rope.

Similarly, an aircraft with its engines turned off and sitting on a backwards running treadmill will go backwards with the treadmill but once an outside force starts acting on its position it will easily overcome the friction and take-off, in the case of the aircraft its not a magic walking tree but thrust generated by a propellor or jet engine.

Now go read my previous post.



[edit on 17-2-2006 by orca71]

but your "tree" is not on the treadmill so again your example is flawed

your "tree" is an outside force indepandant of the forces acting on the plane.

The tree gets to take advantage of a staionary location that then gets to "magically" move into a forward postion.

And because the rope is attached to the plane on the treadmill it imparts the new forward position on the plane to conveyor orientation.

Not the same thing, this new "tree" is an additional third force vector.

Nice idea but completely flawed and just sidetracks from the acutal situation


Chain a car on a dyno and then move where the chains are bolted down forward and , gee what heppens, you pull the car right off the dyno



[edit on 17-2-2006 by robertfenix]

Originally posted by robertfenix
but your "tree" is not on the treadmill so again your example is flawed

your "tree" is an outside force indepandant of the forces acting on the plane.

The tree gets to take advantage of a staionary location that then gets to "magically" move into a forward postion.

The walking tree acts on the stationary ground and the propellors or jets act on the stationary air, neither of which are affected by the treadmill and are "outside" forces.

And because the rope is attached to the plane on the treadmill it imparts the new forward position on the plane to conveyor orientation.

The rope does nothing to the conveyor orientation and in the example I gave its attached to the bicycle not the plane. The bicycle has the rope attached to the magic tree which generates thrust off the ground, the plane has a jet engine or propellor generating thrust off the air. Neither the ground nor the air are affected by the treadmill.

this new "tree" is an additional third force vector.

Precisely. Turning on a jet engine or propellor does exactly the same thing. It introduces a new force vector that is independent of the others.


[edit on 17-2-2006 by orca71]

Thrust does not = lift.

If you take the treadmill out of the equation: Thrust moves plane forward, creates airflow, which in turn causes lift. Airflow creates lift, not thrust.

Which means: the treadmill theory doesn't work.

Unless there's some other force acting which I either don't notice or understand.

Originally posted by Kruel
Thrust does not = lift.

If you take the treadmill out of the equation: Thrust moves plane forward, creates airflow, which in turn causes lift. Airflow creates lift, not thrust.

Which means: the treadmill theory doesn't work.

Unless there's some other force acting which I either don't notice or understand.

Thrust = forward motion = lift

There is nothing keeping the jet engines or propellor from providing forward motion.

The treadmill cannot keep a jet engine or propellor from proviring forward motion to the airplane because the forward motion is created by thrust is created by acting on the air which is unaffected by the treadmill.

But the forward motion (relative to the airspeed) is = 0 with the treadmill in place. The forward motion (relative to the ground) makes no difference... otherwise planes wouldn't need wings!

Originally posted by orca71

There is nothing keeping the jet engines or propellor from providing forward motion.

The treadmill cannot keep a jet engine or propellor from proviring forward motion to the airplane because the forward motion is created by thrust is created by acting on the air which is unaffected by the treadmill.

Sure there is the plane body, which is attached to the gear which is in contact with the treadmill which is preventing positive forward motion to be imparted on the plane body regardless of the source of the "thrust". As the conveyor is equalling the forward input in the opposite direction.

You put a jet engine on some rollers and you put that on the conveyor and you have both the conveyor and jet trust equallized and guess what the cart that the jet engine is on stays in one place

Originally posted by Kruel
But the forward motion (relative to the airspeed) is = 0 with the treadmill in place.

Not if the engines are on and the brakes off. If the engines are on the forward motion is essentially whatever it normally would be without the treadmill because the treadmill does not affect thrust.

The forward motion (relative to the ground) makes no difference... otherwise planes wouldn't need wings!

Youre getting confused here. The ground and the treadmill are seperate. The treadmill track moves backwards, the ground does not. Forward motion relative to the ground is forward motion. Neither the ground nor the air are affected by the treadmill.

Ok, I should have been more specific... when I said "ground" I meant "treadmill", because in relation to the plane, the treadmill IS the ground.

All thrust does is propell the plane forward through the air which creates lift. With the treadmill in place, the plane isn't propelled through the air at all. Therefore, no lift.

Originally posted by robertfenix

Originally posted by orca71

There is nothing keeping the jet engines or propellor from providing forward motion.

The treadmill cannot keep a jet engine or propellor from proviring forward motion to the airplane because the forward motion is created by thrust is created by acting on the air which is unaffected by the treadmill.

Sure there is the plane body, which is attached to the gear which is in contact with the treadmill which is preventing positive forward motion to be imparted on the plane body regardless of the source of the "thrust". As the conveyor is equalling the forward input in the opposite direction.

No because the conveyor imparts force indirectly through friction which is a very small component. For example, if the airplane had its engines off and its brakes off, the treadmill still woudlnt be able to pull the plane backwards at a high speed because the friction through which the conveyor attempts to pull the airplane couldnt overcome the aerodynamic drag.

The conveyor cannot keep the jet engine or propellor from providing forward thrust anymore than it can keep the bicycle from being pulled forward by the magic tree with a rope. This is because the jet engines, propellors and magic trees act independently of the conveyor and are not affected by it.

You put a jet engine on some rollers and you put that on the conveyor and you have both the conveyor and jet trust equallized and guess what the cart that the jet engine is on stays in one place

No, because the jet produces thrust relative to the air which is independent of the conveyor, hence the conveyor cannot equalize the force of the jet and keep it from acting on the air and moving the vehicle forward relative to the air.

You really ought to think this through carefully before posting again. No offense, but I am beginning to wonder if your really serious about your posts or if your just posting nonsense to get a reaction.

Originally posted by Kruel

All thrust does is propell the plane forward through the air which creates lift.

Yes.

With the treadmill in place, the plane isn't propelled through the air at all. Therefore, no lift.

No.

The thrust is generated regardless of whether the airplane is on the treadmill or not and the thrust is always much greater than the friction force the treadmill relies on to pull the plane in the opposite direction.

In otherwords, the thrust form the engines will move the plane forward relative to the treadmill AND relative to the air with no problems whatsoever, which means lift will be generated and the plane will take=off very easily.


[edit on 17-2-2006 by orca71]

Then what you're saying is, the jet engine/propeller creates the airflow which travels over the wings and causes lift.

I can see how this might be possible with a very strong propeller. The propeller would create the airflow required for lift. But I mean, you'd have to have a really powerful propellor for this to work. It would have to move all the air around the plane at quite a speed.

My glider example would not lift however, because the air in relation to the glider would be completely still with the lack of engine.

I'd like to see what would happen if we could get the vehicle pictured below on a set of dynometer rollers.





www.dself.dsl.pipex.com/MUSEUM/TRANSPORT/helica/helica.htm

As you can see from the pictures, the sole means of propulsion for the vehicles pictured derives from the prop attached to the vehicle (as would be the case with the plane described in the example).

Now, granted, these antique cars do not posess wings, so there is no question as to their inablity to fly (fly Apart, maybe!). But their design led me to wonder.

It has been argued that since an airplane's propulsion system "acts on the air" surrounding the aircraft, and not with respect to surface upon which the plane sits, the "counter" effects of a moving runway surface would have no, or at best, little, effect on the plane's ability to generate an airspeed sufficient to fly.

Webster's defines Airsped as "the speed of an airplane with relation to the air as distinguished from its speed relative to the ground".

And so my question: When in motion, would the velocity of the vehicles pictured be measured as ground speed, or as airspeed?

If we answer "groundspeed" since the car is not only not in the air, but has no hope of ever being in the air, how do we discount the equally obvious fact that the sole means of propulsion for the vehicle, like the plane in our example, has no mechanical connection to the wheels and is acting solely 'against the air' to propel the car?

If we answer "airspeed", then wouldn't the vehicle's airspeed be equal to its groundspeed since the vehicle is obviously "grounded"?

If the true speed of such a vehicle should be measured as airspeed (the measure of the speed of the air about the car, as generated by the thrust of the prop) then the car should be able to roll/move forward even if the wheels of the car were mounted on a counter-rotating surface. As are the wheels of the plane in the example.

But planes have wings!

But as all agree, wings only work if enough air flows over them at a high enough velocity to generate enough lift.

If our "prop-buggy" cannot pull itself off the rollers, neither would our airplane be able to. And if the plane is stuck in place on the "conveyor", the only way it will be able to fly is if the "prop wash" alone can provide enough volume of air at high enough velocity to allow the wings to lift the plane; and if that were the case, the plane wouldn't need a runway of any type.

The point was made before, in prior posts, but to re-iterate: An aircraft's propulsion system acts against the mass of the Aircraft, NOT against the air surrounding the aircraft.

Propellors are "miniture wings", airfoils, which when moved through the air generate lift, described as thrust, which "lifts" the plane through the air; picture a helicopter with its nose down, sorta.

A jet more efficiently uses the force of heated expanding gases to push the plane through the air; most jet engines are not, by themselves, powerful enough to lift the weight of the aircraft they are attached to without the added lift supplied by the plane's wings. Some are.

Rockets, generally do not need wings (except for stablization) since they generate enough thrust to counter the weight of the vehicle. But rockets are inefficient for extended flight.

Originally posted by Kruel
Then what you're saying is, the jet engine/propeller creates the airflow which travels over the wings and causes lift.

No.

The jet engine/propellor pulls the aircraft forward, causing airflow over the wings.

Really amazes me at the length some people will go

Take a giant wind tunnel,

Put whatever plane you want inside the wind tunnerl and attach the plane with a cable to the end so that the plane has a full range of motion.

Now crank up the wind tunnel to generate the airspeed needy to get the plane into the air. No power, since the plane is held in place by the cable.

The plane is "flying" right... yes

Because the "airspeed" over the wing is at the right velocity to provide lift.

Consider this to be relative airspeed, with a 0 ground speed.

So you have your plane in the air in the wind tunnel, now turn on the power on the plane.

Let assume the force of the wind to equal our relative airpseed to keep the plane in the air is 50 units

50 counter untis are being imparted on the plane by the cable to keep it in place right. 50 pushing it 50 holding it back.

So now you turn the planes power on and make 10 units of forward power,

Does the plane move forward ... no

50 units pushing against it,, 40 counter units by the cable and 10 counter units by the plane

lets take the plane up to 50% so now

50 units against it, 25 counter units from the cable and 25 counter units from the planes power.

has the plane moved no, why because the plane still has to overcome the counter force keeping it from flying back that the cable is holding onto

Now lets crank the planes power up to 90%

Still the same force pushing against it (the wind) but now the cable is only imparting 10% to hold it in place and the plane is using 90% to push forward, without the cable though the plane would bo forced backwards because it has less forward force then the wind pushing against it

so now you are at 100% equal force from the plane as the wind against it, the cable goes slack and the plane is flying in place.

FOR THE PLANE TO FLY FORWARD INSIDE THE WIND TUNNEL IT MUST CREATE MORE FORCE THEN THE WIND FLYING AGAINST IT. REMBER this whole time the plane has 0 ground speed and the airspeed is only relative to the wing, the "object" has 0 forward momentum

You can not use the downwind turn example because a plane flying the air has postive forward momentum which is different then our example as well as the conveyor belt.

A plane flying at a true airspeed say of 80 knotts is traveling 80 knotts forward, if the plane flys into a wind blowing at 160 knotts the plane does not stall just the relative airspeed over the wing now becomes 240 knotts reletive to the wing and creates MORE lift as well as more pressue on the wing controls. If for example the controls or airframe is only rated for 165 knotts then the plane does not lose control because it stalls but because the total force imparted on the control surfaces is greater then it can handel. This is why planes can take off into the wind and why sometimes they crash after taking off into the wind. If for example your wing lift needs 100 knotts to take off and you have a 40 knott wind you could then accelrate to 60 knotts ground speed and take off but if the 100 knott wind layer was shallow and say a mere 50 feet up it was only 10 knotts instead of 40 knotts then as you rolled up at 60+ knotts but a relative 100knotts at the wingspeed sure you could be in the air but as soon as you hit that 10 knott layer your 60+ 10 knott wings speed being below the 100knott lift requirement would send you crashing into the ground.

And has happend to more then enough private planes, commercial planes, military aircraft etc.

You here it all the time, plane crashed after take off just 1 mile from the airport. Some of these case are on accent with not enough forward airspeed (true forward airpseed) and into the wind, where the plane meets a slower relative airspeed wind and being under powered for the condition, stalls at low altitude where there is not enough time to recover and crash.

It is safe to land into the wind but it is very tricky to take off into the wind


A plane then flying with the wind is still moving forward at the 80 knotts relative to the wind but the ground speed becomes increased slightly due to friction on the plane. Since the air mass as a whole is moving at 160 knotts and the plane is flying within the air mass is moving at 80 knotts. The plane could appear to be flying at 100knotts or more due to the position on the ground relative to it, but airspeed of the wing may still be 80 knotts.

Thats why the problem of powering down causes the plane to stall, the air mass over the land is moving as a whole but that is independant of the wing speed through that airmass which is required to produce lift.