Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
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It does not matter to a plane how fast its wheels are turning. The only way to generate lift is with wind under the wings.
It is possible for a plane to be going backwards across the ground if the winds into which it is flying are greater than the plane's indicated air speed. I've done that.
Rauchy is correct in this regard and as a pilot I explained this, in detail, in the original thread. Groundspeed and airspeed, no matter how it's measured (IAS, TAS, CAS, etc.) are all that matter. Ones speed over the ground is irrelevant to ones speed through an airmass.
One can have a theoretical GS of zero and yet still be flying. In fact I have come close to this in reality. I have flown certain aircraft that when in a strong enough headwind could, and quite literally had, a groundspeed that was near zero yet still showed a positive indicated airspeed.
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Thrust does not produce lift, lift is produced by differential pressure (top and bottom) on the wings. To have this differential pressure, the airplane need to be moving. In a threadmill, the airplane is not moving, so it is not taking off. Rauchy is correct.
I never claimed or made the statement that thrust produced lift (directly). Speed through an airmass is what generates airflow over a lifting surface, in this case a wing, and it is the wing that generates lift.
In addition, the AIRCRAFT DOES NOT HAVE TO BE MOVING RELATIVE TO THE SURFACE. In this case the hypothetical treadmill.
The thrust produced by the engine/s propels the aircraft through the airmass. A car on the other hand is propelled by a tractive force, that being the friction between the tires and the surface. If the surface is moving, such as the hypothetical treadmill, the car will not move RELATIVE TO THE MOVING SURFACE. Since the moving surface itself is stationary, relative to the Earth's surface, the car will not move relative to the Earth's surface. However, the airmass will still be moving past the vehicle. This is the fundamental difference between a normal car and an aircraft.
Silly example #1:
HYPOTHETICALLY SPEAKING, a rocket would launch (takeoff) if placed on a treadmill, for the same reason an aircraft would. I say hypothetically speaking because in the case of a rocket there are inherently impractical considerations due to lateral movement relative to the surface during launch. Principally speaking however, a rocket and aircraft are one in the same since both rely on thrust. Consequently, the outcome is as a result of Newton's third law.
One can take this a step further and use the example of a rocket propelled car, again on the hypothetical treadmill. Since the rocket propelled car does not rely on the tractive force of tire friction against the surface, but instead the thrust produced by its engine, the rocket car would not move along the treadmill. It would remain stationary. However, the airmass flowing past the rocket car would be moving.
The movement of this airmass is what would result in lift being generated if the vehicle had wings, such as an aircraft and thus would enable it to fly, as in takeoff.
Silly example #2:
Take an aircraft carrier that is moving toward the aircraft as it is on final approach, vs. the normal movement in the same direction as the aircraft (and yes, this has been done in reality). Is landing possible? It certainly is! (although I wouldn't want to try it in a modern aircraft) In fact, it was done prior to WWII in experiments conducted by the British, who by the way first came up with the practical concept of an aircraft carrier and were the first to successfully attempt it.
Remember, that in the instant just prior to touchdown the aircraft is still flying, yet the landing surface, in this case a carrier, is moving in the opposite direction of flight, just like the hypothetical treadmill would be. As the aircraft touches down the airmass is still moving past the wings. If the pilot elected to go around after first touching down, it would still be possible.
This example is very similar to the hypothetical treadmill as the surface (the flight deck in this case) is moving in the opposite direction of the aircraft, just as the hypothetical treadmills surface would be.
Before anyone argues the carrier itself is moving and the hypothetical treadmill is not and that only the surface itself is - it doesn't matter. The fact is the surface is moving in the opposite direction of the aircraft and that is all that's relevant.
carrier direction----------->
<---relative wind direction
<------------aircraft direction
----->relative wind direction
Note how the relative wind directions are opposite. However, the aircraft is still flying and can still land on the aircraft carriers flight deck and even takeoff again.
Note: I sometimes use the term "car" vs. "vehicle" so as not to confuse car and aircraft since an aircraft is often referred to as a vehicle.
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
You wouldn't be around. The viewing area is in another, well protected, room. The photos shown are of the treadmill itself. That entire thing will be placed in a wind tunnel. The treadmill was not running in the original photo.
The cars are held by retention devices similar to the manner in which a car is safetied on a chassis dyno.
Oh, and how the Hell could you have possibly missed the treadmill thead!?
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
There are four forces at work on an airplane in flight, lift, drag, thrust, weight. Lift counteracts weight, thrust counteracts drag.
Lift is achieved by the creation of a pressure differential across the top and bottom of the wing. Airflow intersects the leading edge of the airfoil and is split in to an upper and lower stream. The curved shape of the top of the airfoil causes the airstream to flow faster across the top than across the bottom. The air "stretches" in order to do this. It spreads out and speeds up and the pressure goes down. The higher air pressure on the bottom of the wing pushes up towards the lower airpressure on the top and the plane, which is attached to the wing, goes up.
Airflow over the wing, which is required to generate lift, is produced with thrust. The thrust must be greater than the drag of the airplane in order to make the airplane accelerate through the air, which generates increasing airflow over the wing, until there is sufficient lift and the plane takes off.
The drag of the airplane comes from two sources. The airflow over the structure of the airplane produces drag, just like the airflow over the surface of your car. Also, while in contact with the ground, the tires and their wheel bearings generate friction, which "looks" like drag to whatever's generating the thrust, also just like a car. This wheel friction adds to the drag until the wheels come off the ground, obviously, at which point it's all aerodynamic drag.
It's important to note that the airplane doesn't "care" what makes the thrust, since it's not the thrust that makes the airplane fly. The airplane cares about the airflow generated across the wing, as a result of the thrust overcoming the drag and moving the plane forwards through the air.
This is the key to the answer to this question. The plane's wheels are in contact with the treadmill and there is friction there as a result. The plane's engine generates thrust to move the plane forward and the speed of the treadmill is instantaneously increased so that it generates enough drag to counteract that thrust, which by definition is what has to happen if the plane is to remain stationary on the treadmill. And because the plane does remain stationary, with the thrust totally cancelled by the drag created by the treadmill, there is no airflow and no flight.
A rocket on a wheeled dolly would do the same, provided it was mounted perfectly horizontally and the treadmill was able to go fast enough.
Man, I just love this discussion!
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Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
oh my god... the can of worms has opened again..
LOL
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Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
RocketSled, although your educational diatribe is correct for the most part* the slight drag penalty due to friction between the tires and their contact patch with the surface will not result in a lack of lift. As long as the aircraft is capable of accelerating to its liftoff speed it will takeoff. This remains true on a normal runway as well as on the hypothetical treadmill. The wheel bearings don't have to be on fire in order to achieve takeoff speed.
If the aircraft has a takeofff speed of X and the hypothetical treadmill is moving in the opposite direction at speed X the aircraft is still moving through the surrounding airmass at speed X and will takeoff. I explain this further below. Please refer to the section labelled "Friction and Thrust".
Thus I maintain the aircraft would indeed takeoff and I am willing to stake both my academic and professional opinion on that.
*Bernoulli's Theorem, although still widely taught in most academic courses relative to this discussion, IS NOT the reason an aircraft wing produces lift.
I try to steer clear of that aspect as it is somewhat involved and would require much explanation, including drawings, equations and external references in order to even hope to overcome the existing thoughts, beliefs and misconceptions concerning it. I assure you it is not entirely correct.
A Reference:
However, I will refer you to this article (it's actually an online text)...
"You’ve probably been told that an airfoil produces lift because it is curved on top and flat on the bottom. But you shouldn’t believe it, not even for an instant.
Presumably you are aware that airshow pilots routinely fly for extended periods of time upside down. Doesn’t that make you suspicious that there might be something wrong with the story about curved on top and flat on the bottom?"
And yes, it delves into symmetrical airfoils, etc.
Although I could sit here and spend hours explaining angle of attack, angle of incidence, symmetrical airfoils, and so on and so forth, I'm not going to. Although please allow me to reiterate and explain in perhaps a different way...
Friction and Thrust:
A car is propelled by the friction of the tires against the surface. The surface in return applies a force against the tires. The wheels and tires are driven by the engine and drivetrain; neither an aircraft nor a rocket-car however works this way. A normal car would not be moving through the surrounding airmass, nor relative to the treadmill itself or the surface the treadmill is sitting on, but a rocket car or an aircraft would! Again, this is the fundamental difference between a normal car and the aircraft in this hypothetical case.
Thrust is produced by the engine/s and thus Newton's third law of motion states that for every action there is an opposite and equal reaction. The thrust propels the aircraft in this case THROUGH THE AIRMASS. Although its speed RELATIVE TO THE SURFACE (of the hypothetical treadmill) is zero, and it's also zero from the perspective of a casual observer, the aircraft is still moving through the surrounding airmass and therefore the wing is generating lift. And yes, lift overcomes gravity, which we measure as weight. Weight being the term we use to indicate the measurement of gravity's effect on a given mass.
A wing only cares about the airflow past it and thus it generates lift. The speed relative to the surface, or the treadmill, is hence irrelevant. It doesn't matter if the treadmill is capable of travelling at a thousand knots per hour because the aircraft is only capable of several hundred knots per hour.The aircraft would be moving through the surrounding airmass at its takeoff velocity long before excessive (wheel) drag due to frictional heating occurred and thus the wing would be generating sufficient lift and the aircraft would takeoff. This is true whether on a normal runway or the hypothetical treadmill as the speed required for takeoff is the same in both cases.
Again, although a casual observer would see the aircraft as remaining stationary, which it is relative to the surface of the treadmill and/or the surrounding area, the aircraft is still being propelled through the surrounding airmass in response to Newton's third law. This cannot be discounted. It is neither a hypothesis nor a theory. It's not even a simple scientific fact, but a basic law of physics.
A real life example that helps explain:
Again, many others and I have experienced this very thing in reality. Ones groundspeed can be much less than ones airspeed. The example I gave whereby certain relatively underpowered light aircraft can virtually hover, relative to the ground, yet still indicate a positive airspeed is proof of this.
Casual Observation is Irrelevant:
What a spectator sees is irrelevant. The speed over the surrounding terrain the treadmill is sitting on is irrelevant and the treadmill itself is irrelevant.
A suggestion:
Go to one of the numerous academic Physics forums and pose this same hypothetical discussion. I will try to remain silent.
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
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Originally Posted by No Doubt
RocketSled, although your educational diatribe is correct for the most part* the slight drag penalty due to friction between the tires and their contact patch with the surface will not result in a lack of lift. As long as the aircraft is capable of accelerating to its liftoff speed it will takeoff. This remains true on a normal runway as well as on the hypothetical treadmill. The wheel bearings don't have to be on fire in order to achieve takeoff speed.
Ahhh, but you missed the point. The fundamental assumption is that no matter how much thrust the airplane's engines generate, the treadmill can go fast enough to counteract that thrust and keep the plane stationary. If the plane doesn't move forward, it can't develop lift and fly.
It's all about energy. As the plane develops thrust and attempts to move forward, the treadmill drags it backwards. The mechanism is the same whether the tires are rolling or not, the only difference is the amount of friction (which is lower when the tires are rolling). Though it might be easier to think of in terms of tires that don't roll. Imagine if you would rubber blocks that don't "wear out" instead of tires that roll.
It's that friction, where the rubber meets the "road", albeit small, that allows the mechanism of this "thought experiment" to work. The energy being generated at the engines, that would normally overcome drag and inertia to accelerate the plane forward, is completely consumed in heat dissipation at the tires. It *has* to be, or the plane moves forward and eventually flies.
Agreed, the friction is quite small compared to the thrust available from the airplane's engines, so the treadmill has to go *wicked* fast to dissipate enough energy to keep the plane from moving. Much faster, in fact, than the velocity that would normally be required to get the plane in the air. But assuming that the treadmill is fast enough, it will keep the plane stationary irrespective of the amount of thrust generated by the engines. And a stationary plane won't develop airflow and therefore won't be able to fly.
If the friction of the wheels against the treadmill was zero, the plane would move forward regardless of the treadmill's speed. If the friction of the wheels against the treadmill was infinite, the plane would be dragged backwards regardless of the thrust being generated by the engines.
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
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Originally Posted by No Doubt
Again, although a casual observer would see the aircraft as remaining stationary, which it is relative to the surface of the treadmill and/or the surrounding area, the aircraft is still being propelled through the surrounding airmass in response to Newton's third law. This cannot be discounted. It is neither a hypothesis nor a theory. It's not even a simple scientific fact, but a basic law of physics.
Ahh, but here you are mistaken. The airmass surrounding the plane is not in fact moving. The treadmill moves backward, there might be attached flow at the surface of the treadmill that is moving with the treadmill but that's not moving over the airfoil. And the air flowing through the engine or past the propeller is moving backwards, but the air is not moving over the airfoils (except for parasitic flow from the propwash or jet intake, which should to be discounted for this discussion, and in any case will never be enough to get the plane off the ground).
I won't argue that if you put a big fan in front of the plane and blow hard enough you can get the plane to lift off no matter the treadmill speed, or even if the engines aren't running. I can fly my model airplanes backwards in a strong enough headwind. Ground speed doesn't matter, it's all about airpseed. So unless the treadmill has a big fan to move the air with the treadmill, there's no airspeed and no lift and no flight!
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
Interesting, as I too flew model aircraft and was an AMA member and national champion in a couple events when I was a teen. I also won the Boeing Model Aeronautics Scholarship Contest, twice. I competed in control line stunt (pattern), fast and slow combat, Goodyear racing, control line speed (ram jets) and both powered and unpowered Freeflight. I also flew R/C aircraft as well, including both powered aircraft as well as gliders and designed a glider that set the world speed record at the time. Today I play with much bigger toys.
Obviously I can't convince you I'm right henceforth I am no longer going to try. As I said, I suggest posing this question on a Physics forum. Write a letter to the NASA AMES research facility and ask. I can give you contact info.
I (or someone I probably know) will get your head around it eventually, one way or another.
Edit: Here's a thought: let's put a model aircraft on a treadmill and see if it takes off. Trust me, it would although it could (will) get a little 'wonky' during the takeoff roll We can film it and post the video right here, regardless of what happens.
I suggest incorporating some channels for the main gear to track in on the treadmill because at this scale* the aircraft will lack longitudinal stability on the moving treadmill surface. A wide track main gear may also suffice. It would really depend on how good one is with their thumbs.
Aerodynamic surfaces effectiveness, including rudder authority, is nil due to the low Reynold's numbers especially at low speeds.
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
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Originally Posted by No Doubt
Obviously I can't convince you I'm right henceforth I am no longer going to try. As I said, I suggest posing this question on a Physics forum. Write a letter to the NASA AMES research facility and ask. I can give you contact info.
Edit: Here's a thought: let's put a model aircraft on a treadmill and see if it takes off. Trust me, it would although it could (will) get a little 'wonky' during the takeoff roll We can film it and post the video right here, regardless of what happens.
Oh, my friend, don't give up so easily!
We clearly both know what we're talking about, so our difference of opinion must be the result of a difference in the definition of terms...
You agree with me that without airflow over the wing there'd be no lift.
I'm sure you also agree with me that if the plane doesn't move forward on the treadmill, no matter how fast the wheels are going there is no airflow over the wing (discount propwash and parasitic flow induced by the motion of the treadmill itself). A treadmill is not a wind tunnel.
So what we're really arguing is whether or not the treadmill can induce enough drag at the wheels to overcome the thrust from the motor and keep the plane from rolling fowards against the pull from the Prop.
That's not a question in aerodynamics at all!
I like the idea of an empirical experiment. But I think we'd need a very specialized treadmill capable of running at extremely high speeds, which alas, I do not have. Got plenty of airplanes, though!
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
Rocketsled, I have been discussing this subject from your same perspective, and you are correct on the physics. But, (there is always a but) there is a subtle issue that is hard to describe in words, that is what "no doubt" is trying to bring to our attention that changes the outcome of this alleged experiment.
I will try to explain:
We are a group of car enthusiasts that are use to power our cars from the engine to the powertrain to the wheels and reacted by the road. If the road (in this case treadmill) moves at the same speed as the car in opposite direction, the car will be stationary. You see this on your dynos all the time. Replacing the car with the airplane changes this interaction.
The airplane, whether jet or prop, move forward relative to the air with no use of powertrain through the wheels, the airplane wheels do not react against the runway to move the airplane. The wheels are free to spin at whatever rate they need to spin, regardless of the airplane speed.
Once the airplane moves by propulsion relative to the air, the threadmill can do whatever it wants, it would not stop the airplane, the wheels spin faster. The airplane will move down the threadmill and when enough airflow is attained through the wings, it will lift up.
I must now admit, "no doubt" is correct............I missed that minor detail that changes everything. My hat off to you "no doubt".
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
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Originally Posted by No Doubt
You wouldn't be around. The viewing area is in another, well protected, room. The photos shown are of the treadmill itself. That entire thing will be placed in a wind tunnel. The treadmill was not running in the original photo.
The cars are held by retention devices similar to the manner in which a car is safetied on a chassis dyno.
Oh, and how the Hell could you have possibly missed the treadmill thead!?
Yes, of course you are right. With years of working around large industrial equipment, I have to say you must have overlooked the inevitable, sh** happens. As far as the plane on the treadmill thread goes, I must have skimmed past it due to a lack of interest and would not known anything about it had you and Rocketsled not hijacked this thread.
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
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I have to say you must have overlooked the inevitable, sh** happens.
You're right, there is always someone who ignores any cautions or warnings.
There are those who will purposely defeat, override, disable or in some way find a 'workaround' for any safety features that are in place.
There can be signs, flashing lights, locked doors, guards, and virtually any security measure imaginable (and some not so imaginable) and yet SOMEONE will ultimately find a way to get around them.
Why?? One need only ask them and you'll inevitably hear something like "I know what I'm doing" or "Oh, nevermind that, I've been doing this for years." And when they get injured they almost always admit they were being stupid ( ), "wasn't paying attention", or the classic "I didn't think it would happen to me".
Oh, and of course if you ever try and tell them not to do what they're doing or are about to do prepare for an ear full of any of the above rationalizations.
Of course neither you or I are one of those people.
Edit: Almost forgot this now Internet classic (and hilarious) safety film that pretty well sums up what you're saying.
Re: 180MPH NASCAR Treadmill - finally an answer to the jet-on-a-treadmill debate?
Quote:
Originally Posted by No Doubt
You're right, there is always someone who ignores any cautions or warnings.
There are those who will purposely defeat, override, disable or in some way find a 'workaround' for any safety features that are in place.
There can be signs, flashing lights, locked doors, guards, and virtually any security measure imaginable (and some not so imaginable) and yet SOMEONE will ultimately find a way to get around them.
Why?? One need only ask them and you'll inevitably hear something like "I know what I'm doing" or "Oh, nevermind that, I've been doing this for years." And when they get injured they almost always admit they were being stupid ( ), "wasn't paying attention", or the classic "I didn't think it would happen to me".
Oh, and of course if you ever try and tell them not to do what they're doing or are about to do prepare for an ear full of any of the above rationalizations.
Of course neither you or I are one of those people.
Edit: Almost forgot this now Internet classic (and hilarious) safety film that pretty well sums up what you're saying.
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