Airplane: Does it take off?

[joshuatree]

Re: Airplane: Does it take off?

I think this debate really depends on the plane's engines and how well can the conveyor belt counter the engines. If a plane had rocket engines, airflow over wings won't matter, it's gonna fly even against the conveyor belt because the rocket engines will beat out the conveyor belt. Now if we're looking at a typical plane, say 747, and let's say the belt can actually roll that fast to negate the thrust of the 4 engines, I don't think the plane will fly. Yes, the 4 engines will create an airflow over the wings but I don't believe a 747's wings solely rely on just the airflow created by the engines, it also depends on the airflow created by the wings cutting into the air themselves. Of course, there are so many variables you can play with. What if it was an empty 747? Then maybe the thrust from the engines will create enough airflow over the wings to lift the plane off the conveyor belt and then the plane may lurch forward into flight or into an uncontrolled disaster.

This debate about a plane over a conveyor belt is sorta like the inverse of carrier catapult operations where a plane is on full throttle and assisted further by a steam catapult to enable the plane with a certain mass to hit a critical speed that's normally impossible with such a short length of runway.

[Pomai]

Re: Airplane: Does it take off?

Yes, the 4 engines will create an airflow over the wings but I don't believe a 747's wings solely rely on just the airflow created by the engines, it also depends on the airflow created by the wings cutting into the air themselves.

Nope. the (4) engines generate a majority of thrust (air pressure) BEHIND the wings. Not in front, centrally or at the flaps. Perhaps if a 747 had huge a propeller on the nose, this might be possible, but such is not the case. Look at a Hawaiian Airlines Boeing 717 (or former DC-9). Where do you see the engines? They're on the back of the fuselage, firing to the rear with nothing behind it. The horizontal stabilizer and elevator are up on the tail. Like all fixed wing aircraft, it needs FORWARD thrust to create airflow over the centrally-mounted wings in order to fly.

Airflow created by engine thrust over the wings is actually a novel idea, and partially applied to the wing design of STOL aircraft. Of course, jet thrust is too hot to apply over an aluminum wing with fuel in it.

The problem with STOL wing design is that it has a high drag factor, thus increased fuel consumption and reduced cruise speed. The (economic) goal behind commercial aircraft design is maximum lift (payload capacity) with minimum drag, favoring fuel economy over cruise speed. Newer commercial aircraft wing designs such as the Boeing 767/777/787 have pretty much optimized this science to the best of both worlds: speed and fuel economy.


Cezanne, those imaginary Krispy Kreme's sound good, but I prefer imaginary Butter Rolls from Komoda's!

[joshuatree]

Re: Airplane: Does it take off?

Nope. the (4) engines generate a majority of thrust (air pressure) BEHIND the wings. Not in front, centrally or at the flaps. Perhaps if a 747 had huge a propeller on the nose, this might be possible, but such is not the case. Look at a Hawaiian Airlines Boeing 717 (or former DC-9). Where do you see the engines? They're on the back of the fuselage, firing to the rear with nothing behind it. The horizontal stabilizer and elevator are up on the tail. Like all fixed wing aircraft, it needs FORWARD thrust to create airflow over the centrally-mounted wings in order to fly.

Newer commercial aircraft wing designs such as the Boeing 767/777/787 have pretty much optimized this science to the best of both worlds: speed and fuel economy.

You're thinking of the thrust produced specifically from the engines. I'm referring to the airflow created by the massive suction of the air intake to feed the combustion of each of the engines. Just the sheer movement of such massive amounts of air will create an airflow around the wings. The airflow may not encompass the entire length of the wing but definitely around the area where the engines are located. It doesn't matter if it's a rear mounted engine like the 717 or a wing mounted one like the 747, there's gonna be airflow from the massive suction of air. I'm not saying this amount of airflow is enough to make a plane fly but in my last post, I said maybe if the plane is empty weight, maybe on full throttle, it may be enough to life the plane off the conveyor belt, once that happens, there is nothing to negate the thrust from the engines so either the plane lurches forward and takes off or we see a disaster happen right before our eyes.

I wouldn't say the 767 is completely optimized. The last iteration, 767-400, has raked wingtips for optimization. But not the past iterations. Aviation Partner is currently developing winglet retrofits for the 767-300ER.

[Pomai]

Re: Airplane: Does it take off?

airflow created by the massive suction of the air intake to feed the combustion of each of the engines. Just the sheer movement of such massive amounts of air will create an airflow around the wings. The airflow may not encompass the entire length of the wing but definitely around the area where the engines are located. [---]maybe if the plane is empty weight, maybe on full throttle, it may be enough to life the plane off the conveyor belt, once that happens, there is nothing to negate the thrust from the engines so either the plane lurches forward and takes off

I highly doubt engine air intake volume alone would create enough airflow over the wing to lift a 747, or 717 for that matter, even if it only had a 1/4 tank of fuel and 1 pilot on board. Very unlikely to happen.

I see your point though.

It would be interesting to see a 747 at a standstill on a conveyor belt runway with engines at maximum power suddenly lift off as if it levitated. Then the engines' thrust finally gets their way and pushes that big boy up and away like a missile. That would be rad!

**Please Fasten Seat Belts**

[joshuatree]

Re: Airplane: Does it take off?

It would be rad to see something like a 747 take off from a conveyor belt but I seriously doubt it will work, if indeed we are able to get the plane to levitate off the belt so the engine thrust can move the plane forward, I feel it's gonna be one of those disasters you see on those old clips of inventors trying something new. I'm sure if a conveyor belt runway worked, it would have been done a long time ago on aircraft carriers, imagine the prime real estate they can save for launches.

[GregLee]

Re: Airplane: Does it take off?

Plane's forward speed increases to 100mph while conveyer belt's backward speed increases to 100mph. Wheels' speed on belt increases to 200mph. Plane takes off.

[craigwatanabe]

Re: Airplane: Does it take off?

Plane's forward speed increases to 100mph while conveyer belt's backward speed increases to 100mph. Wheels' speed on belt increases to 200mph. Plane takes off.

IMPOSSIBLE!! Because of the friction in those wheels the drag induced by the rotation of those connecting axles, the plane's wheels speed will always be less than the conveyor belt's.

That friction will keep the plane from ever passing the countering momentum of the conveyor belt's rotation. The plane needs it's own source of propultion to overcome that friction difference.

And even so, your theory forgets the one important factor to derive lift and that's air flow over and under the wings. The mass of the perceived forward speed of the aircraft and the air surrounding it remain neutral.

If the speed of the wheels is the critical factor why not just hoist the plane on a lift until the wheels leave the ground then have motors spin the wheels at rotation (speed at which a plane achieves lift). According to your theory the plane should fly. It won't. I guarantee it.

Head wind over the airfoil is what makes an aircraft lift. Or counter-phasing the frequency of the gravitational mass that holds the plane on the ground to 180-degrees will allow for lift while the plane's mass remains at a steady state.

Oops did I just mention the key factor in anti-gravity generators? Damn, wrong thread sorry.

When you jog on a treadmill, the faster you run do you feel any wind resistance? NO. Why? Because your body isn't bucking the mass of the air around you, it's neutral.

[GregLee]

Re: Airplane: Does it take off?

And even so, your theory forgets the one important factor to derive lift and that's air flow over and under the wings.

No, my theory does not forget that factor. The plane's forward speed (with respect to the ground and the air) increases to 100mph (or whatever the critical speed is). The key is to see that the backward movement of the belt does not prevent the plane from moving forward -- it just makes the wheels go faster.

[cezanne]

Re: Airplane: Does it take off?

I think it will take off too. There I said it.

The wheels are freewheeling, so the power of the plane's turbine/prop are an independent variable. Like GL just said and Pomai's been trying to explain, is that there is nothing holding the plane back...the conveyor belt indeed just makes the wheels spin faster thus the requirement of alien manufactured wheel bearings.

So after one year of thinking that it will not take off... I man up and say I was wrong and that yes the plane will take off... final answer.

[GregLee]

Re: Airplane: Does it take off?

If you measure the forward speed of the plane with respect to the ground, you get the answer that the plane does move forward and takes off. If you measure the forward speed of the plane with respect to the belt surface, since any speed-up of the wheels is exactly compensated by backwards speed of the belt, the plane can't move with respect to the ground and air, and so it can't take off. So, I'd like to amend my answer to: "It depends."

The fact that airspeed is ordinarily thought of as the way to measure a plane's speed and that the plane's wheels aren't powered perhaps introduces a bias in favor of the first interpretation. But I don't see anything wrong with thinking of the plane's speed along the belt surface, alternatively.

Thinking of this puzzle as a test of how good we are at recognizing the part our biases play in our thinking, we all failed.

[joshuatree]

Re: Airplane: Does it take off?

If you measure the forward speed of the plane with respect to the ground, you get the answer that the plane does move forward and takes off. If you measure the forward speed of the plane with respect to the belt surface, since any speed-up of the wheels is exactly compensated by backwards speed of the belt, the plane can't move with respect to the ground and air, and so it can't take off. So, I'd like to amend my answer to: "It depends."

The fact that airspeed is ordinarily thought of as the way to measure a plane's speed and that the plane's wheels aren't powered perhaps introduces a bias in favor of the first interpretation. But I don't see anything wrong with thinking of the plane's speed along the belt surface, alternatively.

Thinking of this puzzle as a test of how good we are at recognizing the part our biases play in our thinking, we all failed.

Hey GL, not quite getting your logic. If we assume this belt is capable of matching the speed of the wheels rotating, the plane itself is at 0 speed. If you measure the plane respective to the ground, the plane has no forward speed. Yes, you can say there is speed relative to the belt, but the plane is not flying in respect to the belt. It still needs to fly respective to the environment. And in order for the plane to fly in the environment, it needs to be moving through the environment which it is not. It's like a car on a dyno. You can floor it and the speedometer will read 100MPH but you know your car is not going anywhere. And if it did, heaven forbid, that just means the car's drive wheels are no longer in contact with the dyno but rather the environment.

[cezanne]

Re: Airplane: Does it take off?

The question does seem to be a bit flawed.

I was talking to some friends at the aircraft radio-control field over the weekend. At first we were of the thinking that it wouldn't take off because the plane needs airspeed to take off and that the conveyor belt negated any potential forward momentum.

We ultimately came to the conclusion that the plane will indeed go forward regardless of how fast the belt is moving because there is nothing is holding the plane backfrom going forward.

If for some insane reason the wheels were geared to reach take-off speed without the aid of any other propulsion system, then no it wouldn't take-off.

Not saying that I was wrong at first... but that I just changed my mind.

[GregLee]

Re: Airplane: Does it take off?

It's like a car on a dyno. You can floor it and the speedometer will read 100MPH but you know your car is not going anywhere.

Is it really like a car? If the car is on the backwards moving belt, its speedometer tells how fast the wheels move along the belt. But maybe the airplane's speed is measured differently -- with an airspeed indicator. The two methods of measuring the speed will give different results, and we have to decide which is to be used before we can tell how fast the belt moves backwards.

[joshuatree]

Re: Airplane: Does it take off?

Is it really like a car? If the car is on the backwards moving belt, its speedometer tells how fast the wheels move along the belt. But maybe the airplane's speed is measured differently -- with an airspeed indicator. The two methods of measuring the speed will give different results, and we have to decide which is to be used before we can tell how fast the belt moves backwards.

Yes, it is like a car on a dyno. Car's not physically moving anywhere because the drive wheels forward motion is being negated by a dyno. For the plane scenario, plane's not physically moving anywhere because its wheels which indirectly is being propelled by the thrust from the jet engines, is being negated by the conveyor belt. The plane I am assuming is a large commercial plane like the 747, not a lighter Global Express which could alter the dynamics of the test.

When you are trying to measure the speed of the car vs the plane, yes, the method of measuring is different. Car is measured by a speedometer which usually measures the rotation of the axle. Changing to different rim sizes can alter and make the speedometer inaccurate which is why a speedometer should be recalibrated if the car had it's rims upgraded, but most people skip that in real life. Plane is usually measured by airspeed which is read by a pitot-static system which is those little prongs you might notice near the nose of the plane. Now I don't know if a plane also has a speedometer to measure ground speed or not. If it does, then it should be the same as a car. But either way, the different way of measuring speed doesn't change the physical fact that neither the car nor the plane is physically moving forward if the dyno/belt is able to negate every forward movement generated either by the car's drive wheels or the plane's jet engines.

[GregLee]

Re: Airplane: Does it take off?

... the different way of measuring speed doesn't change the physical fact that neither the car nor the plane is physically moving forward if the dyno/belt is able to negate every forward movement generated either by the car's drive wheels or the plane's jet engines.

But we don't know whether the belt is able to negate every forward movement until we know how fast the belt is moving backwards. And we don't know how fast that is until we know how fast the plane is moving forward (because the speeds are the same). And we don't know how fast the plane is moving forward until we've decided how we will measure the plane's forward speed.

If we measure the plane's speed as we measure a car's, your reasoning is correct. But not if we measure airspeed, and here is why. If the plane doesn't move forward with respect to the air and ground, as you say, the airspeed must be zero. Then the belt speed is also zero. So how can the belt negate forward movement when the belt can't move at all?