Can the Plane Take-Off?

[EricH]

Originally Posted by lemon1324

If you theoretically create a huge fan at the end of a runway, that will shorten the ground roll of the aircraft. However, the aircraft will not have enough speed to fly if it immediately leaves this stream of moving air. Thus you can have a shorter runway but the aircraft will need to remain in the accelerated air for some time.

I dunno...on a simulator, I've been able to take off at about half the speed I'd otherwise need. I finished accelerating in the air. There are these tricks known as high-lift devices. Specifically, flaps on the wing's trailing edge. Once in the air, I'd pull them in.

Edit: Lemon1324, I know how to fly. The flaps would get retracted once I was at a decent speed, and only when I was at a flying speed. I might not even use them until I cleared the accelerated air, an then use them only until I got a better speed.

[lemon1324]

Flaps and other high-lift devices will only make the problem worse--when you retract the flaps, your coefficient of lift drops, and so you get less lift at that particular point in the envelope. Now after you lift off (at half the ground speed, presumably) when you leave the accelerated air, the airspeed of the aircraft drops suddenly. If you also retract flaps, the aircraft will most likely stall and come crashing back down.

[Morgan Gillespie]

The answer is yes; the belt tracks and recreates the planes speed in the opposing direction, yet if the plane isn't moving and just standing still, then neither is the belt. The problem never states it is moving in such a speed that it would cause the plane to not move forward, it only says it would go in the opposite direction at the same speed of the airplane.
Under experiment conditions... it works perfectly fine, even at take off the belt is only moving as fast as the plane is forward, this just means the wheels are rotating at twice their normal take off speed...

[Molten]

The reason I did not source the formula is because it has been around for long enough that it does not need a source. It is common knowledge. Also, the wheels can still be used to measure speed despite the fact that they are not used for propulsion. Furthermore, I am not discussing this anymore until I have seen Mythbusters. I hope that I did a reasonable job explaining some of the misinterpretations of my words. Until then, Good day.

[EricH]

Originally Posted by Molten

The reason I did not source the formula is because it has been around for long enough that it does not need a source. It is common knowledge. Also, the wheels can still be used to measure speed despite the fact that they are not used for propulsion. Furthermore, I am not discussing this anymore until I have seen Mythbusters. I hope that I did a reasonable job explaining some of the misinterpretations of my words. Until then, Good day.

I didn't ask for the source, I asked for a proof. Also, how does it tie in? You did not explain that part. The equation is there with no tie-in.

I don't disagree that the wheels can be used to measure speed, but there is a chance (I've done it in a simulator) that one or more wheels will be off the ground at any given instant, so which wheel do you measure? How do you determine which wheel's data is correct, if you do all of them?

And the conditions are clearly stated in the problem. What conditions did you have in mind? For anyone to analyze your conditions, you have to state them. You don't. Therefore, I cannot fully answer you because my mind-reader is on the blink (and has been for 19 years at this time). If I know what conditions you use, I can either agree or disagree with the conditions. I may or may not agree with you, but not knowing what conditions you have in mind prevents my doing either. I disagree based on the given conditions. You have other conditions. Very well, let's hear them.

[Pat McCarthy]

The Mythbusters have conclusively proven that a plane on a treadmill can take off! (When comparing airspeed vs. conveyor speed)

Warning! Science Content!
This can be explained by comparing the means of propulsion of a plane vs. a car. A car uses the powered spin of its wheels to propel it forward.

Now a plane uses it's propeller to push against the air, while the wheels are left to free spin. And a plane depends on the speed of the air passing over its wings to generate lift to take off. With enough headwind, a plane can take off while not moving relative to the ground. Thus, a plane will pull itself through the air, no matter if it's wheels are spinning (or how fast they are spinning) or even if they are not spinning at all.

There is a corollary to this.
And I'm not sure if I will be explaining it right.
You have to be measuring the plane's airspeed for the experiment to work. If the speed of the wheels was monitored, and you were only trying to match the speed of the conveyor belt to the rotation of the wheels, assuming no friction in the system other than that between the wheels and the conveyor, the plane wouldn't have to exert any propulsion of any kind to remain stationary (Newton's First Law of Motion). However, in real life, the plane would be pulled backwards along with the conveyor, relative to the ground due to the friction of the wheels and their bearings. So to counter this, the plane would have to be pushing some air with its propeller to remain stationary to the ground for the wheels to match the speed of the conveyor.
In this case, the plane would not take off, as the speed being measured was the speed of the wheels matching speed of the conveyor, not the airspeed vs speed of the conveyor. To sum it up, the plane would remain stationary to the ground and thus have zero airspeed (assuming no wind) and not be able to take off.

[nuggetsyl]

Originally Posted by dlavery

If you still think that the plane will not take off, then think through this: imagine exactly the same stating conditions. The airplane engines ramp up to full thrust. If you are assuming that the airplane (relative to an off-conveyor observing position) is "standing still" the explain the following: the pilot quickly retracts the landing gear, so the airplane is no longer in contact with the conveyor belt, the engines are still at full thrust, and the airplane is at zero velocity relative to the ground. What happens next? When you try to reconcile this self-conflicting condition, you will realize that in the original problem, the airplane takes off.

-dave

I will have to disagree. Engines at full power means nothing. If you retract the landing gear you will crash the plane because your air speed is 0. Many people are confusing ground speed with air speed. If air is not going over the wings you can not lift a plane off the ground plain and simple.

[EricH]

Originally Posted by nuggetsyl

I will have to disagree. Engines at full power means nothing. If you retract the landing gear you will crash the plane because your air speed is 0. Many people are confusing ground speed with air speed. If air is not going over the wings you can not lift a plane off the ground plain and simple.

The air speed is ONLY zero if:

1. the wind is directly from one side, or
2. the wind is from dead aft at the same speed as the plane (and a wind that fast would ground half the planes in the area anyway).

The airplane moves forward on the ground. Ground speed is approximately airspeed, in a calm. If the aircraft is moving on the ground, it will have airspeed, unless one or both of the conditions above is met. The airplane will also be able to move fast enough to beat the wind.

Oh, and I have heard of a fighter pilot on a conventional runway who pulled his gear up to take off. No rising of the plane. If you can do the same on a moving runway, you can take off.

This besides the point that Dave was making: Same conditions apply, but the pilot simply pulls up the landing gear and is no longer in contact with the belt. IF you are assuming that the plane will stand still, THEN the plane will not take off.

However, the Mythbusters have shown that the plane CAN take off, especially because it moves, so this discussion is moot, unless you have a problem with their methods.

[Tom Schindler]

Originally Posted by nuggetsyl

I will have to disagree. Engines at full power means nothing. If you retract the landing gear you will crash the plane because your air speed is 0. Many people are confusing ground speed with air speed. If air is not going over the wings you can not lift a plane off the ground plain and simple.

I normally don't post in these kind of threads, yet i feel the need to in this one.

There is an airspeed/ground speed issue here.

Think about the problem in terms of the horizontal forces -- You have the thrust of the aircraft pushing the body of the plane forwards, you have the force of the treadmill being pushed opposite the engine thrust on the wheels.

F=M*A tells us that if these two forces do not equal each other, the plane will accelerate with the direction of the greater force.

As for the force being applied by the treadmill, onto the wheels -- If you think about where that force is being applied, and in what direction, you'll see that it's acting tangent to the wheel, so all (well nearly all) of that force goes into rotational motion on the wheel. I say nearly all because of any wheel bearing friction. So that force goes to almost zero.

Since there is still a force being applied to the aircraft frame by the propeller or jet engine, the plane will accelerate (f=ma)

When the plane reaches normal takeoff speed, the wheels will be spinning at twice normal takeoff speed (due to the treadmill below), but because they are free-spinning and not powered wheels, there is almost no difference between this and a normal takeoff.

my $.02

[296Minion]

Yes. The trust generated by the plane is no match for anything else

[Waiakea12]

I think that it all depends on the size, Power, and overall weight of the plane. The "runway" size also matters.

WHAT ABOUT THE FIGHTER PLANES THAT CAN TAKE OFF AND LAND VERTICALLY AS WELL AS HORIZONTALLY? DOES THIS APPLY TO THEM??

(Don't kill each other over this one!!! lol)

P.S.- I'm a little bit late in this thread, sorry.

[EricH]

Originally Posted by Waiakea12

I think that it all depends on the size, Power, and overall weight of the plane. The "runway" size also matters.

WHAT ABOUT THE FIGHTER PLANES THAT CAN TAKE OFF AND LAND VERTICALLY AS WELL AS HORIZONTALLY? DOES THIS APPLY TO THEM??

(Don't kill each other over this one!!! lol)

P.S.- I'm a little bit late in this thread, sorry.

Actually, plane size doesn't matter. The thrust amounts are calculated to lift each plane type. See, plane weight and size are taken into account when planning for power (thrust is the technical term on aircraft). There is always power to spare--that is, the plane will be able to take off at maximum weight, guaranteed, or the mechanic or designer is looking for a new job.

But yes, the runway size matters. The Mythbusters started with a conveyor that was too short for their R/C plane, and it went off...

And I already suggested the VTOL aircraft. That's cheating!

[BigWhiteYeti]

If any of you have watched Mythbusters recently, they proved it can be done! Think about it. Planes derive their power from propellers or jets, each of which move air. This forces the plane forward relative to the air, not the ground. The wheels are free-spinning, so they will move twice as fast when the plane takes off upon reaching its take-off airspeed. Airspeed and groundspeed are totally different principles.

As for the Mythbusters, they pulled a really long mat with a pickup and had an ultralite take off successfully. Look it up!

[wfoard]

I know that I might bnot seem reliable because this is my first post, but I wish to dispel this fear by mentioning that I have seen this done in real life, not on Mythbusters. First, the airplane did take off. For all of you people who talking about lift, yes the lift comes from the wings moving through the air. As far as the airplane moving in relation to the ground, it does. The reason for this is that the thrust comes from the engines (whether they be old-fashioned propellor or modern turbine) and has nothing to do with the wheels whatsoever. This means that basically the wheels can spin freely. Because of this, the treadmill is basically nonexistant. If you do not believe me, put a model airpland on a big treadmill and notice that it does take off. I have personally seen it done many times.

[Protronie]

Originally Posted by sanddrag

As mentioned previously, a plane's flight is based on it's relation to the air, not to the ground. Thrust comes from engines pushing on the air, not from wheels pushing on the runway.

Yes but... the thrust from the prop pulls the plane forward so air rushes over the wings to produce lift.
Would the prop-wash alone be enough to produce the required air moving over the wings flight surface in order to produce the lift needed?

If the plane is on a belt moving under it at the same speed as the plane then the air above and under the wing would depend on the prop wash.