Airplane on Conveyor Belt Mythbusters 12/12

wfooshee · Dec 11, 2007

tinpusher said:
If you have enough thrust (read "NASA"), you dont need velocity to generate lift.

A rocket isn't flying by lift, it's ballistic. Technically it's not flying, it's falling. The thrust makes it fall a different direction.

If you have wings to generate lift, you DO need velocity, because you need airflow. And the treadmill wil NOT affect jet or propellor generated thrust. In an aircraft, ground speed is irrelevant. It's airspeed that matters.

The plane is not propelled by its wheels, so the surface it's rolling on is totally irrelevant, unless you get to some insane speed where the wheel bearing friction actually does act as a brake. That would be really really fast ground speed. Tires would probably shred first.

tinpusher · Dec 11, 2007

wfooshee said:
Bearly is correct here. The treadmill has absolutely no effect on the power output by the plane's engine. The treadmill CANNOT reduce the plane's forward motion. It can only make the wheels spin way faster.
The only way the treadmill will affect the takeoff is if the brakes are on, which is always how they take off, right?

Now, if you're launching a glider from a truck, then you're no go. The truck won't move fast enough, because it IS wheel-driven, and the treadmill fights its forward motion.

You guys are sort of right. Put wheels on a rocket, put it on a treadmill, it will still fly. It has enough thurst to overcome gravity to fly. An airplane doesnt. That's why it has wings, to generate lift. If it didnt need wings to fly, they wouldnt be there. And it would be called a rocket.

Airplanes dont have enough thrust to overcome gravity without lift.

tinpusher · Dec 12, 2007

wfooshee said:
tinpusher said:

If you have enough thrust (read "NASA"), you dont need velocity to generate lift.

Click to expand...

A rocket isn't flying by lift, it's ballistic. Technically it's not flying, it's falling. The thrust makes it fall a different direction.

If you have wings to generate lift, you DO need velocity, because you need airflow. And the treadmill wil NOT affect jet or propellor generated thrust. In an aircraft, ground speed is irrelevant. It's airspeed that matters.

The plane is not propelled by its wheels, so the surface it's rolling on is totally irrelevant, unless you get to some insane speed where the wheel bearing friction actually does act as a brake. That would be really really fast ground speed. Tires would probably shred first.

Right, a rocket doesnt fly because of lift, it's because it has enough thrust to lift its weight off the ground. And I mispoke, you DO need velocity to generate lift, but not to make an object fly (Rockets DONT use lift to fly).

A jet engine generates thrust, but it doesnt move any air over the wings. Thus, no airspeed. A treadmill will NOT affect thrust, but it will offset the fact that the thrust would move the airplane forward, generating the lift required to make the airplane fly. If the jet engines could generate enough thrust to overcome gravity without the added help of lift, then the airplane would take off.

So take a small aircraft, and put really powerful engines on it (basically a rocket), it will work. Otherwise, it wont happen on any airplanes I know of.

markjenn · Dec 12, 2007

The part about this problem that doesn't make sense is where the treadmill spins backwards as fast as the plane moves forward so as to keep the airplane's speed with respect to the ground zero. For this to happen, you have to assume that the treadmill can somehow affect a drag on the airplane, but the wheels are supposed to spin freely. Essentially the problem is overconstrained and non-sensical. I have no idea why it has generated so much discussion.

- Mark

Toecutter · Dec 12, 2007

I think this one has been beaten up here before.

I can't wait to see that conveyor belt!

IndianaTom · Dec 12, 2007

Hmm... imagine a frictionless environment (wheel bearings are perfect and so on). If the plane is on a treadmill that is turning at 300 miles per hour backwards, the plane will remain motionless, if frictionless. Right? So, the forward thrust of the plane will overcome the treadmill no matter how fast the treadmill is running.

Think of it as you wearing roller skates on a treadmill, then pulling yourself forward by holding on to the hand rails. The forward motion of the plane in this senairo will void the results.

The experiment should be with the plane that is tied down with it's arrestor hook. Full thrust means no forward motion, and if there is enough lift generated by the bernoulli effect of the running engines the plane will lift up, but not forward. I've been in test cells with F-15 running their engines, and there is a good deal of wind generated by the jet engines forcing air out the back of the hush house.

Now, I suspect an F-15 won't lift off that way, probably, but I don't know. The hush houses only ran one engine at a time, allegedly because running both engines would collapse the building, but lets say that with one engine at full thrust, there was an estimated 70+mph of wind. Both engines might be 140 mph, and that is enough to get an F-15 off the ground, I think.

I think the best plane to attempt this stunt would be something like a empty C-130. The prop wash of the 4 turbo props cover the majority of the wing, and that might be enough to get it to lift off if tied down at the tail.

and yea, I'll bet the blow it up before it's over.

ponyfool · Dec 12, 2007

IMO, many are missing the point. If the engine produces forward thrust, the airplane will move forward. Even if the treadmill matches the wheels speed to "negate the forward movement" the airplane will still move forward, and the wheels will just turn that much faster because the wheels aren't providing the thrust.

Unless there is something restricting the forward movement of the plane (ie: tie down) and the wheels are free to move and are not acting as brakes on the treadmill, the plane's thrust will move the plane forward independent of the treadmill.

EDIT: And Tom posted just before I did, and more eloquently I might add!

wfooshee · Dec 12, 2007

tinpusher said:
A jet engine generates thrust, but it doesnt move any air over the wings. Thus, no airspeed.

To continue beating it up :assassin: The thrust doesn't move air over the wings, unless you count pushing the plane forward. That moves air over the wings.

tinpusher said:
A treadmill will NOT affect thrust, but it will offset the fact that the thrust would move the airplane forward. . .

Self-contradictory. If it won't affect thrust (true) it can't possibly affect the thrust's action on the aircraft.

The treadmill can't counteract the aircraft's forward speed, because it has nothing by which to "grab" the aircraft (unless the wheel bearings suck or the brakes are on.)

Now, if the aircraft were parked on the treadmill, which then ran backwards to full speed, and only THEN did the engines light, the plane would have to counteract the treadmill's speed, but it would do so easily. The takeoff would take longer than from a standstill, but there's nothing to keep it down.

Can somebody throw something political here so the thread can be killed?

BobG · Dec 12, 2007

I have seen a Super Cub (with tundra tires) hover at the end of the Juneau Air port waiting for a Alaska air lines 737 to land. There was 25 knot winds at the time.

Why not , planes have been known to fly backwards in too much head winds.

Can't you measure the lift of an aircraft in a wind tunnel?

I saw a picture somewhere of a tied down plane a foot or 2 off the ground because of head winds. Can't remember the airport it was at.

I have no fait in myth busters they have screwed up on many tests. But I guess it is good entertainment.

Bob

SPORT · Dec 12, 2007

oster_stupid:

FJRFencer said:
borrec said:

They are joking right? On a treadmill there is no forward movement, you are running but not moving forward. The wheels could be going 500 MPH but no airflow over wings equals zero lift. :dribble:

Click to expand...

oster_stupid:

oster_stupid:

As any A&P Mechanic would know!

Now, roll the conveyer belt in the direction of take-off and the airplane will take-off an a considerably shorter distance...Talk about a short field take-off. Be similar to a carrier take-off.

FJRottie · Dec 12, 2007

Well it won't work cuz there is no airflow around the wings. Airflow=low pressure above and high pressure below the wings and then you have downwash behind the wings. Lift is dependant on the motion of air. Lift increases as the speed of air increases and is dependant on wing angle.

I am suprised MBs would even bother, it is not a mystery. The plane is motionless, even though the wheels are moving.

There is Bernoulli’s principle and there is the first 3 of Newton's laws. Either way you need airfow.

edited - if the airplane's engines are on (which was not stated), that is a different story.

...So, You are stuck in a room with no windows and no doors.

There is a hole in the floor about a foot deep and just barely wider than the diameter of a table tennis ball. There is a table tennis ball at the bottom of the hole.

You have a fork, a wrench, and a long, thin plastic wire.

How do you get the ball out of the hole without damaging anything?

ionbeam · Dec 12, 2007

See what happened to this thread the last time Randy posted this question.

BobG · Dec 12, 2007

Isn't this what happens when you point an aircraft to the west on take off. The plane is essentially moving back wards and still takes off.

The earth is the tread mill, and the plane still lifts.

Bob

wallypiper · Dec 12, 2007

but it will offset the fact that the thrust would move the airplane forward,

The thrust, whether its created by a jet or a prop or a rocket, acts against the air, not the ground (or treadmill).

As said several times, ignoring the possible effect of friction from the wheels being forced to spin so fast, the treadmill has no effect on the plane's forward movement. And forward movement results in airflow over the wings and that produces lift.

A plane that is teathered, even a jet, won't fly unless it generates downward thrust like a Harrier. The C130 idea is interesting. They do have very large props and the prop wash does cover a large wing area and they are very high lift planes capable of flying at pretty low speeds. Might work but that is not how they are designed to fly. They fly just like all other planes - props generate thrust which causes forward movement which creates airflow over the wings. Note that they are equipped with boosters (I think rockets, but maybe jets) to assist in short takeoffs. Theses don't create airflow over the wings, they add thrust to get enough forward speed for flight in a shorter distance.

Randy · Dec 12, 2007

ionbeam said:
See what happened to this thread the last time Randy posted this question.

now, don't i feel like the dork. guess it's time for me to go install the google whatchamacallit on firefox as i did a search and nothing came up.

oh well...my defense is that i didn't post this to renew the discussion, i posted it to talk about mythbusters.

Politics suck! there - can it be closed now?

FJRottie · Dec 12, 2007

Note that they are equipped with boosters (I think rockets, but maybe jets) to assist in short takeoffs.

JATO bottles (Jet Assisted Take Off). 4 on each side (on the air deflector doors) just forward of the paratrooper doors and point about 30 deg downward.

They are hard on the airframe but JATO can enable the C130 to take off in just under 1500 feet.

kansas transit · Dec 12, 2007

FJRottie said:
Well it won't work cuz there is no airflow around the wings. Airflow=low pressure above and high pressure below the wings and then you have downwash behind the wings. Lift is dependant on the motion of air. Lift increases as the speed of air increases and is dependant on wing angle.I am suprised MBs would even bother, it is not a mystery. The plane is motionless, even though the wheels are moving.

There is Bernoulli’s principle and there is the first 3 of Newton's laws. Either way you need airfow.

edited - if the airplane's engines are on (which was not stated), that is a different story.

...So, You are stuck in a room with no windows and no doors.

There is a hole in the floor about a foot deep and just barely wider than the diameter of a table tennis ball. There is a table tennis ball at the bottom of the hole.

You have a fork, a wrench, and a long, thin plastic wire.

How do you get the ball out of the hole without damaging anything?

That's to easy, you piss in the hole untill the tennis ball floats to the top, no tools needed, well i guess some people call it there TOOL. Just gotta think outside the box.

Stan

ionbeam · Dec 12, 2007

i posted it to talk about mythbusters. Politics suck! there - can it be closed now?

Awww, I still want to talk about props pulling (usually, sometimes pushing) the plane through the air, resulting in air flow over the wings creating lift; and reactive engines like jet engines, ram jets and rockets pushing the air frame through the air, resulting in air flow over the wings. Props are not intended to produce air flow over wings for lift. Heck, the plane I fly doesn't even have an engine :blink: (think gliders).

As far as Myth Busters, the test won't be complete until the test vehicle blows up or is allowed to fall to earth resulting in a crater of ??? size -- they would have to actually drop the test vehicle and measure the hole. The MBs tend not to embrace the whole problem or cover all the related details. I think they do have a spin-off program in them, just plain “Busters”, you name it and they can find a way to destroy it in spectacular fashion. :lol:

sonicspeed · Dec 12, 2007

wfooshee said:
tinpusher said:

A jet engine generates thrust, but it doesnt move any air over the wings. Thus, no airspeed.

Click to expand...

To continue beating it up :assassin: The thrust doesn't move air over the wings, unless you count pushing the plane forward. That moves air over the wings.

tinpusher said:

A treadmill will NOT affect thrust, but it will offset the fact that the thrust would move the airplane forward. . .

Click to expand...

Self-contradictory. If it won't affect thrust (true) it can't possibly affect the thrust's action on the aircraft.

The treadmill can't counteract the aircraft's forward speed, because it has nothing by which to "grab" the aircraft (unless the wheel bearings suck or the brakes are on.)

Now, if the aircraft were parked on the treadmill, which then ran backwards to full speed, and only THEN did the engines light, the plane would have to counteract the treadmill's speed, but it would do so easily. The takeoff would take longer than from a standstill, but there's nothing to keep it down.

Can somebody throw something political here so the thread can be killed?

is it a republican plane or a democrat plane??????

ionbeam · Dec 12, 2007

sonicspeed said:
is it a republican plane or a democrat plane??????

Stick a fork in it, it's done.

Airplane on Conveyor Belt Mythbusters 12/12

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