Topic: Can this airplane lift off?

[Borkunit]

I guess the fundamental issue is a difference in our interpretations of how the force works.

We agree that the forward thrust is created by the props acting on the air, completely independently of whatever the treadmill is doing.

Your argument is that the treadmill exerts an equal and opposite force on the plane than the props do, thus allowing no forward motion of the fuselage, therefore no airspeed, therefore no lift.  This is where we disagree.

Consider a theoretical frictionless world where the treadmill is moving and the plane is not trying to accellerate; I believe the treadmill would simply spin the wheels and exert NO force in a rearward direction on the plane.  The bearing in this situation is a frictionless lubricant separating the plane from the treadmill.

If I understand your position, you're saying that even in the frictionless world the treadmill would still exert a rearward force on the plane.

In the real world, the purpose of the bearings is to minimize the unavoidable friction as much as possible thereby allowing the fuselage to move back and forth on the tarmac as easily as possible.  If the bearings were unable to reduce the force of the tarmac on the fuselage, then the thing might as well have fixed wheels.  The plane would be unable to take off regardless of what the treadmill is doing unless it could generate enough force to overcome the static friction between the tire and treadmill.

So, my summary point is that the bearings serve to minimize force from the tarmac on the plane, therefor there is a net vector of force in the forward direction.

[Markcb750]

As a student pilot myself. (up to 4 full hours!)


The Myth Busters video is conclusive, the physics are undeniable: the propeller will accelerate air reward, air has mass so a reaction force will accelerate the aircraft forward. Without the brakes applied the conveyor will just make the wheels spin faster, as others have pointed out.


When an aircraft engine is tested with the wheel brakes on, no relative motion can occur between the wing and the air because the wheels cannot turn, the reaction force is absorbed into the Earth.


The conveyor has to be pretty long, or the aircraft will leave it, Jamie and Adam where wise in test/demonstration design.

[Markcb750]

Dukie is just f'n with us...seriously...I think...he must be

[tramp]

what about a glider
a glider has a wheel that would spin with the conveyor belt
no forward motion no fly

[SD750F]

Again look at my prior comment on the second page.

The wheels are free spinning. They do not provide the forward movement required to create lift. The prop does that job. But like I said in my comments before, the wheels will free spin at a higher RPM than they would on a fixed surface like a runway. This is a math problem similar to one that has been used for years in a Calculus class. It will demonstrate a basic relationship of a couple of variables that can be calculated at any time. Time being the third variable.

[MCRider]

The wheels allow forward movement of the plane to get airflow over the wings which create lift.
No forward movement, no fly. The theoretical conveyor belt prevents forward movement and matches the thrust of the engine, pound per pound.
The bearings push on the axle, which in turn moves the plane in the rearward direction, they don't just rotate.

But the conveyor belt DOESN't prevent forward movement that's the whole thing right there. Its as if the plane was suspended by little hovercraft feet. It doesn't matter what the wheels do. It doesn't matter what the pavement does. The thrust of the engine is completely independent of the wheels. The wheels are not geared to the engine.

You'll wake up someday and smack your forehead... could've had a V8! I did.

[MCRider]

what about a glider
a glider has a wheel that would spin with the conveyor belt
no forward motion no fly

The glider doesn't have an engine/propeller. With an engine/prop it would have forward movement.  Regardless of what the wheel does.

[MCRider]

Look at it this way - the purpose of the free-spinning wheels on a plane is to NEGATE the influence of forces of the tarmac on the plane.  If the wheels were fixed (didn't rotate) the plane would never take off, unless it generated some massive amount of thrust and was able to drag itself over the asphalt.  The wheels are basically a lubricant that allows the plane to slide over the runway as freely as possible.  

yes- it may be easier to see if one could imagine the wheels as little hovercraft feet not even touching the ground, but providing no forward or backward resistance. So you could literally drag the tarmac beneath it and the plane would still go forward. All the wheels are doing is keeping the plane off the ground.

[cb650]

There are a couple "experts" out there that havent chimed in.  I wonder why. Must be on vacation.       Ground speed "conveyor" dont matter.  You can hover a plane with a good headwind.  

[Duke McDukiedook]

Ok, I'll try and explain this as simple as possible. He says the runway will match the speed of the tires so that means the plane will have zero forward movement.

And that's where you're going wrong in your reasoning, right from the beginning.  The conveyor belt will only match the speed of the wheels, not the plane.  In fact, the conveyor belt will determine the speed of the wheels.  The wheels can and will spin freely without regard for the forward airspeed of the airplane itself.  

The propeller, which is not in any way affected by the movement of the runway, will still pull the plane forward through the air, and moving forward through the air is what causes a plane to fly, not the speed that the wheels are spinning on the ground.  

Keep typing things like "I'll try and explain this as simple as possible", and the crow you'll be eating will just keep getting bigger.

And when you prove me wrong with sound science Gordo, I'll eat that crow quite happily. So far you haven't.

[mattcb350f]

I got a better one, how bout' a helicopter on a turn-table?


Seriously, has anyone else googled this......other than Dukie  

 Matt.

[Sam Green Racing]

I was just going to throw in my two cents but I've just spewed tea all over the screen
Sam.

[shacolaid]

Planes take off into the wind, why? To help get lift. When you run on a treadmill, does  your hair blow back? No. You are running at the same speed as the belt.

The propeller is pulling the airplane through sky allowing forward air speed. Lift is created by the air passing over and under the wings. Only  helicopter blades create their own lift and thrust.  

Every aircraft has a stall speed due to lack of airflow over the wings, this happens when the pressure under the wing and over the wing equilibrate. The wings lose their lift and airplane stalls. ie drops from the sky. THerefore an aircraft travelling at 0mph ground speed on a conveyor belt is below the stall speed. EVen if the aircraft is facing into the wind.

T

[Duke McDukiedook]

You have to keep friction in the equation, you do have to keep this relatively real world.
The plane weighs let's say 10,000 lbs. This downward force is exerted on the frame to the bearings to the wheels. The belt matches the speed of the wheels no matter what speed it goes, the friction force on the bearings and wheels would increase with speed which provides rotational and translation forces to the plane (friction forces would increase with increase in velocity).

The only way the plane could overcome this is to produce more thrust than the weight on the wheels and bearings and friction on the wheels from the belt. If you can produce real world numbers on this I'll gladly eat your crow Gordo.

[Markcb750]

I got a better one, how bout' a helicopter on a turn-table?


Seriously, has anyone else googled this......other than Dukie  

 Matt.

If the turn-table is set to be equal and opposite to the rotary wing of the helicopter this wing will have zero velocity relative to the air, no lift will occur but the pilot will become a little dizzy.

Don't know about Google in this, too much misinformation out there.  Try Wikipedia.

[MCRider]

Planes take off into the wind, why? To help get lift. When you run on a treadmill, does  your hair blow back? No. You are running at the same speed as the belt.

The propeller is pulling the airplane through sky allowing forward air speed. Lift is created by the air passing over and under the wings. Only  helicopter blades create their own lift and thrust.  

Every aircraft has a stall speed due to lack of airflow over the wings, this happens when the pressure under the wing and over the wing equilibrate. The wings lose their lift and airplane stalls. ie drops from the sky. THerefore an aircraft travelling at 0mph ground speed on a conveyor belt is below the stall speed. EVen if the aircraft is facing into the wind.

T

The presumption is that the conveyor belt moving backwards is keeping the plane from moving forward, it simply is not. If the wheels were driven, and that is what the plane depended on for forward motion, then the conveyor would 0 it out.

But the wheels are not driven. they will do what ever is beneath them causes them to do. The plane is driven by its propellor. And it is unimpeded by the conveyor belt, the plane will move forward at the same speed the prop would have it move forward if the conveyor belt wasn't there.

Just as you say: the propellor is pulling the airplane through the sky... exactly!

[Duke McDukiedook]

Read my last post MC.

[Markcb750]

I might be wrong here about who is jerking who.



Friction in the wheels?

Damn

What about a ski plane on a river?

[MCRider]

Read my last post MC.

I've read every post here. Did you see the video? The plane will take off, the wheels are irrelevant.

[MCRider]

I might be wrong here about who is jerking who.



Friction in the wheels?

Damn

What about a ski plane on a river?

Now that's real friction.

[Markcb750]

perhaps I should have said pontoon.

[shacolaid]

propellers give an airplane foward thrust not lift. so if an airplane is on a conveyor belt running at the same speed as the airplane, the airplane is going 0 mph. This means no air flow over the wings to create lift. This is why aircraft carriers have steam catupault systems and runways. I am not sure of the stall speed of the airplane shown on the video but i know it is faster than 0 mph, more like 80 to 90 mph.  An airplane will not fly below its stall speed it will go into a flat spin and fall from the sky like a rock.

[masonryman]

OK I did not read every post in it's entirety but the tread mill dose not matter unless it some how induced drag on the plane it'self and it is impossible for the treadmill to match the wheel speed because the plane is pushing against the air not the ground.

Now if you would have said there was a giant fan behind it sucking all the pressure created by it's power source that would be a whole nother ball of wax

[Caaveman82]

Dookie is an engineer, he takes laws serious you guys.  

But dook, as to your challenge to Gordon, can you do the math to prove it won't work?

Not instigating, but both of you raise good points, so I think you should both be assigned with this homework...

[Duke McDukiedook]

No they aren't irrelevent. If the thrust from the motor is greater than the friction forces created from the weight of the plane on the bearings and wheels from rotation and translation due to forces exerted by the belt, the plane will fly. The wheels and friction forces from them figure into the equation.

Plane in a river flowing will fly due to thrust forces being greater than drag forces from water on the skids.