Will It Take Off?

[YT2095]

crucial for plane to stay aloft is AIRSPEED, if it`s not moving relative to the air at a set velocity it will fall (or in this case not take off).

 

consider an ARV, there props rotate at a fixed speed, plenty fast enough to keep them aloft and maintain a fixed airspeed, but even they have to be either catapaulted or rocket launched at 1`st to Get to that rate, once aloft maintaing that speed is easy

[SkyQueen737]

Keep in mind that an aircraft can stall at ANY airspeed, not just Vso/Vs. Stalls occur when the critical angle of attack is exceeded.

 

Correct me if I'm wrong, but the original post has the aircraft sitting at the end of the "rwy"...as the airspeed accelerates, the belt would equal the speed going in the opposite direction. If there is no forward motion of the aircraft, you wont get any indication of airspeed in the cockpit (IAS - indicated airspeed). The ASI works based on the comparison from the pitot tube (senses ram air) and the static port. We use IAS to base our rotation speed, since groundspeed and IAS can be VERY different due to head/tail/cross winds. I've had an indicated airspeed of 100, but the GPS showed my GS as being 130.

 

Like someone mentioned, the only real airflow will be from the prop...which isn't sufficient enough to produce the lift required to rotate. It does however effect lift...this is why the nose drops when you reduce power...and climbs when you add power.

 

If you don't have enough lift, your not going very far.

 

Did I misread the original post?

[Bettina]

Keep in mind that an aircraft can stall at ANY airspeed' date=' not just Vso/Vs. Stalls occur when the critical angle of attack is exceeded.

 

Correct me if I'm wrong, but the original post has the aircraft sitting at the end of the "rwy"...as the airspeed accelerates, the belt would equal the speed going in the opposite direction. If there is no forward motion of the aircraft, you wont get any indication of airspeed in the cockpit (IAS - indicated airspeed). The ASI works based on the comparison from the pitot tube (senses ram air) and the static port. We use IAS to base our rotation speed, since groundspeed and IAS can be VERY different due to head/tail/cross winds. I've had an indicated airspeed of 100, but the GPS showed my GS as being 130.

 

Like someone mentioned, the only real airflow will be from the prop...which isn't sufficient enough to produce the lift required to rotate. It does however effect lift...this is why the nose drops when you reduce power...and climbs when you add power.

 

If you don't have enough lift, your not going very far.

 

Did I misread the original post?[/quote']

 

This has nothing to do with how a plane flys. The plane is not tied down in any way and he says it is free to move in one direction. So, since a plane moves via its engine, it will reach its TO airspeed and take off no matter how fast the "runway" moves in the opposite direction.

 

Bettina

[SkyQueen737]

That has a lot to do with how an airplane flies. Though certainly, as long as there's enough lift...it will fly. You also don't need to reach takeoff speed in order to get airborne, consider ground effect...the aircraft will get off the ground before having sufficient airspeed to climb. (ie soft field takeoffs)

 

I don't think I get the scenario then. If the belt moves as fast as the aircraft is supposed to be going, how are you getting airflow over the wings? No airflow, no lift...unless you throw in a huge gust or something. That's always fun to handle in a real plane.

[padren]

That has a lot to do with how an airplane flies. Though certainly' date=' as long as there's enough lift...it will fly. You also don't need to reach takeoff speed in order to get airborne, consider ground effect...the aircraft will get off the ground before having sufficient airspeed to climb. (ie soft field takeoffs)

 

I don't think I get the scenario then. If the belt moves as fast as the aircraft is supposed to be going, how are you getting airflow over the wings? No airflow, no lift...unless you throw in a huge gust or something. That's always fun to handle in a real plane.[/quote']

 

SkyQueen, you are correct if the assumptions the original poster made about the conveyor belt are accurate - that the belt could move backwards at a speed capable of negating the forward pull of the aircraft's engine.

 

However this is reasonably flawed, because the airplane does not gain airspeed by accelerating with its wheels, but by pulling against the air with the propeller. Since the backwards moving conveyor cannot counter the forward pull of the engine through the air, the plane will move ahead relative to both the still air the prop is pulling itself through, and to the ground surrounding the conveyer, regardless of what the conveyor is doing.

[SkyQueen737]

I get what you mean...but how are you developing the lift with no airflow then? I think that's why I'm confused here. Thanks.

[padren]

I get what you mean...but how are you developing the lift with no airflow then? I think that's why I'm confused here. Thanks.

 

Lets say, you put on a jetpack, goggles, and rollerskates. Not to fly, but just to get around town.

 

Someone puts you on a conveyor, because they want to test how fast you can go without you having to leave the lab.

 

But, no matter how fast the conveyor goes to counter your forward motion, all it does is make the rollerskate wheels roll faster. Only the ground is moving, but your jetpack is still pushing you through the still air, giving you the same true ground speed and air speed as if you were rolling down the street. Your "conveyor ground speed" would be much faster, since its trying to move backwards to slow you, but failing to do more than cause a slight drag via friction within the rollerskate wheel mounts.

 

Since the jetpack pushes against air, you would need a wind tunnel to test it and have a countering airspeed instead of a countering ground speed, since the ground plays no part in propulsion.

 

So if you did do that with a plane, that airspeed would be moving over the wings and it would lift in place, or if you tried the experiment in how the OP described, the plane would move forward and leave the runway as fast as if it was not a conveyor.

[SkyQueen737]

I get what your saying. Next time I go flying I'll see if they can't build me a big belt and we'll try this - joking of course. :D Thanks a ton.

[Klaynos]

I agree totaly with what padrenis saying and would just like to emphasis some important things here:

 

The wheels are not powered, no matter how fast they go past the ground it bears no relation to the actual velocity of the object/aircraft (within certain bound obv.)

 

The important thing about aircraft take off, is air speed, the velocity relative to the ground is completely irrelevent, and is only a usefull guide for considering situations where the wind can be considered to be negligable.

[SkyQueen737]

Airspeed is everything. If you can get an IAS of around 50, it'll take off. I still can't visualize this happening and still have my doubts, but I'll take yalls word for it. Not like this is something we use in aviation everyday anyway. lol. Yall know physics very well, I don't - heck of a lot smarter than I'll ever be. *bows down*

[YT2095]

that only applies IF the air is moving as fast as the conveyor belt, prop speed alone wont allow the plane to take off.

 

consider this: we have a plane suspended on a tower and kept stationary (thus elliminating the conveyor idea totaly).

we get the props up to take off speed but still keep the plane in situ.

we then let go...

 

what do you think happens at this point?

 

does it fly as if it was never held in the 1`st place?

 

or does it drop like a stone towards the ground with a little forwards motion during, with a pilot PRAYING he can gain enough speed during this drop to get some lift and pull up?

 

my money`s on the latter!

[SkyQueen737]

I'm gonna have to go with YT here. That's the same concept as a power on stall. You can stall an aircraft will full power, we do it all the time for practice. Once the stall "breaks" you have to lower the nose, decrease angle of attack and get the air flowing over the wings again. Maybe if you throw in a 50kt gust you'll get it airborne, that certainly happens in storms.

 

Maybe I'm still looking at things too much through the windscreen here...but I just don't see the thing flying.

[YT2095]

the prop or jet (for some designs) is only to maintain airspeed for power on a craft with a small wing span to weight ratio.

were the situation is reversed (a greater wing ratio to weight) we can have such craft as Gliders they use No power (except for the occasional thermal) at all

 

it`s ALL about wings through Air and LIFT!

prop and jet trust really only comes in to play with VTOL craft such as Harriers and copters

[Jacques]

Me too I don't see how it could lift.

Unless the propeller is big enought and the engine powerfull enought to create a large airflow on the wing. But nothing like that stated in the initial post.

[padren]

Me too I don't see how it could lift.

Unless the propeller is big enought and the engine powerfull enought to create a large airflow on the wing. But nothing like that stated in the initial post.

 

In any example where the air is still, and the plane is still, you will have no lift, I think we are all on the same page with that.

 

In the initial post, they try to keep the plane still, but implement a flawed method for keeping it still, therefore the plane will move forward through the air.

 

Take two boats in the water with exceptionally low water resistance.

One has a propeller, the other is a fan boat.

 

They both try to move against a 10mph current with their engines set to go 10mph in still air.

 

The fan boat moves ahead, the propeller driven boat stays relatively still. Crank the speed up to 90 MPH or so, add some airfoils, and you have the same situation presented in the example.

[[Tycho?]]

Instead of a conveyer belt lets use frictionless ice as an example here.

 

Put a car on this frictionless ice. It will not move, no matter how quickly or how slowly you spin the tires. Without friction, the car will not move forward. This would be the same with a conveyer belt that negates the speed of the wheel; car remains stationary.

 

But what happens when you have a rocket car on frictionless ice? The wheels and the ice dont matter, the rocket is propelling itself forward independantly. This is the same as if it were on the conveyer. How fast the wheels are moving does not matter, the jet or plane or whatever will move forward because it is pushing itself through the air.

[Bettina]

Sorry to be such a pain, but my answer still stands.

 

The original post is not flawed and the example given was clear. The plane is not tied down and free to move. As the propeller screws thru the air, it pulls the plane along....thru the air.....and takeoff speed will be reached......and it will take off. The wheels of the plane offer little resistance against the propeller.

 

What would happen if it was a jet aircraft capable of 27000 lbs of thrust. When that jet lights up, its going to move irregardless of the conveyer belt moving in the opposite direction.

 

Sorry, but I'm positive this is correct.

 

Bettina

[Bettina]

Sorry to be such a pain, but my answer still stands.

 

The original post is not flawed and the example given was clear. The plane is not tied down and free to move. As the propeller screws thru the air, it pulls the plane along....thru the air.....and takeoff speed will be reached......and it will take off. The wheels of the plane offer little resistance against the propeller.

 

What would happen if it was a jet aircraft capable of 27000 lbs of thrust. When that jet lights up, its going to move irregardless of the conveyer belt moving in the opposite direction. Groundspeed has nothing to do with airspeed.

 

Sorry, but I'm positive this is correct.

 

Bettina

[padren]

Sorry to be such a pain' date=' but my answer still stands.

 

The original post is not flawed and the example given was clear. The plane is not tied down and free to move. As the propeller screws thru the air, it pulls the plane along....thru the air.....and takeoff speed will be reached......and it will take off. The wheels of the plane offer little resistance against the propeller.

 

What would happen if it was a jet aircraft capable of 27000 lbs of thrust. When that jet lights up, its going to move irregardless of the conveyer belt moving in the opposite direction.

 

Sorry, but I'm positive this is correct.

 

Bettina[/quote']

 

You are correct, except I do think the original post is flawed, because I am pretty sure the fellow intended the conveyer to stop the plane from being able to move forward.

 

This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

 

If the poster intended this to have no effect on the plane, then you are correct in that it was not flawed. If he intended this to counter the plane's forward motion, than it is flawed. If it was intended as a brain teaser than it makes sense why he'd include a red herring, but I thought he was asking the question genuinely.

[SkyQueen737]

I understand now. I was thinking a little too literal. I fly airplanes, and this is something that's just beyond rediculous. I had to put aside the REAL way to fly a plane and try to visualize this. Once I did, I caught on. The airboat and ice examples helped a lot. Thanks padren and tycho.

[swansont]

Sorry to be such a pain' date=' but my answer still stands.

 

The original post is not flawed and the example given was clear. The plane is not tied down and free to move. As the propeller screws thru the air, it pulls the plane along....thru the air.....and takeoff speed will be reached......and it will take off. The wheels of the plane offer little resistance against the propeller.

 

What would happen if it was a jet aircraft capable of 27000 lbs of thrust. When that jet lights up, its going to move irregardless of the conveyer belt moving in the opposite direction.

 

Sorry, but I'm positive this is correct.

 

Bettina[/quote']

 

 

Bettina is right (other than using "irregardless")

 

The conveyer belt exerts a torque on the wheels but, absent friction, that only causes rotation and not an force on the center-of-mass. The plane moves forward, relative to the ground, due to the force exerted by the propeller, and can take off. Unless the wheels sieze up.

[swansont]

You are correct' date=' except I do think the original post is flawed, because I am pretty sure the fellow intended the conveyer to stop the plane from being able to move forward.

[/quote']

 

 

I disagree. The conveyer doesn't have that effect. I think it was put there to cause confusion and discussion - this same question has popped up in several places on the net.

[Jacques]

I understand the mistake I made...

Looking at the force involved, it is easier for me to see that the conveyer belt will not be able to bring back the plane to a rest position. So the condition in the original post can never be met unless you put the brake on the plane...