777 down.

[blondini]

Megawatt, I've taken all your digs with good grace so don't be turning grumpy on me. It's all good intentions as far as I'm concerned.

In an earlier post you've dismissed rolling resistance of the aircraft wheels.

Me. The conveyor is exerting an equal and opposite reverse thrust against the aircraft solely through the rolling resistance of the aircraft wheels.

You. Nope, the conveyor isn't exerting any thrust at all to the airframe with the brakes off it is simply spinning the wheels. As I've already said, as the engine thrust is increased the aircraft will react as normal ... Only difference will be the speed of the wheels.

Don't you accept that there is some rolling resistance acting against an aircraft even on a smooth surface?

 

[Trazor]

........

 

[imamartian]

.... but !

what if the plane were coming in to land, and the runway was moving in the same direction and at the same speed as the plane.... would the plane land or crash?

Nurse! can i have my straight-jacket, oh and double my medication will you, i've been reading DIYnot again !

 

[megawatt]

Now I find that amusing as I have a broad sense of humour but I'm sure someone will complain to the Mods and get the thread locked now

 

[securespark]

Both!

I assume that the plane would land, but would not slow down sufficiently (as the runway would be expected to be doing a GT of 0mph), therefore it would overrun & crash.

 

[Softus]

what if the plane were coming in to land, and the runway was moving in the same direction and at the same speed as the plane.... would the plane land or crash?

It would land, in the sense that it would touch down, then it depends on your definition of a moving runway.

When accelerating to take-off speed, a plane doesn't rely on traction between the wheels and the ground. When braking (if using the wheels to brake), it does.

To those people who think that a plane on a conveyor wouldn't take off, you're right - as long as you're right in thinking that several hundred thousand Newtons of thrust aren't capable of overcoming the drag of the wheels rotating at double-speed.

That's basically a tug-o'-war between this:

and this:

I know which one my money is on.

 

[blondini]

To those people who think that a plane on a conveyor wouldn't take off, you're right - as long as you're right in thinking that several hundred thousand Newtons of thrust aren't capable of overcoming the drag of the wheels rotating at double-speed.

Where do you get the 'double-speed from' Softus?

In the hypohetical OP there is no limit placed on the speed that the conveyor can achieve as it is simply stated that it will be able to match the wheelspeed of the aircraft.
If there is x amount of rolling resistance at a given wheel speed, is it reasonable to assume that as the wheel speed increases, the rolling resistance increases too?
With no limit on the wheel speed, it is theoretically possible for the rolling resistance to increase to a point where it achieves equilibrium with the engine thrust which does have a limit.

 

[megawatt]

With no limit on the wheel speed, it is theoretically possible for the rolling resistance to increase to a point where it achieves equilibrium with the engine thrust which does have a limit.

Says who?

If there is x amount of rolling resistance at a given wheel speed, is it reasonable to assume that as the wheel speed increases, the rolling resistance increases too?

Marginally but it is by no means directly proportional as it is a factor of resistance to inertial movement (environment) and resistance of components (bearings etc) ... We can assume the former is static and that the dynamic component (the additional speed of the conveyor) is only dependant upon the latter which is negligable.

I was going to draw and post a diagram to show how the inertial forces generated by the engine thrust are not relative to the ground but Softus has explained this better than I could have.

In this scenario there is no way that the rolling resistance on the wheel bearings could ever amount to the order of thrust produced by the aircraft's engine(s) ... Trents produce up to 74,000lbs ... or, in fact, anywhere close and I don't care how fast the wheels rotate.

There is a very real likelyhood that the bearings would seize, the heat packs would catch fire and the whole thing would crash and burn but that isn't the crux of the OP's question.

Practically or theoretically Blondini the aircraft's airborne.

MW

 

[Softus]

Where do you get the 'double-speed from' Softus?

The answer is contained within your very next statement.

In the hypohetical OP there is no limit placed on the speed that the conveyor can achieve as it is simply stated that it will be able to match the wheelspeed of the aircraft.

Therefore, at take-off velocity on a conveyor, the wheels will be going twice as quickly as without a conveyor.

If there is x amount of rolling resistance at a given wheel speed, is it reasonable to assume that as the wheel speed increases, the rolling resistance increases too?

Yes.

With no limit on the wheel speed

The limit is double the take-off velocity.

it is theoretically possible for the rolling resistance to increase to a point where it achieves equilibrium with the engine thrust which does have a limit.

Only if your theory allows you to ignore such practicalities as the inability of wheel bearings to rotate at infinite speed.

 

[bernardgreen]

.... but !

what if the plane were coming in to land, and the runway was moving in the same direction and at the same speed as the plane.... would the plane land or crash?

Aircraft carriers steam into the wind for landings and take off, this is the same direction as the direction the aircraft is flying. The majority of landings are OK

The one place where the "conveyor belt" effect is significant is when a flying boat is taking off from moving water. The drag on the floats is significant and does have to be ovecome by extra power on the motors. Some aircraft ( American military, south Pacific air-sea rescue ) had disposable solid fuel rocket boosters to assist take of from water.

 

[tim west]

He He

 

[Softus]

Aircraft carriers steam into the wind for landings and take off, this is the same direction as the direction the aircraft is flying.

AFAIK, jets that land on aircraft carriers without hovering do so by virtue of a tailhook that catches on arrestor wires, much like this:

See here for an explanation.

The fact that the carrier is moving in the same direction as the jet makes the landing safer, not more dangerous, because it effectively increases the length of the runway from impossibly short to merely ludicrously abrupt.

The one place where the "conveyor belt" effect is significant is when a flying boat is taking off from moving water. The drag on the floats is significant and does have to be ovecome by extra power on the motors. Some aircraft ( American military, south Pacific air-sea rescue ) had disposable solid fuel rocket boosters to assist take of from water.

Oh, very droll.

 

[bernardgreen]

quoting from

http://www.check-six.com/Coast_Guard/Salem_Armed_Forces_Day_Crash.htm

It was a beautiful sunny spring day in Salem and over a 1,000 people had gathered around Salem Harbor to see the Armed Forces Day activities. One of the featured events of the day was to be the Jet Assisted Takeoff (JATO) for one of Air Station Salem's UF-1Gs.

JATO bottles, still in use today by the military, are small solid propellant rocket motors, mounted on the side of an aircraft in multiples. On the UF-1G, JATOs were mounted near the rear, two on each side. A JATO launch allows an aircraft to take off on a short runway with a heavy load, or in the case of an amphibious aircraft, allows them to get in the air faster in rough seas.

 

[Softus]

Don't you think that's on a different planet to the OP?

 

[noseall]

airplanes are steam catapulted off a ship, with the pilot NOT touching the controls and they land with engines at full throttle, so to be able to abort landing and gain altitude with immediate effect.