I've just decided that MY hypothetical conveyor belt has a giant magnet underneath it. So, no, the plane will not take off.

My head hurts.

I think.

There is no airspeed relative to the wing, due to the motion of the rolling road/treadmill countering the forward motion of the plane from the engines thrust, so there will categorically be no lift and the hypothetical beast will remain rooted to the surface.

What the engines are doing is not relevent, if there is no air going over the wing, then there is nothing to create lift.

If you turn the plane around and use the treadmill to generate airspeed without turning on the engines, it will creat lift...but it wont fly for long as once it leaves the ground/treadmill there is no forward propulsion mechanism, and drag will slow it down to the point that there is no longer enough lift to keep it in the air.

This one should have died after the second post.

Iain, how does the conveyor keep the aircraft still relative to the ground?

Airplanes are largely made of Aluminium. So a magnet isnt going to do much, unless you get lucky and it acts on the steel landing gear. Although as its not going to take off anyway it really doesnt matter.

Isnt it obvious?

Its moving in the opposite direction / velocity to the direction the aircraft wants to go...just like the treadmill at the gym has a motor that moves the track in the opposite direction to the direction you are trying to run at whatever speed you set it to go....keeping you more or less in the same spot as long as you maintain the preset pace.


Somewhere along the line someone here got the idea that the conveyer was not powered, but was like the beer store trackway or a rolling road dynomometer, relying on friction with the thing on it to provide motion, but Ive never met a conveyer yet that was not powered by a motor in some way to make it move. Otherwise it cant convey anything, and does not live up to its name

No cos the engine is essentially a giant pump, pumping air from the intake side, through the engine where its accelerated by burning fuel, and expelled through the exhaust...so all those thousands of pounds of air/ combustion products moved to create the thrust is not passing over the wing at all, its being sucked through the engine instead. Any suction into the engine if anything is going to be in the wrong direction over the wing...towards the front, where the engine intake is, so if anything that would reduce the aerofoil effect.

In crude terms I suppose you could think of the engine as a tunnel through the wing.

Think about it this way, Iain. I'll take your scenario. Jet aircraft sitting on powered treadmill. We're well into the experiment: jet is at full thrust, treadmill is running at say 500 mph, and aircraft is stationary relative to the air around it. Now, a big invisible hand slides in from behind and underneath the aircraft and lifts it off the powered treadmill. Does breaking the connection with the treadmill cause it to spring into flight?

If it were frictionless you would have a case, but its not. Shut the aircraft engines down, and the friction between the wheels and the treadmill will move the plain backwards.

Its velocities innit...If the plane is doing 200mph north, but the runway is doing 200mph south, then in reality the two counter each other out, and the plane has no velocity and hence no airspeed...unless this is all happening in a hypothetical hurricane too.

Ok, so if a plane's engines provide thrust ... is it reasonable to imagine a plane as a horizontal rocket?

And if so, do we have runways more because they're required for landings?

Could how planes take off now, on runways, be more about a fuel savings (I'm assuming a horizontal 'blast off' would use a lot of fuel) than for reasons to do with physics?

You mean an act of God

No, because the inertia of several tons of modern jet aircraft will prevent it from magically accelerating from zero to whatever speed it needs to create enough lift...what is that, say 100mph for the sake or argument.

Gravity will make it will bounce back down to the clawing friction of the hypothetical treadmill runway long before it has the chance to build enough airspeed to generate the lift needed to keep it in the air.

Now, if God him/herself reaches down and grabs the plane who am I to say what will happen...thats a theological argument, not a technical/ scientific/ engineering one, so I'd defer to the god squad should that ever happen

Did none of you lot scrape together an O level in physics or what?

Or put another way:

No thrust is being generated because there's no movement of the plane relative to the ground to generate an effective headwind.

What if the plane is on the ground, and under controlled circumstances, a headwind is increased from zero to liftoff velocity. Will the plane achieve vertical lift? Of course, the friction keeping the plane on the ground (horizontally) instead of it blowing backwards would also have to be sufficient.

Much more of the runway length is required to land and stop a plane than to get it in the air.

If you want to save on jet fuel turn the treadmill around, and use it to accelerate the aircraft to air speed without using just the engines. Or use a giant catapult...just like on an aircraft carrier. You are probably familiar with the catch wires on a carrier too, the take off is violent, but the landing is much more so.

But energy being what it is, your overall energy usage by the time you take all the conversions and loss factors into account is probably worse than just converting fuel into heat and thrust?

Its not really a rocket situation. A rocket uses only the motor thrust to counter and overcome gravity. Its a lot more thrust. An aircraft uses Bernoulli's effect to generate a pressure differential, which in effect sucks the aircraft into the air to overcome gravity.

Thrust in the aircraft is used solely to generate enough motion for the airspeed to be high enough for the wing to generate enough suction/ lift to get the plane into the air, and also of course to push it along from point A to point B. If you want to make a vertical take off you need vectored thrust like on a Harrier Jump Jet, but its a lot of machinery and complexity, and a massively over powered engine compared to what you would need for the same performance from a regular setup. The US military also have the V22 "osprey"...a larger vertical take off aircraft. Lots of them crashed in the development phase...it technically very hard to make the transition from straight up thrust to forward motion...rockets use balistic trajectories to arrive on target, but that makes for a bumpy landing.

Rockets dont tend to have very slow, controlled landings. Not important if you are only trying to deliver something thats going to explode anyway, but not good if you intend to have repeat customers on your airline.

If you rely on thrust alone to get you airbourn, then if you have an engine failure you are in deep shit. On the other hand there are several examples of conventional aircraft managing to glide safely back down to earth after engine failure, using the wing and momentum alone to generate lift

Ahh 3 pages already, it's going well...

But with joggers on a treadmill, cars on a rolling road, etc, the overwhelming difference is that the soles of your shoes, the (winter, if it's icy ) tires on your car, provide a friction surface that moves the thing one way as the belt moves it the other because the muscles in your legs, or the engine of your car, are providing motive power through the contact patch with the ground. In the case of an aircraft with free-rotating undercarriage, that is different - the motive power is not being transmitted through the wheels. There is no mechanism other than the friction in the bearings by which the motor in the conveyor system can transfer its force to the aircraft. It doesn't matter how powerful a motor you drive the conveyor with, it can't keep the aircraft stationary if the wheels are free to rotate. The problem can't be solved because the premise used to construct it is false.

You bar steward, you've wrecked an otherwise productive afternoon