Brain Teaser

Agreed. Having read the forum thread I found the teaser in, I also thought that the plane would take off as well.

The only thing I can't figure out is how much faster than normal the plane would have to go in order to get sufficient lift.

A helicopter would have no trouble, nor would a Harrier.

However, assuming the "ground" upon which the plane is standing is always moving in exactly the opposite direction to the plane and at exactly the same speed then a conventional fixed wing aircraft will make nil airspeed and will not be able to achieve lift so will not take off.

If the plane wheels are turning only because it is being driven forward by its engines, and the conveyor belt is responding equally in reverse to the wheel rotation, the aircraft will remain fixed relative to the ground. It will receive no lift because it isn't moving through the air so it will stay on the ground. There won't be much air getting into the engines either.

In theory of the headwind was equal to the take-off speed required the 'plane will take off provided there is enough air flow into the engines. A Fiesler Storch has high-lift anti-stall devices and thus can actually hover in the air relative to ground because its stall speed is less than wind speed, and if wind speed was say 40 mph then it will hover. My late fatherin law actualy did one better than tghat once in an RAF Chipmunk and flew backwards, as wind speed was so strong that as stall speeds was exceeded, the 'plane flew and the speed was high enough to beat the prop thrust! An observer would see the 'plane fly backwards!

Oh! If a 'plane could actually temporarily lift off from the conveyor, the lack of resistance from the tyres would enable it to hop in the air, and then engine thrust would enable forward speed to increase airflow over and under the wings.

Exactly! That's why I want to know how much faster the plane would have to go in order to get sufficient traction to begin moving air over its wings.

Aircraft don't have traction. The wheels are only there to allow a plane to roll along the ground (In some parts of the world, aircraft have floats or skis for operating from water or snow.) Movement is the result of the jet thrust or airscrew acting on the atmosphere.

Hence an aircraft needs air moving backwards (relatively) over the wings to acquire and maintain flight. This is the principle a kite uses to fly.

Thus the "conveyor belt" moving in the opposite direction will prevent the aircraft from moving forward and create an airflow over the wings.

My closing thoughts are well they were but I have risen to a challenge!:

a) There will always have to be a time delay between the aircraft speed increasing through thrust, propellor wash or air flow and the belt speeding up to comenstae. This means that it would be theoretically possible to move forward for a limited period of time. This might be sufficient to take off.
b) Even if the aircraft is stationary relative to ground, if windspeed can be increased it will take off if the lift component is sufficient. It is thus irrelevant if the aircraft is for all intents and purposes not advancing.

As I said before, this analogy does happen in the real world, when forward speed is matched or exceeded by wind or air speed. It is the air flow above and below the wing and the pressure differential that matters. That differential provides lift, or in the reverse direction, groundforce as in a racing car's airofoils. If a powerful fan was put in front of the ensemble, sufficient air flow could be induced to cause the 'plane to lift off. The same would apply if we had to use as I mentioned before an aircraft carrier, or even a fast-moving road vehicle [that has been done..to take off and land]. It's the wind or air speed over the wings that determines whether it can take off or not.

Well, I can say that I have personally witnessed b), well almost! HMS Daedalus, 50th D-Day anniversary [Lee on Solent airfield] : the Fleet Air Arm's Fairey Swordfish took over against a strong headwind in just a few feet at slow speed.

I have also seen a friend's gyrocopter, G-GYRO which he made himself take off instantaneously after he had self-rotated the rotors. However, we are not talking of rotary craft here, presumably.

I also remind that an aircraft can lift off even if tied down! Witness Hurricane Andrew which devastated a USAF base. It is wind or rather more correctly, air speed over a lifting surface that provides lift or creates downforce. I should also add in here that air temperature can also affect lift, as can air pressure. But that's getting off the nub of the Teaser.

Getting back to the conundrum, the aircraft is said to be on a runway [and at this point you have to read it carefully and think laterally]. By that notion it is implied that there is, unless there is a dead calm, some form of air flow is available. Apart from physically moving the whole ensemble, that is aircraft and conveyor, on some form of road trailer to get the flow going [which is of course one asnwer to the condundrum] then that aircraft is not going anywhere until the air flow over the wings reaches critical point. That could be achieved by a fan or series of fans or waiting for wind velocity to increase. Therefore my answer to the question:

Will the plane take off or not? Will it be able to run up and take off?"

is "It is assumed that as it is a runway, and it is in the open air because the aircraft is hoped to take off, there must be a wind available although not necessarily at any given time. Short of moving the conveyor on a road trailer or sending it down an inclined plane or off a cliff, the aircraft will not be able to take off unless its critical lift component can be exceeded by either augmenting airflow by means of a fan or turbine, or by waiting until the wind speed is high enough. The question does not mention types of aircraft but some with supercritical wings and other higher-lift devices would require less air flow than more conventional types".

I should of course have pointed out that a sloping runway, and by golly I have been on one...Compton Abbas is on a hillside...or using a catapult to launch the conveyor would also assist air flow but that's taking things to extreme.

My reply to the question is based on a little bit of aeronautical engineering and a heck of a lot of flying over the years in light aircraft..over land and water!

The OP quite explicitly states that the speed of the belt is matched to the speed of the aircraft. Therefore as the a/c gains speed so does the belt, therefore no forward movement relative to starting point, therefore no increased airflow over the wings.


True, hence sea and ski planes.


Yes it is, because as the a/c speed increases so does the belt's.


Quite true, but until the a/c is airborne it needs to have groundspeed as well. Since the belt is running at the same speed in the opposite direction the a/c is unable to gain groundspeed.


In Canada floatplanes are widely used. Until we built large airports seaplanes were amongst the largest commercial a/c.

en.wikipedia.org/wiki/Flying_boat
www.photovault.com/Link/Technology/Aviation_General/show.asp?tg=TAGVolume06/TAGV06P13_17


True.

There is a forward movement relative to anything that is not attached to the runway/belt (i.e. passengers looking at the plane from the terminal building, signs and lights next to the runway and -very important- the wind).


Why would a plane need groundspeed? It just needs airspeed, if the natural airspeed/wind is not enough (which it hardly ever is) it creates it by pulling itself forward. As a side effect of this it usually also creates groundspeed but that is a byproduct and not necessary for lift.

It is important to note that the plane pulls itself forward by pulling at the air in front of it. The air is not attached to the belt so the belt has no effect on it.

I think it may be pertinant to mention that this question has been discussed in a number of places, with equally diverse views.

www.google.co.uk/search?q=A+plane+is+standing+on+a+runway+that+can+move&start=0&ie=utf-8&oe=utf-8

You'll need to copy the whole url and paste it into the address bar.