This is not exactly car related, but I thought it would be a cool question to post
I’ve seen this on another forum and they had 40+ pages of discussion! LOL
Anyways, here’s the question:
A plane is sitting on runway that can move (some sort of giant conveyer belt). The plane moves in one direction, while the conveyer moves in the opposite direction. 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).
The question is:
Will the plane take off or not? Will it be able to run up and take off?
eh If the plane starts its engine and starts to accelerate, but the conveyor below is doing the same just in the opposite direction the plane would actually be standing still.
a standing still plain with its wheels moving very fast is unless, plane uses down force from the wind to take off, no movement = no wind = not taking off
think of a tread mill you arent moving, therefore the plane cannotfly. The reason why planes accellerate to such speeds is to generate lift, no lift is occuring when the plane is standing still.
The only way i see that the plane will take off is if there is a massive wind to generate enough lift.
it moves by thrust from the jets which is independant from the wheels…
so…
the plane and conveyors wheels could be doing whatever they want…you could even have the planes wheels going backwards… its independant of what the planes thrust is going to do…which is move the plane forward relative to the conveyor…the plane would eventually run off the conveyor…and take off like normal…
it flys on speed relative to the ground…when the aire moves over the wing fast enough to create lift…
this is NOT the same as a boat trying to move forward in a fast river…or a car on a conveyor…
thrust will only determin speed, when the craft is on the ground, it really generates no lift. The question asks if the plan can never move faster than the “treadmill” can it lift off. the answer is no.
no lift is therefore generated, because the plane is not moving, the the casual observer the plane would be standign still. the plane cannot take off.
I should add that a 747 is no harrier, and isnt’t going to beable to take off by thrust alone.
The only reason planes go so fast is to generate enough lift under the wings to lift the planes weight off the ground, it conitnues to build lift and speed via the turbines until it is at a height/speed where the weight of the plane free faling, combined with the speed created by the turbines is enough to let the plane glide, also allowing more fuel effiecient flying. The most amount of fuel is wasted in takeoff.
However ADAMH makes a good point… Because the plane doesnt work like a car, and the wheels are completely independant on the turbines… However i still think that the plane would not generate any lift, if there were some kind of enourmous wind tunnel this might be a possibility, but everybody aboard would die instantly from severe whiplash. As soon as the plane generates enough lift from the wind, since its position is at rest, the plane would jump forward like a slingshot… ORRRRRRRR i could be wrong and it would lift upwards with the nose pointing directly up and it would lift like a helicopter until its out of the wind tunnel’s parameter (thats not the right word) and then drop to the ground due to no lift being created outside of the wind tunnel’s field.
it doesnt have to move faster than the treadmill…the thrust will move the plane forward regardless of what the wheels are doing…and then the plane will eventually just clear the conveyor …
if you take it to the extreme and the conveyor can go infinately fast…then before the plane would achieve enough ground speed.the aircrafts tires might blow apart
its like a plane flying in extreme head wind…the plane may have 0 ground speed but is still in flight due to the airflow over the wings…
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).
Therefore as the plane reaches full thrust and takeoff speed, the conveyor will be at the same speed.
So as the plane is being pushed forward by engine thrust, it is being pulled back by the conveyor. The wheels are simply spinning without actually going anywhere.
BUT:
At the same time, think of an aircraft carrier. If the carrier were moving East at the same speed the plane was trying to take off in a Westward direction, the plane would not have enough speed to take off, it would plummet into the ocean because it runs out of runway fster.
OK no
I got it this time.
Consider take off speed of a plane is 100knots.
Say the conveyor had a static speed of say 100knots. In order for the plane to reach 100knots it needs to use maximim thrust. At this point they are going the same speed and there is no displacement. In order for the plane to overcome the conveyor force, it will need to genereate more thrust. In essence it needs twice the speed to take off.
I don’t know, something to do with vectors right? I’ve hated physics since high school.
lets say you have a rollerblader on a conveyor belt…and the conveyor is at a 45deg angle…and the rollerblades are special and have 0% drag on the bearings…
the rollerblader lets go, and the conveyor belt moves to meet the wheels speed…will he roll down the hill or stay stationary…
he will roll down hill… since gravity is pulling him…and there is 0% drag on the wheels themselfs…now…
same case with the PLANE…its independent of the wheels…it does not rely on its locomotion from the wheels…it uses thrust from the engines…
you have no lift, the plane is not moving. The plane will not fly based soley on thrust, unless thrust alone can lift the body of the plane, ie the harrier.
Use a Bullet for example, or a missle, both need to be aimed upwards and will fall tot he ground without a way to generate lift. The plane is generating no lift because the plane is not moving, fast enough to get enough air to flow over the wings to generate the lift required.
The plane may be “moving” at 500mph, but the conveyor is as well. Wind speed will remain constant. The wind will not provied enough lift at the low speeds it has at the ground to generate lift.
If a plane only needed thrust to fly, we would hoist it off the ground, turn the engine on to full and it would fly. this is not the case, even tough the engine may be at max thrust, the wings are not generating enough lift to overcome gravity. if the plane was held like this from a high enough height, and released, it WOULD eventually produce enough lift as it’s altitude decreased and speed increased. As soon as enough lift was genorated the plane would then fly.
This plane on a treadmill is genorating no lift and cannot fly.
if it was the case of only needing thrust to fly, the land speed racers would fly off trhe ground. But they do not as they are designed like a bullet and generate little lift. However a normal car, because of it’s “natural” shape will generate lift as speed increase, which is why it nededs downforce to remain on the road.
the wheels are INDEPENDANT of the planes locomotion…
this is exactly what will happen…
plane is standing still…conveyor is stopped…
plane wheels at 0mph, groundspeed 0mp, conveyor 0mp…
plane then applies full throttle to turbines…a few seconds go buy…and conveyor keeps compensating
plane wheels 30mph, its “airspeed” over and above the differential of the combined thrust forward and the wheel speed would be lets say 50mph , conveyor is matched to planes wheel speed 30mph… this means the plane is moving forward on the conveyor at a +20mph speed so far…
plane keeps accelerating in the same fashion…ever gaining on the conveyors total length…then the plane finally reaches its take off speed relative to teh ground… the friggen wheels may be spinning at 1,000mph, but it doesnt matter cause the jets are what creates the movement forward
i guess i should comment in this topic since i’m a pilot and an aerospace engineering student…but lucky and boost got the right idea.
The question doesnt specify whether the thrust is created by a gas turbine engine or an internal combustion engine, but it is irrelavent what kind of engine is being used. Since the belt measures the speed of the plane (assume ground speed) and applies the exact same velocity but in the opposite the direction, the plane WILL NOT MOVE.
No matter how much thrust is being applied by the engines, the speed of the wheels will be matched by the belt (assuming that the belt does not have any speed restrictions and the plane stays on the ground). Its all relative motion…if you have equal and opposite ground speeds, they will cancel out (vector summation), hence the plane will stay still and won’t be taking off anytime soon.
Well i really hope nobody designs a jet engine located in front of the wings lol but most likely the plane will not fly. this is of course based on the assumption that the total diameter of the exhaust outlet is much smaller than the wing span. If the diameter of the outlet was almost as large or larger than the wing span then i suppose its theoretically possible for lift to be created.
