I'm off to bed good people. However, I'm not sure I have any more examples to my point. Much love! GT
That, in essence, is what the catapult does. It creates "ground speed" in the direction of the jet's launch. .
GT is absolutely right that the forward movement of the aircraft is due only to the thrust of its engine relative to the atmosphere. Here's the thing, though. As the plane starts to move forward, instead of a progessive, linear increase in wheel rpm there will be an exponential increase that will cause the wheels, tires and bearings to spin so fast they will self destruct long before the plane can reach a speed where the airfoil will create enough lift to take off. Without the wheels the screaming conveyor belt will tear off the landing gear and our bird is grounded. That's what I'm thinkin'.
Not sure what the issue is. The steam-driven catapult on an aircraft carrier propels the jets from 0 to 165 mph in two seconds. HowStuffWorks "Catapults and Taking Off from an Aircraft Carrier" Was it the use of the term "ground speed" in quotes? Perhaps just "motion" would have been a better term to use. (although, technically, from a KIAS/KTAS/GS point of view, the jet's GS would increase by 165 mph during the catapult launch - even given the non-zero GS starting point due to the carrier's speed in the sea). .
Wow, this is trickier than I could have ever imagined, I actually changed my mind a few times over the minutes... It all reduces though to a simple question: is the conveyor able to keep the plane from moving forward? - If it is, the plane will never take off... BUT that would take an unreal conveyor and unreal landing gear on the plane that wouldn't destroy at crazy conveyor speeds. - If the thrust of the plane can overcome the conveyor action then it will fly, slowly gaining airspeed till take off... BUT hey, that goes against initial proposition because at some point the conveyor isn't keeping up. So there you got it, tricky question without a clear answer. Thanks for giving it a try, though
I have no problem with the catapult. It's mechanically connected to the plane and provides...wait for it...thrust. Now...imagine that plane is your hypothetical plane sitting on your hypothetical conveyor belt that's matching wheel speed, turning in the opposite direction. Will the plane move? Is a catapult any different than a pair/set of jet engines? It's that other part I can't believe you actually said...
No air flow over the wings? Stand under the wing of a small craft with the engine revving and tell us what is hitting you in the face. I take it you have never flown on a propeller-driven aircraft. If the engine dies while flying, what happens to the forward speed and altitude of that aircraft? What is the ground speed of an aircraft whose airspeed matches the speed of a headwind into which the airplane is flying? Just a few hints to allow you to reach the correct answer to the question.
Why do you believe that there is no wind moving around the wings? Has the propeller been disabled in your example?
This question first hit the net 10 years ago; Russian > Translate Òåìà ïðî ñàìîëåò. Âçëåòèò èëè íåò? [1] - Êîíôåðåíöèÿ iXBT.com Since then; How many facilities have a conveyor belt runway setup in the same manner? If none . . .
I think it really depends on where the engine(s) is as it relates to the wings. So the conveyor belt and wheel speed exactly match. All you have is a running engine producing forward thrust, but the conveyor belt in the opposite direction counteracts all forward motion. Therefore, the resulting ground speed is zero. I think the plane could take off if the engine was in front of the wings. Even though the ground speed is zero, the lift created by the engine or egines MAY allow the aircraft to leave the ground. For instance, if you tethered a plane by the rear wheel to a stationary object and went full throttle to the point where the tether is fully taught, the ground speed would be zero. Same effect as the conveyor belt. BUT you have so much lift created by the engine in front of the wings (especially if you had twin props) that the resulting airflow over the airfoils MIGHT (that's a big maybe) allow the plane to lift off the ground. If the engines were in the rear, behind the wings, it's like a dog leashed up to a tree. He isn't going anywhere. Just a thought... Flame suit on.
There is nothing in the original question that limits the thrust of the engine. You have interjected a limitation on the engine's thrust to support your opinion. So how about I do the same thing? Let's say that the plane is powered by a Saturn V rocket engine. How are you going to keep it from taking off with a conveyor belt? Think about why aircraft are tethered to the ground when left on the apron. Could it be that they lift off the ground while stationary if enough wind is blown around their wings?
True, the plane would fly if the engine & propeller could produce enough wind over the wings to create lift. I was using a more typical GA airplane such as a Cessna 172 as the example for the hypothetical question. In a Cessna, the engine & propeller do not produce enough wind over the wings for lift. In the typical engine run-up, you can rev the engine and the plane will not lift (due mostly to the prop wash wind being nearer the fuselage and not going over the outer areas of the wings). As far as a Satrun V rocket engine on the plane, I think even in that extreme example, the plane would not fly. The reason is the equally ridiculous speed of the imaginary conveyor belt that would be going even faster than that level of thrust such that the rolling resistance of the wheels would create enough frictional drag to keep the plane from moving forward (relative to the fixed ground surrounding the conveyor belt). However, with extreme thrust AND sufficient wind over the elevator in the tail of the airplane, the plane WOULD fly. As the conveyor belt sped up to a high speed to keep the plane stationary, if the elevator had enough wind such that a sudden pull back on the yoke or stick would cause the nose of the plane to rise, then the wheels would come off the friction-causing conveyor belt and the plane would be airborne and fly away. So, you have uncovered a scenario where the hypothetical plane would fly. Congrats. .
What is the cost of a conveyor belt that matches the speed of the wheels vs. the cost of a concrete runway? Then there's that sticky part about landing on the same conveyor. If a landing strip is required, why not use it for takeoffs as well?
It doesn't matter where the engines are. Thrust is created and wind is created. Consider a wind tunnel. Can a model aircraft fly inside of a wind tunnel? It doesn't matter if the fans are blowing from the front or sucking from the back. The wind created is the same.
In the conveyor belt example it absolutely does matter. If your engines are behind the wings (Beechcraft Starship) and your ground speed is still zero then you can create all the the thrust you want but without lift over the wings you will not take off. As long as enough air is moving over the wings you have a chance of taking off. I totally agree with the wind tunnel example but the original question is about a plane on a conveyor belt or zero ground speed. Forward thrust creates air movement over the wings by allowing the aircraft to roll down the runway. The wind tunnel just substitutes air movement in place of forward movement. Even though the airplane is stationary in the tunnel, the wind is still going over the wings causing lift.
Where is the air coming from that is being sucked by the propellers? Fans create air movement on both sides of the blades. Why do you think it's a bad idea to stand in front of a jet engine under full throttle? Where in the question does it say that the engine does not create adequate airflow moving over the wings to lift off? This is a conceptual physics question. There is no such thing as a magic conveyor, so why do you then limit the capability of the aircraft to conventional existing planes created to take off on conventional runways? What is holding the aircraft in the airflow of a wind tunnel if the model remains stationary? A model has no engines, yet is able to fly if enough wind is available to create lift. So if ground speed is so important to an aircraft's ability to stay aloft, why does it not fall to the ground when groundspeed matches a headwind exactly? If the plane cannot take off if there is no wind, then it certainly must fall from the sky if groundspeed perfectly matches a headwind. Here's a video of a 747 trying very hard to take off while it's wheels are perfectly still. With just a little more wind speed, it would be flying in place. Mount a powerful enough propeller on the front of that 747 and it lifts off, regardless of the speed of the wheels. [ame=http://www.youtube.com/watch?v=cHhZwvdRR5c&html5=1]Amazing video of 747 lifting in place in extreme wind conditions - 1080P HD - YouTube[/ame]
Guys, come on. Stop and think. The wheels on a plane, whether it's a Cessna or a 747, are freewheeling. They do not contribute to the forward motion of the plane in any way. They just support the plane until the wings can generate lift. The plane's engine, the propeller in the case of the Cessna, is what "pulls" the plane through the air. It's just like the propeller on a submarine that "screws" through the water, pushing the ship forward. As the plane moves forward the wheels roll along with it. Now, if somehow the ground were to start moving along in the same direction and at the same speed as the plane as its propeller pulls it forward, the wheels would stop rotating completely. In this conveyor belt scenario the opposite is happening. In essence, the ground is moving counter to the motion of the plane, which will cause the wheels to turn faster. This in turn causes the conveyor to turn faster, which will increase the speed of rotation of the wheels, which will cause the conveyor to go faster still....etc. etc. It's a cumulative effect that will get the wheels spinning faster and faster at an exponential rate. At some RPM the laws of physics say that the wheel assembly will tear itself apart. The question is, will that happen before the propeller can pull the plane to its takeoff speed.
I think you are missing the rolling resistance and friction in the wheels, axles, and wheel bearings. Imagine a Cessna 172 with its wings removed, running down the highway at full speed. Its top speed would not be infinite. It might top out at 100 mph or so. That is the speed the conveyor belt would get up to. The plane would not move forward, the top "rolling speed" might be 100 mph. I'm thinking the wheels will not tear themselves apart at that speed. .
You are right. I did not account for all airflow factors in a pusher prop. Even though the entrance air velocity into the propellor is less than the exit velocity, it can still produce enough air flow over the wings prior to hitting the blade. Assuming thrust is not limited. Thanks!!!
The reason that this has gone 120 pages and was resurrected from years ago is because posters insist upon injecting parameters and constraints not included in the original question, such as the plane must be a Cessna 172. Where is a Cessna 172 mentioned in the question? As I said, there is no such thing as the conveyor described in the original question, so why then do so many insist that the plane must be one that was designed and constructed to be capable of lifting off of a conventional runway? Given the parameters, it's physically possible to install a propeller capable of generating enough lift for the wings to lift the plane off the ground. It hasn't been done because there are much better designs for flight and runways or catapults are more economical launch platforms than a conveyor belt.
Like so many others, you have missed the point entirely. It makes no difference what kind of plane it is or what the engine type or location is. The airplane and the rotating wheels are two seperate entities. The plane will move forward through the AIR. What is happening between the wheels and the ground (or conveyor belt) is a separate issue. We need to have an aeronautical engineer explain this. Perhaps we can ask Bob Parks from Aviatorchat.com offer his opinions.
There is no point. Some people just don't get it and never will, regardless of how many different ways it is explained. I would like to think these people are stubborn but sadly that might not be the case.
I'm not the ultimate expert on this kind of thing but if one looks at the forces required for flight it is obvious that the airplane doesn't give a hoot at what the wheels are doing. Flight requires air flow over the wings to achieve lift. To get this there has to be enough motion through the air of the airplane. To achieve this there must be thrust. So, if the propellors or jet exhaust produce a force to the rear, the airplane will react in the opposite direction and move forward until enough speed is achieved to make the wings work to produce lift, another reaction to pressure differential between the lower surface and the upper surface and down flow aft of the wing. Inertia and drag of the vehicle has been overcome , gravity has been over come, and somehow we have levitation. I hope that this makes sense because I have had a few scotches.
