So a car will take off if you put it in a wind tunnel? I get it now!
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Aaah. I remember being very confused by that episode, this finally cleared it up in my head. Hard to believe it didn’t occur to me that an airplanes wheels have nothing to do with propulsion, but at least I wasn’t alone in this.
Food for thought.
Tailwind Operations in fixed wing aircraft are considered to be takeoffs or landings with a performance diminishing wind component – that is, a tailwind.
- Take Off - The take off run will be longer and the maximum allowable take off weight for a specific runway and temperature may have to be reduced. The climb gradient will be reduced due to the higher groundspeed and could result in a CFIT accident due to inability to out clear an obstacle.
A conveyor belt, because it affects free spinning wheels, doesn’t diminish performance, but a adverse wind might.
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There’s actually an even stranger wrinkle to the plane-on-a-conveyor belt problem. Adam is correct that the plane will take off if it is moving through the stationary airmass as usual, despite the motion of the conveyor belt, due to being propelled by the propeller.
But if the conveyor belt is large enough, and has been running long enough, the plane can still take off even if it maintains its original position (like a car) and doesn’t move forward!
That is because the air that actually touches the conveyor belt sticks to the belt and moves with it. That air drags along air just above it, which drags along air just above that, which drags along air above that. The result is a boundary layer that builds up along the belt. Far away above the belt the air is stationary, but as you get closer to the belt the air velocity moves in the direction of the belt’s motion, and at the belt itself it matches the belt’s velocity. That means the belt will generate a headwind for the plane.
If the belt has run long enough, the boundary layer along the belt will be thick enough that the headwind will be great enough for the plane to take off.
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The engine is pushed by it’s exhaust. The engine thus pulls the plane along with it. umop_apisdn thus is right.
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Came here to say this. Also, in high school physics, you learn about force vectors. The propeller provides a big push forward. How exactly would the conveyor belt hold the plane back? Friction between the runway and the wheels. And everyone knows what happens when momentum in a car exceeds the tires’ grip on the road, such as on ice: It slides. Just like the skis on a snowplane, the wheels of the plane on a moving runway will slip when the engine’s thrust becomes greater than the friction between the wheels and the runway.
The original quote is that the engine operates by PULLING itself. That is wrong for a turbojet engine (or ramjet or rocket engine):
This engine design generates thrust which pushes the engine. You would see an engine design like this is supersonic aircraft. You even stated the engine is pushed by it’s exhaust.
A turbofan:
This design essentially incorporates a turbojet inside of a ducted fan. Here you are both creating thrust from the turbojet section and a suction from the larger ducted fan on the nose of the engine. You could say this design does both push and pull. You would see this on most commercial aircraft since it is more efficient at subsonic speeds.
The concept of pushing or pulling on the aircraft is not linked to how the engine works. You have prop planes that pull the aircraft when the propeller is mounted in the front and push the aircraft when mounted in the rear. In either case the propeller itself is pulling air over itself to create the necessary force to move the plane. It’s semantics to say all engines pull because they are the force generators. Something like the Falcon 9 rocket isn’t pulled by its engine. The entire rocket is placed into a state of compression as the engine pushes forward on the entire thing.
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All runways are on the spinning Earth. This is like having a giant conveyor belt. So, even if the conveyor belt drags some of the atmosphere with it, the plane still takes off.
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So did I. It is fascinating how we can all have deeply entrenched opinions on something we have never really needed to think about. And how readily we wonder whether it is a hoax, and whether the ‘demonstrations’ are faked. Scepticism s good, but I guess opinions like this get entrenched by not being challenged.
I stuck at “but if you are moving at the wind speed, then you feel no wind, so how can you take energy out of it?” for some while.
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If the car’s body is shaped in any way that generates lift - yes.
(Audi had problems with the first generation TTs. The rear end actually generates a bit of lift which, in some situations, could lead to the rear wheels having somewhat less road grip than you’d want to have while going fast and not straight ahead. They “solved” this by adding a lump of steel weighing 30 kg or so to the rear end. Vorsprung durch Technik, my ass. Later models have slightly different aerodynamics and a little spoiler.
On the other hand, modern racecars are designed to generate so much negative lift that at anything over say 120 km/h you could drive them upside down on the ceiling.)
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Vorsprung durch (steinzeitlich) Technik.
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Nah, give credit where credit is due - they did use Iron Age technology, not Stone Age.
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If the vehicle is a Hobie Cat in anything above a Force 3, the answer is “in a fucking terrifying fashion. “
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As a sailor, I am surprised there is any controversy about making a vehicle travel downwind faster than wind speed. Every couple of years lately, there has been a major global celebration of this feat: The America’s Cup.
When hydrofoils were added to the rules and teams began competing with the AC72 and AC40 catamaran rules, with foils and wing sails, the boats began to travel at close to 2x the speed of the wind downwind.
It actually makes the new America’s Cup races less exciting in my mind, because while you get lots of spectacular crashes, you don’t get the beauty of a fleet of boats being pushed downwind behind a cloud of colorful spinnaker sails.
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I see a bit of a problem in his design with the 25mph conveyor + ultralight. I’ve pictured a continually accelerating treadmill (possibly up to light speed), not one that just stops at 25mph.
He’s also discounting the fact that you could stall the airplane for a while with a fast enough and/or continually accelerating treadmill, just from creating drag with the friction of the axles. They won’t drive forward motion, but they can resist it.
Of course, I see that XKCD link there, and I’d say they have the best summary I’ve seen. The question is really premised on something that’s impossible in the first place: The Goddamn Airplane on the Goddamn Treadmill – xkcd
When I’ve argued with idiots about this, I’ve always gone for ‘because it’s a fucking plane’. Pithy, if not particularly explanatory.
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Exactly the same. I was absolutely positive that it was impossible, so must be a hoax. I’m a pilot with a physics degree so it’s not like I was naive about aerodynamics and mechanics. It’s good to have your assumptions challenged once in a while and feel your beliefs flipping. In this example, the more knowledge you have, the more counter-intuitive it is, and it has tripped up University professors of aerodynamics. It did take quite a bit of puzzling before i found an intuitive way to look at it. A bit like Monty Hall in that respect.
The way to think about how it extracts energy when moving at the speed of the wind is to think of that high pressure area behind the spinning propeller (or the low pressure area in front).
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You’re forgetting autogyros =) .
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My two cents worth. Comparison to a car is not helpful. The rotational speed of the car’s wheels is determined by the drive-chain, engine, transmission, diff, wheels. Applying a conveyor at an opposing speed will cause the car to remain stationary.
For a plane with a takeoff speed of 25mph an opposing conveyor speed of 25mph will cause the wheels to spin at 50mph because they free-wheel. They are not constrained by being driven like a car. Add to this the thrust of the propeller, as explained by Adam, and the plane will take off normally. If my plane had wheels and bearings that could not free-wheel at twice the normal takeoff speed I’d be staying on the ground.
