How would a jet engine result in airflow striking the bottom of the wing???
(I’m assuming you mean that pusher props would create a flow towards them, resulting in some minimal airflow over the wing)
The same way any fan creates an air flow. While there is a zone of concentrated air flow in front of and especially behind the prop, there is a more general air flow in the area that is affected by the moving air. Air moves into the prop/jet/etc, some air moves to where that air was. Where did that air come from?
But the answer is that that airflow is insufficient to do anything to help the plane fly. If it were sufficient, then you wouldn’t need a runway and wheels, you could just crank up the prop/jet/etc until enough airflow was established for the plane to float up into the air.
So AskNott, it’s not an assumption, it’s a demonstrated fact by how planes operate.
AskNott said:
You are correct, if one tied the plane at the wheels, then the axis of thrust is not in line with the anchor force, and that induces a rotation. You would need to tie to the fuselage in line with the engines. Preferably some solid airframe structure that could withstand the loads.
What if Monty Hall is the pilot?
First off, I’m quite willing to accept that the airflow of a stationary engine is not enough to lift the plane, and that somebody surely showed that long ago.
I’m still going to disagree with the idea that there’s little or no wind ahead of a pusher prop or jet engine. Both work by shoving lots of air rearward. That air has to come from somewhere, and that creates a tremendous wind in front of the engine.
On a much smaller scale, if you stand on the intake side of a 20-inch box fan, you’ll feel quite a breeze. If you have it in a window, pointed out, and you stand in a doorway on the other side of the room, you’ll be comfortably cool.
In industrial situations, I have stood upwind of 4-foot fans, and the wind will ruffle your hair and clothing. Even that is not near as much air as a pusher prop or a jet pulls into itself. The air crew won’t let you stand in front of a jet engine at full song, but if you could, you’d have to lean into the wind and hold onto your hat. (And leave the goose at home!)
Now is that enough to lift the plane? Alas, no.
Right, in an ordinary plane, there’s only a tiny bit of lift from the propwash over the wings, not enough lift to make much difference.
But let’s modify a plane to see if we can make it work. First, all the wings are doing is taking some of that backwards propwash and pushing it downwards, so if you want lift from the propwash, the most efficient way is just rotate the prop so it’s pointing up-and-down and the wash is straight down. At that point, you don’t need the wings anymore, so just drop them (dead weight). To get enough lift, you’ll need a big propellor, I mean bigger than the main body of the aircraft probably. And maybe another smaller propeller (at right angles) to balance rotational issues. But if you did all that, you would indeed have an aircraft that wouldn’t need to be moving forward to lift off, and indeed could just “crank up the prop until the aircraft floats up into the air”. Most people wouldn’t call it a plane anymore, though.
The problem, sir, is that it’s a thought experiment. In the experiment, we are told that the conveyor belt is moving fast enough to keep the plane stationary. Thus, any live experiment in which the plane is not held stationary (you could simply set the brakes) is not valid as a “solution” to the original thought experiment.
The point of the experiment is to see if the plane will take off while on a conveyor belt designed to keep it stationary despite applied takeoff power.
It’s not a question of whether or not the treadmill “could” hold it from accelerating. The brakes do. MythBuster’s experiment did not.
You didn’t read my post, did you? And I spent so much time on it, alas.
Lift is indeed produced when the wing has a positive angle of attack - meaning that the angle formed between the chord of the wing and the relative wind is greater than 0° on the leading edge.
But, Bernoulli’s “Principle” is not a “theory.” Just as the restriction in your carburetor (if you have one) sucks fuel in to the intake air, and just as the venturi dam under many performance automobiles reduces pressure beneath the car to add downforce, the camber of the wing also generates lift by causing a reduced pressure above. Go look at some dimensional layouts. Many aircraft do not have wings with a positive angle of attack during level flight. How do they stay aloft?
I’m not saying that article is untrue… I’m saying that it is only part of the reason why planes fly, but not the only or the primary reason.
Unless the Federal Aviation Administration and all of the licensed aviation schools in this country are wrong?
But let’s say for a second that lift is generated solely by a positive angle of attack. In the thought experiment, the aircraft is stationary on the conveyor belt. If you hold your hand outside the car window, but you’re not moving, is your hand still lifted in to the air?
The whole premise of the thought experiment is that the plane is stationary because of a conveyor belt moving exactly the same speed in the opposite direction. So when the MythBusters test allowed the aircraft to move forward, they failed to perform the right experiment! They may as well have taken off from a runway and said, “Look, see! It works!”
The problem is that it has airspeed! In the thought experiment, it does not! It is stationary!
Nice post.
I think the premise was simple, that the conveyor belt would be designed to keep the plane stationary. And planes aren’t moving that fast when they take off; many less than a hundred nauts and even those that are higher -
I’m sure we have both the technology and the steady hands to construct such an apparatus.
The idea that the conveyor’s speed will somehow have to ramp to infinity to keep the plane stationary is ludicrous. You can walk backwards up the escalator and maintain the same position…
Dynamometer anyone? Dear God, the universe is imploding!
Horse puckey.
They wanted to solve the riddle. Of course a treadmill could prevent the plane from moving. Nothing magical about it. Are you saying that the truck pulling the tarp on that episode was moving the exact speed as the plane at all times?
Jet engines exhaust behind the wing.
Propeller driven aircraft form a propwash about 1-2 yards wide, and our single engine aircraft will only or mostly be washing on the fuselage… No lift.
Even a twin is not going to make nearly the required airflow to lift the aircraft. You have to figure that this ~2000 lb object (twin recip engine) has to hurtle itself about 90 miles an hour down the runway before it can take off.
Well, again, you could just set the brakes.
Oh, well, I’m sorry. What were your credentials again?
The problem with your interpretation of the riddle is that you are essentially wording it this way:
‘If you could create a situation in which it is impossible for an airplane to take off, would the airplane take off?’
ONLY a magical treadmill could prevent an airplane from taking off.
It wouldn’t have made any difference. The truck could have been going ten times as fast as the plane and it still would have taken off.
Stop. Think.
I’ve already addressed the issue of propellers, and others can do the math for you to establish that the inflow wouldn’t be enough to lift the plane.
But I raised objection mostly to the jet engine theory. HOW are you positioning the jet engine so that it’s intake is coming in via the underside of the wing???
Oh, right, that’s not where jet engines are located (directly behind the wings). 
The problem with your blasé response to the question posed is that it assumes something that may and or may not be part of the question. zut very CAREFULLY addresses this issue.
Put plainly, you simply CANNOT in reality make a treadmill that would be able to accellerate infinitely fast. Nor can you make one that would accellerate to the finite speed needed to actually impart a restraining force on the plane. So unless you want the question to be entirely a thought question, without practical application, you have to accept a meaning for the question that allows the planes wheels to roll as the force of the engines pushes the plane forward faster than the treadmill can roll under them.
You seem to be of the mistaken opinion that this issue hasn’t been thoroughly addressed by really intelligent (and qualified) people here before. Trust me that it has: this subject gets rehashed about every 6 months or so here.
Indeed it has, and indeed it does.