-
There seems to be an uptick in Political comments in recent months. Those of us who are long time members of the site know that Political and Religious content has been banned for years. Nothing has changed. Please leave all political and religious comments out of the forums.
If you recently joined the forums you were not presented with this restriction in the terms of service. This was due to a conversion error when we went from vBulletin to Xenforo. We have updated our terms of service to reflect these corrections.
Please note any post refering to a politician will be considered political even if it is intended to be humor. Our experience is these topics have a way of dividing the forums and causing deep resentment among members. It is a poison to the community. We appreciate compliance with the rules.
The Staff of SOH
- Status
I just rewatched the penny segment on youtube. They determined with the wind tunnel that the terminal velocity of a penny was somewhere between 13.5 and 30 m/s. Adam then says "It's going out on television that we did the experiment that showed exactly how fast the penny goes. No math, no ideas, no timing off a building, we got it right here". For some reason they decided to go with the value that was highest rather than estimate or measure the velocity of the area where the penny was hovering. From then on they referred to the velocity the penny was being fired at as the terminal velocity, when in all actuality the value was totally contrived.
The penny myth is actually not their worst work, the episode where they tested gun barrels getting chopped off with samurai swords was painful to watch.
The penny myth is actually not their worst work, the episode where they tested gun barrels getting chopped off with samurai swords was painful to watch.
T
tigisfat
Guest
I beg to differ. The ultralight episode was the worst. They asked a bunch of people if a stationary airplane could take off from a treadmill. They said no. They laid a tarp onto the ground and placed the ultralight at point x. They moved the tarp backwards at say, 40mph and then accelerated the airplane foward at 40mph, so that the wheels were doing 80. The plane took off, as it had not remained stationary but had accelerated forward from X; they declared all the pilots and others wrong on the spot. I wanted to climb through my TV and strangle them.
K
Ken Stallings
Guest
You'd be wrong then.
Aircraft are not powered with the wheels. They are powered with the prop. That makes all the difference in the world as the free turning wheel would not be rolled back equal to the velocity of the treadmill. The source of velocity has to be countered for takeoff to be affected. The threadmill works for a car, but not an airplane.
This discussion was held at the AOPA forums over a year before Mythbusters did their work. In fact, they did the work because of all the letters they got from AOPA members who wanted to see it tested out. Most pilots on the AOPA forums concluded that an aircraft would takeoff on a treadmill set at rotation speed and the Mythbusters experiments validated that conclusion.
In addition to the actual aircraft, they also replicated the results with a RC aircraft on an actual treadmill. That helped dissipate views that their version of a treadmill for the actual aircraft wasn't valid.
Cheers,
Ken
AckAck
Charter Member 2015
Am I reading you guys wrong? The episode I saw was that they COULDN'T take off a stationary aircraft (scale model from a treadmill, or full size on the ground), but they did take off an aircraft (scale model from a treadmill, and full size on that tarp) once the aircraft reached flying speed (regardless of the wheels).
Ken in his first paragraph says that an aircraft's propulsion has nothing to do with the wheels (correct), and in the next paragraph says that an aircraft takes off when placed on a treadmill at rotation speed. (Um, no, that's not what I remember them finding at all). Two contradictory statements...
Brian
EDIT - apparently I AM reading you wrong - the myth was that it wouldn't take off at all, not that it would take off with 0 ground speed. Nevermind... I apologize - I am misrepresented the myth, and can't figure out what the fuss is about
Ken in his first paragraph says that an aircraft's propulsion has nothing to do with the wheels (correct), and in the next paragraph says that an aircraft takes off when placed on a treadmill at rotation speed. (Um, no, that's not what I remember them finding at all). Two contradictory statements...
I'll have to find the episode, but I think they busted that myth, not confirmed it... If they did confirm, they were horribly wrong, just like the people on the AOPA forums...
Brian
EDIT - apparently I AM reading you wrong - the myth was that it wouldn't take off at all, not that it would take off with 0 ground speed. Nevermind... I apologize - I am misrepresented the myth, and can't figure out what the fuss is about
T
tigisfat
Guest
Exactly my point. I refuse to debate anything that doesn't involve whether the aircraft has lift or not.
T
tigisfat
Guest
Come again? Precisely what am I wrong about? I am 100 percent mostly certainly and positively not wrong in the slightest. Before I get offended and this conversation goes to a wierd place, I'm going to assume that you had to have misunderstood me.
I was saying that an airplane will NOT take off just because the wheels are rolling.
I was saying that the mythbusters episode was obscenely stupid because they claimed to have proven that an airplane will take off from a treadmill, even though the airplane was allowed to move forward through space.
Oh goody! The airplane on a treadmill debate again! This will go on for pages!
Yes it will take off.
My favorite is when a plane that is flying faster than M1.00, and it goes past the observer, who then explaims "wow, it broke the sound barrier just as it passed me!" As if the "Earth Shattering Ka-Boom" is the sound of the sound barrier actually breaking, at that very moment. Boom! It broke! LOLOLOLOL.
Yes it will take off.
My favorite is when a plane that is flying faster than M1.00, and it goes past the observer, who then explaims "wow, it broke the sound barrier just as it passed me!" As if the "Earth Shattering Ka-Boom" is the sound of the sound barrier actually breaking, at that very moment. Boom! It broke! LOLOLOLOL.
AckAck
Charter Member 2015
That WAS what the myth was! The myth was that an aircraft could not take off from a treadmill, not that it couldn't take off without forward motion. That's why it was a stupid myth to prove/disprove. It can take off from a treadmill - myth busted. But so far as I can tell, the real myth was that an aircraft would take off from a treadmill with no forward motion, and that was busted too...
Brian
EDIT - PRB - the problem here is that we all are saying the same thing and agreeing - I'm not sure what the argument is about now... I'm right - no, you're right - no, you're right - no, I'm right...
Brian
EDIT - PRB - the problem here is that we all are saying the same thing and agreeing - I'm not sure what the argument is about now... I'm right - no, you're right - no, you're right - no, I'm right...
T
tigisfat
Guest
There is no debate. Absolutely none.
A stationary aircraft with no relative wind and no means to produce lift will not take flight just because a treadmill has spun the wheels up.
On the other hand, if you set the parking brake and rig up a treadmill so that it moves the entire aircraft forward, and if the treadmill goes fast enough, the airplane will take off. I call this system the 'tig-o-matic fuel saver 9,000'. You do have to run checklists like a mofo if you took off with the engines out though.
Tig, you're stating the problem incorrectly, which is the source of endless confusion on this.
If you “don't let the plane move”, relative to the planet, then it will not take off. It doesn't matter by what means you succeed in keeping the plane from moving, be it a giant treadmill, or a big rope. If the plane doesn't move, obviously it can't take off.
The question is, can you keep a plane from moving, relative to the planet, by means of this giant treadmill experiment? Answer: no. Therefore, the treadmill plane will take off just like any other.
If you “don't let the plane move”, relative to the planet, then it will not take off. It doesn't matter by what means you succeed in keeping the plane from moving, be it a giant treadmill, or a big rope. If the plane doesn't move, obviously it can't take off.
The question is, can you keep a plane from moving, relative to the planet, by means of this giant treadmill experiment? Answer: no. Therefore, the treadmill plane will take off just like any other.
K
Ken Stallings
Guest
Whether you get offended or not is frankly your choice and I won't concern myself with that.
However, since the wheels on the aircraft are free floating, and not tied to the propulson drive, the aircraft's forward velocity from prop thrust is not affected by a treadmill since the wheels will freely turn. Because of inertia, the plane may initially roll back on the treadmill due to Newton's law of bodies at rest tending to stay at rest. But, the prop thrust will counter that by providing the necessary force to counter the inertia.
Once the thrust of the prop takes over, the free turning wheels simply increase their rotational velocity commensurate to the linear velocity of the treadmill.
Again, your choice if you choose to be offended by my statements.
Ken
Theoretically, would this work?
If the tether was taut from the start, and the engine provided enough airflow over the wing surface, would the aircraft lift off?
View attachment 7601
Actually no it won't. As the wing starts to produce lift the weight on the wheels will decrease. As a result the friction force between the wheels and the treadmill will decrease. You will find that they will slip relative to each other before the plane actually flies.
Don't mind me though. I'm just fanning the flames
K
Ken Stallings
Guest
Theoretically, would this work?
If the tether was taut from the start, and the engine provided enough airflow over the wing surface, would the aircraft lift off?
View attachment 7601
Assuming the teather's tensile strength exceeded the max thrust of the prop, then of course not because the teather would freeze the aircraft's position relative the wind and therefore unless the wind velocity exceeded lift off speed, the aircraft would remain firmly on the ground.
This isn't even a debate.
The concept is applied by thousands of aircraft every day. The teathers are called tie downs and it is how a pilot keeps his aircraft safely on the ground when parked.
Cheers,
Ken
AckAck
Charter Member 2015
Sure. Not very efficient, though, and generating enough wind over the wing with propwash might be problematic - easier just to make the propeller thrust down - but then you have a helicopter.
Can you take off by tying a rope to the wing and anchoring it to the ground, and fly in a circle fast enough to generate lift while being held in a circle by the rope?
Again, yes, but it's probably uncomfortable.
Brian
(EDIT - I type slow, I guess. Ken, as long as the engine is generating the airflow over the wing, then of course the aircraft will take off. There is more than one tie down on an aircraft, and they are in opposite directions, and the force is down to the ground. If you have only one tie down on the nose, and a 120 MPH wind, the plane will fly until the wind speed decreases. (Except we might call it a kite.) Similarly, if the plane is generating lift exclusively from the propwash, then as long as the engine is running, it will fly (nowhere.)
Can you take off by tying a rope to the wing and anchoring it to the ground, and fly in a circle fast enough to generate lift while being held in a circle by the rope?
Again, yes, but it's probably uncomfortable.
Brian
(EDIT - I type slow, I guess. Ken, as long as the engine is generating the airflow over the wing, then of course the aircraft will take off. There is more than one tie down on an aircraft, and they are in opposite directions, and the force is down to the ground. If you have only one tie down on the nose, and a 120 MPH wind, the plane will fly until the wind speed decreases. (Except we might call it a kite.) Similarly, if the plane is generating lift exclusively from the propwash, then as long as the engine is running, it will fly (nowhere.)
K
Ken Stallings
Guest
Actually no it won't. As the wing starts to produce lift the weight on the wheels will decrease. As a result the friction force between the wheels and the treadmill will decrease. You will find that they will slip relative to each other before the plane actually flies.
Don't mind me though. I'm just fanning the flames
Not only that, but without prop thrust the flight (even if it happened) would be mighty short because as soon as the wheels broke contact with the treadmill, the thrust would go immediately to zero and the best outcome would be an instant touch down, or worse, a very hard one!
On the other hand, the concept of assisted thrust from a ground source is played out all the time. Imagine a bungee shot for a glider, or a carrier catapault takeoff. It's really a matter of having an additional source of thrust independent of the ground to keep the lift on the wing. For the glider, it is thermal lift exceeding aircraft weight, allowing a bit of forward velocity to translate from lift thrust. For the carrier catapault, it is the aircraft's engine(s) which overcome the drag of air to maintain takeoff speed and accelerate out.
Ken
K
Ken Stallings
Guest
Well, if it's a long enough rope, and skilled enough pilot, it could be a simple pylon turn and the rope could remain just barely taut.
I would NOT volunteer to test the theory out however!
Ken
T
tigisfat
Guest
Actually no it won't. As the wing starts to produce lift the weight on the wheels will decrease. As a result the friction force between the wheels and the treadmill will decrease. You will find that they will slip relative to each other before the plane actually flies.
Don't mind me though. I'm just fanning the flames
Actually, you're wrong. It is quite possible to hold an aircraft down at speeds faster than the speed at which it can begin to fly. If you don't rotate a heavy, it'll likely never get airborne in the runway you have.
- Status