Mythbusters tonight - Page 12

**roastbeef** · #**111** (**permalink**) 12-15-2007, 08:13 AM

the resistance from the friction of the wheels would prevent the plane from moving foreward. the plane must move foreward to create lift.

kennith · #**112** (**permalink**) 12-15-2007, 02:59 PM

Quote:

Originally Posted by roastbeef

i still maintain my theory that the plane would not take off.

there is very significant resistance against the free spinning wheels.
pretend you have a toy car on a treadmill. the treadmill is on, and you are holding it in place. the moment you let go of the car, it will immediately begin to 'slow' and fall off the back of the treadmill pretty quickly.

the same applies to an airplane. even though the airplane is moving by creating thrust, the engines are still working to keep the plane from falling off the back of the treadmill. the engines can not create and infinant amount of trust, and therefore, can not create enough thrust to overcome the resistance against the spinning wheels. im willing to bet the turbines would burn up too.

in a perfect world where there was no friction, the plane would take off. if there was no friction, the toy car on the treadmill would remain stationary when you let go, also.

That is simply not the case at all. I hate to be so darn contrary, and I mean this as nicely as possible, but that is just wrong.

Cheers,

Kennith

Mr. Plow · #**113** (**permalink**) 12-15-2007, 03:35 PM

Well I took the time to read all 11 pages, and being a pilot I will put in my 2 cents (whether my thoughts are worth that much is up to debate). I agree w/ the air flow guys in that the plane WILL NOT take off w/out the sufficient airflow over the wings to produce lift. It does not take too much thrust to get a plane moving so it can be reasoned that the friction created by the wheels against the movement of the plane is low. It therefore doesn't matter if the plane is on a treadmill going 100mph, it will take very little thrust to keep it in a relative position on the treadmill and by adding thrust the plane should accelerate through space while riding along on the treadmill. Now if you have a treadmill as long as a runway, then I agree that a plane should be able to accelerate to take off AIRSPEED even w/ the treadmill moving in the oposite direction; however depending on the speed of the treadmill, the wheel bearings would eventually give out. On the plane I fly the wheels are limited to 170kts Ground Speed. I think it can be agreed that if a plane is maintaining the same speed as the treadmill but is not moving through the air that it will NEVER takeoff because there is no airflow over the wing. In aviation there are two basic speeds: airspeed and ground speed. With absolutly no wind airspeed = ground speed, but say you are flying at 100 knots airspeed into a 10 knot headwind. Your airspeed is still 100 kts, but your groundspeed is now 90 kts as the speed you are going over the ground has been reduced 10kts by the headwind. Takeoff speed has 100% to do w/ airspeed not ground speed; that is why it is prefered to takeoff into a headwind, because through the air you are going say 100 knots at takeoff, but you are only going 90knots over the ground and therefore not traveling as far to get up to 100knts airspeed and therefore not having as long a takeoff distance.

Also, a plane does NOT have to be at maximum power to takeoff, we do it all the time to save the aircraft turbines; its called a reduced power takeoff. So basically it is all about airflow over the wings, not how fast the wheels are moving that determines if a plane will take off. I could go into a big aerodynamics discussion, but it sounds like everyone is sick of this thread as as, so I hope this helps w/ the discussion, and has put some misunderstandings to rest. I guess I put about $2.00 in istead of 2 cents, sorry about that.

YOURADHERE · #**114** (**permalink**) 12-15-2007, 10:58 PM

haha...this thread is still going?

Nitewulf · #**115** (**permalink**) 12-16-2007, 10:49 PM

Thank god a pilot has dropped in and explained it. I tried with the visual links so they could look at it for themselves. Airspeed is the only factor in it! Thrust + Resistance(non-moving air)= Forward velocity. In the intance of the bearings, as long as it wasnt made a habit, I believe a one time take off it would survive. Relatively, look at the hotwheels on the treadmill, short duration, its fine, leave it there for a while and you'll melt those plastic wheels.

Zeek · #**116** (**permalink**) 12-17-2007, 09:38 AM

Quote:

Originally Posted by Nitewulf

Thank god a pilot has dropped in and explained it. I tried with the visual links so they could look at it for themselves. Airspeed is the only factor in it! Thrust + Resistance(non-moving air)= Forward velocity. In the intance of the bearings, as long as it wasnt made a habit, I believe a one time take off it would survive. Relatively, look at the hotwheels on the treadmill, short duration, its fine, leave it there for a while and you'll melt those plastic wheels.

One more time for the last two posters. We're not saying the plane would simply 'takeoff' without going anywhere. The plane would have airflow over the wings because a treadmill simply cannot stop a plane from moving. I understand that you're a pilot, and that sometimes planes don't take off at full throttle, but uhh...we're trying to make the plane take off. We're going to use full throttle. As such theforce of the engines AT FULL THROTTLE would certainly overcome friction from the weight of the plane on it's wheels.

As a side note, I am curious however about the effect of a treadmill big enough to create these kinds of speeds, whether it would create a signifigant draft itself. Heck, if the treadmill really spun up to 300mph, it might be enough to provide an extra element of lift to lessen the friction on the wheels, allowing the plane to take off in a shorter distance than regularly required. That'd really piss off all the 'won't take off' people.

westexasrepublic · #**117** (**permalink**) 12-17-2007, 09:54 AM

why do yall think friction is such a big factor!? what do you think bearings are designed for (reduce friction)? unless ball bearings are made out of sand paper and lubricated with super glue then yall are just sadly mistaken.

Quote:

Originally Posted by roastbeef

the resistance from the friction of the wheels would prevent the plane from moving foreward. the plane must move foreward to create lift.

Mr. Plow · #**118** (**permalink**) 12-17-2007, 09:55 AM

Yes Zeek, you are correct that in this instance w/ a plane on a treadmill you would want to use full throttle as the treadmill is in a way resisting the forward motion of the plane. I was simply addressing an earlier comment that planes even on a normal runway can not take off w/out full power which is simply not true.