Mythbusters Episode

[JeremyH]

Originally Posted by kris

I was going to bump an old thread with this

[slobin3d]

Funny, I look back in the old thread, I was fooled by the misconception too, but since seeing the show it makes perfect sense.

[JasonInAugusta]

I have planes that take off stationary but they're grossly overpowered.

I've got a buddy that's a pilot...he once called for a ground speed check and the tower responded with "four miles an hour"

He was flying that fast backwards

I missed the show, would liked to have seen it.

[slobin3d]

I've always heard that no matter how fast the wind is blowing a plane can not fly "backwards"

[JasonInAugusta]

I just watched the video of the ultralight taking off.

If the aircraft and the "runway"/tarp were moving in opposite directions at the same speed...the aircraft should've stayed in place. The ultralight accelerated faster than the truck...of course it took off.

For the test to be true to the idea of the treadmill myth the "runway" would have to match the throttle response and forward thrust (converted to MPH) created by the prop and move at the same speed.

Looking at that video it seems that they proved that the plane can create forward thrust faster (accelerate faster) than their runway setup can accelerate.

Simply hammering on the throttle proves nothing.

Slobin, you can fly backwards. I've chopped throttle on planes in a headwind and let the airflow over the wings maintain. The plane would drift backwards relative to the ground.

Basically being pushed back by the wind...the same wind that's keeping it in the air.

[Mad Scientist]

Correct, a plane can not actually "fly" backwards - the wing shape does not work like that.

But perhaps a strong enough headwind could cancel out the plane's forward motion, letting it stay aloft without moving forward, somewhat like a kite, or flying in a wind tunnel. The backward motion would then be caused by the headwinds actually blowing the craft in the reverse direction.

Edit: Beat me to it.

Originally Posted by JasonInAugusta

If the aircraft and the "runway"/tarp were moving in opposite directions at the same speed...the aircraft should've stayed in place. The ultralight accelerated faster than the truck...of course it took off.

But did you see the small-scale test they did with an RC plane on a treadmill? They measured the take-off speed of the model, and set a treadmill to match. Then they set the plane on the treadmill, and started the treadmill up. Adam held his hands behind the plane - not really holding the plane at all, just stopping it from being pushed off the back of the treadmill by wheel friction. Then after the treadmill was up to the appropriate speed, Jamie hit the throttle on the plane. It accelerated, and moved forward. Even though it did not have enough treadmill to actually take off, it showed that the treadmill moving at the plane's takeoff speed did not prevent the plane from acellerating forward. And that was with the treadmill already moving - the plane did not get the hole shot.

The treadmill only acts upon the wheels on the plane, which if it were not for friction, would not have any effect on the motion of the plane in it's entirety. It does not effect the surrounding air, which is what the propellor of the plane acts upon.

They then did the same thing as the large-scale test, but with the model plane and Adam dragging a large sheet of brown paper behind a Segway. With enough "runway", the plane was able to get off the ground, with the paper moving beneath it.

[CCFBERG]

Just to add my two cents, Ive been a crew chief in the AF for 10 years working on jet aircraft. If the fully loaded takeoff speed of a planE is ........lets say 90kts and the wind is blowing directly down the pipe or nose of that aircraft, that aircraft will lift off the ground but it will only hover and move backwards as the resistance to the flow of the air across the entire plane creats drag wich will in a sence drag it backwards as the aircraft is forced backwards it will fall due to the loss of airspeed over the wings 90kts- aircraft reverse speed = less than 90kts wich is what is required to sustain lift on the theoretical aircraft Im talking about. If there is a Pilot controlling engine output (thrust) he can keep the throttle at a rate that will sustain enough forward thrust to keep the aircraft hovering in one spot.

Try it on Real flight g3 if you have it and turn up the wind untill you get lift off and then control the power and elevator to keep it in one spot! its FUN!

[JasonInAugusta]

Originally Posted by Mad Scientist

But did you see the small-scale test they did with an RC plane on a treadmill? They measured the take-off speed of the model, and set a treadmill to match. Then they set the plane on the treadmill, and started the treadmill up. Adam held his hands behind the plane - not really holding the plane at all, just stopping it from being pushed off the back of the treadmill by wheel friction. Then after the treadmill was up to the appropriate speed, Jamie hit the throttle on the plane. It accelerated, and moved forward. Even though it did not have enough treadmill to actually take off, it showed that the treadmill moving at the plane's takeoff speed did not prevent the plane from acellerating forward. And that was with the treadmill already moving - the plane did not get the hole shot.

The treadmill only acts upon the wheels on the plane, which if it were not for friction, would not have any effect on the motion of the plane in it's entirety. It does not effect the surrounding air, which is what the propellor of the plane acts upon.

They then did the same thing as the large-scale test, but with the model plane and Adam dragging a large sheet of brown paper behind a Segway. With enough "runway", the plane was able to get off the ground, with the paper moving beneath it.

I only saw the video on the link posted earlier with the 1:1 test.

Makes sense in a way. I still have it in my head that it should stay stationary. I'm thinking that more throttle is being applied than for standard takeoff...creating more thrust.

In the RC setup did they apply the same amount of throttle with the treadmill as without it?

I feel that they had to have done so. On a standard runway the plane is moving forward and creating lift over the wings not just by the airflow generated by the prop, but by moving air over the entire wing surface as the aircraft moves forward. At that point you're using the entire wing area of the plane, which reduces wing loading. (weight of plane divided by wing area) Take a look at planes like the P38 and B25...there's a reason that the rudders are in line with the engines...that's where most airflow is.

If the treadmill setup is performed correctly, and no throttle is given other than to match what it took to lift off on a conventional runway, the effective wing area (in my mind) should be reduced because the only airflow over the wings will be generated by the propeller and it will be concentrated (for the most part) on the area of the wing and fuse that is behind it. This INCREASES wing loading...effectively reducing the usable wing area (less of the wing is being used to produce lift) and will take more throttle to get off the ground.

Seriously...if all it took was a freakin' treadmill to get off the ground without a need for the aircraft to be moving...wouldn't the Navy have figured this out by now?

Just think, an additional advantage to a treadmill that big is that the entire crew of the aircraft carrier could get their PT in.


*EDIT*

Actually, that running on a treadmill thing brings up a perfect illustration of what I'm talking about.

Go outside and run your ass off. Feel that wind in your face?

Get on a treadmill and run your ass off. Where'd the wind go?

That absense of wind means an absense of lift for the aircraft.

That leaves the propwash as the only means of lift - and it's not going to create lift over the entire wing as you would have if an aircraft was in foward motion.

Can some aircraft take off with only the prop to move air over the wings? Yeah...but only those with light wing loading.

The problem lies in the fact that nobody realizes where the difference lies.

[RedBullRockIt]

Originally Posted by JasonInAugusta

I just watched the video of the ultralight taking off.

If the aircraft and the "runway"/tarp were moving in opposite directions at the same speed...the aircraft should've stayed in place. The ultralight accelerated faster than the truck...of course it took off.

For the test to be true to the idea of the treadmill myth the "runway" would have to match the throttle response and forward thrust (converted to MPH) created by the prop and move at the same speed.

No...

And here is why.

There is a difference between ground speed and air speed. In aviation ground speed means nothing except to judge "time to destination" calculations.

Consider this. Each time a propeller turns around, it grabs a certain chunk of air and moves it backwards...or, moves the plane forwards...or really, a combination of both. It depends on the air density and such, just how much "thrust" is created, but the basics are this:

The pilot pushed the throttle to takeoff power. That propeller will move the airplane forward the amount of air that it is displacing, regardless of ground speed. It wouldn't matter if the conveyor was moving at 3 mph, or 3,000 mph, the propeller would still be taking the same size chunk of air and the plane will move forward the same distance (minus a tiny bit for the tire/wheel friction).

Next, someone said that the propeller doesn't create lift. Using that term you're correct...however, the propeller does create deflection, which is wind blast across the bottom of the wings and elevators (horizontal part of the tail). That does create lift and in fact, in our Super Cub, we can lift the tail off the ground sitting still, just by using prop blast and holding it there.

My father taught an extreme flyer so well, the guy (Lonnie Habersetzer) can crawl his cub through large boulders. He has 32" tundra tires and keeps the tail off the ground with the prop blast while he inches forward across 2 foot diameter exposed rock. If you're into crazy flying, check out the video Big Rocks and Long Props, and you'll see Lonnie flying into many locations my father first scouted and eventually landed. Lonnie took it to a whole new level and even lands and takes off from water on his tires, with the use of hydroplaning. He needs 10 feet of beach to take off and land with water.

[badltl510]

At first I also did not think it would take off, but after watching the episode and giving it a little thought it makes perfect sense. It reminds me of a previous episode they did where the myth was if there was a truck full of birds and they all flew at the same time the truck would be lighter. A lot of people thought it would be just because you don't have the weight of the birds but they forgot about the downwards thrust to hold the birds up. With the plane myth you have to remember the wheels don't come into play. The prop or even turbine moves the plane not the wheels.

[JasonInAugusta]

Originally Posted by badltl510

With the plane myth you have to remember the wheels don't come into play. The prop or even turbine moves the plane not the wheels.

All of my points were concerning airflow...not wheels.

Originally Posted by RedBullRockIt

No...

Consider this. Each time a propeller turns around, it grabs a certain chunk of air and moves it backwards...or, moves the plane forwards...or really, a combination of both. That propeller will move the airplane forward the amount of air that it is displacing, regardless of ground speed.

That is making sense I'm looking at lift created by movement over wings.

Dustin might convince me I'm wrong.

I really wish I had seen this show.

[badltl510]

Originally Posted by RedBullRockIt

No...

There is a difference between ground speed and air speed. In aviation ground speed means nothing except to judge "time to destination" calculations.

The pilot pushed the throttle to takeoff power. That propeller will move the airplane forward the amount of air that it is displacing, regardless of ground speed. It wouldn't matter if the conveyor was moving at 3 mph, or 3,000 mph, the propeller would still be taking the same size chunk of air and the plane will move forward the same distance (minus a tiny bit for the tire/wheel friction).

Well put it's all about air speed it's just hard to forget about the ground.

[Norcal crawler]

Originally Posted by JasonInAugusta

*EDIT*

Actually, that running on a treadmill thing brings up a perfect illustration of what I'm talking about.

Go outside and run your ass off. Feel that wind in your face?

Get on a treadmill and run your ass off. Where'd the wind go?

That absense of wind means an absense of lift for the aircraft.

That leaves the propwash as the only means of lift - and it's not going to create lift over the entire wing as you would have if an aircraft was in foward motion.

Can some aircraft take off with only the prop to move air over the wings? Yeah...but only those with light wing loading.

The problem lies in the fact that nobody realizes where the difference lies.

But think about this. When the plane is taking off normaly, there is an absence of wind also. So how would that air be moved over the aircraft's wings? By gradually moving faster and faster. The only thrust to get it going is the propeller.

[JasonInAugusta]

Absence of wind when taking off? No. Just like when running, throwing a ball, etc...air flows over and around the object moving through it. That's why a frisbee flies in no wind. That's why riding a motorcycle at 60mph on a windless day with no glasses pushes water out of your eyes and so on.

It's a good debate with plenty of variables to consider. I still think the tests were flawed, though.


Here's a question for those that saw the show...when they got the treadmill up to "takeoff" speed on the rc test...did Jamie need to apply throttle to keep the plane in place on the treadmill?

Dustin's point of forward bite from the prop is valid, and something I was overlooking. But something is telling me that they applied more throttle than normal to take off on their treadmill tests.

[2JSC]

No - he kind of hit the juice and it took off... that was "more than enough" for them to jump to full scale. I seriously think they f'ed this up as well.

[dhoffroad]

man I realy wish I would have seen the show. but it makes total sence how the plane would take off. for those who have a treadmill start it up put a hotwheel or something similar to one on the treadmill. it will go with the direction of the treadmill BUT the tires will "freewheel" a little, ok now the plane's movement comes from something other than what is contacting the treadmill the friction from the tires and axles will force the plane to move backward (direction of treadmill) but that's it the prop will take care of the foward movment the only "extra" power that the plane should require would be to compensate for the friction between the wheels and axles......

[JasonInAugusta]

Originally Posted by dhoffroad

man I realy wish I would have seen the show. but it makes total sence how the plane would take off. for those who have a treadmill start it up put a hotwheel or something similar to one on the treadmill. it will go with the direction of the treadmill BUT the tires will "freewheel" a little, ok now the plane's movement comes from something other than what is contacting the treadmill the friction from the tires and axles will force the plane to move backward (direction of treadmill) but that's it the prop will take care of the foward movment the only "extra" power that the plane should require would be to compensate for the friction between the wheels and axles......

...and that "extra" shouldn't be added.

[RC-RAMIE]

Once explained on the show and seening the test it makes sense I didnt think it would take off before that either.

[slowrockr]

Wasn't the myth simply if a plane was moving at take off speed on a treadmill moving in the opposite direction at the same take off speed would the plane take off?

Meaning they are both moving at take off speed just opposite directions it wouldn't matter how much power the plane has to apply to get to take off speed as long as it gets to take off speed. The ground or treadmill speed wouldn't make any difference at all, the plane will still reach take off speed even though the ground is moving underneath it's wheels. The wheel speed has absolutely nothing to do with the required air speed needed for take off.
Why is it the simple physics are so hard to describe.

[yoda316]

i think they messed this myth up too. for all of there tests they had to apply more throttle to build up actual ground speed (moving faster than the belt the plane is on) so that they build up lift and takeoff.
i think all they proved is that yes its possible to take off from a conveyor belt, you just have to go faster than the belt....

i have a question to all the people who believe that it is possible to take off in a stationary position like the moving belt.
if they were to put a plan on top of something , say a shipping container , then lock the plane to it , allow it to get to full throttle . what would happen if the shipping container was suddenly took away leaving the plane in midair with no airspeed.? i know i wouldn't wanna be in it.
it is the same thing as the belt , how can a plane takeoff with no lift(or very little from the prop, the prop is just their to get it to a speed that the wings provide lift and it maintains the plane moving forward providing lift so the plane doesn't just glide out of the sky back down to earth.

you can pretty clearly see that the planes in all the tests on the episode were not staying in one spot and then just taking off in the same place.

just cause the plane's tires are spinning doesn't mean that the plane is actually moving forward providing the lift to take off....

so as i see it, if the belt is moving equally as fast as the plane and in the opposite direction, then the plane would do the same thing that it would if its brakes were on so the plane couldn't move across the ground, which is nothing other then make noise and enough wind to mabey lift the rear section of the plane off the ground as planes are lighter in the back.
but not enough that the entire plane is just going to take off with the brakes on in a stationary position and then start flying forward


just my thoughts on it
i would be very interested to be proven wrong however , interesting it would be