BS: The Mythbusters airplane takeoff problem

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 06 Nov 12 - 12:53 PM

Yup. Those wheels would spin like hell. It would make an extra-thrilling ride for a thrill-seeking, wheel-hub-riding hamster.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bobert
Date: 06 Nov 12 - 12:41 PM

Oh, I get it... Duhhhh...

Yes, of course the plane would take off... Might burn up the bearings in the wheels but the treadmill would not counter the props...

B~

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 06 Nov 12 - 12:39 PM

Bobert, take a diecast toy car whose wheels spin freely (like those on an airplane). Put it on top of a treadmill. Move the treadmill rapidly east. Does the toy car move along smoothly in the same direction with the treadmill? No, it doesn't. Its own weight (inertia) causes it to want to remain where it is, and its free-spinning wheels allow it to do just that, despite the moving treadmill. If placed exactly in line with the treadmill (so its wheels are perfectly aligned) it will tend to remain in its original position geographically, regardless of the treadmill.

The same is true of the airplane.

But....If you now add propellor thrust pulling it west, the vehicle will move in the direction the propellor is pulling it...to the west...you will get the necessary airflow over the wings...and you will generate lift sufficient to allow takeoff.

Any hamster with a degree in mechanical engineering could figure this one out! ;-)

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bobert
Date: 06 Nov 12 - 10:43 AM

If the treadmill accelerates equal to that thrust of the props then those wheels can spin as fast as they want... That was the original information, no??? If that is the case then no air speed is created and therefore no lift... The plane has to actually move by one of two ways:

Sufficient air speed over and under the wings or because the prop is out dueling the treadmill or...

...there is one heck of a lot of headwind that day that allows sufficient air moving above and below the wings...

That's the way I see it but, hey, seein' as I wrecked a Piper J-3 Cub on my 16th birthday, I ain't gonna say that I'm in Charles Lindbergh's league... lol...

B~

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 06 Nov 12 - 01:18 AM

That's right, Bobert. No air speed, no lift. However, the wheels on the plane spin freely, unlike the wheels on most vehicles. The static inertia of the airplane means that when the treadmill starts moving East, the plane doesn't move east with the treadmill. It sits where it is, due to its own inertia, and its wheels spin. Now you start up the propellor. The plane will want to move west due to thrust pulling the plane through the air. The wheels will spin even faster while in contact with the moving treadmill. Presently the plane will start moving west because the prop is pulling it in that direction. If its wheels were resisting the treadmill, as the wheels on a car would, then the treadmill would cancel out the plane's westward movement, but the wheels aren't resisting the treadmill's eastward motion. They're spinning freely. The plane will move west regardless of the treadmill, and the wheels will spin ever faster as long as they are in contact with the treadmill. Presently the plane will reach a westward rate of speed sufficient to take off, regardless of what the treadmill is doing.

***

fossil - you're right about the RAF not providing parachutes to their unfortunate airmen during almost all of WWI. The Germans provided parachutes to theirs by about halfway through, and many German airmen's lives were saved by bailing out of stricken aircraft.

Hamsters, however, have to make and provide their own parachutes whenever they use them, even German hamsters! Therefore the RAF policy in this regard was irrelevant in the case of wheel hub riding hamsters. A sensible wheel hub rider didn't need a parachute, because he would leap off just as the plane was leaving the ground...but a tiny parachute would be a good emergency backup in case the hamster for some reason did not jump off soon enough, as could happen with rotational intoxication!

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bobert
Date: 05 Nov 12 - 08:08 PM

Okay, LH... No air speed, no lift... You don't get air speed sittin' on a friggin' treadmill unless the plane has a stall speed of zero... Lowest stall speed I know of is in the 20 mph range...

BTW, speakin' of airplanes... "My Poor Brother"... Remember him??? Well, first of all he ain't never been "poor"... But he has gotten himself into a pickle or two but this ain't one of those...

Reckon about 25 years ago he got this idea that he was gonna be a stunt pilot and was learnin' aerobatics and we were just out bombin' (not literally) over the area where his farm was in a Cessna 172 and reckon he just wanted to scare me so he stalled it at 5000 feet... Big deal... The plane shudders and then a warning sound starts tellin' ya' that yer air speed is too slow to fly the airplane... Like I said, "Big Deal"...

Now every pilot in the world knows that you put the nose down and all is well, right...

So "My Poor Brother" decided, "What the Hell... Think I might as well scare the hell outta the Bobert" so he puts a danged rental Cessna into a purdy serious dive and this plane ain't really up for that kinda stuff but I guess his aerobatics instruction musta told him what kinda ridiculous air speed a 172 could handle... So I'm lookin' at the air speed indicator and watchin' as he hit 100, 110, 120, 130, 140 and we've dropped 3000 feet in what seemed like seconds...

Now, I didn't let on but at a buck forty I was beginning to think, "Hmmmmmm, Poor Brother gonna fuckin' kill us" but I didn't let on... No, I was scared shitless at that point then I look over and this Cessna is diving fast 150 miles per hour... That is fast in a Cessna 172!!! Very fast... These shit boxes ain't F-16s... At about 800 feet he pulls up on the yoke and this Cessna is vibratin' & shudderin' and levels out at no more than 300 feet...

"Uh huh" I was thinkin', "I'll get even one day for that little scare"....

'Bout 5 years later we were flying down to Kitty Hawk, NC (yup, that one) in a rented Cessna 210 Turbo and low clouds kept up bumping along at 115 mph at 1500-1700 feet 'cause "My Poor Brother" ain't instrument rated and that meant no cloud punchin'...

So we're somewhere over the "Northern Neck" in Va and had an hour or so before we had to swing west to avoid restricted air space around Norfolk/Newport News so brother says,' "I'm kinda sleepy and gives me a heading, says "Your plane" and pulls down his ball cap to take a nap...

Well, folks... There are those times in life where the rule that "Get-backs are Hell" and this was going to be one of those days...

I wanted to be sure that "Poor Old Brother" was in downtown sleepsburg as I plotted my egt back and I kept lookin' over for signs and then saw his head kinda fall to one side without any jerkin' so I knew that the bases were loaded...

I wasn't sure I had the balls to do it but...

...WTF???

So I pulled up on the yoke ever so slightly... Felt it fightin' as I put that 210 into a full stall and the plane was vibratin' and shudderin' and then the warning alarms went off...

That was all it took to get poor brother outta his nap...

"What's happening, Bobert?"... "What the hell you done???"... "My plane, brother"... "Shit..."

So I let him put the nose down and take us out of stall... He was pissed... Who cares???

We laugh about it these days....

Get backs are Hell...

B;~)

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: gnu
Date: 05 Nov 12 - 06:00 PM

"When Brittania ruled the waves, you could have sent an invasion force to make us all do it your way...but those days are long over, my good fellow. Learn to live with it. ;-)"

Long over? You read a newspaper lately? Iraq? Afghanistan? Libya? Egypt? Syria?....

And, don't chore me with "details" about why you think I am wrong. Britannia makes the waves. Sail with her or drown.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Fossil
Date: 05 Nov 12 - 05:26 PM

Little Hawk seems to have forgotten that nobody on the British side was issued parachutes during WWI (apart from a brief period at the very end). The reason given was that craven cowards might fail to press home their attacks on the enemy if their plane was damaged. It would be too easy if all you had to do was jump out.

So, elephant or hamster, you would have to contrive a landing, or take your chance on terminal velocity working in your favour. My money's on the hamster in that contest...

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 05 Nov 12 - 02:16 PM

Try to be brief, okay?

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bobert
Date: 05 Nov 12 - 01:14 PM

No...

B~

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 05 Nov 12 - 12:19 PM

Several people HAVE already stated that point, Richard, including me, when I admitted that my initial conclusion was mistaken. The wheels spin freely, therefore the moving conveyer belt does not prevent the plane from attaining forward motion relative to the surrounding body of air. Therefore, the plane does take off in a normal fashion.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Richard Bridge
Date: 04 Nov 12 - 11:15 PM

The point that people are failing clearly to state (and I have had a little look on the link provided) is that the plane actually does move.

The conveyor belt will have little effect on the air above it.

The propellor of the aeroplane (both correct UK spellings) pulls the aeroplane forwards, regardless of the conveyor. The wheels go round like buggery, but the friction is insignificant.

So the 'plane moves forwards in the air. This is stated in the first post on the thread linked to. Air moves over the wings. The Bernoulli effect creates lift. The 'plane takes off.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 04 Nov 12 - 10:53 PM

True! A hamster is far more likely to survive a long fall than a human being or an elephant. If the hamster fell into long grass, for example, I think its chances of survival would be quite good.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: JohnInKansas
Date: 04 Nov 12 - 10:49 PM

Survival after free fall depends almost entirely on the ballistic coefficient of the object, and data is not readily at hand for hamsters. There is a reasonable expectation that a hamster might survive with adequate PLF training.

A well known axiom used to explain "scale factors" in assessing such possibilities is:

"You can't kill a flea by throwing it up against the wall, but throwing an elephant the same way makes one helluva mess."

John

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 04 Nov 12 - 10:37 PM

This would depend largely on whether the hamster had equipped itself with a workable set of little glider wings and knew how to use them. I am told that the famous Corridus (bestselling author of "Seeds of Sin") was trying to perfect the use of such a device when he met his untimely end...

Freddy the White Rat suffered deep depression as a result, and passed away not long after. He had helped to design the hamster wings. He should probably have opted for a parachute pack instead.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Mysha
Date: 04 Nov 12 - 10:31 PM

Hi,

So, suppose a hamster escaped the machine-gun thread-mill in mid-flight - commonly known at the time as "jamming the gun", though nowadays the expression "jumping the gun" seems more popular - and let itself drop down from the aircraft. Would it indeed fall to its death, or would gravity lose its fight with air resistance?

And does it matter whether the hamster had, at the start of the flight, been employed to aid the flying machine in taking off from a moving surface?

Bye,
                                                                  Mysha

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Jim Dixon
Date: 04 Nov 12 - 09:57 PM

Apparently you have to be logged on to the Discovery Channel web site to be able to see the discussion forum for Mythbusters. Here is the link for the first page of the discussion called Plane on a Conveyor Belt/Treadmill (aka PoCB/PoaT).

The discussion began on January 30, 2008 and is still going on – up to 566 pages of messages!

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 04 Nov 12 - 09:54 PM

Oh, come now, John! ;-) WWI airplanes were pretty fine machines from about mid-1915 on. They flew beautifully, carried good firepower (for the time), were highly maneuverable, could attain considerable altitudes...hell, THAT's why the hamsters had faith in those airplanes!!! Listen, my friend, once you have flown an Albatros D.III, an S.E.5A, a Fokker D.VII or a SPAD XIII or Nieuport, you KNOW what a good airplane is! And you could safely land them "dead stick" very nicely in any small field of open ground too. Try that with your modern planes.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: JohnInKansas
Date: 04 Nov 12 - 06:03 PM

Bettynh -

If the wheel has to rotate at a rate corresponding to50 mph for the plane to "hold in one place" on the conveyer, for the plane to move forward on the belt to take off airspeed, which for sake of argument might be 50 mph, the rotation of the wheel is the same as if the plane reached 100 mph on a normal runway. The centrifugal force due to the rotation of the wheel could burst the wheel, or could stretch the tire bead off the rim so that all the air leaks out rapidly with the same effect. More recent design guides call for a "rotation burst speed" of at least 2 times maximum rolling speed, but planes capable of lowest takeoff speeds are often equipped with older kinds with less margin of safety. The requirement that "pressure burst" be more than something like 40 times normal inflation (with additional consideration for "impulse loads" at touchdown), suggests that just pulling the bead off the rim due to rotation rate is unlikely, but for older (and lighter) planes it could possibly happen. Not all tires are without defects/deterioration after even a couple of landings.

LH - In 1916 the hamsters running on the wheels were not there for pleasure. They were the ones that escaped from the exercise wheel that powered the prop (usually at least three or four were used), and they were just trying to get off before the pilot flew into the ground (common enough that even 1/4 of the hamsters knew the statistics).

Many fewer hamsters were seen there once the big rubber bands that powered the 1917-1918 planes were developed. The Brits, of course, claimed to be the inventors, but the French were much quicker adopting them, although by the time the invention was available for use the French only had three airplanes left to convert, and those three were soon lost to "enemy action" - if fences and trees were considered "the enemy," which was a justifiable opinion.

(Lin uses one of the surplus store rubber bands to run her 20 year old S10 mini-pickup, but the ones available are beginning to suffer from oxidation breakdown of the rubber so it's getting hard to find good replacements. I've thought about replacing with a clock spring, but the necessary spring/housing weight exceeds the load capacity - remaining - for her "truck.")

John

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bettynh
Date: 04 Nov 12 - 05:18 PM

JohninKansas, I think they'd be rotating at the same speed, but backward. Even rotating backward, they'd function to reduce friction.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 04 Nov 12 - 04:31 PM

Bonzo, you must face the fact that the entire world does not do everything exactly as you would have them do it. When Brittania ruled the waves, you could have sent an invasion force to make us all do it your way...but those days are long over, my good fellow. Learn to live with it. ;-)

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Jeri
Date: 04 Nov 12 - 04:23 PM

You obviously have figured it out without help.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bonzo3legs
Date: 04 Nov 12 - 04:21 PM

What pray is an "airplane"? We have aeroplanes in Europe!!

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 04 Nov 12 - 03:46 PM

It is a little known fact that thrill seeking hamsters will sometimes ride on the rim (or hub) of an airplane tire in order to experience extreme rotary velocity. By clinging to the rim, the hamster is able to hang on as the tires rotate rapidly during takeoff. More athletic hamsters will even run inside the recessed hub, going either with or against the direction of rotation in order to increase or decrease the effect. The objective is to hang in there until the moment the plane's wheels leave the ground, then bail out and make a rolling landing that you can walk away from.

A hamster that is capable of walking a straight line within a few seconds of bailing out will win the admiration of his fellows.

Wheel riding became a craze with hamsters as long ago as 1916, when many were observed riding the wheels of French, German, and British aircraft in the First World War. It is thought that the common rodent pastime of running on exercise wheels is what led to the craze for riding mechanized wheels on both automobiles and aircraft.

One danger that can arise is rotation intoxication. The hamster becomes so caught up in the thrill of rotational acceleration that he forgets to bail out until the plane has attained an altitude of 100 feet or more! This can and has resulted in the premature deaths of many wheel-riding hamsters.

One hamster attempted to set a distance record by riding a Boeing 747 wheel all the way from Los Angeles to Tokyo. Unfortunately, he either froze to death or was asphyxiated shortly after reaching 15,000 feet. His frozen corpse fell out on the tarmac when the plane landed in Tokyo.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: JohnInKansas
Date: 04 Nov 12 - 01:53 PM

The 50 mph for the belt is sort of arbitrary, but assuming the plane's normal takeoff speed is similar, for the plane to lift off the wheels would have to spin at the same speed as if the plane touched down at twice normal takeoff/landing speed. While airplane tires are intended to have considerable safety margin, the real question is whether the tires would explode at the overspeed that might result - i.e. could the plane land safely after taking off from the conveyer.

Beginner pilots are assured that "no matter what fails, the reserve features will carry you to the point of impact," so it's probably not too important a question.

John

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Mysha
Date: 03 Nov 12 - 09:47 PM

Hi,

Sorry, should have read the replies. But the "Hey, that's wrong!" was so strong that I just jumped to the bottom of the message, and the rest of the thread, to express that.

Bye,
                                                                Mysha

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Mysha
Date: 03 Nov 12 - 09:20 PM

Hi,

"The conveyor belt accelerates at the same rate as the normal acceleration of the airplane, such that, for instance, by the time the airplane would normally be moving westward at 50 mph, the surface of the conveyor belt is moving eastward at 50 mph, and so on."

Trick question. That "normally" suggests that now the plane doesn't move. With my enormous flying experience of once over the Atlantic, and once of USA and the Atlantic to get back (and bits of Canada both ways), I'd say it's the plane moving the wheels, not the wheels moving the plane, so the fact that the wheels are made to spin twice as fast would be irrelevant.

Bye,
                                                               Mysha

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: John on the Sunset Coast
Date: 03 Nov 12 - 09:12 PM

Bettynh @ 03 Nov 12 - 02:15 PM --

I am aware of that...I was following-up someone else's comment about residual wind from propellers or something like that, in trying to understand possible ways the plane could fly according to this problem.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 03 Nov 12 - 03:04 PM

Neat! I was wrong because I forgot one thing. The key to the fact that the plane will take off is this: the wheels on a plane spin freely (unlike the wheels on a car). The only time the wheels on a plane do not spin freely is when you apply the wheel brakes (assuming the plane has wheel brakes...some simpler aircraft do not...they simply roll to a stop when landing).

The freely spinning wheels cancel the effect of the moving conveyer on the plane. Therefore the plane does take off in a normal fashion as its propellor is pulling it forward, despite the conveyer belt moving in the opposite direction.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bettynh
Date: 03 Nov 12 - 02:15 PM

John, one other thing - propellers don't move a plane by creating lift on the wings. They literally screw their way through the air. This may help.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bettynh
Date: 03 Nov 12 - 02:04 PM

DMcG, the Mythbusters explored a similar problem. Then, they took it to full scale. Like good scientists everywhere, they're not afraid of failure, but consider it great fun and/or food for thought.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bettynh
Date: 03 Nov 12 - 01:50 PM

John, they don't matter. There is no power, going forward or back, to the wheels. They're only there to hold the plane up on a tiny footprint to reduce friction.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: DMcG
Date: 03 Nov 12 - 01:05 PM

I remember going to a science lecture when I was around 14 (I was like that!) and the scientist had a small wheeled trolley about 30cm long. At one end was an electric fan, which was pointed at a sail fixed at the other. The question was whether and how the device would move if the fan was switched on ...

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: John on the Sunset Coast
Date: 03 Nov 12 - 12:27 PM

Thanks to Betttynh,I am about convinced...BUT was that trial done such that the speed of the conveyer and the plane were moving at equal but opposite directions at all times, given the primitive conditions of the test? Also, were the wheels in constant contact with the conveyer, again owing to the primitive conditions. Finally, do those things matter?

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: John on the Sunset Coast
Date: 03 Nov 12 - 11:01 AM

DMcG--

Last night after my post I went to Mythbusters to see their explanation. While the video did not come up [I think someone said there was one showing the plane to get off the ground], the comments were very interesting--about propeller created air flow, rolling friction v dragging friction, where friction points are, etc, etc, etc. It seems to me the comments broke about 50/50 on whether the darn thing takes off, about the same as here.

I understand that the propeller pulls the plane forward to cause airflow, and perhaps some of its own wash (not sure on this), and that planes take off into the wind because that assists in reaching air speed more quickly. But an airplane can take off with the wind just requiring a longer runway, assuming the tail wind on the ground is not too great.

So I suppose, like Archemides' lever/fulcrum principle, theoretically if the propeller(s) were large enough to create enough wash over the wings to lift the plane while the plane was in a stationary position relative to the conveyer belt/ground. Once off the conveyer the plane would fly.

So then questions abound: Is it physically possible to actually do this? What other variables might come into play? Given the state of materials are there any available capable of withstanding the massive friction and rotational speeds required? Could this be done with practical size aircraft, so there is some utility to be gained?

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bettynh
Date: 03 Nov 12 - 10:36 AM

The Mythbusters actually built a giant conveyor belt and tested the idea . The plane flew. Here's the theory.



We moan about schools and education, then along come the Mythbusters. Engineering and physics are glamorous and, above all, fun.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: GUEST,EBarnacle
Date: 03 Nov 12 - 09:46 AM

Depending on the aircraft, the amount of lift is dependent on both the amount of wind created by the propeller and the wing area exposed to the wind. In a single engine aircraft, this would be a lot less area than in a multi-engine aircraft.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: DMcG
Date: 03 Nov 12 - 03:33 AM

I don't know how the Mythbuster's tackled this, but one way would be to tether a model plane. I feel pretty confident that that would attempt to take off and bounce around pretty wildly.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: DMcG
Date: 03 Nov 12 - 03:25 AM

The plane should be able to take off, in most cases. Sunset John's analysis is right until he says "as the plane is in the same spot there will be no airflow". There are two sources of the airflow in normal circumstances: the wind from the environment and the wind created by the propellers. The sum of these create the total air flow over the wings and when that is more that a certain amount, the plane will take off. Moving forward increases the effect of the environmental wind, as does taking off into the wind rather than against it, but for a powerful enough propeller, it would not be necessary to require any environmental component.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: BK Lick
Date: 03 Nov 12 - 03:03 AM

Synopsis of Episode 97: Plane on a Conveyor Belt, Cockroach Survival, Shaving Foam Shenanigans at:
Annotated Mythbusters

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: John on the Sunset Coast
Date: 02 Nov 12 - 11:31 PM

This is interesting problem. What little I know of how a plane flies, if I recall correctly, is that air must go faster over the top of the wing, causing the air pressure over the wing to become less than the air pressure below the wing. This causes the lift to allow the plane to rise.

Now I picture this problem as follows; if the plane is going forward at the same rate the conveyer belt is pushing it backwards, so to say, it seems to me the plane would always be in the same spot, so there will be no airflow and the plane will not rise.

My analogy is my using the treadmill at the gym (a treadmill works on the same principle as a conveyer belt. If I set the machine to 4mph, and move my legs at the same pace, I will stay at the same spot on the the treadmill. If my legs tire and I slow down, I will be pushed back by the belt, and conversely if I walk faster than the 4mph, I will move forward.

Where have I gone wrong (as I am not an airplane, and I never get a lift, only tired) in my analogy?

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: JohnInKansas
Date: 02 Nov 12 - 10:50 PM

Until the prop generates enough thrust to spin the wheels at the same speed as the conveyer, it's not doing much of anything to change the airflow over the wings, but if it took much thrust to spin the wheels every plane would tip up on its nose at every landing. The wheels must spin freely with little tangential force required.

Once the wheels are spinning fast enough, the airplane can begin to move forward and take off normally. The additional thrust to take off once the wheels are up to speed should be quite comparable to the thrust required for a normal takeoff.

The problem is inadequately stated for a complete analysis, since the width of the belt is not given (and I haven't seen the movie) but there is a "component" in the problem that could have an effect in the form of "boundary layer behavior." When an object (a surface) moves in a fluid (in this case air) the fluid immediately at the surface moves at the same speed as the surface, so the conveyer belt is effectively an air pump, dragging a layer of air with it. The absolute velocity in a fluid cannot change abruptly, so additional air is dragged along with decreasing speed as one moves farther from the surface. As the speed of the moving surface increases, the "boundary layer" becomes a little thicker.

There are a few airplanes that can "fly" in relative air near or slightly below the 50 mph specified, so that if the conveyer is wide enough (possibly near normal landing strip width?), it would be possible for an airplane of this kind to "fly" (lift its weight entirely off the conveyer) without having forward absolute speed. Once the plane is "off the belt" it could accelerate and take off normally.

Unfortunately the boundary layer is not very thick for relative speeds in the range given, so if the plane tries to ascend more than a little above the belt it will rapidly move into air with lower absolute velocity, and likely will perform the four standard maneuvers** every pilot knows well.

The same fluid behavior is what's well known to pilots as "ground effect." At altitude, with lots of air surrounding, the plane drags a lot of air along with it, and must have enough speed so that the net relative wind is sufficient to lift it. When approaching the ground, the air is "bound" to the ground, so at the same forward speed there is less air moving along with the plane, and lift is enhanced enough that the actual stall speed may drop to lower levels than when in flight. This is the effect that sometimes causes amateur pilots to "float a landing" and touch down much farther down the runway than they expect (or go past the end of the runway and do those famous four maneuvers again). If the plane has slowed below free air stall speed but is still above stall in ground effect, attempting to pull up before getting enough speed to be above free-air stall will inevitable produce all four maneuvers. Going up always reduces speed, and a significantly higher speed is required as one moves out of the ground effect.***

** While nearly all here will already know the four standard maneuvers, for the benefit of the few who might not, they are (always performed in the same order) STALL - SPIN - CRASH - BURN. Every pilot must know them before being allowed to take the controls.

*** There might be a few more who might not know that rate of climb - whether you go up or down - is something you control with the throttle in an airplane. Airspeed - how fast you go - is what's changed when you move the stick (or "steering wheel" in the fancy ones). The other bit of info is that you control which direction you're going with the pedals on the floor. "Turning the wheel" - or moving the stick sideways - changes the direction you're pointing, but failure to use the pedals properly will demonstrate that most airplanes can fly sideways quite nicely - within limits.

John

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: GUEST,Chongo Chimp
Date: 02 Nov 12 - 10:07 PM

The Republican Party would do something that stunned, that's who! Ook! Ook!

Now, regardin' the pig...here's what would happen. The conveyer belt would accelerate rapidly, the pig would squeal and jump off and beat it outta there like a bat outta hell...but not flyin'. Pigs ain't gonna fly till I get elected.

- Chongo

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: gnu
Date: 02 Nov 12 - 10:01 PM

Oh! The conveyor is in "reverse"! I didn't even detect that in my sped reeding. And why would I? WHO would do something that stunned?

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 02 Nov 12 - 09:55 PM

Huh? Wouldn't the air have to be moving in concert with the conveyor belt to generate lift by passing over the wings rapidly?

Here's another example. Put a glider on that conveyor belt, simply resting still in a fixed cradle. Now move the conveyer belt west at 100 mph, and I predict that the glider will get lift and take off, because the conveyor belt is moving it rapidly through the surrounding air.

If, however, the glider is moving backwards (east) at the same speed ON the westward-moving conveyer belt, then no lift is generated.

Now, what if we tried it with a pig? ;-)

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Amos
Date: 02 Nov 12 - 09:49 PM

The ground speed may be neutralized, but the airspeed over the wings will not be neutralized because the air is not moving the conveyor belt. The airflow over the wings that is the equivalent of normal airspeed from ground movement + normal prop thrust will be possible if the engines are up to generating the diffgerence and thus compensating for the lost fground speed. That is a function of total engine thrust capability, which is not specified. Therefore there is insufficient data to solve the problem.


A

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 02 Nov 12 - 09:43 PM

Hmmm. Interesting point, Bee-Dub. I'm not sure how much lift the prop would generate in that fashion, but probably not enough to get the plane airborne, I wouldn't think.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Bee-dubya-ell
Date: 02 Nov 12 - 09:39 PM

I'm going to go out on a limb here and say that some prop-driven planes could take off. It would depend on whether the prop backwash would create enough "artificial wind" directed at the airfoil to provide adequate lift. That's going to depend on the design and weight of the airplane. It wouldn't take much - just enough to get the wheels barely clear of the runway.

We're talking light twin-engine planes for the most part, and maybe a few single-engines like the Cessna O-1 Bird Dog. No jets, no planes with engines mounted above or behind the wings.

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: MarkS
Date: 02 Nov 12 - 09:05 PM

Makes sense Hawk

If you are flying forward at, say, 500 mph and in the jet stream with a 200 mph headwind, your ground speed is 300 mph. You are indeed flying but not going anywhere real soon.   This accounts for the flying time differences during long east/west vs west/east flights.

So if you are moving forward a 50 mph and the conveyor moves you back at 50 mph, you go nowhere, at a ground speed of zero - which will likely not generate enough lift to take off.

Good thing the jet stream does not move as fast as the speed of modern airliners too. A plane at 500 mph going into a headwind of 500 mph will loose lift and act like a brick with wings. I think the correct term is "stall."

Mark

Subject: RE: BS: The Mythbusters airplane takeoff problem
From: Little Hawk
Date: 02 Nov 12 - 09:01 PM

"standing still relative to the surrounding air"

Another example...a plane that is sitting on the ground with its brakes on and its prop going flat out at full power will not generate any potential lift (upward force). It will generate plenty of potential thrust (forward force).

If a heavy wind comes at the front of the plane, THAT will generate some lift, even if the engine is shut off, because it will be passing over the airfoil...and that produces lift.

This is why a plane that's diving with its engine shut down and its prop feathered (not moving at all) can STILL generate lift from the dive, and can make a shortlived climb with that generated lift when it pulls out of the dive. Seagulls do that with absolute ease.