Mythbusters/airplane on a treadmill.

[5-90]

Airplane lift depends on airspeed, not groundspeed. So, you can put it on a treadmill and spin the wheels all you want - if forward airspeed is zero - or below takeoff speed, it's not going anywhere. That's why you can generate lift in a wind tunnel without forward motion.

I hadn't read through the whole thread, but here's the question I consider key - will the wheels/airframe be powered, or will the treadmill be spinning the wheels? If the treadmill is going to spin the wheels, forward airspeed will be zero (or whatever ambient wind is happening,) and no lift will be generated.

The reason that aircraft take off "into the wind" is simple - they get to combine the speed of the wind with the speed generated by thrust - it's additive - and that allows them to take off at a lower ground speed and using a bit less power. If they were to take off "with the wind," it would take more power and fuel to do so. That's why major commercial airports have so many runways - it allows them to work with the weather and prevailing ground wind patterns. Once they're off the ground, friction is reduced (I don't care how good your bearings are - put 50 or 60 tonnes on them, and you're going to get friction) and it's easier to use thrust to generate airspeed.

Say you've got a light airframe that has a takeoff speed of 60knots, and a 10-knot wind aligned right along your main runway. If you take off facing into the wind, you actually get sufficient lift at 50 knots ground speed (you'll show an "indicated airspeed of 60 knots) and you'll take off. Take off with the wind blowing into your back, and you'll actually have to do 70 knots ground speed (same 60 KIAS) to get lift to get off the ground - the airspeed is the same, but the ground speed will differ by twenty knots. That can be quite a bit, and mean shorter range due to extra fuel consumption (and longer takeoff distance as well.)

The fuel and power problems are magnified when you've got 50 or 60 tonnes - vice 2000-3000 pounds - to get off the ground.

So, I don't know enough to give a definitive answer - but I can say with some certainty that, if the wheels are going to be driven by the treadmill (and not by engine thrust,) that plane isn't going anywhere.

 

[JeepFreak21]

A plane is standing on a runway that can move (like a giant conveyorbelt). This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be EXACTLY THE SAME (but in the opposite direction).Will the plane be able to take off?

Doesn't the fact that they say, "the plane's speed" suggest that it's moving???
Billy

 

[oo7ravisXJ]

Can't believe i actually read this whole thread... my head hurts.

 

[jeepboy381]

I think everyone here has valid points, I just think the description of what they are trying to do and how they are going to do it is not described very well. I guess we will all find out what happens in a matter of days!

 

[JNickel101]

Doesn't the fact that they say, "the plane's speed" suggest that it's moving???
Billy

It doesnt specify airspeed, and since the damn wheels are on the ground, how is it gonna measure airspeed? I've never been in the cockpit of a plane - and i dont know if they are measuring the forward speed of the plane itself, or how fast the wheels are being spun....

the more i think about it, the more i want to watch something else on TV. I like it better when they blow stuff up - or shoot stuff - like the dynamite in the paint can

or shooting the frozen chicken/turkey thru the aircraft windshield with the giant air cannon....

 

[JNickel101]

They tie the car down. Are they going to tie the plane down? No? then it can take off.

I say again, it does not matter how fast you move the ground, the AIRSPEED is what gives a plane it's lift, and since the only thing between the treadmill and the plane is a set of free spinning wheels, the plane will roll off of the treadmill and down the runway as the engines pull it forward.

Open your mind a little bit.

hone: The clue phone is ringing, will you pick it up?

Damn, you're as bad as Ron....I feel like a 3rd grader...lol...

I understand what you're saying - and I have the whole time - but their question says nothing about AIR SPEED or GROUND SPEED. Is ground speed measured by how fast the wheels are spinning?

hasta we're both just doing this b/c we're not understanding each other....:cheers:

 

[JeepFreak21]

It doesnt specify airspeed, and since the damn wheels are on the ground, how is it gonna measure airspeed? I've never been in the cockpit of a plane - and i dont know if they are measuring the forward speed of the plane itself, or how fast the wheels are being spun....

the more i think about it, the more i want to watch something else on TV. I like it better when they blow stuff up - or shoot stuff - like the dynamite in the paint can

or shooting the frozen chicken/turkey thru the aircraft windshield with the giant air cannon....

They don't say air speed, wheel speed, ground speed, speeding ticket, or Speedy Claxton... they say "the plane's speed." Why would it be anything but the speed the plane is traveling? :dunno:
Billy

 

[JNickel101]

Forgive my ignorance, but would that be air speed or ground speed or is there no difference???

I guess that should have been my question from the beginning...

 

[5-90]

Doesn't the fact that they say, "the plane's speed" suggest that it's moving???
Billy

The tricky part about that is quite simple - the airplane's wheels aren't powered, like on a vehicle. They're free-wheeling.

The forward speed of an aircraft is generated by engine thrust working against the air and the airframe's own inertia (gross oversimplification, I know.) So, you can't really have a treadmill countering the airframe's "forward speed," since that speed is not provided by the wheels - it's provided by the thrust of the engines. Without knowing more about the circumstances of the test, I have to assume (which I hate doing!) that the treadmill will be driving the wheels. You can drive the wheels of an aircraft as fast as you like - lift is generated by moving air over the airfoil/wing - and without that, it's not going to get off the ground without a hydraulic lift or a cargo helicopter (like a flying banana.)

I didn't read the whole thread, nor do I intend to. However, given the conditions as I've stated - and I've had to make some small number of assumptions and a couple of guesses - I'll say that the airplane isn't going to get off the ground. The converse is true - you can sit the airframe perfectly still and put a big honkin' fan in front of it - if that fan can generate enough airspeed over the airfoil, the plane will lift off (and, if it's tethered to not blow backwards, you can keep it in the air for as long as the fan holds out without the airplane having to move.)

 

[JeepFreak21]

Forgive my ignorance, but would that be air speed or ground speed or is there no difference???

I guess that should have been my question from the beginning...

It's just the speed. It's the distance the plane would be from it's starting point after 1 hour... (usually measured in miles in the US.)
Billy

 

[JohnX]

Ive got a degree and my A&P if that means anything and "techincally" an airplane prop produces lift which is the source of thrust and yes I know all about Trust, lift, drag, weight. Ive also spent a few hours crewing helicopters so if you want to talk about swash plates, gyroscopic precession or centripical/centrifical force, have at it.

Centrifical force....WTF are you talking about?

Maybe you meant to say Centrifugal force (which is non existant)?

If so, please kindly refer to angular momentum and centripetal force for future explanations.


BTW my vote....if the plane doesn't move, the plane doesn't fly. I know little to nothing about planes, but I just cant imagine that a prop alone will flow enough air past the wings to create any lift.

 

[goodburbon]

Let's see if I can relay this correctly.

Airspeed is related to groundspeed.

Airspeed is your groundspeed (grid) plus or minus the Vector component of the wind that is moving parallel to you.

If a plane goes over a moving river, the fact that the river is flowing at 5mph, does not change the airspeed or ground speed of the plane.

if a seaplane tries to take off in a flowing river, the first order of business is to take off into the wind if there is enough room. doesn't matter if it is upstream or downstream, once the pontoons plane off, friction is minimal, just like wheels on a reg plane. Groundspeed in this case is still equal to airspeed minus vector component of windspeed.

 

[ZacSquatch]

I swear I'm seein the SAME SHIT typed by 10 different people... 12 pages worth... enough.. just wait for the damn show you whiners... I already made me bet, I'm set.

 

[cal]

Ok third grader, at a girls school, it goes like this:

Read FAR 91.205

Required equipment, day, VFR.

Mnemonic: A TOMATO FLAMES

A nti collision lights
T achometr
O il pressure gauge
M ainfoild pressure gauge if altitude engine
A ltimeter
T emperature gauge if water cooled
O il tempreature gauge
F uel gauge
L anding gear position indicator
A ir speed indicator
M agnetic compass
E LT
S afety belts

So, you see? It's simple. Wheels are not required for flight. The plane will fly.

You try.



--ron

Wait. So you're saying without safety belts the plane wouldnt fly?

 

[5-90]

Forgive my ignorance, but would that be air speed or ground speed or is there no difference???

I guess that should have been my question from the beginning...

"Ground speed" is the speed of the aircraft relative to a fixed point on the ground. "Ground speed" is also what the speedometer in your rig will read out to you - since it's measuring your speed by reading wheel revolutions, and it's the wheels turning that get you moving.

"Air speed" is the speed of the aircraft in the air (literally,) and bears only a passing resemblance to ground speed. If there is no wind, your airspeed and groundspeed will be identical. If there is a wind, then your groundspeed and airspeed will differ (if there is a crosswind to your desired direction of travel, then we can get into scalar/vector addition to determine "true" speed, and you can also start adding "crab angle" into the mix - where you are actually pointed in a slightly different direction than the one you wish to go. Sounds complicated - but it makes sense, once you start thinking about it...)

Aircraft speed is usually measured in "knots" - short for nautical miles per hour. A "nautical" mile is a bit longer than a "statute" mile (what the speedometer and odometer in your vehicle works with,) being 6,076 feet in length (vice 5,280 for a statute or "land" mile.) This is used for a good reason - it's the length of one degree of latitude along any meridian on Earth (http://en.wikipedia.org/wiki/Nautical_mile) A nautical or "sea" mile is 1.151 "land" miles. Once you get used to them, they're actually quite useful in navigation (since they're derived from a geophysical measurement - like the metre.)

It is entirely possible to measure airspeed on a ground vehicle - same way you do for an aircraft. It's the scalar quantity of the vehicle's ground speed vector added to the wind vector - just like for aircraft.

With a watercraft, you can actually have three speeds to track (right, Ron?) You have "groundspeed" relative to a fixed point on the ground (lighthouses are good for this - but a navigation buoy can also be fixed WRT the ground, relatively speaking. As can things like oil rigs...) "airspeed" (a scalar quantity) and water speed (another scalar quantity, combining groundspeed and the water's current.)

Note that I do not interchange the words "vector" and "speed" - they mean two different things. "Speed" is a change in position over time, and is a measure of magnitude of change. "Speed" does not include a direction. "Speed" would be something like "two hundred knots" (a rate of change of position, but no note of how the position is otherwise changing.) This means you are moving two hundred nautical miles per hour - but nothing about which way.

"Vector" is a measure of magnitude and direction - a "vector" (can also be called "velocity") would be something like "two hundred knots on a bearing of 270 magnetic." This indicates a change in position of two hundred nautical miles per hour moving due west (relative to magnetic north.)

And, of course, all the other things apply. However, I would think that navigation would primarily work relative to fixed points on the ground - and would therefore use ground speed wherever possible. After all, you're trying to get from on fixed point on the ground to another, no?

(This, of course, also avoids the issue of celestial navigation - if you're going to the moon, you have to figure out how long it will take you to get there - and then go to where the moon will be at that time. If you go to where the moon is now, it won't be there when you get to that point...)

 

[in2fords]

jeepboy381, thats what i've been saying for the last 30 minutes....

if you read the way their question is written, it states that the treadmill and the speed of the plane will be the same, just in opposite directions.

everyone seems to imply that i dont realize that if you move the speed of the boat/plane/runner/skate FASTER than the speed of the treadmill, that you won't move forward....well no shit Einstein....

there is problems with your reasoning. If they get the plane to do 1 hundred miles an hour and the tread mill to go 1 hundred miles an hour the plane still moves foward at 100 MPH and doesnt stand still! the wheels will be doing 200 mph but who cares?

everyone seems to be saying the same things, some of us say that the plane will move forawd as long as it isnt tied down. some seem to think that the wheels are what make a plane move forward. If the plane moves forward in realation to the earth/air it will fly!

 

[5-90]

there is problems with your reasoning. If they get the plane to do 1 hundred miles an hour and the tread mill to go 1 hundred miles an hour the plane still moves foward at 100 MPH and doesnt stand still! the wheels will be doing 200 mph but who cares?

everyone seems to be saying the same things, some of us say that the plane will move forawd as long as it isnt tied down. some seem to think that the wheels are what make a plane move forward. If the plane moves forward in realation to the earth/air it will fly!

The wheels don't make the plane move - they're not powered, like on cars. It's thrust from the engines that make the plane move forward - or thrust from the propeller (or fan, for turbofan aircraft.) That's why seaplanes can also move forward - they don't usually have wheels.

 

[JNickel101]

i give up...science isnt my thing...lol....well, okay, science i cant SEE - i cant just read about it and imagine it. my mind doesnt work like that. anyway...

i'll stick to blowing stuff up...its more fun anyway.

 

[5-90]

i give up...science isnt my thing...lol....well, okay, science i cant SEE - i cant just read about it and imagine it. my mind doesnt work like that. anyway...

i'll stick to blowing stuff up...its more fun anyway.

No argument here! "There are few personal problems that cannot be solved by a suitable application of high explosives..."

 

[in2fords]

The wheels don't make the plane move - they're not powered, like on cars. It's thrust from the engines that make the plane move forward - or thrust from the propeller (or fan, for turbofan aircraft.) That's why seaplanes can also move forward - they don't usually have wheels.

Thats what I was saying, by plane moving forward I mean realitive to a stationary object like a building or tree or whatever. so if the plane is moving forward at 100mph but the wheels are moving backwards at 5 billion miles an hour the plane is still moving foward 100mph, in relationship to the stationary item.
Jeremy