hmmm

This is the place for posts that don't fit into any other category.

Moderator: RichardW

Post Reply
jgra1
(Donor 2021)
Posts: 4625
Joined: 27 Nov 2005, 19:07
Location: Kent / Susssex
My Cars: 2010 C5 X7 2.0 hdi 160 exc auto
MG TF 135
Boxer II 2.2 camper conversion
BMW R1200RT
BMW K1300 R
Honda V F R 800 5thG / MT500 Armstrong
x 39

hmmm

Post by jgra1 »

if you put an aeroplane on a free moving conveyor belt (which happened to be the length of a runway), and pushed the thrust to max, would the plane lift?

ok ok i will get my coat...

:)
User avatar
AndersDK
Posts: 6060
Joined: 21 Feb 2003, 04:56
Location: Denmark
My Cars:
x 1

Post by AndersDK »

phew ! :lol:
Anders (DK) - '90 BX16Image
bxbodger
Posts: 1455
Joined: 23 May 2003, 03:34
Location: Lovejoy country (Essex!!)
My Cars:
x 1

Post by bxbodger »

It depends how heavy the plane is and how fast the belt moves!

Planes have been assisted in launching from ships for years by various means- mainly catapults, plus of course gliders are just towed into the air, so as long as the belt moves fast enough to get airspeed around the wings then it'll lift.

What on earth prompted the question??!!!
jeremy
Posts: 3959
Joined: 20 Oct 2002, 16:00
Location: Hampshire, UK
My Cars:
x 2

Post by jeremy »

No is the answer - its the airspeed that provides the lift and has nothing to do with the groundspeed.

You can launch a plane successfully 5 miles in the sky by dropping it from another aircraft (USA Bell experimental program of the 50's comes to mind with a rocket plane dropped from a B29). As mentioned the catapult is sometimes used on ships - simply to assist acceleration to the point where there's enough airspeed for the things to fly.

It also used to be carrier practice to steer into the wind for takeoff which would have the effect of increasing the airflow over the carrier deck and hence the aircraft wings.

The matter is probably complicated by the ground effect which I think is compression of the air between the runway and the wings producing an additional upward force on the plane but this is due to the presence of the surface in close proximity and it would make no difference if it were moving or not (providing it were smooth)
jeremy
User avatar
Clogzz
Posts: 2115
Joined: 15 May 2005, 18:04
Location: Australia
My Cars:
x 36
Contact:

Post by Clogzz »

I’d say yes, because the thrust will push the plane through the air and cause lift-off.
The free-moving conveyor belt, if I’ve got it right, means ‘free-wheeling’, and will be driven along only by the rolling resistance of the loaded tyres and wheel bearings, actually reducing drag by the extent to which it moves along.
What’s on the ground only affects drag, unless you’re a seaplane, where drag is high.
The 737, and probably others, can take off from a ploughed field.
Large wheels easily roll over irregularities.
2002 C5 2.0i AL4 230,000 km 76372389
jgra1
(Donor 2021)
Posts: 4625
Joined: 27 Nov 2005, 19:07
Location: Kent / Susssex
My Cars: 2010 C5 X7 2.0 hdi 160 exc auto
MG TF 135
Boxer II 2.2 camper conversion
BMW R1200RT
BMW K1300 R
Honda V F R 800 5thG / MT500 Armstrong
x 39

Post by jgra1 »

hehe thanks chaps...

the original question should probably been

'Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?'

for more answer see

http://www.pistonheads.com/gassing/topi ... 41&h=0&p=1

and

http://www.runnersworld.co.uk/forum/for ... =1&v=6&sp=

you may need a large cup of something ;)

(I think the answer is yes, as Jeremy says)
jgra1
(Donor 2021)
Posts: 4625
Joined: 27 Nov 2005, 19:07
Location: Kent / Susssex
My Cars: 2010 C5 X7 2.0 hdi 160 exc auto
MG TF 135
Boxer II 2.2 camper conversion
BMW R1200RT
BMW K1300 R
Honda V F R 800 5thG / MT500 Armstrong
x 39

Post by jgra1 »

ok, so heres my thoughts..

the plane will take off, because it matters not at all what the groundwheels are doing, they could be going the 1000 x a second, the plane would still go forwards (using thrust from the AIR) and take off

;)
FrenchLeave
Posts: 359
Joined: 18 Jan 2005, 21:47
Location: 5 miles N. of Boston, Lincs
My Cars:

Post by FrenchLeave »

Reckon you're right.
Sl4yer
Posts: 849
Joined: 12 Apr 2003, 04:29
Location: United Kingdom
My Cars:
x 2

Post by Sl4yer »

If the belt is moving at the same speed as the wheels, but in the opposite direction, the position of the plane will not change. Therefore, the plane will not move relative to the air. No airspeed, no takeoff!

James
Image
Now Citroenless for the first time in 20 years
2008 Mazda RX-8 231
2007 Honda CR-V Auto
User avatar
AndersDK
Posts: 6060
Joined: 21 Feb 2003, 04:56
Location: Denmark
My Cars:
x 1

Post by AndersDK »

Sl4yer wrote:If the belt is moving at the same speed as the wheels, but in the opposite direction, the position of the plane will not change. Therefore, the plane will not move relative to the air. No airspeed, no takeoff!

James
Nope James 8)

The aeroplane does not achieve its airspeed by its wheels.
It does in fact use the propeller to move the air relative to the plane, and then achieve airspeed.
As long as the wheels are free to move, it will then take off. No matter if the freewheeling wheels will have to achieve double speed on an opposite moving ground.
It is possible for an aeroplane to take off not moving relative to ground - in high winds for instance. The propeller is then used to prevent flying backwards - provided the thrust is higher than the wind speed :lol:

I must point out that this discussion is HIGHLY off topic, but since christmas is coming up etc .... :wink:
Anders (DK) - '90 BX16Image
Sl4yer
Posts: 849
Joined: 12 Apr 2003, 04:29
Location: United Kingdom
My Cars:
x 2

Post by Sl4yer »

AndersDK wrote:
The aeroplane does not achieve its airspeed by its wheels.
It does in fact use the propeller to move the air relative to the plane, and then achieve airspeed.
As long as the wheels are free to move, it will then take off. No matter if the freewheeling wheels will have to achieve double speed on an opposite moving ground.
It is possible for an aeroplane to take off not moving relative to ground - in high winds for instance. The propeller is then used to prevent flying backwards - provided the thrust is higher than the wind speed :lol:
Nope Anders! [-X

The propeller mainly works as an aerofoil section (creating low pressure in front of the blade) to drive the plane forwards. There may also be some thrust directed backwards from the prop - this may be enough to affect the aerodynamic behaviour of the aircraft (for example, when going from full power to idle). The wash from the prop will not, on it's own, create enough airspeed for flight. In fact, many modern aircraft get a considerable amount of lift from the fuselage, not just the wings!

The crucial bit is in the detail:

'Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?'

If the conveyor belt exactly matches the speed of the wheels, but moving in the opposite direction, the plane cannot move relative to the surrounding air. Actually, is doesn't matter if the conveyor belt is as long as a runway, or as long as the wheelbase of the plane. The plane will not move relative to the surrounding ground, or air. Therefore no airspeed.

Think of it another way. 'Imagine a car is sitting on a massive conveyor belt, as wide and as long as a motorway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Will the car drive off the conveyor belt?' I suggest it won't! :)

James

PS Merry Christmas to all! Hope your plans are going better than mine....
Image
Now Citroenless for the first time in 20 years
2008 Mazda RX-8 231
2007 Honda CR-V Auto
vanny
Posts: 767
Joined: 16 May 2002, 21:08
Location: BXProject
My Cars:
x 1
Contact:

Post by vanny »

Sl4yer wrote: Think of it another way. 'Imagine a car is sitting on a massive conveyor belt, as wide and as long as a motorway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Will the car drive off the conveyor belt?' I suggest it won't! :)

But cars arent designed to fly, planes are!
Image
jgra1
(Donor 2021)
Posts: 4625
Joined: 27 Nov 2005, 19:07
Location: Kent / Susssex
My Cars: 2010 C5 X7 2.0 hdi 160 exc auto
MG TF 135
Boxer II 2.2 camper conversion
BMW R1200RT
BMW K1300 R
Honda V F R 800 5thG / MT500 Armstrong
x 39

Post by jgra1 »

agghh..

for the plane to be able to move forwards.. the wheels also need to..

and they cant, because the conyeyor it makeing them go backwards..
that's my answer rubbished ;)

John
James.UK
Posts: 1169
Joined: 14 Dec 2003, 23:12
Location: United Kingdom
My Cars:
x 2

Post by James.UK »

I think Anders is correct. :D

Wether it be a prop craft or a jet, it is not propelled via its wheels. :) Therefore once the power is applied by its engine, provided the wheels are free to rotate then they dont effect anything and the craft will move in the direction of the applied power i.e. forward, untill it reaches lift off speed.. :D

Or in the case of a Xantia bi plane, it goes into low mode thus converting the lift into downward thrust and buries itself!! :lol:
.
James. (Nr M67 East of Manchester).
Dark Blue ZX 1.9D Auto 1994 'L' 5 dr (modified) Aura. 98K miles used daily. Ave mpg 40
Wedgewood Blue 75 CTD auto Connoissaur. 2002. 144k. used daily. ave mpg 40 ish.
jgra1
(Donor 2021)
Posts: 4625
Joined: 27 Nov 2005, 19:07
Location: Kent / Susssex
My Cars: 2010 C5 X7 2.0 hdi 160 exc auto
MG TF 135
Boxer II 2.2 camper conversion
BMW R1200RT
BMW K1300 R
Honda V F R 800 5thG / MT500 Armstrong
x 39

Post by jgra1 »

yeah thats what i thought.. but the wheels are free to rotate, the conveyor wants them backwards then thrust wants them forwards..
they cant do both..

;)
Post Reply