Who's right?

squiffy · 20 Jan 2007 at 10:14

What about the thrust from the prop wash? Won't this give enough lift/over the control surfaces? :confused:

gurusan · 20 Jan 2007 at 10:16

squiffy said:
What about the thrust from the prop wash? Won't this give enough lift/over the control surfaces?

? In theory it could I suppose if the engines were placed strategically.

Dave M · 20 Jan 2007 at 10:36

Hypothetical Question

A plane is standing on a runway that can move like a giant conveyor belt. The plane applys full forward power and attempts to take off. This conveyer has a control system that tracks the plane's wheel speed and tunes the speed of the conveyer to be exactly the same but in the opposite direction, similar to a treadmill.

The question is:

Will the plane take off or not?

No, it has no airspeed.

The belt is moving the plane backwards at the same speed the plane's thrust is trying to move it forward - hence no air speed.

singist · 20 Jan 2007 at 10:36

Any popcorn left there Kaiowas? ........... or a sandwich .... maybe?

When this plane gets airbourn, will the light from it's forward facing lights be travelling faster than the speed of the fly in the cockpit that is trying to eat the Nigerian pilot's half sandwich?

xsnv · 20 Jan 2007 at 10:37

Lol...it's the whole "treadmill/freewheel/analogy with cars" thats confusing most people.

The function of the threadmill is to model a frictionless surface. Any rotation in the wheels is immediately counteracted by the treadmill. Thus the wheel cannot rotate relative to the treadmill (as it rotates with an equal but opposite velocity).

All that means is there is no friction between the wheels and the road.

So...to simplify things forget about the thread mill. Imagine the plane was on a very very very slippery sheet of ice. Also imagine that we did have a motor that could actually rotate the wheels of the plane (like a car)

Now we turn on the motor (to rotate the wheels). The plane doesn't move forward becasue i't just slips (much like a car in mud). We're stuck.

Now imagine superman came along and blew an almighty gust of wind at the plan from behind. Would the plane move?

Make it easier....imagein you had a very light toy car on very slippery ice(no engine in the toy car so it can't genrate it's own motion) If you blew it would it move forward?

EDIT:

For the drivers amongst you. Have you ever had you wheels stuck in mud? No matter how hard you rev the engine the car doesn't move - because the wheels can't get any traction on the mud making the surface frictionless

If you and you mates got out and pushed it would it still remain there because the mud dosn't offer the wheels any traction or would it just slide past the mud?

gurusan · 20 Jan 2007 at 10:37

Telescopi said:
No, it has no airspeed.

The belt is moving the plane backwards at the same speed the plane's thrust is trying to move it forward - hence no air speed.

*sigh* you should have read some of the thread before posting that.

gurusan · 20 Jan 2007 at 10:39

xsnv @ overclockers.co.uk said:
Lol...it's the whole "treadmill/freewheel/analogy with cars" thats confusing most people.

The function of the threadmill is to model a frictionless surface. Any rotation in the wheels is immediately counteracted by the treadmill. Thus the wheel cannot rotate relative to the treadmill (as it rotates with an equal but opposite velocity).

All that means is there is no friction between the wheels and the road.

So...to simplify things forget about the thread mill. Imagine the plane was on a very very very slippery sheet of ice. Also imagine that we did have a motor that could actually rotate the wheels of the plane (like a car)

Now we turn on the motor (to rotate the wheels). The plane doesn't move forward becasue i't just slips (much like a car in mud). We're stuck.

Now imagine superman came along and blew an almighty gust of wind at the plan from behind. Would the plane move?

Make it easier....imagein you had a very light toy car on very slippery ice(no engine in the toy car so it can't genrate it's own motion) If you blew it would it move forward?

excellent analogy. Hope you don't mind if I borrow it to help prove the point.

divine` · 20 Jan 2007 at 10:40

Telescopi said:
The belt is moving the plane backwards at the same speed the plane's thrust is trying to move it forward

No it isn't. It's spinning the wheels backwards.

It's then a case of whether that will cause the plane move backwards. Which it won't.

tim_enchanter · 20 Jan 2007 at 12:35

The trouble is not the treadmill or the plane or the amount of friction, thats all fine it's the people with very little understanding of physics and the way forces interact that are the problem

blighter · 20 Jan 2007 at 12:39

lol @ tim for staying up nearly all night!

just to say this thread has made my brain melt several times over but i know fully understand why the plane will take off :cool:

blighter · 22 Jan 2007 at 00:01

what happened to this thread?

it disappeared for a few days :eek:

tim_enchanter · 22 Jan 2007 at 00:04

My God thats one late running plane, it's still on the runway

The thread has been cleaned of it's unpure link

Rich_L · 22 Jan 2007 at 00:07

How did this thread stay open after Scuzi's post at the very start?

Scam · 22 Jan 2007 at 00:15

Poor Scuzi arguing it till the death

I'll stick with my shopping trolley analogy!

tim_enchanter · 22 Jan 2007 at 00:16

Scuzi said:
Yes it will take off. Thrust is not applied through the wheels. The engines provide thrust against the airmass and the plane will accelerate relative to the airmass. When the airspeed over the wing is sufficient, regardless of what speed the wheels are travelling at, enough lift will be produced to lift the aircraft from the ground.

The wheels just freewheel, there's no drive through them. I can't believe how many people argue this case, it's plainly obvious that it will take off.

Any argument otherwise is wrong

Yep he was of course correct, the thing is people still can't believe it

blighter · 22 Jan 2007 at 00:17

tim_enchanter said:
Yep he was of course correct, the thing is people still can't believe it

they must be dog owners tbh

Scuzi · 22 Jan 2007 at 00:26

Christ, I thought I'd seen the last of this! No such luck

fornowagain · 22 Jan 2007 at 00:26

blighter said:
what happened to this thread? it disappeared for a few days

It definitely disappeared, I couldn't think why it got deleted.

tim_enchanter · 22 Jan 2007 at 18:11

Scuzi said:
Christ, I thought I'd seen the last of this! No such luck

lol Just when you thought you'd seen the last of it, there it is again

fornowagain said:
It definitely disappeared, I couldn't think why it got deleted.

Because the OP was a naughty boy and linked to a page with lots of swearing, his post was sanitised and the thread reinstated

fornowagain · 22 Jan 2007 at 19:28

tim_enchanter said:
lol Just when you thought you'd seen the last of it, there it is again

Because the OP was a naughty boy and linked to a page with lots of swearing, his post was sanitised and the thread reinstated

I meant why delete the thread? Why not edit his link? Just wondered if there was a technical reason why it vanished, not even the usual Dons place holder saying why. I was curious if there's like a non public store for "to be see to" threads or something.