Which of the following statements are wrong? / General Chatting / Forums

Forums

4hv.org :: Forums :: General Chatting

« Previous topic | Next topic »

Which of the following statements are wrong?

previous 1 2 3 next

Move Thread

LAN_403

Steve Conner

Mon Jul 24 2006, 09:20AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Here's another interesting puzzler: What happened to "EastVoltResearc"'s "h"?

ragnar

Mon Jul 24 2006, 10:13AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

It's off bonking the E, R, N, A, G and the E.

ShawnLG

Tue Jul 25 2006, 03:20AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

I say B is wrong

JimmyH

Tue Jul 25 2006, 06:00PM

Registered Member #358 Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28

The wing deflects the air downwards. You can make a glider with a styrofoam plate and a penny and the wings will have the exact same size top and bottom and it will still fly.

Beleive it or not Tom, that is not the primary lift producer, even on flat airfoils. Flat airfoils still produce pressure gradients in linear proprtion to their angle of attack

I thought you had a pretty good explanation earlier, but have to disagree with you here. F = MA, end of story. For the airplane to be held up by some force, then it must accelerate some air downwards.

Of course there will be pressure differential across the wing, it couldn't fly without it. The pressure differential is the force that holds the plane/glider up.

You can't have a pressure differential without accelerating some air, and you cant accelerate some air without having a presure differential.

A good analogy would be pushing on a cart. One person argues the guy on rollerblades moved back because he pushed on a (frictionless) cart, and the other person argues it's because he accelerated the cart. Who's right?

You can't push on a cart without moving it, and you cant move a cart without pushing on it.

It's just different ways of looking at the same thing.

Oh, and A and B aren't right either.. what they said..

Ben

Tue Jul 25 2006, 07:55PM

Vigilatny
Registered Member #17 Joined: Thu Feb 02 2006, 02:47PM
Location: NL
Posts: 158

JimmyH wrote ...

The wing deflects the air downwards. You can make a glider with a styrofoam plate and a penny and the wings will have the exact same size top and bottom and it will still fly.

Beleive it or not Tom, that is not the primary lift producer, even on flat airfoils. Flat airfoils still produce pressure gradients in linear proprtion to their angle of attack

That's fine, but the lift is not primarily produced by the wing deflecting air downwards(from the bottom of the wing). The wing creates a pressure differential, which then accelerates the air.

If it was as is implied you would have L/D ratios <1 (i.e. very poor). Another way to think about it is, when a wing fails, the top rips off, the bottom doesn't get blown in.(The usual(aerodynamically induced) failure mode is for the wing to twist off, however. This is why the Kutta condition is a good way to think about it.)

Madgyver

Tue Jul 25 2006, 07:56PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

I am in the batch how says everything is wrong, or at least not truely correct.

Water isn't blue because of scattering I think, but beacuse of absorbtion. Maybe I am mixing up things.

About the wing: I don't know exactly, why or how. But (as I understood it) our professors say that the bernouli effect indeed provides 'some' lift, but it is not the main cause. I'm into mechatronics anyway, not aeronautics...

Simon

Wed Jul 26 2006, 01:38AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Madgyver wrote ...

Water isn't blue because of scattering I think, but beacuse of absorbtion. Maybe I am mixing up things.

Something always transmits, absorbs or reflects light in some combination. Saying water is blue because it absorbs everything but blue is kind of equivalent to saying it is blue because it only scatters blue light.

Edit: I have a feeling I'm just confusing things more with this post.

Bjørn

Wed Jul 26 2006, 05:29AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

A. Oceans, lakes, and bodies of water are blue because they reflect the sky.

This correct in the sense the appearance of large bodies of water is mainly dominated by whatever is reflected in it, the ratio is dependant on the angles between the observer and the lightsources and the smoothness of the surface.

B. Electrons flow through wire at nearly the speed of light.

The speed of electrons is a fractal function. The smaller scale you observe at the faster is the speed. So you can get more or less the speed you want by changing the definitions of wire. If we measure at a scale of meters then the speed is very slow.

C. Aircraft fly because the upper surface of the wing is longer than the lower surface.

No the classic airfoil shape has that shape as much because it is easy to analyse than because of it's lift generating capablilities. If you ask 10 people that design wings you will get 11 different answers to why it generates lift.

JimmyH

Wed Jul 26 2006, 06:19PM

Registered Member #358 Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28

That's fine, but the lift is not primarily produced by the wing deflecting air downwards(from the bottom of the wing). The wing creates a pressure differential, which then accelerates the air.

Agreed. I don't think I ever implied that it 'deflects' air when each individual partical bounces off the bottom of the wing or anything.

A good way to see this is to move your hand underwater and watch it 'suck' the water over the top surface.

If it was as is implied you would have L/D ratios <1 (i.e. very poor). Another way to think about it is, when a wing fails, the top rips off, the bottom doesn't get blown in.(The usual(aerodynamically induced) failure mode is for the wing to twist off, however. This is why the Kutta condition is a good way to think about it.)

Yeah, when the wing stalls, you kinda lose the top of the wing stuff, but the L/D suffers more due to the vastly increased D than the decreased L. I don't like the structural failure argument so much, since wings can be made to fail wherever you'd want without changing physics. The only failure mode I've seen is the wings snapping off at the body anyway.

If I had to give a better explanation I'd start by describing how it basically has to turn the streamlines that touch the wing, and since the mean distance between collisions is small, the streamlines have to be pretty much parallel to eachother.

Bored Chemist

Wed Jul 26 2006, 08:21PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Stunt planes can be flown upside down more or less indefinitley (at least untill the pilot gets bored/ sick).
The aerofoil shape of the wings doesn't need to change to do this. (Of course, the flaps etc are probably set rather differently compared to normal flight.)

previous 1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob