Principles of Flight

[David Webb]

To all aspiring ATPL PoF candidates:

Bernoulii told us that the sum of the energies in a gas is constant.
The Equation of Continuity tells us that air mass flow is a constant. Therefore in a tube, if the cross-sectional area decreases, the velocity will increase.

Therefore, ignoring compressibility, gas heat energy and gas potential energy, if velocity increases then pressure decreases.

Consider a windsock. It is convergent with respect to the airflow.
If this is so, then velocity will increase and pressure will decrease.

Question. Why therefore does the windsock not collapse.....?

David

[Nearly Man]

I have no idea

[Seriph]

A windsock is not a venturi. It is inflated by ram effect.

[gijoe]

ditto Seriph.

The pressure on the outside of the sock is the same as that on the inside - therefore no collapse.

It is the literal force of a block of air smacking into the limp windsock that causes it to fly from its limp position toward the horizantal and, thus, allowing the block of air to escape from the other end- another block of air follows etc and, if at the same speed and force, keeps it at the same angle.

G:cool:

[timzsta]

Because for the purposes of PoF exams, the licensing authorities say the windsock will not collapse, so therefore that is the correct answer!

[Tinker]

The exterior profile of the windsock is a reversed venturi profile (don't know a proper name for it). So the forces on it are in equilibrium. It's got to be a wing.

Any chance of the MLS impression :D


edited to say damn gijoe got there first:(

[Gin Slinger]

The pressure on the outside of the sock is the same as that on the inside - therefore no collapse.

I would suggest the pressure at the 'exhaust' end of the sock is actually marginally higher than ambient.

[Spit15]

It's all a question of energies.

Bernoullies equation : (ignoring compressibility effects - as at normal windspeeds there is negligible compressibility) - states that energy in =energy out (ignoring heat due to friction).

The kinetic energy of the air flowing through the wind sock is transformed in part to potential energy (in raising the weight of the sock through a proportional height); and pressure energy brought about by the ram effect. The sock positions are infinitely variable as is the restriction to the flow of air through the core (within the limits of completely open and completely collapsed). Therefore the ram effect is proportional to wind speed (as we all know) but also proportional to the aperture of the restriction. An infinitely large aperture would cause no restriction to flow resulting in little or no ram effect, thus preventing a pressure differential from existing across the wall of the sock. The smaller the apperture then the greater the restriction to flow of air and hence the greater the ram affect for a given free stream air velocity. The ram effect being created by the sudden slowing down of the airflow due to restriction and hence (here's where bernoulli comes in) for the total energy to remain constant the remaining two forms of energy must increase. Hence the sock inflates and is lifted through a height.

:)

[MorningGlory]

Well done spit15! Excellent explanation. There you go Dave, I've been scratching my head about that one since groundschool last year.

Dave, what about the Canary one eh?? Come on, refresh me on your theory again...

[Ali Ronn]

SPIT15

You've missed your vocation. You should be doing the Christmas lectures

[Dan Winterland]

If you look at larger windsocks, you will notice that the end is gauze. This is to equalize the pressure and stop it collapsing.

[kissmysquirrel]

and I thought they were solid orange tapered tubes, positioned by those in the tower to cause confusion!!!!

[SkyCruiser]

its not RIDGED:D

[David Webb]

Thanks for all your response guys.

Some interesting theories but Spit 15 wins. By the way Dan Winterland, I always thought that the gauze was there to reduce fraying and prolong the life of the sock.

Still, some interesting theories. It is NOT a CQB question from the ATPL, (although if a certain ex-colleague now at FCL sees it, it may well be added!). It is simply one that was presented to me in my days at the RAF Central Flying School, and it's obvious that some of you ex-PPSC guys have heard me ask it before.

Well, after all, it helps to keep a little scientific thinking going on the site.....

Aye, David

[G-SPOTs Lost]

David

Still got that dodgy jag?

And thanks for getting me through POF....:)

[Dan Winterland]

My gauze theory was proffered by an expert as the official answer to a very boring question - at CFS!

My, how those long winter evenings at Scampton simply flew by!

[David Webb]

It always was our policy to tell fibs in order to see how long it took people to work it out! Scampton never was the real CFS.

That was Little Rissington - on the bump in the Cotswolds