propeller slipstream

[baobab72]

hello folks

i might saying a bunch of non sense here but can anybody shed some light on the physics behind propeller slipstream, what is it caused by?
cos i was wondering why considering that the air in front of the propeller disk being accelerated should have a lower pressure than the air behind it being slower and thus having a higher pressure, so normally, and absurdly the air should flow from low to high therefore from the rear to the front of the propeller rather than the opposite way!!!

i know i am wrong, facts do prove me wrong, just wondering what i am mistaken about, maybe the whole mechanics how a propeller work!!

Regards

baobab72

[Runaway Gun]

Just think of the aerofoil of the blade, being exactly the same as the wing.

The Low pressure on top sucks the aerofoil up, and the higher pressure underneath pushes it up (or forwards as in a propellor).

Probably not correct if you are an aerodynamics specialist, but I hope it works for you.

[Dan the weegie]

Try not to confuse yourself too much with pressure gradients.
Start with the basic principal that if a wing deflects air downwards then the opposite reaction is that the wing gets pushed up.

A propeller blade is a small wing, it deflects air backwards by its motion through the air and as such the opposite reaction is that the propeller will get pushed forwards.

The pressure differential is a very localised to the immediate area surrounding the blade and relates specifically to how the aerofoil redirects the air.

The redirection of the air is the bit that generates lift, the pressure differential is the bit that generates the redirection. It's all part of the same process but don't allow yourself to get too confused :).

Someone will be along shortly to tell me I'm talking total nonesense but unless you're studying aero engineering, fluid dynamics or some other branch of physics and engineering that relates to lift and the generation of it :) you can probably work with it ;).

[foxmoth]

Basically differential is there because the prop is driving the air to make it that way, the prop is CAUSING the high pressure behind it and low ahead which is what drives the aircraft forward and the continued rotation means it stays that way -the differential is there because the prop is MAKING the air flow that way, as soon as the prop stops of course you lose that pressure differential and the airflow and pressure difference stops.
Another way of putting it is that the differential is there because of the airflow, rather than the airflow being there because of the pressure differential, also, it is a dynamic situation, as soon as it becomes static not dynamic, all balances out.

[Heston]

Think about where tip vortices come from - the original question is a good one to ask...

H

[Pull what]

should flow from low to high therefore from the rear to the front of the propeller rather than the opposite way!!!

It does try to at the tips as indeed does any airfoil.. You can see the tip votices on a damp day from quite a few aircraft, the Viscount in particular, a sadly missed aircraft (The Whispering Slum)