Simviation Forums

...about aerodynamics.  Today I was pondering about them again, something I often find myself doing.   And I got myself all confused about helicopters.

Okay, so I know that bernoullis principle basically states that the low pressure created over a wing pulls up the wing, which is lift.  But I am also aware that the pitch of a wing affects lift because as air flows under and over the wing it is directed in a downwards direction if th wing is pitched up and in an upwards direction if it is pitched down.

So I was thinking about helicopters, and I just cant figure it out.  As we know, a helicopter blade is basically a spinning wing.  The blades are shaped like an airfoil and as the spin air flows over the blades and that generates lift.  But a helicopter controls its vertical speed by changing the pitch of the blades, which suggests that the blades generate lift by forcing air down, explaining the rotorwash.

And now im getting MORE confused.  Does a prop force air in an aft direction pusing the plane, or dies it generate bernoulli lift pulling the plane.

I AM SO CONFUSED!

[quote]And now im getting MORE confused.

Oh Boy..

Best thing to do is forget about all that nonsense and go flying.

Throttle forward, stick back.  If you fly, it's aerodynamics, if you don't, it's gravity.


That's all you need to know.

Throttle forward, stick back.  If you fly, it's aerodynamics, if you don't, it's gravity.


That's all you need to know.

(Grunts, nods, scratches belly)

I think when you increase the pitch of the blades, the air underneath is higher pressure, causing it to be pushed up as well as sucked up from the low pressure over the blade.

it's both

BTW, the rotor wash will be the lower pressure than the surrounding air.  Next time you fly an airplane, leave the door open during taxi and see what it doesn when you change the RPM (speed of air passing).  You'll notice that it will actually pull the door open a bit.  This is because the faster moving air (just like the faster moving air on top of the wing) creates a slight drop in pressure.  

Another thing to remember when thinking about all of this is that everything flows from high pressure to low pressure.  The are above the rotor is high(er) and the area below is low(er).  

AS for whether the prop is pushing/pulling the plane.  It would be exactally the same as your wing.  All a prop (or rotor) is is a wing that spins.  I have grown up being instructed that the key function is the lower pressure on top of the wing, not particularly the high pressure below.  (Although it certainly does contribute!)


One simple experiment can illustrate the principles, and that is finding a piece of paper, place it directly under your lips, and blow air OVER the paper.  If you use a small enough paper (or blow hard enough) you'll see the paper rise.  This shows how strong that low pressure drop caused by the FASTER moving air is.

All in all, fast air = low pressure.  It applies to weather as well.

Prop blades and wings aren't quite the same thing... when it comes to pressure differential.

Given enough airspeed.. a wing with zero AoA can keep a plane aloft. A prop uses AoA (pitch) to create thrust.

Constant-speed props capable absolute zero pitch (I think it's called beta)(90 gegrees from fully feathered) create almost no thrust. What little they do create is more than cancelled by the drag that a full "flattened" blade causes.

My bet for a prop is that Newton (every action...) is more right on than Bernoulli when it comes to props.

As the prop spins it displaces air imparts a force on it and pushes it back the result is the air imparts a force on the prop pushing it forward.  The fact that air effected by the prop has less mass means that the prop has to displace a great deal of air to get the required thrust.

Skwang

the prop on an air plane (and jets on jet planes), pull (or push, whatever u prefer) the plane through the air. this causes air to flow over the airfoil and creating lift, yada yada yada.... The props only purpose is to move air over the wing.... THE PROPS CREATE [glow=yellow,2,300]NO[/glow] LIFT

The props only purpose is to move air over the wing.... THE PROPS CREATE [glow=yellow,2,300]NO[/glow] LIFT

Sorry, but that is terribly incorrect.  The props do create lift, but not vertical lift, only horizontal lift, which we usually refer to as thrust.  This propels the airplane forward through (relatively) stationary air, which creates the vertical component of lift.

The props only purpose is to move air over the wing.... THE PROPS CREATE [glow=yellow,2,300]NO[/glow] LIFT

Sorry, but that is terribly incorrect.  The props do create lift, but not vertical lift, only horizontal lift, which we usually refer to as thrust.  This propels the airplane forward through (relatively) stationary air, which creates the vertical component of lift.

I wonder if a moving prop could split a hair...  

But it is worth noting, yes... a propellor blade bears an uncanny resemblance to a small wing... because it is an airfoil, and produces thrust by much the same means as a wing produces lift.