Originally Posted by
BIRDIE
can we find something less exciting to talk about?? i'm blinking flashing warning lights over here... i'm about to erupt
sorry we're boring you
Originally Posted by
shdw
Will redo this later I need to do some research on CLmax. You are wrong about indicated airspeed though, the formula is regarding actual not indicated airspeed. That is why the pitot tube is hooked to the static system.
I'm not sure what formula you're talking about. Some engineering textbooks and other aerodynamic books state a little about fluid dynamics (this is after all what we're talking about, when we talk about a "stall" and "lift"). A stall is when a fluid (in this case air and assuming a prior force 'lift' was created) in the boundary layer does not have enough energy to overcome the adverse pressure gradient (or reverse flow). Fluid forces depend on these variables:
(a) mass density
(b) a characteristic area
(c) velocity squared
(d) Reynolds Number (Re) (viscosity)
(e) Mach No (M) (compressibility)
When engineers are windtunnel testing airfoils, they pay special attention to Reynolds numbers because when they vary, they will give different values in CLmax and thus critical AoA. Mach number is not as important to GA aircraft (pertaining to stalls). Reynolds is generally for lowspeed, Mach is for higher.
Reynolds is basically the ratio of inertia to viscosity, especially seen in the boundary layer. Basically it describes part of how much energy is in the boundary layer (and x-amount of energy is needed to overcome the adverse pressure gradient).
Reynolds has a 2D and 3D effect on a stall:
The 2D effect is that a larger Re value means a greater CLmax value, and a higher AoA where it occurs -
because it changes the shape of boundary layer (less or more molecules 'stick' to wing).
The 3D effect is not really relevant to what we're talking about and pertains more towards high-aspect ratio design (depends on planform).
Absolute viscosity is a function of temperature and independent (practically speaking) of pressure.
Indicated airspeed measures stagnation pressure (dynamic / static pressure). Pressure can be constant, but if CLmax changes, the required pressure changes as well resulting in a different
indicated stall speed.
To break it down simple:
Lower Reynolds = Lower CLmax value = Lower critical Angle of Attack = Higher required pressure = higher indicated stall speed
Most of you may think this is all just stupid - making things too technical blah blah blah
...But stuff like this explains how vortex generators and some other high-lift devices work. As VGs provide turbulent (high energy) flow to the boundary layer delaying seperation, etc.