Vivian Meazza wrote:
> Thanks for all that: all looks good - the documentation has got a
> bit astern of station. Could you explain a bit more about the
> "turbo" attribute when used for a supercharger?
Actually, the existing turbo-mul implementation is *more* like a
supercharger than a real turbo. YASim models the boost as a simply
multiplication factor on the input manifold pressure. If it's set to
2.0, then the engine sees twice the static pressure, etc... Real
turbochargers don't have linear boost-vs-RPM curves, and tend to lag
(in time) engine power by a little bit. A gear-driven supercharger is
going to be closer to that ideal.
> MOI = # of Blades * (8.2*(10^-5))*(D^5), slug-ft^2
> then converted to kg-m^2 and finally square root to kg-m for torque
> value
Is "D" diameter? That looks like the right relationship (linear
dimension to the fifth power) for a moment as a function of size, but
I'd be *really* suspicious of using that equation for anything else.
The .000082 constant is pure fabrication, and will change depending on
the shape and density (wood? aluminum? composite?) of any given
propeller. A Lockheed Constellation and a Piper Cub sure as hell
aren't going to have the same constant. :)
Here's (IMHO) a better framework: Think of a propeller blade as a
stick, with a constant density along its length. That's not quite
right, but for most propellers the "non-stickness" is concentrated in
the thick middle, which makes very little contribution to the moment
of inertia.
So the MOI is the integral along the blade length (from zero to "R" --
the propeller radius) of rho*r*dr, where rho (the density) is just
propeller-[M]ass / ([N]umber-of-blades * R). So we do the integral
for each blade and multiply by N:
R M
N * INTEGRAL ------- * r * dr
0 N * R
M, N and R come out as constants (and the N drops out entirely), so we
have just a trivial:
M R
--- * INTEGRAL r * dr
R 0
Which of course is just (M/R) * (R^2/2) == M*R/2
So multiply your propeller mass (which you might have to guess at) by
its radius and divide by two. Much simpler, and no magic constants
needed. And you can do it in native units, without looking up what a
slug is. :)
Andy
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