Andy Ross wrote > > 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 >
Like the math - D was diameter btw. How about embedding it in YASim? I have an accurate figure for the mass of the propeller. Slug: Unit of mass that is equal to the mass that takes 1 lbf to accelerate at 1 ft/s2 - that's the easy bit. Now on with the model until the next question. Thanks Vivian Meazza _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
