tjander wrote:OK I have read a few topics on here about prop pitch and how it controls the RPM not the pitch, now with that said how is feathering a prop not changing the angel of the blades. It is my understanding that feathering a prop reduces the amount of air flow over the blade allowing it to pass through the air with little friction. So if I am wrong, please correct me. However, I have noticed in flying the C 208B at 5000 agl that I gain my most airspeed from a RPM setting at about 16oo RPM's and the MP set at near the red line. What I don't understand is how can a propeller turn slower and produce a larger air speed. Seems inverse to logic or am I completely off base here...
The FSX turbine modeling is a mess.. so it's not worth getting into.. and even if it were accurate; turbine engines have almost 3-dimensional power curves.. biased by not only RPM, but altitude, too. To a point, they get "better" with altitude, where a piston has a linear fall-off with altitude. So maybe, at a relatively low altitude (5000msl), a C208 gets peak IAS pushing the power turbine, while limiting the prop RPM (
they aren't mechanically linked in this turbine, unlike a piston engine, where prop RPM and engine RPM are the same)
In general (especially piston engines), you are correct in assuming that higher thrust at lower RPM is counter-intuitive. Light GA piston engines will always generate more HP, at higher RPM, and the constant-speed-prop "governs" RPM much like a pilot "governs" airspeed while climbing.. ala constanty adjusting wing AoA to maintain a specific airspeed (prop AoA to maintain a selected RPM). Alll these prop-blade pitch changes happen, without the pilot touching the prop-control.
Now... Feathering differs per the aircraft.. and it's well outside of normal operations.
For most turbine engines, it's a setting independent of what you'd use in flight.. as is a genuine, "Beta".
For most light GA piston aircraft, "feathered" is simply the theoretical, minimum RPM... normally used to minimize drag from a "dead" engine.. and must be set while there are still enough RPM (wind-milling), for the oil pressure needed.
Also, if it's not an engine failure, but a prop-mechanism failure, there are spring-loaded settings that will set the prop blades to either full-coarse, or full-fine. Full-coarse for most twins, for an effective feathering (
minimize engine-out yaw).. full-fine for most singles, so that you can still fly (albeit slowly, with the engine screaming).