From: "Curtis L. Olson" <[EMAIL PROTECTED]> > By my observation, the vacuum gauge should read just a little over 4-inhg > at engine idle (600-700 rpm) and approach 5-inhg at about 2200 rpm. > The previous equation returned a value that is far too low at idle and > ramped up too quickly after that (and had a "hard" clamp at 5.)
The value is allowed to droop to 3 and is supposed to be adjusted that way to optimize wear on the vacuum pump. Higher values simply cause the pump to wear out more quickly. Similarly, the definition of idle RPM changes dramatically between aircraft and throttle adjustment. > This means that previously, at idle the pilot received a very > abnormally low vacuum reading. This also affects the attitude > indicator because there isn't enough vacuum to properly spin the gyro > at idle. Indeed, if you manage to idle the engine at 500 RPM or so, which is not recommended for other reasons, the vacuum is indeed too low and the instruments will spin down. That's what is supposed to happen. I chose numbers that reflect an older aircraft, rather than as-new, so that an extended slow descent with the throttle completely retarded will have the instruments spin down. That's what happens to me in flight. > I'd be happy to add in air pressure effects if someone could give me > an indication of what that relationship might be. Read the existing code I created. It is a physics based model. Until the regulator kicks in, the system is mostly linear and operates between the implicit vacuum inside the pump before each stroke and the external air pressure. Therefore, the calculation of the number of inches of vacuum generated is inherently proportional to the external air pressure. The model has that proportionality built into it. That one unknown parameter, the funny number, indicates the ratio between the implied leak in all the instruments and the volume of the vacuum pump. Try adjusting that to meet your needs since it changes as you add instruments and/or change vac pump. The other item with a hard cutoff at 5 is the pressure regulator and has nothing to do with the vacuum pump. The pump model itself should never limit itself to 5 inches because the regulator is essentially a non-linear leak that opens up rapidly when 5 is reached. If the non-linear leak is adjusted around 4, the instruments spin up very slowly. If adjusted at 6 or above, the gyros will overspeed and wear out their bearings faster than scheduled maintenance expects. Hope that helps ... _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
