Collective Bias asked:
I wonder if you could quickly run through the OEI Bias system and the what is shown on the IIDS regarding hashed and un-hashed boxes and what it all means.
Also, define transient and static droop as it applies to the 76.
Finally, A few years ago I read an article which had a 76 on fixed skids and some other mod's courtesy of the russians. Has this progressed at all?
Nick answered:
The OEI bias on the C lets the pilot need to recall the N1 limit as a constant, regardless of the fact that the N1 limit varies with ambient conditions. On other Turbomeca Ariel equipped helos, the pilot must look up the limit on a chart while flying out after an engine failure (yea, right!) We Sikorsky pilots asked for a simple calculator to fudge the indicator to always read a constant number, so only one number had to be remembered by the pilot, and no chart had to be consulted.
If the bias computer fails, the indicator tells you that the chart must be consulted. I cannot recall which way is hatched.
For all engines, the reduction in Nr/N2/Np with power application is called droop. If the engine takes a bit of time to come to power, and the Nr droops but then recovers, we call that transient droop. For many engines with hydro-mechanical or hydro-pneumatic controls (not FADEC's), the long term Nr may not be a constant, as the combination of N2 setting thru the beeper, the collective bias and the ambient conditions might all work out to a number somewhat off from the nominal setting. Most engines must reduce rpm as power is applied for engine control stability, so the natural engine speed trend with increased power is a down droop, usually about 1/2 to 1% for each 10% power. This is cancelled by the collective bias, or by a computer governor, in most cases. The permanent droop with power is called static droop. FADECs make all this go away, as they allow very sophisticated control techniques to keep the output rpm very constant.
S-76 fixed skids were considered a few years back, but I don't think it went too far, as the market seemed to like where we were with retractable. The weight of the retract mechanism is about 90 pounds, and the fuel saved by the drag reduction actually saves about 100 pounds per full fuel tank!
[ 29 July 2001: Message edited by: Nick Lappos ]