MD500 autorotation question
 

Hello all,

I have a question regarding practice autorotations in an MD500D. While established in an autorotation with the throttle set to ground idle and with the rotor rpm in the green autorotative range. My question is should the N2 be at ground idle with the Nr in the green range (ie needles have split), or should the N2 be aligned with the Nr up in the autorotative range?

cheers

We have some very experienced MD-500 Pilots, Instructors, and Engineers (aka Mechanics) that attend here that can provide you excellent answers about such matters.

Hopefully, they shall be along in time and offer their input.

Been a long time, but I don't think it matters so much in the situation you describe. The needles should definitely split upon engine shutdown and as such needs to be checked every time.

If you leave them super imposed then you run the risk of busting the N2 limit with the NR near the upper limit power off. Needles split is surely the power off auto situation.

We need Hughes500 Hughes 500's input here.

The N2 will not be in the ground idle range because idle is set by NG (62-64% or thereabouts depending on the specific machine). 62-64% NG will give different N2s depending on if the drivetrain is being driven (ground idle) or if it is in autorotation. I've usually only seen a bit of a N2/NR split when autoing machines with C20 series engines in them.

Mike

Interesting, thanks for that!

With the throttle at ground idle the N2 needle will be split from the Nr needle. It tends to vary from machine to machine but it is normally around 85 to 90% N2.Tq gauge will be around 12 psi with TOT around 500 degrees.

Many pilots of piston-engine helicopters who are transitioning to a turbine become confused about how turbines operate. They expect to see a needle-split when snapping the "throttle" (really the fuel control lever) from Full to Idle and become concerned when they don't get one. But, depending on the type of aircraft and engine powering it, you do not always get a definite needle-split in a turbine. Helicopters powered by the RR-250 series don't always see a needle-split. At the Idle position, there is nothing controlling N2 rpm. The PT governor is out of the picture, and the N2 will be..."wherever." The N2 turbine is only "gas coupled" to the N1. Without a load on it, the N2 can be quite high even with the "throttle" at Idle. In a practice auto, if you're light and your NR auto-rpm is low at flat pitch, you might not see much of a split with the "throttle" at Idle. (Obviously, the N2 rpm can never be above the NR.) On the other hand, if you're heavy and the NR is high, then yeah, you might see more of a split. This behavior is puzzling for the new turbine pilot. Just remember that the throttle only controls the N1, not the N2.

Unlike piston engine ships, we typically don't do a check of the freewheeling unit operation with the engine running. We do that prior to starting the engine (by moving the main rotor blade), and we also do that when we shut down, where we would expect to see an N2/NR split.

Excellent explanation sir, thanks a lot.

just ensure the split is the correct split, have seen the N2 higher than the Nr when the throttle was at idle, was engaged in book work during the Huey cool down and only woke up to the situation when I noticed I could count the blades passing by. Always wondered what might have been the result with a sudden clutch engagement.

Probably would've been exciting, even on the ground!

It's interesting (and lucky!) that the FWU stayed engaged during the flight but the sprags did not re-engage after rolling to Idle once you landed.