Autos and retreating blade stall

[Xnr]

Fresh back from Flight Safety and was amazed how easily the S76A sim went into retreating blade stall when trying to enter autorotation from cruise 135 kts.

Can anyone tell me at what decreasing Nr this could happen if you allow the Nr to drop while maintaining you forword airspeed.

Our parameters were 4000', standard day and cruise speed of 135 kts.

[vorticey]

doesnt reducing collective stop retreating blade stall? maybe just a glitch in the sim?

[the wizard of auz]

I would have thought the same as Vorticey, but after thinking about it, would think that after reducing the pitch angle as far as you can (bottoming collective), but still decending, your angle of attack is increasing to the point that you cannot prevent a blade stall without decreasing rotor speed, as you cant reduce the pitch angle (or AoA) any further.
But I've been known to be wrong more often that right. ;)

[nonradio]

xnr: What were the symptoms of your "RBS" during auto entry? Difficult to see how LOWERING collective could cause it. Aft cyclic and a reducing airspeed would decrease the aofa on the retreating side.

[Lama Bear]

Xnr,

Are you talking retreating blade stall or Nr decay?

I'm unfamiliar with the autorotation Vne on the S-76? Could you tell me what it is?

If you exceed that speed you will lose Nr.

[IHL]

XNR , what was the NR at when it entered RBS. Minimum in the A is 87 % with a transient down to 78 %.

Were you in the clapped out old A model Sim, it barley meets the level A requirements.

[Shawn Coyle]

Is there a Vne in autorotation?
What is the level flight retreating blade stall airspeed and G like? What sort of G can you pull at cruise speed before you get RBS?
How do you know it's RBS - just cause the sim says so?
I'd be very suspicious of the reaction of an older simulator to something outside straight and level flight, and I'd be even more suspicious if the company were telling you it's exactly like the real thing everywhere.

[slgrossman]

From the S-76A flight manual; Vne Power Off (all weights) at 4000' PA, Std Temp (15 deg C - 2 deg per 1000 ft = 7 deg C) is 141 Kts.

Under the same conditions Vne Power On (up to 10,500 lb) is ~148 Kts.

Like Shawn, I'm skeptical that the aircraft would encounter RBS under those conditions.

-Stan-

[Aesir]

"Shawn coyle said: Is there a Vne in autorotation? "

Yes many helicopters have a published autorotation Vne. The 206L has auto Vne 100 Kts & Bell-222U´s auto Vne is 80 Kts.

[NickLappos]

Xnr,
Take care with judgements about behaviors in the simulator where you are out-of-envelope. The sim is not at all accurate when you get even close to the limits, and it can be very misleading. The FSI Instructors should know better than to train folks there! I was the development pilot for the A and B sim, and can assure you nobody tuned it for accuracy there, and it does not behave like an aircraft there. It is a fine training tool, but almost worthless for any maneuver that is significantly out of the maneuver or rpm envelope. The rotor behaves much differently on the real aircraft, especially in near stall environments. Similarly, the tail rotor failure practice where you can fly away from a high hover with zero thrust are very suspect.

The rotor decay with a total power failure is fast, but not too hairy, at least in discussions with pilots who have experienced it (fuel starvation, mostly).

Regarding Autorotational Vne, it has nothing to do with stall. Quite the opposite, it has to do with the tips going trans-sonic when at the Auto Vne and at max Nr simultaneously. The auto Vne of the S-76 family was set by the team I was on during the end of development, and was based on two items - the max tip mach not exceeding M0.96, and the longitudinal static stability in auto (an esoteric measurement that means almost nothing to a typical pilot in VFR flight.) Note that the Auto Vne drops with cold temp at low altitude, because the speed of sound is dropping, so the speed must come down too. The tip speed is 675 at 100% Nr, so at 115% it is about 776 feet per second. At 141 knots the airspeed is 236 feet per second so the tip speed is 1012 feet per second. At 15 degrees C, sound travels at 1115 feet per second, so the tip is doing 0.91 Mach. By 0 C, sound is down to 1085 ft/sec, so the tip mach is up to .93 at 141 knots and 115%Nr.

In autos, we have trimmed and maneuvered down to about 82% Nr with no difficulty (the bottom of the green arc is 87% IIRC, so we tested to 82% for the whole envelope). I did that testing myself, as well as the touchdown autos.

For the record, you could stall in auto, if you pulled enough g's and trimmed the collective up to keep the rpm within limits.

[Lama Bear]

Nick,

It is my understanding that the autorotation Vne in the Bell 206 (115 mph) is based on an insufficent airflow through the rotor system to maintain rpm with the collective full down. Do you know if this is the case?

Thanks for your time, Jim

[PPRUNE FAN#1]

Did we all misunderstand Xnr's question? He said:Can anyone tell me at what decreasing Nr this could happen if you allow the Nr to drop while maintaining you forword airspeed.I didn't think he was referring to steady-state autorotations. I took it to mean that the scenario he was describing was one of a complete power failure in which the pilot delayed getting the pitch down quickly enough but also let the nose drop to maintain airspeed. In this case, as the NR decreased (with the airspeed and collective up), at what percent would RBS occur?

Granted, such a thing might not ever happen in an S-76 (anymore!) unless the hapless pilots ran it out of fuel. But it certainly could in single-engine helos - you know, the kind we don't have simulators for. I doubt that many 206 instructors would want to explore that region of the flight envelope in the real aircraft anyway.

[Lama Bear]

"In this case, as the NR decreased (with the airspeed and collective up), at what percent would RBS occur"

Shortly before impact! Which may or may not have been proceeded by the rotor blades assuming the vertical position.

Just guessing! Jim

[Xnr]

Thanks PPrune Fan

Thats exactly what I mean.....I have done many, many simulated autos in the S76, including some surprise autos at cruise where the candidate was slow to lower the collective and get the nose up.

But after my latest sim session they were advocating, may I say, an aggressive lowering of the collective combined with an agressive reduction in airspeed to avoid RBS.

I feel that if I treat the real aircraft in such a manner the result would be a battle to keep the rotor from overspeeding.

I am relieved to hear from Nick on the fact that the sim is not accurate in this regime. Maybe with his background and mathematical ability he can give us a ball park Nr figure when RBS could occur if you maintain your forward airspeed at 135 kts and don't lower the collective. (standard day ops.)

Cheers

[donut king]

Been on that A model sim.

Good procedural trainer, BUT it does not fly like a real 76. As Nick L. states, anything involving a/c maneuvers at envelope limits, TAKE WARNING in that sim.

As for RBS in the 76, have you encounterred it in the actual a/c XNR??

D.K

[Xnr]

No...... haven't had the displeasure......and don't know anyone who has....that's why I was so surprised the sim did it so easily....if it happenned that easily in the sim (and the instructor insisted that the sim was accurate) someone should have done it in the aircraft.

Cheers