Bell 525 fatal accident July 2016

[JohnDixson]

Viper 7

Viper, that sounds like pitch-lag instability, not stall.

Part 2: I looked in vain online for an old H-3 flight manual. There used to be a write-up on pitch-lag instability in the USN military manual.

Anyway, re H-3 pitch-lag: The H-3 rotor has some alpha-1 coupling in the geometry. In the H-3, as the blade lag angle increases ( which occurs with increased power ), the pitch angle decreases a bit. Hardly noticeable, but coming back into a hover on the 61, the pilot needs to nudge the collective up a bit more when he applies collective to come to the hover. Really a non-issue.

However, if a blade damper has a problem ( e.g. sticky relief valve ) then at higher speeds, and having nothing to do with stall*, the rotor can excite a pitch to lag angle instability. Pilots unfamiliar with the excitation may well call it a 1:1 or 1/rev, but in fact it is 2/3 per rev. Fix is to look at the dampers and check their timing.

* USN H-3 had a cruise guide indicator. The main rotor servo ( typically highest loads on the newer machines have highest loads on the aft longitudinal servo, but I honestly don't recall which servo on the H-3 had the highest loads ) had an LVDT ( linear,variable,differential transducer ) which measures the loads on that servo and feeds an indicator with range markings. Like the same system on the S-65 series, anything over 30% in indicative of increasing degrees of stall.

On the H-3, with a damper problem, one can see pitch-lag when the cruise guide will be barely indicating anything. It will be at higher power, thus faster speed, but its not a simple 1/rev. An easy way to evaluate the difference is that with a simple 1/rev, ( and assuming it is large enough to get one's attention )there is usually one blade out of track and quite visible from the cockpit. With pitch-lag, the main rotor feels and appears to wobble at a slightly slower frequency, and one can see that.

Rotor heads subsequent to the S-61 have a very flat alpha-1 geometry, that is, almost zero blade pitch change as the lag angle changes, and as a result have been absent this particular instability.