Retreating Blade Stall No 2

[JohnDixson]

Stall Characteristics at SA

SAS, I'm not sure how to address your " different than " with regards to SA vs Bell stall characteristics. Have a lot of experience with the former and zero with the latter.

Over the years, the various new rotors at SA have been subjected to flight into stall, as you can imagine. It is an interesting subject and far too long a story to type in here. The increasing number of rotor blades from the 3-bladed S-55 has softened the roll rate for the cases where the rotor is driven into stall, straight ahead, one " G ". In those instances, one notices an increase in N/rev vibration and a measurable tendency for the lateral stick position at the trim points to migrate toward the right. That agrees with the consistent results we have seen in dynamic cases as well, where the rolling tendency is always left.

I might mention one very different but common rotor behavioral effect, ( that quite frankly does not explain the story as related by Army ), and that is the gyroscopic effects of control inputs. Hence for a clockwise rotor, a pure positive ( nose up ) pitch up cyclic input will result in the rotor/aircraft pitching up, and then, rolling right, with the roll rate a function of the pitch rate. Easiest to observe with all of the electronic stability enhancing equipment turned off.

Allow me to amplify the comment regarding N/rev vibration encountered with stall. If the rotor is flown into the edge of stall, the signature will be N/rev, but if driven further into deep stall, to include stall flutter situations, then the signature will be more complex, with additional blade response harmonics present, which can, and often do, set up beat frequency vibrations sensed in the cockpit as something near 1/rev.

Thanks,
John Dixson