Originally Posted by
megan
Had an email from a reader who provided the following, they assure me the info comes from a reputable source.It would seem that rotor stall as proposed by AnFI is a fact of life, just not something taught to we stick jockeys. Must admit my ignorance. Bit like the meaning of Va in the FW world until the NY A300 crash I assume.
Megan, there is no evidence that the rotors stalled in that crash. Do you know something that we don't? What is obvious is that the rotor head reached a "power required exceeds power available" in terms of thrust vector up not being able to overcome vertical speed down, hence impact with the water. I will suggest to you that the rotor head (Apache does not have a low inertia rotor head, and the engines were still trying to keep it spinning) did not slow down fast enough to stall until after impact (at which point it doesn't matter). Yes, I understand what accelerated stalls are, in both fixed and rotary wing. G loading considered (and we
don't know what the G load was) there is still momentum and inertia in play in the brief time between recognition that the maneuver was balled up and the attempt to use all (we presume) power available to stop the rate of descent short of the water.
So while your generic point is that rotors can stall (aerodynamics makes no special allowances for helicopters in that regard) asserting that the rotors did stall is assuming some facts not in evidence. The coning angle is an effect, not a cause, of the rotors changing pitch and responding to loads on the blades/rotating wings. It is a reaction to forces on the blades.
Megan, why is the airspeed around the time of impact of interest? Power margin and airfoil efficiency. Depending on the day, weight, and such from the charts in the back of the flight manual, there's a sweet spot airspeed. Based on similarity to Blackhawk in terms of weight and propulsion, I'll estimate somewhere around 70-75 knots for an Apache on a standard day. That is the knee in the curve analogous to L/D max for a fixed wing. We used to refer to that airspeed as "max conserve" because you had the most additional power available at that airspeed, or more to the point, needed the least amount of power to fly level (and thus burned the least amount of gas).
If the Apache was below that airspeed as the final pitch and pull was attempted, it was already in energy debt. If it was above, (say 90, or 80 knots airspeed (AnFI's estimated are based on ground speed due to measurement method) then there was a little energy "in the bank" for the flare but there's a catch, which is where your concern about rotor loading comes in. Trading that extra energy in the flare/pull gets you some benefit but you are likely passing through that sweet spot airspeed and into the beginning of needing more energy.
At this point in the high demand maneuver, you've got a variety of issues working against you. In overcoming your energy debt with that flare, you've increased drag as you've gotten a bit of lift (the blades bite harder) and your AoA goes up ...
but your airspeed is still decreasing which isn't helping, and is indeed hurting.
Whatever angle the blades cone to is a reaction to that bundle of forces and effects on the disc/blades.
Put another way, by the time you are wondering whether or not you have reached a critical coning angle you are already far enough behind the aircraft that you are at the mercy of inertia, and altitude available to trade for more energy ... which thanks to the entry altitude of this maneuver the pilot ran out of in a hurry.