DP Davies on Deep Stalls
My understanding of Deep/Super Stall is the stage after the initial stall when the aircraft is not recovered immediately - i.e: held in the stall (in AF447 case through aft control column, and full aft tailplane trim along with full thrust). This prolongs deceleration (due drag rise) and despite fairly benign looking attitude (perhaps 8 deg pitch up) leads to a massive increase in the incidence (effective AoA) as the flight path becomes more of a sinking than forward motion.
DP Davies devotes around 6 pages to the subject in his book Handling the Big Jets. He was chief test pilot for the UK Airworthiness Authority (pre CAA) and tested all manner of machines from ultra lights to the B707, B747 and Concorde. He categorically states that Deep/Super stall recovery is only possible where elevator authority exceeds the nose up pitching moment post stall (caused by wing sweep, change in chordwise lift distrubution after initial stall, forward fuselage lifting body effect and now add in AF447 aft stick, trim and TOGA underslung engines).
To quote DP Davies:
"...aircraft that have been lost to such manouevres finally reached the ground substantially level laterally, having defied all efforts to roll or spin them out of this stabilised condition,....with little or no forward speed, rotating only slowly in yaw, ... exposed to massive angles of incidence (AoA) and enormous vertical velocity"
"At one time it was thought that an attempt to roll or spin the aeroplane would offer the best chance. This idea has now been withdrawn (1971), because even assuming that this (additional) upset could be achieved, the resulting very steep nose down attitude, the lack of proof of spin recovery capability, he very high rate of decent and the large height range required (for subsequent) recovery makes it unlikely this method offers any advantage at all"
" The best recovery technique (super stall) is now considered.......persist in full forward elevator control, put the flaps to the postion recommended by the maufacturer and wait for the aeroplane to pitch down and recover from the stall"
"...... pitch attitude is not enough guide to the recovery.Too early a recovery from a gentle dive, following assumed recovery from a super stall, will again increase incidence and ensure the aeroplane remains locked in."
"Without incidence information, a nose down pitch with an increasing speed is no guarantee that recovery has been effected and an up elevator movement at this stage merely serves to keep the aircraft stalled"
Last edited by angelorange; 10th Jul 2012 at 13:19.