bumper wrote:This has been an interesting thread, with good stuff for thought.
Not yet mentioned are tail plane stalls. This is thought to be responsible for many of the "moose stalls", where the aircraft is circling low and slow, usually dirty (flaps), and crosses its own wake turbulence. If the horizontal stab stalls, without downforce, the tail goes up the nose drops through, often with not enough altitude for recovery.
I've watched amazingly large wing tip vorticies from a Husky just after the start of the take off roll from a wind sheltered dry lake bed (had very fine alkali dust). The vortices had a radius of near a third of a wing span, though neither the Husky or the wind rotation was fast moving at that point - - dirtied up, heavy and slow, I'm certain they'd be significant
The Husky got a bad rap for this when used for coyote control, though it happens with makes as well. Maybe it is more of an issue with the Husky, or maybe an undeserved bad rap, I'm not sure. I've intentionally tried to induce this at altitude, but other than moderate turbulence hitting my wake, no loss of control. So it appears that things have to come together just right (or rather just wrong) for it to occur. I make it a point to avoid flying tight circles at low altitude.
Tail plane stalls are doubtless responsible for many of the icing related crashes.
Bumper,
I don't buy the tail plane stall as a cause for "Moose stalls". I DO believe that "Moose stalls" are caused by wing tip vortices, but they aren't causing a tail plane stall.
Aircraft designers work really hard to avoid designs that will permit a tail stall, and tail stalls are pretty rare.
That said, the two accidents that the Border Patrol had with Huskys probably WERE precipitated by tail stalls, but they weren't caused by wing tip vortices, but rather by a big burble of disturbed air near the surface that the airplane left in its wake. They were doing a pitch up with full power, with very slow speed, flying a race track pattern, then running through their own wake somewhat lower, and close to the ground. This "tripped" the flow over the tail, airplane pitched hard nose down, and went straight in, no rotation.
Moose stalls, on the other hand, almost always involve rotation, in otherwords, they are in fact a stall/spin. And, the ADC accidents with Huskys and Cubs for that matter, were probably stall/spins, NOT tail stalls.
Aviat was able to replicate the tail stall scenario at altitude, using TWO airplanes, one in trail and below the other. I know both the test pilots that ran this experiment. I was told that both airplanes were at very slow speed (like in the range of 45-50 mph), full flaps to replicate the Border Patrol's profile, then the lead airplane pitched nose up hard and applied full power, leaving a big burble of disturbed air behind for the trailing airplane. They had to try this numerous times before they got a tail stall, but when it happened, the test pilot said they lost a LOT of altitude prior to recovery.
Point is, it's very difficult to get yourself into THIS SPECIFIC scenario....think about 45 mph, full flaps, stooging along at 50 feet AGL.....looking out the window man tracking.
I suspect that the Husky exhibits a somewhat stronger than "average" wing tip vortex. That may simply be because the airplane is heavier than many Cubs or it may have something to do with wing tips, etc.
Circling has it's own hazards, no doubt, and this is in fact where moose stalls come from, but I'm convinced that they are precipitated by running into one's own wing tip vortex assymetrically, causing an uncommanded roll, which the pilot then tries to correct by application of opposite control inputs. I've been in this exact scenario, and it's a VERY subtle effect, I suspect, right up till the airplane turns loose... I have an acquaintance who went through one of these, and it did in fact break on him. This is a VERY high time, VERY experienced pilot in Super CUbs and doing the kind of work he was doing. He and his gunner were very lucky and both survived, with a lot of injuries. He is convinced that he ran into his own wake vortex, which induced a rolling moment to the aircraft, which he tried to correct with opposite aileron and rudder (coordinated--he thought). He couldn't just roll out and go on his way because he was up against a steep canyon wall.
Tail stalls are nasty indeed, but the manufacturers of light aircraft make it VERY difficult to actually encounter one in their aircraft.
MTV
