Originally Posted by
Bergerie1
fdr,
Thank you (and all you other knowledgeable posters here too). As you say fdr, the 747 stalls very nicely, I did many on the -100s and -200s during CofA check flights. When the 747 was put on the British register, D. P Davies required a stick nudger (not a pusher) to restore longitudinal stability shortly before the clean stall at aft CG. I quote his words from his book, Handlng the Big Jets:-
"With trim speed of 1.3Vs, after a small elevator force to start the speed reduction, the stick force falls to zero while the aeroplane quietly progresses all the way to the stall on its own .... It is common knowledge that the UK attitude to stalling is quite firm and this degree of instability, although slight, was declared unacceptable..... ."
Here was a simple cheap and reliable fix which had already been used on Boeing aircraft. It would have required no training with only a mention in the manuals. Would this not have been a better solution?
K, for those wanting to stall any of these aircraft, a note first up, there is a maintenance procedure for inspections after stall buffet encounter on most if not all of these aircraft. The amount of buffet that occurs varies from type to type and with configuration. Clean is usually quite light buffet and associated loads on the tail, but heavy buffet entry in the MD11 in the cruise, at high Mach pretty mush shredded the elevators on both sides, and came close to two ripples on the pond. A fully configured stall buffet can be quite impressive, as are the loads to the tail and flap tracks.
25.173 requires 1lb/6kts average gradient over the desired window being assessed, which is within the flight envelope from stall to upper limits. There are two main methods of determining compliance, but basically, taking the load on the control, coming off the trimmed speed, (1.23 or 1.3 depending on the rules applied earlier in the Part, the year etc... ) and slowing down as well as speeding up around that datum. For a stall speed around 100kts, the minimum trim point is Vref... so the loads being measured total are around 4lbs to 6 lbs of elevator force, which is quite light. The requirement is for the average to be that, and the guidance material in AC25.07 shows clearly that a reduction in force can occur towards an extreme of the test points, and still be acceptable. That is open to interpretation in so far as a zero gradient for the last 10kts could be fully compliant, but considered objectionable from an HQ view by the testing driver. My recollection is that the stick gradient in the mid-trim condition was way higher than that, and never encountered reversal, which is where a rationale for a SAS system would arise. D.P. Davies did good work, and perhaps at aft CG limit -0.5% at light weight, high thrust, he encountered an undesirable that Boeing didn't. It is possible. Is it likely? TBC did a comprehensive testing program on the 747 under the gaze of Joe Sutter and coming in the wake of the Trident, and some 727 funnies, they were not without awareness of what was needed. A spring balance to the controls could detect that level of force in the day, today, some of the FBW systems record the forces applied, or force sensing gloves can be applied and calibrated to give the info. (make your own, go grab some Phidgets... ).