SVSUSteve
Well-Known Member
Since it came up in the "How safe is safe enough?" thread, I figure it might be worth bringing it up for further discussion outside of that thread. While everyone agrees that training (initial and recurrent) should include stall/spin recovery, the technology exists and has for some time to prevent an aircraft from stalling under the conditions that lead to crashes (approach or takeoff). To me at least, it would seem that saying that training or AoA indicators are sufficient as a response to a known issue when there's a way to avoid it is a bit like designing a reactor and going "We don't need to design it to avoid thermal runaway even though we could....we just need to train the crews how to respond!" (granted, an extreme example but it was the best one I could come up with that would be a clear and immediate threat to life). The classic and perhaps best example of application of technology to the prevention of stall in a light aircraft is, of course, the Ercoupe. Of course, if one were really comfortable with their programming and electronic construction abilities, one could always go for a fly-by-wire "law" system that would not allow you to hazard the aircraft but that is really beyond the scope of anyone on here so far as I am aware. If you have that kind of skill, I need to talk to you about some much simpler systems.
The only detailed discussion of putting a stall-proof control system I have ever seen is in Thurston's "Design for Safety" in which he discussed two things:
1. The use of a "two-control" wheel/yoke (with interconnections between ailerons, elevator and rudder) like in the Ercoupe to avoid cross-control situations which can lead to a stall even in the hands of a skilled pilot who is startled or momentarily distracted. Wing-mounted force-balanced spoilers/speed brakes like those on sailplanes provide the option of a slip and negate one of the major complaints pilots had about original configuration of the Ercoupe.
2. The best way to alert of an impending stall would be to provide heavy stick forces as the aircraft departs the speed for which it is trimmed. The issue becomes how to do this since neither of the systems Thurston puts forth are ideal (the FAA flight testing requirements for a spring/pulley based system is rather extensive and the potential need for very strong springs that could cause serious complications if you are developing a higher performance aircraft when it comes to accommodating the speed range, the CG range, the effects of flap extension, etc) and the "bobweight" system is largely applicable only to high speed stalls and have little benefit to the low speed stalls we are most concerned with.
Regarding the first point, several issues come to mind:
-How do you make it fail safe? That is, what options are there in case you have a linkage failure other than doubling the linkages? Could you design the system to be controllable (not ideal but enough to get you on the ground safely) by the trim system alone?
-Does anyone have any insight/references/suggestions into how to actually rig the interconnections?
Any thoughts on the second point?
One of the major issues (also brought up in Thurston's book....been reading it hence my mentioning it up) that can lead to a stall is a pitch-up associated with flaps and/or the various changes in pitch that can occur in some aircraft when there are changes in flap position or engine power. To the experienced designers or aerospace engineers on here, as a new designer I would like to ask how one calculates such things so as to minimize discovering such problems only during flight testing.
The only detailed discussion of putting a stall-proof control system I have ever seen is in Thurston's "Design for Safety" in which he discussed two things:
1. The use of a "two-control" wheel/yoke (with interconnections between ailerons, elevator and rudder) like in the Ercoupe to avoid cross-control situations which can lead to a stall even in the hands of a skilled pilot who is startled or momentarily distracted. Wing-mounted force-balanced spoilers/speed brakes like those on sailplanes provide the option of a slip and negate one of the major complaints pilots had about original configuration of the Ercoupe.
2. The best way to alert of an impending stall would be to provide heavy stick forces as the aircraft departs the speed for which it is trimmed. The issue becomes how to do this since neither of the systems Thurston puts forth are ideal (the FAA flight testing requirements for a spring/pulley based system is rather extensive and the potential need for very strong springs that could cause serious complications if you are developing a higher performance aircraft when it comes to accommodating the speed range, the CG range, the effects of flap extension, etc) and the "bobweight" system is largely applicable only to high speed stalls and have little benefit to the low speed stalls we are most concerned with.
Regarding the first point, several issues come to mind:
-How do you make it fail safe? That is, what options are there in case you have a linkage failure other than doubling the linkages? Could you design the system to be controllable (not ideal but enough to get you on the ground safely) by the trim system alone?
-Does anyone have any insight/references/suggestions into how to actually rig the interconnections?
Any thoughts on the second point?
One of the major issues (also brought up in Thurston's book....been reading it hence my mentioning it up) that can lead to a stall is a pitch-up associated with flaps and/or the various changes in pitch that can occur in some aircraft when there are changes in flap position or engine power. To the experienced designers or aerospace engineers on here, as a new designer I would like to ask how one calculates such things so as to minimize discovering such problems only during flight testing.
