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8th Dec 2001, 18:31

IHL,
The higher hydraulic pressure allows more energy to be transmitted, and it allows smaller piston areas in the servo for a given force, so it saves weight. 3,000 pounds per square inch is now considered old hat. The S-92 is 4,000 psi, and the V-22 is 5,000 psi (!!) The tradeoff is against the leak potential and the need to protect against line burst.

ShyTorque,
The real issue is a subject dear to my heart. Our machines should not execute the pilot for an error, I think, but the levels of jack stall we are discussing are really just annoying characteristics, so I really agree with the group, I think.

Helmetfire,

Generally, few helicopters can pull 3 g's, and I'll bet the 350 cannot. High g capability robs hover performance, so designers give it out gingerly. The Black Hawk can pull 3.5, and an Apache can get close to that, Comanche can pull almost 4. I have hit 2.7 in the S-76, doing stuff that would make many pilots toss their cookies (I used to demonstrate helo aerobatics to the Army when they were deciding how to specify LHX maneuvers). Estimating g's is hard, I really can't without a g meter.

Max G capability is actually easily estimated based on blade chord, tip speed and diameter. I can give you a reference, if you'd like.

The issue here is really the nature of carefree handling. As long as jack stall does not cause control loss, it is not a major issue, and we really agree. If it robs the pilot of control, it should be fixed, even if the maneuver is considered extreme. FAR states that we must test to the maximum the aircraft will experience in flight, so if someone comes back and pulled more, we didn't test enough. Flight manual cautions and the "pilot error" rubber stamp are quick outs for poor designs.

We used 2 g's with a servo failure in the S-76 to assure that nobody would ever get jack stall, reasoning that folks would not pull near stall loads after a failure. With both servos on, the controls can never get to jack stall under any case, as is true with the Black Hawk.

Something I must calrify is the notion that any helicopter can hit any g if the pilot gets wild enough. The max g's for a helo are set by the rotor design, especially the solidity (amount of blade area relative to the disk area). The rotor can only pull a few g's before it stalls, and sometimes the maneuver can be quite mild. The typical rotor stall g level drops quickly with altitude, so if the maneuver took 3 g's at sea level, it might stall at 2 g's at 10,000 feet DA, a much more achievable level, and near Vne, it might stall at only 1.5 g's.

If my helicopter could lose control in jack stall at 1.5 g's, I'd find another!

You ask about g stall as opposed to speed stall, I think. There is no difference, really. The relationship between g and speed is that the stall g drops with speed until at some speed beyond Vne, the aircraft stalls at 1 g. I will be glad to email you some charts of this relationship, or give you some references, if you'd like.

You ask, "Does this level of severity have to occur during certification? Or do you restrict it to 2 or so Gs as you indicated for the S76?" Please note that the S-76 is jack stall free with only 50% of its servo strength, and jack stall with both systems healthy is impossible. The manufacturer must test to a maneuver level that he then declares to be the maximum (the phrase is that the test maneuvers are extreme enough so that the probability of exceeding them in service is "extremely remote" which is FAA speak for 1 in 10e9 hours. In other words, if 1,000 helos are building 1,000 hours per year, in 1,000 years we will have one event! Using this definition, it is not good design to experience jack stall (to loss of control) in service, having certified that your tests were so thorough.

8th Dec 2001, 18:31	#62 (permalink)
Nick Lappos Guest Posts: n/a	IHL, The higher hydraulic pressure allows more energy to be transmitted, and it allows smaller piston areas in the servo for a given force, so it saves weight. 3,000 pounds per square inch is now considered old hat. The S-92 is 4,000 psi, and the V-22 is 5,000 psi (!!) The tradeoff is against the leak potential and the need to protect against line burst. ShyTorque, The real issue is a subject dear to my heart. Our machines should not execute the pilot for an error, I think, but the levels of jack stall we are discussing are really just annoying characteristics, so I really agree with the group, I think. Helmetfire, Generally, few helicopters can pull 3 g's, and I'll bet the 350 cannot. High g capability robs hover performance, so designers give it out gingerly. The Black Hawk can pull 3.5, and an Apache can get close to that, Comanche can pull almost 4. I have hit 2.7 in the S-76, doing stuff that would make many pilots toss their cookies (I used to demonstrate helo aerobatics to the Army when they were deciding how to specify LHX maneuvers). Estimating g's is hard, I really can't without a g meter. Max G capability is actually easily estimated based on blade chord, tip speed and diameter. I can give you a reference, if you'd like. The issue here is really the nature of carefree handling. As long as jack stall does not cause control loss, it is not a major issue, and we really agree. If it robs the pilot of control, it should be fixed, even if the maneuver is considered extreme. FAR states that we must test to the maximum the aircraft will experience in flight, so if someone comes back and pulled more, we didn't test enough. Flight manual cautions and the "pilot error" rubber stamp are quick outs for poor designs. We used 2 g's with a servo failure in the S-76 to assure that nobody would ever get jack stall, reasoning that folks would not pull near stall loads after a failure. With both servos on, the controls can never get to jack stall under any case, as is true with the Black Hawk. Something I must calrify is the notion that any helicopter can hit any g if the pilot gets wild enough. The max g's for a helo are set by the rotor design, especially the solidity (amount of blade area relative to the disk area). The rotor can only pull a few g's before it stalls, and sometimes the maneuver can be quite mild. The typical rotor stall g level drops quickly with altitude, so if the maneuver took 3 g's at sea level, it might stall at 2 g's at 10,000 feet DA, a much more achievable level, and near Vne, it might stall at only 1.5 g's. If my helicopter could lose control in jack stall at 1.5 g's, I'd find another! You ask about g stall as opposed to speed stall, I think. There is no difference, really. The relationship between g and speed is that the stall g drops with speed until at some speed beyond Vne, the aircraft stalls at 1 g. I will be glad to email you some charts of this relationship, or give you some references, if you'd like. You ask, "Does this level of severity have to occur during certification? Or do you restrict it to 2 or so Gs as you indicated for the S76?" Please note that the S-76 is jack stall free with only 50% of its servo strength, and jack stall with both systems healthy is impossible. The manufacturer must test to a maneuver level that he then declares to be the maximum (the phrase is that the test maneuvers are extreme enough so that the probability of exceeding them in service is "extremely remote" which is FAA speak for 1 in 10e9 hours. In other words, if 1,000 helos are building 1,000 hours per year, in 1,000 years we will have one event! Using this definition, it is not good design to experience jack stall (to loss of control) in service, having certified that your tests were so thorough.