Sikorsky S-76: Ask Nick Lappos
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212man,
Vne is actually not imposed on any aircraft, it is usually the speed at which we propose to stop testing because any more is unusable. Few machines are qualified at a physical limit due to structural or handling limiting.
The S-76 has Vne at 155 knots mostly because we decided not to spend lots of test time climbing up after we dove to higher speeds. Many helos since then have also qualified that way, I think.
The Vne for the S-76 once set, also set the maximum speed at which we met the Longitudinal Static Stability (LSS) for level flight. We adjusted the pitch bias actuator (a device that reads airspeed and fools the longitudinal stick to build in LSS) to that speed. We needed the actuator (since removed on most models because it was only needed to meet the letter of the requirement, and had little practical value.)
The LSS maneuver in the FAR says to trim level at .9 Vne, then without resetting collective, push to 1.1 Vne and see where the stick trims to hold that new speed. In the old old days, when Vne was 100, the 10 knot push was no problem, and in fact a good measure of how a pilot would work to trim at the new speed. In a fast helo like the S-76, the initial trim was 142 knots, and the push was to 172 knots, and the 30 knot speed increase lead to a screaming dive at about 2800 feet per minute. Some level flight trim check, huh?
At 2800 feet per minute down, the stick trimmed back right where it started, and the flat gradient was deemed unacceptable, so we adjusted the pitch bias actuator to give 1% forward stick there, and called it OK.
I worked with a group of handling qualities professionals a few years back, and we proposed a change to the FARs that is now in the works that basically looks at a 10 knot push and pull around the level flight trim. This seems to work well now.
For Joker's Wild - Regarding the question of very bad behavior in an S-76 in a dual engine cut, I performed most of those back when, and the aircraft handles quite well, about like most modern helos. If you wait too long with the collective up in your armpits, however, the rpm can get low, and things can get busy. I terms of response to engine failure, I believe the S-76 is typical of most modern helos in its rpm decay, but it has exceptionally good control capability at low rpm, and a very wide and forgiving rpm range. I have autorotated it to touchdown at high weight and at high altitude, and I think it is actually quite good, not a 206, but surprisingly easy. The couple of touchdown autos experienced by customer pilots due to fuel starvation and the like have been quite successful, too.
Vne is actually not imposed on any aircraft, it is usually the speed at which we propose to stop testing because any more is unusable. Few machines are qualified at a physical limit due to structural or handling limiting.
The S-76 has Vne at 155 knots mostly because we decided not to spend lots of test time climbing up after we dove to higher speeds. Many helos since then have also qualified that way, I think.
The Vne for the S-76 once set, also set the maximum speed at which we met the Longitudinal Static Stability (LSS) for level flight. We adjusted the pitch bias actuator (a device that reads airspeed and fools the longitudinal stick to build in LSS) to that speed. We needed the actuator (since removed on most models because it was only needed to meet the letter of the requirement, and had little practical value.)
The LSS maneuver in the FAR says to trim level at .9 Vne, then without resetting collective, push to 1.1 Vne and see where the stick trims to hold that new speed. In the old old days, when Vne was 100, the 10 knot push was no problem, and in fact a good measure of how a pilot would work to trim at the new speed. In a fast helo like the S-76, the initial trim was 142 knots, and the push was to 172 knots, and the 30 knot speed increase lead to a screaming dive at about 2800 feet per minute. Some level flight trim check, huh?
At 2800 feet per minute down, the stick trimmed back right where it started, and the flat gradient was deemed unacceptable, so we adjusted the pitch bias actuator to give 1% forward stick there, and called it OK.
I worked with a group of handling qualities professionals a few years back, and we proposed a change to the FARs that is now in the works that basically looks at a 10 knot push and pull around the level flight trim. This seems to work well now.
For Joker's Wild - Regarding the question of very bad behavior in an S-76 in a dual engine cut, I performed most of those back when, and the aircraft handles quite well, about like most modern helos. If you wait too long with the collective up in your armpits, however, the rpm can get low, and things can get busy. I terms of response to engine failure, I believe the S-76 is typical of most modern helos in its rpm decay, but it has exceptionally good control capability at low rpm, and a very wide and forgiving rpm range. I have autorotated it to touchdown at high weight and at high altitude, and I think it is actually quite good, not a 206, but surprisingly easy. The couple of touchdown autos experienced by customer pilots due to fuel starvation and the like have been quite successful, too.
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Thanks Nick
Now, I've not seen this for myself, but I'm told the latest 76 simulator in WP will in fact roll hard immediately following a dual engine failure in cruise (higher speeds).
I'm also told the folks who run the sim are declaring this to be expected behavior. I find that just a little hard to believe. If in fact the 76 behaves the way I think it does with a dual engine failure, then what they're saying in WP is way off the mark.
Now, I've not seen this for myself, but I'm told the latest 76 simulator in WP will in fact roll hard immediately following a dual engine failure in cruise (higher speeds).
I'm also told the folks who run the sim are declaring this to be expected behavior. I find that just a little hard to believe. If in fact the 76 behaves the way I think it does with a dual engine failure, then what they're saying in WP is way off the mark.
A friend of mine recently lost an engine past V2 on a 'C' model and reported a vicious cross coupled LH roll(!).
Sounded really uncomfortable on the phone and I've been thinking about it for a month now.
No doubt there is the lag between event and pilot response; but does this sound correct nick?. They would've been at 97%N1 at least.
Sounded really uncomfortable on the phone and I've been thinking about it for a month now.
No doubt there is the lag between event and pilot response; but does this sound correct nick?. They would've been at 97%N1 at least.
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For Joker's Wild, I will check with my friends down at flight safety, because this is not only wrong, it is beyond the simulator's capability to accurately simulate, I believe. Thanks for the steer.
For steve76, there is little response to a single engine failure at all. We typically demonstrate hands off single engine cuts to show this. Without the engine gages, it is not easy to tell when the cut occurs. I think you might be getting some pilot bravado from your buddy! Was he knitting his parachute with a silkworm and a sewing machine?
For steve76, there is little response to a single engine failure at all. We typically demonstrate hands off single engine cuts to show this. Without the engine gages, it is not easy to tell when the cut occurs. I think you might be getting some pilot bravado from your buddy! Was he knitting his parachute with a silkworm and a sewing machine?
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Nick,
Can I ask a little more about the LSS? Without the actuator, is the Apparent LSS positive up to Vne and what about with the actuator? I am particularly interested with regard to the S76B.
Thanks
Can I ask a little more about the LSS? Without the actuator, is the Apparent LSS positive up to Vne and what about with the actuator? I am particularly interested with regard to the S76B.
Thanks
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Bit,
Most helicopters have marginal or negative LSS in corners of their envelope, the traits are virtually meaningless to pilots, and not detectable unless careful cookbook maneuvers are flown.
The speed stability and trim characteristics of helicopters are created by the rotor, which has excellent speed/stick stability, but can be disturbed by the fuselage and horizontal tail, which react to climbs and descents more than they do to small speed and angle changes. What the regs try to do is measure the cues a pilot gets which tell him that speed has changed. The stick is the last place he looks, especially if the aircraft has "normal" dynamic stability (where the pilot is constantly moving the stick to keep the greasy side down). The most important speed cue is the nose attitude gradient, where the nose goes progressively further down as speed increases. (remember the "60 knot attitude" that the instructor told you about?) With a good attitude/speed gradient, speed is easy to hold, because the pilot just flies back to the right attitude and all is OK. With LSS, the supposition is that the stick position tells the pilot that speed has changed, and that is just not true.
The funny thing is that FAR/JAR dictates LSS based on speed and stick, but never mentions attitude gradient at all. That is because the fixed wing folks who wrote the first CAM 6 and 7 regs were translating requirements from the airplane world, and airplanes have no attitude gradient, just stick gradients.
I could go on here, and you can't nudge me and tell me to stop, so I will just stop.
Most helicopters have marginal or negative LSS in corners of their envelope, the traits are virtually meaningless to pilots, and not detectable unless careful cookbook maneuvers are flown.
The speed stability and trim characteristics of helicopters are created by the rotor, which has excellent speed/stick stability, but can be disturbed by the fuselage and horizontal tail, which react to climbs and descents more than they do to small speed and angle changes. What the regs try to do is measure the cues a pilot gets which tell him that speed has changed. The stick is the last place he looks, especially if the aircraft has "normal" dynamic stability (where the pilot is constantly moving the stick to keep the greasy side down). The most important speed cue is the nose attitude gradient, where the nose goes progressively further down as speed increases. (remember the "60 knot attitude" that the instructor told you about?) With a good attitude/speed gradient, speed is easy to hold, because the pilot just flies back to the right attitude and all is OK. With LSS, the supposition is that the stick position tells the pilot that speed has changed, and that is just not true.
The funny thing is that FAR/JAR dictates LSS based on speed and stick, but never mentions attitude gradient at all. That is because the fixed wing folks who wrote the first CAM 6 and 7 regs were translating requirements from the airplane world, and airplanes have no attitude gradient, just stick gradients.
I could go on here, and you can't nudge me and tell me to stop, so I will just stop.
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Nick,
thanks very much for your detailed reply. I have done a tp course and have an appreciation of the theory you have discussed but having monitored your excellent posts (S92 etc)for a while I was really after specifics regarding the S76 which in retrospect I should not really have asked about here - sorry bout that
Please keep up the good work
Bit
thanks very much for your detailed reply. I have done a tp course and have an appreciation of the theory you have discussed but having monitored your excellent posts (S92 etc)for a while I was really after specifics regarding the S76 which in retrospect I should not really have asked about here - sorry bout that
Please keep up the good work
Bit
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Bit,
No sweat, the S-76 has positive LSS across most of its envelope as measured by the dive till it hurts technique, but at aft CG it is slightly negative. The A had a pitch bias actuator to patch up that corner, and after a short while in service, the FAA agreed that the PBA was unnecessary and allowed it to be pulled. When the B came out, we folded the PBA algorithyms into its Sperry autopilot, and that took care of that. If no autopilot was fitted, then a PBA was required. The B is just like the A, but the people on the program at that time decided the PBA was a good thing, so they kept it.
All the while, the UK authorities resisted removing the PBA on all models. I have lost track of how that stands now.
I was the Chief Pilot on the S-76 in the beginning, and I challanged the FAA lead test pilot to allow a double blind flight eval, where the PBA was randomly switched off as he performed careful trimmed airspeed maneuvers. He refused, saying, "I couldn't tell, so why bother?"
So much for the sanctity of stick gradient requirements. I could discuss the fallacy of lat-dir gradients next, but my fingers are getting tired!
[ 07 September 2001: Message edited by: Nick Lappos ]
No sweat, the S-76 has positive LSS across most of its envelope as measured by the dive till it hurts technique, but at aft CG it is slightly negative. The A had a pitch bias actuator to patch up that corner, and after a short while in service, the FAA agreed that the PBA was unnecessary and allowed it to be pulled. When the B came out, we folded the PBA algorithyms into its Sperry autopilot, and that took care of that. If no autopilot was fitted, then a PBA was required. The B is just like the A, but the people on the program at that time decided the PBA was a good thing, so they kept it.
All the while, the UK authorities resisted removing the PBA on all models. I have lost track of how that stands now.
I was the Chief Pilot on the S-76 in the beginning, and I challanged the FAA lead test pilot to allow a double blind flight eval, where the PBA was randomly switched off as he performed careful trimmed airspeed maneuvers. He refused, saying, "I couldn't tell, so why bother?"
So much for the sanctity of stick gradient requirements. I could discuss the fallacy of lat-dir gradients next, but my fingers are getting tired!
[ 07 September 2001: Message edited by: Nick Lappos ]
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Jokers Wild,
I heard about the FSI C+ sim problem with roll early this year. I was also told it didn't like quickstops either. However, it flew perfectly normally when I did my course a few months back. I believe it was a programming problem and not a representation of the aircraft.
ShyT
I heard about the FSI C+ sim problem with roll early this year. I was also told it didn't like quickstops either. However, it flew perfectly normally when I did my course a few months back. I believe it was a programming problem and not a representation of the aircraft.
ShyT
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Steve 76,
Any roll, yaw or pitch would be due to rotor rpm droop, as there is no other mechanism for the disturbance. How low did your friend let the rotor go? The normal tendency at the 0 to 40 knot range for all US/UK helos is to require right and forward stick to keep the aircraft level and accelerating. Rotor droop with the right stick in (its about 1/2 to 1 inch on most helos) will result in a left roll and a pitch up, both slight. If the rotor is kept within about 5% droop or so, the afcs will handle it and the pilot will not need to correct. If the rpm goes down a bunch more, it would be more noticable. I certainly would not call any handling quality of the S-76 in this regime "vicious".
I have seen documented evidence of one case where an operator got the rotor down to very very low numbers, in flight, and flew home. Very very means way below 60% Nr. I am not sure how many helicopters are that kind to their occupants, but I'll bet not many. So take care what you call my baby!!
Any roll, yaw or pitch would be due to rotor rpm droop, as there is no other mechanism for the disturbance. How low did your friend let the rotor go? The normal tendency at the 0 to 40 knot range for all US/UK helos is to require right and forward stick to keep the aircraft level and accelerating. Rotor droop with the right stick in (its about 1/2 to 1 inch on most helos) will result in a left roll and a pitch up, both slight. If the rotor is kept within about 5% droop or so, the afcs will handle it and the pilot will not need to correct. If the rpm goes down a bunch more, it would be more noticable. I certainly would not call any handling quality of the S-76 in this regime "vicious".
I have seen documented evidence of one case where an operator got the rotor down to very very low numbers, in flight, and flew home. Very very means way below 60% Nr. I am not sure how many helicopters are that kind to their occupants, but I'll bet not many. So take care what you call my baby!!
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A question for Nick.
Does the C model drop the AC Gen in an OEI condition like the A++? If so does this have any effect on the SPZ-7600 or is it powered only by the inverters? I know that on all our A++ (AC Gen equipped) you will get that momentary kick when you go OEI as the Gen drops off line.
Cheers, OffshoreIgor
Does the C model drop the AC Gen in an OEI condition like the A++? If so does this have any effect on the SPZ-7600 or is it powered only by the inverters? I know that on all our A++ (AC Gen equipped) you will get that momentary kick when you go OEI as the Gen drops off line.
Cheers, OffshoreIgor
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Nick,
Going back to the original question for a moment -- While you say the Vne on the S-76 is somewhat arbitrary, is there not an aerodynamic reason for the decrease with altitude and temperature? I'm flying an A++ and find with a light load at altitude I have to back the power off a good 10%-15% from the torque setting at max gross weight to stay under the placarded Vne. It seems like there's more to it than merely correcting IAS to TAS.
Thanks,
-Stan-
Going back to the original question for a moment -- While you say the Vne on the S-76 is somewhat arbitrary, is there not an aerodynamic reason for the decrease with altitude and temperature? I'm flying an A++ and find with a light load at altitude I have to back the power off a good 10%-15% from the torque setting at max gross weight to stay under the placarded Vne. It seems like there's more to it than merely correcting IAS to TAS.
Thanks,
-Stan-
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60% I cant believe my eyes? was this right after the engine failure or something???? Talk about slow reactions, I was taught that letting rpm drop below 90% is hazardous to my health.
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Hey.Moe,
The Vne is based on the test envelop that we determined based on S-76A power, where Vne was just a bit faster than a fully loaded aircraft would cruise at the old max continious. At light weights, you can easily beat that speed in any model, even the original A. Even so, it really isn't arbitrary (anymore) because it is the limit, based on the flight manual, and also now helps set the component stresses and therefore the lives. The reason it is at those speeds was based on power available, but now is is cast in concrete, and is the basis for a bunch of things, so please observe it properly. Rest assured that an incursion into the red range will not cause any major problem, however.
Barafin,
The 60% is actually much higher than the data we have, but I am reluctant to publish the actual number to preserve the specifics. I am pleasantly surprised at the fact that the S-76 brought its folks home after that!
[ 17 September 2001: Message edited by: Nick Lappos ]
The Vne is based on the test envelop that we determined based on S-76A power, where Vne was just a bit faster than a fully loaded aircraft would cruise at the old max continious. At light weights, you can easily beat that speed in any model, even the original A. Even so, it really isn't arbitrary (anymore) because it is the limit, based on the flight manual, and also now helps set the component stresses and therefore the lives. The reason it is at those speeds was based on power available, but now is is cast in concrete, and is the basis for a bunch of things, so please observe it properly. Rest assured that an incursion into the red range will not cause any major problem, however.
Barafin,
The 60% is actually much higher than the data we have, but I am reluctant to publish the actual number to preserve the specifics. I am pleasantly surprised at the fact that the S-76 brought its folks home after that!
[ 17 September 2001: Message edited by: Nick Lappos ]
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SK76B Cat A (Vert).
Can anybody tell me if Sikorsky had developed official Cat A (Vertical)procedures for the SK76B (as distinct from Cat A Clear Heliport procedures)......Nick L are you out there????? ...cheers
Input drive shaft failure S76
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Hmmmm?
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Hmmmm?