Exceeding Vne
Uh Oh.....yet another discussion about "Jack Stall" and remember how we used to do it back when!
Hydraulic Lock /Jack Stall
Hydraulic Lock /Jack Stall
My interpretation of the references courtesy of SAS indicates my understanding was accurate. Thus, since Eurocopter chose to not modify the machine, but to rely upon ( I assume ) flight manual limitations, I’ll consider this question answered and look to further Vne posts.
John, the main thing about jackstall was that it was essentially self-correcting (much like RBS) and you had to hold the cyclic back with the lever up to keep it in the jackstall condition - this was for demonstration purposes.
The correct recovery was to relax the back pressure on the cyclic and lower the lever slightly - then fly around for a bit straight and level to get your heart rate back to normal.
The correct recovery was to relax the back pressure on the cyclic and lower the lever slightly - then fly around for a bit straight and level to get your heart rate back to normal.
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 422 Likes
on
222 Posts
Thanks, Shy, but it seems I totally misunderstood the use of “ jackstall” in connection with several events involving Gazelles. I thought it signified a condition wherein the maneuver control loads exceeded the force capability of the servos, hence the cyclic became stalled or immovable. What did it in fact mean?
It's also referred to as 'servo transparency'.
I may be wrong, but I thought one reason for the demonstration was that the French Army had lost at least one aircraft during NOE flight in the early days of service.
and one had to be more than a little ham-fisted to jackstall a Gazelle
Why ever would you build an aircraft that lacked the ability to control a Rotor System under all expected flight loads?
Especially, in a helicopter that is going to be used in a military application to include actual combat?
Especially, in a helicopter that is going to be used in a military application to include actual combat?
Why ever would you build an aircraft that lacked the ability to control a Rotor System under all expected flight loads?
Who would build a helicopter that had a pitch/roll couple that killed several pilots at low level? BO 105 anyone..
Who would build a helicopter that had insufficient yaw control in many situations within the flight envelope? Step up Bell helicopters............
Shy: Disclaimer-prior to installing the fan tail on the S-67 in 1974, we had the opportunity to fly a short eval in a privately owned Gazelle, and it was pure fun to fly.
212: In fact there was a marketing movie circulated re the Gazelle maneuverability, featuring a Gazelle in French Army colors and with the Light Cavalry Overture by Von Suppe as background.
SAS: re the comment about all flight loads-Yes.
I'm assuming Part 27 mimics Part 29, in requiring the basic design to positive 3.5G, but with an important follow up statement to the effect that less than 3.5 is acceptable if analysis and flight test proves the lower number is extremely remote. The flight loads survey is then flown to that envelope, and the resultant flight loads are utilized in assessing fatigue damage accumulation and component replacement times. It appears that the philosophy used by Eurocopter at the time was to use the limited capability servo system as a flight envelope limiter. I would guess that some in their flight test group foresaw the weakness of that choice, but were overruled. A design choice that was never repeated, however, as far as I know.
Alternative approaches included the Cruise Guide system on military 61's and 53's, later copied into the 47 ( Chinook ). On the S-67 we had a cruise guide AND a collective stick shaker, activated by the same control load sensor in the servo, but much more usable in a heads-out-of-the-cockpit scenario. Cruise Guide was based on a load sensor within the primary servo. The basic idea was simple. Flight test proved the relationship between control loads and stall, so setting some limits on loads thru the indicator kept the usage within the flight loads data. I must add an important note here because the handling of flight loads data is different on US military machines vs the FAA: although the military machines utilized the cruise guide indicator system, the implementation of flight loads data as to component replacement times also included a spectrum of control loads from the military structural demonstration ( which FAA Regs do not require ), where the machine is flown to the 3.5 G limit or to a limiting factor preventing that. ( example of a limiting factor from the UH-60 demo: at 80 KIAS, one can't generate enough pitch rate to get 3.5, so collective must be used. Applying collective at the max rate helped, but we quickly got to the engine limiter, drooping the rotor. With lift being a V-squared term, that didn't help. thus the max we could achieve at 80 was 2.6 as I recall. At the higher speeds, the limit G achievable was typically limited by control loads/stall/vibration-typically occurring together. While we were able to achieve 3.53 at lighter weight, 3.23 was the max achievable at design ( mission ) weight. )
I
212: In fact there was a marketing movie circulated re the Gazelle maneuverability, featuring a Gazelle in French Army colors and with the Light Cavalry Overture by Von Suppe as background.
SAS: re the comment about all flight loads-Yes.
I'm assuming Part 27 mimics Part 29, in requiring the basic design to positive 3.5G, but with an important follow up statement to the effect that less than 3.5 is acceptable if analysis and flight test proves the lower number is extremely remote. The flight loads survey is then flown to that envelope, and the resultant flight loads are utilized in assessing fatigue damage accumulation and component replacement times. It appears that the philosophy used by Eurocopter at the time was to use the limited capability servo system as a flight envelope limiter. I would guess that some in their flight test group foresaw the weakness of that choice, but were overruled. A design choice that was never repeated, however, as far as I know.
Alternative approaches included the Cruise Guide system on military 61's and 53's, later copied into the 47 ( Chinook ). On the S-67 we had a cruise guide AND a collective stick shaker, activated by the same control load sensor in the servo, but much more usable in a heads-out-of-the-cockpit scenario. Cruise Guide was based on a load sensor within the primary servo. The basic idea was simple. Flight test proved the relationship between control loads and stall, so setting some limits on loads thru the indicator kept the usage within the flight loads data. I must add an important note here because the handling of flight loads data is different on US military machines vs the FAA: although the military machines utilized the cruise guide indicator system, the implementation of flight loads data as to component replacement times also included a spectrum of control loads from the military structural demonstration ( which FAA Regs do not require ), where the machine is flown to the 3.5 G limit or to a limiting factor preventing that. ( example of a limiting factor from the UH-60 demo: at 80 KIAS, one can't generate enough pitch rate to get 3.5, so collective must be used. Applying collective at the max rate helped, but we quickly got to the engine limiter, drooping the rotor. With lift being a V-squared term, that didn't help. thus the max we could achieve at 80 was 2.6 as I recall. At the higher speeds, the limit G achievable was typically limited by control loads/stall/vibration-typically occurring together. While we were able to achieve 3.53 at lighter weight, 3.23 was the max achievable at design ( mission ) weight. )
I
Last edited by JohnDixson; 30th Nov 2018 at 21:00. Reason: typos and additional information
The AS365 N3 has a switch on the stbd lateral jack (area of highest aerodynamic backloads) that gives you a caption in the cockpit ( LIMIT Light and audio bong) at high disc loadings so Aerospatiale/Eurocopter/Airbus must have learned some lessons......
Who would build a helicopter that had a pitch/roll couple that killed several pilots at low level? BO 105 anyone..
Just as in other issues like Tail Rotor problems....knowing the effect of flight controls...to include RPM Controls as alternative ways of controlling yaw works to one's advantage.
John, it is a load sensor - as the aerodynamic backloads increase and the non-rotating star is pushed down onto the top of the servo jacks, a force that exceeds 330daN will cause a switch to close, illuminating the LIMIT light and sounding the gong.
This also occurs with pressure loss in the RH hyd system which feeds the upper half of the jacks.
Note that it is the jack body that moves up and down whilst the fescalised shaft is bolted to the MRGB
This also occurs with pressure loss in the RH hyd system which feeds the upper half of the jacks.
Note that it is the jack body that moves up and down whilst the fescalised shaft is bolted to the MRGB
So it is a one-point cruise guide type idea. I was going to ask SAS if the later versions of the 47 have cruise guide, as SA dropped it after the S-65 model line. One of the reasons was that while doing the flight loads survey on the USAF 53C, where a heavy/high alt/“relatively” high speed envelope was needed for air to ar refueling ( and the original S-65 flight loads was done only to 10000 I think-USMC mission definition), we saw problems with the cruise guide signal. The 53C featured the T64-7 engine as well with more power than the USMC 6/6B so it was possible to drive the 53C into stall in level flight at altitudes above 10-12k, and the blades reflected a version of stall flutter, where the basic 6/rev servo load signal would break down into 12/rev. If one looked at the servo load trace on a printout, the very top of the signal was cut off, so the peak servo load was in fact reduced*, but at the same time, the push rod load was increasing exponentially, thus the cruise guide function was subverted in those conditions. I’ve no idea where the 53K is on this subject.
*the resulting indication on the indicator when this occurred was a needle that would slowly oscillate thru a large range on the dial, rendering it unreadable. The N/rev vibration signature told all, however, so seat of the pants to brain connection told you what was going on.
But you have introduced a word that even my Merriam-Webster Collegiate Dictionary did not have, and I’m not at work with NL, so I can’t ask him: what does “fescalize” mean?
*the resulting indication on the indicator when this occurred was a needle that would slowly oscillate thru a large range on the dial, rendering it unreadable. The N/rev vibration signature told all, however, so seat of the pants to brain connection told you what was going on.
But you have introduced a word that even my Merriam-Webster Collegiate Dictionary did not have, and I’m not at work with NL, so I can’t ask him: what does “fescalize” mean?
Last edited by JohnDixson; 1st Dec 2018 at 11:51. Reason: Additional thought
It is the shiny chromed part and the term is applied to motorcycle forks, aircraft oleo legs and the servo jacks.
I have heard the term used many times over the years but googled it to make sure of the spelling - some of the origins attributed to it are bizzare, especially one involving a goat's penis
I have heard the term used many times over the years but googled it to make sure of the spelling - some of the origins attributed to it are bizzare, especially one involving a goat's penis
I am not knowing re the Chinook having a Cruise Guide system using Sensors....must have been after my time.
In my days we had a mechanical computer on the Instrument panel that incorporated Gross Weight, OAT, and Altitude that could be used to hold plastic spoons in anticipation of a C-Ration Lunch.
Basically....just a Vne calculator.....which rarely got used in flight as we planned for Max Weight and our customary Flight Levels mandated by SOP's for Traffic Separation and the OAT which was pretty standard by Season....Monsoon, pre/post Monsoon, and Dry Season.
In my days we had a mechanical computer on the Instrument panel that incorporated Gross Weight, OAT, and Altitude that could be used to hold plastic spoons in anticipation of a C-Ration Lunch.
Basically....just a Vne calculator.....which rarely got used in flight as we planned for Max Weight and our customary Flight Levels mandated by SOP's for Traffic Separation and the OAT which was pretty standard by Season....Monsoon, pre/post Monsoon, and Dry Season.
Crab.....you just had to drag the RN into this didn't you!
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 422 Likes
on
222 Posts
All I can say is that you had to be a lot more careless to get a Gazelle to stall its jacks than to find retreating blade stall in a Whirlwind - having seen the latter at about 110 kts and about 150 feet, the outcome at low level was potentially just as exciting and dangerous - I'm thankful that the Whirlwind I was in rolled upright from the steep turn, rather than inverted.
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 422 Likes
on
222 Posts
Who would build a helicopter that had insufficient yaw control in many situations within the flight envelope? Step up Bell helicopters...........
But that's why we pilots have to be trained to understand and respect the limits of the aircraft we fly - almost all of them (rotary or fixed wing) have their dirty little tricks to keep us on our toes.
The Best Design can always be un-done by a Pilot.
Shytorque - Lucky indeed!! I was given a 'next day' posting from Valley S&R flt to Thorney to replace the winchman, killed with his pilot, as a result of RBS on the South Downs. First job, on watch, was picking up my predecessor's personal effects from the crash site. The Boss arrived shortly after to make a valid point - the pilot involved had been well known (and applauded!) for various similar exploits and the"don't grass up your mates" ethic allowed it to happen. Basically, another 'Bud' Holland episode and yet there's no reason to suppose it won't happen again!
Last edited by Cornish Jack; 1st Dec 2018 at 15:16. Reason: %^*&!! predictive text