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Sikorsky S-76: Ask Nick Lappos

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Sikorsky S-76: Ask Nick Lappos

Old 14th Aug 2008, 02:15
  #1061 (permalink)  
 
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Steering can be a combination of brakes or T/R pedal. Often, just adding pedal will produce enough turn, and then add some brakes to help.

Landing with speed needs to be done carefully - make sure you have less than 10deg nose up to avoid dragging your tail on the ground. Comfortable run-ons can be done in a B model up to 54 kt groundspeed and slow to 45 kt before brake application. (If memory hasn't faded too badly.)
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Old 14th Aug 2008, 03:07
  #1062 (permalink)  
 
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Here are the airspeed numbers:
maximum touch-down speed - 54 KIAS
maximum brake application speed - 34 kts.

Visibility is restricted for tricky confined area approaches, but manageable with a little bit of practice.

The front nose wheel is of castor style - it pivots freely. All steering is done primarily with the individual brakes on the main gear as well as with tailrotor pitch (to a much lesser degree).
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Old 14th Aug 2008, 05:37
  #1063 (permalink)  
 
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I heard that in some parts of the world (eg Hong Kong) the TDP (takeoff decision point) is not rearward but directly over the pad. With limited forward and downward visibility, an OEI on takeoff before TDP necessitates a vertical drop back onto the pad. Peripheral visual reference is mostly water !

Quite demanding especially when it's often hot, humid and heavy.
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Old 14th Aug 2008, 06:30
  #1064 (permalink)  
 
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All steering is done primarily with the individual brakes on the main gear as well as with tailrotor pitch (to a much lesser degree)
Got that round the wrong way I feel.
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Old 14th Mar 2009, 23:44
  #1065 (permalink)  
 
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S-76 MGB run dry time - looking for info

Does anyone have the certification figure?
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Old 15th Mar 2009, 02:10
  #1066 (permalink)  
 
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The need to address oil out was introduced as a requirement in 1988, so it is reasonable to assume that most helos certified before that date do not meet the requirement, and have never been tested for oil out. I have searched the FAA archives, and found that the TCDS (official FAA source for certification basis) lists the below for oil failure qualification:
B412 NOT
S-76 NOT
AS-365 NOT
AW-139 YES
EC-155 YES
EC-225 YES
S-92 YES

Here is the requirement:


29.927 amendment 26, 10/3/1988:

c) Lubrication system failure. For lubrication systems required for proper operation of rotor drive systems, the following apply:
(1) Category A. Unless such failures are extremely remote, it must be shown by test that any failure which results in loss of lubricant in any normal use lubrication system will not prevent continued safe operation, although not necessarily without damage, at a torque and rotational speed prescribed by the applicant for continued flight, for at least 30 minutes after perception by the flightcrew of the lubrication system failure or loss of lubricant
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Old 15th Mar 2009, 05:47
  #1067 (permalink)  
 
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The need to address oil out was introduced as a requirement in 1988, so it is reasonable to assume that most helos certified before that date do not meet the requirement, and have never been tested for oil out.
The requirement prior to amendment 26, 10/3/1988 was,

It must be shown by tests that the rotor drive system is capable of operating under autorotative conditions for 15 minutes after the loss of pressure in the rotor drive primary oil system Amdt. 29-17, Eff. 12/1/78

Prior to 1978 there was no dry run capability requirement as far as I can find. The 76 I assume would have been certified under the 1978 ammendment and further assume that the 76D will be grandfathered to the 1978 requirement. Nick could fill us in.

Question for Nick: A common belief is that providing you operate outside the RFM H-V curve you will have fly away capability in the event of an engine failure. True or false? (assuming you are maxed out, and not single pilot in Antarctica with 100 pounds of fuel )
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Old 15th Mar 2009, 06:46
  #1068 (permalink)  
 
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Question for Nick: A common belief is that providing you operate outside the RFM H-V curve you will have fly away capability in the event of an engine failure. True or false? (assuming you are maxed out, and not single pilot in Antarctica with 100 pounds of fuel )
I'd say false. On a normal departure you are only assured that you should, if you perform properly, be able to land back safely (before CDP) as long as you remain in the unshaded area.
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Old 15th Mar 2009, 11:55
  #1069 (permalink)  
 
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Brian,

If using the Category A procedures and (after TDP in the take-off or before LDP in the landing).

There are statements inside AC29-2C to the effect that the Category A procedure effectively modifies the HV diagram.

There is no compulsion to be within: (a) the take-off manoeuvre to 1000ft; or (b) to be within the landing manoeuvre before LDP. Hence the necessity to make the HV diagram a limitation if full engine-failure accountability is to be retained.

The HV diagram is a sledge-hammer to crack quite a small nut - as has been indicated in the past, there is usually only one provided and under the most conservative conditions.

Most are aware of the necessity to be able to operate like a helicopter and perform useful amounts of work; this is why there are exemptions from excursions within the HV diagram contained in FAR 91.9 and JAR-OPS 3.005.

Jim
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Old 15th Mar 2009, 17:16
  #1070 (permalink)  
 
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Sort of a real world example: several years ago I had a trim runaway to the low side in an A model while taking off from a rig. We were about 10,000lbs, 25C day or so and the deck was about 70-80' high. Drooped to around 96%, the good engine pulled almost 900C and trading some altitude for airspeed we kept it in the air. I don't have any charts in front of me but it worked.
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Old 15th Mar 2009, 18:22
  #1071 (permalink)  
 
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Wherever Nic is, I cannot say but let me try, Brian:

The HV curve for a Cat A helicopter is the boundary for safe landing in the event of a one engine inoperative situation. As JimL says, the Cat A procedures define the fly away and stay up ability procedures, which are always far more restrictive than the HV curve (it takes more power - or less mass- to fly than to fall).

While amendment 29.17 had a 15 min auto oil out requirement, the older transmission test requirement at the time the below table of helos were qualified (amendment 2 and prior) had no oil system failure criteria, so I believe the table is still valid (but defer to any manufacturer's engineers who know otherwise)

js0987, it is good that you made it in that trim runaway, but in fact you had full power available from both engines. The trim unbalanced the engines, and made one carry the lion's share of the power, but if the rotor had been drooped to perhaps 93 or so, the other engine at min N2 beep would start also picking up load, and by 90% Nr, would be at full power too. In other words, the N2 beepers can't cause you to fall out of the sky as long as you are (rightfully) willing to overtemp the high engine.
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Old 15th Mar 2009, 18:32
  #1072 (permalink)  
 
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Ramen,

The Chinook (A-B-C) models had an ineresting engine trim system.

The Primary Engine Trim was an AC system which provided inputs to a DC powered actuator.

The Emergency System was a set of momentary switches on the lower center console just ahead of the Engine Condition Levers which allowed for direct input to the DC Acutators.

The procedure was to pull the Overhead Circuit Breakers to the AC Trim and revert to the Emergency Trim Switches which had no memory and had to be adjusted for any collective movement.

Thus one could balance engine output by means of the Emergency Trim Switches.

The one draw back was pulling the overhead CB's....the field mod was a couple of Hand Grenade Pin Rings safety wired to the CB's and left dangling for easy reach.

It beats rolling throttles and throwing switches as well as allowing one to control high side failures by pulling the affected engine down by use of the momentary switch while getting the overhead CB pulled to disable the Normal Engine Trim.
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Old 19th Apr 2009, 00:17
  #1073 (permalink)  
 
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Returning to a (much) earlier post by Nick regarding the hydraulic check:

2) The pedal/collective feedback -
(2A - one servo turned off) if one side of the yaw servo has a fault where it develops little force (blown seals, etc) it will not move easily while the rotor is turning at low rpm, so when you turn the good side off, this bad side will be harder to move, and the pilot's force on the pedals is more easily transmitted to the collective instead of the (higher force) yaw control system. In effect, the yaw force is "reflected back" from the mixer to the collective.
(2B - both servos on) if the yaw cable system has unusually high force (mis-aligned pullys, misrouted cable) then it will have fairly high frictional force, but might not be felt by the leg/foot when making a pedal check at low rpm. An easy way to detect such friction is to see if the pedal motions cause a motion of the collective, because the collective path "reflected" from the mixer is an easier path for the pedal force to go. Any significant motion of the collective while moving the pedals is a sign that the yaw control system needs to be looked at for the source of the extra friction.
I understand I should be looking for collective movement when I push the pedals during the hydraulic check and take any movement, whether with one servo system off or both systems on, as a sign of a hydraulic issue. But what about the other way around? If I raise the collective with one servo side turned off and I observe a noticeable amount of pedal movement? Does this indicate the same type of feedback mentioned by Nick, possibly caused by a faulty yaw servo?
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Old 17th Jun 2009, 13:26
  #1074 (permalink)  
cpt
 
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S76 acrylic windscreens speed limitation

Is there somebody on PPRuNe, affected by the speed limitation of 109 kts when cast acrylic windscreens are fitted on S76 (in place of the original glass windscreens) ?
How the Sikorsky recommendation letters on this matter could be interpreted ?....
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Old 18th Jun 2009, 00:24
  #1075 (permalink)  
 
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Off the top of my head: It's not a speed limitation; just a caution that the acrylic windshield, when flown above the aforementioned speed, will not provide the equivalent impact protection of a glass windshield. In other words: fly faster at your own peril.
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Old 18th Jun 2009, 00:24
  #1076 (permalink)  
 
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I don't recall any speed limitations when the glass screens were replaced, you just had to be careful when using the wipers! Maybe if non standard ones were fitted, there may have been limitations but I've never seen an amendment in the flight manual.
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Old 18th Jun 2009, 06:57
  #1077 (permalink)  
 
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As an alternative to the use of the Sikorsky approved windshield types, operators with aircraft possessing
cast acrylic windshields can attain an equivalent level of impact tolerance by limiting the maximum speed
of the helicopter to no more than 109 knots. A field check for the presence of cast acrylic is available
and is provided in Enclosure (1).

Up to you !!
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Old 18th Jun 2009, 07:57
  #1078 (permalink)  
cpt
 
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I only would like to know, if you have a knowledge of operators or customers having restricted their S76 with acrylic windscreens, speed to 109 kts.
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Old 18th Jun 2009, 08:25
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There are two thicknesses of acrylic windscreen, the thinner offers the same protection as glass at 109 kts. The thicker 123 kts.

I believe it is up to the operator if they restrict the speed or not.
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Old 7th Jul 2009, 04:55
  #1080 (permalink)  
 
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Question Rotor RPM droop AND stability

To: Nick Lappos, Shawn Coyle, et'al
Although I guess I've fairly understood Static & Transient Droop in rotors (essentialy from John Fay's Book) but for
(i) Its effect on RPM stability with regard to various possible combinations of Static or Transient droop Being more vis-a-vis less.
(ii) specifically what does RPM stability mean in this case.?
(iii)any graphs depicting these two types of droop clarifying the variables along each of the co-ordinate (x- & y-axis).
please enlighten on subject.
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