Sikorsky S-76 [Archive Copy]
Joined: Sep 2001
Posts: 219
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From: South of 60
S76's & Triple Tachs
Just when I thought I'd seen it all, another surprise.
Here's the deal:
Both Nr needles go to zero and stay there (on the ground by the way). Take a look around and note that both CB's are popped as well, ESS Bus and PRI bus.
What I don't know is how the Nr pickup on the MGB is wired inside. If the pickup is generating one signal only, and that signal is ultimately split between two independent electrical buses to their respective triple tachs, what might cause both CB's to pop?
Cheers
Here's the deal:
Both Nr needles go to zero and stay there (on the ground by the way). Take a look around and note that both CB's are popped as well, ESS Bus and PRI bus.
What I don't know is how the Nr pickup on the MGB is wired inside. If the pickup is generating one signal only, and that signal is ultimately split between two independent electrical buses to their respective triple tachs, what might cause both CB's to pop?
Cheers
Guest
Posts: n/a
the Nr tach is dual powered, with a single coil in the pulse pickup feeding the indicators (another coil, in the pickup but electrically independant, serves the other ship's needs for Nr, such as DCA or FADEC.) Diodes isolate one power source from another.
Sounds like a short in the instrument to me. What did it turn out to be?
Sounds like a short in the instrument to me. What did it turn out to be?
Joined: Sep 2001
Posts: 219
Likes: 0
From: South of 60
Problem turned out to be the pilot's triple tach.
Should one be surprised that the copilot triple tach went u/s also?
Seems to me I heard a story a long time ago concerning the way the pilot and copilot triple tachs were wired (linked), or something to that effect, although I can no longer remember what the details were. It's been a while so I might be out in space on this one.
Cheers
Should one be surprised that the copilot triple tach went u/s also?
Seems to me I heard a story a long time ago concerning the way the pilot and copilot triple tachs were wired (linked), or something to that effect, although I can no longer remember what the details were. It's been a while so I might be out in space on this one.
Cheers
Joined: Jan 2001
Posts: 61
Likes: 0
From: England
What you need is a 'B' (or a'C'?)
Then you don't need a triple tacho at all 'cos you just know that Nr's gonna be 107 regardless, and P&W's favourite sons are gonna do what it takes to keep it that way while 4 EECs look after them.....now that's class
Shame about the first try with the C30s!
What was all that about the PBA being a no-go item until the 'B' replaced it with a fixed length rod????
Then you don't need a triple tacho at all 'cos you just know that Nr's gonna be 107 regardless, and P&W's favourite sons are gonna do what it takes to keep it that way while 4 EECs look after them.....now that's class
Shame about the first try with the C30s!
What was all that about the PBA being a no-go item until the 'B' replaced it with a fixed length rod????
Joined: Apr 2002
Posts: 172
Likes: 0
From: Can
S76 Wheel Brakes
Nick
After our chat on the phone the other day I found quite interesting your analysis of the capabilities of the S76 wheel brakes.
You stated that if you were concerned about burning up real estate on a rejected OEI take-off or an OEI landing that placing the aircraft on the ground and applying the wheel brakes would out perform the aerodynamic braking ability of a 20 degree nose up flair.
With over 3000+ hrs on the machine I never believed this to be true. I fact I feel that quite a few other S76 drivers are of the same opinion.
I put this out here so you can share this info with our fellow S76 pilots.
Jump in any time.
Cheers
Xnr
p.s. I am researching a Canadian pub for you regarding VRS vs. SWP
After our chat on the phone the other day I found quite interesting your analysis of the capabilities of the S76 wheel brakes.
You stated that if you were concerned about burning up real estate on a rejected OEI take-off or an OEI landing that placing the aircraft on the ground and applying the wheel brakes would out perform the aerodynamic braking ability of a 20 degree nose up flair.
With over 3000+ hrs on the machine I never believed this to be true. I fact I feel that quite a few other S76 drivers are of the same opinion.
I put this out here so you can share this info with our fellow S76 pilots.
Jump in any time.
Cheers
Xnr
p.s. I am researching a Canadian pub for you regarding VRS vs. SWP
Joined: Dec 2001
Posts: 943
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From: Gold Coast, Queensland
With most wheeled helicopters and certainly with the 76, the shortest stopping distance is obtained by getting the machine on the ground quickly and applying full brakes. Much above 12 degrees nose up will guarantee a tailboom repair job!
Joined: Jul 2002
Posts: 512
Likes: 0
From: Texas
I have to agree that the ground will slow you more quickly than the air. There just isn't that much friction in the air, & the S76 doesn't like to slow down up there. You might need to replace the brakes & the wheels when you're finished, but that's better than the entire machine.
Joined: Feb 2001
Posts: 579
Likes: 0
From: Canada
GLSNightPilot:
My experience has been that on a dry runway the brakes are not powerful enough to lock up the wheels. One maximum performance brake application won't ruin the brakes either.
The only time you risk grinding down a tire is when the runway is wet. Then its easy to lock up the wheels.
My experience has been that on a dry runway the brakes are not powerful enough to lock up the wheels. One maximum performance brake application won't ruin the brakes either.
The only time you risk grinding down a tire is when the runway is wet. Then its easy to lock up the wheels.
Joined: Jul 2002
Posts: 512
Likes: 0
From: Texas
Perhaps, but we used to have a set of wheels above the door to the pilot's lounge here that were worn through the treads when an unnamed pilot taxied to the fuel pad with the brakes locked. The wheels never even tried to turn, while the pilot was wondering why it took so much power to taxi. I think I'd try to have the brakes locked before I hit the ground, in the above scenario. Fortunately I've never had to try it for real.
Guest
Posts: n/a
Comments on breaks and tires and aerodynamic breaking:
Brakes on helos are not exactly like those on cars, but they are powerful. The friction of a tire is large, about 75% of the weight that it supports. For an S76, this means that the two mains can put about 5000 pounds of deceleration on the aircraft, probably about .4 G
The brakes on an S-76 will certainly lock the wheels with full bottom collective, I have left dark black stripes and blue smoke during Cat A and Cat B landings, and the tires made a thumping feel as they rotated after that!
Tires make the most stopping force when they are at 95% of the ground speed, ie just slipping, not stopped. That is how ABS and "pumping the brakes" works. Skidding tires are quite a bit poorer at stopping.
Aerodynamic breaking is a very poor way to slow a helicopter. With normal attitudes, about .1 g is all that is available. Even with ridiculous nose up attitudes, like 20 degrees, the thrust of the main rotor that is actually decelerating you is about 25%, or about .25G, a lot less than the brakes can provide.
If you are on the ground, nose wheel off, mains on, the rotor is even less effective, because it is producing less thrust, and therefore has less decel capability. This situation, (mains on, nose off) is also bad for aft stick on most helos, because of the possibility of making a blade tailcone contact. The 76 has a control limiter to keep this from happening, but many helos do not(including original 76A's that have not had the control system mod - ask maintenance if the limiter SB has been installed!) , and it could be a bit more expensive to replace the blades then to replace the brakes!
Brakes on helos are not exactly like those on cars, but they are powerful. The friction of a tire is large, about 75% of the weight that it supports. For an S76, this means that the two mains can put about 5000 pounds of deceleration on the aircraft, probably about .4 G
The brakes on an S-76 will certainly lock the wheels with full bottom collective, I have left dark black stripes and blue smoke during Cat A and Cat B landings, and the tires made a thumping feel as they rotated after that!
Tires make the most stopping force when they are at 95% of the ground speed, ie just slipping, not stopped. That is how ABS and "pumping the brakes" works. Skidding tires are quite a bit poorer at stopping.
Aerodynamic breaking is a very poor way to slow a helicopter. With normal attitudes, about .1 g is all that is available. Even with ridiculous nose up attitudes, like 20 degrees, the thrust of the main rotor that is actually decelerating you is about 25%, or about .25G, a lot less than the brakes can provide.
If you are on the ground, nose wheel off, mains on, the rotor is even less effective, because it is producing less thrust, and therefore has less decel capability. This situation, (mains on, nose off) is also bad for aft stick on most helos, because of the possibility of making a blade tailcone contact. The 76 has a control limiter to keep this from happening, but many helos do not(including original 76A's that have not had the control system mod - ask maintenance if the limiter SB has been installed!) , and it could be a bit more expensive to replace the blades then to replace the brakes!
Joined: Apr 2002
Posts: 243
Likes: 0
From: canada
to XNR.... a test!!!
Picture this situation XNR.....!
Two 76's side by side...
One on the ground...one in the hover..
15-20 kts forward speed( just an arbitrary number)
wheel taxiing 76 JAMS on the brakes while the hovering one flares 20 degrees.
Which one stops first???
While you're at it...... Two trains leaving at the same time...one travelling east, the other west......!!!!!!!!!!!!!!( just joking!!!)
D.K
Two 76's side by side...
One on the ground...one in the hover..
15-20 kts forward speed( just an arbitrary number)
wheel taxiing 76 JAMS on the brakes while the hovering one flares 20 degrees.
Which one stops first???
While you're at it...... Two trains leaving at the same time...one travelling east, the other west......!!!!!!!!!!!!!!( just joking!!!)
D.K
Avoid imitations
Joined: Nov 2000
Aviation Qualifications: ATPL
Posts: 15,111
Likes: 1,085
From: Wandering the FIR and cyberspace often at highly unsociable times
Nick,
I tried today to leave some black lines behind the aircraft. Failed completely. Grass and sheep all dead. Where am I going wrong?
I tried today to leave some black lines behind the aircraft. Failed completely. Grass and sheep all dead. Where am I going wrong?
Joined: Feb 2001
Posts: 579
Likes: 0
From: Canada
Brother & Nick:
I guess my little legs ain't strong enough, cause I anin't never seen them wheels locked up when the collectived was full down.
Of course if the brakes were locked before touch down, well thats a different scenario.
I guess my little legs ain't strong enough, cause I anin't never seen them wheels locked up when the collectived was full down.
Of course if the brakes were locked before touch down, well thats a different scenario.