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Bendix FCU/GOV fundamental question

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Bendix FCU/GOV fundamental question

Old 20th Jan 2019, 01:40
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Bendix FCU/GOV fundamental question

OK, I'm a retired dude having fun learning to fly the jetranger. But I'm clearly not understanding how the governor works, or I'm missing something basic. Help appreciated!

My assumptions are based on a diagram by Terry Mooney, Northern Lights College. Because I'm new to PPRuNe, it doesn't seem to let me attach or embed the image. (any hints for how to get around this? the diagram is pretty key )

With my current understanding, I imagine these scenarios...

Scenario 0 - steady state
I'm flying along straight and level with 100% N2
The N2 governor must be "on speed", i.e. the N2 governor servo is somewhat open, Pg is less than Pr and the differential is suppressing the FCU from additional acceleration.

Scenario A -
  1. I click the INCR switch
  2. The governor linkage moves so as to increase the spring tension on the N2 governor flyweight assembly
  3. The greater spring pressure closes the Pr orifice, reducing the Pr-Pg differential
  4. The smaller differential on the Pr-Pg diaphragm effectively adds spring tension to the N1 governor
  5. The N1 governor servo closes a bit allowing the governor bellows to contract and meter a bit more fuel
  6. N1 increases which, in turn drives N2 higher
  7. THis returns to equilibrium when the N2 governor again reaches "on speed", i.e. when N2 rises enough to spin the flyweights fast enough to vent Pg enough to suppress the FCU from acceleration.
  8. At the new equilibrium, N2 RPM is now greater than in Scenario 0 say 102%


Scenario B -
  1. I raise the collective a smidge - (suppose just enough to be equivalent to the INCR effect in scenario A)
  2. This creates additional lift and drag which will tend to pull down Nr/N2 shortly, but the governor doesn't know this.
    (From the governor's perspective this looks just like scenario A, and essentially this is why I'm confused, everything looks just like the INCR scenario, but the end result is different)
  3. The governor linkage moves so as to increase the spring tension on the N2 governor flyweight assembly (it's just coming from the collective not the electronic linear actuator)
  4. The greater spring pressure closes the Pr orifice, reducing the Pr-Pg differential
  5. The smaller differential on the Pr-Pg diaphragm effectively adds spring tension to the N1 governor
  6. The N1 governor servo closes a bit allowing the governor bellows to contract and meter a bit more fuel
  7. N1 increases which, in turn drives N2 higher
    (in the interim N2 may have drooped, but it's my understanding that this collective linkage is intended to anticipate and minimize the droop.
  8. THis returns to equilibrium when the N2 governor again reaches "on speed", i.e. when N2 rises enough to spin the flyweights fast enough to vent Pg enough to suppress the FCU from acceleration.
  9. Why isn't the new equilibrium RPM higher than 100%
    Even if the N2 increase and drag-caused droop offset each other during the transient, the governor flyweight assembly now has a higher set point. I expect equilibrium to only be reached at 102% again


Interestingly, Phil Croucher in the "Bell 206 book", compares the Pr-Pg signal to the accelerator pump in a carburetor and calls it a "false droop signal". But an accelerator pump is a momentary fuel injection whereas in this system the N2 set point is changed until the next adjustment.

What am I missing?
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Old 20th Jan 2019, 05:19
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To put things in order -

Flyweight governors have inherent droop - this gives stability to stop it hunting incidentally.

Droop occurs because the spring tension is non-linear and when it is back "on speed" to equal the spring tension it will be in a different place to equal the spring tension = droop. Google "mechanical governor droop"

If you think about it if the tension was linear it wouldn't work.

The linkage from the collective is the "droop compensation" to adjust the new set point with the change in power back to where it should be.

To adjust for more rapid changes the droop compensation is normally "over compensated".

Set 100% N2 in a Bell 206 flat pitch - pull it up into the hover and it should be higher.

The beep trim is there to compensate for atmospherics - the P in DP N1/N2 is a pressure which varies from day to day.

Set the correct N2 with the beep trim and forget about it - if you are constantly playing with the beep trim something is broken or out of rig.

The Bendix system has been around since Pontius was a Pilate and works pretty well considering. The operation and maintenance manual is around online for free.
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Old 20th Jan 2019, 07:26
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N2 is Power Turbine speed, I once witnessed a PT Governor on test, at around 38,000 rpm it was continually being corrected up and down, it was amazing to watch.
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Old 20th Jan 2019, 13:02
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Like a few engineers I spent 30 years adjusting Bendix PTG and FCUs, mainly rectifying misrigged installations on 206 / 105 / 355 Helicopters.

Pilots seem to ask more detailed questions about fuel system components because they are constantly having to "adjust" the engine trim whilst flying, taking off or landing due to engine control rigging being incorrect.

Once an engine control system has been correctly rigged and adjusted for engine characteristics (a twin may have two different engine powers due to having a fresher engine on one side) a pilot (especially low hour) should be able to take off and fly around and hardly ever touch the engine beep switches, concentrating on looking out the window and not always at the RPM or torque gauges.

I have a digital version ( unamended) 10W2 operation and maintenance manual and 10W4 parts manual if you need one, but it is too large to e mail at 53mb on g mail, until somebody tells me how.
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Old 20th Jan 2019, 14:30
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At PHI they had an unwritten policy to not mess with the N2 beep switch. The instructors stressed that WHEN the linear actuator failed, it would be at the "full-decrease" position and then you'd be in a jam. "Just set 100% and leave it alone!" they said. And I did. However throughout my career I've met 206 pilots who just have to constantly play with the "inker-dinker" as they call it. They beep it down for the approach, and then back up for the landing. Their natural left-hand control position is with their thumb on the beep switch. Seems silly to me, but I guess some pilots need to be fiddling with something or they feel that they're not piloting. Whatever. With as many hours in 206's as I've got, my philosophy falls in line with PHI's in that if you have to keep beeping the damn thing up and down, then you've got a governor or rigging problem that should be addressed.

And no, I don't want to know the *how* of the flyweights and ports and springs and things. That's for guys with more brainpower than me.
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Old 20th Jan 2019, 14:40
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Really dumb question: why is it called a "beep switch"? Wouldn't a better name be "Governor Trim"?

Sorry for any thread drift...
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Old 20th Jan 2019, 14:54
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Originally Posted by FH1100 Pilot View Post
At PHI they had an unwritten policy to not mess with the N2 beep switch. The instructors stressed that WHEN the linear actuator failed, it would be at the "full-decrease" position and then you'd be in a jam. "Just set 100% and leave it alone!" they said. And I did. However throughout my career I've met 206 pilots who just have to constantly play with the "inker-dinker" as they call it. They beep it down for the approach, and then back up for the landing. Their natural left-hand control position is with their thumb on the beep switch. Seems silly to me, but I guess some pilots need to be fiddling with something or they feel that they're not piloting. Whatever. With as many hours in 206's as I've got, my philosophy falls in line with PHI's in that if you have to keep beeping the damn thing up and down, then you've got a governor or rigging problem that should be addressed.

And no, I don't want to know the *how* of the flyweights and ports and springs and things. That's for guys with more brainpower than me.
I agree totally.

The actuator will fail, they always do, and it WILL fail when the pilot has beeped it in the full decrease position at a remote location at the weekend, whilst doing his "pretakeoff checks".........let the fun begin.
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Old 20th Jan 2019, 15:07
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Exercising the actuator does help it last longer....at home base first thing in the day....is good enough for that.

But then PHI had some interesting ideas like putting the Engine Start Switch on the cyclic so one could ALWAYS have both hands on the flight controls even during the start procedure......as if a pair of knees would not trap the Cyclic and keep it from moving.

We bought a used PHI Machine....cheap....and that was the first thing we removed and put the aircraft back to the state it was built by Bell.

Then there was the floppy cyclic crowd who removed all friction from the controls giving no mind to commonsense.

In the 212 with the Sperry Helipilot systems installed....there was a preset level of friction to prevent cyclic movement due to vibrations from causing feedback issues in the SAS system.

What was good for Sperry seemed good in general.
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Old 20th Jan 2019, 15:50
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Originally Posted by autojohn View Post
What am I missing?
Sometimes it's better to look at things from the other end in the beginning. The FCU controls the engine fuel flow from start through about 97%-98% N2 (at least on a Bendix). At around 98% the GOV starts to modify the air signals within the FCU. Each FCU and GOV are bench set to specific performance standards with the GOVs having two standards whether it will be installed on a single engine or twin engine aircraft (206 vs BO105).

When the FCU and GOV are installed on a particular engine they are individually "matched" to that engine: FCU max fuel flow setting and idle setting plus ensure the airframe throttle control allows the FCU arm to contact the full open and cutoff stops. The GOV is set to allow a N2 beep range of 97% to 100% with a spread of 3% N2 plus verify the GOV arm does not hit full open stop with the collective at full up travel.

Now with the aircraft running at 100% N2 with flat pitch, when the collective is pulled into a stabilized hover the N2 will droop but should recover to 100% (in a perfect world) within the defined transient time frame (5 sec). If it does not, then an adjustment to the airframe mechanical droop compensation system is required. In general, the droop comp sys is used to match different airframes to different engines. The droop adj is made by changing the mechanical geometry of the airframe droop sys torque tubes, cables/control tubes, and GOV actuator arm positions to allow recovery to 100%.

Since we're dealing with non-linear adjust values sometimes it is not possible to set the droop comp system to exactly 100% recovery. So if necessary you would electrically beep the N2 to 100%. Then leave it alone. Do not chase your N2 after this point. This type of engine control system contains a lot of lead/lag/transient response values that are "features" of the system: fly-weights, control movements, gas coupling between the N1 and N2 turbines, etc.

If you find your N2 erratic or hunting consistently with or without power changes then there is a problem with one of the system components which your beep switch is not used to fix.

Now to answer your original question. How the above takes place is determined by the sequencing of Pg, Pc, Py, P this, P that signals, the control paddles/bellows, fly weights, etc, etc. However, if the internal wiz-bang ops is still important I'll try and post the authoritative source for these FCU/GOV straight from Allied Signal-Bendix. Good luck.

Last edited by wrench1; 20th Jan 2019 at 16:34.
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Old 20th Jan 2019, 18:30
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A few more tips while we are here -

Max N1 - should be checked. There is a tool for the job that goes between the FCU arm and the stop to save you having to run the engine at Max N1. Important on singles operated at high altitude. VERY important on twins.

IDLE setting. Sometimes the airframe rigging masks the true idle setting. You dont want the idle setting too low. Check on shutdown by releasing the idle detent and slowly reduce the N1 and note the N1 at fuel cutoff.

Slam decel. N2 100% flat pitch stabilised, slam the throttle to idle. Check that N1 does not decrease below 65% under 2 seconds. If so deceleration is too lean and may cause flame out in flight.

Check for wear in throttle control. Engine shutdown - check IDLE position is correct on increase direction and is close to the same on decrease direction. There is a limit.
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Old 20th Jan 2019, 19:49
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Wow, so much good info, thanks all!

@RVDT - "Flyweight governors have inherent droop - this gives stability to stop it hunting incidentally. "

This was the fundamental point I was missing. I had heard this but didn't understand it, I thought the governor was designed to return to the previous speed. You're saying that it can't do this without causing hunting and is inherently designed to give up some of the original speed.

"Droop occurs because the spring tension is non-linear"
I'm not quite grocking this yet. I see how a basic droop arm works to modulate the spring tension, but not how the conical speeder springs result in droop. I'll keep trying...


"The beep trim is there to compensate for atmospherics - the P in DP N1/N2 is a pressure which varies from day to day"
This point, (and the similar points made about leaving the INCR/DECR alone) is very illuminating; as I've just been on the receiving end of "DECR on downwind and INCR on final". And the related point about not creating a failure opportunity at a remote sight.
​​​​​​​
@wrench1 and PEASACAKE - I probably don't need to know the internal details. I think you're right that I'm trying to understand how to use it, by understanding how it works. These replies have helped a lot.

THanks!

Last edited by autojohn; 21st Jan 2019 at 00:05. Reason: formatting
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Old 20th Jan 2019, 22:29
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Here's one in pieces......if you're interested.
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Old 21st Jan 2019, 02:10
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Okay, a couple of things...and I apologize in advance for a bit of thread drift.

SASless says:
But then PHI had some interesting ideas like putting the Engine Start Switch on the cyclic so one could ALWAYS have both hands on the flight controls even during the start procedure......as if a pair of knees would not trap the Cyclic and keep it from moving....
You misunderstand why PHI did that - it's actually the opposite. For one thing, the PHI Ops Manual specified that pilots use sufficient friction to keep the cyclic from moving if you remove your hand. (They did not specify whether this was for ground or air ops.) The starter-on-the-cyclic was so that a pilot could "fly" the cyclic during a startup in high winds. Sometimes on an offshore platform you want the cyclic positioned all the way to a stop to prevent excessive flapping during those first few rotations. But as the rotor comes up to speed you need to get the cyclic back toward neutral to prevent the hub-stops from bumping. LongRanger blades start "flying" almost immediately, but those short, square-tipped B-model blades take a few turns before the cyclic becomes effective. It helps to be able to move the cyclic around during offshore starts. After a while this setup seems perfectly normal, and the stock configuration seems awkward and dumb.

I've flown many, many, many 206's that had a different beep range between cold and hot. You'd start the ship up in the cool/cold of the morning and maybe only get 98-99% max N2. Then you make a short flight, everything warms up and now your beep range is a proper 98-100%. If you shut it down and call a mechanic over to reset the beep range, by the time he gets there the engine will have heat-soaked, the beep range will be proper, and the mechanic will send you off thinking you're full of something that comes out of the south end of a northbound bull.

There is a certain amount of artistry or black magic in getting a 206 FCU and governor set up correctly. They're nice when they're new, but (it seems that) almost as soon as the components start accumulating hours things start to get loosey-goosey. I guess FADEC would help there.
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Old 21st Jan 2019, 02:21
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Originally Posted by vaqueroaero View Post
Here's one in pieces......if you're interested.
That's super useful to see. Much simpler than I expected. Is that actually the speeder spring hanging in the housing?

WHere is the arm that vents Py on overspeed?

Thanks! JOhn
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Old 21st Jan 2019, 03:54
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FH,

Fortunately I never flew in the Gulf of Mexico's extreme weather....instead got to fly in the tranquil and balmy North Sea.

Although I did fly the Jet Box in the Mountains of Iran which could also get a bit of wind and turbulence.

The question is why did PHI have the monopoly on that particular bit of wisdom....never heard of it anywhere else?

Did ERA or any of the other GOM companies do that Start Button thing?
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Old 21st Jan 2019, 13:32
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Originally Posted by SASless View Post
The question is why did PHI have the monopoly on that particular bit of wisdom....never heard of it anywhere else?
If I recall correctly, there was a fatal incident involving a blade strike and the start switch was relocated to mitigate future incidents. PHI always tried to prevent a second incident when they could.
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Old 21st Jan 2019, 13:52
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Originally Posted by SASless View Post
FH,

Fortunately I never flew in the Gulf of Mexico's extreme weather....instead got to fly in the tranquil and balmy North Sea.

Although I did fly the Jet Box in the Mountains of Iran which could also get a bit of wind and turbulence.

The question is why did PHI have the monopoly on that particular bit of wisdom....never heard of it anywhere else?

Did ERA or any of the other GOM companies do that Start Button thing?
Not a monopoly, just an STC. And I don't know if any of the other GOM operators implemented PHI's STC for the relocation of the starter button. The reason PHI did was more for straight-line winds than gusts or turbulence which, fortunately are not common over water. Our max-wind limit was 40 knots, and there were plenty of times I started up in...(ahem)...39.5 knots. You do what you gotta do when it's late and you have to get back home and you're on a small platform on which you cannot spend the night. And depending on the particular structure, it's not always 40 knots horizontally. SAS, when you flew in the North Sea I'm pretty sure it wasn't in a 206 with a teetering rotor. I came to PHI with quite a lot of 206 time. I initially thought the placement of the starter button was...weird. But once I got out in the field and had to shut down offshore, I saw the wisdom of it.

BTW, moving the starter button allowed that hole on the collective box to be filled with the Float-Arm switch. The Float-Inflation switch then moved from under the collective to the cyclic, allowing you to pop the floats without removing a hand from the controls. Pretty clever dudes, those PHI guys.
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Old 21st Jan 2019, 14:00
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Bell 212's operated offshore in the North Sea with British and Norwegian Operators.

I flew a Sikorsky Product.

Did the PHI 206's have Rotor Brakes?
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Old 21st Jan 2019, 14:29
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AutoJohn - this is a very good book which covers governing principles, including droop and droop curves, generically: http://www.woodward.com/WorkArea/Dow...?id=2147483987
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Old 21st Jan 2019, 16:51
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Originally Posted by autojohn View Post
That's super useful to see. Much simpler than I expected. Is that actually the speeder spring hanging in the housing?

WHere is the arm that vents Py on overspeed?

Thanks! JOhn
Sorry, I can't answer your questions. This was at a 'mom & pop' maintenance facility in Mississippi.
The mechanic, who started working on Bell aircraft early in the Vietnam war, did give me a detailed description of how it all works and what does what, but I honestly don't remember much of it now. Pretty unusual to see one in pieces outside of the factory though.
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