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PostPosted: Wed Jun 21, 2017 11:21 am 
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Hi all, this is a continuation of the "Supercharger output seems inconsistent," thread. However, we've moved to the subject of propeller governors so I thought this should warrant a new thread.

Its important to highlight again that,
Scott - A2A wrote:
The engines in the Connie (and 377) us the Microsoft built-in tech..


CAPFlyer, you stated,
CAPFlyer wrote:
on electrically actuated props the prop pitch MAY change when actuated while on the ground and the engine stopped. This is because some electric props directly pump fluid in/out of the prop dome and governor instead of moving the governor control only. The prop on the L-049 and B-29 are this type.


Starting with the second statement, the L-049 does not have electric propellers. The A2A manual mentions "Hamilton Standard 3-blade 'Hydromatic' adjustable/constant-speed, full feathering (later reversible)" propellers (p.53). Going beyond this, the FAA TCDS gives two options for propellers... both with the same Hamilton Standard Hydromatic 33E60 Hub.

The first issue is that these propellers require oil pressure to move the blades. The first method of obtaining this pressure is the engine driven mechanical oil pump, which requires engine rotation to function. The next method is via the electrically powered feathering pump, but operation of this pump is via the feathering buttons (not the propeller RPM switches) and it bypasses the governor entirely. The engine isn't turning, so we know the mechanical pump is out, which means only the feathering pump is available. With my testing, the blade pitch only changes with the batteries connected, so we must be using the feathering pump. If I was indeed using this pump, then upon activation it would continue to provide pressure until the blade pitch either reached the feathering stop, or the full fine limit. As it currently functions, moving the RPM switch to 'increase' reduces the pitch of the stationary blades. When the switch is immediately released, the change in blade pitch ceases. This behavior is the same for 'decrease' and increasing blade pitch. This suggests that the governor is controlling the feathering pump (or this is default MS engine coding with an RPM switch instead of lever :) )

The next problem is the workings of a constant speed governor. Governors work by controlling the propeller pitch to change the induced drag acting against the torque of the engine to correct deviations from selected RPM. They do this by detecting either an "Underspeed" or "Overspeed" condition and correcting the error until the RPM returns to an "Onspeed" condition. With the engine stopped, any RPM selection other than 'Feather' will result in the governor detecting and underspeed situation, and the response will always be to drive the propeller blades to fully fine pitch. This is regardless of the selected RPM because all RPM>0.

This excludes Feather operation, but again feather operation excludes the governor anyways.

From some quick research on Curtiss Electric propellers, it does seem that they can be placed into a manual mode where the prop pitch can be changed by the crew, without governor control. So, I fired up my United colors A2A B-377 (The one that I know has Curtiss props). Surprisingly, it has improved governor operation over the Constellation. With an engine stopped, RPM lever (and switch) commands result in no change in visible blade pitch. The blade pitch always stays in full fine, because the governor is always Underspeed.

The only exception is through use of the overhead feather switch, which immediately drives the stationary propeller to Feather. Turning off this switch does the exact opposite.

My next test will be to to try and feather the stationary Hydromatic Constellation propeller with the Feather button. I think that should work, assuming the feathering pump is electric.

This is all purely conversation, now that we know what's going on with the simulation (perhaps I should have moved this out of tech support? Oops)


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PostPosted: Wed Jun 21, 2017 11:30 am 
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Just checked the Feathering of the Constellation, it appears to work as intended.

Here's some online resources talking about some of the function of Hydromatic Propellers

http://aircraftspruce.com/catalog/pdf/13-07569.pdf
http://www.enginehistory.org/Propellers/HamStd/hamstd.shtml
http://okigihan.blogspot.com/p/hamilton-standard-hydromatic-propellers.html


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PostPosted: Thu Jun 22, 2017 8:51 am 
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flapman wrote:
From some quick research on Curtiss Electric propellers, it does seem that they can be placed into a manual mode where the prop pitch can be changed by the crew, without governor control. So, I fired up my United colors A2A B-377 (The one that I know has Curtiss props). Surprisingly, it has improved governor operation over the Constellation. With an engine stopped, RPM lever (and switch) commands result in no change in visible blade pitch. The blade pitch always stays in full fine, because the governor is always Underspeed.
Yeah, and interestingly "many" electric props have this specific functionality: they can be controlled either automatically (in constant speed mode), or manually, in selectable pitch mode.

A classic hydraulic propeller in practice never has such feature, if we don't count the beta-equipped which have the necessary position feedback, for a simple reason. The system actually balances multiple opposing factors trying to alter the pitch either way, and the governor is not actually at on-speed condition when the RPM is at selected. A small but measurable internal leakage needs to be balanced out, and in a fine-acting governor system, the governor is actually a little bit underspeeding to compensate for it. The system constantly, and naturally, re-adjusts, so it is ill-suited for direct pitch control, whereas an electric prop is built so that it is more locked in place.

-Esa


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PostPosted: Thu Jun 22, 2017 2:55 pm 
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Yup,

When I was researching the Curtiss Electric propellers, I found an article describing the function of the hubs and governors. It does state they the blade pitch can be directly controlled by bypassing the governor entirely.

AKar wrote:
...whereas an electric prop is built so that it is more locked in place.

That's exactly how the electric hub was described. An electric motor turning the blades via bevel gearing, with a friction brake to lock the blades when the motor wasn't energized. This prevented inertia from the electric motor preventing fine control, and to resist the aerodynamic forces preventing the blades rotating into a coarse pitch.

Of course, the A2A Electric Prop Strats only have governor controlled blade pitch (no manual selection)... and that's fine.

As you described, the way a hydromatic hub operates with completely different logic from an electric hub. Excluding BETA (which is a turboprop feature and not in any of our simulated aircraft) the system does not directly control blade pitch. It only corrects errors. Interesting remark about the system compensating for small leakages by being slightly underspeed!


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PostPosted: Thu Jun 22, 2017 8:54 pm 
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Maybe there's some bypass switch on the CV-240 or something, but when we used to exercise the props with the engine stopped, if you pulled the lever back and held it, the feather pump would kick on and cause the prop to actually begin moving, separate of the feather button. You could also run the feather switch obviously, but you could also drive the props to their coarse stops without activating the feather pump. You could then either pull out the feather switch (driving it to "unfeather") or you could push the levers back forward and get the same thing happen. We did not have the Curtis Electric props on our planes (all were ex-USAF T-29s), but I distinctly remember several times where the crew checked the props with the engine off that way and were able to use the feature to make sure the stops were properly set.

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PostPosted: Fri Jun 23, 2017 12:43 am 
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CAP,

I don't know for sure, but that sounds like a 'feather' detent in the prop levers. When I flew the PA-44 Seminole, we had no feathering switches, only a feathering range at the bottom of the RPM lever (we also had no feathering pump).

You mentioned that they "pulled the lever back and held it." Did they have to hold it because of spring tension on the lever at that low position?

It's cool you have experience with a CV-240..


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PostPosted: Fri Jun 23, 2017 10:47 am 
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The Convairliners in their piston-powered guises had small levers with microswitches on each end of travel instead of more standard switches (like what is on the Connie and B377). They still just actuated the electric motor for the governor, however as I said, there was a system that allowed the feather/unfeather pump to be activated if you held it for a certain amount of time to drive it to the normal limits (but not to feather). There are separate Feathering buttons on the overhead that drive the props to feather and then when pulled out (past their normal position) activates the unfeather function of the feathering pump.

Short aside - the feather pump is often used with tail-draggers (like the DC-3 or Beech 18) as if you run the pump until it starts to move the prop, you ensure both the prop and the engine have some oil in them prior to start. In effect, it's a "poor man's" pre-oiler. On aircraft like the Convairs it's of less use because the engine sits more or less level and doesn't have the problem of oil draining out of the dome. However, if the engine's not been run in a while, using the feather pump to drive the prop to feather and back has a similar effect.

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PostPosted: Fri Jun 23, 2017 10:56 am 
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If you watch this video of a CV240 takeoff from Miami (I think in N150PA, one of the 240s we had in 2004), you'll see the two little black levers in the center of the pedestal with the yellow lights on either side. These are the prop levers/switches and the lights are the high/low pitch stop lights. https://www.youtube.com/watch?v=NunJaQSiibo

You can also see them in the middle of this pic (kinda) -

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