Prop governor follies

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Rich
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Prop governor follies

Post by Rich »

Along with having my recent prop overhauled, I also had the (Woodward/Ontic) governor overhauled. After installation, there are adjustments that typically need to be made, iterating between flight tests:

1. The safety-wired screw/jamnut that sets the max RPM the governor will allow.
2. The throttle cable linkage so that the forward throttle travel limit does not defeat the max RPM screw setting. This is the combination of where the cable is held by governor arm pinch bolt and the screw fitting for the forward end of the throttle cable.

But there was a complication that baffled me until I was blessed with a middle-of-the-night inspiration that seems impossible but turned out to be correct. The problem behavior was that the RPM was properly limited to 2700 RPM when the engine was "cold" but not after it had flown at cruise power for a bit. Instead it was limiting it to about 2660 RPM. Now when I say "cold", the ambient temperature would be around 40 degF and oil temp 90-100 degF. And this is a multi-weight 20W50 oil.

I'll skip the logic by which I came up with the root cause and instead offer the theory and correction. The theory was that a warmed-up engine compartment caused things to heat up enough to affect the throttle cable linkage such that the throttle forward travel was OK when the engine was "cold" but was limiting the movement of the governor control arm when warmed up. A careful check post flight showed this to be correct. Further adjustment of the throttle cable linkage has corrected the problem.

Here are the adjustment points in question:
IMG_0646.jpeg
But wait, there's more - on a different subject. We've had discussions about reducing RPM in a glide to improve glide ratio by pulling back on the prop control. Some folks swear by it, others (including me) see no effect. I postulate that this may depend on which governor you have. Note the following picture, with the throttle pulled full back:
IMG_0647.jpeg
The screw at the bottom of the picture is the minimum RPM limit adjustment. This shows that for the type of governor I have, there is not enough throttle travel to reach that stop screw. Hence the minimum attainable RPM is not as low as the governor is capable of forcing. This is the reason that my minimum RPM is higher than idle RPM at reasonable glide speeds. Perhaps other make/model governors do not require as much travel to fully traverse the required movement. Something as simple as a shorter lever arm in the design would do it.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Steve
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Re: Prop governor follies

Post by Steve »

Rich:

I have the same model governor, but can't recall whether my prop governor lever reaches the low RPM limit stop with the PROP control pulled all the way back, but if not, it is certainly very close to it. One thing I noticed in your first photo is how far back the threaded adjuster on your cable is set. I am wondering if that is what is limiting the forward travel of the lever. Compare with my installation:
Prop control cable
Prop control cable
Of course, there are also mechanical limits to the PROP control lever in the control quadrant, but these should not limit the control motion - only the stops on the prop governor (or the fuel injection unit in the case of MIXTURE and THROTTLE controls). Is there a possibility that your PROP control lever is hitting the back of the slot before the prop governor lever hits the low RPM stop?

I'm not sure if it is just the photo angle, but it also appears that the movable end of the cable is just barely being held by the pinch bolt. My A&P/IA explained that any control cables not terminated by a screw-on cable eye (like the mixture and throttle cables are) should have a bend put in the end of the wire. The reason for this is to prevent the cable from slipping completely free from the control arm should the punch bolt loosen.

Just some thoughts...
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Re: Prop governor follies

Post by Rich »

Steve,
The relationship between the arm movement and the throttle cable travel is a combination of where the Bowden cable is trapped by the pinch bolt and the adjustment of the forward cable housing mount. I could rework mine so that the pinching occurs further up the cable and then the housing would come down further. Currently there is some availability to this in mine and would result in more downward travel. The throttle lever does hit the limit of the slot on the low end and doesn't quite reach the end of the slot on the high end. There's supposed to be a small gap on both ends. I'll probably try this out, but the effect of engine temperature makes getting the max setting right trickier than expected.

In reality, the gap to the lower stop is larger than it appears in the picture and it doesn't seem likely I'll be able to reach it. I'd be interested in what yours looks like.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Prop governor follies

Post by mhoran »

PXL_20210709_193605522.jpg
This is what mine looked like prior to overhaul. Not sure what it looks like now, but I should (fingers crossed) know soon.
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Re: Prop governor follies

Post by Steve »

Rich:

Unfortunately, I don't have any photos of my governor with the PROP control at low RPM. I'll try to remember to get one during my next oil change (in a few hours). One weird issue I had after my prop governor was overhauled was that I couldn't reach 2700 RPM, even with the high RPM screw all the way out. I called the rebuilding shop and talked with the tech that worked on my governor. Turns out that he was using incorrect data as to the gear ratio of the governor drive gear in the engine, which resulted in him clocking the control lever incorrectly. He gave me instructions on how to correct this, since I didn't want to ship the governor back to the West coast.

Re-clocking the lever forward one spline on the governor shaft solved the issue. In fact, the adjustment nuts came out at the same place on the cable sheath as before the overhaul, the high RPM adjustment screw extended the same amount, and the witness mark on the Bowden cable matched the prior pinch bolt location to get everything within tolerance.

Before I realized that the governor was assembled wrong, I thought that the RPM displayed on the VM-1000 might be off (of course that could have meant that I was overspeeding the prop before :shock: ). So I bought a Trutach optical tachometer:

https://www.aircraftspruce.com/catalog/ ... rutach.php

Turns out that the tach function on the VM-1000 was quite close to what the Trutach reported, with the VM-1000 reading about 5-10 RPM lower. So I adjusted the prop governor so that the VM-1000 reads 2690 at full RPM. The Trutach is a nice unit because you can validate the calibration by pointing it at a fluorescent light, and it can be used in flight - just rest it on the glare shield.

If you, or anyone on the Forum needs to borrow an accurate tachometer, PM me.
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Re: Prop governor follies

Post by Rich »

I've had to re-clock mine in the past. It's not particularly hard but it's best to avoid unhooking the arm from the spring. This time around it came pretty close to needing to be re-clocked. I had to back the stop screw two full turns, leaving only a couple of threads protruding. Because the VM1000 only displays to the nearest 10 RPM it dances a bit. Mine bounces from 2690 +/10 RPM. In general, most of the displays exhibit similar behavior.

What my experience shows it is possible to adjust the cable travel so that the max setting is OK when "cold" and not right when warm. The technique is to get it where is just barely OK when cold, then move the throttle lever back a wee bit and take up the slack with the cable housing adjustment. The symptoms of it not being right are as I've described.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Prop governor follies

Post by Steve »

The variability with temperature is unusual. I wonder if lubricating the cable or installing a new cable would resolve the issue.
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Rich
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Re: Prop governor follies

Post by Rich »

Steve wrote: Tue Feb 07, 2023 6:06 pm The variability with temperature is unusual. I wonder if lubricating the cable or installing a new cable would resolve the issue.
Not likely. The cable moves smoothly with no binding. I confirmed that the arm was up tight against the stop before flight. After landing I popped the cowl off and saw that there was a small gap present. I didn't have a feeler gauge handy but that's what it would take to measure the gap. Repeatedly working the lever fore and aft confirmed that this gap was going to stay until everything cooled back down. This last had been confirmed earlier by the fact that every day I took it out, the behavior was the same. Full RPM on initial climb out, 40-50 RPM lower on subsequent full-throttle climbs. Incidentally, just once around the pattern wasn't enough to cause the problem. It needed to fly for 15-20 minutes before things got warm enough. After my minor adjustment it now behaves the way it should.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Prop governor follies

Post by Rich »

Subsequent to earlier discussions I decided to revisit the installation. There is a procedure described in the AMM for this adjustment. (Like many, I check the manual after the fact, if at all.) This procedure, without explanation, quantifies doing what I found needed to be done on my own. Nevertheless, I also made it a point to try to obtain the lowest possible RPM setting I could manage. Here are the steps:
1. Released the cable clamping fitting on the governor arm.
2. Released the cable housing forward adjustment nuts and moved that fitting as far forward as possible to ensure it wouldn’t factor into impeding the low RPM adjustment point.
3. Pushed the prop lever forward to the stop. It comes up against a transverse bolt across the inside of the quadrant.
4. Rotated the governor arm full aft, sliding the clamp on the cable to where the arm came up against the governor high RPM stop, making sure the prop lever stayed full forward.
5. Moved the prop control lever back the 3 mm specified as a minimum “bounce” (this is the term in the manual). This resulted in about a 2 mm gap between the governor arm and the high-rpm stop. (I used a piece of tape on the quadrant and a sharpie to mark the 2 positions of the control lever.) This 2 mm was just about what I saw in my original tweaking. However, I had somewhat more than the 3 mm at the lever at that time.
6. Moved the governor arm again rearward to the high RPM stop, sliding the clamp along the cable as necessary. Made sure the prop lever was still at that 3 mm “bounce” point.
7. Tightened down the clamp and the two nuts on the forward cable housing. Rechecked the prop lever forward limit was still at the desired 3 mm back.

The result is the prop lever has the maximum possible movement and the furthest low-RPM limit bias I could obtain. The result is that the prop lever still does not come any closer than 8 mm to the low RPM stop. I haven’t had a chance to retest what that gives me in flight as low RPM. But the directions for adjustment in the AMM regarding the low RPM limit of the prop control lever are irrelevant in my case.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Prop governor follies

Post by mhoran »

As luck would have it, I'm having the exact same issue that Rich reported with my newly overhauled governor and engine R&R. It was rather chilly when I took off from Augusta, GA yesterday -- as it was on the day they test flew the plane. I was able to reach 2700 RPM on my first takeoff:

https://apps.savvyaviation.com/flights/ ... 5553e5d4a7

However, on my second and third legs (I failed to re-insert my SD card in time to capture the start of my second leg), I reached a maximum of 2640 RPM:

https://apps.savvyaviation.com/flights/ ... 2e3c4f7779

Hopefully this procedure works for me.
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