Three maintenance questions

[AndreiC]

Hello,

I just finished the annual at Oasis Aero in Willmar on my 1970 M20E, and three squawks ended up not being resolved. May seem small, but I'd like to take care of them and I hope maybe the MS people have some suggestions on how to proceed.

1) My prop governor does not give me more than 2610/2620 at full prop. I asked Oasis to look into this, they said they "gave the rear bearing on the governor one turn" (whatever that means) and that should get me closer to 2700. When I first attempted to fly the plane, it would now only go to 2450. They checked and said the cable was now not allowing the governor control to reach its full stop, rerouted the cable. Back to 2610/2620, no change from before. They suggested that the only option at this point is sending the governor to be overhauled. Any opinions on this? Is there something I can still fiddle with? I read about a screw on the back of the governor that gives you 25 rpm with each turn, can someone explain this?

2)The door seals on both doors were shot (the main door was leaking water in heavy rain), so I asked Oasis to replace them. I ordered T-9088 from Brown Aircraft supply. The baggage door is fine now, slightly stiff but ok. However, the main door is almost impossible to close; from the outside I have to lean into it with all my weight, from the inside I have to pull both the back end of the door and the top with all my strength to barely get it to close. I fear I will damage something. Also, on one recent flight the top part of the door popped open, even though I was convinced I had closed both sides. Not a good experience.

The folks at Oasis said that it takes a while for the seal to compress, and I should just wait. The seal has already been there almost two weeks. Is there something that can be done to speed up the process? Or do I need to trim a bit of the seal? As of now, with the door closed, it sticks above the fuselage about 1/16" all around.

(I am worried because on my old plane, a Piper Cherokee, I installed a seal myself and had the same problem. Half a year in the door was no easier to close. Ended up getting a different seal and having my mechanic at the time put it in, and suddenly all was well.)

3) The ram air door opens in flight uncommanded, from the air pressure. The lever for power boost moves by itself about 1/2" from the closed position. It is annoying because on take-off and landing (when the gear is down) I get this warning light telling me power boost on, and I have to focus on it instead of focusing on one of the critical phases of the flight. Oasis fiddled with it but said there is no clear adjustment that can be made, and the only solution is to replace the cable (??) which is unavailable. Any suggestions?

Thanks to all, this is a wonderful forum to be in.

[EricJ]

52 minutes ago, Andrei Caldararu said:

Hello,

I just finished the annual at Oasis Aero in Willmar on my 1970 M20E, and three squawks ended up not being resolved. May seem small, but I'd like to take care of them and I hope maybe the MS people have some suggestions on how to proceed.

1) My prop governor does not give me more than 2610/2620 at full prop. I asked Oasis to look into this, they said they "gave the rear bearing on the governor one turn" (whatever that means) and that should get me closer to 2700. When I first attempted to fly the plane, it would now only go to 2450. They checked and said the cable was now not allowing the governor control to reach its full stop, rerouted the cable. Back to 2610/2620, no change from before. They suggested that the only option at this point is sending the governor to be overhauled. Any opinions on this? Is there something I can still fiddle with? I read about a screw on the back of the governor that gives you 25 rpm with each turn, can someone explain this?

There's a high rpm stop on the governor that limits the travel of the control to prevent exceeding redline rpm.   It sounds like they're saying that they backed it out all the way and you're still not getting full rpm.   The fact that they had to go back to eliminate an interference issue with the cable is not confidence-inspiring.

The first thing to do is get an optical tachometer to make certain the engine rpm is low and it's not just the tach that's reading low.   In other words, check that the tach is reading correctly.   Optical tachometers are not expensive.

If the rpm is correct, then there are other potential considerations, particularly that the engine is not making full power, or the propeller low-pitch stop has gone out of adjustment (which is very unusual).   What sort of peak rpm do you see during takeoff or climbout?

[PT20J]

As @@EricJ said, always check the accuracy of the tach first.

Since things have been messed with (and assuming that the optical tach confirms that it is not making 2700 rpm), the first thing to check is the control cable rigging. The control arm should be able to contact the high rpm stop on the governor with some cushion between the control and the instrument panel. In other words, you don't the control to bottom out at the panel end. If that is OK, then back the governor high rpm set screw all the way out and fly the airplane setting the prop speed to 2700 rpm in cruise flight. Land without touching the prop control and adjust the set screw to just touch the control arm. Fly again to retest and safety wire the screw.

The governor doesn't take effect until you have some airspeed. During run up the prop will be at its low-pitch stop and should make somewhat less than 2700 rpm at full power. Usually around 2650. 

If the door seal is extruding out the gap between the door and the airframe it was installed too close to the edge. The centerline of the circular part of the seal should lie over the rivet line where the inner and outer door pieces are riveted together. 

Skip

[AndreiC]

@PT20J Thanks for your reply! My tach (in the panel) is a digital one, made by EI. I assume that is expected to be accurate, isn't it?

Regarding the door seal -- what do you mean by it extruding out the gap between the door and the airframe? It is not sticking out at all. It is the door itself that instead of being flush with the airframe, the way it was before, is now raised 1/16" all around. As if the seal is taking too much space between the airframe and the door.

A.

[ZuluZulu]

22 minutes ago, Andrei Caldararu said:

@PT20J Thanks for your reply! My tach (in the panel) is a digital one, made by EI. I assume that is expected to be accurate, isn't it?

Regarding the door seal -- what do you mean by it extruding out the gap between the door and the airframe? It is not sticking out at all. It is the door itself that instead of being flush with the airframe, the way it was before, is now raised 1/16" all around. As if the seal is taking too much space between the airframe and the door.

A.

By optical tachometer, this is what they mean:

https://www.aircraftspruce.com/catalog/topages/trutach.php

It will help diagnose whether there's a problem with the instrument or not.  A local mechanic probably has one, or you might be able to find one to borrow from one of your local fellow airplane owners.

[hammdo]

That tach tool is the same one I used to confirm my lower reading tach gauge. My tach gauge was 50 or so RPM low…

-Don

[AndreiC]

I repeat my question: I can easily imagine how a mechanical tach can be off. But it is hard for me to imagine how a digital tach (mounted in the panel) could be substantially off. Have people experienced such digital tachs being inaccurate? I thought they function by counting pulses from the magnetos.

This is the unit I have in the panel:

https://www.aircraftspruce.com/catalog/inpages/ei-digitalgraphic1.php

[M Terry]

If your ram air is like mine, the cable is easy to adjust. May need to disconnect a light to reach it. Just get is so it goes over center when closed

[Danb]

Andrei although you have a quality electronic tach, any equipment can be off new or old especially electronics. I wouldn’t assume since you have a new digital tach that it is correct. Check and verify twice.

D

[Fritz1]

as stated before make sure governor arm hits high rpm stop before the vernier bottoms out in the cockpit, make sure you cycle the prop 5 times before takeoff, after I bough my Bravo I could not get the engine to make takeoff rpm below 120 kt, had prop overhauled, it turned out flat pitch was set 0.5 degrees too high in the prop, I have heard that the ram air function on the E/F is more trouble than it is worth, some people just close it off at the cowl, the door seal appears to be too thick in some areas and create excessive pressure so door does not close well, it will take a very long time for the rubber to compress sufficiently, start looking for the high pressure areas , see if you can move the seal a tad or remove small amounts of material in these high pressure areas, trial and error, you may damage the door seal trying to get the right fit, after you bought the second seal you will be a door seal fitting expert with 15h expertise under your belt, surprised Oasis did not get it right

[ZuluZulu]

2 hours ago, Andrei Caldararu said:

I repeat my question: I can easily imagine how a mechanical tach can be off. But it is hard for me to imagine how a digital tach (mounted in the panel) could be substantially off. Have people experienced such digital tachs being inaccurate? I thought they function by counting pulses from the magnetos.

This is the unit I have in the panel:

https://www.aircraftspruce.com/catalog/inpages/ei-digitalgraphic1.php

I repeat my answer: the optical tach will eliminate any problem with the instrument.  It will give you an independent, accurate reading, which is what you want to diagnose the problem, instead of trusting whatever the digital instrument says just because it's digital.  I have the exact same tach as you plus a JPI EDM-830 and my mechanic still used a TruTach to troubleshoot prop governor settings.  Digital doesn't mean it's 100% correct every time, every situation, every condition, every installation.  And it's very cheap insurance against throwing money at a problem before you know whether there's a problem or what the problem is.

[Igor_U]

1 hour ago, ZuluZulu said:

  I have the exact same tach as you plus a JPI EDM-830 and my mechanic still used a TruTach to troubleshoot prop governor settings.  

Copy that. I'm curious was there any difference between three tachs? Asking for a friend...  

[ZuluZulu]

1 hour ago, Igor_U said:

Copy that. I'm curious was there any difference between three tachs? Asking for a friend...  

If I remember right they all disagreed by about 10 RPM  

[EricJ]

Just search on Amazon for "optical tachometer".    There are a lot of options that are very accurate.

[MikeOH]

I have two electronic tachs: one in the G3 Insight and one in a Horizon...they never agree; maybe 10-20 rpm difference.  I suspect this is because there is electrical noise on the 'signal' from the mags and the tachs occasionally 'count' noise as a mag pulse.

I'd use an optical tach to be certain.

[PT20J]

11 hours ago, Andrei Caldararu said:

Thanks for your reply! My tach (in the panel) is a digital one, made by EI. I assume that is expected to be accurate, isn't it?

Probably, but it's always a good idea to check the instrumentation before adjusting things.

11 hours ago, Andrei Caldararu said:

Regarding the door seal -- what do you mean by it extruding out the gap between the door and the airframe? It is not sticking out at all. It is the door itself that instead of being flush with the airframe, the way it was before, is now raised 1/16" all around. As if the seal is taking too much space between the airframe and the door.

I misunderstood your original post and thought the "it" that was sticking above the fuselage was the door seal but now I understand that you were talking about a door/fuselage misalignment. I would check the seal placement as I described. If it is right, I'd just keep the door closed for a few days and see if it compresses. When I installed mine it was pretty tight for a couple of weeks until the T-9088 took a set. If your door is like mine on the J, you might also have to adjust the top latch to loosen it a bit.

[FlyingDude]

17 hours ago, Andrei Caldararu said:

is a digital one, made by EI.

I have the same R-1 RPM tach by EI. During a panel reorg, it had to be moved. When it was installed in the new place, it started reading -100!!! The wiring was moved just by aligning it with another bundle. The part was unmounted very delicately and mounted in the new hole also very gently. EI told me that the internal comparators can get corroded over time so it had to be overhauled.  At a charge, of course... I paid $50 more and got a refurb one.

In the meantime, take off rpm were verified with a laser tach to be 2700, so I didn't have to mess around with the prop or governor...

[Jsno]

I replaced my ram air cable with bowden cable that I bought from Spruce.  It sells by the foot and works great.  I replaced my door seal with Aircraft Door Seal, then I closed it a warm hangar for a day.  I had to push from the outside and carefully check that it did not get folded under while closing.  Two days later it works great.

Door Seal, Entry Door. Mooney M20 Series. ADS-M1201

 

$149.95

[Pinecone]

22 hours ago, EricJ said:

Just search on Amazon for "optical tachometer".    There are a lot of options that are very accurate.

And they are easy to check.  Point at a fluorescent light, and in the US and Canada, it should read 3600 RPM.  

[MikeOH]

1 hour ago, Pinecone said:

And they are easy to check.  Point at a fluorescent light, and in the US and Canada, it should read 3600 RPM.  

@Pinecone

Have you done this?  I would, a priori, think you should read 7200 rpm as the flicker rate is 120 Hertz, not 60 Hertz.  A single 60 Hertz sine wave cycle reaches TWO peaks (one positive, one negative) resulting in the 120 Hertz flicker rate.

EDIT:  Never mind.  The tach is dividing by two for the two blades...so, the 3600 rpm makes sense.

[jetdriven]

The T-9088 door seal is for the M20R and later planes. The proper seal is BA-1608 but Brown Aircraft  stopped making it. 

[EricJ]

55 minutes ago, MikeOH said:

@Pinecone

Have you done this?  I would, a priori, think you should read 7200 rpm as the flicker rate is 120 Hertz, not 60 Hertz.  A single 60 Hertz sine wave cycle reaches TWO peaks (one positive, one negative) resulting in the 120 Hertz flicker rate.

EDIT:  Never mind.  The tach is dividing by two for the two blades...so, the 3600 rpm makes sense.

No, you're right.    Mine reads 7200 when pointed at cooperating lights.   Sometimes the manual for the tach suggests putting a reflective piece of tape on one spot on the rotating piece (often provided with the tach) so that it counts only the brightest reflection when the tape goes by.    I've found that with props or model helicopter blades or similar things, it'll often get enough reflection from each blade that you have to account for that in the measurement.   In other words, if it reads twice as high as you think, it's getting both blades so just divide by two.   If you have a three-blade prop, it might see all three and read 3x the actual rpm.   It's not hard to figure out what it's doing, though.

Ten years ago I did a long, very nerdy video with horrible audio on a signal processing topic (aliasing) using an optical tach and a model helicopter.   It's a little tricky to use, but gets the job done.
 

 

[PT20J]

23 hours ago, jetdriven said:

The T-9088 door seal is for the M20R and later planes. The proper seal is BA-1608 but Brown Aircraft  stopped making it. 

I called Brown and the T-9088 is a replacement for the BA-189-139 which was foam core covered with a thin rubber and lost elasticity over time as the foam aged. The T-9088 is called out in the M20J IPC.

[MikeOH]

1 hour ago, EricJ said:

No, you're right.    Mine reads 7200 when pointed at cooperating lights.   Sometimes the manual for the tach suggests putting a reflective piece of tape on one spot on the rotating piece (often provided with the tach) so that it counts only the brightest reflection when the tape goes by.    I've found that with props or model helicopter blades or similar things, it'll often get enough reflection from each blade that you have to account for that in the measurement.   In other words, if it reads twice as high as you think, it's getting both blades so just divide by two.   If you have a three-blade prop, it might see all three and read 3x the actual rpm.   It's not hard to figure out what it's doing, though.

Ten years ago I did a long, very nerdy video with horrible audio on a signal processing topic (aliasing) using an optical tach and a model helicopter.   It's a little tricky to use, but gets the job done.
 

 

Cool video!  

However, I don't think a simple optical tach is a sampled data system so I don't think there is any aliasing going on; there is only one frequency (the prop) involved.

I'm not, having never owned one, familiar with optical tachs but I would have expected ones designed for aviation to at least be designed to divide by two (and, a switch for selecting 3, and 4).  A general purpose optical tach would read 7200 rpm, however.

[Bartman]

I downloaded an acoustic tachometer called Engine RPM for my phone. It is intuitive and seems to work well. I compared it to an optical tach from a buddy and they were spot on.