B377 Calibrating the potentiometers.

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Guus
Airman
Posts: 20
Joined: 10 Mar 2019

B377 Calibrating the potentiometers.

Post Guus »

Hi All,

The "Pilot's Handbook" says on pg. 52(55) "The four calibrating potentiometers
provide a means of setting a uniform manifold pressure on all engines."
But HOW to achieve that ?! WHAT shall I calibrate to what ?! When I follow the chk.list "GROUND TEST" pg. 86(89) # 6: set MAP to ambiënt press. say 30" (2.050 RPM).# 7: Turbo Booster to 7, and ALIGN the POT.METERS !
TO WHAT ?! RPM becomes 2.660 MAP 58,6" Torque 217 PSI Exhaust back press. 48,7". And what about 2" MAP increase ??? I do not get this !

Regards, Guus.

TreeTops
Technical Sergeant
Posts: 539
Joined: 07 Apr 2010

Re: B377 Calibrating the potentiometers.

Post TreeTops »

Hey Guus,
Great to see someone else tackling the mighty Strat.
Check this link from Scott
viewtopic.php?f=80&t=26318&p=200745&hil ... os#p200745
There are a few others posts about this also.

Due to the Strat's old age (a2a) and complexity, there are a few loose ends where the manual needs updating or a system needs some update attention, you will find some little quirks. Fortunately most of these have been discussed on this forum.

I am hoping when A2A get to upgrading it for P3Dv4 these systems will be looked at. I know there will be more knowledgable users at the release than when it was first released, and also at the COTS release.

Unfortunately a lot of the high milage pilots for the Strat now use P3Dv4 and can't fly it anymore. I still have FSX installed just for flying the Strat and some other older planes. Regardless, there is a lot of experience found among the members here who can answer your questions.

A tip for searching the forum.
Turbo will not include Turbos in the search, so use the wildcard *. ie Turbo* will find both Turbo and Turbos.
Or Calib* will find Calibate and Calibration.
Cheers
Trev

TreeTops
Technical Sergeant
Posts: 539
Joined: 07 Apr 2010

Re: B377 Calibrating the potentiometers.

Post TreeTops »

Just a follow up, since I haven't flown the old girl in a while and after a new PC install I ran a calibration on the engines which only have 0.6 hrs. Due to the confusion in the checklist I had never really wrapped my head around this procedure and I mainly flew the Guppy so it was good to revisit this again. I don't think anyone has written in the forums about what we experience in the COTS B377 so I gave it a crack.

The checklist says to do the engines one at a time but I did all 4 together to understand what was going on and compare across all 4 engines.
I set up on an airfield in the centre of Australia YAYE Connellan which is 1635ft elevation. Pressure was standard at 29.92/1013.
According to the checklist after setting the Turbo Boost to 7 percent, we should look for Manifold Pressure (MP) increase of 2", plus 1.5" for every 1000ft airfield elevation. In my case 1635ft / 1000ft *1.5"MP = 2.45"MP or say 2.5"MP.
I need to see 30" (29.92 Atmos. Pressure) + 2" (turbo boost) + 2.5" (elevation component) = 34.5"MP

Note 1 : Point 6 in the checklist says we should see approx. 2050 RPM. Remember the props are constant speed so we cant use RPM to adjust the calibration of the turbos. You can have one turbo cranked right up and the prop will still read the same RPM compared to the other engines.

Note 2 : Remember these are accusim engines and you can have a fresh No. 1 engine, a tired No. 2 engine and No's. 3 & 4 are in different states of damage. What we are calibrating is a common turbo boost effect for all 4 engines. We might need a different throttle setting for engine No. 2 than No. 1 because it is tired. Damage in engines 3 & 4 likewise. So the engine settings to get 30" MP may be different but regardless of the settings, 30" for all 4 engines is what is important.

The calibration : (in my test case all 4 engines together, checklist suggests 1 engine at a time)
1. Turbo selectors set to take-off (no turbo available)
2. Turbo boost lever set to 0 (0 percent)
3. Bring all 4 engines up to 30" MP (equal to atmos. pressure)
4. Turbo selectors set to climb-cruise (turbo now available)
5. Increase Turbo Boost lever to 7 percent.
6. Look for an increase in MP to target number. (34.5" in my case)
7. Adjust each Turbo Boost Potentiometer (Pot) to achieve target MP. (in my case 34.1" was the maximum I could get from engine No. 4 with the Pot set to +50.) I ended up with +38, +25, +20, +50. (seems to be about 5 on the Pot for ever 0.1" MP).
8. Turbo selectors set to take-off (return to 30" MP even with the Turbo Boost lever at 7%, ie no turbo available)
9. Turbo boost lever set to 0 (0 percent)

Calibration outcome :
For my engines (new install) I was not able to achieve my target MP of 34.5" MP. Given that the Pots were all adjusted up high, I went go back and set them closer to the Pots midrange of 0.
I ended up dropping all 4 Pots -30 and looked for 33.7" MP. I ended up with Pots at +8, -3, -8, +20.
With the knowledge I now have, I still hesitate to test each engine individually because if the last engine is at the Pot limit and not reaching the target MP, the other 3 engines would need to be set lower to match. Having said that, the maintenance crew and flight engineer would know the performance of each engine and would already know which engine is limiting MP target.
So in a general flight example the flight engineer know that No.3 Pot is set highest so that is the MP that the other engines would be set to match. As engines age, these Pot settings will change slightly which is normal. If one Pot suddenly needs a large adjustment it would flag some issue that would need looking at.

Interestingly, since the Turbo Boost Selector markings are 0 - 10, when it should be 0 - 100 percent; where all the confusion stems from, I tried adjusting the Turbo Boost lever to 8% and 9%. I got 0.5MP increase for each 1% increase on the Turbo Boost lever. So at 9% I was able to achieve 34.7" MP with the Pots set at +8, -3, -8, -20. Back off the Pots to 34.5" MP would mean I would get new settings -4, -15, -20, +8.
I'm not sure if we should be using 9% Turbo Boost lever to reach the target MP and have the Pots centred around a value of 0 or we should just reach an MP setting at 7% which gives Pots values centred around 0.

After adjusting the Pots and still with Turbo Boost, check that all 4 engines are achieving the same Torque PSI.

I hope this helps and others may chip in with their experiences.

This old plane is so fascinating I really hope it gets a freshen up into P3Dv4 soon.
Cheers
Trev

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Jacques
Senior Master Sergeant
Posts: 2139
Joined: 26 Jun 2011

Re: B377 Calibrating the potentiometers.

Post Jacques »

I remember coming to grips with this a few years ago and just a few months before jumping to P3Dv4. Which kind of underscores the depth and complexity of the Stratocruiser! What an absolutely fascinating aircraft! My final checklist (for the part you are discussing) read like this:
Calibrating turbos two engines at a time:
Set turbo controls to Climb and Cruise
Advance throttles to 30” MP
Advance turbo control to achieve 36”MP (~ 10% )
Adjust pots to equally calibrate engines to 36”
Stop calibration if CHT climbs above 232 C

I usually did this two engines at a time as it was quicker and you could avoid raising the CHT past the 232 C mark. but you still needed to move quickly! All these amazing notes I collected, and wrote, fill a fairly thick binder. I guess I kind of liked the 377!

I think I had her pretty well figured out by that time, but the next challenge was to finally get a firm grasp of celestial nav using Dave Bitzer’s (?) sextant.

TreeTops
Technical Sergeant
Posts: 539
Joined: 07 Apr 2010

Re: B377 Calibrating the potentiometers.

Post TreeTops »

Yes the sextant is awesome to use with the long range planes like the B377.
Cheers
Trev

Guus
Airman
Posts: 20
Joined: 10 Mar 2019

Re: B377 Calibrating the potentiometers.

Post Guus »

Hi All,

Thank you for the Excellent explanation !!!
For me, the "Strat." is not old-fashioned in FSX! I only recently switched from FS9.1 to FSX! (Where I flew for decades via CelNav, dead reckoning etc. with sextant from "Dave Bitzer" ... Jaqcues ;-) ! What comes to my mind when you compare 2 engines can be taken: 1 + 2, 1 + 3, 1 + 4, 2 + 3, 2 + 4, 3 + 4.

Trev, I will further analyze your Excellent explanation - and the attached link -. What wonders me is that you are talking about 7%. The Handbook talks about "Move Turbo Boost to 7". The "Tooltip" indicates the "Turbo Boost Selector to 7" ... 70%! And so almost ALL gauges applicable to the Turbos (at 70%) give the MAX. limit on! So quick calibration is NECESSARY! I assume you ALSO talk about 70% instead of. 7%!

And yes, first I also raised the 4 engines to the MAP 30" ambiënt press. at the same time and made equal them!

Regards, Guus from Belgium.

TreeTops
Technical Sergeant
Posts: 539
Joined: 07 Apr 2010

Re: B377 Calibrating the potentiometers.

Post TreeTops »

No, I repeat the boost should be about 7%. The markings from 1-10 are like 10-100%. The tool tips show correct percentage.
This has caused confusion for many in the past. Rob has posted about this in a previous post years ago.
So while checking the boost the manifold pressure shouldn't rise to more than 35-37".
Cheers
Trev

Guus
Airman
Posts: 20
Joined: 10 Mar 2019

Re: B377 Calibrating the potentiometers.

Post Guus »

Trev,
After a long search I found it (not from Rob). "2 questions. Props @ 2100 RPM and Turbo Calibration." May 2014 !!!
My whole point is indeed successfully discussed by various people there! The 7 versus 70% is not formulated in a handy way !
Thank you for this contribution !

Greetings, Guus.

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