Automatic Mixture - now there's a thought...

[Pimpernel Leader]

Hi all,

I am in the process of writing a de Havilland Dove/Devon ops manual so that our company can operate it on our JAR AOC. As the Dove is one of the aircraft I'm lucky enough to fly, I am fairly familiar with it, but I do have one question which I'm hoping an old piston aficionado will be able to answer for me.

It concerns the aircraft's automatic mixture control funnily enough. Why does the flight manual give not only a Boost Limit but also a RPM limit when stating it's 'Maximum Weak' mixture setting . From my Layman's standpoint, I would assume that the automatic leaning is a function of the Carburettor and I'm not sure how Prop/Engine RPM effects it. It is not a limit that is unique to the Dove either, as there are RPM and Boost limits associated with most of the other old aircraft we get to fly. It's not even an in-line thing as it applies to Twin Pioneer, Pembroke, Prentice, Anson, and several others.

I guess it's a field of knowlege that isn't getting any bigger, so I would be grateful of any insight you may be able to offer.

Thanks in advance.

Pimp Leader

[Intruder]

Many older airplane recip engines use excess fuel to help keep temperatures under control. Using a lean mixture may cause "hot spots" in individual cylinders at high power settings.

[Old Smokey]

The carburettor on the Gipsy Queen 70 MkII operates schedules Lean mixture below a given Boost/RPM threshold and increasingly Richer mixtures above.

We used to climb at +2 Lb / 2400 RPM (34" for the Yanks), and had a lot of Cylinder Head problems, as this was the upper limit of the Lean range. Upon increasing Climb Power to +3 Lb / 2600 RPM, the problem disappeared.

Hope that that helps.

Regards,

Old Smokey

[xsbank]

High combustion chamber/cylinder head temps are the reason why "reduced thrust" take-offs on a piston engine will kill it faster than 'normal' take-off power - the last few inches of throttle opening introduces raw fuel into the combustion chamber to cool same.

[Pimpernel Leader]

Great stuff!
I had an incling it would be a cooling issue. I did read some very interesting 'Warbird notes' which was full of really good gen on round engine handling but didn't quite answer my question. So thank you.

Toodle Pip

[A37575]

I recently had reason to contact Pratt & Whitney Canada re engine handling of the P&W Wasp Junior 985. The reply was prompt and concise and I was highly impressed. One of the recommendations was to shut down the engine on the ground by first going to 1600RPM then select pitch to full coarse and drop the RPM before shutting down the engine. The start up was also done in full coarse pitch until oil pressure stabilised then select full fine.

The reasons why were given in the P&W reply. From all this I strongly recommend you contact the engine manufacturer for your specific query as it minimises being swayed by old wives tales.