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Ram air TIO540

No, they are quite independent.

EGKB Biggin Hill

It has been suggested to me that the effect is not ram air, but reducing the angle of attack of the prop blades, and hence their turning resistance.

Does that make sense?

EGKB Biggin Hill

…don’t think so…finer pitch with same RPM translates to less power…power being a function of work, or distance covered per revolution

Oxford (EGTK), United Kingdom

I don’t think it has anything to do with RAM air but more with pitch changes. Flying in Switzland over the mountains regularly with various degrees of mountain wave strength to maintain altitude either you or your AP will have to change pitch ever so slightly from -50 to +50 fpm which will change your IAS by 5kts easily. I have this all the time in mountain waves.

LSZH

There is no question that pitch and TAS change in mountain wave, but that is not the question.

The question is why MP changes as well.

EGKB Biggin Hill

Here is a lot of food for thought about MP (albeit all in relation to normally aspirated engines): http://www.avweb.com/news/pelican/Pelicans-Perch-15-Manifold-Pressure-Sucks-182081-1.html

Question: What does your RPM do when your speed changes? This information is needed in order to apply Rule #1525: “Manifold pressure depends on ambient pressure, the position of the throttle plate, and the speed at which the pistons are moving up and down. Manifold pressure does not indicate ‘power’, unless other things are taken into account.”

Edit 1: It seems that when you operate a turbocharged engine with the wastegate fully closed (i.e. at high altitudes), the relation between RPM and MP is exactly the opposite of a normally aspirated engine; a decrease in RPM would decrease the MP, and vice versa.

Edit 2: Found some detail regarding the turbo controllers here:

The density controller is the only controller capable of sensing changes in temperature. It relies on a bellows charged with dry nitrogen to accomplish this purpose. Density controllers are found in the Piper Navajo and Chieftain and a handful of helicopter applications. In the Piper, the density controller modulates the wastegate movement at wide open throttle while a differential controller keeps deck pressures from exceeding manifold pressures by more than a specified amount at part throttle settings. Density controllers are extremely sensitive to in-field adjustments. A 1/16th turn of the adjustment screw will result in a 2-in. change in manifold pressures. A thermocouple probe referenced to deck temperature is required when setting up these systems (see Lycoming S.I. No. 1187J).

So another question is: Did you fly with WOT?

[ local copy of above doc ]

Last Edited by Rwy20 at 22 Apr 22:52

Tim, might it be that when you change pitch, i.e. descend without touching any of your levers the prop moves to a more coarse pitch to maintain RPM and that has an effect on your MP.

LSZH

When discussing MP in a turbo-charged engine, you must take into account the exact configuration of the turbo system.

Examples :

- Fixed wastegate :The valve is partly open all the time, and manifold pressure is controlled exclusively by throttle opening.

- Manual wastegate, sometimes called the “second throttle.” : This arrangement typifies early inexpensive turbo systems and requires the pilot to set power using both the conventional throttle (full open at altitude) and the wastegate control

- Automatic controlled wastegate : of which there are several types. Suffice it to say that an automatic controller maintains a manifold pressure the pilot asks for through throttle position. It’s largely a set-and-forget system and reduces pilot work load tremendously. .

FAA A&P/IA
LFPN

May I add that very few pilots of TC’d aircraft really understand their turbo system and the particularities each presents.

FAA A&P/IA
LFPN

May I add that very few pilots of TC’d aircraft really understand their turbo system and the particularities each presents.

I wonder if Timothy’s turbo is like the TB21 which has a reportedly extremely delicate/critical wastegate adjustment. I know nothing about this myself but I know one TB21 owner was “working” with Air Touring for about 6 months with recurrent turbo problems. OTOH some A&Ps say there is nothing to it and the real issue is the lack of turbo-specific engineering experience on the UK (and probably European) GA scene.

What do you think, @Michael?

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Shoreham EGKA, United Kingdom
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