Boost cutout override principle at not full open throttle?

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gulredrel
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Boost cutout override principle at not full open throttle?

Post gulredrel »

Hello,

I think, this goes directly to Killratio :wink:
I guess, with full open throttle and the supercharger and engine being able to produce more than the set maximum boost pressure, the excessive pressure is simply vented outside leaving the MAP a the maximum.
I'm wondering how the boost cut out override is functioning when not having full open throttle.

I was i nthe MkIa 2-position-airscrew at about 13.000ft and having set around 2100 rpm with -1 boost.
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Flipping the boost cutout handle forward without moving the throttle lever, the MAP dropped down so did the rpm.
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What's causing this? I thought the boost cut out override simply stops venting excessive pressure, but without excessive pressure, there should be nothing to vent and MAP should stay the same? I think, I'm totally wring here?

Thanks and regards
Jens
"Give me a ping, Vasili. One ping only, please."

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Killratio
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Re: Boost cutout override principle at not full open throttle?

Post Killratio »

Sorry for the late reply Jens,

I’ve been out of commission for weeks.

The System is not venting any pressure above FTH, around 10500ft for the Merlin III if I remember correctly, so you then have to look at the position of the throttle butterfly. The aneroid controls that whilst the Automatic Boost Control is engaged. At, let’s say, 85% throttle, that may be fully open at the butterfly at above FTH because there is no more to give. Now if you override the control, the throttle position is now at 85% , the butterfly comes back to 85% (straight connection) and the power drops. THAT is what I believe you are seeing, what I am NOT sure of is if it should be that way. Scott did extensive work on the supercharger/throttle and the automatic altitude adjustment, I do know that. So perhaps he can weigh in on the subject?

Darryl
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>


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gulredrel
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Re: Boost cutout override principle at not full open throttle?

Post gulredrel »

Okay, thanks I guess, I know what you mean.
I'm not familiar with the term FTH, is this the same as the German word "Volldruckhöhe", so meaning full pressure height?
"Give me a ping, Vasili. One ping only, please."

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Killratio
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Re: Boost cutout override principle at not full open throttle?

Post Killratio »

FTH = Full Throttle Height...

The point where the full throttle position still gives maximum power. Above FTH the engine maximum power
will reduce as altitude increases, even when the throttle is fully open.

The Supercharger, by artificially increasing pressure, raises the FTH above that of a normally aspirated engine.
But even that has its limits and there comes an altitude at which the air pressure and the supercharger are no longer
able to make the engine deliver maximum power.

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D
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gulredrel
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Re: Boost cutout override principle at not full open throttle?

Post gulredrel »

Thanks Darryl,

the description for FTH corresponds to what I'm used to with Volldruckhöhe in German.

Regards
Jens
"Give me a ping, Vasili. One ping only, please."

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