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jcomm

Manifold Pressure - All you wanted to know about it but were afraid to ask...

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On another "Picky" thread I mentioned one of FLIGHT's bugs regarding piston engine driving variable pitch props. There are 6 in FLIGHT, although only three have a cockpit and allow us to check MP and RPM...

 

Here's a link to an excellent text explaining it all :-)

 

http://www.advancedp...nloads/prep.pdf

 

Check your preferred flightsims for this ... Operate them as real as they can get ;-)

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Great resource. Thanks for sharing that link... Suction not 'pressure' :) Only obvious when you understand the principle... (and still got 60 more pages to read).

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Wow... my goodness! Didn't know there was so much to say about MP! :wink: Very interesting pdf, thanks for the link!!!

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Definitely more than I wanted to know! :LMAO: Thanks for the link.

 

Best regards.

Luis

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I read the articles about MP and prop: very interesting! Nice to know how it all works! :smile: Problem is... that any bugs in Flight will become more apparent now... :wink: What exactly didn't work as it should again...?

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What exactly didn't work as it should again...?

 

- For instance, with a given MP / RPM setting, varying RPM has NO EFFECT in MP.

- MP at ground idle is way too low - should read at least 11/12 Hg

- FF is buggy as in MSFS previous versions (the way it depends in the other variables)

- In FLIGHT we can't use EGT, altough the gauge is there :-(

etc...

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Yes, I see what you mean. I tried various examples from that pdf and indeed things aren't as they should be.

 

Bummer, why did you have to make me aware of this, hahaha! :wink:

 

Still, it's nice knowing more about how it works! I never ever noticed or wondered why MP was almost at 30 with the engines off! Also nice to know it's 'harder' for the engine to run at idle then with full throttle! I thought that on the ground and throttle at idle, the engine was very relaxed and calm while in fact it is sucking itself almost blind! :wink: Now I don't feel so bad anymore to cruise with throttle at full anymore.

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Yes, I see what you mean. I tried various examples from that pdf and indeed things aren't as they should be.

 

Bummer, why did you have to make me aware of this, hahaha! :wink:

 

Still, it's nice knowing more about how it works! I never ever noticed or wondered why MP was almost at 30 with the engines off! Also nice to know it's 'harder' for the engine to run at idle then with full throttle! I thought that on the ground and throttle at idle, the engine was very relaxed and calm while in fact it is sucking itself almost blind! :wink: Now I don't feel so bad anymore to cruise with throttle at full anymore.

 

 

 

EDIT: This was supposed to be an answer to your post #5

 

There is no relationship between RPM changes (by prop-control), and MP...

 

I can look past this, because RPM are changes are usually infrequent, and slight.. I.E.. the increase in MP when you pull the RPM from red-line after takeoff, you're looking at going from ~2700 to ~2500.. and during a climb you'll lose a full inch of MP per 1000 feet of altitude. And when you pull RPM back to cruise (~2300), a small change in MP (compared to throttle setting changes), only comes into play when you're going after PRECISE power settings (RPM/MP), that I've never stuck to, anyway.

 

I'll admit though, it would be nice for realism immersion, if you had to compensate with the throttle, after changing RPM, however subtle it might be.

 

The other bug, that really bugs me, is how mixture is modeled. We can get into that later..

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But for instance, it's more or less ok regarding the 1 inch difference between static MP and MP at Full throttle during takeoff...

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The other bug, that really bugs me, is how mixture is modeled. We can get into that later..

 

Yep, on my link above, on the "Pelican's Perch #18: Mixture Magic" we can read an interesting test that can be run on a twin, by increasing throttle (MP) and reducing Mixture, ending up with the same "Power" output from both engines, with less fuel comsumption on the engine where the mixture was leaned and the throttle increased...

 

I no longer have MSFS X, so I can't say for sure, but I believe it has the same "bug" that ties FF to Power output, thus turning proper leaning techniques useless in as far as their RL counterparts are used for...

 

We lack a twin to do the test in FLIGHT, maybe with Alaska :-)

 

Then, if it ends up being some sort of PT-6 driven aircraft like the Twin Otter, I would really LOVE to see a properly modelled free-running turbine engine. AEROSOFT did a great tweaking work on their Twin Otter, where FF was almost totaly stable for RPM variations. The default MSFS turboprop model desn't account for this :-(...

 

A free-running turbine should show no FF variation (along a wide range of variation) with Prop (RPM) variations.

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I no longer have MSFS X, so I can't say for sure, but I believe it has the same "bug" that ties FF to Power output, thus turning proper leaning techniques useless in as far as their RL counterparts are used for...

 

As to FSX, the importance to lean, kicks in around 6000'. From that point, it's just a listen to the engine sounds, and performance that works in the sim. At least the sim is portraying the effects of leaning mixture or not doing so. Since variations such as LOP (lean of peak) can get complicated, I never expect the sim to faithfully reproduce the results, nor do I think it needs to.

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