Manifold Pressure Values?????

by geoffrey klestadt in 912iS Technical Questions 8 years ago

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Re: Manifold Pressure Values?????

by Glenn Martin » 7 years ago

Glenn Martin

3 281 798

Hi Roger,
thanks that is most unfortunate for the people in the US.

The question was a request for Ball park figures, so the table I replied with should work.It is not "official ROTAX data" although calculated from the Rotax graph, so not good enough for Geoffrey. It also depends on what equipment you have in the aircraft. The fuel flow to power figure still works so long as you are in eco mode and have fuel flow data.

I suspect that with a fixed prop you want approx 5800 (max power) for TO, although the IS engine power curve is very flat CF the carb engine so 5500-5800 is only about 3% difference not enough to loose sleep about.

5500 for sustained climb ( Maximum allowable continuous rpm from Rotax).

and around 4800-5200 as you say for cruise, 4800-5200 should be eco mode.

One of the big differences CF a carb engine is that Carb engines tend to have a "sweet spot" and areas where the engine is "not so happy". The IS engine is happy just about anywhere thanks to the ECU and good programming.

With a fixed prop am not sure why you would monitor MAP ? would you not just use RPM as an analog of power ?

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Re: Manifold Pressure Values?????

by Roger Lee » 7 years ago

Roger Lee

125 5.12k 4.53k

Yes to just rpm on a fixed pitch. From what I've seen setting the ground adjustable prop for 5600-5650 rpm at WOT in level flight at your average altitude is the sweet spot for most. There are a few exceptions.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Manifold Pressure Values?????

by geoffrey klestadt » 7 years ago

geoffrey klestadt

27 94 94

Glenn, I'm sorry that you don't understand why we need fuel flow AND manifold pressure data for use with a constant speed propeller.

These propellers are not just "in flight adjustable" they are controlled automatically to maintain engine rpm at a constant set value. This introduces not just performance gains as you would expect but a new problem - the propeller will maintain its set rpm even if the engine is broken and producing very little power!

To put that another way, with a correctly adjusted fixed pitch propeller, if you get 5800 rpm, you know that the engine is producing 100% rated takeoff power. If you have a constant speed propeller this may not be true!

We get around this problem be measuring manifold air pressure as well as fuel flow. We need both numbers because there are failure modes when one number might trick you (eg fuel or air leak), it is only when all numbers rpm, fuel flow and manifold pressure are correct that we can suppose we have the required power.

I have the numbers taken from a Diamond D42i manual that gives pressure and fuel flow, it is indeed correct when in ECo mode that fuel flow sets power because the engine is running lean of peak however without manifold presssure to confirm the settings you are not going to be able to diagnose an engine problem in flight, for example a collapsing air filter, fuel leak etc. unless you know what the correct pressure number should be.

As for running the engine at any combination of rpm and power setting you like, well, I guess if Rotax wanted us to do that they would have said so in the manuals. Instead they give specific settings for rpm and power values to be achieved and my guess is that they did it for good reason, probably to do with engine reliability.

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Re: Manifold Pressure Values?????

by Glenn Martin » 7 years ago

Glenn Martin

3 281 798

Hi Geoffrey,
thanks for the explanation. If you need both to recognise a failure then how about flying after a service and note the "normal" MAP, fuel flow make a simple table then if you suspect an error you can refer to it. The Ball park figures I supplied should be good enough to see a significant error and by definition a "minor error" possibly is not an error at all.

Secondly can you give me a reference for where Rotax
"give specific settings for rpm and power values to be achieved"

I cannot find these in either my POH or Rotax manuals ???

I like simple things All I can see is my POH and Op manuals is
5500-5800 (max) for no more than 5 min
5500 continuously
1400 rpm minimum idle.

But it is a big document so some direction (manual, page number etc) would be appreciated.

FYI I do have a constant speed prop and see MAP and fuel flow and EGT and fuel pressure etc etc but the EIS flashes at me if any of the T&P's go out of range. I only use full power for take off, so am almost always in ECO mode. I look at both MAP and Fuel as I fly so of course know "what is normal" for my particular engine/airframe installation.

MAP is interesting to watch, mainly how it varies widely depending on the day (temp QNH) and altitude.

Apologies for the slow reply been on a tour flying away for a week

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Re: Manifold Pressure Values?????

by geoffrey klestadt » 7 years ago

geoffrey klestadt

27 94 94

Thank you for your reply Glenn. Page 5-4 of the POH gives power settings for the engine in terms of rpm and torque for WOT. Max continuous, 75,65 and 50 percent power. I assume Rotax selected those rpm settings for a good reason, probably to do with reliability.

However in this latest manual, they don't specify the fuel flows or MAP to achieve these figures. However in an earlier version of the manual they specify fuel flows for these power settings.

Now i was directed to the manual for the diamond D40i which uses the rotax injected engine and that has manifold pressure and fuel flow data for the various power settings, so I can use that.

Then two days ago Rotax released a service letter Sl - 912 - 016R1 dated 30 Dec 2016 which specifies where NOT to run a 912 engine and I assume that includes the injected engines because it says "all serial numbers".

Section 3.1.3, fig 1 of the letter shows where not to run the engine (the 912 ul/Ulsan) for example, never WOT below 5200 rpm and no operation above 27 inHg below 4600 rpm if i am reading the table correctly.

I think the injected engine has better protection against detonation thanks to the ECU and running in eco mode but I don't know about preignition. I don't think we need to discuss rich of peak vs lean of peak operation etc.

The service letter also specifies and manifold pressure gauge must be fitted to their carbureted engine, we get the data from the CAN bus.

I think we are slowly getting a clearer picture of how Rotax wants us to operate the engine.

In my case, since I am building experimental, I don't have an aircraft manufacturer to do this research and development for me. Instead I must find "approved data" to satisfy the authorities myself - that means the source of the data must be Rotax.

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