A quiz for you

[rosslr]

Had a nice flight today out to Yarrawonga on the Murray River to get measures up for a cover. A couple of photos attached for general interest. But here is what I need some input on. I have the 912is sport engine coupled to an Airmaster CSP. The Airmaster is programmed for 5000rpm cruise. (some tell me it should be about 5200rpm but I cant see why to run it at that setting Vs 5000). With the 912is sport the computer organises the fuel/air mixture and has basically 2 settings. 1. Full power (throttle) or 2. economy he's what I dont actually understand. I can reduce the throttle a cm or 2 to achieve the economy setting at about 27"MAP. I can futher reduce the throttle setting to about 21"MAP before engine cannot maintain 5000rpm. So, what is the difference in throttle settings doing between 21"MAP and 27"MAP?? My assumption is that the MAP setting doesnt really matter between that range as the computer will decide how much fuel is needed to maximise the power and the excess is returned to the tank. Does this sound right to you guys?? I find the Rotax manuals to be scant on this info and the rotax owners site to be not much better. Your thoughts appreciated as I have no luck getting any sense from the rotax dealers here!

cheers

ross

[Av8r3400]

I don't know the details of the software, but here's how I'd see it.

At 21" the engine is putting out less hp than at 27". The prop goes flatter to maintain the 5000 rpm setting. Your speed should reduce slightly and your fuel burn should also decrease slightly.

What is your observed speed doing with this change in MP?
Do you have a fuel flow meter? If so, does that change?

[HighWing]

Ross,
I have no insights into your specific question, but a question I have is why you are restricting the RPM? My engine experience has only been with the 80 hp 912, but the manual that came with my engine specified the maximum continuous RPM to be 5500 which is what our group routinely flew.

Then looking at the Zipper kit installation manual, Hal says this:

26. Check static RPM. You may need to repitch your propeller. These engines love RPM so I pitch propeller to run 5400-5500 RPM on take off, 6100 RPM straight and level, and always cruise above 5200 RPM. If I'm in a hurry or bucking a big wind cruise goes to 5900-6000 RPM.
Depending on your altitude, you may find that the Main Jet may have to be changed and or Jet Needle rich or lean setting may have to be adjusted. Optimal EGTs should be 1400 to 1450 degrees at cruise.
With these High Performance Cylinder Sets, you WILL have some oil consumption. It will decrease after breakin and it varies. Not to worry.
PG 4.

NOTE: On the Dyno the HP increases until about 6150 RPM. The torque peaks about 5200 RPM, so as the engines RPM increases, the load on the case and gearbox decreases. You will get better fuel economy with a lightly loaded engine at 5500 RPM, than a heavely loaded engine at 5000 RPM.

(Note) Hal has spent his life maintaining industrial engines.

[jiott]

Here's my opinion, pretty much agree with Lowell, assuming your cruise airspeed stays the same at the same altitude, the engine HP required will also stay the same because the airplane lift & drag does not change. Since HP=TorquexRPM it will therefore require less Torque at a higher RPM. Since Torque is directly related to stress on the components, I believe the engine is under less stress and strain at the higher RPM and will last longer and wear slower. To be honest yes there is some increase in stress at the higher RPM's but it is not nearly as direct a ratio as Torque in the rpm range we are talking about. So IMHO it would be better to cruise at 5200 rpm and not lug the engine as much.

[kitfox2009]

Hi guys
I hope many more rotax fliers chime in here.
On my 912 UL I usually run at 5200 on "fast cruise" and 5800 on takeoff, putting some pitch in for climb out at 5500.
But my manifold pressure in cruise is 27+ . It has always concerned me but everything seems to run and sound right.
I am using a 70 inch IVO IFA Ultralight. Fuel burn averages 15 litres hour. Just over 600 hours on mid 90's engine. No oil use. No repairs just plugs and filters.
In order to get my map lower I have to either reduce rpm or fly down hill. Not always practical choices .
Are these numbers about the same for everyone?
Cheers
Don

[av8rps]

Ross,

If the Airmaster is a true constant speed prop, that means it will maintain the same basic rpm regardless of where the throttle is set. So if you set the prop for 5,000 rpm, at 27 inches you are really giving it a lot of power (throttle) and burning a lot of fuel.

On the contrary, if you reduce the throttle setting to 21 inches MAP that just means the engine is running and working less hard, and fuel consumption will be lower, even though prop rpm is the same 5,000 rpm in the above example.

Not sure I am answering your question or not, but that is th e basic premise of a constant speed prop. I think the 912is may have programming to know when it can run lean of peak to get optimum horsepower out of every litre of gas, making it an extremely fuel efficient engine.

[kitfox2009]

One more observation.
Yesterday I did a demo flight in a 1999 Europa with a 914 c/w Wirlwind hydraulic governed CS prop.
Normal cruise @ 75% power is 5000 rpm/30 inches!! Owner reports 20 litres/hour of 91 or 94 grade unleaded fuel.
I believe this setting is whats recommended in the Rotax manual.

Using my logic I would think running our non turbo-ed engines at 5000 to 5200 and 27 to 29 inches should be well within design limits. Correct?

The Europa was a very nice experience. More like a "quiet RV" than a KF and I saw 130/140 KNOTS @ 3500 ASL quite often. Not enough leg room for me though. Looks like a very fair price @ 40,000 USD.
Cheers
Don

[Danzer1]

I can reduce the throttle a cm or 2 to achieve the economy setting at about 27"MAP. I can futher reduce the throttle setting to about 21"MAP before engine cannot maintain 5000rpm. So, what is the difference in throttle settings doing between 21"MAP and 27"MAP?? My assumption is that the MAP setting doesnt really matter between that range as the computer will decide how much fuel is needed to maximise the power and the excess is returned to the tank.

Ross,

I agree with much of what is said here, but would add (FWIW):

The computer does 2 things - adjusts fuel AND timing to maintain optimum fuel/air ratio and avoid detonation. Generally a higher MAP means higher fuel consumption to maintain optimum fuel/air ratio - that's a Rotax set parameter and their data may not be optimal for your combination.

Add in the constant speed prop (variable) and you might find that even at 5000 rpm (constant prop speed) at varying MAP pressures, that your fuel consumption AND airspeed will change. As the prop angle is adjusting pitch to maintain prop rpm NOT airspeed or MAP or fuel consumption.

It has to be consuming less fuel at 21" than 27", the question is how much less fuel and what is the effect on airspeed as the prop pitch changes to maintain 5000 rpm?

So as every installation and combination is different, here's what I would do. Fly the same patterns under the same conditions at varying prop speeds and MAP pressures and record both fuel consumption and airspeed at each. Graph those to determine your most advantageous settings for takeoff, cruise, ecomomy, etc. Watch CHT's and EGT's too, to avoid lugging the engine.

[rosslr]

Thanks for all the replies guys - I am trying as you suggested Danzer. Am away at the moment and been running 5250 cruise and about 24.5" I am using about 18lph - I think this is a bit higher than I would like some will try lower MAP.

On another matter, this morning I had the low fuel light come on - despite having full tanks. After about 1-2 minutes the light went out. I landed and checked flow to header tank from both L and R tanks - full flow from both. As the low fuel gauge is in the return line from the header tank to the R tank, I assumed either:

1. electrical fault in sensor - not evident
2. electrical fault in warning light -not evident
3. a block in the rtn line from the header tank to the r fuel tank.

I took the seats out to check the flow to header tank and noted that there were some air bubbles/ short sections of air in the clear tubing from header tank to the low fuel tank. I am thinking that I have got an air block in the rtn line that has cleared itself. I did a lot of steepish turns yesterday as we had the BEST day flying (1 pic below - more on that later). The rtn line from header tank to wing tank has a slight S down bend in it that I am thinking needs to be corrected to ensure air blocks cannot occur - does my reasoning sound ok to you guys?

cheers for now
r

[jiott]

Hi Ross,
I believe you thinking makes sense. I would look closely at that vent line from header to right tank. Yes, an "S" bend in the tube from header to warning reservoir could trap air bubbles. I had the same "S" bend issue with mine (although my low fuel light never came on) and could see air in that clear tube. I never liked it so recently I fixed it by getting a 45 degree fitting for the top of the header tank to replace the straight fitting. This allowed a nice continuous upward slope to that tube.

The other possible blockage to the vent line is where it goes into the right wing tank fitting. This is also a straight fitting that forces a rather drastic bend in the clear tube. It doesn't look too bad when the wing is folded back, but when you swing the wing into flight position that tube bumps up against the butt rib covering and could possibly put a kink in the tube. This spot also would be better with a 45 degree fitting but I am not sure if there is room to swing it so I have not replaced it. But I did fiddle with the length of the clear tube so it doesn't stick out so far and get kinked. Anyway, just a few thoughts of things you could check. Maybe take the long vent tube off the fitting at the warning reservoir and blow thru it to see if it is clear. Doing this sort of thing is always a hassle because fuel wants to squirt out of any disconnected fitting since we don't have shutoff valves in the main lines out of the wing tanks. If you are quick, you can get a finger over the squirting fitting before too much comes out (have a towel handy). Or you can clamp off the main lines (which I don't like to do).