Primarily for general aviation discussion, but other aviation topics are also welcome.
#1719232
I agree with all comments on virtually everything except 105 mph 'cruise' and will be doing all suggested items.

The prop is a std. two blade Warp Drive, set to 13 3/4 degrees at the tips and checked as still the same a couple of times this year using the WD bubble protractor device.
16 3/4 l/h was yesterday (agree too short a flight really) the same as for the 4 1/2 hour return from France when it used 70 litres at 5,000 ft mostly, where it was cooler and 4,800 rpm.

As to IAS versus true airspeed.
i) I know Rans suggest 120 mph WOT and so on but they go for full throttle and substantial height so both their Max speed & 100+ mph 'cruise' for their sales advert's was remarkably attractive - but is not my normal mode. My best WOT S & L with any of the several minor prop pitch alterations without cheating has always been hovering around 116/117 mph.
ii) The LAA dictate that the ASI static is taken from inside the cockpit behind the panel, which is approx. correct for the stall and thus pertinent. However as speed increases so does the cockpit pressure reduce: at around 100 one can 'gain' almost 5 mph over true airspeed. Easily verified - many times now - with two way GPS checks.
To get a true 100 mph 'cruise' I find 5,050 rpm works but the consumption (before the present aggro. ) rose to a heady 17 l/h - against comfortably under 14 l/h at Rans figure of 4,800 which as best assessable yields ~95 mph. [When pottering, hours aloft were considered better than arriving a little earlier.]

By the By - I was irked by the speedo exaggerating by that +~5 mph at normal cruise and eventually made a little drilled button fitting and repositioned my static port to the classic aeroplane position down the fuselage side (same as another -116 owner) which largely clears up that cabin suction generated excess reading.

All of which, of course, is virtually irrelevant to my sudden onset of excess fuel use, likewise I'm now worrying that if it's from the carb (floats sinking ?) the richer fuel mix is 'washing' oil off the cylinder walls.
In all ways an unsustainable situation !
By MikeW
#1719285
ivor.phillips wrote:I would never run my rotax 912/4 below 4800rpm because you will get a reduced fuel consumption but to the decrement of the reduction gearbox , so what you save fuel wise you will put back in gearbox overhaul,


I have not heard this before, is there any Rotax reference for it, or recommendations on permitted/advised operating range?
I find that at about 4600 the noise level starts to increase a lot (despite ANR headset) which is a nuisance, so when bimbling rather than journeying I tend to go slow. This is a 912 in a Kitfox 4 Speedster.
By MikeE
#1719325
An engineer in our hangar was working on a rotax engine the other week. It had cut out suddenly, thankfully while on the ground. He said that the floats had absorbed fuel and were no longer working efficiently, apparently a known problem. I see that others have suggested similar so it might be worth checking them carefully. Hope you find the problem.
Best wishes
Mike
#1719340
I have been down this route from purchase new floats and replacements twice during the float sinking affair, but will now be opening the carbs again of course.
However out of curiosity I just re-read the TADs for the adjacent S6 ES model (none done for the short wing Rans) & no mention of the float inspection ?

It makes sense but could you hint at where this instruction might be found ?
Last edited by mikehallam on Mon Sep 09, 2019 11:24 pm, edited 1 time in total.
#1719362
AIUI, the "absorbent" floats were some sort of plastic foam. If that becomes absorbent, you could expect that buoyancy would gradually degrade. Hollow ones are invariably non-porous and only leak when they crack/split.The buoyancy -loss in the latter case is likely to be pretty sudden, tying in with the O.P's problem symptoms. As I posted earlier, a hole in the pump-diaphragm could also result in a hidden loss of fuel.
#1719393
MikeW wrote:
ivor.phillips wrote:I would never run my rotax 912/4 below 4800rpm because you will get a reduced fuel consumption but to the decrement of the reduction gearbox , so what you save fuel wise you will put back in gearbox overhaul,


I have not heard this before, is there any Rotax reference for it, or recommendations on permitted/advised operating range?
I find that at about 4600 the noise level starts to increase a lot (despite ANR headset) which is a nuisance, so when bimbling rather than journeying I tend to go slow. This is a 912 in a Kitfox 4 Speedster.


Having done two Rotax factory visits and asking many questions regarding cruise RPM nobody suggested a RPM below 4800 was good,
The main reason given was increased wear on the slipper clutch and gearbox components,

This is from the Rotax owners website,

Re: Rotax 912ULS Cruising RPM !!!!
by Roger Lee » 3 years ago

Hi Michael,

Your engine is rated to run continuously at 5500 rpm all day long. The majority run usually between 5100-5300 from what I have seen over the years and what is talked about on the forums. I've seen some that run the engine from 5400-5500 all the time. It was never designed to run down in the 4K's all the time. Those people cruising at 4200-4800 are wrong. The 4200-4800 cruise all day people should attend a Service level Rotax class and get some education. This should pull people up out of the 4k's. Many that I talk to on forums use 5100-5300 all the time. The engine runs leaner down in the mid 4k's too.
Where did the 4k rpm people get that type of info because it isn't in print anywhere and it's talked about on all the forums not to cruise in the 4k's, but in the 5k's.

Let the other guys do their thing and you can run somewhere around 5100-5300 and know you're doing better.
Many pick these rpms to cruise at because there is a speed vs fuel crossover point where more rpm doesn't help speed that much, but just uses more fuel.

Having the prop pitched to help your cause is a must too.
Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
#1719403
The reason many 912 are run a low rpm can be traced back to the maximum empty weight of microlights, a calculation which included 1 hour of fuel at MCP. Those seeking approval for new microlight models got the BMAA and LAA to accept 10kgs for a 912UL and 13kgs for a 912ULS, a level of fuel consumption that could only be achieved by running the engine at lower than ideal rpm. Silly prop pitch settings became the norm.
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