maxbc wrote:
Inside cylinders there’s always some 800C burning happening.
There’s something called MEP. The pressure in the cylinder. More pressure, more power, it’s how a piston engine works. To get more pressure you need more heat. This pressure effects piston rings. Obviously a Rotax is a robust little engine, but prolonged running at very low power is not what it’s designed to do. Running to cool over prolonged time, is certainly not something it is designed to do. Let’s just say that running it outside it’s design parameters is not a good thing if you want to reach TBO of 2000 h.
In practice I don’t think Risen pilots will run it at low power settings though.
Dan wrote:
Well, the glide ratio is given as a very high 20, but looking at the wing or the fuse, that would be a revolutionary concept for a glider…
20 is still aircraft territory. Gliders take this to more than 60, so still far off.
LeSving wrote:
The pressure in the cylinder.
The point of running low RPM is to have a similar cycle than at high power (i.e. high engine load, high burnt gas temps, high pressure, just fired less often).
But I agree that, in general, it has to stay where it was designed for. POH for the DA20 has the lowest recommended RPM at 1900, I don’t know if it can go lower. It’s something that may require a lot of (bad) experiences before having a good idea of what is good and not good (just like running too lean on descent / approach eventually damages the engine).
The 1900 RPM they are refering to in the Katana POH is probably propeller RPM (the Rotax has a gear box with a reduction ratio of something like 1:2,3).
europaxs wrote:
The 1900 RPM they are refering to in the Katana POH is probably propeller RPM (the Rotax has a gear box with a reduction ratio of something like 1:2,3).
That’s correct (1:2.27), which means a 4300 engine RPM. Not a “low” number for any gasoline engine, but parking / warm up RPM is at 1100 (2500 engine), which means the engine should be quite safe at these settings for semi-extended periods (you can after all get stuck on the ground for 20min). MP is of course very low in this situation (I would say 5 inHg but not sure).
LeSving wrote:
How well designed is it? A 912 ULS “cruising” at 7.5 l/h. A 912 ULS is not designed to run like that. It is designed to cruise at 12-20 l/h. Perhaps OK with a constant speed prop getting the RPM up, but how is the cooling solved? It will run awfully cold without some vanes closing off. It’s seems to be a very special kind of taste in all respects.
It’s not a 912ULS but a 912iS; you can run it back to 4000rpm and reduce the fuel flow at altitude to around 8l/h but as Rotax don’t recommend running the engine in cruise below 5000rpm, I wouldn’t do that. The consumption figures stated at 75% power match what I see – 17l down low, 14l at FL090, 13l higher
maxbc wrote:
The basic version seems to be €180K new, maybe a little more for the high perf.
Surprises me because I saw a 2020 Risen with 912iS offered for €250k without taxes……….
Steve6443 wrote:
Rotax don’t recommend running the engine in cruise below 5000rpm, I wouldn’t do that.
Is that specific to the fuel-injected 912?
Airborne_Again wrote:
Is that specific to the fuel-injected 912?
No, generic statement from Rotax concerning the 912 series. They recommend flying between 5000 and 5500 rpm although plenty I know fly at 4000 – 4800 to save on the fuel…..
No, generic statement from Rotax concerning the 912 series.
Is that written down somewhere?
Bristell says to cruise at 4800 RPM to keep EGT’s low..
