Peter wrote:
If you just reduce RPM, but increase the MP in order to maintain the power output (i.e. IAS) you don’t get much benefit. Or did I get that wrong?
If you do that, you’d ideally need less advance – in car terms you’re lugging the engine to improve efficiency but if you reach the fuel octane limit you’d need to retard the spark with a counter-acting reduction in efficiency. On an aircraft engine fixed timing is set for maximum MP operation and I don’t think the relatively minor reduction in rpm here is a strong driver.
It’s when you run the engine more like a car or motorcycle that efficiency benefits from variable advance, in other words with RPM highish and MP really low, as in normally aspirated high altitude operation. In car terms that means not lugging the engine, and given high ambient pressure (around the car) you’re prepared to accelerate the car by just opening the throttle. The overall configuration is inefficient because the RPM is higher than needed for the current power output but you make the best of it by advancing the spark.
In a plane at cruise and unless you’re at high altitude, you don’t need to accelerate or climb on a moments notice so you run the engine more efficiently. Efficiency is likely higher if you instead reduce RPM and open the throttle more, less friction losses that way, and you have no need to climb with a single quick control action. Ignition timing doesn’t need to change so much in that low altitude cruise configuration, unless maybe you’re leaning like crazy.
All this stuff could be done with totally automatic mapped control of engine/propeller rpm, mixture spark timing etc based on pilot selected throttle opening (his only input) and altitude sensor inputs. As discussed a million times before the issue is that the automatic control equipment stuff breaks, costs a lot to diagnose, inspect, maintain and replace over decades and doesn’t actually reduce pilot workload all that much because planes don’t need continuous changes in power setting.
Ideally this plane should have diesels, but until those engines are available, this is probably the best they can do. I think the P2012 will be a big success – there are tons of little island hopper airlines all over the world that need replacements for their old BN Islanders etc. Just is nothing in this segment.
And it would probably be a very good candidate for a RR300 engine retrofit should they need a turbine option. The RR300 loses steam up high pretty quick, but since this isn’t pressurized, it won’t fly very high.
I don’t see a single reason for not waiting for the Rotax 915 (other than that those aren’t an american brand).
The 915 would be state of the art.
How many of these state-of-the-art 135 HP Rotaxes does one need to power a 12 seat twin?
… except that the currently proposed TEO540 offers 350 HP, three times as much as the 915iS?
for those “modern” engines why are they not using a lambda probe to get the correct/optimum fuel/air mixture?
Cobalt wrote:
How many of these state-of-the-art 135 HP Rotaxes does one need to power a 12 seat twin?
Okay, now I see this ONE single reason not to choose a Rotax 
Mixed it up with the Tecnam P 2006.
Silvaire wrote:
Efficiency is likely higher if you instead reduce RPM and open the throttle more, less friction losses that way
And lower pumping losses. That is what kills efficiency of car engines and where a lot of the development is – making the engine efficient at very light loads (and it’s what’s tested).
Yes yes, that principle is at the heart of the world’s most efficient piston combustion engines – slow running two-stroke diesels as used for marine propulsion. Yet for aviation the same principle never got further than the very impressive Jumo 204 and its derivatives. Why? I suspect power/weight ratio.
Rotax claim (but I’ve not checked it) their 915 iS offers the best power/weight ratio in the market, yet it runs at 5800 rpm and carries the extra weight of a reduction gearbox. There must be some logic there. Or perhaps some laws of physics.
This “Rotax v. Lyco” thingy has come up many times e.g. here.
I don’t think anybody has shown a Rotax delivering better SFC in cruise. It’s hard to see where it would come from. Obviously they are more efficient in climb (where a Lyco has to run very rich because of the need to keep CHTs down) so overall you have a saving in typical operations. The biggest savings come from the planes themselves being much smaller.
Power to weight ratio is a different thing and is “easy” to achieve – just rev it higher, and better still turbocharge it But you lose efficiency at higher revs.
