separate prop control mod for cirrus

Thanks! I was familiar with that. It sounds improbable to me. I also have fiddled quite a bit with RPM settings on a Bonanza I fly regularly. The effects I saw there don’t give me much hope.

But, if it’s true, well, once the word gets out, the mod will sell like hot cakes.

I have no interest in arguing you away from this move, everybody here uses very creative math to justify their flying, but you are taking an advertised 1 USG saving from a SR22, operated 50F ROP to justify an investment in your SR20, which you fly LOP?

Creative :slight_smile:

Power = Torque x RPM

Power (LOP) = directly proportional to Fuel Flow

Speed = nonlinear function of Power

No way that a reduction of fuel flow (LOP) gives you an increase in power (or speed, that is).

Note that the top and middle diagram of Tamarack is for ROP. Only the third diagram is for LOP - and what does it say? It says that you will save fuel and gain efficiency when you fly slower than what is given in the POH. Who would have thought this… So, yes, if you decrease fuel flow this will decrease power and airspeed.

The Tamarack charts indicate greatest savings between 2000 and 2250 rpms. The problem with that is Continental warns against cruise operations below 2300 rpm, via a service letter. They have experienced crankshaft failures on several engines operated in this manner, by operators seeking increased efficiency.

It’s certain that a broken crankshaft will increase your fuel economy for a short time, until you run out of altitude…

Tim:

I disagree … I always found the Cirrus a little squirrly when setting up for the ILS compared to the C182, specifically due to not having the separate controls. That said, it is not a big enough deal to make a major mod like Norman is suggesting. But I don’t think it is true to say there isn’t any benefit to the separate controls.

There’s response from Brian Willett of Tamarack Aerospace Group in the thread here. I assume that’s the mod we’re talking about, unless there is more than one of them. He clarified that they’re only talking about an increase in fuel economy (gal/NM) at lower TAS, not fuel savings for a given TAS, nor increase in TAS for a given FF. That makes obvious sense and I didn’t see the point, since you can already do that by pulling the RPM back to fly slower and get better fuel economy. I would love to see a direct comparison with the same fuel flow with the only difference being RPM. I would be surprised to see a difference in resulting TAS. Or shoot for the same TAS and compare any difference in FF. The important thing being a comparison with only 1 variable.

But less mass airflow through the engine at lower RPM, resulting in cooling issues. At any particular fuel flow you are running richer at lower RPM because there is less air moving through the cylinders. That’s why it often helps to run at 2600 RPM in ISA+20 temps as opposed to the usual 2500 to keep CHT’s in check.