75% cruise

[dswo]

I bought maybe half of Bill's planes this last winter and spring. Have been flying them a lot recently, and wondering how to interpret something. A lot of the instructions say to cruise at 75%. Of what? The obvious answer is manifold pressure (MP), but MP goes down as you gain altitude. Should I be maintaining 75% of available MP?

[Admin]

I've always interpreted it as pulling back on the throttle about 25% and watching that the important gauges stay in the green. BUT I am not a professionally trained real world pilot.

[ecduggan]

Most of the time that means what Tom said. 75% power is excactly that 75% engine power as long as every thing is in the green you will be ok. If you are worried about fuel burn pull the mixture back to the proper fuel burn.

[jimslost]

Lacking a real cruise chart (which is why I collect aircraft manuals), at one time both Wright and Warner claimed that 90% of max rated rpm delivered roughly 75% power. That rule of thumb still appears usable for aircraft with fixed pitch propellers installed.
Most of the folks who flew behind big round engines tell me they usually pulled about 55% power. On the Wasp powered Goose and Beech 18 for example, that means 1850 rpm and 26" hg with a combined fuel flow of 43-44 gph. However, to get real world performance with those numbers, you may have to tweek the aircraft.cfg files.
jim

[Kofi]

75% cruise is usually for an optimal altitude setting also

[dswo]

That's helpful -- thank you! One more question. Which gauges should I be paying particular attention to for keeping it in the green: CHT? Oil temperature?

[Admin]

I think it will vary from plane to plane. The J-3 only has 5 gauges but the Beech-18 requires a lot of babysitting. Oil temp is important, manifold pressure bears watching in some planes, even RPM should be watched. Basically if a gauge has red, yellow and green lines, it need to be checked.

[jimslost]

Kofi is correct. A power setting (say 2400 rpm) that delivers 75% power at 2000 feet will produce less power at 6000 feet (67% power for a Lycoming 0-235). To maintain the desired power, you must increase rpm (or manifold pressure if you have a constant speed prop) as you climb. Depending upon the engine and installation, you eventually reach an altitude at which full throttle will no longer deliver 75% power.

What I have noticed about FS9 is that it is very easy for (virtual) aircraft designers to duplicate real-world engine performance when working with complex engines with constant speed propellers and superchargers, but doing the same with low-powered engines (under 150 hp) with fixed pitch props is EXTREMELY difficult. It appears that FS9 ties engine power largely to propeller rpm, and that as the plane climbs, the reduction in available engine power is simulated by a corresponding reduction in available rpm. The practical result of this is that if you start with (for example) a 65 hp Continental with a max rated rpm of 2300, by the time you get to 6000 or 7000 feet, your virtual engine might only produce 2050 rpm at full throttle and your airplane will not perform as advertised.

The three most commonly used methods I have seen to step around this problem (in about the order encountered) are 1) increase the max rpm of the engine, usually by several hundred rpm, 2) supercharge the engine, raising the critical altitude to 5500 (or higher), or 3) install a constant speed prop. Applying these solutions, alone or in combination, can make it possible to achieve the aircraft's "real" performance at altitude but at the cost of unrealistic sea level performance or/and engine readings that are outside of "real world" limitations.

I bore you with all this up because I have spent much of my spare time these last 8 months trying to rewrite aircraft.cfg files to resolve this issue, with limited and inconsistent results (what works on one model fails on most others). I am now forced to explore the possibilty that my understanding of FS9 might be flawed (really??!). Can anyone out there shed more light on this subject?

jims still lost

[Admin]

Jim- as far as i know, the only light being shed is the work of Jerry Beckwith and the info in the SDKs and those really reinforce the fact that not everything is modeled in the sim.
There are other wizards of course but they don't publish their secrets.

I have adopted a "close enough" attitude and stopped letting the inconsistencies spoil my fun.
If a plane doesn't fly like I think it should, I just send it to the byte bucket unless I "just have to have it", then I tinker briefly. I don't normally concern myself with performance above 5000 feet as i rarely go there.

We've come a long way from FS4 when a DC3 flew just like a Cessna 172 and used the same panel too. Let's hope for more flight models in the next version.

[jimslost]

Thanks for the lead, Tom, I shall look into Jerry Beckwith's work when I get out from under the pile of work my brother dropped on me this morning.

"Close enough" is, of course, the rational way to look at this hobby, but the mountains we have out here frequently eliminate the option of staying below 5000'. Consequently, depending on which my versions of FS9 I am using, (1930, 1939, or 1959), I usually find myself in Bill Lyon's Goose, S-43, Tamo, or a Hisso-powered Jenny I cobbled together from Willyb's Airmail addon.

And I'm not buying a new FS version unless I can bring Bill's creations with me.
jim

[Admin]

I should have included a link to Jerry's page

www.mudpond.us/

[jimslost]

This will be very helpful. Thank you, Tom.
jim