Takeoff FF vs. OAT (IO-360)

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briankelly327
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Takeoff FF vs. OAT (IO-360)

Post by briankelly327 »

Polling the experts here - should takeoff fuel flow correlate to OAT?

Logically, Id imagine that in wintertime, the negative density altitudes (low temp, high pressure) would draw more fuel flow through the servo to feed more oxygen. But Ive noticed the inverse, that on very cold high pressure days, my takeoff FF barely gets to 17gph. During warm summer months, my takeoff FF is more like 17.5-17.6gph (same RPMs). Also this is with powerflow.

My concern would be that the lower fuel flow combined with more oxygen effectively runs less ROP than desired at high power. CHTs seem to be okay regardless, but EGTs are into the 1400s on takeoff in these cold conditions.
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Steve
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Re: Takeoff FF vs. OAT (IO-360)

Post by Steve »

According to the documentation I have, the RSA-5 injector servo is altitude compensated, but does not have any independent temperature compensation mechanism. I'm not enough of a fluid dynamicist to figure out what effect temperature extremes would have beyond the air density compensation. I haven't noticed this phenomenon on mine, but then again, I live in TX, and have rarely operated the aircraft at surface OATs below 40 degrees F.

Being that it is wintertime, I will pay more attention to the FFs vs temperature to try to get some comparative data for you.
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Rich
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Re: Takeoff FF vs. OAT (IO-360)

Post by Rich »

I have speculated on the fact that in cold-weather at sea-level one starts producing significantly more power than 100%. For example 0 degF at Caldwell would likely mean the engine producing 110-ish percent power. But I don't think FF of 17 GPH would be a problem, especially with this engine.

According to the Lycoming manual for this engine, best power at 2700 RPM is at 14.5 GPH. If we bump that by 10% because of more O2 molecules getting sucked in, that brings us to 16 GPH. The excess is expected to not be burned at all, but used to cool the engine on climbout. And with the cold OAT helping out with CHT's we're golden in that regard.

We're flying over-octaned. The engine is rated for 91, and we're flying with 100-octane. Apparently 100LL tests a bit above that, like 103.

1400's deg EGT isn't in itself a problem. I have historically cruised for hours over many years in the mid-high 1400's. Surefly complicates that at lower MAP/higher altitudes. But it's still normal in some circumstances.

Basically all I concern myself with is CHTs.
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Re: Takeoff FF vs. OAT (IO-360)

Post by Rick »

I haven't paid that close attention either, but ... the DAN does have thousands of data files uploaded from our users, so you could sort through some and look at fuel flow and OAT and see what others are getting if you were so inclined!
2007 DA40 XL - Roanoke, VA (KROA)
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