Hot, High Density Altitude

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Rich
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Joined: Mon Aug 30, 2010 12:40 pm
First Name: Rich
Aircraft Type: DA40
Aircraft Registration: N40XE
Airports: S39 Prineville OR
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Hot, High Density Altitude

Post by Rich »

We're having unusually hot weather around here these days. Today was a very interesting example. 25 Deg C on the ground (3250 ft. MSL) and also 25 Deg C where I leveled off at 7500 ft. That made the DA right at 10,000 ft. WOT (22.3 " MAP) and 2400 RPM yielded 144-145 KTAS. All good, right? Not so fast. It also yielded CHTs at 400 deg F and I had to use 11 GPH to keep it that low. Oil temp was nice and toasty also at around 210 deg F. It's very unusual for my oil temp to even hit 190.

The issue here is we're talking air density (molecules per cubic foot) available to carry off the excess heat from the cylinders that is equivalent to 10,000 ft. BUT at a temperature that is 30 deg C higher than is standard for that air density and 25 deg C higher than is standard for 7500 ft. Since the ability to transfer heat by forced (or unforced) convection is roughly linearly related to the temperature differential between (in this case) the cooler air and the CHTs, equilibrium required that the CHT's rise to reach the required equilibrium.

Getting the CHT's down was done by reduction of RPM to 2300 (no other changes), which brought them down to the mod-380s and reduced fuel flow to 10.4 GPH or so. It cost me a couple of knots. Going lean of peak was the other obvious option but this was not part of the experiment. Plus it wouldn't have been long before the afternoon convection gremlins would rise up and be expected to confound reasonable measurements.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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