From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Sky2high@aol.com
Sent: Monday, September 11, 2006 12:38 AM
To: Lancair Mailing List
Subject: [LML] Do the Dew Point affect the "Do" point - Like "Do" it go slower? Faster?

OK performance fans - recently I became interested in aircraft/engine performance related to air density.  Most specifically, related to the dew point.  I was lucky enough to find an instrument (about $80) that displays the air temperature, %humidity and dew point temperature.  The dew point is important to understand the air density difference from the ISA standard predicted only by air temperature at some altitude. 

I will be constructing a spreadsheet to calculate air density and plot this against performance.  I was inspired this last summer because, after my prop was resealed, I thought the prop shop had dulled the leading edge thus causing slow cruise speeds recorded on a series of muggy days.  Of course, the first slightly cooler day with seemingly drier air returned the performance numbers to their prior levels.

I don't have much data yet, but I must relate an interesting experience as I flew home from Redmond.  I was at 9500 MSL (10300 d alt), 10C near KAIA (western Nebraska, ground about 4000 MSL), flying in smooth air over a broken layer about 2000 feet below me. I was doing 166 KIAS (192 KTAS) and decided to add dew point data.  Using my new instrument and sticking it in the vent air outlet, I first checked that the temperature matched the outside at 10C.  Then, I got a reading of 48% humidity and a dew point (dp) at 0C.

After taking that reading it got more interesting as the cloud layer became quite scattered and the autopilot called for a trim adjustment.  Duly made, I noticed that the airspeed moved up to 168 KIAS (195 KTAS) -so I took another reading: 6% hum and -25C dp.  Hmmmmm, only a few minutes had passed.

Some minutes later, scattered layer now gone, I was at 167 KIAS (193 KTAS) with 18% hum and a -11C dp.  Verrrrrry interrrrrrresting.  The air mass composition, with respect to moisture, had changed quite a bit in just tens of miles.  So had the speed.  Note that the outside air temperature remained at 10C throughout.

I can hardly wait to do more testing on hot and muggy days, if any are still around this year.

I have got to get better flight recording instrumentation.  I hope that the Garmin 430 WAAS upgrade, with 5 readings per second, will report ground speeds in feet per minute instead of the sloppy Knots (Yep, switched to Km, it has twice the accuracy).  That is assuming that ground speed is meaningful when the air mass changes.  Too bad I don't have those numbers for this stretch of flight.