OK performance fans - recently I became interested in aircraft/engine=20 performance related to air density. Most specifically, related to the=20= dew=20 point. I was lucky enough to find an instrument (about $80) that displ= ays=20 the air temperature, %humidity and dew point temperature. The dew= =20 point is important to understand the air density difference from the IS= A=20 standard predicted only by air temperature at some altitude. = =20

I will be constructing a spreadsheet to calculate air density and plot=20= this=20 against performance. I was inspired this last summer because, after my= =20 prop was resealed, I thought the prop shop had dulled the leading edge thus=20 causing slow cruise speeds recorded on a series of muggy days. Of cour= se,=20 the first slightly cooler day with seemingly drier air returned the performa= nce=20 numbers to their prior levels.

I don't have much data yet, but I must relate an interesting experience= as=20 I flew home from Redmond. I was at 9500 MSL (10300 d alt), 10C near KA= IA=20 (western Nebraska, ground about 4000 MSL), flying in smooth air ov= er a=20 broken layer about 2000 feet below me. I was doing 166 KIAS (192 KTAS) and=20 decided to add dew point data. Using my new instrument and sticking it= in=20 the vent air outlet, I first checked that the temperature matched the outsid= e at=20 10C. Then, I got a reading of 48% humidity and a dew point (dp) at=20 0C.

After taking that reading it got more interesting as the clou= d=20 layer became quite scattered and the autopilot called for a trim=20 adjustment. Duly made, I noticed that the airspeed moved up to 168 KIA= S=20 (195 KTAS) -so I took another reading: 6% hum and -25C dp. Hmmmmm= ,=20 only a few minutes had passed.

Some minutes later, scattered layer now gone, I was at 167 KIAS (193 KT= AS)=20 with 18% hum and a -11C dp. Verrrrrry interrrrrrresting. The air= =20 mass composition, with respect to moisture, had changed quite a bit in just=20= tens=20 of miles. So had the speed. Note that the outside air temperatur= e=20 remained at 10C throughout.

I can hardly wait to do more testing on hot and muggy days, if any are=20 still around this year.

I have got to get better flight recording instrumentation. I hope= =20 that the Garmin 430 WAAS upgrade, with 5 readings per second, will report gr= ound=20 speeds in feet per minute instead of the sloppy Knots (Yep, switched to= Km,=20 it has twice the accuracy). That is assuming that ground speed is=20 meaningful when the air mass changes. Too bad I don't have those numbe= rs=20 for this stretch of flight.=20

Scott Krueger=20 AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL (KARR)

A m= an=20 has got to know his limitations.