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Date: Mon, 11 Sep 2006 09:29:47 +0200
From: Alain NOIREAUX <alainoireaux@club-internet.fr>
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To: Lancair Mailing List <lml@lancaironline.net>
Subject: Re: [LML] Do the Dew Point affect the "Do" point - Like "Do" it go
 slower? Faster?
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Sky2high@aol.com a écrit :

> 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.
>  
> Scott Krueger AKA Grayhawk
> Lancair N92EX IO320 SB 89/96
> Aurora, IL (KARR)
>
> A man has got to know his limitations.

Hello Scott,

it is interesting indeed . What is this instrument ? How does it look 
like ? Where did you find it ?Should you get a photo ?
Have a nice day .
Alain NOIREAUX   L 320


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