Lorn,

I wonder whether these two indicated airspeeds are correct.  Have you done static port position error testing near stall?  With flaps extended and retracted?  I'm guessing most people don't.  I wouldn't be surprised if flaps change the airflow near the static port(s)--either due to change in aircraft attitude or more directly--enough to cause a few knots of airspeed error.

If you want to check what effect the flaps have at low speed without the toil of a full PEC evaluation you could do a rather quick evaluation.  While flying at the same altitude and heading the same direction in the same airmass, simply fly an airspeed a few knots above stall (say 85 kias, in your case) with flaps up, and note your groundspeed (from GPS).  Then extend flaps and fly the same groundspeed and note your airspeed.  The indicated airspeeds shouldn't be compared to the groundspeed for accuracy, but the difference in indicated airspeeds should be a good first order approximation of the relative effect of your flaps on Pitot-static error.  This relative error could then be applied to your stall numbers to determine the effect of flaps on your stall speed, for a given GW/CG.

 Cheers,
- Kyrilian


On Sep 8, 2009, at 9:23 PM, Lorn H Olsen <lorn@dynacomm.us> wrote:

On my airplane, I stall at 80 kias with no flaps and 78 kias with 25° of flap. 25° of flap gives my lowest stall speed. I use this setting for my instrument approaches.