Chicken6, I think what you may be seeing is roughly this. The best glide speeds for flaps 0 and flaps 10 are different. In a C152 the figures are (roughly) 70 kts clean and 65kts with flap. Thus if you are gliding at slightly lower than your optimum clean gliding speed and you lower 10 flap, you would probably see the RoD decrease a little. Try increasing your gliding speed a little and repeating the experiment. If your RoD increases this time you've nailed it. If not then I'm buggered if I know.
Cheers DB6
[This message has been edited by DB6 (edited 07 May 2000).]