Tigerman
The C172 manual warns against slipping, with flaps extended because the wing/fuselage configuration can lead to a very unpleasant stall/spin. In other words, when flying out of balance the wings/flaps/fuselage on one side can "blank" the other wing from the airflow causing a stall on one side, which can flip the a/c over into a spin.
Having said that, I know many pilots who have slipped a 172 with flaps and survived, but I wouldn't try it myself.
I find that slipping the 172 when clean gives a good 1500' rate of descent, which can be instantly removed without side effects, unlike flaps. Therefore I'd slip the 172 clean and then feed in flaps later in the approach - as I recall you can extend 10 degrees at up to 135kts or thereabouts and then the limit is 90 kts for the rest, so this is quite practical.
The PA28 can be slipped with 40 degrees of flaps without fear!
[This message has been edited by JamesG (edited 18 January 2001).]
[This message has been edited by JamesG (edited 18 January 2001).]
[This message has been edited by JamesG (edited 18 January 2001).]