More on slips.
Thanks for the excellent article on slips ("Slips ... Who Needs lEm?") in the December 2011 issue. Thanks also for publishing Rich Stowell's follow-up letter in the January issue. Some good points were brought up in both. It was the diagram accompanying Mr. Stowell's letter on which I wish to comment however.
In short, the diagram is misleading. It would have been more accurate to label the diagram "slip" rather than "forward slip." Let me explain why.
As Mr. Stowell pointed out, a forward slip and side slip are the same thing, aerodynamically speaking. A slip is a slip. The words "side" and "forward" refer to the position of the airplane's ground track with respect to the extended runway centerline (assuming no wind).
In a "forward" slip, the ground track coincides with the centerline, while in a "side" slip, it is at an angle to it. For both types of slips, the flight path/relative wind is at an angle to the longitudinal axis of the airplane (as shown in the diagram), but it is impossible to call the slip in the diagram a forward or side slip without knowing the orientation of the runway relative to the aircraft.
Now, regarding a side slip, I'm sure many are now asking, "How can the ground track be at an angle to the runway?" This is an excellent question, and shows precisely why a side slip is a great way to correct for a crosswind. In an (opposing) crosswind, that angle is eliminated, therefore the airplane stays on centerline.
I also wanted to point out a subtlety in the diagram that some may not have noticed. The airplane is in a left-wing-low attitude, as indicated by the shadow on the left wing. It might also have been a good idea to have added the rudder, deflected to the right of course.
Thanks for an excellent magazine! It is always thought-provoking and makes for some great discussion!
David Gill, CFII, MEI Charlotte, N.C.