- Some aircraft are prohibited for slips with flaps
Certified aircraft are not prohibited slips with flaps, the certification requirements state (my bold):
Sec. 23.143
General.
(a) The airplane must be safely controllable and maneuverable during --
(1) Takeoff;
(2) Climb;
(3) Level flight;
(4) Dive; and
(5) Landing (power on and power off).
(b) It must be possible to make a smooth transition from one flight condition to another (including turns and slips) without exceptional piloting skill, alertness, or strength, and without danger of exceeding the limit load factor, under any probable operating condition..................
And:
Sec. 23.177
[Static directional and lateral stability.
]
(a)
Three-control airplanes. The stability requirements for three-control airplanes are as follows:
(1) The static directional stability, as shown by the tendency to recover from a skid with the rudder free, must be positive for any landing gear and flap position appropriate to the takeoff, climb, cruise, and approach configurations. This must be shown with symmetrical power up to maximum continuous power, and at speeds from 1.2
up to the maximum allowable speed for the condition being investigated. The angle of skid for these tests must be appropriate to the type of airplane. At larger angles of skid up to that at which full rudder is used or a control force limit in Sec. 23.143 is reached, whichever occurs first, and at speeds from 1.2
to VA, the rudder pedal force must not reverse.
(2) The static lateral stability, as shown by the tendency to raise the low wing in a slip, must be positive for any landing gear and flap positions. This must be shown with symmetrical power up to 75 percent of maximum continuous power at speeds above 1.2
, up to the maximum allowable speed for the configuration being investigated. The static lateral stability may not be negative at 1.2
. The angle of slip for these tests must be appropriate to the type of airplane, but in no case may the slip angle be less than that obtainable with 10° of bank.
(3)
In straight, steady slips at 1.2
for
any landing gear and flap positions, and for any symmetrical power conditions up to 50 percent of maximum continuous power, the aileron and rudder control movements and forces must increase steadily (but not necessarily in constant proportion) as the angle of slip is increased up to the maximum appropriate to the type of airplane. At larger slip angles up to the angle at which the full rudder or aileron control is used or a control force limit contained in Sec. 23.143 is obtained, the rudder pedal force may not reverse. Enough bank must accompany slipping to hold a constant heading. Rapid entry into, or recovery from, a maximum slip may not result in uncontrollable flight characteristics. ...............
Someone is going to point out that some mid sized single Cessnas are placarded "Avoid slips with flaps extended". That's correct. It's not a
prohibition, it's an "
avoid". It warns the pilot that slipping with flaps could create instability. The aircraft still met the requirements for certification ('cause it was certified) but was less than ideal. The background is that the span of flaps of 172's, and 182's is such that in a slip, the wake off the outboard flap can impinge on the horizontal stabilizer, and upset the smooth airflow there. 150/150 do not suffer from this, as they have less span flaps. This is well described in the book "Cessna, Wings for the World" by Thompson, a former Cessna test pilot. In this book, Tompson writes that the aerodynamic deficiency was corrected in later models, but the placard was retained out of an abundance of caution. A little sad in my opinion, as a slip is an excellent means of controlling descent, once you're well trained to fly them within the aircraft's limitations, cautions, and with good airmanship.
Of course, an "avoid" placard should be carefully considered, and complied. But it is important to understand that it is not a prohibit, it's not an operational limitation. The wording in flight manuals, and warning placards is chosen very carefully ('cause of the lawyers), and must be considered by the pilot very literally - again, not a home made procedure, what the manufacturer says about flying the plane.