DA20-C1 Forward Slip with Full Flaps

When I was training in the DA20-C1, I recall my CFI mentioned
something about slips not being recommended in a DA20-C1 with full
flaps. We then proceeded to practice slips with full flaps. This was
usually not a problem. I did notice, however, on two occasions where
I did not seem to have much elevator authority afer exiting a forward
slip with full flaps at idle. It was a little un-nerving: I pulled
back on the stick and the plane kept heading toward the ground. All
it took was a little power to get the elevator control back. I tried
to demonstrate this problem to my CFI at altitude, but was unable to
reproduce the effect. Have you heard of this before? Is this reason
they don't recommend the slip-to-land in the DA20 (is that even the
case)? Is it a function of the t-tail getting disturbed airflow over
the tail?

BTW, thanks for a great website!

-Ray

-- Ray Letulle, March 13, 2007

Answers

There is nothing in the DA20 POH as far as I know that talks about slips with or without full flaps. A lot of instructors get confused because they fly many different types of airplanes and your CFI was probably remembering something from some other airplane's POH. I myself was telling students that it was illegal to run the DA40 at more than 2400 RPM because that was a POH limitation (put in for noise certification in Europe). I hadn't noticed that buried in the supplements for the two-blade prop was a change of this limitation to 2700 RPM.

If you were concerned about airflow over the elevator, a plane with a T-tail should be more stable given power and configuration changes. The whole point of the T-tail is to get the elevator away from the propwash and disturbed airflow from the wing.

-- Philip Greenspun, March 13, 2007

When I learned flying in a PA-28 thirty years ago I was told by my CFI never to sideslip a low wing airplane on landing to throw off altitude. Contrary to high wing airplanes like the Cessna family of aircraft, the airstream over the a low wing becomes disturbed by the cabin in a side slip. This causes a reduction in lift of that wing. There have been known cases in which the maneuver caused the wing in the cabin 3shadow3 to stall, making the airplane roll over. If this happens at low altitude - which is usually the case since sideslipping is used to increase sink rate during approach - this is potentially deadly. Side slip produces a higher rate of sink but, as you describe, sometimes cannot be controlled by the elevator. If this happens close to the ground you're getting yourself into a problem. The golden rule I was taught by my CFI never to forget: NEVER SIDESLIP A LOW WINGED AIRPLANE WHEN LANDING, just use your flaps and fly a normal approach.

-- Herman De Wulf, March 13, 2007

I have side-slipped many airplane types on short final, mostly low winged types. I tend not to use the maneuver with the Cessna 172 using full flaps, as the POH warns against it - not because it is dangerous, but because it induces some vibration in the elevator, which translates to the wheel and that is a little uncomfortable. The only unplesantness I have experienced doing sideslips in low-wing airplanes is that it is not effective in all types. I don't find that sideslipping to loose altitude on final works well in a Mooney, for example, but it is fun and effective in European types like Grob 115 and TB-20 (-10, -9). It is many years since I flew the DA20 (Rotax version), but as I recall, it sideslipped normally. The sideslip will blanket a small part of the stabilator/elevator in all airplanes, and for this reason the nose will tend to drop when the maneuvre is employed. Conversely, when the aircraft is straightened, the nose will tend to rise, and the attitude and airspeed should be checked. Like all maneuvres, it could be dangerous if executed at low altitude without previous training.

Aerodynamically, the sideslip is no different from landing in a cross-wind, wing-low method. For this reason, I would not expect an aeroplane that exhibits dangerous manners in a sideslip to obtain a CoA.

30 years ago most PA-28's still had the Hershey bar wing, which is not as forgiving as the newer tapered PA-28 wing. Stalling the old PA-28s with "crossed" controls (as in a sideslip) could provoke more of a wing drop than for newer types. Could this be the origin of the strange advice never to sideslip a low-winged aeroplane? I don't know - maybe there was a landing accident once that was attributed to sideslipping. I find it a useful and safe maneuver in all types of aircraft, although I hesitate to do it when passenger comfort is an issue.

-- Henrik Vaeroe, March 13, 2007