February 1, 2008
By Barry Schiff
December’s column discussed how to perform a turn with the shortest possible radius while maintaining altitude ( “Proficient Pilot: What, Me Spin?” December 2007 Pilot). A conclusion was that such a minimum-radius turn is achieved by rolling into a 75-degree bank while maintaining maneuvering speed. This is true for most airplanes, those with a limiting load factor of 3.8 Gs. During such a 3.8-G turn, the airplane is on the verge of a high-speed or accelerated stall that would occur at almost twice the speed of a 1-G stall.
Although theoretically accurate, there are two problems associated with such a turn. The first is that most pilots would be uncomfortable in a 75-degree bank while maintaining 3.8 Gs on the verge of a stall. Most have never performed such a maneuver. The second problem is that many airplanes do not have sufficient power to maintain altitude under high-lift conditions.
Paul Rorden, a general aviation flight instructor and a pilot for a foreign air carrier, sent an e-mail suggesting an alternate and more practical solution to the problem of making a minimum-radius turn such as the type needed to extricate one’s self from a narrow box canyon. (The best solution is to avoid getting into such a predicament in the first place.) Rorden’s suggestion seemed to make sense, so I tried his recommended maneuver in a few airplanes and conclude that he is correct.
Turning at as low an airspeed as possible while banking as steeply as possible reduces the radius of the turn. The problem is that low airspeed and steep bank angles are incompatible because of the rise in stall speed associated with steep bank angles. But an interesting compromise is available.
This time we are going to turn using a bank angle of 60 degrees instead of 75. This is a better choice for most pilots because it is not much greater than that typically used to practice steep turns. Also, the resultant load factor is only 2 Gs, which is more easily obtained and tolerated than 3.8 Gs.
Another advantage of limiting the load to 2 Gs is that this typically is the maximum-allowable load factor at which wing flaps may be deployed. This means that we can turn with flaps extended, something we should not do during 3.8-G maneuvering. Lowering flaps during the turn reduces stall speed, which allows us to fly more slowly and further reduce the turn radius.
In some airplanes, full flaps can be used because sufficient power is available to maintain altitude and airspeed with so much additional drag. In others, it might be necessary to use only partial flap deployment, depending on power availability. The decision to use partial or full flaps depends on how well the pilot knows his airplane.
If you are uncertain about how much flap to extend, experiment with different flap settings to determine how much flap can be used in a 60-degree bank and still have sufficient power available to maintain altitude and a safe airspeed. It gets a little complicated, though. There might be enough engine power available with the flaps fully extended when flying a lightly loaded airplane at low altitude, but this does not necessarily mean that you could repeat that performance in a more heavily loaded airplane at a higher density altitude.
My limited experience in practicing this maneuver indicates that partial flap extension should be used in most cases. This results in some stall-speed reduction without the quantum increase in drag that occurs during the last portion of flap deployment. Lowering the flaps to 20 degrees in a Cessna TU206G, for example, reduces the stall speed by 6 knots, but lowering them an additional 20 degrees (40 degrees total) reduces stall speed by only an additional 3 knots. You get the majority of stall-speed decrease with minimal drag rise. During a 60-degree bank, stall speed is 41 percent greater than when the wings are level, for a given gross weight and flap setting. The airspeed during a minimum-radius turn, therefore, should be 1.41 times the wings-level stall speed for the amount of flap selected.
For example, the stall speed of a fully loaded Cessna 206 with 20 degrees of flap extension is 59 knots (calibrated airspeed). Using 40 degrees it is 56 knots. The target airspeed in a 60-degree bank, therefore, is 83 knots when using 20 degrees or 79 knots when the flaps are lowered to 40 degrees.
It is worth emphasizing that this maneuver requires practice. Do not wait until such a turn is needed during a potential emergency. Develop the proficiency needed to confidently perform a minimum-radius turn at a constant altitude while the airplane is on the verge of a stall.
Depending on the airplane, turn radius while banking 75 degrees and pulling 3.8 Gs at maneuvering speed is approximately the same as when banking 60 degrees and pulling only 2.0 Gs with flaps extended and maintaining 41 percent above the wings-level stall speed. Again, this presumes that the airplane has sufficient power to maintain altitude during the maneuver.
Although it might seem that I am suggesting that you fly the airplane at stall speed, this procedure actually includes a nice airspeed buffer. This is because the power-on stall speed is significantly lower than the power-off stall speeds published in the pilot’s operating handbook.
If you really need to turn on a dime, this might be the best way to do it.
Safety and Education,
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