On "flat turns"

August 8, 2007 edition
Steve Seibel
www.aeroexperiments.org

* A great deal of discussion is spent in hang gliding circles on the question of "flat turns". By quickly yawing his body hard to one side, a hang glider pilot can transfer a small yaw torque in the opposite direction to his glider. This may be useful for helping to overcome adverse yaw and for some specific thermal-centering techniques.

* However, I think that in general "flat turns" are vastly over-rated. Just because something can be done does not mean it's efficient.

* A turn is a curvature in the flight path. A curvature in the flight path is always caused by some sort of centripetal aerodynamic force (pointing toward the center of the circle). A wing is optimized to produce aerodynamic force with a minimum of drag. That's why we usually use the wing to turn. Banking the wing tilts the lift vector so that it includes a centripetal component (pointing toward the center of the circle) as well as the upward component that opposes gravity. Generating that centripetal aerodynamic force by any other means is almost surely less efficient-- i.e. it will make more drag for a given turn rate or radius, even if the wings are less banked. For a given airspeed, if we are achieving a given turn rate with a lower-than-normal bank angle, we must be generating the centripetal force that drives the turn by some means other than the banked wing, and we can expect the sink rate to rise. The wing is the most efficient way to generate aerodynamic force, so using the wing to generate all the needed aerodynamic forces (the upward force that opposes gravity plus the centripetal force that causes the turn) is always the most efficient way to go-- a normal, banked, turn surely delivers the least drag for a given turn rate or radius.

* If this were not the case those pilots who do have lots of yaw control (those flying planes with rudders) would choose to make flat (skidding) turns. In actual practice, they don't.

* Turning by yawing sideways is exactly analogous to flying along in a knife-edge vertical bank in a straight-line flight path in an aerobatic airplane-- in both cases drag goes way up because we are using some surface other than the wing to generate some of the needed aerodynamic forces (the upward force that opposes gravity and/or the centripetal force that causes the turn.) The optimum aircraft for knife-edge vertical flight is one with a broad, flat-sided fuselage. Note that this is the shape used by the RC models that are designed for "3D"-style aerobatics. If such a model is a biplane, the struts between the wings are often flat surfaces that run the whole width of the wing chord, rather than the conventional narrow struts. This further increases the side area of the aircraft. When all this area moves sideways through the air, it generates a significant aerodynamic sideforce that can be used to support the aircraft's weight during vertically-banked knife-edge flight, or can be used to make a "flat" non-banked turn. Again, these maneuvers are less efficient than normal wings-level flight or a normal banked turn, but they are possible. The fuselage side and other surfaces of the aircraft act like inefficient wings and generate significant aerodynamic forces when they meet the air in a sideways manner, i.e. at a non-zero "angle of attack". A hang glider, on the other hand, has the opposite shape-- it has very little surface area as seen in a side view.

* Because a hang glider or other flying-wing aircraft has little surface area as seen in a side view, the drag penalty for a given slip or skid angle (a given sideways angle in the airflow) will be less in a hang glider than in a "conventional" aircraft with a fuselage. The other side of this coin is that hang gliders generate relatively little aerodynamic sideforce in the presence of a sideways airflow. In other words, when we fly a hang glider sideways, very little happens. Very little turning force is created. This means that a "flat turn" technique will be much less effective in a hang glider than in a "conventional" aircraft with a fuselage. This also means that you won't see hang gliders-- even with souped-up Mosquito engines-- successfully flying vertical "knife edge" maneuvers!

*There's one sense in which it does make sense to seek a "flat turn"-- by flying as slowly as possible, we minimize the turn radius for a given bank angle, or we minimize the bank angle needed for a given turn radius.