Tailwheel thoughts



This article is not intended to be a comprehensive instructional text for tailwheel pilots.  Rather, as the title implies, it is a collection of informed ramblings that should be of interest to tailwheel pilots.


Stability and the tailwheel


Aerodynamic stability can be defined in terms of degrees of an aircraft's resistance to movement around its center of gravity. Examples of static stability around the pitch axis (lateral stability) are presented in most aviation texts. An aircraft that displays positive stability returns to its original pitch attitude after it has been displaced by a gust or an elevator input. Negative static stability is characterized by a tendency to increase the deviation from the original pitch attitude after the initial displacement.


Although lateral stability is the most widely discussed aspect of this subject, it must be recognized that relative stability around the other axes plays a significant role in an aircraft's performance characteristics. Of particular interest to the tailwheel pilot is the subject of stability around the vertical or yaw axis (longitudinal stability). An airplane whose configuration or flight characteristics favor longitudinal stability will resist yawing motions. The opposite is also true.


Let's compare tricycle-gear and tailwheel airplanes in terms of longitudinal stability (Refer to Figure 1). A Cessna 182D and a Cessna 180K are nearly identical airframes, except for their landing gear configurations. The Cessna 182 has two large main landing gear legs aft of the CG, and one, somewhat smaller nose gear located forward of the CG. In flight, the large amount of drag created by the main gear, which is only partially offset by the drag of the nosegear, combines to create a “center of drag” that is aft of the CG and increases longitudinal stability. On the Cessna 180K, the main gear are located forward of the CG, and the very small tailwheel is located far aft of the CG.  The “center of drag” created by this configuration forward of the CG destabilizes the aircraft longitudinally.  Since this instability generates in the tailwheel airplane a tendency to fly backwards, it is logical to incorporate into its design a rudder with greater effectiveness (compared to the equivalent tricycle design) so that adequate control can be maintained.  Tailwheel sages commonly declare that a taildragger flies exactly like a tricycle-gear airplane once it is in flight.  Obviously, that concept is not entirely accurate. 


Figure 1


Once on the ground, however, the relative longitudinal stability of the different designs really plays an active role in their individual performance characteristics.  In the case of the tricycle-gear configuration, the aft “center of friction,” created by the surface contact of the main gear behind the CG, increases longitudinal stability during the rollout after landing and during the takeoff roll before liftoff (Refer to Figure 2).  The tailwheel airplane, by comparison, has a forward “center of friction” which enhances the possibility of longitudinal excursions which, if not promptly and correctly countered, may result in the often-expensive and always-humbling ground loop.



Figure 2


Steering on the ground


There are two ways to steer a tailwheel airplane on the ground.  The rudder can be used for aerodynamic steering, or the steerable tailwheel can be used for frictional steering, or both.  Successful ground handling, including takeoffs and landings, require at least one at all times.


An analysis of the takeoff roll will illustrate this.  Before power is applied and forward motion is begun, the only relative wind is provided by the headwind component and the slipstream of the idling propeller.  The effect of any crosswind component is countered almost entirely by the friction of the tailwheel as it rests on the runway surface.  The conventional-gear configuration allows the horizontal stabilizer to be situated in a positive angle of attack.  As power is applied and the airplane accelerates, the increasing relative wind provides a constantly-increasing effectiveness of the rudder.  At the same time, the lift provided by the horizontal stabilizer unloads the tailwheel, decreasing its frictional steering effectiveness.  When the lift is sufficient to lift the tailwheel clear of the surface, the rudder is fully effective.  Some tailwheel pilots and instructors advocate applying full forward elevator control simultaneously with the application of takeoff power.  This increases the lift generated by the horizontal tail surfaces, prematurely unloading the tailwheel.  As the “center of friction” is shifted forward, frictional steering is drastically reduced  before full rudder effectiveness is achieved.  This works as long as the pilot is absolutely perfect with all control inputs, and nothing (like a crosswind gust or a large p-factor/gyroscopic input) displaces the airplane around the yaw axis.  The new tailwheel pilot will probably achieve better results by allowing the tailwheel to lift off at an airspeed that will allow sufficient aerodynamic steering by the rudder.


Crosswind technique


All pilots have been properly trained (presumably) to employ traditional crosswind techniques when performing crosswind takeoff, landing and taxi operations.  This technique includes the practice of deflecting the aileron controls into the crosswind.  The common explanation given for this action is that it minimizes the chance of the aircraft flipping over in a strong crosswind.  This explanation is pretty feeble, since a crosswind strong enough to put an airplane on its back would better be handled by a PIC decision to avoid taking off in those conditions, or (in the case of landing) selecting a runway more closely aligned to the wind.  Since most pilots wisely avoid flying in winds of this intensity, and since the ground stability provided by the tricycle configuration reduces the adverse effect of crosswinds, this technique has fallen into general disuse. 


The real reason for using proper crosswind technique seems to have been lost to time.  The most common explanation flight instructors offer when insisting that crosswind takeoff and landing procedures be used in crosswinds of any intensity is that it is important to develop good habits in preparation for the unexpected occurrence of an upsetting crosswind.  In fact, a deeper understanding of the aerodynamic forces involved illustrates the importance of always employing proper crosswind takeoff and landing procedures any time a crosswind is present. 


Refer to Figure 3.  Before beginning the takeoff roll, the crosswind strikes all the vertical surfaces, most of which are located aft of the “center of friction,” creating a rotational force vector  (shown in blue) that would cause the airplane to weathervane, except that  the friction of the tailwheel on the runway surface prevents it.  The pilot has deflected the aileron controls fully in the direction of the crosswind and applies takeoff power.  The airplane accelerates, and relative wind increases.  The downward-deflected aileron develops more lift, with the associated exponential increase in drag at the outboard section of the downwind wing.  As the relative wind increases pressure on the lower surface of the horizontal stabilizer (which is at a positive angle of attack in a tailwheel airplane), frictional control  provided by the tailwheel is reduced.  As tailwheel steering is lost, the drag differential between the ailerons generates a rotational force (shown in red) that partially counteracts the weathervaning  effect of the crosswind.  During the acceleration, rudder effectiveness increases, and the pilot neutralizes the aileron controls so that the airplane does not actually enter a bank while still on the runway. 





Figure 3


The crosswind landing is the reverse of this procedure.  The pilot touches down in a side slip (in which the longitudinal axis of the airplane and the direction of travel is aligned with the runway centerline) on the upwind main landing gear.  The power is reduced, and as the aircraft decelerates, the ailerons are deflected in the direction of the crosswind at a rate necessary to stay on the upwind main gear.  As relative wind is decreased, the ailerons will become fully deflected and the differential drag will decrease, helping to prevent weathervaning.  The lift on the horizontal stabilizer will decrease, transferring weight to the tailwheel.  Full up elevator then maximizes frictional steering.  The airplane is now in the recommended crosswind taxi configuration. 


Note:  This article, because of its open-ended nature, has been posted as a work in progress.  The reader is encouraged to check back for additions to its content.