EXPLORING THE STOL MODE

(Excerpts from a two part series published by FLYING Magazine - June/July 1974)


In the corner of every airplane's flight envelope, there's a regime that too few pilots know well.

By Robert B. Roe


The immutable laws of aerodynamics dictate that a quid must be exacted for a quo, and this stern reality applies to STOL aircraft and STOL conversions as well as to more conventional designs. Short takeoffs in STOL airplanes are quite safely accomplished by pilots with average skills. What ordinary pilots must remember, however, is that a STOL machine can achieve its STOL-mode landing performance safely only after a careful checkout and a complete understanding of what is happening during approach, flare and touchdown. Most pilots do not realize that habit patterns learned as part of conventional experience have a way of complicating STOL landings, and they must understand at the onset that dangerously high sink rates can occur if a thorough understanding of the fundamentals is not mastered before practice begins.

The throttle becomes a primary flight control for operations in the rarified field of STOL. Not only is routine climb and descent accomplished with power, but effective throttle use during flare directly affects landing safety. Quick response for control of sink near the ground is imperative. The effectiveness of using power to maintain a desired glide slope also determines the touchdown error in spot landings, where accuracy is vital to STOL performance.

The process of decreasing the rate of sink until it is low enough to allow a safe landing requires energy. During conventional landings, this energy is supplied by the airplane itself, which maintains sufficient airspeed in giving up altitude during its descent. As long as such speed is maintained, enough added lift can be produced during rotation so that the rate of descent is arrested and the airplane landed without use of engine power.

If the airspeed is too low, however, airspeed change during rotation will not yield sufficient lift to slow the descent enough for a safe touchdown. In this case the pilot must use engine power to augment the lift being created by rotation. This combination of conditions is the STOL mode of landing, which can be defined as any landing during which the engine must be used to avoid a hard landing. This applies to conventional aircraft as well as those designed for STOL.

The degree of power that must be used in the landing is, in general, determined by the approach speed. The lower the speed, the more likely power will be necessary for flare. Thus, in anticipating a STOL-mode landing, the pilot can control the degree of commitment to power. In total STOL commitment, negligible lift increase is developed in the rotation, and practically all deceleration in descent is the result of engine power...The STOL mode includes all conditions in which lift during flare comes from any combination of rotation [and] engine power...the pilot has control of this mixture and decides at what time in the approach to make the commitment and to what degree.

STOL-mode landings involve techniques quite different from those used in conventional landings. Careful attention to holding constant airspeed and aircraft attitude, rapid recognition of sink, accurate management of power in the flare, quick response in holding the desired glide path and knowing through experience when not to close the throttle are skills that must be developed...

There's no way one can learn from a text how to "perceive sink" or "adjust the power to land." This can be imparted only by training in the airplane. Two common errors that plague pilots must be recognized before flying begins, however. Loss of airspeed control is one; the other is premature reduction of power.

Effective airspeed control is a matter of practice making perfect, but beating the problem of premature power reduction is a state of mind as much as anything. It is the overcoming of old patterns learned in normal-mode landings, and it's accomplished through understanding the throttle and its nonlinear relation to the rate of sink. You must say to yourself, over and over, "STOL-mode landing means landing with power." You don't cut the throttle and land, you land and then cut the gun. You must recognize that the throttle is a primary flight control in this mode, and you must resolve never to initiate flare with less than zero-thrust manifold pressure when approach at STOL-mode speeds - no matter how much you may be overshooting. It is good practice, especially if the airplane is heavy, to enter flare with not less than the average manifold pressure used for the power-on STOL descent. This procedure will prevent large sink rates from developing suddenly and will also keep the engine ready to deliver more horsepower quickly if required in landing. Minimum power at flare is one number that the pilot should have firmly in mind for every STOL approach.

The ability of airplanes to take hard landings varies widely. Generally, an airplane designed for STOL operations will be stressed to take harder arrivals than a conventional airplane...

The flare and landing maneuver constitute a dynamic transition from one equilibrium situation in the approach to another at ground contact. During this transition, the aircraft is not in equilibrium, because airspeed, lift, drag and power all changing...

Successful STOL-mode landings are a combination of an accurate spot landing resulting from a well-controlled approach, and a flare performed to insure safety in the touchdown itself. It is the successful blending of these two phases of STOL mode that separates the beginner from the experienced STOL pilot...

There are a number of subtle considerations that have a significant impact upon the STOL approach performance by influencing landing touchdown accuracy and, consequently, the final landing distance. Probably the most critical of them is the ability of the pilot to hold airspeed at STOL speeds with the cues that are available to him. In the STOL 210, for example, the pilot soon learns that to hold airspeed requires practiced skill and undivided attention, and the slower the speed, the more difficult the task becomes - especially in turbulence. The "squeezed airspeed scale on the low-speed portion of the dial doesn't help matters.

The problem is seriously compounded by the fact that the difference between indicated and calibrated airspeed changes markedly over the lower speed range. In the case of the STOL 210, at an indicated 70 mph, the error is only one mph; at 60 mph IAS, however, it increases to three -mph difference; at 50 mph IAS, to seven mph; and at 45 mph IAS, to 10 mph. In turbulent air, a portion of the change in indicated airspeed as the ASI inevitably fluctuates is due not to the airplane's changing speed but to the error's changing, and the error is a significant portion of the speed being observed. As the speed decreases, the pilot's difficulty in interpreting rates of change and anticipating what to do becomes more difficult...

A common misconception is that a STOL student must first work at obtaining the shortest landing-roll distance. Not so! Priority must be given to learning safe spot-landing techniques with the emphasis on reduction of the touchdown dispersion around the desired landing spot. The speeds will gradually reduce as proficiency increases, and the total landing distance will improve. ST