Air University Review, March-April 1968

What is STOL?

Lieutenant Colonel Walter P. Maiersperger, USAF (Ret)

The question in the title suggests that the term “short takeoff and landing” needs definition, and so it does. But since the word “short” has only relative meaning, an explicit definition in few words is difficult, if it is to have engineering values. Since the term has been variously and loosely applied, this is my attempt to put the STOL concept into a tighter frame of reference.

When the Wright brothers made ready on the day of their historic flight, they realized that the wind velocity was greater than the stalling speed of their machine. They could have risen vertically that day had they so chosen. Instead they completed the laying of 60 feet of wooden 2˝x4˝ rail on the sands of Kitty Hawk, to be used for a takeoff runway so they would be at a comfortable speed margin above stalling speed when they lifted off. They chose to do it this way so they would have additional control over their machine in the air.

Ever since, the rules of safe conventional flight have defined a necessary airspeed margin over and above the power-off stalling speed of the airplane. At first, the low power-to-weight ratio caused the early flyers to realize that any attempt to climb at too steep an angle would cause the machine to lose speed and settle back onto the ground. Aerodynamically speaking, the high angle of attack produced so much additional induced drag that the total drag was greater than the thrust available, so the machine slowed and then settled down.

After World War I, engines were more powerful but still none too reliable. To be safe, a climb-out after takeoff had to be made at sufficient speed so that, if the engine quit, the airplane could be nosed over and a glide established at an airspeed high enough above the stalling speed to permit a successful flare, or transition, for landing to be made. If the climb-out was too steep, or if the engine quit too close to the ground, the climbing speed would be too low to permit nosing over into a good gliding speed, and a crash would follow. Thus the conventional flight rule was established that a safe climb-out speed is the stalling speed plus the margin for a safe gliding speed (to permit a landing flare to be made) plus the margin needed to nose into the glide.

These early pilots normally made their landings power-off so as to obtain the necessary practice for the frequent occasions when a real-life emergency power-off landing had to be made. The pilot always cautioned himself never to try to stretch the glide by raising the nose, as this would cause a loss in airspeed and preclude his ability to execute a proper landing flare. It was recognized as being more dangerous to stall at a low height and hit the ground than to sail straight ahead into whatever obstruction presented itself at proper gliding speed. The stretched glide, the slow glide, was called the “graveyard” glide.

As commercial aerial transportation developed, flight operating rules were codified by government regulating agencies, and proper climbing and gliding speeds were established by formulas. The Federal Aviation Regulations specify the proper climb-out and approach speeds as a percentage of airspeed margin above the power-off stalling speed of the airplane for the condition under examination. For multiengine airplanes, conditions governing performance with at least one engine inoperative are specified. For example, at the correct climb-out speed (i.e., margin above power-off stalling speed) with one engine inoperative, the airplane must demonstrate a certain rate of climb or angle of climb. Airfields from which the airplane is licensed to operate must be long enough to allow the airplane time to reach the specified climb-out speed, or it must be shown that the takeoff can be aborted and the airplane can be stopped without going off the end of the runway.

All these traditions and regulations for safe flight are now known as the conventional mode of flight, which the term CTOL (conventional takeoff and landing) now designates.

The invention of the helicopter brought into being a new mode of flight, vertical takeoff and landing (VTOL). Quickly it was realized that the safest way to operate this aircraft was very nearly the conventional airplane way. For safest operation, the vertical climb is limited to a few feet above the surface, quickly followed by acceleration to climb-out speed; and in the landing approach, deceleration from approach speed back to hover is also done close to the earth’s surface. There is little basic difference between safe airplane and safe helicopter practice.

Instead of a dread of stalling, as in fixed-wing airplanes, the dread in helicopters is to lose rotor rpm. This happens when gliding power-off at too low a forward speed to keep the rotor going fast enough to store the requisite energy for the flare. The cure is the same as in recovering from a graveyard glide in an airplane: dive to regain proper airspeed and proper rotor rpm.

What can happen when the helicopter makes a straight vertical climb or descent, with no forward airspeed? In such operations the pilot gambles that the engine will not fail during certain portions of the vertical flight. If engine failure occurs close to the ground, the helicopter simply pancakes back onto the ground without damage. If the failure occurs above a certain height, the helicopter in falling can gain forward speed and maintain rotor speed sufficient to execute a landing flare. At intermediate heights, engine failure will result in a crash because of insufficient height to achieve the proper forward speed to maintain the rotor rpm to complete the landing flare.

There is a combination of height and airspeed from which a helicopter can lose its engine and enter safe autorotational flight. Such a curve is plotted for each model of helicopter. In pilot’s terminology, it is known as the “dead man’s curve,” not too different from the “graveyard glide” terminology of airplane flight. The helicopter manufacturer naturally prefers that this curve be known as the height-velocity curve. Whatever it is called, the condition of less height or less forward speed than called for on the curve is not considered safe by conventional flight standards and is avoided as much as possible in helicopter operations.

Multiengine helicopters reduce the area under the curve in proportion to the number of engines they carry, their overall power-to-weight ratio, and other factors. Ideally, there is a requirement to produce a multiengine helicopter that can suffer the loss of one engine and continue forward flight without having to enter a controlled descent. For central city operations and for low weather minimums, this is really the only safe way. Meanwhile, vertical flight in helicopters more than a few feet off the ground is practiced mostly in commercial or military “crane” operations, not in passenger transportation. Government regulations governing climb-out and approach conditions for transport helicopters contain provisions regarding engine-out conditions and heliport size, generally similar to those which apply to airplane operations. The minimum-size heliport is one on which the operator can demonstrate a safe return to the ground following engine failure. The minimum heliport varies according to the model helicopter in use and the environmental conditions prevailing at the site. It is interesting to note that one source indicates the minimum heliport should be 700 feet in length, assuming a vertical climb to 35 feet at time of engine failure.

When VTOL airplane developments started, following the successful development of gas turbine engines during World War II, there was a big hue and cry from the helicopter proponents that such an airplane was unsafe because it could not autorotate. This undeniable “special case” logic forced the VTOL airplane proponents to install a sufficient number of engines in their designs and connect them in such a way that the loss of any one engine in vertical flight would not prove fatal. Also, one of the VTOL designs that seems most ideal for short-haul transportation, the tilt wing, has transition characteristics that reduce its power requirements drastically almost as soon as transition from vertical to horizontal flight is started. In other words, the time interval during which a single engine failure could have serious consequences is reduced to a very few seconds. In any case, the dangers inherent in the VTOL mode of flight were and are fully recognized, having been learned from twenty years of helicopter experience.

No sooner did the VTOL airplane prospects appear promising than a new rash of proponents of another kind of airplane appeared. These voices argued (and no one denied it) that a VTOL airplane was not only less efficient than a conventional airplane but also less efficient than something they proceeded to call an STOL airplane. To this day the STOL term and the STOL airplane remain undefined except in a very general sense.

Be assured by STOL proponents that STOL does not mean the World War I airplane, or even a Ford Tri-Motor or a Bellanca, all of which most certainly made short takeoffs and landings. All sorts of airplanes can be found that were designed to take off over a 50-foot obstacle in less than 3000 feet, 1500 feet, 800 feet, and even 500 feet. These distance requirements are all to be found in various government specifications seeking to identify a particular design as STOL. But none of the older designs qualify. In fact, few of the commercial designs that are advertised as STOL designs meet government requirements. What is the nature of this paradox?

Government STOL specifications usually exclude the older designs and the newer commercial designs by combining an airspeed requirement with a given takeoff requirement in such a way that they cannot qualify. In other words, the government STOL specifications require a speed range—a ratio of top speed to power—on stalling speed—plus a takeoff requirement that necessitates a special design. What kind of a design is it?

To begin with, the STOL airplane requires far more power than a CTOL airplane, and so it is less efficient and more expensive. Since it cannot VTOL, it is not directly comparable in mission capability to either the VTOL airplane or the helicopter, although it is constantly compared to them. What else is distinctive about the design? When takeoff and landing performance is computed for the STOL design, one discovers that a new reference airspeed may be in use. Not power-off stalling speed but power-on stalling speed may be the reference. Also, the margins above power-off stalling speed for climb-out and approach are reduced. An examination of this STOL mode of flight reveals a serious compromise of both CTOL and VTOL flight traditions. I shall discuss here only the longitudinal aspects. In practice, lateral, directional, and cross-coupling and thrust-coupling effects provide additional complications. Assuming all these are brought under control (though in practice they have not been yet), how do STOL operations compare with traditional flight along the longitudinal axis?

In STOL takeoff and landing operations, lift is produced by the direct or indirect application of thrust to augment the lift produced by the forward motion of the wings of the plane. In its usual form, the lift obtained from power is produced by the action of the propeller slipstream on highly flapped wings. It could take other forms. Jet lift engines could produce direct lift to augment the lift of the wings. The point is that the takeoff or landing is made at an airspeed less than the traditional margin of airspeed above power-off stalling speed.

In a single-engine airplane STOL takeoff, if the engine is lost during takeoff the airplane cannot enter a safe gliding speed unless it has reached a considerable altitude. A multiengine airplane making an STOL climb will start settling immediately after an engine failure. The perilous difficulty of an airplane operating in the STOL mode of flight is that the only way it can regain lift is to reach a higher airspeed. It cannot do this by lowering its nose, hoping thereby to reduce its induced drag and accelerate, for instead the result will be loss of aerodynamic lift and consequent faster settling. If the airplane raises its nose, it will create more induced drag, slow down even more, and settle faster. In either case contact with the ground is inevitable unless the plane is high enough to dive and thereby regain a conventional speed margin above the stall.

In the landing condition, the same predicament exists. The STOL landing is made at speeds below the normal gliding speed. In some recent military STOL designs, the approach was to be made at speeds 20 knots below the power-off stalling speed of the airplane. The rate of descent was to be held to design limits of around 10 feet per second by the use of engine power. In this particular design, the wing was totally immersed in the propeller slipstream, which gave the necessary lift when power was applied. Unfortunately, such lift vectoring also produces an associated thrust vector, which tends to speed up the plane. To prevent this, the flaps came down more than 90 degrees. Thus thrust was neutralized in the landing configuration to the degree that the airplane had a total drag greater than the thrust available from both engines. The airplane had a negative rate of climb with full-down flaps under the fun power of both engines. During an approach to landing it is obvious that the airplane could not execute a missed approach, even with both engines operating, unless the missed approach procedure was started at sufficient height to raise the flaps to their best lift-over-drag ratio, possibly at some slight sacrifice in altitude. If the airplane lost an engine during the approach (and if only the pitch axis is considered), it would immediately sink at a rate exceeding its landing-gear design vertical sink speed. Its only chance to recover would be to regain conventional gliding speed by lowering its nose and raising its flaps. At low altitudes this would only result in hitting the ground harder.

Aircraft companies that advertise STOL airplanes take a more conservative approach. They do not base their performance figures on climb-out and approach speeds below power-off stalling speed, but usually they reduce the conventional margins by 30 to 50 percent.  Since this is not a government-validated performance criterion, some manufacturers publish two sets of takeoff and landing performance figures, one labeled as government-approved certification figures, the other labeled “STOL” performance. One company, to its credit, even publishes the margin above stall at which the charts are calculated. The latest trend among STOL manufacturers is to maintain a conventional margin above stalling speed during the approach for landing and to reduce the glide distance from the 50-foot obstacle to the point of flare by depending on a so-called Beta control of the propeller. This is a variable pitch control between the normal cruise settings and full reverse pitch which allows the turbine engine to be maintained at a high power rpm setting, while at the same time adjusting the propeller pitch to produce either positive or negative thrust. In this way the angle of descent can be regulated while maintaining a full margin of speed over the stall. It is obviously a far safer procedure than reducing stall margins because neither an engine failure nor a sudden gust can appreciably affect the pilot’s control over the airplane. Full reverse pitch and power are applied after the flare is completed, to stop the airplane. The total landing distance over an obstacle may be increased by this technique, but nonetheless this is the technique preferred by the manufacturers and the one they are selling their customers. The point about Beta control is that in itself it is a recognition on the part of the STOL manufacturers that high lift devices and reduced margins above the stall may be advantageous theoretically, but Beta control is safer.

With this background, how can STOL be defined? One definition of STOL might be “that mode of flight in which part of the lift is induced by power.” Since this definition would also satisfy the powered flight of any airplane at high angles of attack, it is obviously too broad. My suggested definition is to the point:

The STOL mode of flight is one during which an airplane taking off or landing is operated at climb-out and approach speeds lower than the conventionally accepted margins of airspeed above the power-off stalling speed of the airplane.

Where does my definition of STOL leave the several airplanes being manufactured and advertised as STOL airplanes? It leaves them as excellent airplanes when operated CTOL, that is, with recognized safe certificated margins above the stall. They are a class of airplanes which, to be certified at takeoff and approach speeds lower than traditional CTOL margins, must approach the full measure of reserve power and control necessary for VTOL certification. It remains to be seen whether this can be done without making the STOL airplane just as expensive as the VTOL.

When the STOL adopts conventional margins of control, the title question, “What is STOL?” can be answered: Safe STOL is short CTOL.

McLean, Virginia

References

Brown, D. A. “727 Investigation Underscores Sink Rate,” Aviation Week, 14 March 1966.

Quigley and Innis. “Handling Qualities and Operational Problems of a Large Four-Propeller STOL Transport Airplane,” NASA TM D-1647, Ames Research Center, Moffett Field, California.

Quigley, Innis, and Holzhauser. “A Flight Investigation of the Performance, Handling Qualities, and Operational Characteristics of a Deflected Slipstream STOL Transport Airplane Having Four Interconnected Propellers,” NASA TM D-2231, Ames Research Center, Moffett Field, California.

U.S., Department of the Navy. “Request for Proposal for Light Armed Reconnaissance Airplane (COIN),” 5 December 1963.

U.S., Federal Aviation Agency, Federal Aviation Regulations, “Part 23, 25, 27 and 29 Airworthiness Standards for Airplanes and Rotocraft.” Washington, D.C.: U.S. Government Printing Office.

Wimpress, John K. “Short Take-Off and Landing for the High-Speed Aircraft,” Astronautics and Aeronautics, February 1966.


Contributor

Lieutenant Colonel Walter P. Maiersperger, USAF (Ret), (B.M.E., College of the City of New York) is Senior Aeronautical Engineer, Research Analysis Corporation, McLean, Virginia. He completed flying training in 1940 and served throughout World War II in engineering assignments in Australia, Far East Air Forces, Netherlands East Indies, and at Wright Field, Ohio, as Chief Engineer in charge of reconstruction of captured foreign airplanes. In 1946 he was USAF Technical Observer on Canadian Arctic Exercise “Musk-Ox.” After a year with Trans World Airlines, he rejoined the Air Force and served as Chief, Special Projects Branch, Weapon Systems Division, Wright Field, 1947-53; and Staff Engineer, Directorate of Research and Development, Hq USAF, 1953-57. Since his retirement he was with All American Engineering Company and International Resistance Company until his present position in 1964.

Disclaimer

The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.


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