The definition of V/STOL is somewhat fuzzy. It has been defined by NATO’s Advisory Group for Aeronautical Research and Development, the U. S. Army, and various symposia—all of whom have agreed in principle, but not in detail, as to the exact capabilities an aircraft must possess before it is considered a V/STOL. A logical definition, and one that appears to be capable of withstanding most criticism is as follows: Vertical Take-Off and Landing (VTOL), is the ability of an aircraft to take off and land vertically, clearing an obstacle 50 feet high which is 50 feet from the aircraft; and Short Take-Off and Landing (STOI.) is defined as the ability of an aircraft to take-off and land within 500 feet of ground roll, i.e., the distance from the touchdown of the wheels to the stopping of the aircraft.
There is much potential worth in an aircraft which can hover as efficiently as a helicopter. If we further supplement this hovering ability with the capacity for achieving great speed and carrying heavy loads, we can see that such a hypothetical aircraft would most certainly be a tool of prodigious capability for the military planner.
“Great speeds” and “heavy loads” are, of course, relative terms, subject to change, and dependent upon the background of the user. Still, in any man’s language, 36 combat troops comprise quite a load, and .92 Mach is a respectable speed. We have, or shortly will possess, this potential in two different Vertical/Short Take-Off and Landing aircraft.
There are essentially seven different types of VTOL aircraft, categorized generally by the way lift is maintained in a hover or vertical mode of flight, and there are numerous STOL airplanes employing a great number of lift principles. The European community, for example, has developed a startling array of multi-talented aircraft. Beyond the basic seven types, there are or will be other types, as this rapidly expanding art is able to make use of larger thrust-producing, lighter-weight engines—the most serious U. S. deficiency in the advancement of V/STOL technology.
A list of these seven types should begin with the best known—the helicopter. Although it is in a class by itself, it is, nonetheless a true VTOL. The helicopter utilizes rotating airfoils, called rotors, in various configurations to produce lift or to counteract torque. Examples are the Sikorsky SH-3A, Kaman UH-2A, Vertol CH-46A, Bell UH-1E and the Sikorsky SH-34J. The helicopter’s advantages are its great hovering efficiency, low downwash velocity, and good weightlifting capability (by comparison with other VTOL). Its greatest disadvantages are its slow speed (210 m.p.h.), relative complexity, and short range. The helicopter was the first practical VTOL, because the low power-to- weight ratios of conventional piston engines required the lift derived from a rotating flying wing (rotor) to be at a maximum if even hovering flight were to be attained. The present helicopter is certain to be followed by variations of its present configuration, designed to increase its speed and range. The compound or unloaded rotor helicopter appears to hold great promise in this area. The Piasecki 16H compound helicopter is a prominent example, featuring a “ring tail” for directional control and torque compensation.
It is still premature to fix firmly the advantages or disadvantages of the following types of aircraft except to say that all of them will in all probability be able to exceed the helicopters’ top speed. It is probable that no one VTOL principle will satisfy all operational tasks, thus necessitating research in at least several types.
Tilt Wing aircraft employ engines with propellers mounted in the wing(s) which are designed to tilt up to 100° or more so that the propellers are producing thrust vertically. The wing is tilted more than 90° in order to facilitate rearward flight. Pitch control in a hover may be provided by a horizontally disposed tail propeller that would stop rotating once sufficient forward flight speed is attained. A similar, pitch-control fan or reaction nozzle could also be used for this purpose. A few examples are the Vertol VZ-2, Vought-Hiller-Ryan XC-142A, and the Hiller X-18. The VZ-2 and X-18 have previously flown, whereas the XC-142-A, a tri-service transport, is now scheduled for first flight in July 1964.
Tile Propeller aircraft are similar to the tile wing except the small wings remain stationary while only the propellers and nacelles tilt upward, usually more than 90 degrees from the horizontal. Control is maintained by varying the thrust and tilt angle of each of the propellers. The propellers may be shrouded in ducts to obtain greater efficiency, as in the Bell X22A, or be unshrouded as the first of the tri-service v/STOL transports to fly, the Curtiss-Wright X-19. The Doak VZ-4 and Curtiss-Wright unshrouded X-100 are among the forerunners of these two triservice transports and employ the tilt-propeller principle.
Deflected Thrust machines are usually unconventional-appearing jet aircraft with the capability of deflecting the tremendous thrust of the engine(s) vertically and thus sustaining lift. Hovering control is provided by reaction nozzles at the wing tip and the tail, which bleed air from the compressor or combustor section of the engine. Some examples are the Bell X-14A, a distinctly unconventional appearing jet, and the English Hawker P.1127, a highly publicized strike fighter being purchased for evaluation under a tri-partite agreement (United States, United Kingdom, Federal Republic of Germany).
Lift and Thrust aircraft have engines for each task, i.e., pure-lift engines are mounted vertically in the wings or fuselage and are used to provide lift when the airflow over the wings is no longer sufficient for flight. These pure-lift engines can be shut down as the thrust engine(s) produce sufficient forward speed to maintain the required airflow over the wings. Notable examples are the English Short SC-1 and the French Dassault Balzac. The U. S.’s Bell D-188A was to have employed vertical, horizontal, and rotatable axis engines to achieve vertical flight. This project, however, was discontinued.
Lift Fan aircraft utilize internally mounted fans with controllable louvers to produce the lift necessary for hovering. As forward speed is attained, the thrust is diverted from the fans rearward to near horizontal thrust. An example of this aircraft is the General Electric-Ryan XV-5A Army research vehicle.
The Jet Ejector principle involves an aircraft exhausting primary air vertically from its turbojet engines and entraining secondary air by opening ejector doors in the top and bottom of the fuselage to attain vertical flight. The doors are closed and the thrust diverted rearward as speed is gained during transition to forward flight. Hovering control is provided by the reaction nozzles previously described. The Lockheed Georgia XV-4A Army research aircraft is an example.
Numerous other examples of vertical flight machines have been omitted in order not to cloud the picture further. The tilting rotor Bell XV-3, the “tail-sitters,” the Kaman K.-16B, the Ryan VZ-3 deflected slipstream, and the McDonnell XHJD-1, and combinations of many of the listed principles involved in these aircraft, have served to advance the art of vertical flight. The seven areas of investigation discussed seem to offer the most promise for military application at present. But this does not preclude the resurrection of machines previously “discarded” if the flight principle should prove worthy of a particular mission.
With all these varied types of VTOL vehicles, the question might properly be asked, “How can V/STOL aircraft help the Navy control the seas?” In ASW they could reduce even further the time to datum—heretofore a primary attribute of the helicopter. They could loiter on station with full kill potential, perhaps land on the water if feasible, and they could operate from small ships and auxiliaries. It appears that they would not replace the helicopter for hovering tasks, such as ASW task force screening, but they might assist the helicopter.
In the attack function, provided the predicted range and load-carrying capabilities are achieved, they could be used as STOL aircraft, for take-off, proceed to pinpoint hidden targets and perhaps drop ordnance in a hover if required. They could then return to the carrier or small, cleared field for a STOL or vertical landing. Factors such as the virtual elimination of arresting gear and a reduction in the fixed-wing landing accident rate present strong arguments for the adoption of V/STOL attack aircraft.
In close support, V/STOL aircraft could provide the Marine Corps with the accurate information needed for directing fire into the enemy’s ranks. As potential tank-killers they would seem extremely promising. They could, in addition, be based close behind troops, thereby minimizing the en route time, with a consequent increase in payload owing to decreased fuel requirements.
Utility applications seem limitless. Because of the myriad tasks they perform, VTOL aircraft employed on board auxiliaries of almost every type—even submarines and destroyers— would offer the Fleet mobility and flexibility. v/STOL aircraft could help reduce the workload and increase the efficiency of these overworked ships. Replenishment at sea might well be eased, quickened, and made safer.
In transport, they could provide for the changing of nuclear submarine crews at sea; the establishment of a Marine beachhead far inland; the ferrying, dropping and recovery of UDT personnel to areas not otherwise accessible; or they could even provide tons of cargo at a given point, neither encumbered by parachutes nor spread apart by paradrops.
For whatever the Navy mission, then, it seems difficult to find some facet that would not be at least improved by aircraft possessing the unique feature of being able to land and take off vertically.
Intense Navy funding and interest has, for a naval service that stands to benefit so greatly, been somewhat restrained in the recent major research efforts in V/STOL machines, excluding helicopters. The approach in terms of Navy self-interest appears to be one of following closely the developments sponsored by other services and foreign nations with a view of “jumping in” when the time is propitious. Because of the limited funds available and present technological progress, this may seem to be the best course of action. But today many of the V/STOL aircraft are research vehicles only; there will doubtless be more than a few VTOL principles that will be untested if the airframe manufacturers cannot secure Navy interest at an early stage. If one of these principles or modes of achieving vertical flight proves superior for a certain task and at the same time is not developed, we will have slipped even further behind in the advancement of this art.
This “make haste slowly” attitude would seem more costly as we attempt to “buy in” at a stage where the airframe or principle has been proved. In addition, we throw a developmental burden on the other services that, were it practiced by them in turn, would mean the total lack of guidance to industry concerning specific service requirements for V/STOL aircraft.
It is apparent that today most V/STOL aircraft cannot accomplish missions better—or even as well—as present-day vehicles. They may not carry the payload or go as far or as fast as today’s machines. This should not discourage research and developmental effort consonant with the current state of the art in this field.
Criticism of V/STOL aircraft in the United States could be set forth in the form of three questions:
“Why carry all that extra power just to achieve vertical flight?”
“Is there any present V/STOL machine that can go as fast or as far or carry as much payload as the—?”
“Won’t V/STOL be much too costly.” Valid though these questions may be, we cannot afford to neglect the potentialities of machines that may render such invaluable service to Navy and Marine missions.
Of the present V/STOL development programs, the U. S. Navy is, by direction of the Department of Defense providing funds for one-third of the cost of three tri-service V/STOL transports, the X-19, X-22A, and XC-142A, and is jointly financing with the Army the research of a “rigid” rotor helicopter, the Lockheed XH-51A.
There have been modest V/STOL development programs in the recent past that were harbingers of the future for those astute enough to heed the faint call. Few did. The distant past recalls the XOP-2 Autogiro in 1936, the XF5U in 1947, the XHRH-1 in 1951 and the “tail-sitters” and flying platforms of the early fifties. They were abandoned, perhaps justifiably so. The Hiller X-18, Bell D-188A, and X-14 are but a few vehicles that suffered from insufficient service funds, but they may well serve as a reminder that some of the flight principles involved are now being successfully pursued.
The U. S. Army, in addition to engaging in almost all V/STOL research activity in which the Navy is engaged, has shown a wise interest in the lift fan, deflected thrust, and in XV-4A jet-ejector VTOL aircraft. The high-speed helicopter received its initial impetus through Army interest, and the Hughes hot-cycle research helicopter is Army-sponsored. The STOL aircraft with various means of achieving low-speed, high-lift flight have blossomed with Army and Air Force funding. Meager is the most descriptive word for Navy interest in STOL aircraft, as shown by assignment of monies, in the past few years.
A current witticism among Navy fixed- wing pilots seems to symbolize the Navy view of V/STOL technology: “Now that helicopters have begun to sprout wings (i.e., a compound helicopter employs wings to ‘unload’ the rotor system to achieve faster flight), all they have to do is to rotate the rotor mast forward 90° (i.e., like a propeller), and then they might have something, i.e., an airplane.” If the witticism is something less than funny to the pilot actively engaged in V/STOL testing, it is only because he smarts at the smug opposition to change and advancement.
The Navy record in helicopter development has been vigorous and, happily, runs contrary to this probable attitude. This development helped make early, successful contributions to some peculiarly Navy missions. But the Autogiro, the tail-sitters, the flying platforms—largely Navy programs—have all had their day. They are the past. What about the future?
Today we stand at a crucial point in the determination of how Navy aircraft will operate in the near future. A continuation of our present research and development activity will mean virtually disregarding V/STOL aircraft in their still embryonic stage. We will almost certainly see other services and nations continue to nourish the V/STOL with tender loving care. Evidence of this abounds in the technical magazines, active research projects, and international air shows. If we do not face the apparent reality that some day all aircraft will have vertical flight capability, we may find the Navy flying yesterday’s aircraft in tomorrow’s operations.