Air University Review,  July-August 1968

Why the VTOL Fighter?

Lieutenant Colonel Kennith F. Hite

History credits the Chinese with development of the first vertical takeoff and landing (VTOL) vehicle some 2000 years ago. Called the Chinese top, the device was really a toy consisting of a short stick with two or more feathers inserted at the top to serve as rotor blades. When the stick was spun between the hands and released, the toy would rise vertically like a helicopter, then descend slowly in the autorotation mode. Although this simple Chinese device was not referred to as a VTOL, the term today includes, technically, a wide range of such vehicles, from the now commonplace helicopter to exotic space systems such as the Gemini and Manned Orbiting Laboratory. In more general terms, however, the VTOL is thought of as a conventional type of aircraft with special features added to enable it to rise vertically during takeoff and to land from a vertical descent. The successful and imaginative employment by the military of the first VTOL type, the helicopter, is now part of aviation history. Currently, the military focus of attention is on a new VTOL type, the VTOL tactical fighter aircraft.

Progress in the development of a practical and capable VTOL fighter has been exceedingly slow over the years, largely due to propulsion limitations and related problems. Among the first and most interesting of the VTOL fighter forerunners was the Chance Vought V-173 conceived in 1933 and first flown in 1942. Called the “Flying Pancake,” the V-173 was a tail-sitter intended to rise vertically, then transition to normal flight attitude. Unfortunately, however, the airplane weighed 3000 pounds and had only 2000 pounds of propeller thrust. As a result, the Flying Pancake never made a vertical takeoff or landing, although it made some 210 successful flights in the conventional aircraft takeoff and landing mode.

The project was canceled by the Navy in 1948 when its partial successes were overshadowed by conventional fighter aircraft powered by the newly developed turbojet engine. Interest waned, and efforts toward further development of this type of aircraft over the succeeding decade were sporadic.

Today, however, resurgent interest in the VTOL fighter is in evidence. The aircraft industries of some six or more major countries are currently active in the testing, development, or production of VTOL aircraft. For example, West Germany and Italy are presently engaged in a joint venture in the development of a VTOL fighter. This project is now in its early stages and the prime contractors, Vereinigte Flugtechnische Werke (VFW) of Bremen, Germany, and Fiat Aircraft Company of Turin, Italy, have established a development program that will result in the initial construction and testing of six prototype V-191B tactical fighters by mid-1969.

In Great Britain, the Hawker Siddeley Company has designed, developed, and placed into service the world’s first production VTOL strike fighter, the P.1l27 Kestrel.

In France, a version of the well-known mach-2 strike fighter, the Mirage III, has been converted for VTOL operations.

In the Soviet Union one model of the MIG-21 jet fighter, which is now making a dubious history over North Vietnam, has been converted for VTOL operational tests. The VTOL performance of this and two other converted Soviet fighters was demonstrated in the July 1967 U.S.S.R. Domodedovo Air Show. At this same air show a Soviet fighter designed by Yakovlev specifically for VTOL operations also performed.

In the United States interest in VTOL has been demonstrated by all three military services. Most recently Ryan Aircraft, a pioneer in the VTOL field, has been engaged in tests of its XV-5A Vertifan VTOL, developed for the Army. The U.S. Air Force recently signed a contract with North American Aviation to gather data utilizing the Lockheed-developed XV-4B Hummingbird VTOL fighter. North American has subsequently scheduled more than 300 flights in fulfillment of this project.  Triservice testing of a later version of the British P.1l27, called the Kestrel in the United Kingdom and designated the XV-6A in the United States, has recently been completed at Ft. Campbell, Kentucky; Patuxent NAS, Maryland; and Eglin AFB, Florida. Four P.1l27’s are now under test by USAF at Edwards AFB, California. Tests have included extensive Navy aircraft carrier qualification flights as well.

Although these examples represent only a partial survey of world aviation industries presently engaged in the design, development, testing, or production of VTOL fighter aircraft, the widespread interest some 34 years after the conception of the Flying Pancake is significant. But how did industry arrive at this point in VTOL development? Why this sudden intense international interest? Of what practical application is the VTOL fighter today?

Historically, three basic types of propulsion have been used to power VTOL fighter aircraft: propeller, ducted fan, and turbojet. A fourth type of propulsion, the rotor, although used as the prime lifting means for the helicopter, has never been seriously considered for VTOL fighter application because of its relatively low efficiency in forward flight and resultant low forward speed capability.

There are two factors basic to VTOL fighter operations. First, in order to attain hovering flight, sufficient thrust must be directed downward to exceed the weight of the aircraft. Second, there must be a means of transition to normal flight attitude and aerodynamic flight speed. Of course this transition to normal flight requires a means of control during hovering and low-speed flight. The old V-173 Flying Pancake possessed all these capabilities except sufficient power to rise vertically.

As with the V-173, a majority of the subsequent VTOL efforts in the United States have been subsidized by military contracts. A few years after the Navy’s cancellation of the V-173, two more Navy contracts were let for tail-sitter VTOL fighters—the Convair XFY-l and Lockheed XFV-l. Both these aircraft were powered by the Allison 5000-horsepower turboprop engine, both made their first flight in 1954, and both were terminated in 1956 following two years of testing plagued by engine and propeller problems. Flight control of both was exercised during hovering and transition flight through the use of conventional aerodynamic controls, inasmuch as the wings and tail surfaces were bathed by a high-velocity flow of air provided by the large propeller during takeoff, transition, and landing. The Convair XFY-l was successful to the degree that it was the world’s first propeller-driven aircraft to make a successful vertical takeoff, transition to conventional flight, and land vertically. As later described by witnesses:

It was truly a magnificent performance, the first time in history that any VTOL aircraft except the helicopter had accomplished the complete VTOL operation.1

The XFY-l had a cruciform “sitter” platform with a very wide tread, whereas the less successful XFV-l had such a narrow tread there was concern that it might tip from its tail-sitter position during takeoff or landing. As a result the XFV-l never made a vertical takeoff or landing, but it was used to gather data during airborne transitions to vertical flight utilizing conventional takeoff and landing technique. State-of-the-art engine power-to-weight limitations required both aircraft to be equipped with propeller blades longer than optimum for efficient cruising, in order to provide adequate thrust for vertical takeoff. This of course compromised in-flight speed and maneuverability, two factors so vital to a military tactical fighter. Additionally, the tail-sitting takeoff and landing attitude of these two aircraft required the pilot to perform these most critical flight maneuvers from a position flat on his back with his feet up in the air and looking over his shoulder—a somewhat less than optimum position for aircrew safety and comfort.

Experiments with ducted-fan propulsion for VTOL fighter aircraft have been somewhat more rewarding. The ducted-fan propulsion principle incorporates a propeller imbedded within a shroud or duct. The propeller or fan may be powered by either a reciprocating piston engine or a gas turbine. One of the more successful aircraft of this type, which is still undergoing service testing, is the Ryan XV-5A Vertifan. This aircraft rises vertically while retaining a level-flight position. The Vertifan is powered by two J-85 turbojets of about 4300 pounds’ thrust each, which operate three ducted fans through a diverter valve for vertical operations. Two fans are imbedded in the wing roots and one in the nose of the low-wing aircraft. Control during VTOL is achieved through variable louvers. Once airborne, the louvers covering the ducted fans are deflected to produce sufficient forward thrust to attain sufficient speed for aerodynamic flight. When flying speed is reached, the fans are covered over, and the full thrust of the two turbojets is diverted and ejected out the aft tailpipe in a conventional manner. This propulsion principle, with its mechanical advantage, shows considerable promise for certain applications.

Since the fans deal with a much larger airmass than the engines, the [Vertifan] system produces about three times more thrust than the basic engines, reducing the usual requirement for very powerful engines for VTOL operation.2

A major problem that detracts from the potential of the Vertifan for VTOL fighter application, however, centers around the structural weakness inherent in the louvered wing design, which tends to restrict the G-forces that can be applied. Of course wing loading and G-forces are directly proportional to maneuverability, a key factor in tactical fighter performance.

The third and most promising type of propulsion utilized in VTOL fighters is the pure turbojet. There are two systems of turbojet VTOL propulsion in common usage today. The first involves the use of small, vertically mounted turbojets of the J-85 type, installed either in the fuselage behind the pilot or on booms attached to the fuselage aft of the cockpit. These vertically mounted engines provide the thrust for vertical takeoff, hovering flight, and vertical landing. The aircraft is maintained in a level flight attitude throughout vertical operations and transition maneuvering. Aircraft control during vertical operations is provided by small jet outlets, powered by compressor bleed air, installed in each wingtip and in the aft section of the airframe. Forward speed for aerodynamic flight is provided by a separate conventional turbojet, exhausting from the rear of the aircraft. When aerodynamic flight speeds are reached, the vertical jets are shut down, retracted if necessary, and covered over in a streamlined manner for normal flight operations. The French Mirage III-V used this propulsion system to enable it to become the world’s first mach-2 VTOL in 1966. Although the vertical-thrust engines are dead weight when VTOL operations are not required, the engines can be removed and replaced with fuel cells for long-range, higher-payload operations requiring the conventional takeoff and landing mode. 

The real breakthrough in jet VTOL operations came, however, with the development of the vectored-thrust turbojet designed specifically for VTOL aircraft installation.

The vectored thrust principle was originated by the French designer, Michel Wibault, who conceived the idea of deflecting the thrust from centrifugal compressors, driven by the Bristol Orion engine. This idea was further developed by Dr. Stanley Hooker and resulted in the first vectored-thrust turbofan, the Bristol Siddeley 53 Pegasus 5. This engine is basically a turbojet driving a ducted fan. A part of the relatively cool compressed air of the ducted fan is expelled through the front pair of cascaded nozzles; the rest of the air is passed on to the compressor of the turbine. After combustion the exhaust gases are expelled through the aft pair of nozzles.3

The Pegasus engine also incorporates a twinspool contraroting compressor that minimizes the gyroscopic coupling effect inherent in some early VTOL engine installations.

To date the most significant application of this new engine design concept has been attained by the British in the development and subsequent production of their P.1127 Kestrel. As noted by one observer:

The decision of the [United Kingdom] Ministry of Defense, taken in November 1966, to start mass production of the Kestrel was however a milestone in V/STOL history, as it was the first time a V/STOL project had resulted in an operational aircraft. Finally the V/STOL concept had reached maturity.4

Operational test and evaluation of the Kestrel were accomplished by a special Tripartite Evaluation Squadron, commanded by RAF Wing Commander David Scrimgeour and composed of United States, United Kingdom, and West German air force pilots. The production P.1127 is a swept-wing VTOL fighter equipped with the latest version of the Bristol Siddeley Pagasus 6 turbofan, producing a thrust of about 19,000 pounds. Takeoff weight is in the neighborhood of 14,000 pounds, which gives the aircraft a payload of about 5000 pounds or, in military terminology, six 750-pounds. This payload is comparable to that of several first-line jet-powered military fighters in use today, including the well-known F-100 and F-104 aircraft. The maximum speed of the Kestrel is reported to be just under mach 1. In recent trials this aircraft demonstrated the capability to land day or night in a 150΄x 300΄ clearing surrounded by 60—foot-high trees. The only special equipment required was an aluminum 50΄x40΄ in size, upon which to touch down. The purpose of the pad was to prevent the jet exhaust from blowing up debris or high-speed pellets and rocks that could damage the aircraft skin or be ingested into the jet engine, with resultant compressor damage.

The British have indeed made a breakthrough in turbojet engine design with their vectored-thrust Pegasus. At the same time, United States jet engine technology and state-of-the-art advances are producing some outstanding results. For example, Pratt & Whitney is now marketing a high-bypass-ratio conventional turbojet engine designated the JT9D, which has a takeoff rating of 42,000 pounds’ thrust and has a growth potential to 47,000 pounds’ thrust. The application of United States technology to the vectored-thrust principle could undoubtedly provide some truly amazing VTOL power plants.

What are United States intentions? The Commander, United States Air Force Aerospace Systems Command, General James Ferguson, has stated:

We plan to press for solutions to the current problems and generally stay ahead of the expressed needs of operational commanders by demonstrating V/STOL systems that will be practical for their purpose.5

The VTOL has finally come of age and appears to be here to stay. There remain many problems to solve in the development of concepts for employment and logistic support; but the military applications of this type of aircraft are many, and the potential for further development and improved performance is great. Let us consider one example of VTOL application:

The problem of developing an economical and flexible manned tactical weapon system that can survive a surprise air attack and continue to operate effectively against the enemy has long been of paramount military importance. During the June 1967 Israeli-Egyptian air campaign, the use of air power by the Israeli Air Force (IAF) was classic. The first wave of IAF attack aircraft flew in low, underneath the radars, and used a newly developed lightweight “dibber” bomb to interdict Egyptian runways. According to one on-the-spot report:

. . . this bomb has retrorockets to brake it almost instantly upon release. The bomb tilts downward and another rocket blasts it vertically into runway surfaces to blow a hole several feet deep. . . . in every case the first attacks were passes along the runways. Only a few craters, judiciously placed, put them out of action.6

Following interdiction of the runways, the Israelis then strafed the grounded Egyptian aircraft with withering accuracy at will, thus reducing the Egyptian Air Force to relative impotence within a breathtaking and decisive three-hour time period. Six days later organized Arab military opposition to the advance of Israel’s armed forces ceased.

How would a VTOL tactical fighter have helped Egypt? With sufficient military foresight, supporting aerospace technology, and a well-dispersed and adequate force of VTOL tactical fighters, independent of the requirement for long concrete runways, the outcome of the Arab-Israeli campaign could well have been reversed.

And so it could be with other farsighted nations on another day.

Ramstein, Germany

Notes

1. John P. Campbell, Vertical Takeoff and Landing Aircraft (New York: Macmillan Company, 1962).

2. “VTOL Ups and Downs,” NATO’s Fifteen Nations, No.12, June-July 1967, p. 57.

3. Ibid., p. 59.

4. Ibid., pp. 59-60.

5. General James B. Ferguson, “Providing the Means to Meet Aggression—at Any Level,” Air Force and Space Digest, November 1967, p. 92.

6. Robert R. Rodwell, “Three Hours—and Six Days,” Air Force and Space Digest, October 1967, p. 58.

Additional references

The following useful sources supplement those cited in the notes.

Riccius, Rolf, and Guira, Franco. “VFW/FIAT VAK-191B,” Interavia, September 1967.

“The P.1127 Goes into Production,” Interavia, November 1966. “VTOL Aircraft 1966,” Flight International, 26 May 1966.


Contributor

Lieutenant Colonel Kennith F. Hite (USMA) is Chief, Attack Branch, Offensive Operations Division, Fourth Allied Tactical Air Force, Germany. After flying training in 1952, he flew 90 F-86 combat missions over North Korea. Other assignments have been as fighter weapons instructor, Nellis AFB, Nevada; instructor in military studies, U.S. Air Force Academy, 1957-60; student, Air Command and Staff College; Plans Officer, Kadena AB, Okinawa; in Operations Plans Division, Hq PACAF, Hawaii, 1963-65; as Chief, 4520th Combat Crew Training Wing Command Post, and F-I05 instructor, Nellis AFB; and as squadron commander, 388th Tactical Fighter Wing, Thailand, flying 100 combat missions over North Vietnam.

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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