Vertical and Short Takeoff and Landing Aircraft V/STOL
Shortly after the airplane was invented, its disadvantage of requirement of a significant runway for takeoff and landing was quickly noticed, which significantly limit the airplane's utility. The operational benefits of an ability to takeoff and land vertically are self evident. Conventional aircraft must operate from a relatively small number of airports or airbases with long paved runways. For commercial transportation, the airport is rarely where you actually wish to go, and is usually crowded causing delays in the air and on the ground. For military aircraft the airbase is vulnerable to attack, and the time expended crusing to and from the in-the-rear airbase increases the necessary range of the aircraft increases the response time.
Like most technical subjects, the area of vertical flight has spawned its own arcane terminology. VTOL refers to a capability for Vertical TakeOff and Landing, as opposed to Conventional TakeOff and Landing (CTOL). VTOL is the broadest terminology and can be applied equally well to a helicopter or an Apollo space capableaircraft which has the flexibility to perform either vertical or short takeoffs and landings is said to have VSTOL capability (Vertical or Short TakeOff and Landing). An aircraft which has insufficient lift for vertical flight at takeoff weight but which can land vertically at a reduced weight is called a STOVL (Short TakeOff and Vertical Land).
The helicopter was introduced in order to overcome the limitation of the airplane. However, the helicopter has not received wide spread use but only in special roles that strictly require VTOL capability, and the helicopter numbers is but {fraction (1/10)} that of the airplane. The helicopter flies too slowly and too inefficiently, with speed and range 1/2 to 1/3 that of the airplane, with 2 to 3 times the fuel consumption and cost of operation per passenger-mile.
The helicopter is less safe per passenger-mile basis. According to NTSB statistics, the fatality rates for piston helicopters is 3-4/100,000 hrs and for light turbine helicopters 2-3/100,000 hrs where as the rates for a typical high wing airplane such Cessna 172 is 0.5 and Cessna 182 is 0.7/100,000 hrs. Light turbine helicopters have purchasing cost 2-4 times that of comparable piston airplane, but recently, the Robinson piston helicopters with their simplified rotor head design has brought down their purchasing cost to a level comparable with piston airplane.
In order to maintain the VTOL advantage of the helicopter while overcoming the helicopter's inefficiency and slow speed, there have been at least 50 different projects experimenting with high-speed VTOL aircraft by a large numbers of well known aerospace companies, proposing at least 12 different configurations in the last five decades. Today, there are only two VTOL transport airplanes that have sufficient merits to achieve production status, the military tilt-rotor Bell-Boeing V-22, and the civilian tilt-rotor Bell-Agusta BA-609. Rotary wing aircraft, such as helicopters, have had more commercial success than the military designs, but still rely predominately on turbojet engines to provide the power for vertical lift. Their advantage in vertical lift capability is offset by their poor flight stability characteristics, and very high initial and maintenance costs.
Helicopter designs fall into two broad categories; a single lift rotor with a tail rotor to control yaw, and two lift rotors rotating in opposite directions to control yaw. The single lift rotor is much more popular since it is simpler, but it yields roughly 30% less direct vertical lift for a given horsepower than the dual rotor system. Typically, helicopter rotor systems provide a lifting capability of 10-15 pounds per horsepower, with top speeds in horizontal flight of less than 200 miles per hour. The flight and maintenance problems associated with helicopters are well known and bear no repeating.
The history of non-rotary wing V/STOL (vertical/short takeoff and landing) aircraft development has generally proceeded along two separate paths. STOL (short takeoff and landing) aircraft development has centered on conventional aircraft operation, using enhanced lifting devices to shorten takeoff and landing runs. VTOL aircraft development has centered on the use of powerful jet engines to provide the required vertical thrust, and as such has focused on military applications. Latest developments, such as the Osprey aircraft being procured by the US Marines, use two widely spaced proprotors, similar to large propellers, but without shrouds or ducts. These proprotors rotate during flight approximately 90 degrees, from a horizontal to a vertical position to transition from vertical lift to horizontal thrust. The prop-rotors are too large in diameter for the plane of the rotors to be rotated to a vertical position when sitting on the ground.
STOL designs reduce the takeoff and landing runs for aircraft by primarily reducing the stall speed of the aircraft. This is accomplished by increasing either the available wing lifting area or increasing the lift coefficient the wing is capable of producing, by means well known to practitioners of the art. Regardless of the design. STOL aircraft must provide forward movement of the aircraft in order to produce lift, with the notable exception of the Custer channelwing aircraft, which could lift vertically while tethered. STOL aircraft have the potential to be particularly safe aircraft, since there is a direct relationship between the severity of injuries sustained in crashes and the speed at impact. With a lower potential impact speed due to lower stall speeds, STOL aircraft can be designed to provide complete protection from injury in most crash landing situations.
Autogyros provide for lift using an unpowered overhead rotor system similar to the helicopter, with auxiliary wings to provide supplemental lift. Gyrocopters do not have auxiliary wings. Most designs provide pusher propellers as the primary means of propulsion. The Cartercopter is a recent example.
Current gyrocopters provide prerotors to spin up the main rotor system to high speed, and then use a "jump" takeoff to lift almost vertically. The gyroscopic inertia of the main rotor is used to lift the aircraft into the air, where it then accelerates forward using the pusher propeller. Once moving forward, the rotor speed is maintained by contact with the moving air. Gyrocopters with prerotors do not have hovering capability in a loitering mode.
Tilt Rotor (Prop-rotor) aircraft provide two or more very large propellers mounted on wingtips or wing pylons. The propellers rotate through over 90 degrees of angle, from a horizontal axis for forward flight, through to a vertical axis for vertical lift thrust. Due to the need for redundancy in case of a single engine failure, complex crossover shafting is required. Current aircraft of this type are military, with smaller civilian versions in the planning stages. These aircraft have lower thrust output for a given horsepower than helicopters, but have the capability to exceed 300 miles per hours in horizontal flight.
Ducted rotors, also known as ducted fans, are more efficient and quieter than exposed propellers of the same diameters. They are also safer than exposed propellers on the ground. VTOL aircraft using ducted fans have been the most commonly developed prototype, but with little success. While the ducted fan provides greater lift than a simple unducted propeller, the drag produced by the duct shroud in forward flight at speeds over 150 mph has greatly limited the success of this type of VTOL aircraft.
There are numerous fan-in-wing configurations, which have been patented, but none has proved to be successful. The primary problem has been the high required disc loading causing high exhaust velocities. A good example is where fans are located in each wing, with the wings having a retracting feature to cover the fans in forward flight.
The greatest military success in VTOL aircraft has come with the alternate, in the form of the Marine Corps Harrier. Other aircraft, including the Moller SkyCar and Soltrek XFV, are in flight testing. The Skycar uses eight separate engines in four deflected thrust lift pods to provide both vertical and horizontal thrust. Many of the VTOL designs have the capability of STOL operation. In particular, the Harrier is used by the British with a jump takeoff ramp on aircraft carriers to enhance range and payload.
In the field of vertical and short take-off and landing aircraft gas turbines are often adapted to provide thrust for both normal wing-borne flight and for lift. This is achieved using vectorable nozzles for changing the direction of the engine thrust. For stability and ease of aircraft control an existing engine of this type employs an arrangement which uses four side mounted nozzles which can be rotated on simple bearings through an angle of over ninety degrees. Two of the nozzles are located forward of the aircraft centre of gravity on port and starboard sides and receive air from the engine compressor. The other two nozzles are located rearward of the centre of gravity on port and starboard sides and receive exhaust gas from the turbine section.
The absence of a conventional axial jet pipe in the basic arrangement enables the rear structure of the engine to incorporate a permanent transverse portion which acts as a gas deflector and strengthens the structure with respect to torsional loads. The rearward pair of nozzles can then be carried by the engine casing which greatly eases problems of sealing the nozzles against hot gas leakage. In a development of this type of arrangement which incorporates an axial jet pipe the transverse portion of the rear structure no longer exists and the torsional loading of the rear nozzles can no longer be tolerated. The nozzles can be carried by the airframe in which case a solution must be found to the difficulty of controlling hot gas leakage from means connecting the engine to the nozzles.
Typical VSTOL aircraft, particularly those capable of attaining high speeds, use a combustion engine which acts as a pressurized gas generator which generates pressurized gas for propelling the aircraft. A problem faced by VSTOL aircraft is that the temperature of the pressurized gas is quite high, typically in the range of 1,000.degree. F. and higher, and the pressure is also considerable. When the pressurized gas is directed downwardly, the high temperature and pressure tends to melt tarmac, erode concrete and even heat metal plates below the aircraft to unacceptably high temperatures. The result is that operation of VSTOL aircraft is restricted, and take-off or landing must often be performed with some forward movement to reduce the damage.
In the area of propeller-powered aircraft, the tilt-rotor concept, as tested in the Bell XV-15, seems to offer the best compromise between helicopter-like vertical flight and efficient wing-borne cruise. The tilt-rotor concept is the basis of the V-22 Osprey which is currently under development.
To date, there have only been a few operational jet VTOL designs, the British Harrier and the Russian YAK-36, both of which are subsonic aircraft. While at least one supersonic VTOL design has flown, the Mach 2 Mirage III-V back in 1966, there has yet to be an operational supersonic VTOL aircraft.
This is largely due to the need for increased internal volume for the vertical lift apparatus and fuel capacity. Also, most concepts for vertical lift tend to increase the aircraft's cross-sectional area near the aircraft's center of gravity, increasing the supersonic wave drag. Finally, the state of the art in engine thrust-to-weight ratio has imposed an excessive weight penalty on VTOL designs. It has simply been impossible, up to now, to provide an operational aircraft having both vertical and supersonic forward flight with any practical range.
Modern supersonic jet fighters have a thrust-to-weight ratio exceeding one, so it would seem fairly easy to point the jet exhaust downward and, therefore, attain vertical flight at "no cost". Unfortunately, this is complicated by the balance problem. Many subsonic jets and virtually all supersonic jets are designed with the engine at the rear, the cockpit and avionics at the nose and the payload and fuel near the center of the aircraft. This traditional layout places the expendables at the center of gravity, co-locates the parts of the aircraft requiring cooling (crew and avionics), and keeps the avionics away from the hot and vibrating engine.
If the thrust exceeds the weight, vertical flight could be obtained simply by deflecting the thrust downwards. However, some vertical upward force is required forward of the aircraft's center of gravity. In order to balance the vertical thrust force at the tail. This balance problem is possibly the single most important problem encountered in the design of a VTOL jet fighter.
There are two conceptual approaches to solving the balance problem. Either the thrust location must be moved to the center of gravity, or an additional thrust force has to be applied near the nose. Both of these approaches will tend to move the design of the aircraft away from the optimal layout. Generally, jet VTOL concepts can be divided into those which utilize fairly conventional engines and those which use modified engines so that the fan and core air are split, with the fan air ducted and exhausted from some location separate from the core air.
The conventional engine VTOL concepts which do not use additional lift engines for vertical flight must have a net takeoff thrust-to-weight ratio in excess of one. If the jet exhaust is not diverted to some other location for vertical flight, the aircraft must either be a tail sitter (VATOL), or it must have the engine exhaust located at the aircraft center of gravity and capable of vectoring downward for vertical flight. This can be accomplished by using a vectoring nozzle or nacelles which tilt.
The X-14 research aircraft had vectoring nozzles at the center of gravity, with the engines out in front. This is probably not a good arrangement for most applications because the cockpit winds up in the rear for balance, which does not provide acceptable visibility for the pilot. Also, in forward flight, the jet exhaust scrubs alongside the fuselage which causes thermal and acoustic problems. Tilt nacelles are heavy, but may be the best compromise for some applications. Grumman Aircraft Corporation has been pursuing a tilt-nacelle concept for Naval applications for a number of years.
Some VTOL concepts provide a means of diverting the exhaust flow to provide vertical lift. This is generally done by a retracting blocker device in the engine which shuts off the flow through the rearward-facing nozzle. The flow is then diverted forward through internal ducting. All of these VSTOL approaches, however, exact significant penalties in weight, cost, and completely when compared to a conventional jet aircraft design.
