VTOL

VTOL means vertical take-off and landing. The A-4, E-10, E-11 and W-6 use main-engine thrust for atmospheric VTOL. Reaction control (RCS) supplies attitude control and a small amount of vertical translation. The P-13 uses its separate TGD drive; the T-19 has no VTOL system.

Controls

The default F9 binding requests upward translation and F8 requests downward translation. Equivalent requests can be assigned to a game controller. Main-engine throttle and forward/reverse selection continue to request longitudinal acceleration. Autoflyte requests use the same propulsion limits as manual controls.

With TVM engaged against a ground reference in BCL, the E-10, E-11, A-4 and Manx use F9 for maximum upward demand, F8 for a height-scheduled descent, and key release to request hover. This does not select a fixed altitude, steer to the reference's position or guarantee sufficient lift. See TVM vertical landings for the descent schedule and limitations.

No separate nozzle or VTOL-door control is required. The propulsion system combines the vertical and longitudinal requests automatically. Requesting lift can increase main-engine output even with the longitudinal throttle at idle. Allow for engine thrust buildup before attempting to leave the ground; nozzle or door travel alone does not provide full lift.

Engine arrangements

E-10, E-11 and A-4

The front and rear outlets on the two side engines tilt down through 54 degrees. Full nozzle travel takes about 1.5 seconds. Each engine can supply its full rated thrust through each opposing outlet. At 54 degrees, about 80.9% of each outlet's thrust acts vertically and 58.8% acts longitudinally. Equal opposing flows cancel longitudinal force while adding lift; both flows consume fuel. Unequal flows permit braking or acceleration while producing lift.

The side-engine nozzles can move while the engines are firing. Applying or releasing lift starts nozzle travel immediately; thrust buildup and rundown still take time. Forward or reverse thrust can develop during the transition, without waiting for both wing engines to extinguish.

The E-10 and E-11 tail engine remains fixed and can provide longitudinal thrust during VTOL. It represents approximately one third of the three-engine axial rating. The A-4 has two engines and no tail engine.

W-6 Manx

The Manx routes thrust through protected downward outlets. Its underside doors take about 0.6 seconds to open. Downward exhaust is inhibited until the doors are fully open, and the downward flow is shut off before they close.

Each engine shares one thrust budget between its forward, reverse and downward outlets. Full downward thrust leaves no thrust available for longitudinal acceleration. Partial lift can be combined with forward or reverse thrust. The engine rating is unchanged: a heavy Manx may be unable to hover.

Atmospheric operating envelope

To enable or re-enable main-engine VTOL, descend below 18 km MSL and slow below 90 m/s air-relative speed. Once enabled, it remains available up to 20 km MSL and 100 m/s air-relative speed. These limits use airflow in any direction, rather than indicated forward airspeed.

The Manx also requires dynamic pressure below 3.2 kPa to enable its doors; above 4 kPa downward flow is cut off and the doors close. This depends on local atmospheric pressure, temperature and speed. In Vieneo's dense surface atmosphere the pressure limit can be reached well below 100 m/s.

Above the operating envelope, during suborbital coast and in orbit, vertical translation uses RCS only. The main engines remain available for forward and reverse thrust; Manx VTOL doors remain closed. Returning to the lower limits permits atmospheric VTOL again.

Hover capability and balance

Available lift depends on loaded mass, engine condition, power, atmospheric performance and the torque that can be balanced about the centre of gravity. The allocator varies front/rear and left/right output, with RCS supplying the remaining attitude correction. It reduces thrust when the requested combination cannot be balanced. Maximum gross weight is not a guarantee of VTOL capability.

Vertical RCS on these four vehicle types provides approximately 0.5 m/s² at maximum gross weight at full power and condition. Its acceleration increases at lower mass and decreases with power or wear. It is a small supplement to atmospheric main-engine lift and the source of vertical translation in space. The P-13's TGD operation is separate.

See also TVM, Autoflyte, Airspeed and Vieneo.