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I saw a video of a F-35 landing vertically using its fans and was curious as to how they do that without a tail rotor? Why don't they spin out like a helicopter does when it loses its tail?

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The F-35-B STOVL variant of the Lockheed-Martin F-35 Lightning II gets its V/STOL / STOVL capabilities from the Rolls-Royce LiftSystem, which consists of four main components:

  • The Three Bearing Swivel Module (3BSM), which allows to turn the nozzle downwards 90° in order to act as a vertical lift system.
  • The two roll posts on the wings, which divert bypass air from the turbofan engine in order to provide roll control during hover.
  • The 29000 hp driveshaft and clutch, which allow diverting power from the turbine to the …
  • Rolls-Royce LiftFan, a pair of contra-rotating fans providing lift forward of the center of gravity to counterbalance the lift generated at the aft.

The important aspect for this question is that the two fans of the LiftFan are contra-rotating, i.e., spinning in opposite directions. Therefore, just like any helicopter with contra-rotating (e.g. Kamov Ka-27 or the most famous of them all, NASA JPL's Ingenuity Mars Helicopter) or counter-rotating (e.g., Boeing CH-47 Chinook, Kaman K-MAX, Bell Boeing V-22 Osprey) rotors, the net torque between them is zero, ergo, there is no need to cancel out the torque using a tail rotor.

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The tail rotor of a helicopter is primarily there to counteract the torque of the main rotor.

The F-35's lift fan is counter-rotating such that it cancels its own torque. In addition to the lift fan, lift and control is provided by the engine exhaust nozzle at the tail as well as two roll posts off the side of the engine.

It is interesting to compare a video of a Harrier in hover to an F-35. The Harrier was manually controlled, the human is constantly adjusting the controls to maintain a hover. Consequently, the Harrier seems to wiggle around -- like balancing on top of a ball.

The F-35 is fully fly by wire under digital closed loop control. Its hover is rock-steady. Rather than the pilot directly controlling the thrust vector angle or control surface deflections or any other control effector -- the F-35's pilot's controls express intent. Go forward, go back, go up/down, go left/right. The computer figures out how to make that happen.

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The contra-rotating liftfan that creates no torque is the answer to the question.

But if you're wondering how to yaw the aircraft, the F-35B has a three bearing swivel duct (3BSD) providing the aft post of thrust and is capable of vectoring thrust not only in pitch but also in yaw, with a yaw angle of -/+ 12 deg.

The following figure shows all the four Propulsion Control Effectors, and their deflection angles:

F35B effectors

Now regarding how to stay stable in a VTOL process, the control allocation of those effectors are provided by the Incremental Nonlinear Dynamic Inversion Control Law (INDI CLAW). As distinguished from legacy FBW schemes, which use preprogrammed (scheduled) gains, the NDI/INDI approach allows the aircraft to determine a control solution on the fly, based on a detailed onboard flight dynamics model (OBM) of the aircraft’s thrust profile, mass properties and aerodynamic data such as stability and controllability derivatives.

In short, the regulator and command modules implement the desired dynamics for the pilot controlled axes (pitch, roll, yaw, height, and axial). The effector blender allocates desired acceleration commands optimally to the control effectors (e.g. propulsion posts and conventional control surfaces), best suited for the particular axis.

F35 INDI CLAW

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