Skip to content

Tandem rotor

CH-47, CH-46. First conversion built and flying (an RFM conversion of the H-47 mod), not released yet. Start from the Mi-8 prefab for its wheels and articulated hub setup.

Heads up, I haven't flown one of these on RFM yet. Everything you need is in there, but expect to spend some time on signs and tuning.

How a tandem flies. There's no tail rotor. The two rotors turn opposite ways, so their twisting cancels out.

  • Pitch comes from differential collective. The front rotor loses pitch while the rear one gains it.
  • Yaw comes from differential lateral cyclic. The front disc tilts one way and the rear disc tilts the other.
  • Roll comes from both discs tilting the same way.

Steps:

  1. Power group. Two sources. Outputs shaft.fwd (role 1, ratio 1, front rotor inertia) and shaft.aft (role 1, ratio 1, rear rotor inertia). Since the shafts are geared together the rotors stay in sync, same as the real cross shaft.
  2. Rotor units. rotor.fwd and rotor.aft, both Role 1, opposite Spin Direction (CH-47: front anticlockwise −1, rear clockwise +1, but check your type). Put each hub on its pylon. The rear hub sits higher. Give each one its real forward shaft tilt. On the clockwise rotor set Cyclic Lat Sign to −1: RFM counts blade position in each rotor's own turning direction, so a clockwise rotor needs its lateral cyclic flipped to tilt the disc the same way as the other one. Cyclic Long Sign stays −1 on both.
  3. Control maps. Start from the tandem rows in the control map table. Keep the differential gains small (0.1–0.2 for collective from long, 0.2–0.4 for lat from yaw), then raise them until pitch and yaw feel right. Each rotor's collective is clamped to 0–1, so one rotor will max out first at the ends of travel. That's normal.
  4. No anti-torque rotor. Leave Anti Torque Rotor Id empty.
  5. Wake links.
    • rotor.fwd → rotor.aft (going forward, the rear rotor flies in the front rotor's wake), sample mode 1.
    • If you want, rotor.aft → rotor.fwd with a low gain (0.2–0.3) for where they overlap in the hover.
  6. Wheels. Four struts, and the rear pair steers (Steering Mode 1).
  7. Visuals. Spin each head from RFM: in RFM_VisualsComponent → Extra Rotor Visuals, add an entry for each rotor with its Slot Name or Model Token, and set the graph's MainRotorAngle / TailRotorAngle rows to Scale 0 so nothing spins twice. Graphs built for a single rotor often move the blades with the rotor angle as well, and that makes the hub run round in a circle when two rotors share it. If a head wobbles, its mast isn't on the physics shaft, so set Spin Axis and Spin Pivot (Lining up an extra rotor visual). For blades that flap, use Blade flap on the model, and check the blade numbering on both rotors, since a clockwise and an anticlockwise rotor can be numbered differently. Set Phase Lock Rotor Id on the rear rotor so the blades stay in step (Rotor units). Pick the Phase Lock Offset Deg by looking at the parked blades: the H-47 uses 0, because the other choice left two blades crossing low over the fuselage.
  8. Damage. Rear rotor damage goes in the damage bridge's Rotor Zones (Damage). Leave the tail rotor fields empty.
  9. Flight control system from the start. A tandem is really hard to hand-fly without one, and the real CH-47 relies on its own. Use a Flight control system profile: it works on the pilot's four controls before the control maps, so tandem mixing doesn't get in the way.
  10. Longitudinal cyclic trim. The real CH-47 tilts both discs forward on its own as it speeds up. Turn on Use Scheduled Long Bias on both control maps (Control maps), with the rear rotor moving further than the front.
  11. Wheels behind the CG that steer need Reverse Pedal Steer (Ground contact).