Prerotator Safety and Reliability

Part 3

By Greg Gremminger

Strong prerotators are a necessity on the new generation 2‐place gyros that are increasingly popular today.  Prerotators come in a multitude of designs and configurations.    Hydraulic prerotators are intuitively popular, but without very heavy large diameter hydraulic lines and large motor/pumps, prerotation without wind help can rarely exceed about 150 rotor RPM.    Electric prerotators are also intuitively attractive, but they require large batteries and some sort of “soft” actuation to avoid sudden damaging motor start torques applied to the rotor and mast and prerotator gear.    Electric prerotators are often good for only one pre‐rotation before requiring re‐charge of the battery on the ground or with an hour or so of flight – one good prerotation at a time! 

Mechanical coupling from the engine to the rotor is the most popular configuration.    There are two prominent types of mechanical coupled prerotator systems.  Historically, the “flex cable” type prerotator has had the most success and use.    The commercial versions of the “flex cable” prerotator systems were developed mostly for lighter single place gyros with lighter rotors.    The “flex cable” itself is the limiting factor for both the size of the rotor and the top speed of prerotation due to the higher torque stresses required through the flex cable.

For larger rotors on some new generation gyroplanes, some designers have reverted to straight torque shaft drives, employing right angle gears and a “U”‐joint coupling at the rotorhead.    Intuitively, such a system can be designed to handle very high prerotator torque applications.  However, good intuitive ideas do not always resolve all problems.  Such systems that use a “U”‐joint at the rotorhead, to allow pitch and roll movement of the rotorhead, create limitations on how the system may be used.  To prerotate with a “U”‐joint at the rotorhead, the rotorhead (and rotor) must essentially be held in a level condition – keeping the “U”‐joint essentially aligned straight with its driving shaft.  Otherwise damaging stresses can be applied to the “U”‐joint when the shafts are rotating – prerotator is engaged to the engine.

For such shaft drive / “U”‐joint systems, the rotor cannot be pitched or rolled from level during prerotation – must be held level with forward and centered cyclic stick.   That means it is difficult to taxi with the prerotator engaged.    That means that any wind or wind from forward movement cannot be utilized while the prerotator is engaged – can’t tilt the rotor back to catch some prerotation helpful wind – such as on initial roll to shorten the takeoff roll.  That also means the pilot essentially must roll onto and align with the runway before starting prerotation.  The ability to prerotate before crossing the hold short line before entering an active runway, the ability to allow forward movement wind to help accelerate the rotor RPM during roll onto the runway, the ability to have takeoff ready rotor RPM as soon as you are aligned on the runway, is difficult or impossible with such shaft drive / “U”‐joint prerotator systems.  On busy runways, especially if/when a controller asks you to expedite or trying to fit into runway traffic, you do not want to have to stop on the runway and only start prerotation at that point, with your back toward oncoming traffic.   

The ability to achieve a higher prerotation rotor RPM shortens the takeoff roll. But with a prerotator system that must be held forward until the prerotator is disengaged, the higher prerotation rotor RPM must be achieved solely with engine power – cannot tilt the rotor aft to collect some helpful wind for prerotation. That means that the rotor can only be tilted back for building rotor RPM to takeoff RPM after the prerotator is disengaged – when roll is first initiated. With all of the mechanical restrictions, it can also be difficult or damaging to have to re‐engage the prerotator if ever needed when the rotor might have slowed down below a safe RPM to begin acceleration down the runway. All of this easily leads to the opportunity to “flap” the rotor – outrun the rotor RPM because the rotor RPM is below, or allowed to dissipate below a safe speed for the takeoff full power acceleration.

Some prerotation systems employ a “push button” prerotator engagement system.    These systems are intended to automatically engage the prerotator clutch at a rate that engages the Bendix mechanism at the rotor head, but avoids excessive torque on the system before the rotor RPM builds.  These systems can make it difficult or impossible to re‐engage the prerotator until the rotor is slowed to a stop – not necessarily safe to do on a busy runway with possible traffic behind you.

The Magni rotor system employs a large diameter flex cable to handle the higher power of the larger rotor and higher prerotation rotor RPM.    The flex cable allows full cyclic control range of the rotorhead during all phases of prerotation and taxi.    This allows prerotation to full available prerotation RPM before or during taxi into position on the runway.   When done right, and using the wind through the tilted back rotor during this taxi, the gyroplane is ready for immediate takeoff acceleration as soon as it is aligned on the runway – no stopping and worrying about traffic behind you while you prerotate!    No prerotation techniques and procedures, or third hand required buttons to push, after you are on the runway!    The Magni prerotator system allows prerotator engaged initial takeoff roll, with the rotor tilted back to use the wind to build rotor RPM quicker and higher. Prerotator engagement is applied by the pilot with a lever that allows re‐engagement of the prerotator at any point or RPM, while also allowing the pilot to build rotor RPM as aggressively as needed.   

The Magni prerotator system is very robust and capable of no‐wind prerotation rotor RPM up to around 300 RPM.   (Normal takeoff prerotator RPM is 220 RPM.  This RPM allows for immediate safe application of full power, full aft stick acceleration down the runway.  Higher prerotation RPMs are available for Short or Soft Field takeoff procedures.)   The Magni prerotator system is designed to be a robust and reliable system, with considerations of maximum takeoff performance, assured prerotation to avoid possible blade “flapping” on takeoff roll, and without complicating procedures and mechanics that can lead to damaged parts in a hurried or stressful situation.