Jump to content

Main Rotor Divergency


Heli-Ops

Recommended Posts

MAIN ROTOR DIVERGENCEY

 

By Mott Stanchfield.

 

Rotor divergence is a state of instability along with partial or total uncontrollability of a helicopter rotor system.  It can be the result of, but not limited to, a violent control input, failure of a rotor damper or bearing, damage to a rotor blade or during flight-testing of a newly designed rotor hub and system.  It is in fact an anomaly, which can happen.  I know because I had the condition occur during the flight test phase of a new helicopter development I was in charge of flight certifying.

 

As Chief Pilot, my first three weeks were spent in the Engineering Department with the Chief Engineer, studying all design drawings, vibratory and previous flight test data.

 

The first prototype had crashed killing the pilot who had no training or time in experimental flight test; a bad situation.  Through interviews with personnel who witnessed the flight, my opinion was control of the craft was lost and could not be regained.  I reasoned pilot error was not a factor.  

 

By the end of the three-week study, there was no evidence indicating failure of airframe, power plant, drive or control systems.  My initial suspicion was the three bladed, hinged free to flap, feather, ridged in-plane rotor system.  It was a unique concept.

 

The rotor blades were also unique:  weight 51.3lbs, blade-loading 2.4lbs, length 174 inches (14 ½ ft), constant chord with a 1 ¾ inch thick leading edge extruded spar for high in-plane stiffness and low span wise bending.  With a disk diameter of 384 inches (32ft) rotating at 350 rpm, each blade imposed 18,000 lbs of centrifugal force on the retention spindles of the hub, a very substantial blade.

 

With a new prototype completed and a flight test program written, it was time to begin the long phasing of flight evaluations.  An eight-track data recorder measured forces produced from the placements of eight strain gauges in location selected for evaluation.  The ship was securely tied down for first run up.   Of prime interest was rotor stability and stress load evaluations during cyclic, collective, and tail rotor control inputs.  All dynamic measurements being satisfactory, first flight is scheduled.

 

Hover and hover to translational lift okay, 25 mph in increments of 5 to55 mph okay, strong feedback at 62 mph with slight roll and pitch up tendencies. This condition required immediate reduction of collective pitch and airspeed and a return to engineering for data recorder read-out measurements.

 

The read-outs presented a normal sinusoidal wave value of low alternating forces, followed by an unacceptable instantaneous increase of powerful alternating forces.  Later I developed a rhythm pattern where extraction of powerful significant data became routine through precision control inputs.  This resulted in many months of gathering data, which mostly dealt with penetrating the onset of rotor divergence, a very critical and treacherous process.  We were determined to achieve success with this rotor system, a very large task for a small company.  A decision was mandatory.

 

A meeting was held with management and board directors.  Engineering wanted to steer the course and continue with the development of the ridged-in-plane rotor.  I insisted on adding a lead-lag offset hinge to fully articulate the hub.  The VP of Sales convinced the board (over my strenuous objection) he make a flight with me to experience the 60 mph divergence onset phase and report his findings to the board members.  My concern was adding his 240 lbs to my cockpit and altering the geometric dynamics of the rotor to unknown values.

 

Taking off and climbing to 500 ft, I gradually accelerated to 50 mph and incrementally to 60 mph.  Suddenly and loudly the blades sounded BANG, BANG, BANG, shaking the ship violently in the vertical mode.  The nose pitched up and the collective sprang to the up stop with great force.  I opened the throttle to full and tried to lower the collective, but could not.  The engine was unable to support 350 rotor rpm, which dropped rapidly and held around 220 rpm, not a good number.

 

The three-week study of design drawings now paid off.  The cyclic and collective control bell crank anchoring points were designed for loads to 165 lbs.  Keeping a downward collective pressure, estimated to be close to the yield point, with nose attitude high and airspeed dropping, I reasoned control should be regained, and it was.  We leveled out intending to land, when the now self-exciting divergence began again unleashing the same tremendous violence from the rotor system.  Recovery was a 50 ft. airspeed 20-25 mph.

 

My concern now focused on the remaining structural integrity of the control systems, but while reporting to engineering the longitudinal cyclic control completely failed.  To create a desired fuselage pitch angle, I reduced rotor rpm from 350 to 320 with graduated increases of collective pitch for added coning and flapping angles thus modifying pitch attitude by torque.  It worked and our landing appeared normal.

 

As the rotor slowed to a stop, I looked up and was shocked to see the blade deformation.  Measurements determined each blade was bent near the 18% position outboard placing the blade tips 40 inches forward in the plane of rotation.

 

During an immediate conference, it was agreed to add lead lag and dampening hinges to fully articulate the system, which developed into the fine ENSTROM rotor system of today.

 

The events and success of this flight further validates the interrelationship

between control systems.  If one system becomes ineffective, a pilot usually can impose partial control of the incompetent system through productive inputs from the remaining systems.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...