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      Is Your Contact With Earth Dynamic?

Recently, I was asked to do a mock ‘oral exam’ in preparation for a check ride. To his credit, the applicant was rock solid on almost all the subjects. However, questions about aerodynamics and critical situations posed the usual challenges. In light of some of the accidents we have seen, clearly, he isn’t alone.

So, let’s review dynamic rollover. Helicopters with narrow skids and a high center of gravity (CG), as with a Robinson R22 or RotorWay, are far more susceptible to dynamic rollover than those with wide-set skids, such as a Bell 47. Also, this is one of the challenges a pilot might face when trying to pick up for the first time in a helicopter, home built or otherwise.

Dynamic rollover can occur whenever a skid becomes a fixed pivot point for a lateral roll around the aircraft’s CG. This can occur in any number of ways. To mention but a few, a skid can get stuck: in soft asphalt or mud; frozen to the ground; on a clump of grass; a taxi light, a tie down, etc. and during slope operations, when the skid is intentionally “stuck” to the uphill surface.

One must also consider that roll rate, tail rotor thrust, any crosswind component, which skid is fixed and any lateral offset in the CG can all change the critical angle at which recovery is possible.

However, in discussions with Tom Smith and John O’Neill, former RotorWay employees, a key factor in determining the critical angle is certainly the maximum angle that the swash plate can move laterally. Using this information and empirical data, we all agreed that the critical angle in a RotorWay is very small - less than 5 or 5.5 degrees! Combine that with the other considerations discussed above and the critical angle may be considerably less. However, the final fact is, once the rolling moment moves the helicopter past the critical angle, recovery is impossible.

Lateral cyclic, for several reasons, cannot stop a rollover once the helicopter exceeds the critical angle. First, lateral cyclic res-ponse is not as effective as in a hover. Second, you will run against the stops or induce mast bumping long before effective input can be achieved.

A sudden increase in the collective (hoping to break the helicopter loose) will likely result in one of two occurrences. If an increase in pitch does not break the skid free, the more horizontal lift vector will produce a disastrously increased rolling moment and an inevitable machine rebuild. If a sudden increase in pitch does successfully free the skid, an uncontrollable rolling movement in the opposite direction can occur as a result of pendulum effect. That is to say, the very natural pilot reaction of opposite cyclic and/or up collective can have disastrous consequences.

So, with all this in mind, the safest way to avoid dynamic rollover is to never allow a lateral rolling moment to develop when a skid is in contact with the ground or with an object such as a taxiway light while in a hover. The best way to avoid the situation is to move the controls slowly during pickups and set downs so that you have time to correct any lateral movement before a skid can get stuck.

When in a hover, again, move the helicopter slowly enough that you can keep a sharp eye out for objects sticking up out of the ground, particularly when flying in unfamiliar/undeveloped areas.

Having said that, we must nonetheless react quickly and correctly if a rolling moment does occur by lowering the collective smoothly, gently and immediately whenever a stuck skid results in ANY rolling moment. It is imperative that we discipline ourselves to this action before loss of control occurs. I emphasize 'smoothly and gently' as radical, rapid down-collective inputs can cause the main rotor to contact the tail boom or, in the case of a slope landing, can produce a roll in the opposite direction with equally catastrophic results. I say 'discipline ourselves' because this is not a maneuver you want to "practice"!

So move carefully and in control at all times with your helicopter; fly safely and have fun.

Charmienne Pohlman (Mother “Goose”) is a Certificated Flight Instructor in helicopters, with over 2000 hours of instructing. She is a past President of the Mid-Atlantic Helicopter Association (MAHA), served as MAHA’s Safety Officer and newsletter editor for ten years, edited the “Rotorheads Newsletter” from 1999 - 2003, is a lifetime member of the Women’s International Pilots’ Association (also known as the “Whirly Girls”). She has served as an FAA Safety Counselor, during which time she conducted Safety Seminars, counseled pilots and helped to establish operational safety agreements at local airports. She is also a charter member of the Helicopter Museum located at the Brandywine Airport in West Chester, PA.

© Experimental Helicopter magazine - On Line
2008
Last Update 6/25/2008