Game capture operation went pear shaped
the throttle your friend. When little to no height is available and no extra power is available, the helicopter will simply lose height. Instinctively the pilot will demand more power by increasing the CP to avoid hitting an obstacle or the ground. In the meantime, the retreating blade stall may be recovered, but the reduction in NR necessitates an increase in power, which in turn actually increases the blade angle of attack and we are back to square one. With excessive drag in the rotor system and no more power to stop the reduction in NR (RPM), the ground will rise up to smite thee. The die is cast and as the ground approaches very fast, the only reaction can be to stop this is by increasing the CP. Loss of NR at height is no big deal as potential energy, or sky space in height and distance, can give us the break we so desperately need.
We know that wind and temperature as well as height above sea level, or rather density altitude, determines a helicopter’s performance in a very defined way. What is not very well defined, is the roughness of the pilot’s actions as they are mostly driven by reaction when close to the animal. To boot, flight techniques used during hard manoeuvring can make an enormous difference. Look, when power is unlimited and I have never flown such a machine, then rough and tumble is on. So close to the ground, with so little spare power in the helicopter, time to correct for overindulgence is simply not there. Here we must understand the three energies with which we work: kinetic, potential and mechanical energy. Not so simple and straight forward when a closer look reveals that flying a helicopter entails two different sets of kinetic energy. Remember the Lift formula? Well, for the sake of a newbie, here it is: . The ‘V’ part equates to speed and for simplicity of argument, directly relates to kinetic energy. The one speed component is the main rotor blade speed and the other component is forward flight speed. The only energies available to feed the need of the rotor RPM, is the engine or mechanical energy and the height or potential energy. Height as a source of energy can be discarded as flight below 50 feet hold little to no usable energy that can be converted into rotor energy. So, the chopper pilot at low level is stuck with the ability of his powerplant to serve the required energy to keep the revs up, but it is limited and there are many factors that will increase the power required to maintain main rotor RPM. Density altitude is a big role player and must never be underestimated as a limiting factor. So, the higher we are, the less the performance of our power plant. This is a big deal if the excess power is limited for starters. When wind is considered, there are one or two pointers. Wind flowing over the disc adds to rotor efficiency. Wind from the side wants to weathercock the nose of the helicopter into wind and as such requires more yaw pedal input. That is what the tail rotor is for and this is what I want to talk about a bit more. The tail rotor will, let’s for argument sake say demand around 5% or less power from the engine in forward cruise flight. This amount will increase rapidly as forward speed is reduced until in the hover, let us say we now need 8% of engine power. To counter any momentum that was allowed to develop in the hover, would easily increase this requirement to around 15% in order to re-stabilise the helicopter. Add any harshness and the power required will spike. Nevertheless, soon, as the margins are small, the power required becomes more than the available power. This now leads to the main rotor RPM (NR), to reduce. This is very serious as the energy required is just not enough and as the NR decreases, the pilot must increase the collective pitch lever to compensate for the loss in Lift. This in turn increases the drag, which in turn requires more power to control the helicopter. Yet the sequence of energy requirements has not stopped. The main and tail rotor are directly connected and any drop in the main rotor RPM will lead to a decrease in the tail rotor RPM of an average factor of four. This then requires more yaw pedal, more power required and this madness power loop ends in the helicopter becoming uncontrollable and falling out of the sky. Take note that this sequence had to start somewhere and that is the point to avoid at all cost. When reacting hard to animal behaviour, the helicopter and the power plant will pay the price.
Recommendations
Aerodynamics is a truth from which we cannot escape. Overstepping the mark leads to instant results. Rough and mishandling of flight controls is the mark of a game pilot with stage fright. Unfortunately, we all go through this phase of getting used to the stage, which leads to smoother control inputs with mostly the exact same helicopter manoeuvrability. All helicopter pilots have run out of revs at some time. Some could recover and some not. Game pilots will experience NR droop quite regularly and must always be able to regain RPM by some or other means. A little NR droop is always quite possible from which to recover, but when it was due to a harsh and overactive input, the results are normally tragic. To fly hard is one thing, but to fly the animal is something else. Animals will change direction in less than the blink of an eye and over reaction, just to stay with the animal, is madness. Do not fly the animal, fly your chopper. I know that getting the animal to yield to your wishes is the mark of a good game pilot, but to accomplish that whilst keeping the helicopter in one piece, is even better, much better.
Density altitude is well known and never seriously considered by pilots flying smaller machines all the time. Momentum is quite a different aspect to consider as density altitude increases. Harshness of manoeuvring must be reduced. When hot and high, cool it with the tough handling. Relax a bit more and give the helicopter a chance. Feel her out before forcing anything.
When I hear about such an accident, I always ask where the pilot has flown most of his or her hours and doing what? Game pilot operations are the hardest flying environment, followed by crop spraying (AG) operations and to just try to dabble in that environment is stupid. Fly what you have trained for and never mistake many hours for task fitness. Five, ten, fifteen or twenty thousand hours sounds very experienced, but experienced in what? One pilot during a revalidation test said the following: “Charlie, I flew 3000 hours game work accident free in my Robbie 22 and thought that it was impossible to run out of revs, then I did. I must confess that I was arrogant and had to pay the price.” No, you do not have to pay anything if you stick to the rules and gain experience, but never believe you can walk on air. Keep your place in the bigger picture. Ensure that your confidence never surpasses your experience.