AOPA Hover Power

In a recent blog (“Standing on a Basketball”) when discussing hovering, I stated that a helicopter is dynamically unstable. A reader commented, “Helicopters have neutral dynamic stability. They are not unstable.” This made me think that perhaps I should dig a little deeper into the subject. Sometimes a mathematical model or an engineer’s definition can be perceived differently when used in a practical application.

I’ll start with a basic definition of stability. An object is unstable if, displaced from its position, it continues to oscillate with increasing amplitude. It would be considered stable if it oscillated with decreasing amplitude, eventually returning to its original position. In the case of neutral stability, it does neither of these. That is, the amplitude does not increase nor does it decrease.

To get an engineering perspective on helicopter stability, I reviewed what Ray Prouty has written. Prouty has contributed to the helicopter industry for more than 50 years. He has done work ranging from preliminary design to performance and flight testing. He has also been honored for his contributions to the industry by the prestigious American Helicopter Society (a group that emphasizes engineering excellence in rotorcraft design www.vtol.org ), which named him an Honorary Fellow in 1983. He has written three books on helicopter aerodynamics and his writing is some of the best in terms of taking a complex engineering concept and explaining it in easy to understand language.

Chapter 8 of his book titled Helicopter Aerodynamics addresses dynamic stability. Here he provides a great explanation of what happens when a hovering helicopter is displaced by a gust of wind. He states, “A typical helicopter will go back and forth across its starting point with an ever increasing swinging motion until the pilot (or someone else) stops it.” According to Prouty, the rate of growth from one cycle to the next is a measure of the degree of instability. Finally he concludes that a hovering helicopter is unstable. Based on my experience teaching students to hover, I agree.

However, in a quest to produce a more stable helicopter, engineers designing early rotor systems developed devices that acted like gyros. While this made hovering much more stable, it reduced controllability. It was later determined that with practice a pilot could learn to hover without these stability enhancing devices. Today, systems like electronic forced trim and Stability Augmentation Systems (SAS) provide increased stability without sacrificing controllability. Helicopters equipped with these systems would behave more dynamically stable or neutral in a hover.

More information about SAS and other topics is available in Prouty’s books and I would recommend them to anyone who is interested in learning more about helicopter aerodynamics. He is a long time columnist for Rotor & Wing magazine and still writes for them occasionally. For more information on how to obtain his books or read his past columns, visit their website at www.aviationtoday.com/rw/

When I tell people that I fly helicopters the comment that I hear a lot is, “Isn’t that dangerous? If something happens to the engine you can’t glide like an airplane.” Well, I explain that is not true, helicopters do glide, it’s called autorotation. Without going into too much detail about the aerodynamics, I describe how it works with the concept of stored energy.

For discussion purposes, a helicopter on the ramp switched Off contains zero energy. However, when a pilot starts the engine, the fuel is converted to energy that is used to start spinning the rotor system. The rotor rpm is brought up to 100 percent; the pilot then lifts off and begins accelerating and climbing. Once established at cruise altitude and airspeed, the helicopter has two kinds of stored energy—Potential energy (energy because of position) in altitude, and kinetic energy (energy do to motion) in airspeed and rotor rpm. Essentially, this is money in the bank to be used in an emergency. It is the successful manipulation of this energy that will bring the helicopter and its occupants to a safe landing during a loss of power.

When a helicopter’s engine stops in flight, a freewheeling unit disconnects the engine from the rotor system to prevent the engine drag from slowing the rotor rpm. In addition, the pilot must immediately lower the collective pitch allowing the helicopter to start descending and forcing the airflow up through the rotor system. Basically, the helicopter begins consuming altitude energy to maintain rotor rpm. This is a very important step because waiting too long to lower the collective will allow drag to slow the rotor system and stall the blades. If this happens we destroy the helicopter’s ability to manipulate energy and it will simply fall out of the sky with fatal results.

Once established in autorotation the descent rate is normally 1,200 to 1,500 feet per minute and the pilot should maintain about 60 knots and maneuver the helicopter to the best landing area available. Approaching 50 to 75 feet agl, the pilot begins to rapidly decrease airspeed with a flare. Airspeed energy is used to arrest the descent rate. If timed correctly, the helicopter should momentarily end up about five feet above the ground with little to no airspeed. With all the altitude and airspeed energy gone, the only energy left is in the rotor system. The helicopter will start descending and the pilot should then raise the collective pitch control and use the rotor rpm energy for a gentle touchdown. As the rotor system slows to a stop the helicopter returns to a state of zero energy.

Of course all this assumes ideal conditions. We all know that in the real world it doesn’t always work that way. There are certain combinations of airspeed and altitude that simply do not have enough stored energy to make a safe landing. For example, hovering at 150 feet the pilot must rely mainly on rotor rpm to cushion the landing. The vast majority of helicopters do not have enough energy stored in the rotor system to completely stop the descent rate. Most likely the landing will damage the helicopter and injure its occupants. If a pilot is hovering higher, say 500 feet, there is enough altitude energy to trade for airspeed and complete a successful autorotation.

This explains why helicopter pilots prefer to take off by moving forward to gain airspeed first, instead of going straight up.

Having both a fixed-wing and rotor-wing ATP, I have often thought about the different skill set required to fly each type of aircraft. The skills and traits that make a good VFR airplane or helicopter pilot are not necessarily the same.

Airplane flying is more procedural, at least when you are thinking of larger airplanes. (Ultralights, kits, and Piper Cubs not included.) Approaches are flown at specific speeds and angles. Helicopters can, and many times do, land in different environments, sometimes making very steep and sometimes near vertical approaches (although there is some very serious aerodynamic considerations to a vertical approach and that’s a subject of future blogs). However, at airports and during emergencies a helicopter can make very shallow approaches. Without any type of vertical guidance (VASI or glideslope) helicopter pilots must become good at determining their own approach angles and controlling descent rates and airspeed. This is very hard at night when little visual clues are available as any EMS helicopter pilot knows. To aid in these types of approaches many EMS operators are starting to use Night Vision Goggles.

A different kind of preflight planning is generally required to fly a helicopter. Airplane pilots must give careful consideration to things like runway lengths, airspace requirements, and performance characteristics. Not to say that helicopter pilots do not think about those subjects, but generally the length of a runway is of little concern. A helicopter pilot’s performance exercise involves a different kind of planning like checking hover power with in-ground-effect and out-of-ground-effect charts.

Hovering is not something airplane pilots think about. In fact, getting an airplane too slow should be downright scary. Whenever I had an airplane pilot learning to fly helicopters, I can always remember the first time I ask or sometimes demonstrated slowing the helicopter to an out-of-ground-effect hover at 500 feet. I say demonstrated because some students just can’t hold the nose up and allow the helicopter to slow past about 60 knots. They get real nervous, but nothing bad happens; it just comes to a stop and will then start moving backwards.

Learning to hover is probably the biggest challenge. It’s all seat of the pants and both feet and arms are doing something at the same time. Throw in the fact that a helicopter is dynamically unstable and it feels a lot like trying to stand on a basketball. It can take some students as much as 10 hours to figure it out, and some never get it at all. Add gusty wind conditions and even experienced helicopter pilots will have their hands full. One of the reasons it’s so hard is that the flight controls are sensitive; you have to think in terms of control pressure not movement. Hand to eye coordination is the skill to have. In fact, one of the smoothest helicopter pilots I know rides a unicycle in his spare time.

Flying lower allows helicopter pilots to avoid crowded or controlled airspace. Of course flying lower adds a new concern; power lines and towers. Best to fly at least 500 feet agl. Although helicopters have great visibility, they normally are not IFR capable. Encountering inadvertent IMC can be deadly in a helicopter. The plus side here is the added maneuverability; more landing options and the ability to hover make it easier to stay out of the clouds.

Some emergency procedures are so different that it forces high time airplane pilots to unlearn certain reactions. For example, a low rotor rpm horn can sound a lot like a stall warning. An airplane pilot’s ingrained reaction would be to lower the nose. In a helicopter that would just force the rotor rpm lower. Unlike stalling the wing of an airplane, if a pilot stalls the rotor system it is not recoverable. Of course, stalling an airplane close to the ground is probably not recoverable either. Have no fear, should you decide to learn to fly helicopters all of these critical differences will be emphasized.