AOPA Hover Power

One big advantage to a helicopter’s rotor system is the vertical thrust that allows the aircraft to hover. However, when this same rotor system is flown edge wise through the air it creates an aerodynamic problem that limits the helicopter’s forward speed. The term used to describe this is dissymmetry of lift.

To generate lift a helicopter’s rotor blades spin to create airflow. A rotor system’s RPM is fixed at a certain value for all phases of normal flight and increasing the blade’s angle of attack (the angle between the relative wind and the blade’s cord line.) with the collective control generates lift. In a hover with no wind, lift is essentially equal across the entire rotor disc. However, as the helicopter begins to move forward it creates a relative wind. One side of the rotor disc has a blade that is advancing into the relative wind (think headwind) and the other side has a blade that is retreating (think tailwind). The difference in airspeed each blade encounters between the two sides grows as the helicopter gains forward speed. This creates an imbalance of lift problem that early helicopter engineers had to solve to maintain controllability.

To help understand how they did it, consider the equation for lift.

Lift = CL ½ p S V2

CL = Coefficient of Lift, which is a function of angle of attack and blade shape

p = air density

S = total blade area

V = airspeed

At a given moment in time, air density, total blade area and blade shape are all fixed values, so as each blade’s airspeed changes the rotor system must respond by changing the blade’s angle of attack to keep total lift constant. This is done primarily by allowing the blades to move up or down in a process known as flapping. Two bladed rotor systems (known as semi-rigid) use a single teetering hinge that allows the blades to flap as a unit (one go up, the other goes down). Rotor systems with more than two blades (typically known as fully articulated) use a flapping hinge on each blade allowing the blades to move up or down independently of each other.

How flapping works is by changing the angle of attack in response to the varying airspeeds the blade encounters as it moves around the rotor disc. When the advancing blade experiences a higher airspeed, the lift on that blade increases forcing it to move up. This upward movement changes the direction of the blade’s relative wind reducing its angle of attack. On the retreating side just the opposite happens. The reduced airspeed causes a decrease in lift causing the blade to move down, increasing its angle of attack. Each blade’s angle of attack changes in direct relation to its relative airspeed. As each blade’s relative airspeed increases, angle of attack decreases and vice versa to maintain equal lift across the disc as the helicopter’s airspeed increases.

As you might have guessed, this creates a problem on the retreating side. You can only increase an airfoil’s angle of attack so much before it stalls. As the helicopter continues to fly faster the retreating side must continue to increase its angle of attack to compensate. At some airspeed the retreating blade stalls and this is what limits the helicopter’s forward airspeed. This is referred to as retreating blade stall.

There is more to discuss on this subject so part 2 is coming up next time.

Over the past year 28 people have died in EMS (emergency medical services) aircraft crashes. The industry is experiencing one of the worst accident rates in its history. Solving this problem is a complicated issue for sure, however I have some very basic thoughts on how this problem can be fixed.

Flying an EMS helicopter was some of the most demanding flying I have done. Flying at night and landing on streets or other confined areas, having to make quick weather decisions sometimes with little information available, and having to block out the pressure to fly. Yet many EMS helicopter pilots receive the minimum amount of required training.

Conversely, when I flew a corporate helicopter it was normally airport to airport or heliport. The occasional off-airport landing was performed, however it was planned and I had plenty of time to assess the area. This was far less demanding and risky than flying an EMS helicopter. Yet, it was also where I received the best and most consistent training. We had the time and resources available to practice our skills and FlightSafety training every six months.

Corporate helicopters are not expected to make money and the person who has the authority to cut training expenses normally rides in the back. That’s a strong motivator to ensure that the pilots know what they’re doing. EMS helicopter operations by contrast need to make money and that means keeping a close eye on costs. Also, because of the competitive bid process hospitals use when selecting vendors, margins are thin. Training costs come right off the bottom line. If a vendor increases its training costs and the others do not, then that vendor is at a competitive disadvantage. Hospital-owned programs are also in business to get patients to their hospital and make money.

To level the playing field, I think two requirements are needed. The first is more frequent and comprehensive training. Not just training in maneuvers but scenario-based training that addresses issues such as crew coordination, judgment, and accident chains to name a few. Additionally, more IFR and inadvertent IMC training, even for VFR-only programs, is needed. Pilots need to be very comfortable initiating a climb and not descending if they get caught in bad weather.

This type of training can be done in simulators. Simulators are not only good for showing pilots how to do things correctly, but can also show how quickly a bad decision can degenerate into a serious problem. That’s a powerful learning tool.

Second is better equipment, such as terrain avoidance and warning systems and night vision goggles. In addition, important in adding new equipment is providing the appropriate level of training on how to use it effectively.

Another issue that should be addressed by the industry is pilot salaries. I have known many very good pilots that have left EMS for better paying jobs. This has made EMS a steppingstone for pilots to get to something better. EMS flying requires a very specific skill set and experience level. It should be the job that pilots aspire to get. Higher salaries will keep turnover down and keep experienced pilots in the industry.

I realize that all of my solutions cost money and that some operators will claim they cannot afford these programs. That is why training and equipment should be mandated for everyone who wants to operate an EMS helicopter. The difficult part is figuring out how the industry will get there.

The FAA has tried the quick and inexpensive solutions and they do not work. Case in point is the risk assessment matrix. Three years ago EMS pilots began filling out a questionnaire before each flight to determine a score that related to a risk level. The accident rate has gotten worse in the last three years.

As with most things in life, to get the best results one needs to spend the effort and money required. Cheap solutions are just that.

Not visible from the cockpit, a helicopter’s tail rotor is perhaps the most vulnerable component to striking objects in a hover. EMS pilots are especially at risk, as their job involves routinely landing in obstacle rich environments.

In 2003, a Bell 430 was substantially damaged when its tail rotor hit a roadway sign during an off-airport landing at night. Prior to touchdown, the pilot said he rotated the aircraft and landed on an easterly heading, at which point the medical crew departed the helicopter. Then, the pilot decided to reposition the aircraft to face west for departure. During the hovering turn the tail rotor hit a steel reflector post. The aircraft touched down on the left rear skid first and came to rest 180 degrees from its initial heading. The tail rotor and gearbox assembly had come apart and departed the helicopter.

Darkness certainly makes objects harder to see. However, two years prior to this accident, during daylight conditions, a Bell 222UT was substantially damaged when its tail rotor hit a barrel while landing on a paved traffic turn-around area. The pilot said that while hovering, he decided to reorient the aircraft to help load the patient easier. During the right pedal turn, the tail rotor struck a 55-gallon trash barrel. The helicopter yawed to the right and the pilot brought the throttles to flight idle and landed the helicopter. The tail boom was twisted, the tail rotor blades were damaged, and the tail rotor gearbox was nearly separated from the airframe.

Although a tail rotor strike in a hover can cause serious damage, the potential for personal injury is low compared to what can happen in flight.

In 1999, a Bell OH-58A, on a photo flight with doors removed, was destroyed on impact with the terrain and the private pilot and passenger sustained fatal injuries. A witness reported that he saw the helicopter flying at an altitude of approximately 350 to 400 feet. He saw what was possibly a large bird hit the rear rotor of the helicopter. The helicopter made three to four rotations during its descent.

Examination of the tail assembly revealed an elastic material with navy blue yarns wrapped around the tail rotor. The material, along with a sample of a navy blue warm-up jacket found along the reported flight path, was sent to the NTSB’s Materials Laboratory for examination. The color, size, and texture of the navy-blue yarns in the elastic material were consistent with those found in the navy blue warm-up jacket. The NTSB concluded that the jacket exited the helicopter and became entangled in the tail rotor.

Removing a helicopter’s doors places the tail rotor at increased risk. In 1993, an R22 helicopter flying with its left door removed crashed after an aluminum kneeboard exited the helicopter and struck the tail rotor.

There have been numerous cases where objects have come out of the cabin or an unsecured baggage compartment and struck the tail rotor. In some cases the pilots have been able to enter autorotation or otherwise land with minor damage or injury. However, as with the two preceding accidents, the tail strike inflicted enough damage to cause the tail rotor assembly to come apart. In these cases, the resulting center of gravity shift will make recovery nearly impossible. The importance of protecting the tail rotor cannot be emphasized enough.