AOPA Hover Power

Many times you will hear helicopter pilots refer to hovering in ground effect as resting on a cushion of air. Technically speaking, what they are referring to is the extra performance that hovering in-ground-effect (HIGE) provides versus hovering out-of-ground-effect (HOGE).

Ground effect is defined as a condition of improved performance that results from operating near a firm surface. A helicopter is normally considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground. However, the amount of benefit varies as a function of height. A lower hover will generate more efficiency and as the helicopter climbs the advantage decreases reaching zero about one and one-quarter times the rotor diameter.

A helicopter requires less power to hover in ground effect for two reasons. The main reason is the reduced velocity of the induced airflow caused by the ground. (Induced flow is air flowing down through the rotor system and is also called downwash.) This reduced velocity results in less induced drag and a more vertical lift vector. As such, the lift needed to sustain a hover can now be generated with a lower angle of attack in rotor blades, which requires less power.

The second reason has to do with vortices generated at the rotor tips. The close proximity of the ground forces more air outward and restricts vortex generation. This reduces drag and increases the efficiency of the outer portion of the rotors.

The maximum benefit is achieved from hovering over a hard surface such as concrete. When a helicopter hovers over an area such as tall grass or water, energy is absorbed by displacing the surface, allowing the induced flow to increase, thus reducing the lift vector. This will require the pilot to add power to maintain that hover height.

When a helicopter is in a high hover, or out-of-ground-effect, it requires a lot more power because there is no obstruction to slow the induced flow or force it outward. This results in a more vertical downwash and also allows the formation of stronger rotor tip vortices, reducing efficiency.

Helicopter pilots need to consider this when making very steep approaches as it has caused accidents. Typically, what happens is while a pilot is attempting to land, they allow their airspeed to get too slow and their approach too steep. They then realize they do not have enough power to slow the descent rate. In this case, the helicopter begins settling from a lack of available power. This is not the same as an aerodynamic condition called “settling with power,” which involves the generation of a vortex ring state (subject of future blog).

The helicopter’s Pilot Operating Handbook (POH) has both In-Ground-Effect (IGE) and Out-of-Ground-Effect (OGE) hover charts. This allows the pilot to take the density altitude and gross weight into account to predict hover performance. It is very important for pilots performing some missions such as ENG (Electronic News Gathering) or external lift operations to know if their helicopter can hover out of ground effect. Safe helicopter operations depend on good performance planning.

The idea of a human-powered helicopter has intrigued many engineers and pilots. Although a practical application really does not exist, it is a good exercise in the development of highly efficient airfoils and light weight structures. As such, many colleges and universities have put together teams of engineering students to develop and build a human-powered helicopter.

A human-powered aircraft is defined as a vehicle that can carry at least one person using only what power is provided by the person(s) on board, usually by pedaling. Early attempts mainly involved airplanes. For example, the best known human-powered airplane is the Gossamer Albatross, which flew across the English Channel in 1979. Helicopters which require much more power to hover present a much bigger challenge. The two biggest problems are weight reduction and designing a highly efficient rotor system. Efficiency means that the rotors must generate a lot of lift with very little drag.

In 1980, to help further and support the idea of a human powered helicopter, the American Helicopter Society established the Igor I. Sikorsky human-powered helicopter competition. A prize of $20,000 was offered for a successful controlled flight lasting for 60 seconds and reaching an altitude of 3 meters while remaining in an area 10 meters square.

The first vehicle that actually got airborne was the Da Vinci III in 1989, designed and built by students at Cal Poly San Luis Obispo in California. For high rotor efficiency, the students knew that it would be important for the blades to work with as much air as possible. A big rotor handles a large amount of air and thus requires less energy to produce lift. The Da Vinci III had a 100-foot rotor diameter and a tip speed of 50 feet per second. In order to reduce weight, rotor tip propellers provided thrust. This eliminated the need for a transmission and anti torque system. The approximate weight of the aircraft with pilot was 230 lbs. It flew for 7.1 seconds and reached a height of 8 inches. However, the helicopter was unstable and required students on the ground to assist with control. No attempt was ever made to correct the instability.

The current world record for human-powered helicopters is held by an aircraft named Yuri I, built by a team from the Nihon Aero Student Group (NASG). It used four two-blade rotor systems (10 meter diameter each) operating at 20 rpm. In 1994, it achieved a height of 20 cm for 19.46 seconds unassisted and unofficially reached 70 cm during a flight lasting 24 seconds.

As far as I can tell, the most recent attempt, although unsuccessful, at a human-powered helicopter was on August 10, 2004, by a group of engineering students at the University of British Columbia. Although their project seems to be on hold, their Web site is still up.

Many have attempted to fly human-powered helicopters both before and after the creation of the Sikorsky Prize. So far no one has met all of the Sikorsky prize’s requirements.

One important method for determining a helicopter’s health is vibration analysis. All helicopters have an inherent vibration. The type and intensity varies as a function of rotor design and isolation systems. Understanding basic vibration levels and being alert to changes is an important safety tool for helicopter pilots.

This is exemplified by the crash of a Bell 212 helicopter equipped with a cockpit voice recorder. About 18 minutes before the accident, the passenger (who was also a mechanic) stated to the pilot, “Boy, those catfish are going crazy down there, aren’t they?”

“Yep,” the pilot responded, “must have been the vibrations from the helicopter.”

About one minute 30 seconds before the accident, the pilot asked the passenger, “Has this vertical just gotten in here or has it been here for a while?”

“We haven’t had any verticals at all,” the passenger replied.

“We do now,” the pilot said.

“Yeah, well it started right after we left back there,” the passenger said. About 20 seconds later, the passenger stated that another person had tracked the helicopter’s blades before they left and that he was commenting on how smooth it was. Forty seconds after that, the pilot said, “This stuff is getting worse.”

The recording then ended.

According to the National Transportation Safety Board, the helicopter was allowed to continue in service with a loose pitch-change horn, which led to separation of the pitch-change horn from the blade grip and the in-flight breakup of the helicopter after the main rotor struck the tail boom. Contributing to the accident, the safety board said, was the pilot’s failure to respond to increased vibration in the main rotor system and land immediately.

Many components on a helicopter can fail and still allow the pilot to make a safe landing. The main rotor system is not one of them. Thus, any abnormal low-frequency vibration felt in the airframe or through the flight controls should be treated with extreme caution and investigated on the ground until the source is found and corrected.

Another telltale sign of a potential problem is difficulty with tracking and balancing the rotor system. Two accidents involving Robinson R22 helicopters, one in Israel and one in Australia, should never had happened. In both cases, investigations revealed that corrosion from water penetration initiated a fatigue crack in the main rotor blades. Both helicopters experienced an increase in main rotor vibration prior to final blade failure. In both aircraft, the vibrations were corrected with track and balance, only to reappear a short time later.

 It’s normal for a helicopter to require periodic tracking and balancing as paint and bearings wear over time. However, when a vibration reappears or abruptly changes, helicopter pilots need to take notice.

 Fortunately, failure of a critical component in the main rotor system is rare–it would be like a wing spar failure in an airplane. The good news is that a lot of times the helicopter will try to tell you. Many sharp pilots and maintainers have been alert to this and corrected an issue before anything happened. I think vibration awareness and analysis should be an important part of the helicopter private pilot curriculum. If everyone understood the importance of vibrations, we could virtually eliminate these kinds of accidents. 

As a parent, I often wonder if my children will develop a passion for flying. I have two daughters, Madi who is 8 years old and Lexy who is 13. Both of them have been around airplanes and helicopters from the time they were babies. In fact, Madi got a helicopter ride before she was born. My wife was four months pregnant, when we flew an R44 from California to Pennsylvania (see AOPA PILOT, September 2000).

Sometimes I think spending so much time around aircraft has given them a different perspective than their friends. Like the time my wife and I decided to rent a C172 and take them around the area to look at the fall colors. They were excited, however, after takeoff we looked in the back and they were both sleeping. My wife commented, “I am not sure they understand the difference between this and the car.”

Last year I had an opportunity to take Lexy with me on a ferry flight from Dallas, TX to Long Beach, CA in an AS350 Astar helicopter. This time she wasn’t content just being a passenger, she wanted to try flying. The first day was a short flight to El Paso, TX. So once clear of DFW’s class B airspace, I gave her the controls. She could hold it steady for a short time before I would take over, straighten it out, and give it back to her. She was determined to make the helicopter do what she wanted and after about 20 minutes she could basically hold it straight and level. After an hour or so she was bored and started asking a lot of questions. I showed her how to read the Garmin GPS, hold heading and altitude and after practicing the rest of the day she got pretty good.

The next day she started out flying right away and flew almost the entire day. I watched, somewhat amazed, as she held a steady course and would tell me how she was using the information she was reading off the GPS. After a while she was eager to learn more and I showed her the sectional map. She would take a break from flying for 5 or 10 minutes and study it. Along our course about 20 nm south of Deming, NM was a tethered balloon to 15,000 feet msl. It was marked on the sectional as a restricted area and she noticed we were heading right for it. When we got closer, she spotted the balloon glimmering in the sun and turned north to avoid the area.

By the time we arrived in the Los Angeles area, she was really comfortable flying and a big help with threading our way through the crowded airspace. We parked the helicopter at Long Beach airport and flew home on the airlines. The next day she came home from school looking very sad. I asked her what was wrong and she said that none of her friends believed that she flew a helicopter. Luckily, we took lots of pictures for her to show them.