Technique Archive

Power source

Thursday, May 20th, 2010

What is the best power source for a helicopter? The two choices are a turboshaft or a reciprocating engine. A turboshaft engine has the same basic structure as a turbojet; however, the energy produced by the expanding gases is used to drive a turbine instead of producing thrust. The turbine is connected to a gearbox that drives the helicopter’s main rotor transmission. Likewise, the reciprocating engine’s output drives the main rotor transmission; however, these engines have traditionally been viewed as less reliable.

To understand where that reputation came from we need to look at early helicopter designs. Helicopter manufactures took piston engines used in airplanes and installed them in their helicopters. However, these engines didn’t quite have enough horsepower for hovering. So to increase the power, manufactures ran the engines at a higher rpm, and as a result reliability suffered. So much so that Lycoming reduced the TBO on the O-360 from 2,000 hours to 1,600 hours for engines installed in helicopters. This fueled the unreliable reputation of the piston engine.

In 1979 Frank Robinson introduced the two-seat R22. His idea was to reduce the helicopter’s weight to reduce the power required. For example, the T-bar cyclic system is simple and weighs less than the conventional dual control system. He then took the reliable Lycoming O-320 engine and reduced the rpm from 2,700 to 2,652 and de-rated the maximum horsepower from 160 to 124. Lycoming then approved the same 2,000-hour TBO it had for fixed-wing installations. He did the same thing with the R44’s Lycoming O-540 engine. The engine’s reliability proved so good that Lycoming increased the TBO to 2,200 hours for both airframes, giving these helicopter installations a higher TBO than the same engine installed in a fixed wing. NTSB accident data supports the higher reliability achieved by derating a reciprocating engine.

Even with the vast improvement in reliability, reciprocating engines suffer from a low power to weight ratio. So for helicopters above about 2,500 lbs gross weight, a turbine engine makes sense. It is compact, light weight, and has a simple design that gives it excellent reliability. However, perhaps the most important feature is its high power-to-weight ratio. This makes turboshaft engines the only choice for large single and all twin-engine helicopters. However, the downside to these engines is the high cost to acquire, maintain, and operate them.

Disc loading

Friday, May 7th, 2010

Disc loading is defined as the ratio of a helicopter’s gross weight to its rotor system’s disc area. A large disc area allows the rotor system to work with more air creating a higher efficiency in a hover. A smaller rotor system compromises hover efficiency for speed and a compact rotor system.

An example of a production helicopter with low disc loading is the Robinson R22. This improves the R22’s hover performance using the relatively low power of its Lycoming piston engine. Taking the concept of low disc loading to an extreme is human-powered flight in a helicopter. The low power output of a human requires a very large rotor system. Students at California Polytechnic State University at San Luis Obispo designed a human powered helicopter that weighted 250 pounds including the pilot/power source. It had a rotor diameter of more than 100 feet and was only designed to hover. In December 1989 it flew for 7.1 seconds reaching a height of 20 cm. It was built to compete for the Sikorsky Prize offered in 1980 by the American Helicopter Society. The award is $250,000 to the team whose human-powered helicopter can stay airborne for 60 seconds and reach an altitude of 3 meters. To date, the prize is unclaimed.

In contrast, a helicopter with high-disc loading requires a lot of power to hover. For example, the Sikorsky CH-53E Sea Stallion uses three General Electric T64-GE-416/416A turboshaft engines producing 4,380 shp each. Its gross weight is 73,500 lbs and has a rotor diameter of 79 feet. The CH-53’s rotor downwash in a hover is so strong that standing near it is nearly impossible. In addition, high disc loaded helicopters have rapid descent rates making them more challenging to autorotate. Taking high disc loading even further is the V 22 Osprey tilt rotor. It has two 38 foot diameter rotors and a max gross weight of 60,500 lbs. In order to hover it uses two Rolls-Royce Allison T406/AE 1107C-Liberty turboshaft engines producing 6,150 hp each.

Tail boom strakes

Thursday, April 22nd, 2010

On some helicopters, running the length of the tail boom are “L” shape (or something similar) brackets that protrude about an inch. These are known as tail boom strakes and they act like spoilers.

Because the tail boom is underneath the rotor system, at a hover, very low airspeeds or sideways flight rotor down wash passes around the boom. Like an airfoil, this produces high and low air pressure areas that exert a force along the tail boom. This force decreases the tail rotor’s capability during hover and slow flight. At higher speeds the down wash moves to the rear and passes above the tail boom. Strakes control the airflow around the tail boom, thus increasing the tail rotor’s efficiency and decreasing the turbulent air, which improves yaw control.

During the late 1980s, NASA and the U.S. Army performed wind tunnel and flight tests to analyze the performance gain from adding a tail boom strake. The tests were performed using a Bell 204B helicopter. Published in 1993 the NASA Technical Report 3278 stated a 5-percent improvement in pedal control margin will provide an additional 2,000 feet of altitude capability or 500 lbs. of payload. The report concluded that the strakes improves handling qualities, reduces tail boom fatigue, improves climb and cruise performance, and increases yaw control safety margins for all single rotor helicopters with enclosed tail booms.

Thoughts on EMS training

Thursday, March 4th, 2010

The helicopter EMS industry is struggling with a high accident rate. Several months ago the NTSB published recommendations ranging from equipment requirements to increased training. There seems to be no doubt in the helicopter industry that the FAA will mandate one or more of the NTSB recommendations this year. In the past the FAA has been reluctant to act; however, the feeling now is if the FAA does not come out with something strong to stop the accidents, Congress will.

In my opinion, increasing the amount and type of training will do the most good. Using technologies such as HTAWS and NVGs are helpful as well, but I think the most benefit will come from better training.

EMS is a tough business with lots of cost pressures, and spending more money on training can be hard to justify sometimes. I was told by one EMS vendor that watching costs was paramount to survival, if he couldn’t bid a competitive price and lost contracts they’d be out of business.

An interesting dichotomy was when I flew a corporate helicopter. I was trained at FlightSafety every six months and could take the helicopter (a Bell 430) out once a month to practice. The corporate mission was nowhere near as demanding as EMS flying, yet there was considerably more emphasis placed on training. Sometimes I wonder if the difference was because the person who ultimately approved the training budget also rode in the back of the helicopter. Those passengers certainly had a vested interest in the proficiency of the pilots.

It will be interesting to see what the FAA does. If operators can afford the technology and the increased training then that’s the best scenario. However, if it’s one or the other I believe the best improvement in the accident rate will come from enhanced training.

Drive link

Monday, February 15th, 2010

Connecting the rotating swash plate to the rotor shaft is an assembly known as the drive link. Because the swash plate needs to move up and down and pivot, the drive link has a joint that acts like a scissor – as such it is sometimes referred to as a scissors link. I have had several students ask me why it is needed.

The swash plate has a rotating and non-rotating side. The non-rotating side is on the bottom and is connected to the flight controls. The rotating side is on the top and is connected via pitch links to each rotor blade. The collective control moves the entire swash plate assembly up and down to change the pitch on each blade equally. The cyclic control tilts the swash plate, changing each blade’s pitch independently depending on its position around the rotor disk. This tilts the rotor disk in the desired direction.

Since the rotor mast runs from the transmission up through a sleeve that the swash plate moves around, there needs to be a method of turning the rotating part of the swash plate. This is the function of the drive link as it connects the mast directly to the swash plate. It is critical that this part be functioning correctly.

During preflight it should be examined closely as the failure of the drive link has caused several accidents. On the Bell 222 an improperly sized bolt that attached the drive link to the swash plate allowed play which caused the bolt to fail. As you can imagine without the drive link the blades will continue turning the swash plate through the pitch links. This stresses the pitch links in a manner they were not designed to handle and can result in a pitch link failure. In this case with the Bell 222 it caused an in-flight break up.

In 1988 the pilot of a Bell 47 spraying a field reported an extreme vibration followed by a loss of control and hard landing. Then in 1992 a CFI and student flying another Bell 47 also felt a sudden and severe vibration and managed to successfully autorotoate to a field. In both cases the center bolt connecting the drive link was missing and disconnected drive to the swash plate.

Low-G pushovers

Friday, January 29th, 2010

A two-blade or semi-rigid rotor system (such as the Robinson or some Bell series helicopters) is susceptible to a phenomenon called mast bumping. To avoid mast bumping it is important to fully understand the limitations and performance capability of this type of rotor system.

In order to produce thrust a helicopter’s rotor system must be loaded. Controlled by the cyclic, the swash plate changes the pitch angle on each blade separately. This creates an imbalance of thrust across the rotor disc forcing the disc to tilt, which causes the helicopter to roll or pitch in the desired direction.

Pushing the cyclic forward following a rapid climb or even in level flight places the helicopter in a low G (feeling of weightlessness) flight condition. In this unloaded condition rotor thrust is reduced and the helicopter is nose low and tail high. With the tail rotor now above the helicopter’s center of mass, the tail rotor thrust applies a right rolling moment to the fuselage (in a counter-clockwise turning rotor system). This moment causes the fuselage to roll right and the instinctive reaction is to counter it with left cyclic. However, with no rotor thrust there is no lateral control available to stop the right roll and the rotor hub can contact the mast. If contact is severe enough it will result in a mast failure and/or blade contact with the fuselage.

In order to recover the rotor must be reloaded before left cyclic will stop the right roll. To reload the rotor immediately apply gentle aft cyclic and when the weightless feeling stops, use lateral cyclic to correct the right roll.

The best practice is to exercise caution when in turbulent air and always use great care to avoid putting the helicopter in a low-G condition.

Safer night ops

Tuesday, January 19th, 2010

Threats, clearly visible during the day, are masked by darkness. In fact, controlled flight into terrain (CFIT) at night is a major problem for rotor-wing operations. CFIT is defined as colliding with the Earth or a man-made object under the command of a qualified flight crew with an airworthy aircraft.

During the 1970s, CFIT became a major problem for commercial aviation. In response the FAA mandated the installation of ground proximity warning systems (GPWS) in commercial airliners. Although this resulted in a drop in CFIT accidents, these earlier systems were plagued with false and late warnings. Improved versions, called enhanced ground proximity warning systems (EGPWS), were introduced. These systems have made a valuable contribution to the reduction of fixed-wing CFIT accidents.

CFIT at night during VMC has been especially troublesome for helicopters in the air medical industry. According to the Air Medical Physician Association, half of all EMS accidents happen at night. EGPWS have been discussed as a solution to reduce the air medical helicopter accident rate. However, because of the unique low-flying operation of helicopters the effectiveness of current EGPWS is unclear. This prompted Honeywell to introduce the Mark XXII EGPWS, specifically designed to address the needs of helicopters. Moreover, the company is developing a database of power lines to add to the system. As computer memory capability grows, databases will be able to contain more detailed maps.

However, by the time the EGPWS activates, the pilot has probably already lost situational awareness. A method to help with situational awareness is improving the pilot’s ability to see obstructions at night. That’s the technology behind night vision goggles (NVG). They work by detecting and amplifying existing visible light, so there must be at least some light available for them to work. Originally NVG were only for military use, but recently they have been allowed in the air medical industry, and more than half of the EMS helicopters are flying with them.

Another technology that holds promise is enhanced vision systems (EVS) which detects and displays thermal energy not visible to the naked eye. In this arrangement a camera is mounted in the nose and feeds the image to a monitor in the cockpit. Some glass cockpit systems will project the image behind the attitude indicator for better situational awareness. These systems are effective in smog, smoke, duststorms, and other limited visibility situations. Likewise, they can help in brownout and whiteout conditions. The U.S. military uses thermal imaging systems in combination with NVGs.

The air medical industry is expecting the FAA to possibly mandate additional equipment requirements like they did with earlier with commercial aviation. With the different technologies available it will be interesting to see what happens.

Servo transparency

Friday, January 8th, 2010

Pilots who learn to fly in smaller helicopters probably hear very little about servo transparency, yet this phenomenon has caused or played a role in several accidents. When giving flight reviews I have found some helicopter pilots who totally misunderstand why and how it happens. However, the concept is not too difficult to understand.

Because of the higher control forces in larger helicopters, hydraulically boosted servo actuators are used to assist the flight controls. The maximum force that these servo actuators can produce is constant and is a function of hydraulic pressure and servo characteristics. Engineers design the hydraulic system to adequately handle all aerodynamic forces required during approved maneuvers. Even so, with certain aggressive maneuvering it is possible for the aerodynamic forces in the rotor system to exceed the maximum force produced by the servo actuators. At this point, the force required to move the flight controls becomes relatively high and could give an unaware pilot the impression that the controls are jammed. To prevent servo transparency, pilots should avoid abrupt and aggressive maneuvering with combinations of high airspeed, high collective pitch, high gross weight, and high-density altitude.

The good news is that this phenomenon occurs smoothly, and can be managed properly if the pilot anticipates it during an abrupt or high-G load maneuver. On clockwise-turning main rotor systems the right servo receives the highest load, so servo transparency produces an un-commanded right and aft cyclic movement accompanied by down collective. The pilot should follow (not fight) the control movement and allow the collective pitch to decrease while monitoring rotor rpm, especially at very low collective pitch settings. The objective is to reduce the overall load on the main rotor system. It normally takes about two seconds for the load to ease and hydraulic assistance to be restored. However, be aware that if the pilot is fighting the controls when this happens, the force being applied to the controls could result in an abrupt undesired opposite control movement.

Many of these accidents have happened while aggressively flying the helicopter at low altitudes, leaving very little time to recover. Most important for avoiding this kind of accident is to follow the aircraft limitations published in the helicopter’s flight manual.

Above reproach?

Wednesday, December 30th, 2009

Commenting on my gross weight blog, Harold wrote:

“Leave the flying to he who is in the cockpit and the finger-pointing blogs to another publication please.”

That got me thinking, when is it (if at all) appropriate to comment, criticize, or even intervene on another pilots actions or behavior? I understand and agree with Harold to a point, but I don’t believe the complete answer is all that clear.

I have studied and written about helicopter accidents for many years. I think most of them have a lesson that can help us all be better pilots. I try to write about these in a way that states the facts without expressly passing judgment (gross weight included) and let the readers draw what they want from the situation. Believe me, I have made my share of mistakes but I have been lucky because they didn’t result in an accident. I have viewed them as learning experiences, because had something been just a little different I might not have been so lucky. I like to tell people that I can’t promise I won’t make a mistake, but I can promise I won’t make the same one twice. Having studied many accidents it is clear that there are no new accidents only the same ones repeated over and over, just in a different manner.

I also believe that simply being a licensed pilot does not make you above reproach. Listed below are three examples of pilot behavior that other people knew was dangerous. A link to the complete NTSB report is included because all the details can’t be listed here.

A pilot flying a news helicopter was well known as a hotdog and the photographer riding with him had expressed concern. His last radio transmission was “watch this” as he pulled the helicopter vertical and severed the tail boom killing himself and the photographer.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001212X20685&key=1

A very experienced tour pilot flying in the Grand Canyon was well known for being a skilled pilot and for his aggressive flying. He had earned the nickname “Kamikaze.” At high density altitude he slammed into a canyon wall killing himself and six passengers.

http://www.ntsb.gov/publictn/2007/AAB0703.pdf

A pilot continued to fail phase checks, check rides, and pre-employment rides. He eventually got a job where his flight skills were not evaluated prior to being hired. He crashed an R22 killing himself and a passenger on an introductory flight.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20060228X00255&key=1

I really appreciate all the professional comments that people post. So if this subject interests you please take the time to read all the details and let us all know your thoughts. I believe that approaching this topic in the correct way can be a powerful learning tool for those so inclined to listen.

My intent is not to point fingers but to get pilots thinking about how easily an accident can happen. I know that reviewing accidents has helped me be a better pilot. However, I am very curious if other pilots find this helpful.

One final thought. I have been involved as an expert witness for helicopter accident cases in court and believe me the intense scrutiny pilots endure is not pleasant. Seeing that has given me another reason to believe that being ultra conservative to avoid an accident is well worth it.

Wire strike protection

Thursday, December 10th, 2009

I fly a Bell 206 JetRanger helicopter as a demonstration aircraft for my company’s autopilot and glass cockpit systems. It is equipped with a Wire Strike Protection System (WSPS) and many times I am asked what it is and how it works.

Bell 206 with Wire Strike Protection

Bell 206 with Wire Strike Protection

The system on the Bell 206 has three main components: an upper cutter, lower cutter, and deflectors. Each cutter has a deflector that forces the wire into sharp high-tensile steel blades (they are rubber coated to prevent inadvertent injury to service personnel). An additional deflector strip runs vertically between the pilot and copilot windscreens to guide the wire to the upper cutter. On different helicopters other deflectors are mounted as necessary to protect critical areas. For example, on the toes of the skids to force a wire to go under the helicopter and stop it from getting caught between the skid gear and the fuselage.

It is a passive protection system that reduces the chances of an accident in the event that the helicopter is flown into horizontally strung wires. The key phrase is “reduces the chances” as the system is not 100-percent effective. In order to work properly the helicopter needs forward speed; faster speeds increase the probability of cutting the wire. Also the level of effectiveness is a function of several other factors including where the wire impacts the fuselage, the cable tension, and the diameter of the wire.

The US Army evaluated the WSPS by performing pendulum swing tests using a Bell OH-58 (basically a military version of the Bell 206). The tests went well and they adopted the system for use on U.S. Army helicopters. Since then several Army helicopters have hit wires that were then cut by the system resulting in no injuries and minimal to no aircraft damage. Several civilian helicopters equipped with the WSPS have cut wires and avoided an accident as well.

Of course the best protection from wire strikes is prevention. Some things to consider are only flying below 500 agl when it’s necessary, looking for poles because they are easier to spot than wires and when you need to fly low over wires cross at the poles or supporting structures. Additionally, when landing in unapproved areas be sure to perform a complete aerial reconnaissance. If your helicopter is equipped with wire strike protection it should be viewed as a last line of defense.