Technique Archive

Stored energy

Monday, August 2nd, 2010

 

One way to think of autorotation is the effective use of stored energy to safely land the helicopter. Like the slow and careful release of the energy stored in a wound spring as opposed to allowing a quick high-energy release.

 

 

 A helicopter sitting on the ground has no stored energy (battery excluded). After start up, the energy in the fuel is converted to motion via the engine. During lift off and climb out the engine continues to add energy to the system. Once established in cruise flight, the helicopter has three sources of stored energy: kinetic energy (from motion) in airspeed and rotor rpm, and potential energy (known as gravitational potential energy because of its position in a gravitational field) in altitude.

 

  

When an engine quits, the conversion of fuel to energy stops. When this happens the first step is to lower the collective control to reduce drag on the main rotor blades, which prevents rotor rpm from slowing down. This causes the helicopter to start descending and this begins the consuming of altitude energy to keep the rotor system spinning. In an autorotative descent, at a fixed airspeed, lowering the collective will increase rotor rpm. To spin faster, the rotor system requires energy, so energy is removed from stored altitude and the helicopter’s descent rate increases. In reverse, raising the collective takes energy from the rotor system (it slows down) and transfers it to altitude and the helicopter’s descent rate slows. Using the collective, the pilot can move energy between rotor and altitude to assist in maneuvering the helicopter to a landing spot.

 

 

However, it is extremely important to not let the rotor rpm get too slow. Allowing this to happen will cause the rotor blades to stall and completely eliminate the pilot’s ability to control and slowly use the stored energy. The helicopter will free-fall and release all its energy at impact–enough energy to destroy the helicopter and its occupants.

 

  

In an autoraotative glide the pilot can also control airspeed and the same energy transfer concepts apply. Increasing airspeed requires energy and it needs to come from somewhere. In this case from altitude and rotor rpm, so when increasing airspeed the helicopter will descend faster (loss of altitude energy) and rotor rpm will drop (loss of rpm energy). Basically, the pilot is transferring energy from altitude and rotor rpm to airspeed. Decreasing airspeed puts energy back into altitude and rotor rpm. It is the skillful manipulation of all this stored energy that will allow the pilot to make a successful power off landing.

 

 

As the pilot maneuvers to a landing spot the helicopter gets closer to the ground and is running out of stored altitude energy. That’s OK as the goal is to land. Maintaining approximately 60 knots airspeed leaves a healthy amount of energy in airspeed to stop the descent rate and this is done by flaring at a low altitude, normally less than 100 feet agl. During the flare, the rotor system will also absorb energy causing rpm to increase and can be controlled by raising the collective. Care must be taken to not flare too much or add too much collective as this can cause the helicopter to gain altitude. The objective is to bring the helicopter to about a 5- or 10-feet hover above the surface. Timed right, all or most of the airspeed energy will be consumed and the helicopter will momentarily be close to the ground with no descent rate and little or no forward speed. However, it will start descending again and here is where the pilot will raise the collective to provide a burst of lift to cushion the touchdown. Raising the collective uses the energy stored in the rotor system and rpm rapidly slows down. Done right the helicopter will once again be sitting on the ground with all of its stored energy depleted.

 

 

AS350 flaring during a practice autorotation

AS350 flaring during a practice autorotation

 

 

Helicopter CFI

Friday, July 23rd, 2010

Flight instruction is some of the most demanding flying a helicopter pilot can do. A CFI must allow extremely inexperienced people to manipulate the flight controls, typically in a light, highly responsive, and unforgiving Robinson R22 (the most popular helicopter for primary flight instruction). As such, the briefest bit of inattention can turn a helicopter into a pile of twisted metal. This reality has haunted anyone who has ever worked as a CFI. Yet, as an industry, we rely on the least experienced pilots to do the vast majority of primary flight instruction. It should be no surprise that flight instruction has the highest accident rate among commercial helicopter operations and many of these accidents happen while trying to teach hovering.

Most CFIs are good pilots, however the skill set required to effectively and safely teach primary flight instruction is different. One of these skills include being prepared to handle a student’s unexpected and incorrect control movement, especially while hovering. In September 2002, a CFI was giving a student an introductory hovering demo in a R22. The CFI stated, “The helicopter caught a wind gust and the passenger accidentally pushed the cyclic left. I was surprised and tried to grab the cyclic back. It was too late.” The aircraft caught the ground and rolled over.

However, more docile training helicopters can also challenge instructors. In March 2003, a CFI had his student practice hover taxiing before concluding the last of three flights in a Bell 47D–a model known for its docile flight characteristics and forgiving nature. The student had trouble that day maintaining rotor rpm during maneuvers, so while lifting off the CFI looked inside to check the rpm gauge. When the CFI looked back outside, the helicopter was nose high and rolling to the right. He tried unsuccessfully to recover. The main rotor blades struck the ground.

Instructors must know when to guard the controls and continually assess when the time is right to take over from the student. A student can benefit from correcting his own mistakes, but an instructor should be careful not to jeopardize the helicopter for that benefit. Yet accident reports from the NTSB consistently list delayed remedial action and inadequate supervision as probable causes in training accidents. Such reports offer a wealth of information, and their complete review would bode well for CFI applicants.

Hot or cold

Tuesday, June 29th, 2010

Helicopter pilots refer to certain operations as hot or cold. A hot operation is one where the engines are kept running during the procedure and cold is with everything shut down. The arguments for and against hot operations center around safety vs. time savings.

For EMS operators, one of the more common hot operations is loading the helicopter. Many times at an accident scene the pilot will keep the helicopter running while the medical crew gets out to retrieve the patient. Since the idea of helicopter EMS, especially with trauma patients, is to save time, the hot loading of patients is performed routinely. However, there have been studies that have shown very little time difference between hot loading, shutting down the helicopter, and then restarting it to depart. The argument for shutting down is that maneuvering around a running helicopter can be hazardous. For example, people have walked into tail rotors and objects have come in contact with the main rotor system. On the other hand, helicopters are mechanical machines and there have been cases where the helicopter failed to start. On an accident scene, this could shut down a highway for a much longer period of time and delay getting the patient to a trauma center.

Another hot operation that is performed is refueling. Pilots trying to save time or an engine start (turbine engines have start cycle TBOs) will ask to be hot refueled. For trained personnel this can be preformed safely on most helicopters especially when the fueling point is low and below the engine. A fueling port high up on the fuselage and above the engine increases the possibility of a fire if fuel spills. Also, climbing on a ladder or other object to reach the fuel port can place personnel dangerously close to a spinning rotor system.

The case for proper training was apparent several years ago when I was watching a Bell JetRanger giving rides at an air show. When the pilot needed fuel, I watched someone drive a pickup truck, with a fuel tank in the bed, up close to the helicopter. The driver climbed out, ran around to the back, and jumped up into the bed. He stood completely up and then quickly ducked. He obviously felted how close his head came to the spinning rotor system. I turned away because I thought he was going to get hit. I remember thinking, wow, he was lucky!

LTE

Friday, May 28th, 2010

The pilot of a Bell 206B helicopter approached a construction site located at Baltimore-Washington International Airport (BWI) and brought the helicopter to a 250-foot out-of-ground-effect hover with a quartering left tailwind. Once in a hover, the aircraft made a rapid right 180-degree pedal turn, stopped momentarily, and then began another rapid pedal turn to the right. The helicopter continued turning at a fast rate and entered a spinning vertical descent impacting Alpha taxiway abeam Runway 15R. The FAA’s examination of the helicopter found no mechanical anomalies.

The NTSB determined the probable cause was the pilot’s improper decision to maneuver in an environment conducive to loss of tail rotor effectiveness (LTE) and his inadequate recovery from the resulting unanticipated right yaw.

So what exactly is LTE? According to FAA Advisory Circular AC90-95, any maneuver that requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where an unanticipated right yaw may occur. It also advises of greater susceptibility for loss of tail rotor effectiveness in right turns and states the phenomena may occur to varying degrees in all single main rotor helicopters at airspeeds less than 30 knots.

Allowing a loss of translational lift results in a high-power demand with low airspeed and can set the helicopter up for LTE when certain wind conditions are present. Using the nose of the helicopter as a 0-degree reference, main rotor vortex interference can occur with a relative wind of 285 degrees to 315 degrees and cause erratic changes in tail rotor thrust. Moreover, be aware of tailwinds from a relative wind direction of 120 degrees to 240 degrees as this can cause the helicopter to accelerate a yaw into the wind. A tail rotor vortex ring state can also occur with a relative wind of 210 degrees to 330 degrees and cause tail rotor thrust variations.

To recover if a sudden unanticipated yaw occurs, apply full pedal to oppose the yaw while simultaneously moving the cyclic forward to increase speed. If altitude permits, power should be reduced.

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.