Technique Archive

Heroism

Friday, December 10th, 2010

I learned to fly helicopters in the early 1980s. Back then I read a story about a helicopter pilot who rescued an airplane pilot who had crashed on the ice of Lake Erie. The helicopter pilot was flying along the shore of the lake in a Hughes 300C when he heard on the radio that the Coast Guard helicopter had turned back because of weather. He decided to head out over the lake and look for the downed pilot.

The traffic-reporting helicopter did not have any navigational radios so the pilot asked the Coast Guard for directional information. Ice had built up on the blades and airframe, and his skids contacted the water a few times when he lost depth perception because of fatigue. After several attempts, in total darkness, he found the pilot. After getting him onboard, he flew back and landed on the shoreline with less than five minutes of fuel remaining. The helicopter pilot was awarded the Avco/Aviation/Space Writers Association Heroism Award.

There is no doubt that his actions probably saved the downed pilot’s life. However, had he crashed while attempting this rescue would he have been viewed as an excessive risk-taker or a hero who tried despite the odds? I wonder in today’s environment of risk assessments and safety management systems if perceptions would have been different?

Eurocopter’s quest for speed

Monday, November 22nd, 2010

In the 1980s, Bell and Boeing Helicopters began developing a twin-turbo shaft military tilt rotor aircraft called the V22 Osprey. Bell then teamed with AgustaWestland to develop a commercial version known as the BA609 and it achieved its first flight in March 2003. During this time the helicopter industry was excited about VTOL aircraft reaching higher speeds. However, Eurocopter was quiet about its plans only saying it had no plans to develop a tilt-rotor aircraft.

On September 6, Eurocopter began test flights of its high-speed, long-range hybrid helicopter concept, which combines vertical takeoff and landing capabilities with fast cruise speeds of more than 220 knots. Called the X3, it is equipped with two turbo shaft engines that power a five-blade main rotor system and two propellers installed on short-span fixed wings. The engines are RTM322s, which power the company’s NH90 military transport. The main rotor gear box is a derivative of the yet-to-be certified EC175 medium size twin helicopter with a modification of two output drives for the propellers.

In cruise flight the rotor pitch is reduced to provide minimal drag and the small wings provide lift. Thrust comes from the propellers. There is no tail rotor so yaw and anti-torque are controlled by a standard pedal configuration that varies the thrust on each propeller separately. The aircraft can be flown like a traditional helicopter until 80 knots, then the main rotor pitch is reduced as the propeller thrust is increased.

According to Eurocopter, the hybrid aircraft will cost about 25 percent more per hour to operate than a conventional helicopter. However, with the increased speed the company points out that when measured in a per passenger/mile basis the operating costs will drop 20 percent. The X3 is currently a technology demonstrator, but Eurocopter says the concepts could be ready for production models in less than a decade.

Tail-boom strikes

Thursday, November 4th, 2010

Some helicopters, like the Robinson R66, have tall masts putting the rotor system high above the tail boom; others, like the MD500, have a more compact design. Engineers take into account flight characteristics of a design when considering the distance between the tail boom and the rotor disk. Even so, tail-boom strikes can and do happen.

One of the more common scenarios is when a helicopter makes a hard landing following an autorotation. Touching down too nose-high on the aft part of the skids can cause a nose-down pitch that instinctively causes the pilot to pull back on the cyclic to counteract it. This action in combination with the fact that the blades are still moving downward can result in the blades contacting the tail boom. An in-flight entry to an autorotation will also cause a nose-down pitch because of the advancing blade seeing a greater reduction in lift than the retreating blade. A pilot who overreacts with sudden aft cyclic will cause the rotor system to flap back while the tail boom is still rising, which can lead to the blades to come in contact with the tail boom. In-flight blade-to-tail boom strikes are normally fatal.

Strong wind gusts can also create a problem. A tail-boom strike can happen as rotor rpm gets lower and the centrifugal force holding the rotor stiff drops. A helicopter that is starting up or shutting down in high winds or near another hovering helicopter is particularly vulnerable. Manufacturers have used droop stops in teetering rotor systems to support the blades at slow speeds. One design uses spring-loaded droop stops with weights that pull them out of the way when the rotor speed–and related centrifugal force–gets high enough. Even so, many manufacturers and operators have maximum wind speeds for start up and shut down.

Max performance take-off

Friday, October 8th, 2010

A maximum performance takeoff is used to climb at a steep angle to clear barriers in the departure flight path. To perform this maneuver successfully a pilot must consider the wind velocity, temperature, altitude, gross weight, center-of-gravity location, and other factors affecting performance of the helicopter.

The textbook procedure is this: After performing a hover power check to determine if there is sufficient power available, position the helicopter into the wind and begin by getting the helicopter light on the skids. Pause to neutralize all aircraft movement. Slowly increase the collective and position the cyclic to break ground in a 40-knot attitude. This is normally about the same attitude as when the helicopter is light on the skids. Continue to slowly increase the collective until the maximum power available is reached. Keep in mind this large collective movement requires a substantial increase in pedal movement to maintain heading. Use the cyclic, as necessary, to control movement toward the desired flight path and, therefore, climb angle during the maneuver. Maintain rotor rpm at its maximum, and do not allow it to decrease since you would probably have to lower the collective to regain it. Maintain these inputs until the helicopter clears the obstacle then establish a normal climb attitude and reduce power. As in any maximum performance maneuver, the techniques you use affect the actual results. Smooth, coordinated inputs coupled with precise control allow the helicopter to attain its maximum performance. Also, the helicopter will most likely be inside the shaded area of the height-velocity diagram where there is very little energy available to perform an autorotation in the event of an engine failure. Using maximum power would decrease the time the helicopter is exposed to a low energy situation.

However, other pilots I have talked to prefer a slightly different technique. Instead of using maximum power available, use the minimum power necessary to safely climb and clear the obstacle. The theory being that in the event of an engine failure the less power the pilot is using the more rotor rpm will be recoverable to help in the autorotation. Also, demanding high power might increase the probability of a part failure at a critical time. I would be interested in hearing comments on the two theories.

Robinson R66—First flight

Monday, September 13th, 2010

I just spent the last two days flying Robinson Helicopter’s new light turbine helicopter, the R66. Although it is still in experimental category, FAA certification is expected in the next 30 days as Robinson and the FAA work out some final details.

Having a couple of thousand hours in the Bell 206 light turbine series helicopter made for an easy direct comparison. Last year Bell announced that it would cease production of the five-place Bell 206B JetRanger, citing the R66 as one reason. Company founder Frank Robinson’s design goals are not just well-engineered products, but cost effective as well. The R66, the company’s first turbine helicopter, exemplifies this objective extremely well, and after flying it, I think Bell made the right decision.

The R66 is powered by a Rolls-Royce RR300 (model number 250-C300/A1), a new engine based to the proven 250-series engine (same engine used in the 206B). It is mounted below the transmission deck at a 37-degree angle which gives easy access for maintenance. The engine produces 300 shaft horsepower and is derated to 270 shp for a five-minute take off rating and 224 shp for max continuous operation. Starting is simple; igniter switch to enable (a nice feature that allows you to motor the starter without firing the igniters–no more need to pull the igniter circuit breaker); press-and-release the start button (it’s latched so no need to hold it down), at 15 percent N1 push the fuel control in and monitor engine light off and acceleration. At 65- to 67-percent N1 the starter disengages and the generator is switched on.

Picking the R66 up to a hover is smooth and it feels a little bigger and a little heavier than the piston-powered R44, which it is. I flew with Doug Tompkins, Robinson’s chief pilot who did all the experimental test flying on the R66. We were hovering at 64-percent torque and as we approached 60 knots during the take off Doug suggested pulling 100-percent torque. I started raising the collective, before I got to 90 percent the VSI was pegged at 2,000 feet per minute and at 100 percent we were climbing like a banshee. It didn’t take long to feel comfortable with the helicopter and we moved on to autorotations. These were predictable and basically a lot of fun. I did 180-degree, 90-degree, and out-of-ground-effect hovering autorotations to a full touchdown. It is just like the R44, only easier.

Another noticeable feature is comfort; the cabin is eight inches wider than the R44. The cyclic flight control retains Robinson’s T-bar arrangement. Not only does this ease transitioning from the R44 to the R66, but the T-bar is exceedingly comfortable in flight.

There is not doubt this helicopter will do very well. Once again Frank Robinson has found a need and filled it. The agile and turbine-powered R66 will do the jobs that a piston engine simply can’t, such as high-altitude flying. It will also find great acceptance in parts of the world where avgas is hard to get or just not available. And for those operators and contracts that require a turbine engine, the R66 will fit perfectly.

There is a lot more to say about this helicopter so look for a full feature article in an upcoming issue of AOPA Pilot magazine.

Low fuel

Tuesday, August 31st, 2010

I have studied and written about helicopter accidents for most of my career. I believe many accidents contain valuable lessons that can help all of us be safer pilots. However, one kind of accident that I find hard to believe–because it defies common sens–involves fuel exhaustion. Not a situation where something prevents fuel from getting to the engine, but rather when a pilot knows he is low on fuel, keeps flying anyway, and then experiences an in-flight engine shut down.

A helicopter pilot who allows his fuel to get too low has many more landing options than an airplane pilot. Maybe that contributes to a complacent mindset when in a low-fuel situation. I am sure it’s embarrassing, or maybe even places ones job in jeopardy, to land in a field low on fuel, but to me it sure beats the alternative. Moreover, some of the reasons pilots give for not stopping for fuel seem bizarre.

Case in point, according to the NSTB on October 15, 2002, a CFI was providing night VFR cross-country instruction to a student in a Schweizer 269C helicopter. They had discussed their low-fuel situation, but elected not to stop and refuel because neither had a credit card. On the last leg of their flight, the low-fuel light illuminated, followed a few minutes later by complete loss of engine power. During the autorotation the helicopter was substantially damaged when it struck trees and the tail boom separated from the airframe. Miraculously, neither pilot was injured.

This is not the first accident of this kind and, unfortunately, probably will not be the last.

Collective control

Thursday, August 19th, 2010

The T-bar cyclic control in the Robinson series helicopters is a departure from the typical flight-control design, however, its collective control conforms to industry standards. I know of only one manufacturer that has designed and installed a collective control that differs from the norm. Bell Helicopter certified the twin-engine model 222 in 1979 with a collective control that moves in more of a horizontal arc as opposed to up and down. The pilot pulls the collective rearward to increase pitch and pushes it forward to decrease. Bell used this arrangement on all subsequent variants including the model 430.

When designing the model 222 in the early 1970s, Bell was targeting the business jet community. Thinking the helicopter would appeal to corporate operators, the company believed the new collective design would feel more like a corporate jet. Additionally, Bell claimed the unique collective lever reduced pilot effort and enhanced safe operation. Also according to company documents, the near horizontal arc is perpendicular to any vertical rotor system vibrations which eliminate any pilot induced oscillations.

The twist-grip engine throttles are located on the collective, however, they are perpendicular (canted aft about 10 degrees to make for a comfortable grip) to the collective shaft as opposed to the standard in-line arrangement. This left/right design works well with the cockpit engine instruments making identification of the correct engine with its corresponding throttle very easy.

When I started flying a Bell 430, the collective movement was natural and instinctive from the beginning. Throttle friction is adjustable, so I never had an issue with inadvertently twisting the throttles while making collective changes. Although the standard up and down collective control works fine, I do like the arc motion better.

T-bar cyclic

Tuesday, August 10th, 2010

 

In 1978 Frank Robinson was granted a patent for a T-bar cyclic flight control system in a helicopter. His concept was a departure from the conventional helicopter flight control design where the cyclic control came up between the pilot’s legs. During the last 30 years the T-bar cyclic in Robinson helicopters has generated a lot of comments.

Conventional cyclic control in S300R66 with T-bar cyclic control
 

Robinson’s objective when designing the R22 helicopter was low cost and mechanical simplicity and the T-bar cyclic fits this design goal by reducing the complexity, weight, and cost of the conventional flight-control system. Other advantages include ease of getting in and out of the helicopter and a comfortable flying position as the pilot’s arm can rest on top of his leg. Moreover, not having the control between the pilot’s legs allows for a narrower cabin, which lends to a more aerodynamic fuselage design. Robinson used this arrangement on the larger R44 and the soon to be certified turbine- powered R66.

I have flown many different helicopters with the conventional cyclic design and do not see any difference in the T-bar’s ability to control the helicopter. Also, I agree with the claims of increased comfort while flying and find entering and leaving the helicopter much easier as well. I view it as simply different and believe with just a small amount flight time with the system most pilots will discover that it works quite well.

Still, like all systems, there are unique issues that need to be addressed. One is that the horizontal control bar pivots, allowing the pilot to lower the hand grip to his leg. I have seen pilots struggle with the control’s ability to move up and down while in flight. Once they get comfortable resting their hand on their leg, that goes away. Also, when giving dual instruction–especially to primary students–the instructor needs to pay close attention to the controls and keep his hand close to the cyclic grip, which can be fairly high up when the student is flying. Another point, even with the dual controls removed, is the access the front-seat passenger has to the flight controls. I have had pilots tell me that when giving rides they have had passengers grab or bump the center stick.

The T-bar cyclic works well, however, as with any aviation system, a complete understanding of limitations coupled with good student/passenger briefings is important.

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.