Minimum autorotation rpm

September 28, 2013 by Tim McAdams

When a helicopter is in autorotation (that is, gliding without the benefit of engine power) rotor rpm must be maintained. This is done when entering the autorotation by lowering the collective control. If the rotor rpm approaches an upper limit, the collective is raised to add pitch. This increases drag and slows the rotor rpm. A low rotor rpm situation is just the opposite, lower the collective pitch to reduce the drag and allow the rotor rpm to speed up.

Rotor rpm in autorotation is a function of several factors like density altitude, gross weight and airspeed. An over speeding rotor is easy to manage since the collective control was lowered on entry there is plenty of movement upward to add drag. However, if a pilot enters autorotation and lowers the collective control all the way down and the rotor rpm is still too low this could be a problem. Typically, in this case, the main rotor pitch is set incorrectly. The helicopter’s maintenance manual has a procedure to adjust this by either lowering the collective control’s down stop or adjusting the main rotor blades’ pitch links. To accomplish this, a mechanic will note the helicopter’s weight and the density altitude and then reference a chart to get the correct rotor rpm. A flight test will then be performed at those conditions and the actual rotor rpm will be noted with the collective all the way down. If it is not at the correct rotor rpm stated in the chart, the mechanic will make an adjustment.

This is done to insure that in the worst case scenario (light helicopter, high density altitude) the pilot will be able to lower the collective control far enough to guarantee an acceptable rotor rpm in autorotation.

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Berkut VL

September 14, 2013 by Tim McAdams

Typically, coaxial rotor systems (one rotor system stacked on top of another that spin in opposite directions) are used on larger helicopters. The advantages are higher speed and more lifting power as a tail rotor is not needed. An aerospace start-up company in India (DASYS), a manufacturer of unmanned aerial vehicles, has designed a light two-seat helicopter with a coaxial rotor system.

Called the Berkut VL, the company plans to certify the helicopter in compliance with US FAR Part 27 standards. Currently there are two prototypes, one for testing and the other for demonstrations. These two airframes are equipped with a Russian ConverVAZ engine, but production models will have the option of a 150 hp Lycoming O-320 engine. Helicopters with American engines will get the designation Berkut VL M. The planned take-off weight is 1,830 lbs with a maximum speed of 108 mph and a range of 527 miles.

The helicopter will be produced at a plant in central Russia. Although no price has been released, the company has stated it will be affordable and plans are for it to compete with the Robinson R-22. As such, the company has announced a four-seat version will follow. First deliveries are scheduled for mid 2014.

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Cobra attack helicopter

September 2, 2013 by Tim McAdams

The first military helicopters were used for medical evacuation and supply missions. Although, their primary mission was utility, handheld weapons and some side-mounted guns were used for offensive tasks. At that time, the U.S. Army did not have a dedicated attack helicopter in its fleet. Bell Helicopter recognized the advantages of using a helicopter for offensive missions and began developing a new model designed specifically for these kinds of operations.

The first design was a modified Bell 47. It first flew in 1963 and used a two man crew arranged in tandem with the gunner in front and the pilot seated directly to the rear. The gunner operated a nose mounted machine gun with an assembly that resembled the pilot’s cyclic control. Since the gunner was also trained to fly, a small cyclic control was installed on the right side as a side arm controller. Yaw was controlled by twisting the grip on the side arm control. This model was called the Sioux Scout and was used as a test bed for the design of an advanced attack helicopter.

Bell took these concepts and applied them to the more powerful turbine-powered UH-1B, known as the Huey. The design work started in early 1965 and the prototype was flying a year and a half later. The newly designed attack helicopter carried the designation AH-1G and was named the Cobra or HueyCobra. The U.S. Army ordered 529 of these and by 1967 they were in action in the Vietnam conflict.

AH-1G Huey Cobra

AH-1G Huey Cobra

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Full down autos

August 17, 2013 by Tim McAdams

When practicing autorotations, the maneuver is initiated by reducing the engine to idle causing the freewheeling clutch to open, which then disconnects power to the rotor system. As the helicopter glides toward the ground, there are two ways to terminate the maneuver. One is to add the engine power back in and bring the helicopter to a hover, this is known as a power recovery autorotation. The other is to leave the engine at idle so the freewheeling clutch stays open, keeping the engine disconnected from the rotor system. Known as a full touch down autorotation, the pilot will increase collective pitch at the right time to create a momentary burst of lift to cushion the touch down.  In the helicopter industry, there are differing opinions on the value of practicing autorotations to the ground. 

The touch down requires precise timing because as the pilot adds collective pitch, rotor rpm begins to decay. If this is done too early, the rotor rpm can get too low causing controllability issues, excessive blade coning and loss of ability to cushion the touch down. By avoiding ground contact with a power recovery autorotation the risk of damaging the helicopter from a hard landing is reduced considerably. Some instructors and companies believe the risk of damaging a helicopter during touch down is too high and the benefit of actually landing does not justify the risk. The thought being that if a pilot performs the proper entry, maintains rotor rpm, maintains appropriate airspeed and then flares at the correct altitude the autorotation will be survivable. In reality, accidents from practice autorotations rarely cause serious injury or death, however, there have been many helicopters damaged from practicing autorotations. In fact, the US Army stopped practicing autorotations to the ground because they were damaging too many helicopters. 

I understand the risk vs. benefit analysis that leads to the decision to only perform power recovery autorotations. However, I think it is beneficial to practice autorotations in the most realistic environment that can be safely done, including full touch down to the ground. The risk can be minimized by using an experienced instructor with proper and extensive training in autorotations. Factory schools like Bell and Eurocopter have been doing full touchdown autorotations for many years with a good safety record.

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Power limits

August 1, 2013 by Tim McAdams

Helicopters powered by normally aspirated piston engines use manifold pressure as an indicator of power levels. Typically, pilots calculate limit manifold pressure for each day which is the maximum power setting allowed by the helicopter’s manufacturer. It is not necessarily the maximum rated horsepower limit for the engine. In many cases, the helicopter manufacturer de-rates the engine to reduce internal stress levels and extend TBOs. However, the pilot can exceed the limit manifold pressure (depending on factors like air density etc…) and still have available power.

In a gas turbine engine, the pilot must monitor three different indicators. Turbine outlet temperature (TOT) which refers to the temperature of the gas as it is exiting the engine, when the ambient air temperature is high this can be a limiting factor. Another is torque, which refers to the amount of torque the engine is applying to the transmission and is normally shown as a percentage. The third one is gas producer rpm, referred to as Ng or N1. When the air density is low, this section of the engine can reach its maximum operating rpm because it needs to spin faster to move the same amount of air.  A pilot of a turbine helicopter must monitor all three of these gauges and stop adding power when the first one reaches its limit.

Eurocopter uses something called a first limit indicator (FLI) to simplify the monitoring of all three parameters. One large gauge with a fixed yellow arc (indicating take-off power range) monitors all three parameters. So when the pilot adds power and the needle enters the yellow arc, then one of the three parameters has exceeded its maximum continuous power limit. To the right of the gauge, are the three values shown digitally (TOT, torque, Ng) and whichever one is the limiting value will be underlined in yellow.

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Coaxial rotors

July 19, 2013 by Tim McAdams

Traditional helicopter designs use a main rotor for lift and thrust, and a tail rotor to counter the torque applied to the fuselage. Another design, known as coaxial rotors, uses a pair of helicopter rotors mounted one above the other to produce both lift and thrust. Sikorsky’s high speed technology demonstrator the X2 uses this design as well as many Russian helicopters.

To neutralize the torque, the rotors spin in opposite directions creating equal and opposite torques that cancel each other and eliminate the need for a tail rotor. Yaw control is achieved by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. Coaxial rotors also reduce the effects of dissymmetry of lift. Because they spin in opposite directions, both sides of the rotor disc have a retreating blade and an advancing blade.  

Another benefit of a coaxial design is a higher payload for the same engine power. A tail rotor consumes some of the available power produced by the engine. With a coaxial rotor design that extra power can be devoted to lift and thrust. Moreover, eliminating the tail rotor reduces noise, allows for a more compact design and increases safety on the ground.

The major disadvantage of the coaxial rotor design is the increased mechanical complexity of the rotor system. Two swash plates and their related linkages for both rotor systems need to be constructed on the same mast, which in itself is more complex because of the need to drive two rotors in opposite directions. This is offset somewhat by eliminating the intricacy of a tail rotor system. It would seem that the complexity of the rotor systems would increase the risk of a catastrophic failure. However, helicopters with this design have a good reliability record.

Sikorsky X2

Sikorsky X2

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Uncommanded yaw

July 7, 2013 by Tim McAdams

In cruise flight, should a helicopter experience an uncommanded yaw the cause in most cases would be an engine failure or tail rotor failure. The direction of the yaw indicates the type of failure.

In clockwise turning rotor system (like the Eurocopter AS350) the engine torque causes the fuselage wants to spin the opposite way (Newton’s third law – for every action, there is an equal and opposite reaction). In this case, from the pilot’s prospective, the nose of the helicopter wants to go to the left. The tail rotor applies a thrust that counters this reaction and pushes the nose back to the right. The pilot varies the amount of thrust with the pedals to control yaw. In powered flight, everything is in balance.

Should the engine fail, the engine torque that the tail rotor is opposing goes away. However, the tail rotor is still producing thrust that is trying to turn the nose right. In this case, the pilot will experience a right yaw and will use left pedal to neutralize it. With a tail rotor failure (loss of drive, producing a complete loss of thrust) the force opposing the engine torque ceases allowing the fuselage to spin the opposite of the rotor system. As such, the pilot will experience a yaw to the left. However, since the tail rotor is no longer effective applying opposite pedal does not work. Airflow over the vertical fin will prevent the helicopter from completely spinning and allow the pilot to fly the helicopter to a suitable area and perform an autorotation. Shutting the engine off eliminates the torque allowing the nose to come back to the right.

In a hover, it is a little different due to the reduced air flow over the vertical fin. Instead of just yawing, the fuselage will start spinning. In the case of an engine failure, opposite pedal will work. With a tail rotor failure, the pilot must immediately enter autorotation.

In a counter-clockwise turning rotor system (like the Bell 206) the theory is the same, but the nose moves in the opposite direction for each failure.

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Ground effect

June 30, 2013 by Tim McAdams

Hovering in ground effect results in a condition of improved performance that comes 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.

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.

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Ground resonance

June 16, 2013 by Tim McAdams

A helicopter’s rotor system, engine(s), and other dynamic components generate vibrations in the airframe. These components will vibrate at a natural frequency which in turn causes other parts like the landing gear, tail boom, cabin etc… to vibrate as well. Each part’s frequency will vary according to its weight, stiffness, shape, etc…. As such, a helicopter contains a complex set of vibrations that add up to a resulting airframe vibration. Engineers attempt to reduce the overall vibration level by tuning the natural frequency of all the components. 

When a helicopter is in flight, the airframe’s natural frequency (the sum of all the components’ frequencies) will vibrate without interference. However, when ground contact is made with the landing gear it can interfere with the airframe’s ability to vibrate at its natural frequency. Ground resonance happens when ground contact alters the natural frequency of the main rotor system. This unbalanced condition triggers vibrations that are augmented with every blade revolution causing a reflected impulse that increases in amplitude very quickly. The only rotor systems susceptible to ground resonance have three or more blades. This is due to each blade’s ability to lead and lag (speeding up and slowing down) independently. If something causes the blades to depart from their symmetry, the rotor system’s center of gravity shift causes it to become out of balance allowing divergent oscillations to rapidly become strong enough to cause serious damage to the helicopter. In some cases, complete destruction  can occur with many componets coming loose and being thrown from the helicopter. 

Engineers design damping systems for the main rotor and landing gear to absorb the energy and prevent the oscillations from accelerating. Still, a sudden shock to the airframe, like a hard landing, can unbalance the main rotor system beyond the damping system’s ability to absorb the energy and ground resonance can start. Improperly serviced or malfunctioning dampeners are usually the cause.  Ground resonance happens very fast, however, if the pilot recognizes it in time and there is enough power and rotor RPM available to lift the helicopter off the ground the divergent oscillations will stop. This is the quickest way to stop it and hopefully will result in little or no damage. If the situation is such that there is not enough power to lift into a hover, then a full power reduction is the only option. However, this approach will take time for the vibrations to fade and significant damage can occur.

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Gemini ST

June 3, 2013 by Tim McAdams

The problem with very light twin-engine helicopters is payload and range. The extra weight of the second engine, combining gearbox, extra fuel tanks and related systems severely limits the airframe’s payload.  Also, that smaller payload capacity will need to include the weight of more fuel. As such, carrying enough fuel for an acceptable range normally leaves little weight capacity for passengers. Operators of small helicopters who wanted the redundancy of a twin have to make some big trade-offs. 

In the late 1980s, Doug Daigle, a 14,000-hour helicopter pilot and owner of Tridair Helicopters, came up with an idea that tried to solve this problem.  He took a popular single-engine helicopter, the Bell LongRanger (206L3), and removed the 650-shaft-horsepower Allison 250-C30 engine (the 500-shp C28 for the older L1s) and added a pair of 450-shp Allison 250- C20R engines to create a twin with good single-engine performance. However, what was different about his design was it was the first twin to be certified for normal operation on one engine in all areas of flight. Certified by the FAA in 1994, he named it the Gemini ST conversion. 

When operating both engines, the Gemini consumes fuel at 45 gallons per hour, compared to the LongRanger at 38 gph. Because both helicopters have the same fuel capacity (he did not add extra fuel tanks), endurance drops from 2.9 hours for the LongRanger to 2.5 hours for the Gemini. However, the Gemini’s C20R engine burns only 28 gph in single-engine mode, and this increases the endurance to 3.9 hours. Daigle believed long trips will normally have some extended cruise flight, where the pilot could choose to fly on one engine. The other engine can be started for critical maneuvers that require the redundancy of a second engine. 

The extra weight of the engine and related systems does increase the empty weight, so there is some trade-off with the Gemini. It has a smaller useful load of 1,610 pounds, compared to 2,175 pounds for the LongRanger. The Gemini can carry 740 pounds of fuel (the same as the LongRanger), which leaves a payload capacity of 870 pounds. 

Bell Helicopter then entered into an agreement with the company that supported Tridair’s conversion of existing LongRangers while Bell would build a new production model, the 206LT TwinRanger, under Tridair’s STC. Bell only delivered 13 airframes and the concept of shutting one engine down in flight never caught on. This model was replaced by the Bell 427 which was under development for the EMS market. The 427’s cabin proved too small and Bell canceled the program and introduced the larger, single-pilot IFR certified 429 in 2009.

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