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Power To The Piston!

Story by Mark Ogden, Photos by Ned Dawson

Page: 1/2

The piston engine originally provided the helicopter with its lifting power - but it was the turbine that allowed the helicopter to realise its full potential. However, the piston engine has made a comeback, championed by the world's most popular helicopter - the Robinson.

For years the promise of turbines was to have signalled the death-knell of the piston or reciprocating engine for the helicopter. Power-to-weight ratios and reliability were just some of the advantages promised, but Robinson seems to have turned many of those theories on their heads. The future availability of leaded aviation gasoline and developments in diesel engines may see a significant evolution in piston power plants. The reciprocating engine looks as if it will be around for some time yet!

Performance

Helicopter performance is basically a combination of weight, rotor system design and the power available to drive the system. There are other issues such as the type of engine, fuselage and tail rotor design, but in the end performance comes down to one thing - power margin; the difference between the power required and the power available.

Power Required

To understand power, there are some principles that help.

Power is defined as the rate of doing work. Consequently, the power required to balance drag will vary with the cube of the velocity and will also increase with any increase in drag. One of the variables in determining the amount of lift produced is the angle of attack of the aerofoil. If the angle of attack increases, lift increases and consequently drag also increases. Essentially, the total power required to operate a helicopter's rotor system is made up of three elements; rotor profile (RPP), parasite (PP) and induced power (IP). The integration of the three curves provides the classic bucket-shaped power required curve. Increasing wind across the rotor reduces the power required. Increasing density altitude (DA) makes the rotor less efficient, modifying the curve position and shape. With a helicopter there are always variables, such as in the case of the tail rotor, single main rotor design, the power to drive the tail rotor can have a significant effect on the power margin. In US design-based helicopters, increasing left pedal (anti-torque) increases the power required, reducing the power margin, and right pedal provides the opposite effect.

Power Available

Although both turbine and piston engines use combustion to generate power, they are very different in handling and operation. The power available curve of a turbine engine in a helicopter is generally flat, and its position in relation to the power required curve changes with DA. The power available from a piston engine, however, changes with DA, RPM and mixture. A piston engine can run on a lean mixture or a rich mixture, each producing different power outputs. A piston engine can also be run on a best-economy mixture which will provide different power availability again. For example, it is generally accepted that a piston engine will produce maximum power with a fuel-to-air ratio of .08. A ratio of .074 will allow the engine to produce maximum power for the particular RPM selected; any further leaning below this figure will cause the power to reduce. Best-economy usually occurs around .060 which will provide about 87% of maximum power. As the DA increases, air becomes less dense, and to maintain the fuel/air mixture ratio, the amount of fuel entering combustion must be reduced, but a reduction in the amount of fuel will also cause a reduction in the power. Normally aspirated piston engines suffer from increasing DA more dramatically than turbine engines. Super and turbo-chargers are needed to cram more air into the piston engine to permit more fuel to be burned to maintain power, but these come with increased complexity and reduced reliability. Full-throttle height in a normally aspirated engine is the height at which the required power is only available with the throttle fully open. In a non-derated engine, full power would generally occur at around sea level. In a de-rated engine which is limited by the Manifold Air Pressure (MAP, also known as Manifold Pressure), this would occur at a higher altitude.

Differences

There is a perpetual argument about engine reliability - turbine versus piston. Much of the piston's poor reputation was gained in the days when helicopter manufacturers needed every ounce of power they could wring from the piston engine. Although an engine may have been rated at say 180hp, the manufacturers would run the engine at a higher RPM to obtain more power. The downside of this was that the engines would fail more often, hence the reputation for unreliability. Indeed, Lycoming reduced its engine TBOs from 2,000 hours to 1,600 hours when they were used in helicopters. However, with the R22 a different approach was taken, de-rating the engine from 160hp to 124hp (continuous). This was done by running the engine at a lower speed and dictating a lower MAP limit.
The R44 engine maximum continuous rating is less than 80% of the engine's rated power - it is never working hard provided it is operated within the handbook limits. Engine reliability improved remarkably and Lycoming reinstated the 2,000-hour TBO. Engine failure rates in the R22 and R44 models are lower than comparable light turbine helicopters, but it will take a long time for facts to overcome perception in this argument. However, if more power is available to pilots, they tend to use it, and Robinson highlighted its concerns about this in its Safety Notice SN-37 issued in December 2001. Following an explanation of metal fatigue, the notice included, among other warnings, a warning not to exceed placarded manifold pressure limits. There is no doubt however that the turbine power-to-weight ratio is higher than the piston. Piston engines are heavy and are comparatively complex. Modern piston engines have achieved a power-to-weight ratio of about one horsepower for each pound of weight, while some turbine engines produce six horsepower for each pound. Turbines use more fuel though and are more expensive to buy and own. However, above the 350 to 400hp rating, the turbine becomes the powerplant of choice. There are some important differences in operating light piston and turbine-powered helicopters. For example, the pilot of a turbine helicopter typically references power to a torquemeter, or refers to engine parameters such as turbine temperature and compressor speed. The piston engine helicopter pilot uses manifold pressure and engine RPM, which are less direct in indicating performance. Fuel consumption generally improves with increasing altitude in turbines, and there are some good gains to be made if the helicopter is flown at a higher altitude. Although the power required increases with altitude, the engine must also work harder. Turbine engines operate more efficiently at high power settings, and so operating at higher altitude improves specific fuel consumption. Piston engines do not have the same characteristics as turbines when it comes to range variations with altitude, again it is a matter of balancing power output with the mixture setting.




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