Archive for September, 2013

Minimum autorotation rpm

Saturday, September 28th, 2013

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.

Berkut VL

Saturday, September 14th, 2013

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

Monday, September 2nd, 2013

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