Archive for April, 2012

Single rotor blade

Tuesday, April 24th, 2012

One of the more unique helicopter designs was built in 1940 by Austrian, Bruno Nagler. In an effort to reduce weight, he built the first helicopter with a single rotor blade. Named the NR 55 it was powered by a 40 hp engine that was mounted opposite the rotor blade and acted as a counter weight. Surrounded by an aerodynamic housing, it was located about four feet from the rotor hub.  A drive shaft from the engine passed through the hub and powered two small counter-rotating propellers (mounted on the leading and trailing edges) and located just over midway along the 18 foot blade. The single seat helicopter weighed 418 pounds and had a tripod style landing gear. Nagler was able to demonstrate hovering flight (inside with no wind) with a payload as high as 243 pounds.

However, the design had several problems. Gyroscopic precession from the propellers mounted on the rotor blade interfered with the blade’s ability to flap (a rotor system function needed for forward flight). Also, the rotor could spin up to 135 rpm producing large centrifugal forces acting on the engine causing ignition problems and fuel flow issues. Finally, high vibration levels proved difficult to reduce.

Nagler eventually retired the design in favor of a smaller design called the NR 54 V1. This version weighed 176 pounds, had a 13 foot rotor blade and a 24 hp engine. Plagued with many of the same problems, Nagler never got the NR 54 V1 to work and abandoned the single rotor blade concept in favor of a traditional two bladed design. Although, he kept the rotor blade mounted engine design because he felt it offered better weight and anti-torque advantages. Known as the NR 54 V2, it had a small 8 hp engine on each blade. However, it never flew due to centrifugal force issues with the engines.

After World War II, the British took the NR 54 V2 prototype into custody and it is now on display at the National Air and Space Museum. The original NR 55 was put in storage at the Nazi glider club in Vienna until a bombing raid in 1944 destroyed it.

Autorotation training

Tuesday, April 10th, 2012

On April 10, 2003 in Auburn, California, a Hughes 269B helicopter was destroyed when it collided with terrain while on an instructional flight. Prior to the accident, a witness on a farm about 3 miles away heard and observed the helicopter performing maneuvers consistent with multiple practice autorotations to a power recovery. The helicopter then flew off in the direction of the airport. Witnesses near the accident site observed the helicopter in level controlled flight about 500 feet above ground level flying in the direction of the airport. They observed the helicopter’s nose drop and then it dove toward the ground and disappeared behind a tree line. Ground scars and the wreckage exhibited evidence consistent with the helicopter colliding with the ground at a high rate of descent in a level upright attitude and rolling on its right side.

The National Transportation Safety Board determined the probable cause of the accident as the misjudged flare maneuver by an unknown crewmember during a likely practice autorotation that resulted in an in-flight collision with terrain.

Ironically, more accidents happen each year from practice autorotations than from actual engine failures. Surprise throttle cuts are especially critical because they can startle students and cause them to make sudden incorrect control movements. Inadvertently raising collective, pressing the wrong pedal or lowering the nose can drop the rotor rpm and risk stalling the rotor system. A student who simultaneously performs two or more of these movements could quickly stall the rotor system.

Before introducing forced landings, it is important that the CFI and student establish a strong understanding of what is expected and what can happen. Then, the CFI should introduce simulated power failures slowly by telling the student in advance of rolling off the throttle. At first, this should be practiced at very low power settings to allow extra reaction time. Only after the student’s reactions are correct and predictable should the difficulty level be increased. Even then, the CFI should always plan to initiate the autorotation and completely guard all the flight controls.

As the following accident demonstrates, guarding the flight controls is even more important for pilot examiners as they are probably not very familiar with the applicant’s background and habits. According to the NTSB, on June 28, 2003 a R22 helicopter impacted the ground and rolled over during a practice autorotation during a private pilot check ride. After completing a series of maneuvers, an autorotation was initiated. According to the designated pilot examiner, the entry and flare were uneventful, and as the pilot applied power to recover the engine and rotor rpm needles were in the green. The pilot then began to cushion the descent with collective and the low rotor rpm horn activated. The check pilot expected the pilot to lower the collective slightly and roll on additional throttle; instead, the pilot lowered the collective almost all the way down and rolled some throttle off. At this point the helicopter was very close to the ground. The check pilot told the pilot “I have the controls.” The helicopter entered a right turn, then the check pilot felt the right pedal move against his foot, and the helicopter settled to the ground at an angle. After bouncing once, it touched down on the left skid and rolled over.

Neither the pilot or the examiner were injured and after the accident, the DPE asked the pilot if he had been rolling the throttle off as he was pulling the collective up during the cushion portion of the autorotation. The pilot reported that in the past he had been over-speeding the engine, so he would slightly reduce the throttle to compensate. The density altitude at the time of the accident was 8,393 ft., leaving little margin for errors.