Archive for June, 2009

Main rotor systems

Monday, June 29th, 2009

There are several different main rotor system designs that are used on modern helicopters. The three basic designs that have traditionally been taught to students are semi-rigid, fully articulated, and rigid. Today there are versions that make extensive use of composite materials and are known as hinge less systems.

A fully articulated system normally has more than two blades. In this design each blade is attached to a hub with hinges that allow it to move independently of the others. A feathering hinge is used to change the pitch of each blade. A flapping hinge allows each blade to move up and down to compensate for dissymmetry of lift. Blades are able to move fore and aft or lead-lag, (called hunting) by use of a drag hinge. Normally a damper is attached to the blade and hub to restrict excessive movement. The drag hinge is used because, when a rotor blade flaps up, its center of mass moves closer to the axis of rotation. This causes the rotor system to spin faster, much like a spinning ice skater speeds up when pulling her arms in closer. Allowing the blades to lead-lag reduces this tendency.

A semi-rigid system refers to a two-blade system where each blade is mounted to a hub that has a center teetering hinge. In this configuration, when one blade flaps up the other one flaps down – like a see saw. As with the fully articulated system, each blade has a feathering hinge. The two blades are mounted in an under-slung position, that is where the teetering hinge is mounted above the plane of rotation. The geometry of this arrangement minimizes the change in distance between the center of mass and the axis of rotation during flapping. This allows a semi-rigid system to not need a drag hinge.

In a slight departure from the traditional semi-rigid design, Frank Robinson used a coning hinge on each blade (some refer to this as a flapping hinge, but it is used for blade coning). When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). This places a high stress load at the blade’s root, so in order to relieve this stress Robinson’s design allows the blade root to cone about a hinge. This reduced the amount of reinforcing required at the blade root making for a lighter easier to manufacture rotor blade.

Rigid rotor systems do not use hinges and limited movement is absorbed through the hub and rotor blades. Many of the modern composite rotor systems also do not use traditional hinges, but have elastomeric and specially designed composites structures (flextures) that allow the blades to flap, feather, and hunt. Manufactures do not use the term rigid rotor system, opting instead to describe these systems as a fully articulated hinge less rotor system. These systems do not require lubrication and are less maintenance intensive. The extensive use of composite materials also increases reliability and helps absorb vibration.

Arthur Young and the Bell 47

Friday, June 19th, 2009

 

There are many early pioneers who contributed to the development of the helicopter. The 1940s was a decade with many flying prototypes from Sikorsky, Hiller, and others. However, the first commercial helicopter certified was a Bell model 47. The design is credited to a young self-taught inventor who learned about helicopters by reading everything he could find in public libraries. His name was Arthur Young.

 

To test his ideas, Young set up a small aeronautical laboratory on a farm his family owned in Radnor, PA. Throughout the 1930s he experimented with many different designs and powerplants. He built so many models that crashed that he became very good at repairing them and could quickly resume flying. His biggest problem was stability. He first tried a pendulum device that could sense gravity and adjust the rotor system. It failed because any aircraft acceleration would affect the pendulum.

 

He finally found success with a stabilizer bar. The device used two weights on a bar mounted perpendicular to the blades, in the same plane of rotation. Acting like a gyro, it controlled cyclic pitch to keep the rotor plane fixed in space. With this system Young was able to hold extremely stable hovers with his electric-powered model.

 

Now that his model was flying well, he set out to interest a manufacturer in building a full-scale prototype. He was finding very little enthusiasm for his project until a friend mentioned his flying model to an engineer at Bell Aircraft Company. This led to a demonstration and a meeting with Larry Bell. Impressed with the concept, Bell gave Young a contract to build two prototypes. On November 24, 1941, Young and his new assistant, Bart Kelly, arrived at the Bell plant to start work.

 

After a series of political and technical issues were finally worked out, Young learned that Bell had withheld funding because of a concern about the aircraft’s ability to land safely with an engine failure. Young decided to demonstrate an autorotation with a raw egg as a passenger in his model. He started it at the top of a 30-foot ceiling and the small helicopter autorotated to the floor without breaking the egg. Bell restored the $250,000 funding and approved Young’s request for a bigger facility.

 

After relocating to a garage in Gardenville, NY (about 10 miles from Bell’s main plant) things started happening fast. Six months later the model 30 was ready to flight test. It was powered by a 160-hp Franklin air-cooled engine and had a 32-foot rotor diameter. Young would hover the model 30 while tethered to the ground. Many of these flights helped solve vibration issues. When it became time to release the helicopter’s tether, Bell assigned a test pilot name Floyd Carlson to the project.

 

After Carlson figured out how to hover the new machine, he began trying faster airspeeds. At different speeds he would encounter vibrations and Young would fix them. While flight testing continued, ship number two was being constructed. In September 1943, Carlson wrecked the helicopter when he struck the tail teaching himself autorotations. Ship number two, which had an enclosed cabin and a passenger seat, took over as the test aircraft.

 

In the spring of 1944 ship number one had been rebuilt and Young and his team decided to build a third prototype. The third model 30 was not authorized by Bell, but Young wanted to build it to make improvements he felt were needed. Some of these included a four-wheel landing gear, an advanced instrument panel, and a tubular tail boom.

 

The third model proved to be the best flying prototype, but it did not have an enclosed cabin. Young then came up with the idea of heating a large piece of Plexiglas and blowing it up like a bubble. Bell liked this model and gave Young the go ahead to produce a production prototype. On December 8, 1945, the first model 47 was completed. Shortly thereafter 10 more helicopters were built for training, product improvements, and demonstrations. Then on March 8, 1946, Bell was awarded the first commercial helicopter certification by the CAA.

 

The model 47 went on to star in several TV shows and movies. When production stopped in 1973, more than 5,000 versions were built.

Speed limits – Part 2

Monday, June 8th, 2009

How exactly does flapping change a rotor blade’s angle of attack? That was a great question with many good explanations provided by readers. I think to fully understand it is important to know the difference between pitch angle and angle-of-attack. Pitch angle is the angle between the rotor blade’s chord line (a straight line intersecting the leading and trailing edges of an airfoil) and a reference plane of rotation. Angle-of-attack is the angle between the rotor blade’s chord line and the relative wind (the airflow that results from, and is opposite of, the velocity of an airfoil. Velocity is used here as a vector to mean speed and direction.)

When the rotor blades stay in the reference plane of rotation the pitch angle and angle-of-attack are the same. The pilot controls the pitch angle with the collective control and thus the angle-of-attack as well. However, when a rotor blade leaves the plane of rotation (flapping causes this to happen) the direction component of its velocity changes. Since relative wind is a function of velocity, it changes as well. In the case of a blade that flaps up the relative wind moves opposite the blade’s new direction. This change in relative wind direction reduces the blade’s angle-of-attack. The opposite is true for the blade that flaps down on the retreating side.

As the helicopter’s forward speed continues to increase, the retreating, or down flapping, side encounters higher angles of attack. Eventually, the rotor system encounters retreating blade stall.

From the pilot’s perspective, when this happens an abnormal vibration will be felt, the nose can pitch up, and the helicopter can have a tendency to roll in the direction of the stalled side. The amount and severity of pitch and roll will vary depending on the rotor system design.

The tendency for the nose to pitch up is because the spinning rotor system acts like a gyroscope and therefore experiences gyroscopic precession (a physical property that states when an external force is applied to a rotating body the effect will happen approximately 90 degrees later in the direction of rotation). As such, when the retreating blade stalls and stops producing lift, the effect of this happens toward the rear of the rotor disc. This causes the disc to tilt back, and the nose to pitch up.

Conditions like high density altitude, steep or abrupt turns, high blade loading (caused by high gross weight), turbulent air and low rotor rpm will increase the likelihood of encountering retreating blade stall when operating close to a helicopter’s Vne (never exceed speed). Helicopter flight manuals contain a chart or textual description in the limitations section that reduce the helicopter’s Vne at higher altitudes and temperatures. This is the airspeed limitation chart from a Bell 407.

Should a pilot encounter retreating blade stall, lower the collective and reduce airspeed. Other actions that will help are increasing rotor rpm and decreasing the severity of any roll or pitch maneuvers. Taking immediate action at the first sign will normally result in a quick recovery. However, if a pilot attempts to increase speed a severe stall would develop with possible loss of control.