Archive for April, 2010

Tail boom strakes

Thursday, April 22nd, 2010

On some helicopters, running the length of the tail boom are “L” shape (or something similar) brackets that protrude about an inch. These are known as tail boom strakes and they act like spoilers.

Because the tail boom is underneath the rotor system, at a hover, very low airspeeds or sideways flight rotor down wash passes around the boom. Like an airfoil, this produces high and low air pressure areas that exert a force along the tail boom. This force decreases the tail rotor’s capability during hover and slow flight. At higher speeds the down wash moves to the rear and passes above the tail boom. Strakes control the airflow around the tail boom, thus increasing the tail rotor’s efficiency and decreasing the turbulent air, which improves yaw control.

During the late 1980s, NASA and the U.S. Army performed wind tunnel and flight tests to analyze the performance gain from adding a tail boom strake. The tests were performed using a Bell 204B helicopter. Published in 1993 the NASA Technical Report 3278 stated a 5-percent improvement in pedal control margin will provide an additional 2,000 feet of altitude capability or 500 lbs. of payload. The report concluded that the strakes improves handling qualities, reduces tail boom fatigue, improves climb and cruise performance, and increases yaw control safety margins for all single rotor helicopters with enclosed tail booms.

Wind

Monday, April 12th, 2010

 To a helicopter pilot, a brisk wind can provide a nice performance boost. However, gusty winds can sometimes be tricky to manage. According to the NTSB, during the last 10 years there were 48 helicopter accidents in which the pilot said a gust of wind was a factor.

On March 27, 2002, a student and instructor were hovering a Hughes 269 helicopter (a small two-seat piston powered trainer) at the Fort Collins Downtown Airport in Colorado. The instructor pilot reported that the wind was 2 or 3 knots at takeoff, but forecast to be gusty in the afternoon. While hovering at about 3 ft, with the student pilot at the controls, the helicopter encountered a very strong gust and began to wobble. The instructor took control of the helicopter and climbed to about 15 feet. Then another gust hit the helicopter, turning it sideways and then downwind. The instructor stated he was attempting to get it on the ground, but the wind continued to drive the helicopter forward with excessive nose-over tendency.

With the tail rotor into the wind, creating a high power demand and limited tail-rotor authority, the helicopter skipped along the dirt two or three times traveling about 200 feet. The right strut failed, and the helicopter rolled over on its right side.

Ten minutes after the accident the reported weather at the Fort Collins-Loveland Municipal Airport, 8 nm south of the accident site, was wind from 260 degrees at 13 knots, gusting to 25.

It is not surprising that many accidents were reported while hovering with students. However, like the following, some happened while in flight.

On June 12, 1997, the pilot of a Bell 206L-3 was flying his fourth trip of the day over a proposed gas pipeline route 15 miles southwest of Weston, Colorado. The flight was for the purpose of conducting a bird study and required low and slow flight over wooded areas.

The pilot said he crested a ridge and descended into the next valley, following the terrain downhill. As he approached the far side of the valley, he stated he was going to need a climbing 360-degree turn to clear the next ridge. After completing about 90 degrees of the turn the helicopter was hit by a gust from the rear, causing a significant decrease in airspeed. The pilot said he had to decide whether to attempt to complete the turn and risk striking a tree with forward speed, or try a landing with zero airspeed. He chose the latter. The helicopter settled into trees and rolled over on its right side. The commercial pilot and four passengers escaped injury.

Reported winds don’t always tell the complete story. The NTSB report listed winds from 100 degrees at 13 knots. Ridges and valleys can cause different and strange wind patterns. Flying low and slow at high density altitudes and high gross weight can make even a small wind change very demanding.