Archive for January, 2013

Tip driven rotors II

Saturday, January 26th, 2013

Placing a small jet engine (such as a pulsejet, turbojet and ramjet) on a helicopter’s main rotor blade tips for propulsion never developed into a major commercial success. Despite the advantages of no heavy transmission or anti-torque rotor, several major problems could not be solved. One was the high centrifugal loads acting on the engines. However, from a design standpoint, driving the rotor system from the blade tips was so attractive that during the 1940s engineers came up with another concept called the pressure-jet rotor. 

The pressure-jet rotor operates by forcing compressed air out aft facing nozzles at the blade tips. An engine driven air compressor located in the fuselage pumps air through a rotating seal and into hollow rotor blades. Initially, this solved the noise issue that was associated with engines at the blade tips. However, compressed air alone did not provide enough thrust for flight, so fuel was added and then ignited at the blade tips. This added more thrust, but resulted in higher noise levels. Several prototypes were built as compound aircraft. This design uses the noisy rotor for take-off and landing, and then a propeller system with a small wing for forward flight. In an attempt to solve the noise issue without adding complexity, the French built a small helicopter with a large compressor that was successful without burning fuel at the rotor tips. It was quieter and worked well enough that the French Army ordered about 200 of them. However, using just compressed air was not powerful enough for a larger design. 

The final attempt at a tip driven design directed exhaust gasses from a turbine engine through the rotor blades. Hughes Helicopters built a working prototype during the 1960s; however, pumping 800 plus degrees (F) air through a seal in the rotor was problematic. Eventually, the tip driven concept was abandoned in favor of the engine driven main and tail rotor design that is used on the vast majority of helicopters flying today.

Ramjet powered helicopters

Tuesday, January 15th, 2013

Early helicopter engineers were looking for ways to increase power available and decrease weight. Placing the propulsion system on the tips of the rotor blades eliminated the need for a power consuming anti-torque rotor (tail rotor) and a heavy transmission. Although, propellers mounted on the blade were tried it was the ramjet developed during World War II that launched a major effort to build a successful rotor tip driven helicopter. 

In 1946, McDonnell developed a single-seat, 285 pound (empty) helicopter for the U.S. Air Force. Called Little Henry, it used two ramjets (one on each blade tip) producing about 10 pounds of thrust each. A fuselage mounted tank supplies fuel to each engine through a rotating seal in the rotor hub. After a long flight test program, the Air Force decided not to purchase the helicopter. 

Hiller Aircraft also developed a ramjet powered helicopter. The XHJ-1 Hornet’s larger engines developed 40 pounds of thrust each. Flight test began in 1950 and the U.S. Military expressed interest provided the helicopter and its engines received FAA certification. Hiller worked hard to overcome several design challenges like excessive centrifugal loads acting on the engines and high fuel consumption. Ramjets are also noisy and need a substantial airflow to start, requiring a small ground engine to spin the rotor system. Although Hiller eventually received certification for the engines, the FAA would not sign off on the high level of drag caused by the engines during autorotation. Without the Military, Hiller ended the project.   

A Dutch company solved the drag issue during autorotation with a larger high inertia rotor system. The company’s NHI H-3 Kolibrie (Hummingbird) received certification in 1958.

CFIT

Tuesday, January 1st, 2013

Pilots can learn a lot from reviewing accident reports. The idea is to understand the mistakes that led up to a particular accident so as not to repeat them. Yet, several types of preventable accidents occur over and over. One of these is Controlled Flight Into Terrain (CFIT) in IMC conditions. According to the NTSB, October 2012 was a bad month with three fatal helicopter CFIT accidents in poor weather. 

On October 5, 2012, about 0758 central daylight time, a Bell407 helicopter was substantially damaged when it collided with terrain shortly after takeoff from CentralIndustriesAirport(2LA0), near Intracoastal City, Louisiana. The commercial pilot, who was the sole occupant, was fatally injured. Day instrument meteorological conditions prevailed for the post-maintenance flight. One witness reported that she saw the helicopter depart on the runway heading and disappear into fog or a low cloud ceiling. Several witnesses reported hearing a sound consistent with a ground impact shortly after the helicopter had departed toward the southwest.

The closest weather observing station was located at the Abbeville Chris Crusta Memorial Airport (KIYA), about 13.6 miles north-northeast of the departure airstrip. At 0755, the KIYA automated surface observing system reported the following weather conditions: calm wind, visibility 1/4 mile with fog, overcast ceiling 200 feet, temperature 20 degrees Celsius, dew point 20 degrees Celsius, and altimeter setting 30.14 inches of mercury. 

OnOctober 9, 2012, about 2000 eastern daylight time, aBell407, was substantially damaged when it impacted trees and terrain inCoolbaugh Township,Pennsylvania. The airline transport pilot and one passenger were fatally injured, and one passenger was seriously injured. 

According to a limousine driver who was supposed to pick up one of the passengers at HPN, at 1938 he received a text from the passenger stating that they were running late. Then at 1953, he received another text instructing him to go back to MMU to pick up the passenger. After arriving at MMU, the driver waited but the helicopter never arrived.

A search by Federal, State, and Local authorities was initiated. On October 10, 2012 at approximately 0230 the helicopter was discovered in a heavily wooded area approximately 1.3 miles northwest of Pocono Mountains Municipal Airport (MPO), Mount Pocono, Pennsylvania. The recorded weather at MPO, at 2003, included: wind 100 degrees at 6 knots, visibility 1 1/4 miles, light rain, mist, overcast ceiling of 200 feet, temperature 09 degrees C, dew point 09 degrees C, and an altimeter setting of 30.10 inches of mercury.

According to Federal Aviation Administration (FAA) records, the pilot held an airline transport pilot certificate with ratings for airplane multi-engine land, with commercial privileges for airplane single-engine land, and rotorcraft-helicopter. His most recent application for an FAA first-class medical certificate was dated June 1, 2012. On that date, he reported 19,000 hours of flight time.

 On October 17, 2012, about 0640 eastern daylight time, an Aerospatiale AS 355 F2, was substantially damaged when it impacted trees and terrain shortly after takeoff from Brigham Heliport (4PN5), Erwinna, Pennsylvania. The certificated airline transport pilot was fatally injured. Dark night instrument meteorological conditions prevailed, and no flight plan was filed. Several witnesses reported hearing the helicopter as it over flew the residential neighborhood about the time of the accident, and one witness observed two lights that she presumed to be the accident helicopter as they descended into trees behind her home. Each of the witnesses described the lighting conditions at the time of the accident as dark, and reported that visibility in the immediate area was restricted due to fog.

No pilot takes off thinking that it’s highly likely they will have an accident. In these kinds of accidents most pilots know it is not ideal conditions, but probably believe it is manageable. However, what is missing is the realization that an unpleasant condition can quickly escalate into an unrecoverable and fatal situation. These kinds of accidents are worth thinking about every time you are faced with a decision regarding continued VFR flight in IMC.