By Ray Prouty | January 5, 2018
Unexpected technical problems challenge almost every new engineering program. It’s the curse of the “unknown unknowns.” Sometimes the problems are serious enough to be fatal to the program. At the very least, management and customers bite their fingernails.
Helicopter engineers face these problems like all other engineers. The following account of the successful solution of a real problem can be taken as typical of how engineers earn their paychecks.
The problem was on the Aerospatiale (now Eurocopter) HH-65A Dolphin, which the U.S. Coast Guard began procuring in the early 1980s as its Short Range Recovery helicopter.
Since snowstorms can’t be ruled out during a rescue, the Coast Guard required that the aircraft must have “unrestricted operation in falling and blowing snow.” Therefore, the Dolphin was equipped with a state-of-the-art engine-intake-anti-ice-system. But during the demonstration tests, a few cases of engine surge were unexpectedly encountered.
The problem took two years to correct. How it was solved was reported by John Murphy of Aerospatiale Helicopter Corp. at the 1988 American Helicopter Society Forum in a paper titled, “Dolphin Eats Snow."
The first task was to determine just what the problem was. In the HH-65A’s two Textron Lycoming LTS 101-750s as first installed, the air entered forward of the rotor mast and reached each engine by making a right-angle turn at the engine scroll.
To research the difficulty, the system was set up in Lycoming’s test cell and windows were installed in the ducting. When fresh snow was gently thrown into the intake, some of it piled up in the turn at the engine scroll. The snow would then come loose and go through the engine in lumps at unpredictable times.
When this happened, the airflow to the compressor was so affected that the engine surged with an audible “bang,” and there was the possibility of damage to the compressor.
The first attempted fix was to try to melt the snow with electric heating pads. Several modifications were made, but they did not do the job. Engineers found that the snow took a finite time to melt, and even as it was doing so, fresh snow was piling up on top of it. Also, it was impossible to increase the heat without damaging the scrolls, which were made of polymide [sic] plastic.
Since heating failed to fix the problem, the next step was to try to modify the internal contours of the scroll. The idea was to increase the air velocity in the regions where the snow piled up. Flights in the French Alps and tests in the Climatic Hangar at Eglin AFB in Florida subsequently showed that the modified contours didn’t work either.
A more successful approach was to replace the engine-supplied scroll with an aircraft-supplied “plenum chamber” that had a much larger volume and enclosed the engine accessory gearbox.
Even this was not straightforward. Some heating had to be supplied to critical areas, and deflectors, drains and insulation modifications had to be made. This solution was also expensive, since it involved changes to the engine.
Several alternate solutions were investigated. The team even returned to scroll-heating schemes both with compressor bleed air and with electrical spray mats. Neither proved practical.
Another promising solution investigated an alternate inlet. For this, the air is taken from the main-rotor gearbox compartment during operation in snow, with the normal intakes being used in all other conditions. The air going to the engine is pre-warmed by the oil coolers and by the gearbox itself.
The scheme was tested not only in the Climatic Hangar, but also in Montana, Wyoming and Michigan. (Engineers must be prepared to travel when faced with a problem involving cold weather operation.)
The results were “generally satisfactory,” though there were a few loose ends to be worked out if this system was to be chosen.
The solution was less expensive than the plenum chamber, because the engines would not require modification. But there was a safety-of-flight concern: The engines might suck in oil or hydraulic fluid should a malfunction occur in the gearbox compartment. Following tradeoff studies, the plenum chamber was chosen by Aerospatiale and approved by the Coast Guard. Tests subsequently had to be conducted to prove that the modification would indeed work. This involved looking for the right kind of weather through Colorado, Wyoming, Minnesota and Michigan during the winter of 1984 to 1985.
The tests were satisfactorily completed and the plenum-chamber system is now standard on all HH-65s.