When one watches a helicopter hovering over the water, the spray churned up by the rotor wash looks spectacular. However, if you are the pilot of the helicopter, dangers are lurking.
For the record, hovering takes more power than flying in level flight. How much more depends on the temperature of the air, the amount of wind and weight of the helicopter at the time. The higher the temperature, the thinner the air and the less lift and power from the engines. If the wind is below 15 knots, the helicopter is out of translational lift – the speed at which the blades act as a “solid” disc and generate 10 – 15% more lift.
As a rule of thumb, figure 25 – 30% more. Put in simple terms, if the helicopter needs 500 horsepower to cruise at 90 knots, it will need somewhere between 125 to 150 more to hover on most days. We’ll get to “most” days in a minute.
So what is the enemy? There are several. First, there is water ingestion. The spray billowing out from the rotor wash gets sucked into the engine intake by an engine or engines hungry for air to mix with fuel to create more power. More spray, means more water into the engine(s) intake and the greater the chance of reducing the power output of the engine, or worse, putting the fire out. If the pilot doesn’t pay attention, he might wind up swimming.
Most modern jet engines used in helicopters have a “debris” or “particle” separator that separates the globules of moisture from the air just as they do blades of grass. But, like anything else, there is a limit on what they can do.
However, if the helicopter hovers within 10 to 20 feet of the water, the debris separation system may become overloaded causing compressor stalls which announce themselves by loud, attention getting, bangs. Sometimes, compressor stalls are accompanied with fire coming out of the intake or exhaust. At night, besides being alarming, flames coming out of either the intake or exhaust pipe can be exciting to see.
If the compressor stalls are really bad, the engine will lose power, or worse shut down. One bang is a warning meaning the pilot should resume forward flight as soon as possible. If the pilot hears a series of bangs, he should depart the hover immediately.
For those who have never heard a compressor stall, think of a 12 gauge shot gun fired about five feet above and behind your head. Even with helmets designed to protect one’s hearing, the bang of a compressor stall is very distinctive. Once you hear a compressor stall, you’ll never forget it.
If the helicopter is hovering over saltwater, there’s another problem which comes from the salt spray generated by the rotor wash. As the salt water dries on the turbine blades, it leaves a coating of salt that changes the aerodynamic profile of the turbine blade. This reduces blade efficiency and degrades engine performance. As the salt deposits build, the engine begins to lose power, compressor stalls and other bad things happen.
In the U.S. Navy, we hovered at 40 feet for hours in the H-3 with a sonar ball at the end of the cable often hundreds of feet below the surface. Typical mission profiles had us fly 75 – 100 miles from the ship, hover for 10 – 15 minutes and listen. No contact, pull the dome up fly to another location and repeat the process. If a submarine was detected, we’d stay in the hover for as long as we could.
Why 40 feet? Good question. One, at 40 feet, we were above most but not all of the salt spray. Two, 40 feet was about the top of the cushion of air called ground effect generated by the main rotorblades. However, the higher the sea state, the less “ground effect.” And, the less ground effect, required more power.
And three, at 40 feet, one had a reasonable chance of flying the helicopter out of the hover if something bad like an engine failed happened. Or, enough time to try to ditch with the “wings” level.
Net net is that we would often fly double cycles, i.e. about eight hours of which we would be hovering for four to six hours. Even at 40 feet, salt spray would coat the blades and require us to wash them as often as possible.
Lose power on one engine in a hover with the dome in the water and you are probably going to test your water landing skills. This is why we would after, wash down the turbine blades with fresh water as often as we could.
Morale of the story. Beware of the spray, it can knock out of the air!