Humidity, High Temperatures, Altitude and Helicopters Are a Bad Mix
Let’s start with the basics. Helicopters are designed to be operated close to the ground that is at or near sea level. If you don’t believe me, go spend a few hours with the pilot operating handbooks of any particular U.S. built helicopter.
While you are poring over the charts and tables, take a gander at the helicopter’s performance numbers when the temperature is above 90oF and 70% humidity. Add in a modest amount of altitude and a helicopter that could hover at maximum gross weight at sea level, can’t hover at all at 5,000.
Allow me to explain. As one goes higher in the atmosphere, the air gets thinner and there is less oxygen. If you every skied or hiked or walked around at 10,000, you’ll know what I mean.
Here’s how the lack of density or to use the aviation term, density altitude affects a helicopter. Heat and humidity make the air less dense which translates to less power and less lift. So, since the air thins as one gains altitude, by definition to get the same amount of liftat 5,000 or 10,000 feet a helicopter enjoys at sea level, requires more power. However, if you study the charts long enough, you’ll notice that very quickly altitude reduces helicopter ability to fly and more importantly, its ability to hover.
The helicopter pilot faces a double whammy in hot and high operations. His engines are generating less power and the rotor blades are creating less lift. Combine high temperatures and high altitude and very quickly, a very capable machine becomes a slug.
The solution is more blade area and lots of excess power. The two U.S. helicopters that seem to excel at high altitudes are the H-47 and the H-53E. Why, lots of blades – the twin rotor H-47 has six large blades and powerful engines. The H-53E has three engines and six blades. The 53E’s replacement, the H-53K will have bigger engines still and seven main rotor blades.
I’ve flown the H-2C/D as high as 6,000 feet around the mountains of Vermont where there are several peaks that get close to 5,000 feet. At that altitude, the controls were becoming sloppy and we were limited to a maximum true airspeed of 90 knots.
The H-3 models I’ve flown (A/G/Ds) in the mountains as high as 9,000 feet, we were limited to 80 knots true air speed. The controls were really sloppy and hovering with much more than 1,000 pounds of fuel with two pilots and two air crewmen and a modicum of search and rescue load of equipment was out of the question. To pick someone up, we needed a clear field so we could do a rolling takeoff!
In the HH-60H, at 9,000 feet we could make 100 knots, but the controls were really sloppy. You could move the cyclic in a six inch circle and nothing happened. Like the H-3, hovering to pick someone up with more than half fuel could be a problem. Landing was preferred, particularly if there was some ground along which we could make a rolling take-off.
So, think about the guys flying in Afghanistan. They’re operating where it is hot during the summer and they’re flying at 12,000 – 14,000 feet all the time. They’re operating in a wartime environment dodging surface-to-air missiles, RPGs and bullets. This takes skill and practice flying the helicopter at the absolute limits of its performance envelope. My hat is off to these guys.
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