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Friday, October 1, 2004

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Density vs. Pressure,

Apples vs. Oranges

In reference to Tim McAdam's September column on density altitude ("Watching Your Ps & Qs--and DA," Safety Watch, page 62), helicopters come in two basic flavors--piston engine and turbine engine. The power available from these two types of engines is affected differently by the atmosphere. The power output from a piston engine is dependent on density altitude, all other things being equal. The power available from a turbine engine is dependent on pressure altitude and temperature.

Now you can get density altitude from pressure altitude and temperature, but the same density altitude can be found from a very cold day at high pressure altitude (say, 9,000 ft. and -40 deg. C) or a hot day and a low pressure altitude (2,000 ft. and +40 deg. C). The power available from a piston engine will be the same in both conditions, but the power available from a turbine engine will be greatly different.

In most turbine machines, the flight manual hover capability chart will have air temperature shown twice to take care of this phenomenon.

The power required to hover (at the same weight and hover height above ground) for both types is a function of density altitude. But the other half of the equation, the power available for the turbine-powered machine, is not a function of density altitude.

Shawn Coyle
Chief Helicopter Pilot,
National Test Pilot School
Mojave, Calif.

All Boeing, All the Time?

In the August issue training section, we referred to "two Boeing C-130 Hercules transport planes." (U.S. Military Training Iraq Helo Pilots, page T2). Some in the fixed-wing world may believe that there are two types of U.S. aerospace workers: those who work for Boeing and those who will.

But reader John Armstrong of the FAA points out that "Boeing did not, does not and probably never will manufacture the C-130." That aircraft was built originally by Lockheed and currently by Lockheed Martin. Our news brief columns are collected from a wide variety of sources and carefully culled to avoid this type of error--unfortunately, sometimes not carefully enough.

Plenty of Blame

Maj. Tom Parker offers a poignant assessment of the price America pays by thrusting regular Army missions on National Guard units without giving those units the training or resources needed to do the job properly. Who knows how many combat deaths and casualties have resulted from this practice in Iraq and Afghanistan?

But Rotor & Wing let Parker gloss over the Nov. 2, 2003 downing of the F Co. Boeing Chinook and the resulting deaths of 16 soldiers. The countermeasures may well have suffered from old age and lack of use. The question is what was done with that knowledge. If no one discovered those flaws until after the downing, that is shameful. If the flaws were known, but tactics were not altered in recognition of that, lives were sacrificed needlessly.

Robert Davis
Arlington, Va.

Seed-Corn Nonsense

James T. McKenna's editorial equates Sikorsky Aircraft's "Buy American" campaign in the VXX competition with a sellout of U.S. rotorcraft technology ("Selling the Seed Corn," September 2004, page 4). That's nonsense.

Sikorsky's contender in that competition, the S-92, is a shining example of what can result when industry devotes money and manpower. Sikorsky invested in the future of helicopters when it launched the S-92 program. The result is an impressive aircraft that has won recognition from the aerospace world in the form of the Collier Trophy and surely will achieve success in the commercial market.

Robert Donahue
Boston, Mass.

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