afghanistan’s thin mountain air power issues challenges u

GETTING ALOFT

ROTORHEAD AND ROTOR BLADESThe rotorhead, where the rotor driveshaft connects to the rotorblades, carries the hugecentrifugal forces created by the turning blades, as well as transmitting control forcesto the blades so the helicopter can hover, move in any direction, turn, climb and descend.

The angle at which the blades meetthe air can be changed in flight.Increasing or decreasing that anglecollectively changes the lift from the rotor to make the aircraft climb or descend. L

IFT

Air flows faster over the blade’s uppersurface, creating a lower-pressure areathat is its lift force.

Faster air

LIFT

Air flows faster over the blade’s uppersurface, creating a lower-pressure areathat is its lift force.

Changing the angle that airflows around the blade — its“angle of attack” — changes the amount of lift the blade produces.

Slower air

The Chinook uses the Lycoming T55 turboshaftengine, which produces 3,750 horsepower.

The power turbine extracts energy fromthe hot gases and then expels themthrough the exhaust. The rotationalenergy from the turboshaft engine iscaptured through a system of gears,transmissions and driveshafts.

As the blades rotate, their angle of attack can be changedcollectively — the same amount at the same time — to makethe helicopter change altitude.

POWER ISSUES

LANDING PROBLEMS

A helicopter can have difficulty carrying a full load in a mountainousenvironment like Afghanistan and areas with high temperatures.

Helicopters can get into trouble by descending too fast withtoo little forward speed. Called “settling with power,” the aircraft gets caught in its own rotor’s powerfuldownwash of air. Trying to arrest the descent with more power only pushes the helo toward the groundfaster. Helicopters can also enter this turbulent “vortex-ring state” when landing with a tailwind.

Helicopter decends at a fast rate, exceeding the normal downward flow of the blades.

The full power of the engines is not enough to overcome the sink rate, resulting in a crash.

Airflow

Normal vortex createdby the blade tip

Turbulent air isgenerated, pushingdown on the rotor.

Mountain slopesand rough terraincan start a chain ofevents that sends ahelicopter topplingover on its side, asituation called“dynamicrollover.”

Main rotorthrust

Crosswind

Afghanistan’s thin mountain airchallenges U.S. helicopter fliersAfghanistan’s thin mountain airchallenges U.S. helicopter fliers

At sea level, a helicopter’sblades are acting on denseair, more easily creating lift.

Afghanistan’s thin mountain airchallenges U.S. helicopter fliers

Heat and humidity lowerthe air density, makingtakeoffs, climb and hoveringlandings difficult.

Afghanistan’s thin mountain airchallenges U.S. helicopter fliersAfghanistan’s thin mountain airchallenges U.S. helicopter fliers

Heat and humidity lowerthe air density, makingtakeoffs, climb and hoveringlandings difficult.

Engines are, in effect, air pumps.When air density is low — becauseof high elevation, high temperatureor high humidity — engines have lessair to work with and the power theycan make decreases. To compensatefor reduced power, the helicopter’sload — troops, supplies or fuel —has to be reduced.

Sand and dust sucked intoturbine engine air intakeserode the delicatemechanisms and sharplycut an engine’s flying life.

Unpredictable aircurrents off mountainslopes can causean aircraft to dropunexpectedly.

DYNAMIC ROLLOVER

SOURCES: U.S. Army, U.S. Coast Guard, FAA, NASA, Federation of American Scientists JOHN G. OWNBY/TIMES-DISPATCH

Shock absorbers

Armcontrols

pitch

Rotorblade

Faster air

Steep angle of attackNo angle of attack Slight angle of attack

Their angle of attack can also bechanged cyclically — differing amountsat differing points in the circle ofrotation — to turn the aircraft.

Air enters through theengine intake and isducted to thecompressor section.

Then the compressed air flowsinto the combustion section, ismixed with jet fuel and ignited.

As the helicopter starts to rollsideways as it touches down on asteep slope or by catching itslanding gear on an obstruction,the pilot attempts to use the rotor’slift to stop the roll. But the rotorcan only tilt so much. Should thehelicopter exceed this “criticalrollover angle,” the aircraft willcontinue to roll, dangerously outof control.

CH-47 CHINOOK CARGO HELICOPTER