SIFT

The Army Aviation Information Test (AAIT) is one of the seven subtests on the Selection Instrument for Flight Training (SIFT). It contains 40 questions to be completed in 30 minutes and assesses your knowledge of fundamental aviation concepts, helicopter flight principles, Army aircraft systems, aerodynamics, and basic flight operations. A strong AAIT performance demonstrates the foundational aviation knowledge that Army flight training builds on.

The Atmosphere

The atmosphere is the envelope of air surrounding the Earth. Aircraft performance varies with atmospheric conditions, so understanding atmospheric properties is essential for flight. The atmosphere consists primarily of nitrogen (78 percent) and oxygen (21 percent); the remaining 1 percent comprises argon, carbon dioxide, water vapor, and trace gases. Air has mass and weight, which creates atmospheric pressure.

Property	StandardValue
Temperature	59°F(15°C)
Pressure	29 .92 inches of mercury (inHg) = 14 .7 pounds per square inch (psi)
Density	Approximately0002377slugspercubicfoot .
Temperaturelapserate	About2°C(35°F)per1000feetinthetroposphere . ,
Tropopause	Approximately36000feet ,

As altitude increases, atmospheric pressure decreases because there is less air above to exert downward pressure. Temperature also generally decreases with altitude until reaching the tropopause. These standard values serve as baseline references for calculating aircraft performance.

Density altitude is the altitude at which the aircraft "thinks" it is flying based on air density rather than actual altitude. High temperature, high humidity, and low pressure all decrease air density and raise density altitude. Higher density altitude reduces performance: the engine produces less power and the rotor blades generate less lift in thinner air.

Pressure altitude is the altitude indicated when the altimeter is set to 29.92 inHg; it represents altitude above the standard datum plane. Pilots use pressure altitude for performance calculations and when flying above 18,000 feet in the United States.

Remember the three conditions that raise density altitude with the phrase "hot, high, and humid." Each one thins the air, and every one of them reduces engine power, rotor lift, and hover performance.

Principles of Flight: The Four Forces

Four fundamental forces act on an aircraft in flight: lift, weight, thrust, and drag. Controlled flight depends on how these forces interact.

The aerodynamic force that opposes weight and supports the

The force of gravity on the aircraft and everything it carries,

aircraft. In helicopters the main rotor generates lift, acting

acting vertically toward the center of the Earth. Total weight is

perpendicular to the relative wind and the rotor disc plane. Lift

empty weight plus fuel, crew, passengers, and cargo. For

depends on air density, airspeed, blade area, and angle of

flight, lift must equal or exceed weight.

attack.

The force that propels the aircraft through the air. A helicopter

The aerodynamic force that opposes motion through the air,

creates thrust by tilting the rotor disc in the desired direction;

acting parallel to and opposite the relative wind. The two main

the horizontal component of total rotor thrust drives forward,

types are parasite drag and induced drag.

rearward, or sideward flight.

Parasite drag results from the aircraft moving through the air. It includes form drag (caused by the aircraft's shape), skin friction drag (air flowing over surfaces), and interference drag (mixing of airflows around intersecting surfaces). Parasite drag increases with the square of airspeed.

Induced drag is a byproduct of lift. The pressure difference between the lower and upper surfaces of a blade or wing causes air to flow from the high-pressure area below to the low-pressure area above, creating tip vortices that produce induced drag. Induced drag decreases as airspeed increases.

Total drag is the sum of parasite and induced drag. Induced drag dominates at low airspeeds; parasite drag dominates at high airspeeds. The airspeed at which total drag is lowest is the most efficient speed for flight.

Airfoil Terminology

An airfoil is any surface designed to produce lift when air flows over it. Rotor blades, wings, and tail surfaces are all airfoils.

Term	Defi nition
Leadingedge	Theforwardedgeoftheairfoilthatfirstcontactstheair
Trailingedge	Therearedge whereairflowfromtheupperandlowersurfacesrejoins ,
Chordline	Animaginarystraightlinefromtheleadingedgetothetrailingedge;thechordisitslength
Camber	Thecurvatureoftheairfoil Upperandlowercamberdescribeeachsurface;themeancamberline .
	lieshalfwaybetweenthem Symmetricalairfoilshaveequalupperandlowercamber;asymmetrical .
	airfoilshavegreateruppercamber
Angleofattack	Theanglebetweenthechordlineandtherelativewind;theprimaryfactorthepilotcontrolstovarylift
Pitchangle	Theanglebetweenthechordlineandtherotorhubplane(notthesameasangleofattack)
Relativewind	Theairflowtheairfoilexperiencesbecauseofitsmotion alwaysparallelandoppositetotheflight ,
	path Forarotorblade itreflectsboththehelicopter'smovementandtheblade'srotation . ,

How Lift Is Produced

Two complementary explanations describe lift production: Bernoulli's principle and Newton's third law. Both are correct, and the pressure differential and the reaction to deflected air work together to produce the total lift force.

As the velocity of a fluid increases, its pressure decreases. Air

For every action there is an equal and opposite reaction. When

flowing over the curved upper surface of an airfoil accelerates,

an airfoil deflects air downward (downwash), the air exerts an

lowering the pressure above the airfoil. The higher pressure

equal and opposite upward force on the airfoil, which

below pushes upward, creating lift.

contributes to lift.

L = ½ × ρ × V² × CL × S

increases with denser air, higher airspeed, a larger lift coefficient, and more surface area. Because V is squared, doubling airspeed quadruples lift.

Stalls

A stall occurs when the angle of attack exceeds the critical angle of attack, causing airflow to separate from the airfoil and lift to drop suddenly. The critical angle is typically between 15 and 20 degrees for most airfoils. As angle of attack increases, the lift coefficient increases until the airflow can no longer follow the curved upper surface; the separated, turbulent flow destroys the low-pressure area above the airfoil and lift falls sharply.

A stall is related to angle of attack, not airspeed. An aircraft can stall at any airspeed if the critical angle is exceeded, but because higher angles of attack are needed to maintain lift at lower airspeeds, stalls typically occur at low airspeeds or during aggressive maneuvering.

In helicopters, retreating blade stall is a significant concern. In forward flight the retreating blade (moving opposite to the direction of flight) meets a slower relative wind, so it must operate at a higher angle of attack to keep lift equal across the disc. At high forward airspeed the retreating blade may exceed its critical angle and stall, causing vibration, pitch-up, and possible loss of control.

Answer choices that tie a stall to "low airspeed" alone are a common trap. The defining cause of every stall is exceeding the critical angle of attack.

Helicopter Aerodynamics