Army Aviation
How does air pass through the airfoil on top? - ANSWER: The top of the airfoil is
cambered, or curved, allowing air to quickly get past the airfoil and create a low
pressure.
How does air pass through the airfoil on bottom? - ANSWER: The air that goes below
the airfoil has a shorter distance to travel and is slower, which results in high pressure.
Define Lift - ANSWER: Lift is the upward force created by the difference in air pressure
above and below the blade. This is what allows a helicopter to fly.
How does the airfoil utilize pressure to allow the helicopter to stay airborne? -
ANSWER: The reduced pressure above the airfoil pushes downward with less force
than the increased pressure below pushes upward, and lift is created.
What does the amount of lift created depend on? - ANSWER: A pluthera of factors from
the material and shape of the airfoil to the density of the air in the vicinity.
What 2 factors that affect the amount of lift created can a pilot influence? - ANSWER:
The angle of the incidence (pitch angle) and the angle of attack (AOA).
Define Pitch Angle - ANSWER: The angle between the chord line (a line drawn between
the leading edge and the trailing edge of the airfoil) and the direction of the rotor's spin.
How can a pilot control pitch angle? - ANSWER: By using the collective on a helicopter
Define the Angle of Attack (AOA) - ANSWER: The angle between the chord line and the
direction of the incoming air.
On a windless day, AOA can potentially equal pitch angel when hovering, why? -
ANSWER: Because the only relative wind will be that created by the rotation.
What would happen if the AOA was zero? - ANSWER: no lift will be generated. In order
to create lift, a positive AOA is needed - the front of the blade needs to be raised above
the back, relative to the wind.
negative lift is technically possible by flying at a negative AOA.
EXCEPTIONS: some airfoils (those that are either cambered or non-symmetrical) can
be designed to generate lift with a zero or even negative AOA. Though increasing the
AOA will still result in more life.
,Define weight - ANSWER: The force acting on a body due to gravity
Define gravity - ANSWER: the acceleration acting on a body due to the Earth's mass
The average lift must be a _______ force than the loaded weight of the aircraft
(including the helicopter itself, all its passengers, cargo, fuel, etc.). - ANSWER: a larger
force
What factors can either decrease the effectiveness of lift or increase the potency of
weight while mid-flight? - ANSWER: Strong winds and other adverse weather
conditions, but a more frequently observable example is banking the helicopter
Why does banking the helicopter affect lift? - ANSWER: When banking the helicopter,
the lift is no longer generated directly downwards; rather some of the vertical lift is
redirected into horizontal movement.
This results in a descent, unless the total airfoil is increased to produce the appropriate
amount of vertical lift once more.
How, at varying angles, does banking affect the need for increased lift? (e.g. banking at
a 25 degree angle vs a 34 degree angle vs 48 vs 90). - ANSWER: Tame at first, but
quickly gets out of hand.
EXAMPLE: a bank of 25 degrees would result in a 10% increase in required lift, but
increasing the angle by just 9 more degrees would result in a 20% increase.
at 48 degrees, lift will have increased by half again of what an upright helicopter
requires.
at 90 degrees, the airfoils would be generating only horizontal lift and so the helicopter
would be essentially free falling.
What is the added strain created by banking called? - ANSWER: The Load Factor or the
G Load
What is the formula for the Load Factor, AKA G Load? - ANSWER: G = L1 - L0
G = the load factor
L1 = the actual load on the rotor blades
L0 = is the resting load
This is expressed as a percentage
(see highlighted section on pg 55 for good example)
, Define Thrust, how it's measured, and how it's primarily created - ANSWER: The force
needed to move the helicopter horizontally, measured in horizontal motion.
Is it primarily created by converting lift via alterations of the pitch angle and AOA.
What requires more lift? To move horizontally or vertically? Why? - ANSWER: Vertical
movement, since horizontal movement does not have to contend with gravity.
Only a small portion of lift needs to be sacrificed to achieve some movement.
What force must lift overcome? - ANSWER: Drag - a composite force made up of 3
categories
What are the three subcategories of Drag? - ANSWER: Profile, parasitic, induced - all
three forces change in varying amounts with the speed of the helicopter
How does increasing speed affect drag? - ANSWER: Usually increasing speed leads to
increased drag.
Why does increasing velocity also increase drag? - ANSWER: Both air and an object
attempt to occupy the same space, and the object must exert force to displace the air
even as the air tries to do the same to the object.
The faster the wind is moving relative to the object, the more air that object has to push
out of the way and the more air that pushes back on the object. This is why increasing
velocity also increases drag.
Define Parasitic Drag - how is it caused? - ANSWER: It is caused by the non-lifting
portions of the helicopter, such as the fuselage, engine cowlings, hub, mast, landing
gear, and external loads.
Parasitic drag increases with airspeed and is the dominant type of drag at high speeds.
Define Profile drag - ANSWER: Similar to parasitic drag with one main difference - it
only concerns the drag created by the frictional resistance of spinning airfoils (see pg.
56 for good example - high lighted sentence)
Profile drag is especially significant at lower speeds.
Define Induced Drag - ANSWER: Induced drag is a byproduct of the lift created by the
rotor.
As the blades spin, they briefly leave a small gap of empty space behind them where
they once were, which is quickly filled by the air above and below the blade. Since the
air above and below the blade is at different pressure, the air creates a spiral directly
behind the blade, which pushes air in the opposite direction of lift.
How does air pass through the airfoil on top? - ANSWER: The top of the airfoil is
cambered, or curved, allowing air to quickly get past the airfoil and create a low
pressure.
How does air pass through the airfoil on bottom? - ANSWER: The air that goes below
the airfoil has a shorter distance to travel and is slower, which results in high pressure.
Define Lift - ANSWER: Lift is the upward force created by the difference in air pressure
above and below the blade. This is what allows a helicopter to fly.
How does the airfoil utilize pressure to allow the helicopter to stay airborne? -
ANSWER: The reduced pressure above the airfoil pushes downward with less force
than the increased pressure below pushes upward, and lift is created.
What does the amount of lift created depend on? - ANSWER: A pluthera of factors from
the material and shape of the airfoil to the density of the air in the vicinity.
What 2 factors that affect the amount of lift created can a pilot influence? - ANSWER:
The angle of the incidence (pitch angle) and the angle of attack (AOA).
Define Pitch Angle - ANSWER: The angle between the chord line (a line drawn between
the leading edge and the trailing edge of the airfoil) and the direction of the rotor's spin.
How can a pilot control pitch angle? - ANSWER: By using the collective on a helicopter
Define the Angle of Attack (AOA) - ANSWER: The angle between the chord line and the
direction of the incoming air.
On a windless day, AOA can potentially equal pitch angel when hovering, why? -
ANSWER: Because the only relative wind will be that created by the rotation.
What would happen if the AOA was zero? - ANSWER: no lift will be generated. In order
to create lift, a positive AOA is needed - the front of the blade needs to be raised above
the back, relative to the wind.
negative lift is technically possible by flying at a negative AOA.
EXCEPTIONS: some airfoils (those that are either cambered or non-symmetrical) can
be designed to generate lift with a zero or even negative AOA. Though increasing the
AOA will still result in more life.
,Define weight - ANSWER: The force acting on a body due to gravity
Define gravity - ANSWER: the acceleration acting on a body due to the Earth's mass
The average lift must be a _______ force than the loaded weight of the aircraft
(including the helicopter itself, all its passengers, cargo, fuel, etc.). - ANSWER: a larger
force
What factors can either decrease the effectiveness of lift or increase the potency of
weight while mid-flight? - ANSWER: Strong winds and other adverse weather
conditions, but a more frequently observable example is banking the helicopter
Why does banking the helicopter affect lift? - ANSWER: When banking the helicopter,
the lift is no longer generated directly downwards; rather some of the vertical lift is
redirected into horizontal movement.
This results in a descent, unless the total airfoil is increased to produce the appropriate
amount of vertical lift once more.
How, at varying angles, does banking affect the need for increased lift? (e.g. banking at
a 25 degree angle vs a 34 degree angle vs 48 vs 90). - ANSWER: Tame at first, but
quickly gets out of hand.
EXAMPLE: a bank of 25 degrees would result in a 10% increase in required lift, but
increasing the angle by just 9 more degrees would result in a 20% increase.
at 48 degrees, lift will have increased by half again of what an upright helicopter
requires.
at 90 degrees, the airfoils would be generating only horizontal lift and so the helicopter
would be essentially free falling.
What is the added strain created by banking called? - ANSWER: The Load Factor or the
G Load
What is the formula for the Load Factor, AKA G Load? - ANSWER: G = L1 - L0
G = the load factor
L1 = the actual load on the rotor blades
L0 = is the resting load
This is expressed as a percentage
(see highlighted section on pg 55 for good example)
, Define Thrust, how it's measured, and how it's primarily created - ANSWER: The force
needed to move the helicopter horizontally, measured in horizontal motion.
Is it primarily created by converting lift via alterations of the pitch angle and AOA.
What requires more lift? To move horizontally or vertically? Why? - ANSWER: Vertical
movement, since horizontal movement does not have to contend with gravity.
Only a small portion of lift needs to be sacrificed to achieve some movement.
What force must lift overcome? - ANSWER: Drag - a composite force made up of 3
categories
What are the three subcategories of Drag? - ANSWER: Profile, parasitic, induced - all
three forces change in varying amounts with the speed of the helicopter
How does increasing speed affect drag? - ANSWER: Usually increasing speed leads to
increased drag.
Why does increasing velocity also increase drag? - ANSWER: Both air and an object
attempt to occupy the same space, and the object must exert force to displace the air
even as the air tries to do the same to the object.
The faster the wind is moving relative to the object, the more air that object has to push
out of the way and the more air that pushes back on the object. This is why increasing
velocity also increases drag.
Define Parasitic Drag - how is it caused? - ANSWER: It is caused by the non-lifting
portions of the helicopter, such as the fuselage, engine cowlings, hub, mast, landing
gear, and external loads.
Parasitic drag increases with airspeed and is the dominant type of drag at high speeds.
Define Profile drag - ANSWER: Similar to parasitic drag with one main difference - it
only concerns the drag created by the frictional resistance of spinning airfoils (see pg.
56 for good example - high lighted sentence)
Profile drag is especially significant at lower speeds.
Define Induced Drag - ANSWER: Induced drag is a byproduct of the lift created by the
rotor.
As the blades spin, they briefly leave a small gap of empty space behind them where
they once were, which is quickly filled by the air above and below the blade. Since the
air above and below the blade is at different pressure, the air creates a spiral directly
behind the blade, which pushes air in the opposite direction of lift.
