Advanced - Aero Exam

What does the collective do?

Changes the pitch angle of all the main rotor blades equally and collectively regardless of their position around the 360 degrees of rotation.

What are the 3 primary flight controls?

Collective

Cyclic

Anti-Torque Pedals

What does the movement of the cyclic do to the TPP?

If the cyclic is pushed in any direction, the rotor tip path plane tilts the same way.

What is the TPP? What force is perpendicular to it?

The Tip Path Plane (TPP) is the plane inscribed by the tips of rotor blades at a given instant.

Rotor thrust acts perpendicular to the tip path plane.

What is the Virtual Axis?

The axis of rotation perpendicular to the tip path plane

What two dimensions does cyclic control?

Forward/Aft (Pitch)

Left/Right (Roll).

What does increasing the collective do?

Increases the magnitude of rotor thrust.

Increases altitude

What does decreasing the collective do?

Decreases the magnitude of rotor thrust.

Decreases altitude

How does Temperature affect Density and Density Altitude?

As Temperature Increases, Density Decreases, DA Increases

As Temperature Decreases, Density Increases, DA Decreases

How does Humidity affect Density and Density Altitude?

As humidity increases, density decreases, & DA increases

As humidity decreases, density increases, & DA decreases

What is the 10% Rule?

Add 100 feet to your DA (based on PA and OAT), for every 10% relative humidity above 0% RH.

What is Pressure Altitude?

True altitude corrected for non-standard pressures. Its the altitude with 29.92 set.

Know how to use the DA chart!

EX from page D-3 in FTI:

• OAT = 30 deg C

• PA = sea level

• Dew Point = 30 deg C (RH = 100%)

The question on the test will give you OAT, Pressure Altitude, and Humidity.

Don't forget to add the 10% Rule using humidity to the density altitude on the chart.

• DA (corrected for RH) = DA (from chart) + (100’ x RH/10%)

Result:

• DA (chart) = 1800’ approx.

• 10% rule:

  • 1800’ + (100’ x (100%/10%))

  • 1800’ + (100’ x 10)

  • 1800’ + 1000’

  • 2800’ = DA (corrected for RH)

Know the characteristics of the blade element diagram!

RW=Linear Velocity + Induced velocity

Lift is always perpendicular to RW

Profile drag is always parallel to RW

What is Relative Wind the sum of?

Linear Velocity

Induced Velocity

What will happen to AOA on the Power On Blade Element Diagram if Induced Velocity is increased with no blade pitch change?

AOA will decrease

What is linear velocity the sum of?

Rotational Velocity

Translational Velocity

What is Droop Snoot airfoil?

TH-73A Airfoil

Mostly symmetrical with a slight camber on the first 25% of the chord

Combines the best qualities of Cambered/Symmetrical foils

Are cambered airfoils desirable or undesirable for helicopters?

Undesirable

Does Cambered or Symmetrical airfoils generate more lift at same AOA?

Cambered Airfoils

How does lift relate to relative wind?

Lift always acts perpendicular to relative wind

How does rotational velocity change along the rotor blade?

It always increases along the span from root to tip

How does profile drag relate to relative wind?

Profile drag is always parallel to relative wind

What is pendulum effect?

a specific case of pendular action

As the helicopter transitions from forward flight to a hover, the moments generated by pendular action will tend to pitch the nose up

What is pendulum effect a specific instance of?

A specific instance of pendular action

What rotor design is pendulum action most prevalent in?

Semi-Rigid

What is the purpose of the vertical stabilizer?

Contributor to the directional stability of a conventionally designed helicopter.

It can also assist in stabilizing the yaw attitude in forward flight in the event of a loss of tail rotor thrust.

It can provide anti-torque, which is proportional to the square of velocity

What is weathercocking?

The tendency of a helicopter to yaw into the wind while in a hover or air taxi.

What does the vertical stabilizer do in sidewards/rearwards flight?

The vertical fin tends to make the helicopter weathercock into the wind.

What does the vertical stabilizer do in a crosswind or tailwind?

In a crosswind or tailwind, the vertical fin gives the helicopter negative stability with respect to yaw.

What does the horizontal stabilizer do?

The horizontal stabilizer gives the helicopter a more level attitude from a hover up to normal cruise airspeed

• negatively cambered (which creates lift vectored downward) IOT pitch the nose up during fwd movement

• This counteracts the nose pitching down due to the cyclic input that moves the A/c fwd

What happens to AOA in the Linear Blade Diagram if you increase linear velocity in Power on flight?

AOA increases

Which direction does the nose yaw after Engine Failure in Cruise Flight?

Left

What is the definition of blowback?

The tendency of the rotor to pitch up with an increase in forward speed due to dissymmetry of lift combined with phase lag.

What does total drag consist of?

Parasite Drag

Induced Drag

Profile

What is Power Required?

The amount of power necessary to keep the rotor turning within normal limits while achieving specified performance

• such as:

  • level flight,

  • climbing flight,

  • hovering out of ground effect,

  • etc

When is Power Required the highest?

In a hover or VNe

What assumptions is Power Required based on?

1) A specific density altitude

2) A specified gross weight

3) Level flight

4) 100% rotor speed

5) Operating out of ground effect.

What is Power Available?

The maximum amount of power the pilot can use, as limited by either the engine or the drive system.

What does Power Available vary with?

It can vary with duration, as;

• continuous power available,

• time limited power available,

• and contingency power available.

How does Density Altitude affect Power Available?

As DA increases, Power available decreases.

What is Excess Power?

• Engine power above that which is required for a given airspeed

  • i.e. “At any given A/s, Power Available minus Power Req = Excess Power”

• Can be expressed as:

  • Ratio of Power Available to Power Required (<1 means Power Req exceeds Power Avail.)

  • Difference between Pwr Avail and Pwr Req (negative # means Pwer Req exceeds Pwr Avail)

• Allows the pilot to climb, accelerate, maneuver, or take on more weight in the form of fuel, cargo, ordnance, or passengers

What does Excess Power allow you to do?

Climb

Accelerate

Maneuver

Take on more weight (Fuel, Cargo, Ordnance, Passengers)

What is Bucket Speed?

The lowest point on the Power Required curve

V_emax = Max Endurance A/s = “min power required A/s”

What is Bucket Speed also known as?

Max Endurance

Min Rate of Descent in auto

Max Rate of Climb

How does increasing collective affect torque?

Torque Increase

How does left pedal affect torque?

Torque Increases

How does right pedal affect torque?

Torque decreases

What is the Mechanical Axis?

The extension of the centerline of the rotor mast

How does rotor thrust relate to vertical axis?

Rotor Thrust acts perpendicular through the vertical axis

How does the virtual axis relate to the TPP?

The virtual axis always acts perpendicular to the TPP

If you increase torque, which way does the helicopter yaw?

Right

What is torque effect?

The yaw reaction of the fuselage to the torque applied to the main rotor.

If the rotor turns counterclockwise when viewed from above, the torque effect would cause the fuselage to turn in a clockwise direction (right yaw)

Which way is torque effect in conventional U.S. helicopters?

Yaws right

What is translating tendency?

Tendency for a helicopter to translate laterally due to tail rotor thrust.

How do you counteract torque effect in conventional U.S helicopters?

Left pedal

How do you counteract translating tendency in conventional U.S helicopters?

Left cyclic

What does translating tendency cause in conventional U.S helicopters?

right drift, that is corrected for by cyclic input, and thus causes a left leaning attitude

What does the horizontal stabilizer do to control yaw?

Nothing

What is Loss of Tail Rotor Authority?

Occurs when torque being produced by the engine(s) is greater than the anti-torque capability of the tail rotor and results in an uncommanded right yaw

What is loss of Tail Rotor Effectiveness (LTE)?

A sudden reduction in tail rotor efficiency that occurs in hovering or low speed flight when wind interacts with the aircraft from certain angles

What is the biggest difference between LTA & LTE?

LTE is caused by wind interaction with aircraft

LTA is caused by a power deficiency.

What does LTE cause in US Helicopters?

Uncommanded right yaw

What are 5 potential causes of LTE?

1. Main Rotor Disk Vortex Interference

2. Tail Rotor Vortex Ring State

3. Weathercock Stability

4. Tail Rotor AOA Reduction

5. Loss of Translational Lift

What wind azimuth (relative to the nose) does Main Rotor Disk Vortex Interference cause LTE?

285-315 Degrees (left quartering wind)

What wind azimuth (relative to the nose) does Tail Rotor VRS cause LTE?

210-330 Degrees (Left XW)

What wind azimuth (relative to the nose) does Weathercock Instability cause LTE?

120-240 Degrees (Tail Wind)

What wind azimuth region causes LTE due to Tail Rotor AOA Reduction?

060-120 Degrees (right XW)

What angle of the azimuth does Loss of Translation Lift cause LTE?

All Azimuths

How does Blowback affect LTE?

Nothing (Trick question)

How does ground effect work?

Decreases induced velocity within one rotor diameter

What type of surface maximizes ground effect?

Smooth & Hard

Not grass

What is Translational Lift? What airspeed does it continue up until?

The increase in rotor efficiency due to any horizontal airflow across the rotor system (aircraft movement or winds).

This effect starts @ 1 kt and continues up to Maximum Endurance Airspeed (Vemax).

What is Effective Translational Lift?

• The pronounced increase in lift during transition to forward flight due to the rotor disk experiencing a significantly decreased induced airflow

  • The flow of air through the rotor disk is more horizontal

  • reduced induced velocity and induced drag

  • increase in angle of attack and lift.

• the rotor disk completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air.

• 13-24 kts

When does ETL typically occur?

13-24 kts

What is translational thrust?

Translational thrust occurs when the tail rotor becomes more aerodynamically efficient during the transition from hover to forward flight.

What is the biggest benefit of translational thrust?

Leaves more power to the main rotor

What counteracts Dissymmetry of Lift?

Flapping

What is Dissymmetry of lift?

The asymmetric lift between the advancing and retreating halves of the rotor disk due to airflow across the entire rotor disk.

• The advancing blade sees higher linear velocity and therefore higher lift

• while the retreating blade sees lower linear velocity and lower lift.

How does the advancing blade behave in dissymmetry of lift?

It increases lift & flaps up

What is Phase Lag?

The angular difference between:

• the point where a cyclic pitch change is made on the rotor disk,

• and the point where the maximum flapping results from the change.

To tilt the TPP left, what blade pitch angle increases and which one decreases?

12'o Blade Pitch Angle decreases

6'o Blade Pitch Angle increases

To tilt the TPP right, what blade pitch angle increases and which one decreases?

12'o Blade Pitch Angle increases

6'o Blade Pitch Angle decrease

What happens as a blade pitches up?

As blade pitch increases, AOA increases, Lift increases.

As that blade cones up, the CG moves closer to the rotor hub.

Thus, blade tends to speed up due to the Law of Conservation of Angular Momentum.

What is geometric imbalance?

A shift of the rotor CG away from the mechanical axis due to aerodynamic effects of excessive flapping or hunting, or mechanical failure (binding) of rotor hinges or dampers.

What can geometric imbalance result in?

Moderate to severe one-to-one lateral vibrations from the rotor.

What is transverse flow effect?

The slight right roll and airframe vibration, which occurs as the helicopter, accelerates through 12-20 kts.

It results when the forward half of the rotor disk is leaving the vertical column of air, and experiences a drop in induced velocity and induced drag compared to the back half of the disk.

What is the result of transverse flow effect and how do you counter it?

Slight right roll

Left cyclic

What causes blowback?

Dissymmetry of lift & phase lag

When does blowback occur?

• Hover to forward flight (i.e. transition to fwd flight)

• Any time airspeed increases

Def of blowback:

• The tendency of the rotor to pitch up with an increase in forward speed due to dissymmetry of lift combined with phase lag.

What is geometric blade twist?

• Geometric twist - improves rotor blade efficiency by adjusting blade pitch to account for the increase in rotational velocity from root to tip.

• Beginning with an increased angle of incidence (i.e. pitch angle) at the root of the rotor blade, which decreases from the root to the tip.

• Geometric twist helps to distribute lift more equally across the rotor blade.

What is a retreating blade stall?

• A high-speed flight phenomenon in which the AOA on the retreating blade stalls at the tip, progressing inward and to the rear of the rotor disk.

• an aggravated case of dissymmetry of lift where flapping of the retreating rotor blade forces the AOA past stall

What causes retreating blade stall?

High Pitch & Downward Flapping

Aggravated case of dissymmetry of lift where flapping cases retreating blade’s AOA past stall

How to recover from retreating blade stall?

1. Decrease the severity of the maneuver.

2. Decrease collective pitch.

3. Reduce airspeed.

4. Descend to a lower altitude, terrain permitting.

5. Increase rotor rpm

Should you increase or decrease rotor rpm to recover from a retreating blade stall?

Increase

What is VRS?

A powered flight phenomenon characterized by an uncommanded descent as the helicopter settles into its own downwash

What must be true for Vortex Ring State to occur?

1. Airspeed at or below ETL (approximately 30 KIAS or less)

2. Excessive rate of descent

3. Power must be applied to the aircraft (rotors generating thrust)

Does VRS cause right yaw?

No

How to recover from VRS using the forward lateral displacement method?

Lower collective

Fwd cyclic

Adjust the flight controls after the helicopter passes through translational lift and its own downwash

How to recover from VRS using the lateral displacement method (Vuichard Recovery)?

Raise collective to max continuous power (MCP)

Enter sideslip with left pedal/right cyclic

Maintain HDG with pedals and increase AOB to 15-20 degrees

As the helicopter exits its downwash and starts to climb, adjust flight controls to forward flight