Aerodynamics CH-47D

  1. Describe the three types of drag?
    1) Induced – Drag produced from induced flow. Greatest at a hover reduces with airspeed.

    2) Parasite – Resistance of air flow from non-lifting surfaces. Nothing at a hover increases greatly with airspeed.

    3) Profile – Parasite drag of the rotor system. Relatively constant until onset of retreating blade stall.
  2. What is Total Aerodynamic Force (TAF)?
    A vector that represents a force on an airfoil. Air flow on an airfoil generates a pressure differential between upper and lower half producing lift, but also drag. TAF is a vector representing the sum of the lift minus drag. It acts on the center of pressure and is normally inclined up and to the rear. Drag is parallel and opposite the RRW and Lift is perpendicular to RRW.
  3. What is the Angle of Incidence?
    An Angle at which the rotor disk is inclined to the helicopter’s longitudinal axis. Through the use of Longitudinal Cyclic Trim, this is adjustable in a CH-47.
  4. Explain induced flow?
    Air moving across the rotor system is deflected downward ( a change is induced in the direction of the airflow (Hence the term “induced flow”). As the airfoil moves through the relative wind it induces the air to flow downward yielding a Resultant Relative Wind.
  5. Explain Airflow at a hover (IGE/OGE)?
    • In ground effect (IGE) – Less power required due to the
    • reduced velocity of induced airflow and less rotor tip vortices as
    • as the airflow is obstructed by the ground and moved down and out.

    • Out of ground effect (OGE) – More power is required to
    • To produce lift due to the higher velocity of induced flow and
    • rotor tip vortices which disrupt lift production of the blades.
  6. What is the slope limit for CH47D?
    Pilot discretion, no slope limits.
  7. Explain Dynamic Roll?
    Helicopter rolls over when there is a pivot point, rolling motion, and the critical roll over angle exceeded. Correct with moderate smooth collective reduction.
  8. What is Torque and how does it affect the CH-47?
    Tendency of the fuselage to rotate opposite the rotor blades (Action and reaction) CH-47 rotors turn in opposite direction to eliminate torque effect.
  9. Explain Dissymmetry of lift? Definition, Compensated for, Tandem rotor considerations.
    Definition: The difference in lift produced by the advancing and retreating blades. The advancing blade produces has higher relative wind and produces more lift.

    Compensated for: Blade flapping (low airspeed) / LCT cyclic feather (high airspeed)Tandem Rotor Considerations: LCT’s are provided to accomplish cyclic feathering at designated airspeed (70 knots). This allows for increased airspeed and level fuselage attitude at high airspeed. If the cyclic-feathering system fails to properly feather the blades at higher airspeeds, greater blade-flapping angles and nose-low flight attitudes induce bending forces on the rotor-driving mechanisms.
  10. Explain blade flapping and cyclic feathering?
    1: Blade flapping – blades flap to maintain rotor equilibrium. Max up flap occurs in the advancing blade at the 3 o’clock position manifested 90 degrees later (gyroscopic procession) over the nose. Max down flap occurs in the retreating blade at the 9 o’ clock position manifested 90 degrees later over the tail.

    2: Cyclic feathering – For FWD flight the lowest point in the rotor system must be over the nose however blade flapping affects this. The CH-47 helicopter utilizes differential collective to rotate the helicopter about the pitch axis. LCT’s based on AFCS computer input; rotate the swash plates at different points in the forward and aft rotor systems to accomplish cyclic feathering in the pitch axis.
  11. Q: Explain Retreating blade stall? (Definition, Indications, Conducive conditions, Corrective actions)
    Definition – Dissymmetry of lift produces a differential in the amount of lift produced in the advancing and retreating blades. The angle of attack in the retreating blade is increased to produce the same lift as the advancing blade with airspeed the no lift areas of the blade increase. When this critical angle of attack is exceeded the blade stalls.

    • Conducive Conditions - Corrective action -
    • - Turbulent air - Increasing RPM
    • - Low rotor RPM - Reducing power
    • - High gross weight - Reducing airspeed
    • - High density altitude - Reducing the severity of the maneuver
    • - Steep or abrupt turns - Reduce Altitude
    • - Trim
  12. Explain Settling with power? (Definition, Indications, Conducive conditions, Corrective actions)
    Definition – Settling with power is a condition where the helicopter settles in its own downwash.

    • Conducive Conditions -
    • - 300’ or more vertical or near descent
    • - Low forward airspeed below ETL
    • - Using 20 to 100 percent of power

    • When it can occur-
    • - Steep approaches >30°
    • - Mask / unmask operations
    • - Downwind approaches
    • - Formation approaches/ takeoffs

    • Corrective action –
    • - Lateral cyclic / pedal inputs to make the transition to directional flight. Fore and aft cyclic may aggravate the situation
  13. Explain Ground Resonance? (Define, Conducive Conditions, and Corrective Action)
    Occurs in fully articulated rotor systems, most common in three bladed helicopters with wheels. When one wheel strikes the ground ahead of the others a shock can be sent through the fuselage to the rotor system causing a rotor to displace out of angular balance. When the next wheel contacts if a similar shock is sent a self generating resonance can occur ripping apart the fuselage.

    • Conducive conditions-
    • - Bouncing landing
    • - Ground taxi over rough terrain
    • - Hard one wheel landing
    • - Defective dampeners or struts

    • Corrective Actions –
    • RPM in normal range: Take off to hover and land somewhere else.
    • RPM below normal range: Reduce power, use rotor brake if installed.
  14. What is Differential Collective Pitch?
    Equal but opposite input to the forward and aft rotors.
  15. How does the CH-47 move forward?
    Through differential collective pitch, forward cyclic decreases thrust of the forward rotor and increases thrust of the aft rotor.
  16. Explain what action occurs with each pedal and cyclic input?
    Pedal inputs - cause the forward rotor to tilt the specified direction and the aft the opposite. Nose always moves in direction of the pedal.

    Cyclic inputs – cause both rotors to tilt the specified direction.
  17. How do you turn around center cargo hook?
    Left or right pedal input cause differential lateral tilting. I.E. Left pedal input tilts the forward rotor to the left and the aft to the right equally.
  18. What does left pedal and left cyclic do?
    Left pedal tilts the FWD rotor left and the AFT right. Left cyclic tilts the FWD rotor more to the left and the AFT back to level causing a left turn around the tail of the aircraft.
  19. What does left pedal and right cyclic do?
    Left pedal tilts the FWD rotor left and the AFT right. Right cyclic tilts the FWD rotor back to level and the AFT more to the right causing a right turn around the nose of the aircraft.
  20. What is translating tendency and what happens in the CH-47?
    A tendency of a single rotor helicopter to drift laterally to the right. Tandem rotor helicopters that have counter-rotating rotor systems do not exhibit the lateral thrusting tendency of single rotor helicopters.
  21. Explain Transverse Flow?
    The difference in the lift produced by the forward and aft portions of the rotor. This occurs at max during10-20 knots. As airspeed increases the airflow through the forward half of the rotor becomes more horizontal creating less induced flow, a greater angle of attack, and more lift then the rear half. Due to gyroscopic possession these tendencies are manifest 90 degrees later causing a right rolling motion. Tandem Rotor systems counteract this rolling motion. Causes a vibration during approaches and takeoffs.
  22. Define Gyroscopic Procession and how it is accounted for in the CH-47?
    1) It's a phenomenon occurring in rotating bodies in which a force applied manifests itself 90 degrees after (in the direction of rotation) the point in which the force was applied.

    2) The swash plates apply a force 45 degrees ahead of the intended point and the offset of the pitch horns applies the remaining 45 degrees, for a total of 90 degrees.
Card Set
Aerodynamics CH-47D
Aerodynamics for Army aviation CH47-D