This document discusses airfoil and rotor blade terminology. It defines symmetrical and nonsymmetrical airfoils and their characteristics. It also defines the angles of incidence, attack, and describes how collective and cyclic feathering changes these angles to control the helicopter. Flapping, lead, and lag are also summarized as important motions of the rotor blades that help control the aircraft.

1. Airfoil Terminology
Span Center of Pressure
Upper Chamber Leading Edge
Mean Chamber Line
Chord Line
Lower Chamber
Trailing Edge

2. Types of Airfoils
•Equal chamber on each side
Symmetrical •Each half mirror image of other
•Mean chamber line and chord line are coincidental
•Produces zero lift at zero angle of attack
•Constant center of pressure with varying angles of
attack
Nonsymmetrical •Greater curvature above the chord line then below
•Chord and chamber line are not coincidental
•Produces useful lift even at negative angles of attack
•Produces more lift at a given angle of attack than
symmetrical
•Better stall characteristics than symmetrical
•Good lift to drag ratio
•Limited to low relative wind velocity, <300 knots
•Excessive center of pressure travel up to 20% of chord line

3. Airfoil (Rotor Blade) Angles
Angle of Incidence
(pitch angle)
rd Line
C ho
Tip Path Plane
The mechanical angle between the chord line of the airfoil
and the plane of rotation of the rotor (tip path plane).
Changed by collective and cyclic feathering. Any change in
the angle of incidence changes the angle of attack.

4. Airfoil (Rotor Blade) Angles
Angle of Attack
(aerodynamic angle)
ine
rd L
Cho Resultant R
W
Induced Flow
Tip Path Plane
The acute angle formed between the chord line of an airfoil
and the resultant relative wind. As an aerodynamic angle the
angle of attack can change with no apparent change in
angle of incidence.

5. 6° Angle of Attack 12° Angle of Attack
18° Angle of Attack 24° Angle of Attack
CL Max Stall

6. Questions?

7. Enabling Learning Objective #5
From memory, the student will identify, by writing or
selecting from a list, the principles of cyclic and
collective feathering and the importance of rotary-
wing flight, the significance of blade flapping and the
significance of blade hunting and the forces
involved with hunting IAW FM 1-203

8. Rotational Airflow
(no forward movement)
Tip Speed
700 FPS
Circular movement of the rotor blades...
...Produces basic rotational relative wind.
Tip Speed Maximum speed is at the tip of the blade
700 FPS and decreases uniformly to the hub

9. Feathering
Feathering is the rotation of the blade about its
span-wise axis
•Feathering can be uniform throughout the rotor through
collective inputs.
•Feathering can be adjusted differentially through cyclic
manipulation
Lets look at some examples of feathering...

10. Collective Feathering
• The changing of the angle of incidence equally and in the
same direction on all of the rotor blades simultaneously
• Changes the angle of attack, which changes the
coeffiecient of lift, which changes the overall lift of the rotor
+
+
+
+

11. Cyclic Feathering
Differential change in angle of incidence around the rotor
•Fore or aft cyclic movements result in changes in angle of
incidence at the 3 and 9 o’clock positions around the rotor
•Lateral cyclic movements result in the angle of incidence
changing at the 12 and 6 o’clock positions around the rotor

12. Forward cyclic inputs
+ -
A forward cyclic input increases pitch angle at the 9 o’clock
position, and decreases it at the 3 o’clock position. Due to
phase lag, the greatest upflap occurs at the 6 o’clock
position. Total aerodynamic force inclines forward.

13. Aft cyclic inputs
-
+
An aft cyclic input increases in the pitch of the blade at the
3 o’clock position while decreasing it at the 9 o’clock position.
Due to phase lag, the highest upflap occurs at the 12 o’clock
position. Total aerodynamic force inclines to the rear.

14. Lateral Cyclic Inputs
-
+
Lateral cyclic inputs change the pitch angle at the 12 o’clock
and 6 o’clock position. Due to phase lag those changes are
manifested in the rotor system 90 degrees later. The resulting
rotor attitude change causes the helicopter to move in the
desired direction

15. Flapping
Flapping is the up and down movement of the rotor blades
about a flapping hinge (or flexible hub)
•Blades flap in response to changes in lift caused by
changes in velocity of the relative wind across the airfoil, or
by cyclic feathering
•No flapping occurs when the tip path plane is perpendicular
to the mast
Contributions
•Helps prevent dyssemmetry of lift
•Allows the rotor system to tilt in the desired direction in
response to cyclic inputs

16. Lead and Lag
Rotor blades in an articulated system lead ahead
and lag behind their normal position in the rotor
system
Causes
•Angle of attack changes and drag forces
•Coriolis force, or the change in the relative
center of gravity along the span of the blade

17. Sequence when blade flaps up
Blade CG
R2
R1
As the center of gravity moves inboard, a smaller radius of travel is
produced. This causes the advancing blade to speed up or hunt. A vertical
hinge pin (articulated rotor) allows the blade to sweep forward and
absorbs stress that would otherwise be transmitted to the blade.