Fundamentals
The blade as a rotating aerofoil, blade angle and twist, geometric/effective pitch and slip, and the forces on a blade.
Fundamentals, summary notes
- A propeller is a set of rotating aerofoils (blades) that converts engine torque and power into thrust, each blade section produces lift (thrust) and an accompanying drag that the engine must overcome as torque.
- The blade angle (pitch) is the angle between the blade chord and the plane of rotation. It is greatest at the root and smallest at the tip (blade twist/washout) because the tip travels much faster than the root, the twist keeps the angle of attack roughly constant along the blade.
- Geometric pitch is the theoretical distance the propeller would advance in one revolution; effective pitch is how far it actually advances; the difference is SLIP (the blade needs an angle of attack to make thrust, just as a wing does).
- The blade's angle of attack is the blade angle minus the helix (advance) angle, which is set by the ratio of forward speed to rotational speed. On a fixed-pitch propeller the angle of attack therefore changes with airspeed and RPM.
- Blades carry large loads: thrust bending, torque bending, centrifugal tension, an aerodynamic twisting moment (tends to INCREASE pitch) and a centrifugal twisting moment (tends to DECREASE pitch), these are exploited by pitch-change mechanisms.
- ⚠ Exam trap: blade twist (angle decreasing root-to-tip) exists because the fast-moving tip needs a smaller blade angle to hold the same angle of attack as the slow root; slip = geometric pitch − effective pitch.
- Slip
- slip = geometric pitch − effective pitch
- Blade angle of attack
- AoA = blade angle − helix (advance) angle
Why is a propeller blade twisted so its angle is large at the root and small at the tip?
Every part of the blade advances forward at the same speed, but the tip moves through a far greater circle each revolution than the root, so the tip's rotational speed (and thus its helix angle) is much higher. To keep a roughly constant, efficient angle of attack all along the blade, the blade angle must be reduced from root to tip, the built-in twist.
Blade twist & angle of attack
Forces on a propeller blade
Fundamentals concept map
Fundamentals
Fundamentals quiz
Fundamentals, quiz
1. Blade twist (washout) along a propeller blade is needed because:
The tip travels faster than the root, so needs a smaller blade angleThe root travels faster than the tipThe blade is made of two materials2. Geometric pitch is:
The distance a propeller would advance in one turn with no slipThe distance it actually advancesThe blade width3. Propeller slip is the difference between:
Geometric pitch and effective pitchBlade angle and camberTip speed and root speed4. The blade angle of a propeller is normally quoted at:
The 75% radius stationThe blade rootThe very tip5. On the propeller blade section shown, the angle marked P lies between the plane of rotation and the chord line. It is the:
Blade angleAngle of attackHelix angle6. On the same section, the angle marked Q lies between the relative airflow and the chord line. In the cruise it is:
The angle of attack, which falls as forward speed risesThe blade angle, which is fixed by the hubThe angle of incidence, set on rigging7. A propeller converts engine power into:
ThrustElectrical powerHeat only8. Blade angle (pitch) is measured between the chord and the:
Plane of rotationRelative airflowFuselage axis