Cast your mind back to the last time you went flying. Poised at the threshold you performed a quick run through your final checks, then with a smooth application of power and a dab of rudder your craft surged forward and off into the wild blue yonder.
Now I’m willing to bet that, for most of us at least, applying the aforementioned dab of rudder is so hard-coded into our subconscious that we’re barely even aware that we do it. And if I asked you why you do it? Other than the obvious answer, “To keep the nose straight”, a hazy recollection of “propeller effects” may come to mind… It’s time to refresh our collective memories and talk about the P-factor.
At first glance most aeroplanes appear to be symmetrical, and when they are obviously not, such as in the Rutan Boomerang, they tend to attract a fair amount of attention. However, the reality is that, despite appearances, all single propeller aircraft are inherently asymmetrical. If you cut a plane in half along the centreline and place it against a mirror a major problem with the prop will be immediately apparent. Propellers are definitely not symmetrical. This leads to a whole host of propeller effects, requiring adjustments in both design and piloting technique to accommodate the quirks this asymmetry introduces.
Quite often all the “P-effects” get lumped together as a single thing, but they actually come from a whole variety of different sources so I’m going to separate them out and look at them individually, starting with why we have to apply rudder when we power up for take-off.
For those of us that fly relatively low powered aircraft – which is virtually all of us given the shortage of Rolls-Royce Merlin powered craft in the ultralight fleet – slipstream effect is the most noticeable of the prop effects. The airflow passing through the propeller is not just accelerated backwards but also develops a helical motion matching the direction of the propeller rotation, causing the air to flow around the fuselage in a corkscrew path. When this airflow arrives at the vertical stabiliser the resulting angle of attack generates a side force causing the aircraft to yaw, unless opposite rudder is applied to cancel the effect. This slipstream effect is strongest at high power settings and low speeds so is most noticeable during take-off and climb out. The pilot is simply expected to apply an appropriate amount of rudder to correct the yaw, but the designer does have the option of making the pilots life easier by offsetting the vertical tail to a slight angle; using a cambered aerofoil; or adding a fixed trim tab to the rudder to minimise the effect at cruise power and speeds.
At this point the more thoughtful (or argumentative!) of you will probably be thinking, “Hold on a minute! If the airflow hitting the fin makes the plane yaw, why doesn’t the same airflow hitting the two halves of the horizontal stabiliser cause the plane to roll?”
This is a very good question, and answer is, “It does”, or at least it would, if it wasn’t for the Torque Effect which is more powerful and acts in the opposite direction.
If the propeller is forcing the air to rotate clockwise our old friend Newton tells us the air must be reacting by trying to rotate the aeroplane anticlockwise – think of ‘cartoon physics’ where grabbing a propeller causes the whole aeroplane to spin in the opposite direction! Torque effect is constantly trying to make the aeroplane roll in the opposite direction to the propeller’s rotation, but it is usually easily counteracted with an application of opposite aileron. However, for aircraft with thousands of horsepower travelling slowly, torque effect can be lethal. During WWII there were several accidents attributed to pilots rapidly applying full-power for a go-around at low airspeeds only to have the torque-roll overpower the ailerons and flip the aircraft inverted with fatal consequences. Thankfully in an ultralight with 100hp or so the effect is much more manageable, to the point that it may not even be noticeable, being easily trimmed out or countered with a fixed tab on one of the ailerons.
Asymmetric Yaw Effect and Propeller Normal Force
Whenever the prop disk is not perpendicular to the incoming airflow, such as when an aircraft is operating at high angles of attack or when yawed, the relative motion of the blades and airflow will cause thrust to be generated asymmetricly. As an example, in a nose high attitude a downward travelling blade will also be moving forward and an upward travelling blade travelling rearward relative to the airflow. This affects both the airspeed and angle of attack of the blades as they rotate, with a downward travelling blade seeing increased airspeed and angle of attack and an upward travelling blade seeing reduced airspeed and angle of attack. This leads to two outcomes:
For a clockwise spinning prop the right hand side of the prop will generate increased thrust whilst the left hand side thrust will be reduced, this force imbalance results in a yaw to the left and is called the Asymmetric Yaw Effect.
Secondly, there will be a change in the direction of the force generated by the prop blades as they rotate. The downward travelling blade will incur more drag along with its increased lift whilst the upward travelling blade will incur less, when these two effects are combined the result is a net force acting upwards in the plane of the prop disk, known as the Propeller Normal Force. For tractor configured aircraft the propeller normal force will be destabilising – pitching up produces a force that induces further pitch up – for pusher aircraft where the prop is located behind the C of G the effect is opposite improving the stability. Either way the effect has implications for stability and needs to be considered when sizing tail surfaces.
Fig3 – The Propeller Normal Force is a result of the incoming airflow not being perpendicular to the propeller disk. (click for larger image)
Last of all, not all propeller effects are a result of aerodynamics. As a rapidly spinning mass a propeller also acts as a gyroscope and so, like all gyroscopes, it experiences precession when a torque is applied to tilt its axis of rotation. Precession takes an applied torque and precesses it 90 degrees in the direction of rotation of the propeller. Gyroscopic precession is far from intuitive, so I’ll give you an example:
When a tail-dragger with a conventional clockwise rotating propeller (as seen from the cockpit) raises its tail during the take-off run it pitches the nose down. Precession rotates this applied nose down torque 90 degrees in the direction of prop rotation, so as the tail comes up the aircraft also yaws left requiring the pilot to counter with rudder. Unfortunately this behaviour is a perfect recipe for a ground-loop. If the tail is raised quickly early in the take-off run (before there is enough airspeed for the rudder to become effective), the yaw can be more powerful than the rudder can counteract. Of course the same thing may happen at any time if the pilot happens to have slow feet!
Figure 4 – The propeller is a spinning mass and so acts as a gyroscope. (click for larger image)
So What Does It All Mean?
Fortunately for low powered aircraft with small lightweight propellers most of the effects just described are small in magnitude and can be easily trimmed out, that is assuming they are large enough to actually be noticed; even in the worst cases the controls are highly unlikely to be overpowered so all that is required in most situations is correct piloting technique.
As a pilot you don’t need to know in detail why you have to counter an application of power with rudder, but you do need to press with the correct foot at the right moment, so understanding prop effects will at least save you some embarrassment when you find yourself piloting a plane with an anticlockwise spinning prop for the first time!