P-factor and slippery airstrips
|PC-6 right wheel bogged in|
Before I answer that question, I’m going to try and explain the theory on why tailwheel aircraft pull left on take-off. P-factor is just one of the things causing this, however there are a couple of other forces at play which contribute, gyroscopic precession and engine torque. For the purposes of this article, we will assume a clockwise spinning propeller when viewed from the pilot’s seat.
The simplest way to understand this is to remember each propeller blade is an aerofoil and when spinning they’re moving through the air producing lift, just like the wings of an aircraft (abet that lift is actually thrust here acting along the length of the aircraft).
When on the ground, tailwheel aircraft have a naturally nose high attitude. The spinning propeller (prop disk) is mounted on the front of this up-pointing nose and thus is presented to the air at an angle. Because of this, the descending propeller blade (the one on the right when viewed from the cockpit) is moving through the air at a higher angle of attack than the ascending blade. That higher angle of attack means the descending blade produces more lift (forward thrust) than the ascending blade. That extra thrust on the right side of the aircraft causes it to yaw to the left. And the faster the propeller spins, the more extra thrust is produced on the right side and the more the aircraft pulls to the left.
Once the nose is level (i.e. when you’re flying), p-factor is zero as all blades are moving through the air at the same angle of attack relative to the airframe. This is why nose-wheel aircraft are not affected by p-factor as their propellers are usually not nose high when on the ground.
|Typical nose high attitude on the ground|
Simply put, the torque of the engine spinning the propeller clockwise acts along the lateral axis (nose to tail) to cause the whole airframe to twist in an anti-clockwise direction when viewed from the cockpit. In addition to this, the left wheel is pushed into the ground more than the right wheel causing more drag on that wheel which also causes the aircraft to pull to the left. The only way to compensate for this is opposite (i.e. right) aileron. However, until you’ve got the speed up, that will have no effect on things.
|PC-6’s long nose housing a 550SHP P&W PT6|
We all remember our gyro theory right? Probably not, so rather than going into all that dry theory I’ll try to explain it purely with relation to tailwheel aircraft. To understand how this affects the departure, imagine you’re sitting in the cockpit looking at the propeller disk spinning clockwise in-front of you. Now, gyroscopic precession dictates that a force applied to a spinning disk will act at 90 degrees in the direction of the rotation.
As the tail of the aircraft is raised before rotation, you are effectively applying a pushing force to the top of the propeller disk. This force moves through 90 degrees in a clockwise direction which is to the right side of the propeller disk. It’s this force that then yaws the aircraft to the left.
With the Porter, this is less of a problem as we tend to depart in a nose high attitude but still worth considering on departure. P-factor is by far the biggest force causing the leftwards tendency on departure.
|Lined up for departure at Idedua, Papua|
So, going back to the original question, what happens when departing from a short, very muddy, slopping airstrip with zero grip in a large, powerful taildragger?, in the case of Kegata it gets pretty hairy, very quickly is the answer!
|View looking up Kegata as the occasional cloud rolls across|
The weather here in Papua these past couple of weeks has been very wet with a lot of rain falling every day. As a result we’ve been forced to close a few airstrips as they’re just too dangerous to operate from when the ground is so wet. Kegata has just become the next one on the list until things dry out.
The landing at Kegata was uneventful although the turnaround resulted in getting stuck side on to the airstrip requiring half an hour of digging to un-stick the left tyre. As Kegata has a decent slope to it, that meant a lot of fuel slowly transferred from the right wing tank and into the left tank. The Porter has no way of preventing this as both wing tanks are interconnected via a collector tank.
|Digging the left wheel out|
The usual way of departing from a slippery airstrip in the Porter is to start with the nose pointing to the right, to help counteract the p-factor and engine torque. However, on this departure I didn’t have it pointing quite enough to the right, as I was soon to discover. As I started rolling and increased the power, the aircraft began to slide left not helped by the extra fuel in the left tank.
Now, there’s a split second window once you start rolling in which you have two choices, continue the take-off or abort. The later would have resulted in the aircraft going off the left side of the airstrip. Once rolling down the slope with zero braking action there is no way of stopping. So, with full right rudder, toe brake and right aileron we slide down the airstrip fully sideways as I increased the power enough to get us airborne but not so much as to pull the aircraft off the left side of the airstrip.
|Parked up at Kegata, Papua|
It was a major lesson in the effects of p-factor and engine torque. Chatting with one of Papua’s most experienced Porter pilots, he mentioned he usually starts with the nose pointing 20 degrees off to the right on really slippery slopes. That result’s in a fairly straight take-off roll, abet fully sideways, which in his words is “scary as hell!”. I’ll remember that next time!