Friday, February 1, 2019
“The Front Office” answers questions about jump pilots and piloting. You’ll learn what pilots do behind the scenes to make your favorite time of week happen, and you’ll get a one-of-a-kind view from the one seat in the airplane you never get to be in.
The last installment of “The Front Office” (December 2018 Parachutist) discussed airplane stalls and how they develop. This month’s installment expands on the concepts covered in that column—angle of attack, the wing’s chord line, the relative wind and what happens when the angle of attack exceeds its critical limit—and examines what happens after a stall.
When an airplane is fully stalled, the angle of attack is beyond the critical range, and the airplane’s wing is desperately seeking an opportunity to get back to flying again. It will try to turn, get its nose down and regain that precious airflow it needs. A few forces will also fight against this recovery.
The propeller (or propellers) is the enemy in this situation. In most single-engine planes (to simplify the example), the engine turns the propeller counterclockwise (clockwise from the pilot’s perspective). This causes several twisting and rotational pressures against the airplane that tend to become dominant once the wing no longer is there to resist them.
The first force is a torque reaction from the spinning weight of the prop, which causes the airplane to tend to roll counterclockwise (from the pilot’s perspective) against the prop. Another is gyroscopic precession, where the air pushing against the prop causes a force that yaws the airplane’s nose to the left. Still another is the result of the propeller creating a spiraling slipstream, which exerts pressure on the wing and the tail, pushing the nose of the airplane still farther left. And finally, P-factor causes the prop to pull harder forward on its descending blade than the rising side, which happens to be to the right. This pulls the airplane’s nose, yet again, to the left. If you guessed that the airplane will most likely turn left after it stalls and begins to spin, you get a gold star! (If the right wing happens to be lower or stall first, or the right side of the airplane is heavier due to cargo, etc., the airplane may still spin to the right, like in our illustration.)
Next, the wing that has the most stalled condition relative to the forces acting on the aircraft will drop aggressively down, causing the nose of the airplane to momentarily rise and then yaw in the same direction as that wing. This begins the incipient stage, and the airplane begins to fall to the earth rotating like a corkscrew with the nose down. The lower wing is stalled and the higher wing, in effect, regains a slight amount of lift during the rotation (since it is going forward, it gets some airflow back) causing it to fly around the lower wing that is still stalled.
The developed stage results from all of this turning and falling, and the airplane stabilizes in this rotating, freefalling state. The airplane can be descending as quickly as 10,000 feet per minute in this stage! The recovery stage is when the pilot applies corrective inputs to stop the airplane’s rotation, regains even and smooth airflow over both sides of the wing and eventually raises the nose (once the aircraft’s indicated airspeed reaches a suitable value).
The inputs, which the pilot can perform simultaneously, are:
Lower the nose by releasing aileron pressure or even letting go. Doing this will most likely lower the wing’s angle of attack and allow the plane to regain lift using the tail (which should have enough airflow during a spin to do so).
Use opposite rudder from the spin. If the airplane is spinning left, the pilot uses the right rudder (again, using the airflow over the tail to fight the spin).
Reduce throttle. This tends to eliminate the torque and turning tendencies described earlier (at the expense of some airflow over the tail), which are fighting against us.
Luckily, most DZs give their pilots an opportunity to enter stalls and find the full range of their aircraft’s normal flight profile (with flaps up and down) during initial training, while remaining safely away from this spinning state. Most jump aircraft are not approved for intentional spins, so pilots aren’t trained to perform them as a full maneuver. In fact, pilots in the U.S. aren’t even required to receive spin training unless they become an FAA Certified Flight Instructor.
Still, it is possible to enter a stall or spin in jump aircraft. With a full load of jumpers who may be hanging off the side of the airplane near the tail, the aircraft is near the lower end of its flight capabilities, heavy and at high altitude and has a very aft center of gravity. Your pilot is expected to follow the aircraft flight manual or pilot’s handbook, the DZ’s training profile with jumpers and the training they have received to avoid and escape from stalls. Encourage them to stick to those standards if you want a safe place to jump!
Chas Hines | C-41147
FAA Certified Flight Instructor and Airline Transport Pilot