Bill Crawford’s Flightlab Blog

I keep promising not to tell flying stories, but the spirit is weak. This month I want to write about the aerobatic maneuver called the hammerhead, yet am drawn by strange alien forces to the subject of swimming pools. There’s a lot you can tell about the tenor of a community by flying around and checking out its backyard swimming pools. If you know anything about pools—or ever had an ambivalent dominion over one, as I have—you know that the natural tendency of small, self-contained bodies of water is to become vile. Rather than signaling moral failure on the part of the man of the house, as my wife once suggested, a green, algae-rich pool actually reveals conformity with nature’s plan: that the fittest slime should take control. Obviously, pool water that’s transparent and sparkling means the presence of folks to whom nature’s wishes carry little weight. These are the same people who deny global warming. They believe the universe is 6,237 years old, at most, despite the fossils dug up when the pool was installed. More about swimming pools in a bit.

Last month I wrote about my first encounter with a Pitts S1S—a little trouble steering on the ground combined with a little trouble making the smoke system stop. Yet I grew to love that airplane, and more importantly, I learned to land it. Come to think of it, I destroyed my first prop in that Pitts, actually a loaner prop from my buddy Hans—a man of serene forbearance, fortunately. Plymouth Airport threatened to but never billed me for the taxi light that was also destroyed, a fact (assuming that a statute of limitations applies) I now feel comfortable flaunting in front of management.

Anyway, a Pitts does a fine hammerhead. In this maneuver, for reminders, you pull to the vertical, slow down almost to a stop, and then kick left rudder. The airplane yaws 180 degrees and heads back down toward the dirt. Before it hits the dirt, of course, you pull back to level flight. I’m going to reconstruct the most satisfying Pitts hammerhead I ever flew. But first a more specific description of how the maneuver is performed.

A hammerhead either requires lots of entry speed in order to gain altitude during the initial vertical line, or should be started high enough so that altitude is irrelevant. The problem is that you slow down to almost nothing at the top of the maneuver. So when you come back down, you have to generate sufficient airspeed to pull the Gs necessary for recovery. That means trading potential energy (altitude) for the kinetic kind (airspeed). An airshow pilot starting a hammerhead at low altitude and low airspeed might not be able to gain the altitude necessary for recovery. Pilots have died this way. My little S1S required an entry speed around 140 mph to drive the vertical line high enough so that recovery at or above entry altitude was guaranteed.

So now we are in the vertical line, with plenty of altitude beneath. As the aircraft slows, slipstream effects increase and cause the nose to yaw to the left. Consequently, a gradual increase in right rudder is needed to keep the flight path vertical. In fact, if we just left our feet flat on the floor, the aircraft would slowly yaw around in a lazy arc and head back down on its own.

At the same time, as we climb and lose airspeed, the torque generated by the prop becomes more apparent. The aircraft starts to roll to the left, against the prop’s rotation to the right. Right aileron may be necessary to counteract torque. If so, aileron deflection will have to increase as airspeed drops and the control surfaces become less effective.

Learning when to kick left rudder takes practice. We go to the left to take advantage of the leftward yawing tendency generated by the slipstream. Kick too soon and the aircraft flies around in an arc, with the pivot point somewhere beyond the wingtip. Wait too long and the aircraft will pivot around its center of gravity, while the center of gravity itself begins to sink. A pivot point somewhere within the span of the left wing usually gives a good-looking turn around. Aesthetics matter.

As the aircraft yaws around the top, the right wing goes faster than the left and tends to produce more lift. Compensating right aileron is necessary to keep the aircraft from rolling. Some of the rolling tendency is still due to torque effect, however, and if the ailerons are giving up the ghost it may be necessary to pull the power back to reduce the torque.

Now that we’ve kicked and are headed back down, the subject of swimming pools returns. My best ever hammerhead happened one hot, summer afternoon, when I pivoted around the top and headed vertically down exactly centered on a circular, above-ground swimming pool. “Dude, way cool!” I thought. This was over a sparsely settled, less fashionable outskirt of Taunton, Massachusetts—a town not fashionable to begin with. There was a mobile home, and cars and pickups scattered around the yard. Clearly a redneck enclave. Perfect!

In my experience, folks with above-ground pools and redneck landscaping invariably fit the airshow attendance profile. They’re what make airshows such classy affairs. This isn’t snobbishness; it’s simply the truth. So I gave them a four-point roll on the down line. On the first point, I must have gotten their attention. By the second point, I bet they were questioning the future. By the third point, at least four individuals could be seen climbing out of the pool and heading off in all directions. The fourth point probably went unobserved by the runners below. Aerobatics doesn’t get any better! In fact, life doesn’t get any better.

The regulations prohibit aerobatics over an open-air assembly of individuals. Do four excitable rednecks swilling Budweiser in an above-ground pool constitute an open-air assembly? Of course I recovered above 1,500 feet AGL, as required by FAA regulations. I will take a polygraph, if necessary, to confirm the entire story.