Engine operations during normal flight may be straightforward, but what goes on when an engine is pushed to its limits? We asked air show pilot Bill Stein (www.billsteinairshows.com) to analyze performance of his Zivko Edge 540’s Lycoming IO-540 as he spins and tumbles his way through his high-energy aerobatic sequence.
As soon as the airboss calls me into the show box, I twist the prop control forward until I show 2,900 rpm. Smoke goes on and I start a dive from 2,000 feet AGL, accelerating as close to 300 mph as possible. Nearing the deck, my first pull is about 10 G’s, and after eight vertical rolls and a 3,000-foot up-line, I’m stopped and ready for a hammerhead entry into a knife-edge spin. I kick the rudder and push the stick forward, beginning the sequence’s most uncomfortable maneuver for me and my pumped-up Lycoming AEIO-540 D4A5. Rotating knife-edge once per second while descending 10,000 fpm and sustaining -5 G’s, I can’t turn my head very far, because if I do, and if my head remains attached to my body, I won’t be able to turn it back to see the panel until I recover from the spin. So instead, I focus on the altimeter and oil-pressure gauge (barely peeking sideways at the ground rushing up). Almost immediately, oil pressure drops from 70 to 35 psi, and then it descends slowly. When I’m at 1,700 feet AGL, or if oil pressure dips below 20 psi, I recover from this insanity with full right rudder, neutral elevator and some left aileron—suddenly my Edge 540 has violently whipped around and I’m in a nice, rolling vertical down-line, accelerating back to 250 mph. Back at the deck again, I pull level and quickly glance at the engine instruments. Oil temperature is about 210 degrees; CHTs have exceeded 400 degrees (because of reduced airflow in the previous maneuver); oil pressure is back to the high 60s psi; and I’m burning 100 LL at around 38 gph. It’s 45 seconds and one figure into my show sequence, so I take a moment to relax…10 minutes and 26 figures to go!
Throughout the remainder of the sequence, my engine encounters a predicable set of challenges. An inverted flat spin will slow the engine down to 2,400 rpm, and oil temp and CHT rise due to lack of airflow through the cowl. My spiraling tower concludes with an upright flat spin, and the engine bogs down again, but there’s no drop in oil pressure. Many people assume that the end-over-end tumbles are the most violent figure I fly, but inside the cockpit it’s relatively smooth and I rarely exceed -2 G’s. However, tumbles do exert a huge side load on the prop and the airframe, and the engine moves around significantly. (I once had to relocate a cowl attach screw after the engine was pushed so far to the left that the starter ring teeth had not only eaten up the screw, but had also gotten close enough to the cowl to destroy the nut plate as well.)
Of any figure, I’m told that snap rolls put the biggest load on the crank, due to the rapid, simultaneous pitch change and yaw motion. When the stick is yanked back and the rudder is stomped on, the spinning propeller (attached to the engine crank) gyroscopically resists these position changes. Snap rolls are always a part of my sequence, but they’re also great fillers, especially on vertical lines, so I usually sprinkle a dozen or so inside and outside snaps throughout a show.
Most pilots are concerned with shock cooling; I’m one of them, except when I’m flying at a show. Toward the end of my sequence, I fly an inverted low pass, push for an outside half loop to a couple of revolutions of a centrifuge and then park the airplane midair for a harrier. It was during this that I obtained my highest CHT readings to date. In a harrier, the airplane hangs on the prop at a very high deck angle, and it feels like I’m trying to balance while standing on top of a basketball. With a good setup, the airplane can stay parked in one spot for a long time—so long, in fact, that most of my attention is devoted to CHT, and I’ll exit the harrier as the temps reach 430 to 450 degrees.
Only two figures remain in my sequence after the harrier, so I often land with a very hot engine. During practice, I have the luxury of slowly cooling engine temperatures before entering the pattern, but air shows mean entertainment, so in front of the crowds I must get on the ground as soon as possible to keep the show rolling. It’s this kind of engine abuse that explains my engine rebuild and overhaul schedule of every 500 to 700 hours.
During a performance, my airplane and I both take a lot of punishment. But that’s only the tip of the iceberg—because I treat every practice like it’s an air show, this scenario repeats at least 300 times per year. I’m glad that I fly the best aerobatic airplane built, equipped with the best aerobatic engine from Lycoming and the best ignition system from Unison. I’m often asked what the riskiest thing is that I do in the Edge, and my reply is flying cross-country from show to show. Every time I look down at the Sierra Nevada range or the Rockies and all I see are trees and cliffs with no place to land, I think about the trust and confidence that I have in my airplane and especially my engine.