9/23/07

While riding today, I was reminded how bicycling is like flying aerobatics.

I met the mountain bike crew, Glen, Tim and Larry at the Circle Beanery. I had my cyclocross bike, they all had mountain bikes. No problem, I planned to ride with them uphill into the forest and then find a way home that fit my bike. But Glen brought a car and they planned a loop on the other side of the forest. After some discussion, I settled on hitching a ride in Glen’s car to their start point then a solo ride home via Poison Oak Road, etc. I’d never been up Poison Oak on the cross bike, but it sounded doable. Just a gravel road uphill.

I used to be a pilot. I got my flying license in ’83 and bought a plane with some partners in ’86. It was a Citabria 7ECA. Citabria is airbatic spelled backwards, and after rebuilding the airplane and some thorough inspections one of my partners and I got into competitive aerobatics. It involved a lot of practice and a few meets a year. Aerobatics is a lot like gymnastics, where you’re judged on executing the maneuvers as viewed by the judges. It was a blast, a combination of physics and kinesthetics. Imagine a roller coaster that you control in real time. The Citabria was adequate for the second lowest (Sportsman) class of aerobatics which involved positive or zero G maneuvers. The plane’s low power to weight ratio, high drag and non inverted systems (the engine doesn’t produce power under zero or negative G) actually made aerobatics more challenging than in a more capable airplane like a Pitts or an Extra.

After I left the mountain bikers, I headed toward Poison Oak on Tampico then Sulfur Springs. It was all good until I got to the point where Poison Oak turned off the paved road. There was a gate and then a gravel road that headed steeply uphill. After going around the gate, my legs quickly reminded me of yesterday’s ride. Heading up Poison Oak, I ran into a bunch of limits all at once. I had my heart rate monitor and watched my heart rate quickly climb to my maximum for that day, which was limited by my recovery time since yesterday. The cardiovascular limit, combined with recovery time and fitness set my power limit, and weight and physics controlled my speed up the hill, about 4 miles per hour. It was close to the stability limit. The cross bike has a 34-28 gear ratio, so at the low rpm set by my power limit, I was just about leg strength limited. But the ultimate limit was the coefficient of friction as the Ritchey Speedmax 38C’s at 60 PSI began to slip. I was just able to keep riding up the hill, at the edge of control, heart rate at max, low rpm, pushing each pedal stroke as hard as I could without spinning the wheels in the gravel. A little steeper and I’d be walking.

That’s when I thought of flying a split-s. It’s an aerobatic maneuver where you do a half roll followed by a half loop. Sounds easy, but what’s really happening is you’re starting with potential plus kinetic energy and then using the control forces of the airplane to change direction and exchange potential for kinetic energy. In the Citabria, you should enter at 60 mph, exactly and full throttle. That’s the minimum airspeed to be able to control the half roll and you don’t want extra kinetic energy at the bottom. You’ll finish the half roll upside down at about 40 mph and 0 G’s. It’s quiet, you’re light in the seat and the world fills the skylight overhead as you pull the throttle smoothly to idle. You’re trying to make a perfect half circle when viewed from the ground, so you gradually pull more G’s as the speed builds up, in order to keep the radius constant. The rate at which you add G’s is crucial; if you pull too hard to soon, the maneuver will look like an egg rather than a circle, or if you pull way too hard you can stall; if you don’t pull hard enough you’ll build up too much speed and either exceed the maximum speed of the aircraft (the speed limit is not a law; it’s an aircraft specification; if it’s exceeded the control surfaces could flutter and then bad things happen like the wings or tail fall off). Another limit is the engine redline. The throttle is pulled all the way back to idle, but as the airspeed builds up the air drives the propeller and the engine faster and faster. You’re committed at the start of the maneuver; kinetic plus potential energy is set by airspeed and altitude at the entry. Things get exciting halfway through. At the half way point, you’re pointed straight down, pulling G’s so you’re forced into your seat, the speed (and noise) is building and the engine is revving faster even though the throttle is at idle. The speed is past the maneuvering limit (120 mph), which means that a hard pull on the stick can exceed the structural limits of the aircraft (and potentially remove the wings). You just continue to pull (not too much, and continue to coordinate rudder) and fly the balance of the maneuver. At the bottom you find out how good you’ve flown the maneuver; if it’s well done you’re at maximum G’s (5.0 on the Citabria), redline airspeed (162 mph) and redline on the engine (2750 RPM). If it’ not done well, either it was an egg shaped split-s or you’ve exceeded one of the redlines and increased the probability of something bad happening. The beginning kinetic and potential energy also set the amount of altitude lost, so you needed to start with enough of one and not too much of the other or you’ll make a hole in the ground. In the split-s you’re at all the limits at once. It’s just like being on the edge of blowing up on a steep hill, or pushing a slippery off-camber corner in cyclocross. Putting a foot down on a hill, or sliding out on a muddy slope in cross has a lot less consequences than going past the limits in aerobatics. That’s okay with me. Bicycling is better for your health.