Wednesday, January 24, 2007

Looping around.

Diving for 140, I pulled on a stick and the little Citabria shot into the skies. Reaching vertical I pulled a bit harder and then released some pressure to coast over the top of the loop. Except I released too much and suddenly my instructor and I were flying inverted, suspended in the air hanging from our harnesses, as all the dust and mud that accumulated in the plane over the last few weeks showered down from what used to be the floor generously coating out heads in the process.

I pulled on a stick again and soon we were getting re-acquainted with positive gravity as all the dust and mud made the reverse trek from the ceiling and back to the floor coating our heads ones more for a good measure.

We spoke almost simultaneously: “I guess I released the stick too much” and “Hmm, I guess I should have vacuumed the plane”. This was my second acro lesson and we were working on loops and hammerheads.

Loop in a basic acro trainer is an exercise in energy management. In my few hours of practice since that lesson, I discovered that it was very easy to fly a loop, but it was hard to fly a perfectly round one with no loss of altitude.

The 7CAB Citabria that I fly has a 150 hp engine which is a bit more powerful compared to other Citabria models, but still not powerful enough for doing acro maneuvers from straight and level flight, thus the requirement to dive to get speed (energy).

At the speed of 140 mph (recommended entry speed), you commence a pull up. As you pulling up, you are converting the energy into altitude. The trick here is not to run out of energy before reaching the max altitude at the top of the loop, otherwise, the plane might stall on a way up. 3.5 to 4 Gs in the pull up usually do the trick in the Citabria. You are supposed to relax the pull a bit over the top in order to keep the loop round – keeping the same pull pressure will result in a weird-shaped loop. Relaxing the pressure too much and/or pushing on a stick will transition you into an inverted flight and could lead to an inverted, if not corrected right away.

At the top, the altitude is highest and the energy is the lowest and then it is all reversed with plane gaining energy as it is loosing altitude. You have to pull up almost the same amount of Gs as you used on the first half, because it you do not you’ll end up lower than your starting point.

The above goes half way to answering the question that bugged since my intro to aerobatics flights - why would the airplane not stall and fall out of the skies when it is upside down? As I discovered, it can and would stall and fall out of the skies if you run out of energy before the top part of the loop or pushed the stick too much forward at the top of the loop. However, in the properly flown loop, the relative wind has time to catch up with the change of direction and therefore the critical angle of attack is never exceeded through any part of the maneuver. Likewise, it is possible to stall the plane at any speed and in any position, but that’s a whole separate topic.

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