Categories: all aviation Building a Biplane bicycle gadgets misc motorcycle theater

Sat, 23 Mar 2019

Crosswinds and Turning Stalls

Image by Chris Kennedy, used under CC BY-NA 2.0 license

I found myself with some free time a couple days ago, and decided to take Norbert the Champ up. There was an occluded front due in the afternoon, so I had to abandon my original plan to fly over to Port Townsend (0S9) for a late lunch. Instead, I decided to do some pattern work and possibly some turning-stall work in the practice area. I wanted to stay close to the field so any nasty weather that turned up wouldn't catch me away from home.

According to the new weather robot at Harvey Field (S43), the wind was blowing about 180-190°, and between 10 and 15 knots in gusts. The closest runway is 15L, so there was a bit of crosswind, but nothing terrible. The gusts made things more interesting, but fine practice for me -- I rarely get to take off and land with crosswinds and need all the practice I can get.

One of the members of my EAA chapter has been developing a pretty cool program aimed at experimental (homebuilt) aircraft, to determine and then correct low-speed stall characteristics. Specifically, he's worried about the base-to-final turn, which is the closest to the ground most pilots will ever turn, and thus the one most fraught with danger should anything go wrong. He recently lost a friend to a likely stall-spin accident on a base-to-final turn, so his idea has received fresh momentum.

Something he mentioned recently was that most pilots haven't explored their aircrafts' stall characteristics except the most basic straight-ahead power-on and power-off stalls. Stalls while turning can be very exciting, easily leading to a spin -- a condition which may be unrecoverable at low altitude, and a prolific killer of pilots in the beginning years of aviation. I realized that not only did I not know my plane's behavior in this condition, I'd never done a single turning stall in my entire flying career.

The setup for these stalls is exactly the same as normal stall practice, except the plane is turning. Add at least another thousand feed of altitude compared to normal stall practice, just in case. A spin can develop very quickly, and the extra space gives you a bit more breathing room to recover if it surprises you. It would be best to have experience recovering from spins and recognizing incipient spins before trying this yourself, but read this handy article on spin recovery at a bare minimum. I spent several hours doing spin recovery training with a CFI a few years ago, which makes me barely competent, but I felt safe enough to give turning stalls a try.

The first thing I tried was power-off turning stalls. I figured, correctly, that with less energy involved, things would be a bit calmer. So I set up for my normal descent to landing -- carb heat on, power to idle, and enter a 20-30° bank to turn from downwind to base. Since I didn't know exactly when the stall might happen, I put myself into a constant rate turn, kept the ball centered with the rudder, and pulled back on the stick. With the hand grip buried in my belly (I really need to get rid of that thing; the belly, not the handgrip) and maintaining a nearly 45° bank angle, the plane simply refused to stall. Just to eliminate the possibility that the 7EC Champ is more resistant to turning stalls to the left than to the right, I climbed back up to 4000 feet and tried again, this time circling to the right. Nope, no stall.

Surprised, I climbed back up to 4000 (the Champ didn't seem to be stalled, but it was definitely going down quickly, losing 700 feet in what felt like maybe 45 seconds) to try power-on turning stalls.

This time, I set up for a fairly unrealistic 45° bank coordinated turn at full power, and held the stick full back until I got a definite stall break. To my complete surprise, with the stall, the plane rolled sharply away from the direction of the turn, trying to roll into an opposite-direction turn and possibly stall/spin (I stopped it before it could develop). I tried in the other direction: same thing. Weird.

I haven't yet figured out the aerodynamics of what might be happening with the power-on turning stall, but I was interested to see that it also seems to happen with the 7AC Champ model in X-Plane. My understanding was that X-Plane treats stalls in a somewhat unrealistic manner, since the aerodynamics get pretty tricky around stalls, and it's hard to simulate them properly. It's cool that the simulator mimics real life in this situation.

My EAA member's idea (I'm not naming him because the program isn't official, and I'm not following it, just inspired by the discussion) with his base-to-final stall/spin reduction, as I mentioned earlier, is that pilots of homebuilt aircraft should explore their planes' stall characteristics, including in a turn, like I did. Once it's determined whether the plane wants to drop a wing in a stall (leading to a spin), apply appropriate anti-stall modifications to the wing, such as vortex generators, stall strips, etc. to correct the behavior. This should lead to a safer and more predictable plane. It's a great idea, and I'm glad he's working on it.

I'm equally glad that I tried out a couple of turning stalls to see what would happen in my plane. The results were very surprising to me, both the fact that the plane didn't want to stall in a turn with the power at idle, and the manner in which it stalled with power on. I may spend some time exploring the power-on stall a bit more, to see if I can figure out what's going on with the airflow that causes the plane to flip around like it does. I'll continue flying well away from the potential danger zones of the stall. I'm glad to learn another bit of knowledge about how my plane behaves.

Posted at 23:00 permanent link category: /aviation

Categories: all aviation Building a Biplane bicycle gadgets misc motorcycle theater