Vortex Generators for the Empennage

I have been flying for a while with the VG’s installed on the wing. During the test flight I found significantly lower stall speeds and better control, but still the elevator and rudder still get washed out at low speed and I wanted to get better crispness and more positive control authority out of them. Before I start trying to get absolute crazy short Takeoff and landing distances out of the bird I want to make sure I have good control ability of it and have confidence in the planes handling.

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The Horizontal Stabilizer VG setup is quite well understood and even the 650 project I have has VG’s on the elevator, its not just a STOL thing. It helps with control authority. Installing them was setting up a reference line 100 mm ahead the hinge line and setting the VG’s at a 30MM pitch across the bottom side of the Horizontal Stabilizer. I used the clear STOLSpeed VG’s sanded flat. The standard VG’s they ship have a curve for the profile of the wing, not the flat profile of the stabilizers.

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The Vertical Stabilizer presented more of a challenge. The instructions were very unclear on how to set them up and I could find no pictures of any installed on Zenith Cruzers. It was late Sunday so I called STOLSpeed in Australia where it was Monday morning and I was able to get the owner on the phone. We discussed the options with him and he had some suggestions. Looking at the rudder, there was a rake back on the hinge line and I decided to have all the VG’s follow 100mm in front of that parallel to each other and orthogonal to the hinge line. After sketching out the design I headed over to pick Halden up from his end-of-course exam at the FBO and checked out the STOL kit on the Cessna 172 I fly. I looked at the Vertical stabilizer and surprise! It was almost exactly what we had decided on for our setup.

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This setup would give them a good angle to create the vortexes when the plane was at a high angle of attach when they were needed most. I was hoping this would give me more control authority at low speeds with the rudder.

After getting them all installed I decided to take them for a test flight. I left KPAE and headed out over Lake Stevens. After gaining sufficient safe altitude I started first a steep climb with full power and full elevator back pressure. The rudder had plenty of control and I was getting over 600FPM climb with full back elevator and under 40kts speed. I then went back to 75KTS and 800+FPM climb.

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At 4000 feet over the lake I slowed to 40kts and went full power and full back pressure. Again total control and crips rudder control without having to go to lock like before. I recovered and setup for flaps up stall with no power. Very crisp control with some shuttering on the roof window and 500FPM decent with no break. Very controllable.

I then setup for a high-alpha slow-flight with power and flaps and got down to 30 KTS before any sign of shudder. I set at 33/34 kts and did 360 turns in both directions. They were very clean. No loss of control or loss of altitude and super tight!

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I then tried some aggressive slips and it felt way better than it used to with all the VG’s working for me. I headed back into KPAE and alone in the pattern I setup for a STOL approach and came in a tight pattern. Slipped very low approaching the threshold and went high alpha with power. I hit a few feet onto the pavement with a solid plant and threshold braking to slow roll. I could have exited off the end intersection! I did put some power and it took for ever to taxi to the intersection where the touchdown target normally is.

This is roughly the landing spot and brake run. I think it is time to put the 80″ prop on the Viking 130 and see what we can do with it!




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