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!





One thought on “Vortex Generators for the Empennage”

  1. Jonathan, wow. This is what I hoped VGs would do for performance. That landing is definitely less than 120 ft. I would be interested to see what your short field takeoff roll on a grass strip proves to be, once you get the variable pitch prop installed


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