Stolspeed VGs Installed on Avid MK4 Undercambered Wing

Amazing results! I accidentally installed the 15 VGs at each wingtip at 30 mm instead of the recommended 60 mm, and the remainder at 60 mm instead of 90 mm. The results are unexpected and truly amazing.

More tests today in 75°F OAT. Idle clean: 47 IAS is 38 mph TAS – 19.1%. Idle flaps: 42 IAS is 36 TAS – 12.1%.

Cruise power clean: 41 IAS is 33 TAS – 19.5%. Cruise flaps: 34 IAS is 28 TAS – 17.6%.

All speeds above are TAS. Touchdown was 40 MPH IAS, which is 35 MPH TAS – 12.5%. Not too shabby.

John Miller
Avid MK4 undercambered wing
Jabiru 2200 Engine

I have added VGs to both the tail and the main wing on my Avid. I feel much more solid elevator authority from the VGs on the horizontal stabilizer. The VGs on the wing allow me to push the envelope for very slow STOL landings without suddenly dropping in. Very satisfied.

Paul, Australia

I have been doing more testing on my Avid Flyer Model B STOL plane using VGs.

Stall speeds with the flaperons on have been reduced only 1 or 2 MPH, but stall speed with no flaperons has been reduced 5 MPH, so that with and without flaperons stall speeds are almost the same at 39 MPH and 41 MPH respectively at idle RPM. My climb has definitely increased by about 200 FPM. My glide ratio has improved, although this is harder to quantify. I have 3,970 hours in this plane, so I can sense the improvement.

Now here is the big surprise. When my plane was built back in 1988, the vertical stabilizer was offset to the right (passenger side) to compensate for the P-factor of the two-stroke Rotax engines that turned the opposite way of normal aircraft. When I installed the Jabiru 2200, which turns the normal way, I really had to hold right rudder on takeoff and climb out. So I put a trim tab on the rudder bent at a 45-degree angle to relieve the rudder pedal pressure.

I had a brain fart and decided to try putting your VGs on the left side of the vertical stabilizer, 4 inches in front of the rudder hinge. Today, when I took my feet off the pedals, the plane would turn to the right, and now I had to hold left rudder. So I bent the trim tab straight back so it would have no effect and found that I could fly feet-off. Tomorrow the trim tab comes completely off.

I have to believe that the VGs have less drag than the trim tab at 45 degrees, and I will see if I notice a better cruise speed. I suspect it will be too small an increase to notice.

John
California