Looking into VG's

[C5Engineer 1170]

Who's got em? Worth it on a short chord wing like ours? The Rans S-7 guys swear by them. Thoughts?

[ski-bum 0]

Joey,

Here is an article written by Larry Martin, a friend of mine, on his experience with CCI VGs.

Mike

CCI VG’s on the Avid Flyer STOL by Larry Martin

This is a report of my findings using CCI VG’s on the Avid Flyer STOL wing aircraft.

Background:

The aircraft is an Avid Flyer TD STOL wing with a Rotax 582, 3:1 gearbox, Culver 74×48 prop. Built according to the kit/plans with no modifications except for "Grove" landing gear. (heavier) Published allowable center of gravity is 11.185 to 16.5. This aircraft was at 13.5". Designed stall speed at approximately 900lbs in flight weight is 32.5 mph.

The landing characteristics of this airplane were such that in order to affect a comfortable flare, power and additional speed was required beyond what I consider to be normal. In addition, there was insufficient nose up trim available on approach. On take-off, it was evident that the wings could fly sooner, but the airplane would not lift off. The tail could not be brought up before lift off speed, and would take off quicker from the 3-point attitude. The stall characteristics were such that when the airplane was gradually slowed to minimum speed, the airplane would sink, well above the designed wing stall speed. (Not with standing of course that indicated airspeed is most inaccurate at high angle of attack) The wing would not "break" at the stall, as most wings will. When the nose of the aircraft was "lobed" up and speed was allowed to bleed rapidly, the wing would exhibit traditional stall characteristics. It was determined that the elevator was not working sufficiently.

After discussion with the designer of the aircraft, the center of gravity was moved to the published aft limit of 16.5" with the assurance that the aircraft was tested well beyond the published limit. For reasons of liability at that time, the published limit was set very conservatively. The aircraft was tested beyond 19.0", which can be safely used. (37% mac) The change in center of gravity helped considerably, but still not sufficient. Further to the designer’s recommendation, the elevator area was increased approximately 15% by extending the elevator aft. There is no room physically to change the horizontal stabilizer angle of incidence. Short of making a new one, modifications were complete.

The flying characteristics at this point were much improved as measured by the following: A much shorter take off ground roll, the tail could be raised on take off with full flaps; almost sufficient elevator trim was available on approach with full flaps; and power off, full flap approaches could be safely made at normal speeds. The stall characteristics were similar, but at a slower indicated airspeed. Almost content with the airplane, but still not to the standard of performance that I thought it was capable of. I followed the studies of vortex generators.

In the pursuit of finding the truth of if and how VGs would enhance performance, I read many studies and opinions. Some say that they help, others called it snake oil, etc. Art at CCI (http://www.vortexgenerator.net) was not in a hurry to sell me anything, but offered me aerodynamic and physics proofs and rebuttals. To say that he was patient is a huge understatement. Finally, he managed to overcome my skepticism, and I ordered a set with a money back guarantee. Art was first concerned about getting more lift from the tail, and we placed the VGs per his instructions on the flat surfaced (non airfoil) horizontal stab.

The results of the VGs on the stab were nothing short of incredible and better result than the sum of what was done before this point. The tail was now producing more lift at all angles of attach. Subjectively, the tail felt more "alive". The take off run was even shorter, with the tail coming off the ground with the application of power. The increase of lift was objectively measured by comparing the neutral trim, hands off, level altitude minimum speed with and without the VG’s on the tail surface. A 10mph slower speed could hold level altitude with the VGs on. This was a dramatic increase in tail effectiveness. So much so, that I was able to remove 5# of ballast and move the CG .80" forward and have the same control feel. The stick position was physically further forward at all speeds, thus proving the elevators were working more effectively. The trim was effective at approach speed even with the cg further forward. (.5" cg change affects a considerable difference in this aircraft) The stall characteristics were such that the wing would stall at a higher angle, a noticeably slower IAS, and much more aft stick movement remained during the stall. Art then wanted to test the effectiveness of stabilizer end plates. There was a small subjective gain in effectiveness, but no objective, measurable results could be recorded. Most likely this was due to installation error inherent in the airspeed system. In addition to the enhanced low speed qualities, an unexpected gain in normal cruise speed of approximately 11% was attained. The top speed was approximately the same, but this is due to the large drag caused by the undercamber of the STOL wing. The airplane reaches a drag point similar to mach drag, in which the airplane will not accelerate above, or in the case of the Avid Stol, a disproportional increase in power only yields a minute increase in speed. It is not practical to fly at this speed. However; in the normal cruise range with a cruise rpm of 5400 yielded 84 mph with the vg’s on the tail. Without the vgs, the same rpm yielded 74mph. This is explained by the efficiencies of the tail.

The next step was to install VG’s on the wing. Research shows that there is a wide range of "ideal" or "sweet spot" placement with respect to percentage of wing cord. Experience gained from others suggest that in a range from 4% to 12%, one would find that spot. CCI suggested 10% as a starting point. I initially placed them as per the supplied directions except I started at 7% and planned to move forward to 4% then aft to 10% to test the effectiveness. The results of the VG’s at 7% were immediately noticeable. The take-off roll was shorter. In slow flight, the stall occurred at a much higher angle of attach and the IAS was 5 mph slower. Again, I caution putting much emphasis in IAS, especially at high AOA due to position error. There is no argument that the AOA was much higher than ever before, and the airspeed needle was slower than previous witnessed. At 75+% power the nose attitude was extremely high and could remain this way flying at the verge of the stall. The Avid has "flaperons" which trail the wing. They are always flying, even in a stall. Therefore a subjective opinion of aileron control cannot be rendered. The Avid does not spin well and enters into a spiral instantly, so fast that I choose not to spin. I did fly a spin series, hoping that due to the higher AOA that was possible, it may spin. Unfortunately the spin qualities did not change, and with rudder application and a pivot of the wings, it becomes unstalled and spirals picking up speed very rapidly. Thus I still don’t spin.

There was a gain in airspeed at the high end. It appears that the "stol wing drag point" was been changed to a higher speed. The old "aerodynamic drag point" is now easily overcome. I also noticed by way of control feel, and wing attitude that the center of pressure is moved forward on the wing. I replaced 5# in the tail to the original CG.

The airplane was designed to be flown STOL. Dean Wilson designed the flaperons to enhance this ability. I use full flaperons for all take off and landings. I have landed with complete control in 20+ mph direct crosswinds with no problems. (grass strip) The VG’s do not detract from landing control nor do they negatively affect any of the flight characteristics. I am completely satisfied with the VG’s and help from CCI. Due to the satisfaction with the results, and my too busy schedule, I have not changed the VG placement from 7%. I would still like to, but it has to take a lower priority for the time being.

Please feel free to contact me for further information. I heartily endorse CCI VG’s (http://www.vortexgenerator.net) if you want to fly the Avid the way it was made to fly!

Larry Martin My93Avid@yahoo.com

[C5Engineer 1170]

Awesome response thanks!!

[flattie45 17]

First off, let me say that I have no experience with VGs (sorry!) I read an article about a fellow who installed VGs on the bottom side of the wing to eliminate some of the drag associated with the airfoil used on the Avid Flyer. He was in contact with Harry Riblett, who helped with the proper placement. Apparently at cruise speed an area of turbulent air builds underneath the wing, and the VGs help to break it up.

This was sometime in the 90's, and he didn't do a whole lot of pre and post testing, but he figured he gained at least 5 mph, maybe more. I'm not sure if or how it affected slow flight characteristics. He called them more of a "question mark generator" than a vortex generator, since it looked quite odd.

I think I still have the article on my home computer. I'll be home next week and post it when able.

I know you are looking for more slow flight/control authority, but I always thought that it would be interesting to try.

Edited 15 Jul 2010 by Nate