Avid is as perfect as an airplane can get, but there is no
end to creativity of the Avid builders.
Below are mods and improvements sent to me by other builders:
Fairing The Struts -by John Wright
I am sending files attached of my strut fairings. These were needed
because of the higher speeds traveled enroute now and upon descent. I
was getting a bunch of buffeting just behind the strut attach points which
in time was only going to fatigue the fabric. This was more a necessity
than a desire, however it did get me around 3 miles per hour. After all the
fairing and speed mod improvements I have done to date, the improvements
done from now on won't get me a lot of speed. Like Mr. Huff said,
sometimes you win, and sometimes the hours of work don't get you much
speed at all. At any rate, these draggy STOL wings and fuselages leave a lot
of room for improvement if speed is your thing. However, you can only
make a box go so fast through the air. I am only after economy. After all,
the price of Avgas is a bit much these days.
The material is carbon fiber and epoxy. I used these materials because
I needed strength without the weight. I molded the area with clay and
balsa wood then waxed the surface so the epoxy would not stick. After
removal of the shells I used Bondo to smooth and paint. They are held on to the
wing with nut plates through the inspection plates and metal tabs where
possible. To access the tie down rings I remove the front cover held
in place with a camlock and replace the cover prior to departure. They
look slick and they are very effective.
click on each image to see a larger version
503 Cabin Heater - by Mark Kunz
After flying a few years in the winter and trying different cabin heat installations on my 503,
I think I have found one that works fairly well.
The 503 has two cooling exits, one for each cylinder.
I made a mold out of cardboard and car putty, then fiberglassed it and came up with a fitting that quick
fastens to the rear cooling shroud exit, then exits with a 2" round end that is connected to a length of 2" aero duct.
This is then connected to a heat shroud around the exhaust, then into the cabin heat box, which dumps the heated air
overboard or directs it into the cabin.
The coldest I have flown with it is around 9 degree's. At this temp you still want to be dressed warm, but it
does take sum of the chill off. When the temps reach around 18-19 degree's on up, it stays comfortable in the the cockpit.
A simple but effective heating system.
Manual Mixture Control Rotax 582 - by Chris Bolkan
The manual mixture control system Ernie Hamilton and I have come up
with uses a vacuum (low pressure) source to reduce air pressure in
the carb float bowls, which reduces fuel flow through the main jets,
thereby leaning the mixture. This is beneficial for 1)compensating
for altitude variations, 2) Compensating for temp variations
(winter/summer, morning cool/afternoon heat), and 3)compensating for
engine load variations (variable pitch prop or changing pitch on
ground adjustable props)
It can be installed on any Rotax engine that has Rotax High Altitude
Compensating carburetors as an option. It should work on any engine
that the concept can be applied to, but with Rotax it is easy
because they have already figured out most of the jetting
The vacuum (low pressure) source is provided on newer bing carbs
from the factory, or you can drill your own carb to install the
port. The following link leads to
Denny Cox's website where we
posted pictures that explain how to drill the port in a bing carb.
I don't know if the link will embed or if it will require a cut and
paste to work, but once you get there, the pictures are self
Once the low pressure source is created, the carbs need to be jetted
per Rotax HAC specifications. I will post this information a little
The last thing to do is get a suitable needle valve, a few
plastic "t"'s and plumb the system together. I've posted a couple of
VERY rough sketches of the system in the files section of this
website to show the concept of operation. I'll add more info later.
When the needle valve is closed, the float bowls are vented to
atmosphere (air pressuer inside the air cleaner)through the
restriction. As the needle valve is opened, low pressure from the
carb port pulls against the atmospheric pressure reference located
in the air cleaner. Because of the restriction, air pressure lowers
in the float bowls. That's all I have time to post right now. I'll
post actual photos of the system and carb jet information next
chance I get.
Avid Speedwing Electric Trim Modification by Allen Aaron
After flying my speedwing (1993 model kit, first flown in anger in 2002!) for about 100 hours I decided to change the elevator trim from the original cable system to an electric servo mounted in the elevator. Unlike some earlier model kits which use flap as a proxy for elevator trim, mine has a trim tab built into the elevator actuated via a cable to a lever in the cabin.
My main motivations for changing the system were:
- the trim system, as installed was never very usable
- little trim lever motion got translated into trim tab movement
- I suspect because of flex or friction in the cable
- the cable (actually wire) had broken at the lever once previously and although it didn't result in flutter of the trim tab/elevator (probably due to friction .. see above) it was inconvenient and not particularly confidence inspiring
- most critically, there was a quasi AD issued on the avid trim tab that suggested making a spring type mechanism to contain movement of the tab in case the cable broke at the tab. This had been experienced at least once and resulted in elevator flutter that the pilot was able to overcome by immediately slowing down and carefully bringing the plane to a safe landing. Elevator flutter is NOT a pretty thing to experience. I didn't need to be warned twice and figured if I had to make the mod to the cable system, I may as well go all the way.
I understand that later kits have the electric trim as standard. I have also seen a mod that involved welding an extra tube into the elevator to mount the servo. This would be better than my mod, but not practical since my plane was already covered.
My objectives were to install the trim tab in a simple and safe manner. I've now used the system for about 15 hours and it seems robust and very workable.
Legal Disclaimer - I make no representations that this system is safe for your aircraft. If you follow my approach and it works, great! I only provide this information for educational purposes!!!! If you are silly enough to copy my approach and your elevator falls off or your electrical system fries or your trim tab comes loose and your plane flutters uncontrollably then DON'T BLAME ME!!! Follow this approach at your own risk!!!! I can't guarantee it will work beyond 15 hours flying time or that it will work at all!!
Here's what I did
1. I cut a hole in the bottom fabric towards the front tube of the elevator. I ground off the tubular lugs that previously held the old trim cable. I used a sharp razor blade on the fabric and a dremel cut off wheel on the lugs. I painted the lugs with a little primer to prevent corrosion.
2. I trial fitted the MAC (or Ray Allen) 4A servo in the corner formed by the longitudinal and diagonal tube of the elevator. Doing it this way resulted in the mounting lugs of the servo fitting in the gap between the tubes and the fabric at the top. The bottom of the servo only sits a few mm proud of the fabric at the bottom of the elevator. Looked like it would fit well and not require a massive fairing over the servo. Servo arm also lined up nicely with the new trim tab arm that I would fabricate.
3. Next I made a small hole in the top fabric in line with the longitudinal mounting holes in the trim tab and over the longitudinal tube in the elevator. I think I burned the hole in the fabric with a soldering iron to stop it from fraying. I (VERY) carefully drilled out the holes in the elevator tube with a 3/32 inch drill bit - being careful to make sure the hole was centered in the tube. I then cleco'd the servo in position. I finally riveted it permanently in position with suitably sized stainless steel pop rivets. In hindsight, I could have installed a couple of RIVNUTS there instead and made the assembly more readily removable for servicing. But then I figured, if I even needed to remove it, it would be because the servo was shot and I'd be able to drill out the rivets anyway.
4. Next I positioned a suitably size adel clamp into position to provide a third mounting point for the servo. I had to remove the rubber portion of the clamp since it interfered with the fabric and would eventually wear through. I wrapped some fabric tape around it instead. I then drilled a new mounting hole in the servo flange. I installed a stainless steel machine screw and locknut in there through the top fabric to securely mount the servo. While four positions would be nicer than three, the servo appears to be very secure as is and I avoided drilling extra holes in the elevator tubes or fabricating a mounting plate. If my plane hadn't already been covered and painted, I would probably go with a separate mounting plate welded to the elevator tubes. As is, the servo does not extend above the top fabric of the elevator (fits nicely just below it) and only two SS rivets and one machine screw are visible from the elevator top.
5. I then drilled out and discarded the original trim tab arm and fabricated a new one. This was made to in one piece and designed to rivet in three places - two rivets in the longitudinal tube in the trim tab and one in the lateral tube.
6. Finally, I laid up a few plies of fibreglass approximately 6" x 4" on a glass panel and shaped it into a teardrop shape. I carefully glued this inside the bottom fabric around the servo and neatened up the hole in the fabric around the servo. I shaped a piece of urethane foam into a similar tear drop shape (see photo - not mine bit similar) to cover the servo. It's really easy to shape this stuff - rough cut it with a razer knife then sand it with another piece of urethane. I glassed this with a few layers of lightweight bid cloth and epoxy resin. I cut a slot in the back of this for the actuating rod and drilled 5 holes in the flange which would screw using SS self tapping screws into tinnerman nuts that are slid onto the fibreglass stiffened fabric around the servo. Sorry, no photos on this but I could take some of the finished installation if anyone wants. Just email me.
7. Finally, I mounted the servo switch on to the instrument panel. I didn't bother with the trim indicator. I check the tab on pre-flight and find that it's easy to gage if it's out of trim. I mounted the switch in an empty 2" instrument hole (left of CHT/EGT rotary switches) but will change its position to a more permanent one soon.
I haven't yet painted the servo cover - having too much fun flying. The trim tab isn't used often but seems to work quite well and gives me more confidence than the old cable system did.
Feel free to email me with any questions - aaaron_at_tvp_dot_com.au.
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.
The aircraft is an Avid Flyer TD STOL wing with a Rotax 582, 3:1 gearbox, Culver 74x48 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!