Modifications incorporated into my A series Sonex.
The modification shown in this folder were created for my own requirements and preferences, and to match my available shop equipment and limited skills. The drawings and photos were created primarily to make a record of these modifications, and not as any instruction for others.
Technology, like civilization itself is built on all that preceded it. Information in these files is provided as a source of ideas for customization by others in the hope that further improvement can and will be made. These files might also serve to to convince others that is best to leave the original designs well enough alone and not to mess with them.
Most of the following modifications were created during my build process, 2010 through 2012. The new B series Sonex now incorporate some of these features, but the designs shown here may be of use to those still building to original design, especially scratch builders.
The designs in these files have not been reviewed by, or approved by, anyone with or without any aeronautical engineering expertise. Any use of the information in these files is strictly at your own risk.
Note: Most all modifications add to aircraft weight. My intent was to make my plane suitable for a couple of normal sized older folks to use for cross country flights, and for that it has worked out quite well.
One major benefit of these modifications is that when hydraulic brake actuator, electric flap drive and panel mounted fuel shut-off are combined, is that they allow me to fly all stages of flight with shoulder belts fully tightened, and thus minimize chance of head trauma in emergency off field landing.
Drawings are provided as .pdf files. My CAD system is a very good, but uncommon one. If wanting any drawings as starting point for your own designs, I can provide them in several formats, PM me for more info.
To conserve file space on this site, I have omitted photos and drawings of some items. Feel free to contact me for additional info on designed covered here, and on most anything seen in photos that appears non-standard.
Information on modifications made to engine installation can be found on
the Jabiru / CAMit forum: https://jabcamit.groups.io/g/main
By: David A.
Sonex Forum ID: DCASonex
Sept. 25, 2021
These drawings and photos show how I installed Grand Rapids Technologies roll and pitch servos for their auto-pilot option with their Sport SX EFIS. My Sonex has single center stick, design will need to be modified for dual stick applications, but the concepts shown should still be applicable.
Most installations mount the roll servo to the aircraft frame as was done here with the pitch servo. However, when that is done, a change in pitch position induces a small change in roll as well. This installation avoids that condition. If mounting the roll servo to the airframe, use as long a linkage as possible between servo and control stick to minimize unwanted roll input resulting from pitch change.
The roll servo that is mounted on the base triangle of the stick assembly, is positioned so that its weight is near the pivot point, and its weight actually helps even out pitch counterbalance forces resulting from the bungee cord. The cable to the pitch servo is positioned near the pivot point for minimum motion of the cable.
Have seen several reports of difficulties in tuning response of auto-pilot control to Sonex with its rapid response and neutral stability. My installation worked right out of the box. Do not know if my design plays any roll in that or if it is Grand Rapids Technologies good design work.
Sonex forum ID: DCASonex
May 21, 2016
NOTE: This design has not been reviewed or approved by anyone. Use at your own risk
NOTE: This write up and design applies to the original A series Sonex.
The panel is an integral part of the Sonex fuselage structure. Large screen all-in-one electronic flight information systems are becoming ever more common. Bisecting the panel with one of these displays destroys the structural integrity of the panel.
This section explains how I addressed that concern while keeping panel layout as clean as possible.
My overall objective in designing the interior of my Sonex was to make it comfortable for wife and I to fly places in, while retaining Sonex fun to fly and aerobatic capabilities.
Single center stick was chosen as that seemed to leave slightly more leg room, and was wife’s preference. Wife is also a pilot, so the ability to fly the plane from either seat was another consideration. I also wanted to keep the space below the panel as open and free of knee banging obstructions and to make it easy to access under the panel.
I selected Grand Rapids Technology’s ( GRT) Sport SX with their 8.4” screen. This completely bisected the standard Sonex panel leaving the structure compromised. A channel was added below the standard panel, and replaced the standard lower panel cross member, and as noted below, it was further re-enforced by bracing to a new bulkhead behind the fuel tank. To maximize usable space in the panel, the top flange of the new channel needed to be kept short, and lower flange could be only slightly longer to maintain aces to connections of circuit breakers to be installed in that channel.
Grand Rapids uses their popular EIS as an interface between the Sport SX and various engine sensors. Access to the EIS’s display is not required when connected to the Sport SX, but is useful for setup and trouble shooting. That is why the EIS is mounted facing straight up on the glare shield and not taking up space on the panel. I may eventually cover it with a piece of same material as now covering the glare shield. Also worth noting that engine limits can be set in both the EIS and the Sport SX, but having both is a PIA, omit them from the EIS as well as the warning light for the EIS.
Locating circuit breakers and other small components in the new channel freed enough space in main panel to install large eye-ball vents in it keeping below panel space clear.
To accompany the single center stick, dual throttles were added, and these were also installed in the new channel.
Note: Photo of completed panel was taken before flap lever was replaced by the electric flap operator.
The new channel partially makes up for the loss of structural integrity from the original design, However, the failure mode of a channel that is long relative to its cross section is to collapse its unsupported flanges, and to twist itself so as to bend in the direction that it is weakest. The way to address that is to keep the channel from twisting and to lessen the span between supports to its open flanges. As can be seen in photos, two braces were added one each side of the large screen display unit, extending forward to a new bulkhead directly behind the fuel tank.
The new bulkhead fills three objectives. It provides an anchor point for the two braces to channel under panel. It provides a metal partition between electrical bit that might go Fittzzz, and the plastic tank full fuel and its fumes. And it also better resists the forces imposed on the upper longerons by the fuel tank straps. Those straps tend to pull the longerons inward when doing high G maneuvers or hard landing.
Important to note that I do not consider the new channel by itself to have sufficient structural strength without including the two center braces connecting it to the new bulkhead. With those braces, the new structure is strong enough that I added a hand hold under the center portion of the panel.
The design of those two braces is also intended to allow them to crumple in event of severe frontal impact, hopefully without penetrating the fuel tank. (I have no plans for testing this.)
Every one has his own design for this, I have included mine here for those looking for ideas.
Odds and ends:
Due to file space limitations on this forum, I have not posted details on the following, e-mail me if any are of interest. Photos and/or drawings are available on the following:
Mounting GRT magnatometer
Mounting ACK-04 ELT
Electrical interconnect wiring of the various avionics
Adding felt seal to Van’s large, low cost plastic eye ball vents
As with all other material in my files of modifications to Sonex #1327, none has been reviewed by or approved by anyone. It is posted to aid others in further improving the breed. Use at your own risk.
Drawings and photos in this section are how I installed a dual throttle to complement the single center stick configuration of my Sonex.
I am using a Bing carburetor on a Jabiru / CAMit 3300. The Bing carburetor’s throttle is spring loaded to the full open position, and cable pull is required to retard it to idle. The design will require modification if using other carburetors or the Aero-injector.
The concept is very simple, a bell crank spread out across the full width of the fire wall using a torque tube with throttles connected to arms at each end. Throttle cables should be routed as close as possible to a straight line to minimize friction as one is pushed or pulled by motion of the other.
Throttles must be non-self-locking, and have sufficiently stiff wire that one can be pushed back out from firewall side when the other is pulled out.
I used ACS type A-820 throttles with friction lock and plain solid wire ends. The friction lock plastic bushing was removed from, the co-pilot’s throttle and a warning notice was posted that pilot’s throttle must be unlocked before co-pilot’s throttle can be used.
NOTE: This design has not been reviewed or approved by anyone. Use at your own risk.
Sonex Forum ID: DCASonex
May 21, 2016
Sonex now has electric flap operation as standard in their new B series. The design shown here was created in 2012, and can easily be retrofitted into existing airframes, especially those where the weldment on the flap torque tube still exists on left side of cockpit.
I decided to install an electric operator for flap drive due to original lever being beyond my reach when shoulder straps tightened. Also, I have a single center stick configuration which means stick cannot be braced between knees when reaching for flap lever and force required to lower flaps caused my upper body to twist moving the control stick and disrupting approach attitude. I was also quite concerned that I might someday not get the manual lever fully into its holding notch, and sudden release from a down position could be catastrophic.
Note: When combined with modifications for hydraulic brakes with operator on stick or via toe pedals, and panel mounted fuel shut off, pilot should be able to fly all phases of a flight with shoulder straps tight minimizing chance of head trauma in emergency or off field landing.
By dumb luck, there was just space enough left on panel beside the pilot side dual throttle to install the up-down switch. Flaps can now be operated keeping right hand on control stick and left hand on throttle.
For an operator I found a low cost 12 volt linear actuator made for automotive and robotic applications. (Specs and sources are on the drawing.) The 6” stroke unit fit nicely in available space and approximated the distance moved by hand, and thus the required force would be approximately that applied by hand. The unit is rated 107 pounds dynamic load and 500 static load. The actuator has nonadjustable limit switches built in at both ends of stroke, so design used operating arm long enough that full 6” stoke equaled 30 degrees flap deflection.
Operating time to full down is about 10 seconds and back to full up about 9 seconds. Normal operating current is listed as about 3 amps with stall current of 10 amps. A 7- 1/2 amp circuit breaker was used so as to trip if actuator is stalled. (THIS HAS NOT BEEN TESTED.) Breaker selected was a Tyco push-pull type W23, so unit could be switched entirely off in event of any problems.
Unlike some flap operating units, this contains no overload clutch, so clevis on end of operating arm is designed to contact face of actuator in event of flap up position limit switch failure before over travel can cause structural damage to control linkages. Flap down travel is essentially unlimited on the Sonex design so no mechanical over travel protection was provided for this.
There is a version of this actuator available with built in potentiometer that can be used to operate a flap position indicator, however, that adds to length and weight of unit and I deemed it unnecessary since flaps can be viewed from cockpit. In practice, I simply count time switch is pressed, about 3 seconds equals 10 degrees flap deflection. Position of flap lever can also easily be seen by pilot.
The ability to add flaps without taking eye off flight path or moving body in a way that affects control stick position has made me at least LOOK like a better pilot in the pattern. It also allows me to operate the flaps at times when it would have been inconvenient to be reaching for the flap lever. To me this modification was worth the approx. 2.7 pound net weight addition.
There are two versions of the flap drive assembly included here. The “A” version is what I built; the “B” version I recently created as a suggestion for someone not having access to milling machine. Other variations on mounting the actuator have also been seen.
CAUTION: Failure of actuator, or electrical system could leave you with flaps down and no way to retract them if a go around is required. Inadvertent lowering of flaps at speeds in excess of 100 MPH is possible as there is no force feedback to pilot. Both situations appear to apply to the actuator in the new B series Sonex as well.
As with all modifications I have posted, none have been reviewed or approved by anyone.
USE AT YOUR OWN RISK.
Not related to the actuator, but have recently found, thanks to postings by others, that the Sonex flaps benefit greatly from addition of rigid stops installed on the rear spar. I made adjustable stops, but only used those to determine best setting, then replaced those with simple plastic angles (wood blocks would work) each about 3-1/2” long with clearance holes for rivets in trailing edge of top wing skins to as to mount them as high as possible on rear spar channels. Two stops per flap, each about ¼ the way in from ends of flaps.
Sonex Forum ID: DCASonex
May 21, 2016
Wanted to be able to reach fuel shut off with shoulder belts tightened in case of emergency landing. Also wanted to be able to see position of that shut off.
With this, plus toe operated brakes, and electric flaps, there are no controls needed in flight that cannot be reached with shoulder straps tight.
Push pull operator used is ACS model A-740, ratchet type to hold position, ACS part number 05-14172.
Photos show first version. Decided that operating arm on valve could be shorter and bracket scaled down to suit. When I installed the infamous “oops” fitting in fuel tank, shut-off valve moved about ½” farther from panel and cable no longer reached.
To solve that made new attach bracket designed for shorter valve operating arm, and that is the one for which the drawing is provided.
Note: My fuel shut-off ball valve had the flats of the hex exactly oriented to axis of valve stem. This may not be true of all valves, which may require altering the design of the clamp grooves to suit. Also size of hex on valve and fuel strainer may vary.
NOTE: This design is has not been reviewed or approved by anyone. Use at your own risk
Sonex Forum ID: DCASonex
May 21, 2016
Full capacity, anti-splash back, anti siphon, fuel tank vent and filler box modifications to catch spills
Sonex plans show two options for fuel tank vent for tail draggers.
One is a drain out the side and down which will spill fuel from a full tank in a tail-dragger when brought back to level.
The other is a simple extension tube with its end facing into the wind. Some have voiced concerns about this splashing fuel back on to the Lexan windshield and destroying it.
This design is intended to address both shortcomings. However, this design is not intended for negative G operations.
It allows full capacity without spillage, is very unlikely to allow any excess fuel to blow back onto windshield includes a siphon break, and openings are aimed into the wind to help pressurize the tank.
It is constructed from hobby store brass tubing to allow assembly with high strength solder.
The unit is connected to tank with a flexible hose to allow small amount of tank movement.
Line from upper tank connection is connected to vertical tube open at top. A second tube is secured over that, closed at the top and open at the bottom, in a manner that lets the inner tube vent out the bottom of the second tube.
Over the assembly of the two tubes, a final tube that is given a streamlined shape (anneal first) and closed at the top is fitted. This outer tube has a vent hole drilled facing forward near its top.
The base of the assembly is fitted with a drain connection that exits the side of the filler box like standard Sonex design. Note: To maintain positive pressure in the fuel tank, the bottom end of the drain tube is bent to face forward into the wind where it exits the bottom of the cowl.
Assembled photo also shows a seal around fuel tank filler neck. This has not been as effective in catching small spills as it would seem due to small movement of the tank breaking bonds to the silicone covered glass cloth seal material. However, the filler box was also modified to incorporate a catch trough and drain, and the combination has been very effective. The modification consisted of a new bottom for the box with catch trough and drain.
As with all else I have posted, this design has not been reviewed or approved by anyone. Use at your own risk.
Sonex Forum ID: DCASonex
rev. May 22, 201
Tank Vent Installed (JPG)Download
Tank Vent Parts (JPG)Download
Filler Box w Drain (side) (JPG)Download
Filler Box w Drain (front) (JPG)Download
Filler Box with Drip Flap (JPG)Download
Filler Box with Top Seal (JPG)Download
Filler Box Mod (side view) (pdf)Download
Filler Box Pattern w/ Dimensions (pdf)Download
FW Fitting for Fuel Vent (pdf)Download
Tank Vent Anti-Syphon (pdf)Download
Note: The designs here were created prior to Sonex introducing their own hydraulic disk brakes.
The differential toe pedal assembly should be applicable to Sonex and other disk brake units, and the Great Plains brakes might be desired by some who prefer floating caliper design to that of the push on one side of disk design that Sonex uses.
The 6” tail wheel design is very low cost and light weight, and includes dual sealed precision ball bearings designed for high speed operation unlike the full complement conveyor roller bearings found in many caster wheels (and some “tail” wheels).
When hydraulic brakes actuated by foot pedals or by a grip on control stick are combined with other modification such as electric flaps and remote fuel shut off a pilot should be able to reach all necessary controls with shoulder belts fully tightened to improve change of surviving an emergency landing.
Lots of small grass strips around these parts, so thought I would want the larger 5.00 x 5 tires at some point, and that would mean modifying the wheel pants to fit, so decided to go with that from the start. Pairing the larger main tires with a larger tail wheel seemed logical.
Prior experience with old style heel operated differential brakes saving my behind from a ground loop in a Luscombe many years ago left me wanting to have that option available in my TD Sonex. Also, many comments posted about deficiencies of the standard Sonex drum brakes, left me thinking that they would be even less effective with the larger tires. So looked into some form of hydraulic disk brakes.
My Sonex being a tail-dragger I was particularly concerned about any tendency for brakes to grab. Lots of comments for and against various brands of brake options but never saw anything really bad about the Great Plains Aircraft brakes so went that route.
I have flown in another Sonex that had a different brand of hydraulic brakes operated by bicycle type hand levers on forward side of each control stick, worked very well and would be a good option to save a bit of weight if not wanting differential braking.
So as not to apply brakes accidentally while dancing on rudder pedals, especially since the rudder pedals hinge from above while my ankles hinge from near the floor, I designed open brake pedals where I must deliberately lift feet up from floor until heal catches under rudder pedal cross bar before ball of foot is positioned to use the brakes.
The open design also makes for a very simply but strong design since each pedal consists of two arms welded to two torque tubes. No welding was done to the Sonex brake pedal weldments, and holes drilled are through neutral webs of the tubes of the rudder pedals to have minimal effect on strength of the rudder pedal assemblies.
For tri-gear aircraft, this pedal design might easily be modified to bring one of the vertical arms in from end of torque tube to clear the link to nose wheel.
The brake caliper mountings require some form of backing plate to bolt to, and also makes installing wheel pants problematic as the usual mounting point will no longer exist. Decided to incorporate wheel pant mounting points into the caliper backing plate. As can be seen in drawings and photos.
Found that the original welding on my stock Sonex welded axles left the mounting surface distorted, so trued this up on a lathe before adding the backing plate. Used a tube placed over axle and clamped with axle nut to keep caliper backing plate pressed tight to seat of Sonex axle assembly while welding backing plate to axle.
Have seen reports of wheel pants coming loose in flight, and one local friend had his rotate down under his tire which made for an interesting landing. If the cutout on inside face of wheel pants fits snuggly to the caliper mounting casting as shown in photos, the upper bolt will keep the pant from rotating, so that bolt is safety-wired, as is to one into the end of the axle.
I also installed a dual circuit Matco parking brake valve to hold plane in position while climbing out of the plane on slight slopes, such as sometimes found at fuel pumps, Note angled position that eliminates trapped air when bleeding the system.
The GP brakes take a real effort to hold for run-up with my CAMit 3300 engine, and that is so powerful that I have had it try to pull the plane over on its nose with brakes locked so I no longer do full power run-ups without tail tied.
6” Tail Wheel:
Use of a 6” tail wheel paired with the larger 5.00 x 5 tires keeps ground attitude about about same as for standard tires and 4” tail wheel. There have been many reports of other benefits of larger tail wheel.
Found a good 6” wheel in local surplus shop. It already had sealed precision ball bearing. I added the stepped internal spacer bushing so that axle bolt could be tightened to inner races without putting axial load on the bearings. Later found that the wheel, complete with bearings is a Colson Performa, part # 5.00006.5P5, grey round tread, with precision sealed bearings #6203-2RS, available from stock for about $14. Found a minor difference in bearings spacing between my original junk yard wheel and the new ones, both are shown on new drawing. Hub cap was hammered, like an ashtray at a kids’ camp, into a dish turned into a piece of scrap plastic. (Wood should also work for a mold.) After 250 hours, still using the original wheel, most landings have been on abrasive grooved concrete runway.
New tail wheel bracket was formed from single piece of 4130 steel and includes a replaceable sintered bronze bushing for the swivel. Sonex now offers their own 6’ tail wheel with bracket that raises the tail even higher than what is shown in my drawings, so modifying the bracket for more lift might be practical. (A 5” wheel would might be good with standard tires, and place less stress on tail wheel swivel bracket.)
Tail Wheel Steering Control Link: Is simply a single piece of 3/8” x 0.083” wall 4130 steel tubing tapped each end for CM-4S Aurora bearings. The integral stud rod-end bearings allow for a greater angular misalignment than standard rod-end bearings, and they are positioned so that the balls of the bearings are in approximately the same horizontal plane as the ends of the tail wheel spring so as to minimize the tendency of vertical flexing of the spring to steer the tail wheel side to side.
Larger tires, larger tail wheel, hydraulic disk brakes, and differential brake pedals all add weight. My plane was intended for somewhat comfortable cross country cruising rather than maximum performance and for me the weight penalty of all that has been worthwhile. Make your own decision on what your plane’s mission will be before deciding to add anything.
End result for me has been: Reliable stopping. No contorted reaching for brake lever and shoulder harness kept tight. Good ground handling. No tendency for brakes to grab or fade.
NOTE: None of the designs here have been reviewed or approved by anyone. Use at your own risk.
Update: Sept. 25, 2021: My designs date from 2010-11, when Sonex did not offer their own hydraulic disk brakes nor a 6” tail wheel.
Brakes: After 9 years and 450 hours I still prefer the full Great Plaines (Now O’Keef) floating caliper design to the Sonex hydraulic brake design which pushes on one side of the disk. The tiny friction pads did not look like they would hold up long, but I think I am still using the original set of pads and the disks still look like new, while hearing complaints of short pad and disk life from the Sonex design.
6” Tail wheel: Today you can purchase Sonex own 6” tail wheel, I am posting my design for anyone who decides to make their own, and because the ball swivel control link may still be applicable to other designs. Length may need to be modified to suit different designs. The low cost caster wheel looks like it might not hold up, and I purchased a couple of spares for about $14 each, but after 450 hours of use and a lot of not too pretty landings on rough grooved concrete runway, I am still using the original wheel.
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