Home > Articles > TruTrak Install in RV-8


Installing the Tru-Trak Digiflight II in a Van’s RV-8 by Bruce Lane [brucelan 'at' microsoft.com]


Before I get started, I have to say this thing is truly amazing.  I have had it in the airplane for about a month and it is fantastic.  No more holding the stick between my knees while I look at my chart and try to keep the plane from turning by looking out of the corner of my eye.


I also need to say that the support from the people at Tru-trak has been outstanding.  They answer the telephone quickly and give clear, easy to understand guidance.  Plus, they make you feel like a really valued customer just like Van’s does.




 I did not build my RV-8 and had not previously had much of it disassembled.  I thus was learning as I went along on many things.  It took a friend and me about two full weekends to do all the work.  If you built your plane you may finish some of these tasks more quickly than we did.  I strongly recommend that you work with a partner – I can’t tell you how many times I was inside the plane on my back when I realized that I needed a particular tool that was sitting on the ground next to the airplane.


Note: my airplane’s floor and baggage compartment uses pan head machine screws to hold it in place.  Some builders may have used rivets instead.  This will likely add a ton of time to your install.


Make sure you have the install manual for your GPS available.  You will need to know what pins it uses to output data (serial or ARINC).  You will also need to configure your GPS to output data, by default it does not.


Estimated Time:


            Remove baggage compartment floor (2 hours)

            Remove bell crank and bell crank brackets (1 hour)

            Measure and drill brackets for pitch servo (1 hour)

            Drill bell crank, install new brackets and servo (2 hours)

            Route servo harness to servo; install connector (2 hours)

            Reinstall baggage compartment floor (1 hour)


            Remove seats, floorboards, etc. (2 hours)

            Install roll servo bracket (2 hours)

            Route harness to servo; install connector (1 hour)

            Reinstall floorboards (1 hour)


            Install circuit breaker / toggle (1 hour)

            Install programmer in panel (assumes it replaces existing instrument) (1 hour)

            Wire power leads (1 hour)

            Wire data leads to GPS (1 hour)

            Connect pitot system (2 hours)


            Configure GPS and autopilot (1 hour)

            Ground test (1 hour)


            Flight Test (1 hour)


Total time: 24 work hours


Additional items that you will need to install the Digiflight:


- 7.5 amp toggle/circuit breaker (part no. W31X2M1G-7.5 at Wicks Aircraft Supply)

- Wiring harness (SteinAir, Inc. Rosemount, MN 1-651-322-7565) – do not have the ends for the servos already on.  It will be much easier to route the cables without the ends on them.

- ¼” ID poly tubing

- 1/8” NPT connector to connect to the pitot system (only required for IIVS)

- Crimp style pins for the connector on the GPS

- ½” long brass instrument bolts 632

- Grommets to route the servo harnesses through different aircraft structures

- Wire ties

- ¼” heat shrink (used for abrasion protection)

- Loctite 242 (blue)

- Primer (to paint brackets for servos)

- Wire terminators (mini spade style) to connect to electrical system

- Grease (to hold the washers in place when you reassemble the bell crank)




            Unibit (1/8” to ½”)                     not required, but really nice

            Crimping pliers                           required

            Portable vacuum cleaner required

            90 degree angle drill                   required

            Electric screwdriver                    required


The Digiflight two consists of three major assemblies:


            A pitch servo that moves the elevator bell crank in the tail section

            A roll servo that moves the torque tube

            The programmer controller in the panel


Pitch Servo:


To install the pitch servo you will need to remove the rear seat back, the baggage shelf and the baggage area floor.  The bell crank is actually under the shelf, but it will be all but impossible to route the harness unless you remove the floor as well.


You need to remove the bell crank from its brackets.  This will require you to remove both the fore and aft push-pull tubes.  I recommend you cover the end of each push-pull tube to keep them clean.


You will need to drill a hole in the bell crank to connect the servo arm.  The Tru-Trak drawing shows you exactly where to drill it.


You will need to drill out the rivets that hold the existing brackets in place.  There are five rivets on each of the two brackets.  Take care not to enlarge the rivet holes in the aircraft structure since you will be reusing these holes for the new brackets. 


You then use the existing brackets to mark and drill the holes in the bracket that Tru-trak supplies.  Be careful here, it’s easy to get the brackets mixed up.  I replaced the rivets with machine screws and fiber lock nuts (it would have been really difficult to pound rivets back there).


I used both Loctite 242 and internal star washers to attach the servo to the brackets.


You will need to put in the front bolt that attaches the servo to the bracket before reinstalling the bell crank because the bell crank covers up that bolt.


The bell crank itself is a bit of a pain to reinstall because there are two washers for the bottom hole (the one that is used to connect the front push-pull tube) and there is not a lot of finger room.  I used a bit of grease to hold the washers in place while getting the push-pull tube and the bottom of the bell crank lined up.


Check for smooth range of motion.


You will need to run the wiring harness back to the servo.  I debated exactly how to do this for a while before deciding to run it underneath the floorboards.  You could run it along the longeron on the side of the fuselage, but since you will have to run the harness down to the roll servo too, it made sense to run them both down here.


I added a bit of heat shrink for extra chafe resistance and also used grommets where the harness went through the structure (four places total).


Running the harness sounds harder than it is.


Note: The front of the airplane is to the right in this picture.

Note: The front of the airplane is in the lower left corner in this picture.



Roll Servo:


The roll servo is easier to get to, but has a few extra challenges compared to the pitch servo.


It is located under the front pilot’s seat in front of the aileron trim springs.  It is attached with a Tru-trak supplied bracket that is bolted (or riveted) to the left hand longitudinal structural member.  On my airplane there were some wires that I had to reroute (a com antenna and the wires for the wingtip lights).


It is important to note that there isn’t a whole lot of room to mount this bracket and getting it right is very important because the clearances are very tight.  The Tru-trak bracket is supposed to fit without modification but I found that it hit two different flanges.  I had to change the bracket (something that Tru-trak wasn’t very happy about).



Also, the harness connector on the servo is the forward most part of the assembly.  It will hit the aileron push-pull tube if you mount the servo too far forward.


Even more important is making sure the servo is mounted low enough since the top of the servo arm will hit the floor boards if you mount the bracket too high.


I also found that there is a stiffener bracket on the bottom of the floorboards that will interfere with the servo arm unless it is bent or trimmed.  You won’t find this out until you put the floorboards back in (with about 50 machine screws).


I used machine screws with fiber lock washers to attach the bracket to the frame.  I then attached the servo to the bracket with AN bolts, internal star lock washers and Loctite 242.


The servo arm needs to be “clocked” so that the servo arm is perpendicular to the linkage arm when the ailerons are in a neutral position.



Note: The front of the airplane is at the top in this picture.




The Programmer:


The programmer has several connections (though only three connectors):


            A static port

            A pitot port (IIVS only)

            A multipin computer style connector


Tru-trak says that you can just leave the static port open to the cockpit.  My early tests suggest this is true.


I thought the pitot connection was going to be difficult but it turned out to be pretty easy.  The standard tubing that Van’s sells is ¼”OD poly tubing.  It has a milky white color and is pretty stiff.  I initially thought I was going to make up a tee fitting for this tube and cut it in the center.  The A&I that I worked with pointed out that the pitot pressure is very low (something like 2 psi) and using a larger tube that would fit over the existing tube would work well.  I bought ten feet of ¼”ID tubing (available at Home Depot for $2).  This tubing is thick walled, pliable and clear in color.  It says not to use it for an ice-maker, but nothing about pitot tubing.


I cut three 4 inch lengths of the new ¼” ID tubing.  Then I used a standard tubing tee connector with barb connections on each arm to connect the three pieces. I added the 1/8” NPT connector with a barb connection to one of the tubes (this is the one that threads into the back of the Digiflight II).  I then cut the existing pitot tubing inside the left hand vertical structural beam.  I slid the two other arms of my “adapter” over each of the existing pitot lines making sure that about three inches of the tubing had slid over the pitot tubing.


I checked to make sure that the connections couldn’t slide off.  I was unsure if the added tubing would make my airspeed indicator inaccurate.  I have not checked it scientifically (with GPS or against another airplane) but it seems to be accurate from personal experience.  I have flown about twenty landings and it seems on the money.


The electrical connections are very straight forward.  The unit needs:


-          a 12-14V power source via a 7.5 amp fuse

-          A data input from your GPS (either serial or ARINC)

-          A ground


It has additional connections for a stick disconnect and two unused wires.


I powered the unit off the avionics bus and grounded it to the airframe near the unit.  I used a toggle switch / fuse that I mounted on the panel next to the Digiflight II.


I connected the single wire to the RS232 style connector on the back of the GPS (A Garmin 250XL in my case).  You will either need to splice it into the existing wire for the appropriate pin or add an additional pin to the connector.


I wire tied everything and taped the ends of the unused wires.


If you can’t resist the temptation to turn the unit on at this point (like I couldn’t), be prepared for it to go through a bunch of gyrations.  Before it has had a chance to calibrate itself, it will likely want to run the controls into the stops.  Luckily the servos have a clutch that will release and it doesn’t hurt anything.  Just verify that it moves everything in the correct direction.


After you check your work, put everything back together and do all the ground testing, its time to go flying.  I hand flew up to 5000 feet AGL and turned on the unit.  It immediately wanted to pitch up and then down and seemed all over the place.  It wasn’t dangerous, but it was a bit disconcerting.  After about three or four minutes, the unit settled down and it was rock solid.



Settings for my RV-8 (your settings may be different):


            LAT Activity        3

            LAT Torque       12

            Baud                 9600

            Bank Angle        MED

            Microactivity       0

            Half Step           off


            Vert Activity       5

            Vert Torque       12

            Static Lag          1

            Min Speed         75

            Max Speed        200

            Microactivity       0

            Half Step           Off






Comments or suggestions:


bruce_a_lane 'at ' hotmail.com