Rans S-7 and S-7S with Rotax 912

Assembly topics

Last updated 2011/02/25


The material presented here has been provided, for the most part, by Rans S-7 builders and owners. The intention is to provide a useful supplement to the Rans documentation. This material has NOT been edited by Rans personnel and in some cases may only represent the opinion of the builder. Exercise normal caution and judgment before accepting this material for your own use.


Most of the following material is based on the S7S airframe. Much of it applies to earlier models as well. If you are working on an earlier kit, make sure what you are reading applies to it. Wherever possible we have tried to indicate if it is for the “S” only or all models (A). Some general topics could also apply to other aircraft.



2007/03/03      General Assembly:  text and pics (wing structure, aileron bellcrank)

                                    Battery Location:    PowerSonic battery note and other wording

2007/03/13      Pre- S7S Topics

2008/11/29        Pulled rivet technique

2011/01/23         Panel, firewall access

2011/01/28      Wiring: diagrams and Master issues


1.    General assembly instructions.  (A)

2.    Landing gear seating in socket

3.    Fitting fiberglass boot cowl       (S)

4.    Windshield                                  (S)

5.    Panel and firewall accessibility    (pre S model)

6.    Wiring                                         (A)

7.    Battery location                          (S)

8.    Ground connections                   (A)        

9.    Propeller choices

10.                       Cold weather modifications       (A)

11.                       Lifting rings

11. Pre S7S Topics

              a. Exhaust Systems




General Assembly Instructions


You will find a useful reference document covering metal work, riveting and painting topics at: 



This document covers aspects of solid riveting as well but there are no solid rivets in Rans kits; only pulled rivets. See below for some comments on dealing with pulled rivets (contributed by Gordon in the UK).



Different components and materials require different assembly techniques. There is an excellent section on the proper sizing and tightening of bolts in the above document. For example a bolt holding the motor mount to the firewall needs to be torqued to a specific value to achieve the correct strength. It is in tension and needs a full sized  tensile nut”. Bolts retaining many other parts, for instance the landing gear in its socket are not in tension but in shear and so nuts can have fewer threads (shear nuts) and need not be tightened to maximum torque values. Similarly castle nuts with cotter pins usually indicate parts in shear and so do not need to be torqued.


Concerns when bolting aluminum

The wing has several aluminum tubes bolted to U brackets on the spars. Here excessive tightening can distort the tube and bracket and/or bottom the nuts on the unthreaded parts of the bolts; both situations must be avoided. In the picture, the bolt  has been over tightened deforming tube and bracket and probably bottoming nut on unthreaded part of bolt:


 Other joints such as the strut attach plate to wing spar utilize internal tubes to prevent distortion allowing proper torque values to be used.


Also in the above picture you can see that one of the rivets in the clip has a dark ring around it indicating it was not seated properly before pulling. In this case the head needed to be filed to clear the adjoining clip.



The “pulled” rivets used throughout the Rans products are quite simple to use but still require correct technique.  The Murphy document covers proper drilling and preparation of holes.  You must ensure that the two parts being riveted are mated tightly together. This sometimes requires appropriate filing. For example the aileron bellcrank is riveted to a spindle. The spindle is machined with a shoulder that prevents the bellcrank from seating tightly to the spindle. You need to round or chamfer the edge of the hole in the bellcrank to achieve proper mating or you will end up with improperly set rivets.  See picture below:



This is an S model bellcrank but a similar problem existed on earlier models.


Tips for working with pulled rivets


General aircraft sheet metal practice is to use the drill diameter in
decimal rather than fraction. So 3/8" is .375", 1/8" is .125" etc.
Fortunately there is not too much that can go wrong when setting pop
rivets if you have them square to the hole and the head in contact
with the materials being fastened together. It is good practice to
use the correct size clearance drill. These are identified in the
build manual by a number rather than a size, e.g. #11, #30. The
clearance drill ensures that the rivet can expand just the correct
amount during setting without buckling thin materials or over
stressing thicker materials. Deburring the hole is important too as
any external burrs will hold the rivet head off the material and
result in a poor job.

The rivets should all look pretty much the same and if there big
differences then it could be that you are not drilling the holes
squarely with the material. A slightly elongated or oversize hole
will cause the rivet mandrel to pull further through the rivet and
break off outside the rivet head – much the same as using a rivet
that is too long for the material being fastened together. Sometimes
this happens anyway when the rivet size is not optimum, in this case
I carefully grind the exposed shank off with an air powered die
grinder if the rivet is somewhere the mandrel needs to be flush with
the head. A Dremel should work Ok but be very. very careful not to
knock the mandrel head into the rivet as this part provides a large
percentage of the shear strength of the rivet.

If – and when – you need to drill out a badly formed rivet get a
small diameter pin punch, or use a discarded rivet mandrel from the
same size of rivet, and knock out the steel mandrel from the formed
rivet then drill the head off the rivet. Aim to simply drill the
head off the rivet then use a punch to knock the rivet tail out of
the hole. This helps keep the original hole size from being
oversized and in turn giving a poor set on the new pop rivet that you
install. Don't deliberately drill all the way through knocking the
tail out with the drill bit, and don't try to drill the rivet out
with the mandrel in place as the drill will run off centre and spoil
the work piece.



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The Lexan provided by Rans is designed to provide a one piece windshield and sky light. Tipp City Plastics can provide a formed plexiglass windshield which is easier to install and does not have the Lexan characteristic of cracking when under bending stress and in contact with gasoline. Installing this formed windshield requires a joint with the skylight above the main spar carry through.


Picture of formed windshield prior to installation:


Skylight only installed as per manual.


Installation of formed windshield:





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The S-7 build manual says: “Set the instrument panel in place and wire it according to the diagram”.  Perhaps there are a few more issues that you should consider before you start stringing wires. Further, the Rans wiring diagram differs a little from the one Rotax publishes for the 912 in the area of the regulator wires and fuses and if you are dealing with a pre2000 kit and followed the diagram exactly you could end up with some things being powered without fuses or breakers.

The 912 wiring diagram also shows the provision for an indicator light to show when the system is not charging (or the master left on after shutdown) yet the Rans diagram leaves this very important item off.


Everyone would be well advised to look into current aircraft wiring practices before just wiring as per the diagram.


Diagrams:                      Early 90’s S7            Rans S7S  S7S Legend             Rotax 912                 My version

Discussion:                                                                                           Why fuse the C line?     including

other comments on Rans vs Rotax diagrams.


Here are some things to consider and make a decision on.


1.    Maintainability.          Instruments and switches sometimes break and need to be replaced. You may want to add an instrument or other device later. In both cases you need access to the back of the panel. Access is very easy without the boot cowl and windshield in place but once they are installed, the only way to get at the panel is from the cabin and unless you plan to make the panel removable, it won’t be. The air line tubing, wiring and mechanical controls have to allow for panel removal by having sufficient slack in their runs to allow the panel to tilt away from its mount after the bolts are removed.  While the best way to connect two appliances together would be with the fewest connections and the shortest length of wire, accessibility and serviceability suggests using longer runs and perhaps plug type connections.    Battery access on the S7 is not easy either and improving it could be important.


See topic on “Panel and firewall accessibility


2.    Adherence to accepted electrical practices such as:  ability to fully disconnect the battery,  avoiding negative effect of heat on components, fusing all uses of power.        The wiring diagram shows what is connected to what using the correct size wire and terminal ends but where you actually place components is open to debate. The placement of some things is fixed but many others are positioned at your discretion. You can decide where the starter solenoid, rectifier/regulator, ground and power busses, fuses and switches physically sit in the aircraft. Where you put them must take into account certain basic rules such as:  


a.     As an electronics part, the rectifier regulator should not be mounted inside the cowl where it is exposed to heat; behind the firewall is a much cooler place to put it. Thus the 2 wire AC output from the engine stator can be routed through the firewall to connect to the rect/reg, capacitor, 30amp fuse etc. mounted on a plate aft of the firewall and making it accessible from beneath the panel. Here is a picture of a typical firewall at the top right side with starter solenoid, regulator, capacitor and fuse all on the engine side as per manual:                                                                                                                                                                          


Next is a firewall with most of the electrical inside:



And here is what is behind the firewall:


Below is another similar panel that will mount behind the instrument panel. You can see the two red power wires (R and B+) combined with the green C line at the top fuse post. The charge indicator lamp is connected to the same fuse post;  the other side of the lamp goes to the L terminal at the regulator. Also the +ve side of the capacitor goes to the same post. The  -ve side of the capacitor goes to the ground bus on the other side. The lower terminal of the fuse goes to the +ve buss and fuses on the other side of the panel where the battery lead from the master switch will also end up..

The two yellow leads from the engine will go into the right side of the connector. They come with enough length to get 8” or so aft of the firewall.

This next picture shows the other side. There is an automotive fuse block with lines going off bottom right to switches for radios, instruments, accessories etc. When mounted in the plane, the main #10 line from the master switch will also go to this bus.  The smaller bus at the top is for ground connections. The ground bus is electrically connected to the plate and that #10 wire bottom left goes to the frame for ground. The regulator itself calls for mounting on a grounded surface.


b.     The starter solenoid is commonly mounted on the engine side of the firewall but there is no significant reason for having it there except that it provides a convenient connection for generator output and the power source for appliances. If you put the reg/rect etc behind the firewall then the starter solenoid could also go there. Many people believe (me included) that it is best placed right at the battery so that the #4 starter cable is not permanently hot.  Placing it beside the battery would then require a #10 wire to carry power to the main bus. In the picture above there is no solenoid because it has been moved to the battery box.


c.     Most certified aircraft include a contactor/solenoid for totally disconnecting the battery. You should decide if this is important to you and, if it is, install one near the battery. Such a contactor can break either the ground or positive cable. In the next picture the panel beside the rear seat holds the master battery contactor (a manual rotary switch that cuts the ground cable), the starter switch and a boost/charging jack.


d.     A fuse or breaker protects the subsequent cable run from burning in the event of a short. The fuse/breaker is mounted very close to the source of power (bus) followed by a switch, then the wire going to the appliance.


3.    Plan for a dead battery at some point. Consider adding an accessible boost/charge plug like the one on the top left of the above plate.

4.    Battery accessibility is an issue on all S7 models so consider a better access hatch or moving the battery forward. See below for one option.


                   Note about solenoid contactors.

                   One of the reasons for going to a manual contactor is that it requires no current to keep it engaged. Thus, if you inadvertently left it on, the battery drain would only be due to whatever other items are turned on; if nothing else was on, there would be no drain. If an electrical one is used it could draw up to several amps depending on which one you chose so, on its own, it would drain the battery.


For example, the Rans supplied starter solenoid draws 2.7 amps; a common automotive solenoid draws 4.0 amps; an Amco RV battery separator solenoid draws only 1.0 amps but it may be designed to carry only a 30 amp charging load not the full starter current load.


The function of the master contactor is to cut off all power anywhere in the system unlike the default wiring proposed by Rans which has the large number 4 cable always hot from the battery all the way forward to the firewall where it goes to one side of the starter solenoid. One compromise would be to implement the above suggestion putting the starter solenoid at the battery, then run the # 10 wire from a 25 amp fuse at the battery to a “master switch” at the rear seat (such as on the above pictured plate). This way, when that switch is off, there is no power ahead of the rear seat plus it is a fused wire. This arrangement results in much less fire hazard in a crash than the always hot, unfused # 4 cable and eliminates the constant current drain from an electric solenoid.


In the next picture, an access panel has been installed beside the battery. The starter solenoid is mounted right to the flange of the battery box as is a 25 amp resettable automotive fuse. The short cable from the battery to the solenoid was originally up front going from the starter solenoid to the starter. The heavy white lines take power to the fuse then forward to a master switch. The small white line will go to the starter switch. With this arrangement, the number 4 cable going forward is only hot when starting. The number 10 cable going to the master switch is always hot up to the switch which in this installation will be beside the rear seat with the starter button.




In the picture below, the starter solenoid is bolted right to the battery post: This PowerSonic battery has vertical, drilled lugs on the battery which make this approach easy. Batteries like the Odyssey which have bolts going into the top would need short 90 degree brass brackets to connect to the switches. 



The switch on the right is a manual rotary disconnect switch breaking the ground run. Most installations would not allow it being mounted so close to the battery.

This is an option if you were thinking of putting the battery right up front as some people have done.



5.    Once you have positioned all the components, you are ready to connect them together.


Excellent reference material on wiring is available from Bob Nuckolls at the Aero Electric Connection. A related company, B and C Electronics, sells wiring hardware and devices such as over voltage relays.



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Alternative battery access hatch

Getting at the battery and the cables through the belly access panel on a short tail or even through the side hatch on an S model is not so easy. Here is an alternative on the right side. First you’ll need a frame to provide support for the fabric and a base for fasteners:


Next it is glued inside to the fabric

Next the starter solenoid and a fuse for the main power going forward will mount here.


And then the door fitted


Battery Location


Most people are quite happy to position the battery in the tail of the aircraft where Rans has provided the appropriate attachment points. The manual adequately covers this installation. There are both benefits and concerns for mounting it further forward as some builders have chosen to do. 


Some of the concerns over the factory location are accessibility, length of cable runs and quality of grounding. Although there is a large battery access panel, it is not positioned opposite the battery so it is quite difficult to get both arms holding tools inside to install or remove cables and the retaining rod. Mounting the battery closer to the firewall greatly simplifies access to it and also improves the electrical characteristics by shortening cable lengths, providing a more direct, through the firewall engine ground and potentially reducing the number of cables and joints.


For these reasons, more than one builder has chosen to mount the battery between the rudder pedals. You should not attempt to do this without carefully examining the center of gravity issues caused by moving such a heavy item forward. One result is likely that under some load conditions you would need to carry maybe 25lbs in the baggage compartment. Some people have chosen to add a baggage compartment fuel tank to handle the cg issue along with the benefit of added fuel.


Other builders have positioned the battery beside and under the rear seat and in the baggage compartment (in the case of a metallized baggage box).


Whether or not you are considering repositioning the battery you may also want to think about including a battery master cutoff (electric or manual) and where it should be placed as well as where the most effective position for the starter solenoid would be for your installation.


This picture shows the stock battery box modified to provide support for the manual master switch (made by Hella) mounted on the negative post and the stock starter solenoid mounted on the positive post. Flanges have been bolted and riveted to the sides to allow for securing it to the floor. The manual master is easily reached by the pilot with the battery on the floor between the rudder pedals.




The starter cable coming off the solenoid goes through the firewall to the starter. The negative cable from the manual master also goes through the firewall to one of the drilled lugs on the back of the heavy duty starter for grounding of starter and engine block.


The frame is then grounded by running a smaller cable from the other started lug to the firewall using a brass bolt through the firewall to connect to a multi terminal ground bus bar inside. (See Bob Nuckolls articles on grounding which use this approach) Most electronics equipment should be grounded back to this bus. Other devices such as lights and pump can be grounded to the frame. The next picture shows the lugs on the starter:


Ground wires need to be the same size as the wire supplying current to the device.


Here is a picture of a ground bus from B and C followed by a picture of the ground cables to the starter and firewall.



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Pre- S7S Topics


Exhaust Systems

Early 90's muffler had slip joints not ball joints and was hung off the mount only. Next iteration had 1 ball joint per pipe outboard of muffler. Next iteration had 2 ball joints per pipe between cylinder head and muffler and had a stainless steel version (maybe they all were in this style) and were supported below. Many people have had issues with the alignment and durability of the lower concentric rubbers support fitting as well as water pump and firewall clearance issues. Some of us have lengthened one or both of the front to rear stacks to help with water pump clearance (but making firewall clearance worse). I lengthened the right one and had to put some right to left tension on the muffler and used a combination of springs to mount and one lower support. Not everyone did this.