Graupner CIrrus Electric Conversion

Acknowledgements

Many have electric-converted their Cirrus before me. I have read many on-line posts/threads by others, and also lots of people have given of their knowledge in forums. However, for this particular conversion I'd like to thank John Chandler and Peter Pine for many very helpful emails around motor selection. Also, John chopped the nose off his the year before I did mine, which helped me finally make the decision to do it and provided helpful pictures of the details of his method.

I would especially like to thank Peter Van Alkemade whose skill, excellent tool shop, generous giving of time, and good company was fundamental to the successful outcome of this conversion. Pete did a lot of of the motor install work as I watched and learned. I did cut the nose off myself though…

Why Do This?

We don’t have a convenient slope to fly from at our club. Further, we rarely get the bungee (HiStart) out for thermalling days. So the Cirrus was not getting much flying at all. For a long time I contemplated an above-the-wing power pod. Peter Van Alkemade had the idea of dual self-contained wing mounted power pods (rubber banded on). However, although the proto-types did work well on the 2m glider we tested on, they added too much weight to the wings and the performance was poor.

The traditional above the fuselage power pod is a poor solution really and in the end I decided that if I wanted to fly the Cirrus, the nose had to come off.

Choice of Parts

• The motor is a DualSky XM2838EG-9 outrunner - note NOT the L model. This has 28mm can diameter, 380W at max power on 3S with 10x6 prop. It is specifically designed for narrow fuselage gliders. See http://www.dualsky.com/Xmotor_Outrunners/XM2838EG.shtml. The small diameter can was a big advantage to the install.

• HiFei 60Amp SBEC (I have rudder, elevator, spoilers and ailerons so needed an ESC that could drive all of those) speed controller

• Aeronaut Folding props (10x6 and 10x8) - 8mm yoke

• Aeronaut cool nose 38mm spinner and 42mm centre piece

• Aluminium sheet from JayCar

• Turnigy 2200 mAh/25C battery, XT60 connectors (HobbyKing)

• Rudder and elevator servos are JR ES539

• Aileron servos (in wing) are HiTec HS65HB

• The spoilers are from HobbyKing

• The receiver is a Spektrum AR620, one of their new range with no external antenna.

The motor, ESC, spinner and props were purchased from Peter Pine at http://flyelectric.com. The servos and receiver are not specifically relevant to the electric conversion, I just list them for completeness.

Cutting off the Nose

• Remove the lead shot from the nose

• Take a deep breath

• With a hack saw, cut about 1cm (to leave plenty of margin) in front of the internal ABS plastic bulkhead

• Trim off excess and sand back to the bulkhead.

• Now is also a good time to carefully cut out the 2mm ABS plastic bridge that retains the canopy pin. Extract the pin and spring and save somewhere you won't lose it!

Fig 1 - cutting off the nose

Fig 2 - nose cut off

Fig 3 - Nose fully removed and sanded back to bulkhead. The canopy pin has been removed also.

Bulkhead Plates and Attaching the Motor

Because ABS plastic is quite soft, front and rear tempered aluminium (about 1.2mm thick) plates were created. I did not want to attach the motor directly to the ABS. The Aluminium used was that sold at JayCar electronics (Australia). The approach is to glue one plate on the bulkhead front, and sit one plate behind the bulkhead. The motor is behind the rear plate, which is just held in place with the motor screws. I did not glue it in, because if I should damage the nose sometime, having some chance to access the plastic bulkhead from behind is useful.

With this approach, you have to contend with an extra 2.4mm of thickness (the two Aluminium plates). You have to make sure that the motor you purchase will work with this configuration. That is, there must be enough motor shaft protruding through the triple bulkhead so that the propellor can be well attached.

With this approach we are not building in any down-thrust. It looks terrible and is hard to do well. You will use the elevator to manage this when flying (either manually which is what I prefer or with some mixing).

Fabricate the plates (use a band saw to rough cut) to fit. The exterior profile is obviously easy to draw up from the fuselage. The interior plate was made by starting with the exterior profile minus the width of the fuselage. Then you just file it down to fit. It's not critical that it's a perfect fit. However, you do want the interior plate to butt up against the plastic bulkhead with no gaps - this is essential.

The interior plate needs notches cut for the fuselage ''ribs'' that run longitudinally along the fuselage. You also need to notch out the top around the canopy pin hole so that the pin spring can butt up against the extant ABS bulkhead. You will probably need to use a dremel tool to sand down any ABS molding protrusions that would stop the plate from butting up flush to the ABS plastic bulkhead.

There was a bit of moulding at the top of the interior fuselage against the bulkhead on mine which was a bit annoying as it’s difficult to get to.

Although a method that drills the motor screw holes in situ with the two bulkheads in place is very appealing, we did not take this approach. This is really only because you have to drill free hand (perhaps then having a skittering drill bit or not perfectly vertically drilled). Instead, we took an approach which allowed us to use a drill press with the plates not installed. We did still end up having to fiddle the holes a bit.

The Cirrus nose profile is not circular. It's close to circular at the bottom, and more elliptical at the top. I picked a spinner that fitted the lower circular profile (38mm). Therefore, there is 0.5cm or so of bulkhead that the spinner does not cover. I think this is a better choice than picking a bigger spinner which would then protrude over the fuselage sides at the bottom. You could also locate the centre hole upwards a little so that the 38mm partially covered all of the bulkhead. However, there are advantages to not doing this. In particular, replacing the canopy pin (see section below) and maximising the space for the motor exit wires (since this is an outrunner). It would be no issue with an inrunner motor. This means that the centre hole for the motor is at the centre of a 38mm diameter circle that perfectly overlaps/fits the lower section of the fuselage.

Now proceed by

• Make a 38mm circular cardboard template with a compass, and put a small hole (just a pencil tip) at its centre.

• Trim (with a file) the exterior plate lower half to match the circular template and fuselage. You can stick it onto the fuselage with two-sided tape to do this or just make it match the template. Check alignment issues at every step, because the final result critically depends on this.

• Mark the centre hole of the motor shaft on the exterior aluminium plate.

• Find an X-mount that matches the motor screw hole pattern

• Use the X-Mount to mark the screw holes on the exterior aluminium plate. In doing this you have to decide if you want to rotate the motor a bit. We rotated the motor about 30 degrees so that the connection leads from the motor run towards/along the fuselage side (because the ESC is mounted on the fuselage side).

• Use a drill press to drill a pilot hole (smaller than the it will ultimately be when the motor shaft and collet are located in the hole)

• Drill 4 holes of the correct diameter (for the screws of your motor) in the exterior plate.

• Locate the plate on the front of the fuselage with two-sided tape

• Drill the 5 holes through the ABS bulkhead via the extant holes in the exterior plate. Try hard to hold the drill perpendicular to the bulkhead.

• Locate the interior aluminium plate inside the fuselage - again it MUST fit flush against the bulkhead.

• With a pencil, mark the centre and screw positions through the holes in the exterior plate and ABS bulkhead.

• Extract the interior plate and drill the 4 screw holes and pilt centre hole with your drill press.

• Make sure all holes/plates are aligned correctly..

• Remove the exterior plate and carefully widen the central hole to match what you need. It depends on your motor and collet. You can do this by using ever increasing diameter drill bits. The hole should be just big enough so the rotating component has no friction. There will probably be only a small width of aluminium plate between the drill holes and centre hole.

• Do the same for the interior plate

• Reattach the plates, and use a round file to file out the ABS central hole to match those of the plates.

Fig 4 - Exterior aluminium plate with all holes drilled and temporary bolts (not counter sunk) for test alignment

Fig 5 - The interior aluminimum plate shaped, notched and drilled

At this point you are ready for a test assembly of the motor. Because we have added 2.4mm of thickness on top of the ABS bulkhead, the bolts that came with your motor may not be long enough. Ultimately, you will want counter-sunk hex-head bolts.

With bolts that are long enough, fit the motor (endless fiddling). You may have to enlarge screw holes slightly (perhaps in one direction) in case of alignment issues. The motor should be perpendicular to the ABS bulkhead. If it’s not, most likely the interior aluminium plate is not fitting exactly flush against the ABS bulkhead.

Fig 6 - Testing the motor alignment. You can see it is rotated so the motor wires aim to the side of the fuselage

When you are happy with the motor install alignment, it’s time to glue on the exterior aluminium plate. Make sure you have removed any temporary two-sided tape. Roughen the ABS and aluminium plate, and affix with a slow cure epoxy. Use sticks (e.g. round tooth picks) to ensure the plate aligns with the holes in the ABS and hold it all on with rubber bands looped over the wing rods and tow hook..

Fig 7 - Epoxying on the exterior aluminium plate. There are alignment sticks through the screw holes and penetrating into the foam.

Once the plate epoxy is cured, you can trim the plate around the fuselage with a file. Put tape on the fuselage so you don’t file it away also !

Continue the motor install by counter-sinking the exterior plate screw holes just a little so that your bolt heads will not interact with the spinner (there will be a gap of about 1mm). The length of the bolts is of course critical. They must not be so longer that they intrude into the motor windings and not so short that you don’t have enough turns in the motor threads to hold on securely. More fiddling to get this right and you may have to grind down some longer bolts to the right length. For the DualSky motor, 8mm length (M3) countersunk bolts were fine.

Finish the motor install by screwing in the bolts and making sure that there is no friction with the centre hole.

This is by far the hardest part of the process. It’s easy from here !

Fig 8 - The exterior plate with countersunk bolts attaching the motor (the interior aluminium plate is in place also of course). The collet can be seen to extend to just shy of the the surface of the aluminium plate. Therefore, although two of the bolt heads (the bolt holes are not all equidistant from the shaft on this motor) extend over the central hole, it doesn't matter. You In this picture you can also see the hole for the canopy pin (see next section)

The Canopy Pin

Use a small diameter drill bit to drill a hole though the exterior aluminium plate (remember the interior plate is notched around the canopy pin) so that it perfectly aligns with the existing canopy pin hole/track. Do this very carefully ! Locate the canopy pin and spring back in the fuselage and make sure the pin slides freely. When it is all the way out, it will extend out past the exterior aluminium plate. Here is the cunning bit. With the choice of 38mm diameter spinner, the spinner outer surface sits just a tiny bit below where the pin extrudes. The pin will extrude over the top of the spinner surface when it’s all the way out (only when you are not flying!). We found we needed to grind off a tiny bit of the pin under-end and bend it up ever so slightly. If your spinner is bigger or you located it higher, this is not going to work and you will have to attach the canopy in some other way.

When you are happy with the pin location when its fully extended, find a small diameter piece of brass tubing to fit neatly in the slot that you made when you cut out the canopy pin bridge. Cut a section that is about 3mm long. Put the canopy pin, spring and tube assembly in place. You will need to use a piece of cotton thread to hold/lift the pin up when you are trying to do this. It’s all very fiddly of course, like everything else.

Once it’s all sitting in place, and the pin slides smoothly out (compressing the spring), you can epoxy it in.

Just place a small amount of epoxy over the top of the brass tube, taking great care not to epoxy the pin ! Locate also a piece of fibre-glass thread over the top of the tube and down the interior sides of the fuselage (see picture - the sides that mate with the canopy) and epoxy that thread to the fuselage. This is just a little insurance to hold the pin in.

Fig 9 - The canopy pin and brass tube in place (not epoxied yet). The cotton thread used to manouver it there can be seen still.

Fig 10 - The canopy pin extending though the hole drilled in the aluminium plate. It's tip has been ground and bent a little. The spinner plate can be seen sitting just below it.

Fig 11 - The canopy pin tubing epoxied in place with glass fibre thread epoxied over the top of the tube and to fuselage.

Removing the Internal Bulkhead

Next you will need to remove the interior ABS bulkhead nearest the motor. This is because the battery will be too long to fit in the compartments that are left. Later you will make a new bulkhead close to the motor. Cut this out with some combination of modelling knife and dremel. I tried to leave a couple of mm all the way around so to leave some strength in the moulding. This was also useful for fashioning the battery plate later.

Fig 12 - The front section showing bulkhead removed.

Locating the Battery and Speed Controller

The choice of battery is critical. You need enough Amps (C rating) to correctly drive your choice of motor. You need a battery with sufficient capacity to last you long enough. You need a battery with physical dimensions sufficiently small to fit. The Turnigy 2200mAh 25C battery meets all of these needs for the motor I have chosen. I could not find any other battery to work (mainly the height of the battery is the issue). The Zippy 35C 2200mAh (if you need extra Amps) is just a little bigger physically. It can still work but is MORE fiddling !

The best physical arrangement is with the ESC mounted with velcro on the side of the fuselage. The ESC I picked, although relatively large, does fit nicely between the horizontal ribs of the fuselage wall. With the velcro stuck to the fuselage, it fills the space so that the velcro stuck to the ESC base mates well with it. I had to extend the ESC battery leads and shorten the motor leads. Because the ESC wires are relatively large (because of the current it is rated for), it was difficult to coil up the excess motor leads - so shortening was better. The ESC battery leads are positioned around the rear bulkhead with the connector emerging on the opposite wall. I had to lengthen the ESC battery lead to achieve this.

The battery is located on its side with the battery leads emerging from the rear of the compartment - they then coil around to plug into the connector from the ESC.

I made a ply plate to velcro the battery to, and also added a narrow velcro battery strap. The ply plate sits on balsa feet that match the shape of the fuselage contours (I used a little pressed in clay to get the shape and then cut the feet to that shape). The feet are epoxied in and the plate epoxied to the feet. I used the couple of mm of height of the remains of the removed bulkhead to abut one of the feet. I angled the plate a little to provide a little more space where the ESC was thicker. Getting the feet all the right height so that the plate was touching all four was a bit tricky. I had to add thin ply shims to a couple. I didn’t fully cover the plate with velcro because that would be likely to rip the whole thing out when removing the battery.

With the ply battery plate in place, you lose some vertical space, but I prefer the security of affixing the battery (you could rely on gravity). With the Turnigy battery, I had to shave a little off the canopy moulded cross piece (the bit the ‘control panel’ is glued to) to make it all fit.

The last part of this installation is to make a new forward bulkhead. I fashioned mine from 5mm ply and located it so as to allow a sensible fit with the battery. I allowed some extra space for the Zippy battery if I ever need to try to use it (its a little longer than the Tunigy) incase the Turnigy C rating was inadequate (should be ok). The ply bulkhead is shaped to fit the fuselage contours with notches for the horizontal ribs in the fuselage wall. It is about 1/2 the height of the compartment.

The new forward bulkhead, apart from re-stiffening the fuselage, also serves as a ledge to hold the motor wires up so that they won’t touch the spinning interior part of the out runner motor.

With the Turnigy battery, the CoG of my Cirrus was perfect with no additional weight. If you do need to add weight somewhere, it won't be too much.

Fig 13 - The feet for the ply battery plate and forward ply bulkhead.

Fig 14 - the ply plate epoxied to the feet, with velcro and velcro strap. The ply bulkhead has been epoxied in place.

Fig 15 - The ESC velcroed to the fuselage showing extended battery wires located around the back and side. The motor wires have not yet been shortened

Fig 16 - The shortened motor wires connected to the ESC. You can also see the new front ply bulkhead.

Fig 17 - All electronic components installed. The receiver (Spektrum AR620 - no external antenna) can be seen (yellow light) velcroed to the tow hook bearer (with tape over the top). There is good clearance from the push rods partly because the AR620 has front located pins. The ply ledge above the receiver serves to look after the wires for the spoilers and ailerons. All the extra wire lengths from the ESC throttle wire, servos, and aileron/spoilers are curled up under the rudder and elevator servos.

The canopy latched in place (there is a cavity under the interior canopy moulding so the motor wires are up in there), cool-nose spinner (I didn't do anything extra to take advantage of its extra airflowy) and 10x6 prop. You can see the gap above the spinner because of the non-circular profile of the Cirrus nose

First Flight

The motor has tons of power as you can see in this short video. In fact, I had to back off the max allowed throttle as the max draw is 31Amp for 15sec and I really didn't want to burn it out. So the max throttle setting I have doesn't allow much more than that.