My 5 year old Super Chipmunk "Pennzoil" from Apache Aviation now supplied by SLEC had always had an ASP180FS up front. Beautiful sound but I now feel that the model really needed something more substantial power wise. The ASP180 should be producing around 2.6 hp when tweaked and tuned and I have to say I have tried many props on this motor and never felt really happy. Plus the fact that the glow fuel residue inside the cowl would slowly weep out and eat the "so called" fuel proof paint on the cowl. But what could I replace it with. DLE30 or DLE35RA or OS GT33? After much deliberation I went for the OS GT33. Coing close to four horse power from this tidy gas engine. However, the conversion to petrol was not as simple as imagined!
First I would need a new cowl, stand-offs for the OS (they are not supplied as standard unlike DLE), optical igntion switch, change the throttle control and much more. The cowl fortunately is still available from SLEC, the stand-offs were an issue but discovered my redundant Wots Wot EP kit had the right length of aluminium stand-offs. In my spare parts store I had a Dubro Quik fill for petrol, lengths of Tygon fuel line, On to the internet for a Rxel opto switch and I should get by with these bits to keep the cost down especially after the purchase of the OS GT33.
After removing the ASP180 and its Apache anti-vibration mounting this left the firewall with four large 18mm holes. The OS engine mounts partially lined up with some them and that was not good. I made up some 6mm ply plates and these were glued on to the backside of the firewall which closed off the offending holes. with all that thickness of ply I could now drill the stand-off locating holes in the correct positions. The stand-offs were 4mm thread holes so the next step was to drill out the holes to 4.5mm. Then the new holes were tapped out to 5mm. Through these holes the 5mm socket head bolts complete with oversized flat washers would secure the stand-offs.
Now that the stand-offs were loosely located I proceeded to fit the GT33 engine to check where the throttle control rod would need to be and mark the position on the firewall. At the same time temporarily fit the exhaust to verify clearance. Close but acceptable to the firewall and pleased that OS had kept the muffler compact and close fitted to the engine casing.
- Engine Removed Engine Removed
- OS GT33 Gas Engine OS GT33 Gas Engine
- Stand Offs Stand Offs
- Firewall Firewall
- GT33 GT33
With the throttle control hole position marked the exhaust was removed so that the cowl could be slide over the new engine to verify the prop driver was just a few millimeters proud of the cowl. With this checked out I could now refit the exhaust permanently and tighten down all the mounting bolts.
The throttle control rod was made up from Sullivan semi-rigid snakes to ensure there was little slack or give when operating the throttle. Unfortunately the control needed two slight curves which together created to much friction and resistance when using the original throttle servo position. The ideal position found was up nearer F2 (see image). This meant creating a new servo mounting tray between F2 & F3. (see image) The control snake ran alongside the tank and lined up with the carb throttle lever with virtualy no curve. A ball link will be used at the servo end and the carb end uses a spring clevis which enables some adjustment when necessary.
In Former number two, I drilled two large holes to allow for wiring and to reduce some weight as these models are a little over built but very strong. With the 20 oz tank still removed I installed the ignition module inside the tank area secured below the tank position by cable ties and insulated by foam rubber. The powerlead coming through one of the large holes. This model has a hatch on the underside known as tank access which makes life a bit more easier when installing the tank and lines.
Before installing the tank, the bung was replaced with a gas stopper and lengths of Tygon fuel lines and clips. The breather line was looped and stuck into place on the top of the tank sitting in foam packaging which not only acts as a tank insulator it also locates the breather in the desired position. Each side of the tank area has foam sponge to help locate the tank. The tank is held well clear of the firewall by a shelf so using the Dubro style tanks is ideal. The lines were then fed through to their locations. Once the tank was located the line's excess lengths were trimmed off. Fill line fitted to a Dubro Quik-Fil the other side blanked off. This will act as fill and drain point. The breather line would be fitted with a dedicated breather which will fitted into the tank access hatch.
With that part all done it was time to fit the throttle servo permanently. Choke rod was made up from 3mm control rod with a 3mm clevis and locknut at the choke lever end, passing through the a dedicated hole by OS in the crankcase casting and the end bent in a slight Z just short of the prop driver height. When the cowl gets fitted the bend will be adjusted to line up with the cowl air intake.
Here is the fun time! Trial fit the new GRP cowl and using a 12v bulb on a lead inside so I could determine what areas needed cutting out to clear the cylinder head, exhaust etc. With these areas marked out with a felt tip it now was time to gradually, a bit at a time to dremel out the grp with a cutting disc. With the cowl now clearing the cylinder head the cowl could now move back further so I can mark out the exhaust. The exhaust did not need the cowl to cut out much at all. With the cowl now sliding back to near it's final position I then refitted the spark plug to make allowances for the plug and cap. With the cowl now fitting nicely the next job was to mark the mounting screw positions, the originals which could be seen through the grp. Marked up with a felt tip and then th ecowl removed and drill 4.5mm holes. The mounting screws would be 4mm button head/allen drive machine screws. These screws give a low profile and would be painted to match the cowl colour.
Running a sanding wheel over all my cut outs to clean up edges it was time to get painting! I used "Halfords" Etch Primer for the first coat. After it had dried I then lightly 600 wet n dried the paint to remove any nibs, etc. The Chipmunk cowl is two tone, Red top half and white lower. I masked up off the top half creating the required line along the side of the cowl. I then gave the cowl two coats of "Halfords" Appliance White. I used an electric heater to speed up the drying process. With the white now dry I now masked of the new paint and gave the top of the cowl two coats of VW Mars Red. This red is the closest I could find to the original Oracover flame red. Why did not use the matching enamel? Takes too long to dry, smells and I much prefer using car acrylic paints anyway. So many colours.
- Fuselage Fuselage
- Cowl Painting Cowl Painting
- Masking Off Masking Off
- Cowl Fitted Cowl Fitted
- Cowl View Cowl View
Whilst the cowl was off I thought it might be a good idea to replace the tatty film covering at the front of the fuselage, although most of it is covered by cowling. A lot of the film issues were down to the oily blow-by from the ASP180 or when it kicked back throwing neat fuel up into the cowl area.
Being a slighty lighter engine and the Chipmunk tending to be tail heavy, I decided to move the twin Life RX packs right up behind F2. This meant making up a custom battery tray. Undeneath this tray would be the igntion NiMh4.8v pack (another 100 gms). This tray would laso serve as a tank securing device, so in reality it serves three purposes. With the removal of two screws the tray can be lifted out with RX packs still secured, access to Nimh pack and removal of tank when necessary. The original set up of the Chipmunk was twin lipos through a regulator. To keep things as simple as possible and having less items to go wrong, during this engine swap I moved over to LiFe power for the RX, my Hitec servos would have no issue with 6.6 volts. (Not so lucky with my Me109 - Fitted 4 years ago with JR DS589's 4.8 volt only) Two HD switches with LEDS turn on main power and igntion. These have charge sockets built in so I can monitor from the outside their capacities. Can also charge igntion pack from outside and if need be in desperation the LiFe packs too.
With all systems go and the cowl left off it was time to start the engine. Initially I had a bit of trouble with flooding which meant a few times of taking out the plug and drying off. This was mostly my fault as the new engine pumped fuel through quicker than I thought. (Unlike my DLE's) On the third attempt it started to fire and run albiet very rough. A bit of tweaking of the low needle improved this somewhat and now the motor was starting to run better. I allowed it to warm up then retightened the exhaust bolts whilst still hot. Thinking of my neighbours the final tuning can be done on the dayof the re-maiden flight.
Maiden Flight w/ OS GT33
Sunday 11th September at our Cashmoor field the Super Chipmunk was fired up again without cowl and with the help of Jon Tappin our chief test pilot and always up for a challenge, we tweaked the low and high needles to get a steady run and it was then flown by Jon. It had 8 mins or so of various throttles, Jon making continuous rollings, verticals, big wide loops, inverted flight and stall turns. Sound a little rich but it is a new engine but never gave up. Back down on the ground a bit more tweaking. It will probably need some more fine tuning after the cowl is refitted as the air flow will have changed.
I am very pleased with the outcome. It is not particulary any faster but does have pulling power to pull wide loops and high verticals easily more like it's full size version. The OS exhaust is efficient and not overly noisy at all. Jon's 8 minute flight used 25% of the tank. 125maH of the ignition pack and just 140 maH of the LiFe twin 2100 maH RX packs