Chipmunk Diary

DHC Chipmunk Project Diary

by Trevor Hewson

Having built two of Ivan Pettigrew's designs (the Sealand and Rapide) I made the pilgrimage to the Chilliwack fly-in in May 2010. We were made very welcome by Ivan and his clubmates and he insisted on us taking away a complimentary plan. I chose the Chipmunk.

This choice was partly motivated by having seen three examples of this model fly equally well and partly because, compared to the T61 Falke which was then dominating my workshop, the Chipmunk, at 75in wingspan seemed a little less daunting.

The following monthly diary entries detail the build progress, which was also recorded as a thread on RCGroups.

November 2010 (More on November)
It's always nice to do something a little bit different so I am contemplating a few changes to Ivan's plan:

i) Oleos: This will be a first for me. After a helpful dialogue with Untract, I ordered a pair of their sport oleos - more details on the November page.

ii) Flaps: I have enjoyed playing with the flaps on the Astro Hog although it really is too light to need them. The Chipmunk will hopefully be on the light side too but we shall see.

iii) Hatch: The plan shows a long hatch from motor bay to wing trailing edge which certainly gives great access but is it really necessary? The only access needed when the wing is in position is for battery changing so a hatch in the anti glare panel will hopefully suffice. Everything else should be accessible through the wing aperture.

iv) Power: Even though the three examples mentioned all fly perfectly well on 3 cells, it still looks to me like a 4 cell model (and I've got too many three cell models already!).

Anyway, as is becoming my standard practice, I tackled the wing first. The spar is from 1/8in balsa re-inforced with hardwood strips for the inboard section. The basswood strips I used are slightly oversize - my supplier didn't do 3/16 x 1/8. Because of the dihedral, they have to be spliced so, when it came to the doubler shown on the lower spar, I laminated this out of two pieces of 1/16 so that it could be bent round the dihedral angle, avoiding a join at the centre. The science behind using one lamination of spruce and one of basswood is complicated - it just happened to be what I had in the scrap box.

The rear section of the wing was built first, with paper tubes built in to accommodate the aileron servo leads.

Because the wing is built in one piece, the flap control rod also has to be built in as the ribs are fitted.

With the front rib sections, undercarriage bearers and false leading edges added, the wing framework is really taking shape.

By the end of the month, the underside of the wing was sheeted in and I could test-fit those oleo legs.

December 2010 (More on December)
After building the recessed leading edge centre section,the upper surfaces of the wing were next sheeted and capstripped.

Then it was time top separate the ailerons and flaps from the wing:

The next stage of fitting the missing end ribs, facing the front of the control surfaces and false trailing edge of the wings, fitting hinges and control horns, sorting out linkages and adjusting the gaps always seems to take a lot longer than I expect. Although having just listed what is involved, I suppose it's probably not altogether surprising!

Whilst the ailerons are top hinged with exposed horns on the underside, the flaps are bottom hinged with internal linkages.

I decided to press on and fit the leading edges and wingtips before moving on to the fuselage.

Whilst I didn't want to bother with working nav lights, I felt that some non-working LEDs set into the wingtips would be a nice detail. I was unsure what size would be best so I ordered some red and green LEDs in 8mm and 10mm. Just to cater for any mishaps, I ordered two of each. While I was at it, I also ordered a couple of 5mm white LEDs for the light in the tail fairing. At least, that was the idea.

Despite double checking the order, it was only when I received the despatch notification that I spotted I had ordered yellow 8mm instead of green. When the package arrived, it was surprisingly heavy. The reason became clear when I found that the 5mm white LEDs which I (correctly) ordered had somehow morphed into reels of solder!

When I found that the appropriate size was indeed 8mm and furthermore that the mounting clips I had ordered didn't seem to enhance the appearance, the net result was that, out of an order for ten LEDs and eight clips, only one LED was of any use. Just as well I wasn't ordering anything expensive!

At this point the wing was put aside and a start made on the fuselage. The construction is reminiscent of the rubber powered models of the 1960s and starts with two side frames built on top of each other over the plan. These are then joined together with more 3/16in square balsa sticks.

The resulting framwork is quite flimsy - it will be stiffened up by the 1/16in skins to be applied later but I wanted to get as much of the internals sorted out first.

Once the tailplane and fin were built, the next priority was the rudder, tailwheel and elevator linkages. These are all hidden away inside the fuselage so space is a bit limited. After a fair bit of fiddling though, I was quite pleased with the final setup.

There are more detailed shots, as always, on the December diary page, especially the rudder, tailwheel and elevator linkage arrangements.

I am hoping to get the basics of the wing fixing and motor mounting arrangements sorted before embarking on sheeting the fuselage but it may be that some skins have to be applied to make the structure rigid enough before that can be done.

Anyway, that's a challenge for the new year!

January 2011 (more on January)
The rounded fuselage underside and the wing trailing edge are not exactly natural partners but, after a bit of head scratching, I decided to ignore the contouring of the fuselage for now on the basis that the wing fairing would cover this area eventually. A 3mm ply plate was used to take the T-nuts for the 6mm nylon wing bolts.

The servos are still accessible through the wing seat. The balsa former visible in the top left of this picture marks the approximate rear limit of the wing fairing so hopefully it will be possible to blend everything in without the corners of the wing plate sticking out!

The remaining formers were added, including one immediately behind the wing plate, then the 3/16in square balsa stringers. All of a sudden the fuselage is beginning to look a lot less boxy.

Sheeting in of the underside and rear sides came next.

The sides were sheeted to the front of the wing seat. . .	and then the underside of the nose section.

This enabled the wing seat to be finalised (see January diary entry for more details) which brought things to the point where the wing could be bolted on and the model stood on its wheels for the first time.

Tail surfaces, canopy and firewall are only dry fitted at this stage. The spinner is a modified three inch plastic one (again see detailed January diary page for more details). First indications are that the battery will have to be well back so I opted to fully sheet the first bay behind the firewall - and hope that it would be possible to fit the motor from the front without any access to the back of the firewall.

The last major task of the month was the wing fairing. This gets quite complicated both at the rear end and around the leading edge, mainly because the fairing has to blend into the curved fuselage side. It was a slow and at times tedious job (more pictures on the January page), but the end result is well worth it. I just don't really look forward to the challenge of covering it!

February 2011 (more on February)
	After building up the noseblock out of layers of 12mm and 6mm balsa, the tailplane was glued on and the rear turtle deck sheeted in. This was followed by a few details in the form of the tailplane strakes and the fillet which sits between the elevators.

	With the fin glued in place and the fin strake / canopy runner added, the tail assembly suddenly looks a lot better.
Trial balances indicated that the battery would have to be located under the cockpit. Although the prototype had the cockpit area removeable, I had hoped to get away with a much smaller hatch and leave the cockpit in place. So I set about making a sled to enable the battery to be inserted and slid into position. It seems to work okay on the bench but only time will tell how it works out in the field. More details on the sled arrangement are on the February diary page.

With the hatch arrangement finalised, there was really only the cockpit area left. Two 1/5 scale "Pete's Pilots" just fit into the widened fuselage (the nominal scale of the model is 1:5.5). The shorter chap was glued to a block of 12mm balsa to lower the cockpit floor a bit and the big guy, who is in full WW2 outfit, had his oxygen mask surgically removed. They were then given to fellow modeller and brushmaster Mike for painting.

Instrument panels were again 1/5 scale and needed trimming down to fit.

Once the cockpit surrounds were finished, I couldn't resist putting it all together one more time before taking it to pieces again and preparing for covering.

March 2011 (more on March) Covering started with the wing. Because Profilm (Oracover) is so expensive, I used the cheaper film available from GiantCod. It goes on pretty much the same but it remains to be seen how well it stands up to the sun. I ground the flange off the base of the wingtip LEDs and applied a couple of turns of matt black vinyl to simulate the mounting for the light.

Covering the wing fillet took a lot of patience. The dark blue stripe on the inboard section of the wing does though help to make the joint look neater. By contrast, the hatch join lines are rather conspicuous in the white film so I plan to bring the antiglare panel down to the bottom of the hatch.

After applying the antiglare panels and registration letters from vinyl, the pilots were permanently installed. Cockpit coaming is from electrical insulation.

The canopy could now be fitted and was trimmed with white vinyl. Then the motor was installed and the model prepared for taxi trials. There was something of a delay when it turned out that the battery had to be moved forwards by seven inches (about 180mm). There is plenty of room for it and arguably this position is more convenient but it does mean that all the work on the cunning sliding battery sled was a waste of time! Still, with the battery re-located, I spent a happy five minutes taxiing the Chipmunk round the garden. Ground handling seemed good, although I didn't see any movement on the oleos, so I'm still not convinced that they are set up right.


Unusually for the UK, a weather window opened up the very next day, so it was maiden flight time.



As you can gather from the pictures, it all went very smoothly. The only snag was that a full power ground test blew the hatch off - hence the sticky tape visible in the last picture! I have since added a couple more magnets and that problem is solved. The model flies very well, as I have come to expect from Ivan's designs and I instantly felt at home with it. The video clips from the second flight of the day confirm the suspicion that the oleos are too stiff so I ordered some weaker springs. That said, the model was well mannered on the ground and you can see on the video how it handled the touch and gos.
With the maiden flight out of the way, the remaining detailing could be approached in a more relaxed fashion. I started with the 'No Step' legends beside the wing walkways. Next came the airscoops on the engine cowling followed by the undercarriage fairings. Other details which may be tackled later include the exhaust and the landing light on the port undercarriage leg.