The De Havilland DH88 Comet is a classic 1930's racing aircraft designed to carry two people. It had a maximum speed of 237 mph and even in model form is easily recognised. The prototype I have built is modelled on the winner of the McRobertson England to Australia air race of 1934 in its famous fire red and white colour scheme with the Grosvenor House lettering.
When Airborne asked me to build this prototype, I was keen on the model but had some reservations as I had no previous experience flying a twin, especially one with a wing section like this. The entire project was a real challenge to me - so first to the building.
I chose to start this model with the wings because of their complexity compared to other aircraft I have built to date. The twin engines, retracts and flaps are all located in this section. Great care must be taken to create the correct thrust lines for the motorsŐ incidence and washout. First, cover the plans with clingwrap as this material is clear and cyno does not adhere to it. Start by pinning the 3mm x 6.5mm spruce main spars and the drag spar. The ribs can next be glued down taking care to put the dihedral braces in at the correct angle. Put the false leading edge on now along with the tip and leading edge blocks. Next I cut out the ailerons and glued in the leading edges while leaving the ailerons in place. Do the same with the other wing panel and then join with epoxy. When finished, turn the wing over and finish the aileron linkages before sheeting. I also set up the flaps and hinged them at this stage. Mark the aileron position on the top of the wing and then complete the bottom sheeting, checking for correct washout. Cut the areas out of the leading edge where shown for the engine nacelles. Next, move on to the engine nacelles with the retracts and tanks.
I started by constructing the plywood boxes which the engine, retracts and tanks all mount onto. Take care to make these with the correct offset for the inboard and outboard mountings. Next, I built up the balsa sides and then mount the complete nacelle on the wing, making sure of the correct down and right thrust. The retracts I used were Robarts mounted on the rails as shown on the plan but the legs were turned at 45 degrees as the undercarriage folds rearward inside the engine nacelles. Take your time with the retracts and they will give reliable service.
I decided to use separate servos for the throttles and mounted them in the nacelle under the retract. I used Hobby Electronics micro servos coupled with Dubro servo savers. Take time to adjust these for the transition of the throttles from idle to full throttle, especially keeping the engine revs the same at low throttle. Nylon radial mounts worked well with the engines side mounted and I fitted 180ml tanks in the space provided. The cowls are made from balsa block as shown on plan, carved, shaped, and mounted with three screws. Finish off the nacelles and then epoxy the entire inside to seal for fuel leakage.
The fuselage on this model is relatively simple to construct as there are no complications of engines or tanks. I began construction with sheet balsa sides, added the stringers then the formers F3 - F9. After the sides were aligned correctly, I added the top stringers and the turtledecking. The easiest way I have found to curve sheet balsa is to first use a squirt bottle to soften the wood by spraying it with water and ammonia (50% mixture).
After this, glue the wood to the airframe using thin cyno. Fit the wing next and drill the holes for the front wing dowels through the former F3 with the wing in place. This makes the job easier and more accurate. Then add the wing bolts.
Next the wing fairings are added and then shaped to suit. The canopy frames can now be added as well as the bottom sheeting. The nose is built simply in balsa and is a good area to add ballast because of the space available forward of the centre of gravity.
All the tail surfaces are of built up construction of balsa and then covered with 1.5mm sheet. This is simple and lightweight with care needed particularly with the tailplane to keep it true and the correct shape. I deleted the wire joiner for the elevator and made the push rod forked at the back of the model instead. After all the tail surfaces are completed, the tailplane can be fitted to the fuselage carefully checking incidences. Now with the tailplane fitted in an accurate manner, the fin can be added along with the balsa blocks.
After the entire airframe was finished, I covered it with 21g fibreglass cloth. To help fill the cloth weave, automotive primer surfacer is brushed over the cloth and then dry sanded with free cut 240 grit sandpaper.
Then I primed the plane with acrylic primer surfacer and filled any imperfections with stop putty.
The Comet was then treated to two coats of acrylic lacquer in Fire Engine Red.
Next I had all the decals computer cut in vinyl.
The canopy is made from flat 1mm plastic and was done in four sections. Two pieces for the top and then two wrapped over the sides.
I then used 2mm pinstripe to simulate the canopy frames.
After all the decals were fitted, the plane was given one coat of Protec 2 pack clear to seal all the decals and paint. This also gives an excellent durable finish with a brilliant gloss.
Radio gear used was all J.R. 507 and 517 servos with the exception of the throttle as previously described.
The retracts fitted were Robart with the valve fitted to the top of the wing. The weight was 5.4 kg which was rather heavy for a model of this size. Weight could have been reduced with less surface finishing but I opted for strength and durability.
Engines used were two O.S. 46 FXs with the standard mufflers and bolly 10.5 x 6 propellors as I have found this a very reliable combination.
With no experience in flying a twin engined model, I waited for a calm day with the wind straight down the strip so at least the weather wouldn't be an unknown factor.
I spent some time tuning the engines one at a time utilising a tacho for accuracy.
The model was originally built to scale with a tailskid but I later changed it to a tailwheel to improve the ground handling under power.
The moment of truth had arrived!
With engines tuned, all controls adjusted and the retracts checked, I had finally run out of excuses.
The model stood ready at the end of the strip. It was time to fly!
I opened the throttles gradually and the model tracked fairly well.
I figured with a wing section the shape this aircraft has, I should get as much ground speed as possible to avoid possible tip stalls which are notorious on highly tapered wings with high wing loadings.
With the model gaining speed, I opened the throttles and we were instantly airborne. No tip stalls, but the most obvious difference to all models I have flown before was that this one is a racer.
At full throttle, this plane lives up to its heritage - a very fast model.
After settling into flying, the Comet seemed to fly very well so I gained some height to check the stall properties. With the throttle idled and the elevator stick coming back, once again I thought, itŐs not a trainer, so slowing it down to land could be interesting.
The stall was very definite and the wing did drop dramatically. Recovery was predictable but exponential function of about 25/35% is a help to avoid snap rolls. Flyers beware!
After many fast and slow flybys with the wheels up, I thought it may be a good idea to land before I ran out of fuel.
The way I had the flaps set up meant I could only get about 50 degrees. The flaps actually seemed to hold the nose of the model up and slow it down quite well.
Next as I throttled back to about 50% and put the wheels down, it still seemed fairly quick.
With the model on final approach at about 25% power with the flaps down, it was looking good.
I had to keep the power on to maintain a good approach. Once over the threshold, I idled the motors and flared it out for a smooth mains landing.
In later flights, rolls and low flybys were about my limit and the model has survived one engine failure. With the engine stopped it is a handful - I needed all opposite rudder and aileron trim and the model crabbed its way back to the strip.
Not dissimilar to the real thing!
All up, this project was a great learning experience with the model flying very well when sorted out. It is not a project for novices.
It is a moderately complex model to build and fly but seeing the Comet in the air with the sound of each motor doing 12000 rpm and imagining what took place back in 1934 is a very satisfying experience.
If good looks, nostalgia and a good burst of speed ever came together in one aircraft, the DH-88 Comet has to be one of the all time classic machines from aviations golden era. A modeller with moderately good skills will have little trouble recreating and flying this beautiful aircraft.
SPECIFICATIONS DeHavilland DH-88 Comet
Dehavilland DH-88 Comet plans are available
Plan No. 672
To return to Airbornes
This page was last modified on the 19 May, 2002