de Havilland Comet 4C

DH 106 Comet 4C

1/7th scale DH 106 Comet 4c

Own Design
Span 192"
2 x Jetcat P120 Turbines (26lbs thrust each)
115lbs weight at takeoff (with 14lbs of Jet A_1 fuel)

This project was started in October 2001 and was a joint venture between myself and my father, John.

Click on images to Enlarge.

Acting as a two man team, but located in different parts of the country, it was agreed that John would build the fuselage and tail components and Steve, the wings. Each would construct as much of their respective parts as possible before mating the wing centre section to the fuselage centre section.

While this may initially sound a recipe for error it actually all went smoothly. The critical part was in ensuring that the dimension between the wing spars was exactly the same as the mating fuselage formers.

Comet 4c sections

To ease construction and subsequent transportation, the model was designed to be broken down into a one piece centre-section that carries the engines, fuel tanks, undercarriage and most of the radio.  The front and rear fuselage sections, outer wing panels and tailplane halves would be detachable for transport.

The main components are all less than 6ft long and will comfortably fit into a medium sized van.

Part glassed

Traditional balsa and ply, construction methods were employed. The spars were made from cypiras. The entire model was sheeted in 1/8" balsa and then covered in fibreglass cloth. Cellulose white primer was applied prior to the finishing coats of Japlac enamels. A final topcoat of Aerokote gloss fuel proofer was applied overall.

The model was completed and test flown in May 2004, it had taken 2 1/2 years to build, but there had been some quite lengthy inactive periods during that time.

One of the design objectives was to be able to assemble and disassemble the model quickly (this seemed to be important knowing the vagaries of the British weather).

The wings panels each have two projecting aluminium tubes which slide onto corresponding tubes in the centre section. When the wing panels are slid into position the electrical connections make via multi-pin plugs and sockets. The wings are then retained by two cap screws.

The fuselage sections are joined by seven cap screws per section accessed through (non-scale) hatches in the lower portions of the fuselage; again the electrical connections automatically make when the sections are screwed together.

The tailplane halves slide onto aluminium tubes and are retained by elastic bands passing through the fuselage. Having had quite a lot of practice now in putting the model together they can do it in about 10 minutes, that sounds good, but you can still get very wet in ten minutes.

Another design objective was for a slow flying (and landing) model. It was reasoned that if the model could land slowly the complication and weight of brakes could be avoided and hence the model could be built lighter, which would help it to land slower!

The radio installation has been upgraded from the standard twin RX, Opto-isoltor route that used to be so common to a single Weatronics RX. This has helped cut down a lot of weight, removed many extra connctions in the servo wiring and still incorporates twin rx's, twin battery supply and basic features that the opto-islolators used to do.

Even better, now, we can 'download' all flying data after each flying session to determine if we had a Failsafe occurance, status of the main flight batteries servo postions etc etc….a very useful feature that helps put your mind at rest…

Comet 4c landing lights

Landing Lights are powered from a 12v Nicad battery right in the nose of the model with 4 35watt Halogen bulbs being operated by a speed controller….the only reason for this speed controller is that they are currently so cheap and reliable!

With a power to weight ratio of just under 1:2 the Comet is more than adequately powered. The model will fly for over 15 minutes on 8 liters of fuel. The take-off run is relatively short and the climb-out good.

Aileron response is adequate but not exceptional so it's best not to fly in very windy weather, the small fin also makes the model slightly unstable in yaw whch doesn't help thos cross-wind landings we keep seeing at the shows!

Comet - engine fit

The two Jetcat P120's have been exceptionally reliable, I don't think I would consider putting a different make of Jet engine in my models!

There is only so much you can do with a model such as the Comet, A typical display includes a gentle, scale take-off just as you might have seen in the Fifties…tuck the gear away and accelerate to a comfortable speed.

There is not a lot of difference to the spectator between a fast pass and a slow pass, though in fact the model is probably flying around 80-100mph during a fast pass and down to around 30 mph for a slow pass, it is just the size of the model that gives few clues to it's really speed.

The biggest problem with displaying the Comet is keeping the speed down as the Comet is quite a slippery airplane, It would be very easy to allow the speed to creeep up to such an extent that airframe damage may start to occur.

Thrust is only ever at max during takeoff and those steep climbs, generally I have about 1/2 to 2/3 power for a display pass and down to idle for any sort of descent.

Comet 4c landing

Landings too require a fair bit of planning, full flap is pretty much a must and I tend to be at idle thrust for the final turn and all the way to the ground (except blustery conditions)

The model has surpassed expectations – we were hoping for a realistically slow flying and stable aeroplane and have not been disappointed. With a model airliner there are not many manoeuvres that can be put into a flying repertoire if realism is to be retained.

Fortunately the low weight enables the Comet to be cruised around at low speeds and low angles of bank,  just like the real thing. On calm days, landings can be accomplished with the nose held high off the ground until the speed has decayed – now that does look realistic and has made the whole project worthwhile!

To date, we believe the model has flown over 80 displays and visited Bruntingthorpe, Cosford, Woodvale, the British Nationals at Barkston Heath, Hucknall, Longhorsley, Winterton, East Kirkby, and Euroflugtag in Germany.

More photos..

Centre-section covered	Fairing
Fairing 2	Fairing 4
Sheeting top of centre section	Comet with Concorde