I have often thought about going in for a "large scale" model with a petrol engine and, in 1999, found that Greatplanes were offering a package of the Dynaflite kit of the Bowers "Flybaby" homebuilt light airplane John bought a late 1999 from "GreatPlanes". They were offering a package deal of the kit together with the U.S. engines 25cc petrol engine, so that's what I bought. The Dynaflight Flybaby is ¼ scale with a wingspan of seven feet (2135mm) and an overall length of 4' 8" (1420mm).
The design is that of a conventional model, with built up wing, elevator & rudder, and a lite-ply and balsa fuselage. Dynaflyte recommend that the wing be built in one piece; the two halves being joined with the usual fibreglass bandage. I elected to keep the wing in two halves, joined using brass tubes and carbon fibre rods. The joined wing is then bolted to the fuselage in the usual way.
The only other change I made was to the undercarriage, where I introduced a full width axle to take the sresses of the flying wires that I added to support the wings.
The 25cc motor is pretty heavy, at 4.2lbs (1.9kg) so I put the 2 elevator servos and the rudder servo in the rear of the fuselage using neat plastic mouldings. (I think they come from SLEC)
The model still turned out nose heavy and required over 200 grammes of weight in the tail to balance properly. I've used a Futaba FF6 radio and standard "148" servos throughout.
The colour scheme is that suggested by Dynaflight. I covered the model with white "Solartex" and then spray painted the blue areas. I then used "Solartrim" to create the light blue, red, orange, and yellow areas. This hasn't been 100% successful because the adhesion between the solartrim and solartex isn't very good. (I hate finshing my models anyway!)
The 'plane flies quite well but is somewhat heavy at just over 15lbs. The petrol engine is very practical and economical. Starting is easy too, thanks to a recoil spring starter. Antony Redding is building the same kit, but will be using a much lighter glow-plug motor. If his model gets closer to
"10 to 12 lbs"
then I might consider changing to a lighter engine too.
(I did, see September 2003 note below)
The engine was pretty noisy with the standard muffler. I have a metalwork lathe and made up an additional muffler, just an expansion box really, with a small outlet hole, that hangs under the wing where a luggage pod is fitted on the full size Fly Baby. This has enabled my FlyBaby to get the maximum noise level down to 81dBA, swinging a 16" x 10" Classic propeller at 7,200 rpm.
September 2003 Update:
My friend Antony Redding's Flybaby came out much lighter than mine, and he even changed engines from an SC91 four-stroke to an old OS61FP two-stroke. I decided to follow suit and bought a new OS 65LA two-stroke motor for the job.
This is the US engine 25cc petrol engine. You can see the magneto, spark plug, and exhaust extension pipe that my secondary exhaust attached to. The all-up-weight of this lot was a hefty 4lbs 8oz. The OS 65LA weighs just 1lb 9oz.
I stripped the 'plane of engine, fuel tank, and radio gear, also taking nearly 8 ounces of lead from the tail!
I then made up a liteply box to take the OS engine, tank, engine servo, and receiver battery all forward of the main bulkhead as shown on the left. The three remaining servos (2 for the elevators and one for the rudder) and the receiver were re-fitted just behind the firewall.
I used a brass propeller nut to add nose weight where it's most effective and had to add just 2 ounces more nose weight and one ounce of lead on the starboard wing-tip to achieve the correct CG. The all-up weight has been reduce from over 15 lbs to 10lbs 13oz. It feels and handles much better -- just waiting now for a club noise test and test-flight. I've run a couple of tanks of fuel through the engine (10% nitro) and she swings a 14" x 6" APC propeller at over 9000rpm. There seems to be plenty of power available.
The picture below shows Antony starting his Flybaby together with my flybaby on the flying field when Antony's was noise tested and mine was being run in.
26th October update: Well, the flybaby's passed her club noise test today and has taken to the skies once more. She's SO much better to fly having lost 5lbs weight. I've calculated that the wing loading is down from 30 to 20 oz/sqft.