Precedent FunFly 200% (Mega FunFly?)

This is a project that was kind of spawn through a conversation. My beloved Precedent FunFly (although a copy, not the genuine article) has served me well over the last year, and as I was slowly progressing to larger models, and with a keen bent to design my own, I wondered if a scaled-up FunFly was feasible.

(I've found it difficult locating information on the history of the UK model market, so the following passage maybe incorrect. Anybody who can help with this, please do.)

The Precedent range of models were marketed by BalsaCraft based in Norfolk UK, managed by John Rudd (or Hook?) who later became one of the Ripmax directors when it was purchased by them. Unfortunately, Balsacraft went into receivership in the later part of 2000, just after John had died from a fatal heart attack. The Precedent range was continued but manufacturing was moved to the Czech Republic. To date, my understanding is that SLEC (Sun Lane Eng. Company) continue the distribution of the Precedent range. However, the FunFly model was dropped with little sign of being re-released. This was a strange decision as the FunFly was recognised as a quality products with excellent flying characteristics. Although there are still many in existence, they are slowly disappearing. In 2005, the FunFly was revived in a similar design by Black Horse, known as the 'Twister'. People who have bought this aircraft have made favorable comments.

I don't have plans for the FunFly, and are not sure if they are freely available. However, the airframe shape isn't complex and should be easy to reproduce. I decided to wait for an opportune moment with my FunFly that would give me access to the innards of the aircraft (dare I say another encounter with the ground). Mainly to verify the fuse internal build (I was familiar with the built-up wing, after carrying out a number of repairs).

Sure enough, fate answered. I had a failure of the wing bolt mounting on a flying session. The wing was intact but the front of the fuse had an encounter with the ground. Right! An opportunity! Out with the calipers, protractor and steel ruler, lets get some measurements!

Work Started October 2005:

General specification: The fuse length is 36 inches long, and the wingspan is 55 inches wide.

The fuse is conventionally made using ply bulkheads with 1/16 balsa sheeting. At the firewall and wing roots there is liteply reinforcement (ply doublers). The tail section and control surfaces is made from 1/8 sheet balsa.

The wing section is built-up. Spars are made from 5/16 hardwood, ribs are 1/16 balsa, and sheeting 1/32 balsa. Spacing between ribs about 2 inches, the tip of each wing is capped with a 60 degree slope downwards.

With paper and pen, I commenced to capture it's vital statistics. Scaling up the measurements is no great deal, simple maths. However, there are a number of issues that make the design of a larger aircraft problematic. Doubling the measurements does not mean that you will get an aircraft that is bigger but flies the same. The build of aircraft is going to have to change, for sake of structural integrity and dismantling for transport. Weight: we need to keep this as low as possible, but without sacrificing structural integrity. And finally choice of engine, glow or petrol?

So, the measurements are doubled. We are know talking about an aircraft that will be approximately XX length and a 110 inch wingspan. Not massive when compared to some of the LMA stuff flying around, but big in my world. I guess the weight is expected to be around the 16-22 lb mark, but this is a guess based on my Glens CAP232 30% experience. I have decided to use a Zenoah G38 engine, for the purposes of suitable power, economy and reliability.

Now to the build.

Many builders start at different parts of the aircraft. Some say start at the wheels and build your way up (probably because many builders purchase wheels, so wheel scale is limited and is difficult to manipulate). I decided to start with the fuse.

I don't have a large workshop, and what space I have is greatly reduced by it's contents. The flattest building table I have is my saw table, which is extended by a sheet of MDF on top (6' x 4'). The expected fuse length is longer than this, overhanging the table. So, I decided to employ a technique that would overcome my small building table issue.

By using stiff wire rods as guides and supports, a semi-rigid frame can be made from the fuse bulkheads or wing ribs. These act as excellent guides and reduce potential warp or distortion in the aircraft shape. Thus, I planned to use 3mm steel rods through the bulk heads and wing ribs. Once the assembly was rigid, the rods could be easily removed.

The fuse length is XX. I decided to use the same number of bulkheads as in the original, but add strength and shape through the use of stringers. To reduce costs, I had a plank of beech (6' x 4" x 1") that I ripped into 8mm and 10mm square section lengths. Cutting my own wood allowed me to be selective, rejecting material that had knots, warp or cupping. The bulkhead and firewall shapes were carefully drawn on plywood and cut using a bandsaw. Receptacle notches were cut to accept six stringers, each placed on corners of the bulkhead.

The aircraft thrust line and general symmetry was used to calculate were the steel rod guide was to be placed. Look at the bulkheads above, the pencil lines intercept at the point where the rod passes. This was more rule of thumb rather than pure science. With the metal rod inserted and bulkheads correctly spaced along the rod, the stringer could be glued. Once the stringers are in place, the balsa sheeting could commence. Sheeting was 1/8 around the firewall and wing root, and 1/16 towards the tail. I decided to use thin sheeting because I had plans to improve the rigidity internally. All of this process was lengthy and slow, constantly checking the aircraft for symmetry and signs of unwanted distortion.

Unfortunately, I was so engrossed with this part of the build that I forgot to take some photographs . So I only have a picture of the sheeted fuse. Sorry! Picture shows a view of the inside. I decided to leave the bottom of the fuse open for accessibility. This would allow me to reinforce the fuse walls and put servo mounts in.

At this stage, the fuse is semi-rigid, but the sheeting is poorly supported between bulkheads. So, the tail section of the fuse was reinforced using a lattice arrangement of 8mm square balsa. Each unsupported area received two sections of 8mm balsa crisscrossing between bulkheads. This adds strength while minimising weight increase.

This view of the fuse shows the completed sheeting and the vertical stabiliser. The stabiliser is made from 1/8 sheet balsa embedded into balsa blocks within the fuse.

The horizontal stabiliser is made from 1/8 sheet balsa. The centre section is sandwiched between plywood sheeting, which will form part of the bottom of the fuse.

At this stage I decided to pause with the fuse build and concentrate on the wing section. Although I wanted to scale up the original wing, I decided not scale the existing airfoil profile. Trying to copy the shape by tracing around the wing proved difficult. Alternatively, I decided to use a NACA standard airfoil that provided a close match to the original. This was done using computer software, so the scaling process is very accurate. As the wing is so large it was decided to make the wing in two parts, which joined together at the fuse. Liteply ribs would be used at the wing roots for strength, the other ribs being made from 1/16 sheet balsa. 4" spacing was used for the liteply ribs, and 5" spacing for the balsa ribs. Four liteply and twenty balsa ribs were needed in total.

As all of the wing ribs have an identical profile throughout the wing, it is possible to mass manufacture them. Thus, the wing ribs were cut using a sandwich technique. First, the rib template is glued (3M 77 adhesive) onto liteply and cut out on the bandsaw. A stack of twenty sheets of balsa were sandwiched between the liteply ribs, held together with plastic bolts. The bandsaw was used to cut the rib outline and spar slots then followed by sanding. Lightening holes were cut using a electromagnetic saw, followed by hand filing for finishing. The photo shows the finished product with liteply ribs removed. This is a delicate exercise that requires patience and care, the ribs are quite delicate and easy to damage.

Wing assembly. From my plank of Beech I had ripped and selected sixteen lengths of 8mm square spar material. Using the wire rod assembly technique, I skewered two liteply and ten balsa ribs together to form one wing. Eight 55" spars were mounted on the ribs, the spacing distance and orientation being carefully checked as each rib was glued.

The wing was fitted with leading and trailing edge strips, hinge blocks, servo mounts and finally sheeted over. The ribs at outer ends of the wings were set at 60 degrees tapering inwards and downwards as in the original funfly design. Between the liteply ribs at the wing root, liteply blocks were test fitted for wing mounting purposes. I had decided that the wings would be film covered, so the centre part of the wing was left uncovered, for lightening and access purposes.

As always, my designs seem to evolve as I build. And I quickly realise that I should have included an alloy tube to act as a wing strengthener, as good practice more than need. This required me to drill a straight hole through the wing ribs along 3/4 length of the wings. Not an easy task, as this should have been done at the rib manufacture stage. Fortunately, the existing bolt holes used in the sandwiching process formed a useful position and guide. The picture shows the finished result of the hollow alloy tube epoxied into the wing. Once the tube was mounted the liteply wing mounting blocks were glued into place.

It has taken a considerable amount of sanding to get the wing into this condition. Not bad for my first attempt at building a wing, although little imperfections are obvious to my eye and niggle at me. My biggest fear was warp of the shape, as many people tell horror stories of their building experiences. Left wing completed, let's start on the right wing! I won't bore people repeating the detail, as expected the process is similar. One warning, wings are generally sided, don't accidentally build two wings for the same side!

Attaching the wings to the fuse requires the lower wing profile to be cut out. This is something I decided to put off until I saw the outcome of the wings. I had kept a sample liteply rib that I intended to use as a template for this activity. The funfly is a mid wing aircraft, so only the lower half was required to be cut into the upper fuse. The upper half of the wing has a shroud and a canopy.

In the existing design, the shroud component forms part of the upper wing surface and aileron servo housing, pictured far left. As the wings on this aircraft intended to be separated, the complexity and fragility of a split shroud would be suspect. Thus, I decided to combine the shroud and canopy together, which are mounted on top of the wings after they are bolted down. The canopy has a unique locking mechanism idea that I commandeered from HaliMal. It is two spring-loaded bolts, one bolt at each end, made from servo horns, 2mm rod, 2mm cap bolts and expansion springs. Very effective and seclusive.

December 2005:

With the canopy fitted, the model is now starting to take shape! Beginning to get a feel for it's size, and it's BIG. Well, big to me. The glue container is there to hold the canopy down as I hadn't finished the rear locking bolt when the shot was taken. Wings are still unattached to the fuse and require to be mounted.

Each wing has two liteply blocks in the wing roots, the intention is to have a 6mm capped bolt per block holding the wing to the fuse. The fuse will have a substantial ply block that will surface mate with the underneath of the wing blocks, having capless nuts in the interior. Both wings are meet butt-flush with no dihedral. I decided due to the size of the wing area and low wing loading that dihedral may not be needed for additional stability. However, if proven wrong then the changes needed to implement dihedral should not be taxing. The photo's show the underneath of the fuse with wing mount (and some additional strengthening) clamped. The other photo is the top view of the fuse wing mounting block. More sanding was needed to ensure this block was flush with the sides of the fuse.

Back to the tail section. I had decided that both ailerons, elevators and rudder are to be driven using separate servos, so five servos in total. The horizontal tail would have three servos, one on each side of the split elevator, and a large quarter scale servo for the rudder. The linkages would be 3mm rod for elevators and pull-pull wire for the rudder. To minimise complexity, and allow for horizontal tail detachment, the servos were mounted directly on the tail. Both servos are mounted opposed, so operate in reverse to each other. Thus, either a reverser circuit was required on one of the servo, or more simply, having the servos on individual channels. The latter was attempted, as I planned to use an eight channel radio system. Servos were mounted directly on the tail, and apertures were cut out of the fuse for clearance.

The rudder was to be controlled using a single servo, with good pulling power. The quarter scale servo was mounted close to the rudder within the tail section to minimise pull-pull cable runs. Next I manufactured the elevators and rudder. Plastic hinges with metal hinge pins were used on all control surface movements. To improve strength and rigidity, the horizontal tail received additional layers of balsa.

Aileron control surfaces were made from 3" trailing edge stock. This was built up by layering additional balsa (cross-grain) to the thickness of the wing. With the aileron being so long (unnecessary), it was decided earlier that it would be split: 2/3 outer aileron and 1/3 inner as flaps. As this aircraft is somewhat experimental, flaps maybe useful to reduce airspeed when landing. So each wing has two servos driven by three channels: one channel per aileron and a shared channel for flaps.

The underside of the fuse is closed up with 4mm ply and undercarriage is fitted. Rudder pull-pull system is fitted and elevator linkages. Aileron and flap servos fitted. The bigger jobs are replaced with smaller jobs as the aircraft beings to take shape. Balsa bashing is replaced with the hardware and radio fitting. I intent to leave the engine installation and covering as the last process.

January 2005:

Well, I've taken the airframe down to the club for a cursory inspection before I start to close it up. Nothing like another pair of eyes to look over the work, as complacency leads to accidents. Apart from suggestions to strengthen up the horizontal tailplane all looks OK.

I've bought some covering film from Steve Webbs on their recommendation. I normally would use Profilm, but they were promoting a brand called 'Toughlon'. Rolls are wider, supposedly similar quality to Profilm but lighter and cheaper. Decided to give it a try. Planning to cover the fuse and upper wings in white, red-white checks on the underside for contrast. A colour scheme will be chosen later to overlay the white.

One of my other concerns is the engine. The Zenoah 38 came with the standard exhaust, which is probably not adequate to pass the 82dBA. Have to decide if I'm going to build or buy a suitable exhaust system.

(To be continued...)

Balsacraft (International) Ltd
Unit 10 Norwich Rd Ind Est
Watton
Thetford
Norfolk
IP25 6DR

01953 884665

This site is © Copyright KMMFC 2004, All Rights Reserved