Minifox Blog

FLYING FOR FUN – CHOOSING, ASSEMBLING AND FLYING AN SSDR

PART ONE – THE PLANNING

Why an SSDR?

For those who don’t know, SSDR stands for ‘single seat deregulated’. This is an aircraft design that has been approved that has a maximum take-off weight of 300kg and a maximum stall speed of 35kts (UK regs, other countries differ slightly). You still need to have a microlight PPL licence but can take over all responsibility for its build/assembly and maintenance. This is personal flying at the most basic level and at the cheapest cost. Initial purchase isn’t necessarily very cheap but with hourly costs of around 5 litres/hr fuel burn and annual costs just a spark plug and some oil, flying doesn’t come any cheaper. The CAA do require at least third party insurance (around £90 annual premium). Just because it is minimal cost it doesn’t reduce the fun element. I also own a full sized 4 seater which is great for touring but for local ‘bimbling’ on a sunny day a small open cockpit aircraft is hard to beat.

The design options

There are several factors which help the potential purchaser to narrow down his/her choice from the current approved list of SSDR aircraft. Within the regulations there is nothing to stop you designing your own SSDR but unless you have the skills and time it is probably better to choose from the approved list.

One of the biggest issues is usually storage, whether at an airfield or in a trailer should you decide to trailer/fly. In my case I am looking at storing in a trailer on an airfield. Unless you have full hangarage ease of rigging and derigging will be important. Also the size of the trailer you require along with its cost is a consideration. Some SSDRs don’t have removable or folding tailplanes which can be an issue with internal trailer width. Folding wings ease the rigging process but, for a pusher design, it means that it will then only accept two blade propellers. As these are single seat aircraft it is important to be able to rig/derig single handed.

The next issue is weight. With a MTOW of 300kg and average pilot weight and 18 litres fuel, that leaves an aircraft empty weight up to 200kg. However not all SSDRs are designed to a MTOW of 300kg.

Other choices include that of high or low wing, 2 or 4 stroke engine, pusher or tractor engine position, taildragger or tricycle gear. All have their followers. Some of the pro’s and cons for each of these choices change slightly for SSDRs. I normally prefer low wing aircraft but the all around visibility with the pusher layout makes the wing position unimportant. I normally prefer the taildragger layout for grass strips but by opting for tundra tyres a lightweight trike can cope well with unprepared or soft fields. Flying at 50-60kts rather than 100-120kts reduces the added drag from larger wheels and the less efficient pusher layout.

Some SSDRs require significant building time. Other more basic designs quote a short assembly time. You may have or wish to develop building skills and take a year or two for your pride and joy to take to the skies. Alternatively the designs that enable straightforward assembly can take a only few months. Take the quoted assembly/build times with a pinch of salt. Unless you have already built the type before you would need to add at least 50%. If you have never been involved in this sort of task before add 100%. Be aware that you are the final arbiter and responsibility for the aircraft’s correct construction and your safety is in your own hands. Getting flying friends with engineering knowledge to critique you work as the aircraft evolves from a pile of hardware into a flying machine provides reassurance.

Why the Minifox for me?

As discussed above we all have our own priorities and preferences. There are a couple of designs from the USA that attracted my interest. The Kolb and the Aerolite. The Kolb has been away from the market for a while and has recently been reintroduced. The Aerolite is popular and has a busy kit production line. Be aware that Part 103 (the USA equivalent of SSDR) has different rules which could affect suitability for SSDR (max empty weight 254lb, max speed 70mph). I preferred the idea of dealing with a company in the UK or Europe. As I found out later this eased design modifications and reduced delivery cost and time.

As my field is soft in winter, part of the reason for an SSDR is to be able to fly when our heavier footprint aircraft are grounded. A design that could accommodate tundra tyres will help this requirement. This might become more of an issue if the climate changes continue!

I am not a fan of two stroke engines so a design that can accept the extra weight of a 4 stroke is important to me. Although SSDRs have very slow stall speeds, flying at relatively low altitudes with poor glide performance provides limited options in the event of engine failure. Burning oil as well as petrol doesn’t seem good to me given the current concerns for the climate. The fuel efficiency difference on such low usage is fairly academic but a 5 litre/hr fuel burn from a 39hp engine is still very satisfying.

The landing gear layout for these STOL slow flying aircraft is less critical. Tundra tyres help soften the loads on the landing gear legs. On narrow strips the ground turning circle of a trigear can actually be less than a taildragger. In the case of the Minifox both types of gear are available.

Eurofly offer the Minifox with a choice of two MTOW design limits, 230kg and 350kg. Working within the 230kg limit is quite restrictive. Adding weight with a 4 stroke engine, tundra tyres and possibly an all enclosing canopy means that, all but the lightest of pilots are likely to exceed that limit. I opted for the 350kg design which provides weight flexibility and a good strength margin based on the 300kg UK SSDR limit.

The Minifox design offers both easily removable or folding wings. Also, after discovering that the tailplanes span is too wide for the average enclosed trailer, Eurofly agreed to modify their design for folding tailplanes and sent me a video of it within a few days of enquiry. The wings only weigh about 10kg and can be rigged/derigged by one person.

Having viewed the Minifox UK demonstrator the design and quality of materials look very good. These lightweight open designs look quite flimsy but close examination in this case gives confidence that a lot of thought has gone into its design and development.

Having decided to go for a four stroke engine I realised that I would be breaking new ground for this type. Although I believe that the EOS engine is now being considered, its 30hp output is too low for the 350kg airframe. I have therefore decided to choose the Helvenco Aero1000 which produces 39hp. I have confirmed with Eurofly that its extra weight of about 12-15kg is acceptable to the Minifox airframe. This created two issues. It will require an adaptor plate to mount the engine to the airframe. Also the Aero1000 requires its fuel pump to be mounted below the fuel tank which needs to be allowed for in the layout of the fuel system. Neither of these issues are difficult to overcome.

Although I chose the open cockpit version I have made a suggestion to Eurofly to slightly modify the airframe chassis to enable anchor nuts to be fitted to make installation and removal of the enclosed canopy easier. At present, as demonstrated in the enclosed canopy installation video, access to the nuts when installing or removing the enclosed canopy is not easy.

I also asked for the elevator trim lever mounting bracket to be mounted on the port side. With a port side throttle this avoids changing hands on the stick to adjust elevator trim inflight. Eurofly are very amenable to suggestions like this. This shows a very can-do company looking to further develop their product. However as mentioned below this led to complications.

In the next chapter I describe the decisions I made with regards to the required extras. Sorting and ordering these whilst waiting for delivery is a good use of time and helps to keep the project rolling once the kit arrives.

PART TWO- PREPARATION BEFORE THE KIT ARRIVES

Apart from organising the workspace there are two areas where preparation can smooth the build/assembly stage.

1. Reading the Manual

This enables you in advance to gain an understanding of what is required to complete the project. As well as applying the principle “Read three times, measure twice, cut once” getting an overall mental picture before starting work gives time to understand the manufacturer’s way of thinking. It also draws attention to any special tools required . A few special drill bit sizes and the need for a nicropress crimping tool are examples in this case. It also enables you to predict parts of the assembly process that requires assistance. In particular the wing covering is a job requiring two people.

Whilst the manufacturer may have some reasons to carry out the assembly process in the order published, it doesn’t make any allowance for limited workshop space or facilities. As space is often a problem many kit manuals advocate assembling the flying surfaces first. After completion these can often be stored against the wall or hung from the ceiling. Conversely the fuselage once put together takes up space. Adding the wheels to the airframe makes it more manoeuvrable and carrying out as much as possible on the forward part of the fuselage before adding the tail tube helps to delay the point where the workshop becomes crowded.

2.  Purchase of parts not provided with the kit

Parts of the assembly require purchase of extra items. The main case of this is the instrument panel but any changes from the standard such as an alternative engine leads to further required changes and amends choices. I will use my particular case to explain the thought process. As a change away from the standard engine will be the biggest difference this will need the most careful consideration.

a. The Helvenco Aero1000 engine

This is one of the few 4 stroke engine alternatives suitable for the Minifox. As this involves an increase in engine weight and possibly increased vibration, confirmation from Eurofly that the engine was suitable for the airframe was required. The increased weight may also affect the CofG but, as the engine is in a pusher aircraft, it is nearer the CofG. As a result this shouldn’t cause much of a problem. However particular attention to CofG becomes essential at the weight and balance stage.

 The two engineering aspects that came to my attention from the Helvenco manual revolve around the fuel and electrical systems. Because the Aero1000 uses fuel injection a dedicated electrical fuel pump is provided with the engine. This must not run dry and needs to be placed 40mm below the fuel tank to ensure a constant head of pressure. To minimise the risk of leaks not all nylon tanks have a bottom mounted outlet. When the kit arrived the supplied tank had a bottom mounted outlet so it is suitable for use with the Aero1000 engine. The latest tanks come with threaded inserts doing away with the strap type fixing. This is a good upgrade.

The Helvenco ignition and starter switch cable needs extension to reach the instrument panel. As the ignition switch is a toggle switch I intend creating a guard from aluminium U channel to prevent inadvertent switching off in flight.

As the single fuel pump and EFI system are essential to the running of the engine some redundancy for the electrical system is advisable. The optional higher output generator is still only 6A so, in the event of generator failure, ensuring the energy from the starter battery is available exclusively for the engine is essential. Helvenco say that although a 4Ah battery is the minimum, a battery capacity an 8Ah is recommended. Adding extra electrical equipment such as avionics and anti collision lights can easily double the electrical loads. Using a Brocott split charge relay with a small 0.8Ah auxiliary battery on its output will ensure that, in the event of a generator failure, the engine gets exclusive use of the main battery energy with the instrument panel having an emergency supply for about 15 minutes from the aux battery. To keep the pilot informed, twin digital voltmeter/ammeters display loads on both sides of the split charge relay.

b. Engine Instrumentation

The Helvenco engine is a little unusual for a 4 stroke engine in that it doesn’t have a conventional oil pump. No oil pressure or oil temperature measurement are provided so checking the oil level before every flight is essential. The engine ECU has its own sensors measuring manifold pressure, throttle position, coolant temperature and crankshaft position. Apart from rpm the only parameter available for external measurement is coolant temperature and that needs a separate third party sensor which can be positioned in the T provided in the coolant hose. As so few parameters are measured there is little point installing an EMS. I decided to buy an electronic rpm gauge (KOSO D60) which displays analogue rpm with a digital display for coolant temperature or voltage. This greatly simplifies the wiring. As electrics are so critical I have added a pair of digital volt/ammeters to show the state of engine and panel electrical systems at all times. As the supplied nylon fuel tank isn't compatible with either float or capacitive fuel quantity senders I have decided to use a fuel flow system to calculate fuel contents based on consumption. The MGL 57mm gauge and plastic inline fuel flow sensor integrate well with this particular application. As a backup, experience will show what is an approximate maximum engine running time based on the engine and fuel tank size used.

d. Anti-collision lighting

Being such a slow aircraft anything that makes you more visible to others is worth considering. The small power output from the electrical system limits the choice of anti-collision lighting. As the wings are removable I have decided to fit lights on the nose and tail. On the tail I closed off the tail tube with a 133mm cap providing a surface to fit a flashing LED light. To ease the cable installation for the tail light I will route the cable within the tail tube before installing the elevator pushrod. As the nose cone has tight curves multiple small strobe lights were positioned to cover the front 180 degrees.

e. Avionics

(i) VHF Comm

To avoid aircraft radio licencing costs and greatly reduce purchase price many pilots find ways of mounting handheld radios. These can also save a few hundred grams in weight. The position of their controls makes them difficult to panel mount so a compromise between neat mounting and ergonomics has to be made. The Yaesu FTA-250 is a very small and neat unit. Using its belt clip screw mounting inserts it can be bolted direct to the panel. They are usually sold complete with external mic/tel lead with PTT socket. To keep the wiring neat 90 degree plugs for both power and antenna enable the cables to disappear immediately behind the panel using accurately positioned holes. The antenna uses a BNC type connector and the right angle power plug with attached lead are readily available on eBay. The radio is designed to be powered from a 6-10.5V DC power supply so a 12v-9v DC stepdown regulator is required. For such a small aircraft with limited antenna placement opportunities the Lynx Avionics M090 VHF antenna makes a good choice. It comes with self adhesive strips to create a ground plane so can be mounted on the nose cone.

(ii)GPS

For navigation, using SkyDemon with either a large mobile phone or a mini tablet as a display is usually the preferred option. As even mini tablets take up a lot of panel space I used a 7” mobile phone.

(iii)SkyEcho

    To improve safety a SkyEcho was fitted. Using this provide the GPS position to the phone provided a more reliable position than relying on the phone GPS.

In the next chapter the kit arrives and the work begins in earnest. One Polish owner completed his Minifox assembly in 18 days. I shan’t try to compete with that but will log the workshop time to estimate the build time for other prospective builders. I will try to highlight anything which causes difficulty or through lack of preparation slows down the assembly process.

PART THREE - THE KIT ARRIVES AND WORK BEGINS

The kit was ordered on 28th October so a delivery from Italy on 19th December (7.5 weeks) was pretty good. The kit arrived in three boxes. If they can be delivered to the unpacking point no extra handling is required. However if you need to move them the following information might be helpful.

The wing box and the the spar box, although long, can easily be carried by two people. The main box is heavier  and due to its bulk is a 4 person lift.

The box sizes are as follows:

Main box - 3m x 1.33m x 0.42m

Wing box - 4.3m x 1.32m x 0.23m

Spar box - 4.5m x 0.35m x 0.35m

The packs with parts and fixings are in bags labelled with SET numbers as indicated in the Assembly Kit file. Due to kit developments this file is a little out of date. The factory now fits the brackets for the tailwheel and the end fittings for the elevator pushrod. As mentioned above, the tank now has inserts allowing the tank to be bolted directly to the chassis. Slight differences in other parts will be discussed during the assembly process. A piece of the cardboard packaging was positioned against the wall and the parts bags were mounted to it in SET number order to create a parts wall.

I have changed the order of assembly slightly to make it easier. The wings need two people to assemble so that will be done as soon as a helper is available. Fitting the top beam and landing gear before the tail tube makes the fuselage more manageable. Fitting the tail tube also easier with assistance as the access to the rear mounting bolts is restricted.

The project is now ready for assembly. As Christmas approaches Day One of the build will be in the new year.

INITIAL FUSELAGE WORK

The initial part of the fuselage work includes fitting the top wing support beam, the tail tube, the landing gear and rudder pedals. Some updates in the kit created some confusion. Extra time was needed to remove powder coating from unmasked parts of the chassis. Also a significant amount of reaming of plastic bushes was required to enable them to be fitted. 

Aligning the main landing gear took a while and with lack of guidance I intially chose one degree nose-in for each wheel. After viewing the wheels from the front I  decided that 0.5 degree would be sufficient. This is decided by the exact drilling of the main wheel fixing to the leg. As it is a one-time decision it is important to set this up accurately before drilling.

The standard kit places the throttle and elevator trim levers on opposite sides of the cockpit. I had asked for the trim lever to be repositioned to the same side as the throttle. This is the normal arrangement for good ergonomics and I fed this information back to the factory. Although I had spotted that the elevator trim cable guides also needed swapping sides, the factory missed this requirement. It also turned out that swapping the trim tab to the port tailplane is also not a trivial modification. To overcome this I will use some Bowden cable to route the trim cable across the fuselage behind the seat.

I routed cable down the tail tube in preparation for fitting flashing LED tail lights. This was done before fitting the gaiter at the forward end of the tail tube. I have bought an end cap to cover the aft end of the tail tube. This also provides a surface for mounting the tail lights.

To get the fuselage to its initial state below took about a total of four days work. 

FLYING SURFACES STAGE 1

The flying surfaces are made up from aluminium frames covered in a translucent reinforced synthetic fabric. As this is a slow flying aircraft the moveable surfaces are relatively large for the size of airframe. Assistance is required to cover the wing frames so I started with the other flying surfaces. The first part I completed was the fin. This took half a day but once the technique was perfected the remaining surfaces could be completed in about half the time. The covering is a tight fit making final fitting of the cross tubes a challenge. However the design is good allowing a drum tight finish with hinges attached in a much shorter time rather than alternative  techniques offered by other kit aircraft. Apart from in the hinge locations the gap between the hinged flying surfaces is closed by Velcro seams. The elevator trim tab covering is included as part of the starboard elevator covering simplifying assembly. As a guideline the fin, rudder, horizontal stabilisers, elevators and ailerons can be covered and hinges fitted in three easy days or two hard work days.

FURTHER FUSELAGE WORK

As the wing covering needs assistance I have returned to fuselage work whilst I waited. This included initial temporary seat and nosecone fitting. Temporary due to waiting to fit the aileron torque tube/ joystick and brake cables to the toe-brakes. The aileron torque tube needed turning down in the middle to allow it to be fitted after the bearings. I had this done to ease the bearing fitting and to enable the torque tube to be removed at a later date should a bearing need replacement. 

The standard kit comes with a single hand operated brake lever attached to the joystick. I chose the toe brake option which has the advantage of differential braking but does not, unlike the hand brake version, have a parking brake. The toe-brake cables were supplied without end fittings at the pedal end so there was a short delay whilst I obtained  them. With the nose cone fitted the routing of the brake cables were confirmed. There was space between the lower nose cone and the floor panel for the cables so the floor panel was trimmed and fitted using the 4 tapped and welded brackets on the airframe. 

I bought a 133mm end cap for the rear end of the tail tube. This provided a suitable mounting point for a rear LED strobe light. Smaller LED strobes were fitted around the nose cone either side of the pitot tube. The factory provide a gaiter for the front end but leave the rear end open. 

No pitot tube was supplied but sleeving a piece of tube into the centre of the nose cone was an easy job. The outer sleeving was glued in using JB Weld allowing a removable inner pitot tube. This convenient position for the pitot tube is an advantage available with a pusher propeller design resulting in a very short pitot tube. As this is an open cockpit aircraft I will assume that the static errors created by not using a static port will be minimal so will omit that initially.

The nose cone was also be used as a mounting point for the VHF comm antenna. As the nose cone is made from glassfibre a groundplane using 'Speed Tape' was required on the inside. Placing the antenna here avoids the need for a longer cable to a more difficult mounting point. 

Having sat in the aircraft the instrument panel seemed unnecessarily small. Although this is a minimal 'fun to fly' aircraft, a bit more panel space would help with ergonomics and provide a bit more flexibility with instrument choice. The top of the panel is high enough to lower its bottom edge without legroom problems. I will be using 3mm plywood with a single coat of glassfibre on each side. Whilst the provided tiny'carbon fibre' instrument panel weighs next to nothing, a plywood panel is only slightly heavier, much cheaper and easier to obtain. Version 2 of my panel design is included below.

FLYING SURFACES STAGE 2

Having had a few tricky moments with the smaller flying surfaces the work to cover the wings was surprisingly quite straightforward. I have passed on some learned techniques to future builders but getting both wings basically complete in one day by two people was a notable result. The work to complete all the flying surfaces was achieved in 5 days. Not many 3 axis kit aircraft achieve this so quickly.

The method for arranging the elevator trim was innovative. The aluminium trim tab is inserted into a fabric pocket which is already sewn in as part of the starboard elevator. This fabric join acts as the pivot point negating the need for a hinge. A spring loaded pushrod is attached to the underside of the elevator to keep the cable under tension. Friction at the lever end enables the trim tab to be controlled by a pull cable. 

INDIVIDUAL COLOUR SCHEME

With an open frame aircraft there is limited opportunity for an individual paint scheme. The flying surfaces are translucent with a limited choice of pinstripe reinforcing fabric inserts. There is a choice of powder coating for the frame and the nose cone can be gelcoated in a range of colours. I decided to etch prime the tail tube, paint it black and add a bright yellow self adhesive reflective stripe as a backdrop for the Eurofly Minifox decals. This colour is a fairly close match for the nose cone (colour ref RAL1026). I should have done this before riveting on the cable guides but it was easy enough to drill out the rivets and refix the guides later.

MORE ON ERGONOMICS

A seen from the photo above the joystick looks a bit long. I decided to reduce its height by 50mm. Similarly the throttle sits up a bit high and both these items slightly impede access and egress from the cockpit. The hoop that provides an attachment point for the instrument panel as supplied is a bit tall. It is possible to see over it but the change in angle for the windscreen between it and the nose cone appears to be out of alignment with the nose cone rebate angle. I reduced its height by 20mm. This slightly reduced the instrument panel size but that is OK provided I allowed for it in the panel layout at that stage. The seat position is just far enough forward for my height (5ft 9in) but a bit on the limit for applying brakes in the turn so I added up to three 20mm spacers to the seat back attachment points to put it further forward. This also required extra holes drilled in the seat base to accommodate each seat position. The forward limitation is the clearance between the seat and the joystick. For shorter pilots it would be possible to tilt the rudder pedals a little further aft be lengthening the steering pushrods.

THE HELVENCO AERO1000 ENGINE ARRIVES

The engine with a four bladed carbon fibre propeller arrived a few weeks after order. It was well protected in its crate lying on its back with its 8 rubber engine mounts securing it to the pallet floor. The initial unboxing involved taking off the surrounding box, adding temporary feet to its frame and installing the exhaust. It could then be released from its base and stood up without damaging the underslung exhaust. Given that its mounting plate is much larger than that welded into the airframe it required an adapter plate. I prepared for this whilst I finished of the airframe and instrument panel. Designing the plate was easy enough but getting the drawing converted to a computer file for water jet cutting took much more time an effort than I expected. Anyone repeating this engine fit is welcome to a copy of this file. My blank plate cost about £40 with another £40 required for the water jet cutting.

FUEL SYSTEM INSTALLATION

The 18 litre fuel tank now has threaded in its front face allowing it to be bolted direct to the airframe. As it is mounted well below the engine fuel supply to the engine is totally reliant on a fuel pump. The Helvenco engine uses injection and has a dedicated fuel pump which has to be mounted at least 40mm below the tank to ensure positive pressure to the pump inlet. This was mounted to the port side of an airframe cross tube. Within the length of this tube it was just possible to fit a water drain tap and the fuel filter. A fuel shut-off valve was mounted diagonally between he tank outlet and the water drain tap. The fuel pipe from the pump outlet routed aft and upwards using an airframe strut. The fuel flow sensor was mounted just forward of the engine bulkhead. The pump has an outlet to feed excess fuel back to the tank.

THE FINAL INSTRUMENT PANEL DESIGN (v3)

Having used a cardboard cut-out as a template the instrument panel was cut from 3mm plywood. this was covered on both sides using thin glass fibre cloth and a relatively smooth finish created by peel ply and a glass panel during cure. The layout of instrumentation was then drawn and cut out.  After a trial fit of the instruments they were removed and several coats of primer were sprayed  to fill the remaining weave followed by the same using satin black top coat. As there was spare panel space at the bottom of the panel this area was allocated to a couple of slots for phone and sunglasses. Later it was found that these boxes provided mounting behind the panel for cable support, ammeter Hall Effect sensors and by interconnecting them with a piece of angle aluminium a ground busbar was created. This busbar was drilled and tapped to provide 17 ground mounting points. Originally this was considered a bit excessive but, although this is a fairly simple panel, 11 of these were used for electrical connections and one to support the antenna cable. Multiple plugs and sockets ensured that the panel could be removed later. The panel fixing used six 24mm plastic P clips secured by 4mm Allen bolts with their heads hidden by plastic caps. As after the windscreen was fitted access to the nuts behind would be difficult it I decided that it would be easier to gain access behind the panel by removing the windscreen. 

WINDSCREEN FITTING

In most homebuilt aircraft the fitting of the windscreen is often considered to be one of the trickiest jobs so I approached this with trepidation. Although this is called the windscreen most of this panel is in front of the panel linked to the nose cone. Many aircraft use acrylic plastic for windscreens which is difficult to cut and drill without cracking. This kit uses polycarbonate which is softer and easier to cut and drill. As this is only 1.5mm thick it is also easier to bend. The sharp curves helped stiffen it. As the pilot only looks through the top 100-150mm of the windscreen the rest was covered in vinyl wrap which extended aft of the panel to provide a glareshield for the instruments. Fixing used 4mm countersunk Allen bolts into rivnuts mounted in the nose cone rebate. Being softer polycarbonate is much easier to scratch so great care is needed to protect the area which remains clear.

WING FITTING

This proved to be an awkward job made worse by one incorrectly predrilled upper wing strut hole. The angle (both in pitch and laterally) of the welded lower wing strut mounts is critical to allow the struts to line up at both ends. In my case the fit is so tight that wing rigging could not be done solo and it looked like rigging and derigging for each flight was not a practical option. This is the only real disappointment in the whole build process. The subsequent removal of the port wing to apply the registration letters proved easier so I will look again at the ease of rigging at a later date to see if solo rigging might be practicable. This may be a one-off problem but the tolerances of the lower strut weldment positions is fairly critical.

ENGINE PREPARATION AND INSTALLATION

The engine bulkhead as provided by the manufacturer is optimised for the commonly used two stroke engines. To accommodate the four stroke Helvenco Aero1000 I needed to design and make an adapter plate. I ordered a 500mm x 500mm x 5mm aluminium plate. Using hardboard I made a template to match up with the outline created by the rubber engine mounts. They offer eight rubber mounts with a choice of 14 mounting points. I ended up using seven of these which is three more than standard engine mounts. Due to conflict between the upper mounting points and the aileron tubes the height of the adapter plate was reduced. The total overlap with the airframe bulkhead enabled seven mounting points to be used between it and the adapter plate. My biggest problem was getting the adapter plate drawing converted to a CAD file so that it could be cut using a water jet. This took a fair bit of time to resolve but resulted in a perfectly cut plate and was much easier than trying to cut 5mm plate with hand tools. 

The existing wiring loom needed some work. Due to the pusher design the ignition switch and starter button needed major cable extension. As the extra engine weight would affect the C of G I relocated the battery under the pilot's seat. This freed up the normal battery location (just in front of the engine bulkhead) for a power distribution box. The ECU and diagnostic socket were mounted on the lower front face of the adapter plate. As supplied the loom was obviously designed for a different application. It looped around creating an untidy and clumsy layout. I pulled off much of the insulating tape, grouped all the on-engine cables together and pushed the rest including the ECU and diagnostic socket through the hole in the bulkhead. It was then a case of shortening some cables, regrouping and boxing it up and adding the connections to the airframe loom. I added an extra guarded ignition isolation switch next to the power distribution box. This has three purposes. Firstly it enables the engine to be turned over without starting. This is used to expel the last of the oil during and oil change. Secondly, by leaving the main ignition switch on with this switch off it is possible to leave both batteries on charge whilst leaving the engine electrics isolated. Thirdly, as there is no ignition key, it helps prevent unauthorised starting of the engine. A small socket was mounted underneath the distribution box to connect a trickle charger during winter storage.

REGISTRATION

Registering an SSDR can be done directly with the CAA. If you wish to choose an out of sequence registration there is an extra charge. You will need the UK dealer to supply a reference number for the aircraft to put on the form. The UK Eurofly Minifox dealer (Marce Colucci) has a contact for supplying registration letters. These were very good quality and easy to apply. The fin mounted letters are easy to add at any stage but the one mounted to the underside of the port wing is much easier done with the wing derigged and placed inverted.

FIRST ENGINE START (Helvenco Aero 1000 engine)

The breaking in oil is Helvenco W-001 racing oil 0W30 or similar. This is used for the first 3 hours. Afterwards Mobil 1 FS X2 5W50 is used. To ensure the oil level is accurately measured the engine needs to be level and the oil warm. The angle of the engine mounting plate is such that the engine is slightly pitched up the nosewheel is on the ground on level ground. I used a downward sloping parking area to get the engine level. The maximum level is with the oil level 2/3rds the way up the sight glass. The minimum is halfway up the sight glass. The recommendation is to add 0.7 litre initially, turn over the engine a few revolutions with the ignition and fuel injector disabled and then add to the correct the level (800-900ml total required). 

The layout of the cooling system allows it to be self-venting. It was filled to just below the filler neck (about 0.7 litre). The initial engine start is done with the cap in the first detent. Afterwards the level is checked and coolant is added if necessary. From then on the engine runs are done with the cap fully tightened. 

Initial engine runs can be done without the propeller. All starts are done with the throttle fully closed. With no propeller load the engine must not be allowed above 2500rpm.

FITTING THE PROPELLER

I opted for the 4 blade carbon fibre E-prop and the engine was supplied with a 4.4:1 reduction drive. The torque setting for the bolts is 16Nm. This torque should be checked after each engine run and periodically afterwards.

Even with OATs in the high 20's it proved necessary to blank half of the radiator to prevent overcooling. Due to the small oil quantity warm up from cold is fairly quick. This can be made even quicker by idling just below the centrifugal clutch engagement speed (about 1900rpm).

WEIGHT AND BALANCE MEASUREMENTS

As this aircraft is the heavyweight version of the Minifox (aircraft MTOW 350kg, regulation limit 300kg) there is good flexibility with regards to adding extra items such as Tundra tyres, enclosed cockpit and 4 stroke engine. In my case the empty dry weight came out at 159kg. With 18 litres of fuel and a pilot weight of 87kg the take-off weight is about 260kg. More important though is the need to keep the centre of gravity (CofG) within limits. An enclosed cockpit with a light engine will create a nose heavy CofG whilst an open cockpit aircraft with a heavy 4 stroke engine will result in a rear CofG. The CofG datum is the leading edge of the wing with an acceptable CofG of 300-600mm after of the datum. In my case with full fuel the CofG is 524mm aft of the datum. 

ENGINE RUNS AND FIRST FLIGHTS

The initial engine runs revealed a problem with the propeller selection. Normally the maximum static rpm is limited by the propeller. In this case the engine speed was limited by the engine ECU so I asked E-Props to send another propeller with a greater pitch angle. This did not make enough difference to reduce the rpm below the ECU limit. Subsequent discussion with both the engine supplier and E-Props suggested that a larger PTO pulley was needed to raise the prop rpm to create more load on the engine. This arrived 2 days before the Popham Microlight Aerofair so time was a little tight to enable the Minifox to fly to the show. Having changed the pulley it was evident that the initial 4.4:1 reduction ratio was far to high. The new pulley reduced the ratio to 3.724:1 which transformed the performance. Due to the change of propeller the engine was now 'over propped' proving that the original propeller was correct in the first place. Changing back to the original prop provided a good overall performance. The rpm still reaches 9000-10,000rpm during the take-off run but stabilises to about 8000rpm during the climb. With 7000rpm set (1880rpm prop) the aircraft cruised at about 45-50kts whilst using about 6 litres/hr. This was a single test so further tests will be needed to confirm these figures. With full power the aircraft will achieve 60kts but the drag and buffetting  makes for inefficient operation. Setting 6000rpm produces 40kts but as this is fairly close to the bottom of the drag curve I think that it is a little low as a cruise setting. The engine dealer tells me that there are some engine resonances to be avoided at around 6500rpm so 7000rpm is a good setting for cruise power.

There was also some interference on the radio from the USB power sockets which I needed to sort out before flying to other airfields. Some of this was caused by the USB power sockets so these were changed for ones with a metal casing and capacitor filtering added. Also the default settings on the radio do not provide sidetone so this was selected from the advanced menu options provided in the Yaesu FT-250.

REFLECTIONS ON THE MINIFOX BUILD

The major parts of the Minifox kit were of high quality giving confidence to the builder. This is of particular importance for an SSDR where there is no engineering oversight. The way that the airframe is designed also makes for a quick build and the kit is being upgraded all the time. As I was assembling it part time it was difficult to log the time spent. I was not rushing and personalised the build which extended the time. Even so the airframe was completed in two months and the instrument panel, wiring and engine installation took another month. The completion of the flying surfaces within a week was particularly impressive demonstrating a well thought out assembly process and the close fit of the fabric to the flying surfaces showed the close tolerances achieved with the fabric stitching. Unlike many enclosed homebuilt aircraft the open fuselage made access easy for every task. 

The one area where the tolerances were too tight was when it came to fitting the wing struts. In my case the struts needed to be pushed into alignment with their end fittings making it impossible to rig singlehanded. The welding of the tubes to accommodate these fittings at the fuselage end has to be very precise so that the other end of the strut lines up perfectly wing the wing fitting. As these tolerances are so tight it is difficult to see how this can be overcome without redesigning the lower fitting. Relying on weld alignment to be so perfect so that the error over the length of the strut is less than 5mm is not practicable. Subsequent removing and refitting of the port wing to apply the registration letters proved easier so perhaps singlehanded rigging will become easier as the airframe beds in.

The most difficult task (as is often the case with homebuilt aircraft) was fitting the windscreen. Working with polycarbonate rather than acrylic made the task easier but being softer it is more vulnerable to scratching. Also the change in radius of curvature around the top edge of the nose cone does not allow the windscreen to perfectly follow both the curves of the nose cone and the top of the instrument panel. This would have been easier if the nose cone had a semi circular shape resulting in a simple curve for the windscreen. As most of the windscreen is forward of the instrument panel and the need for a glareshield for the instruments, only a small amount at the top needs to actually be a windscreen. Vinyl wrapping the rest is an obvious technique to improve appearance. 

Deciding on the heavyweight airframe was a good decision giving flexibility in options. Even with a 4 stroke engine and Tundra tyres I still had about 40kg to spare below the UK limit of 300kg MTOW. Both these options were very important to me. The extra reliability, durability, lower emissions and reduced fuel consumption of a 4 stroke engine make a lot of sense. With the variable weather conditions we seem to be getting, the Tundra tyres will extend the operating season.

Having bought and assembled the aircraft, the ongoing costs are minimal. The hourly cost is just the fuel at about 6 litres/hr and annual costs including third party insurance amount to about £120/yr. Flying doesn't come much cheaper than that!

Take-off from Popham after the Microlight Aerofair