Cricri aerobatic aircraft and other complex projects Page
Fitting Canopy Hoops and Turtledeck Frames
Diary 6 (final)
Previously fabricated Klegecell hoops with fibreglass epoxy outer layer were primed and painted in Delfleet white.
Secured to rear polycarbonate turtledeck and front panel cover was by drilling a recess for threaded aluminium tubes which were then glued into holes in the klegecell. Attachment is by screw fasteners - much neater than gluing. (Threaded inserts made on lathe 10 x 6 mm with thread 4mm )
The tubes were set into hoop and epoxied in place, providing a secure anchor for the attachments to the hoop.
Canopy frame tubes bent of wooden wheel system described earlier to fit on plans, Fiddly, but not much trouble achieving the major bends. The 45 degree angles carefully cut and filed to snug fit, then appropriate angle cut from 4mm ali with 30mm legs was epoxied into the butted tube ends to provide rigidity at the junctions ( and for later canopy screws) With the epoxy cured the angle flanges were riveted in placed as per plans.
Canopy latches fitted, with nutplates secured. Slots filed to correct slight inaccuracy in fitting
Canopy & its Frame and Turtledeck
Turtledeck polycarbonate (1mm) cut out and fitted to its angle brackets. After selecting a nice Delfleet 2 pot white for the fuselage section covered by the turtledeck, I painting the skin prior to fitting these items.
Elevator and rudder deflection angle setting
Elevator stops adjusted extensively to give the -11degress +6 degrees shown on the plans. This was a stretch to achieve, then I find the written description only requires -9 and +4 degrees, as long as the setting at +0.5 has the stick at 5 degrees forward of vertical. Clamps on the reducing bar were move out by 10 mm to give rudder through of 20 degrees each direction. Full and free movements without play or friction.
Completed earlier, these had been alodined by Paul Muller. Slots enlarged to give a good accurate fit..
Rudder reducing bar clamps moved 12 mm laterally cf as specified in plans - to give 20 degrees of rudder deflection R & L.
Front fuselage hatch:
Bits and pieces done—something different every day. I decided to create a large hatch in the front fuselage upper surface as there are so many maintenance items in this front section—bungees, battery, access to circuit breakers, wiring and fuel hoses. Neville Hay said lack of access was one on his most troubling features. The lower air scoop hatch is partly covered by the rudder tray and doesn’t give much access anyway. Ultimately, after much cognitation and discussion I realsie the skin of the nose is stressed and is therefore quite critical to integity. Therefore the hatch is thicker 0.032 2024 aluminium, with nutplates and a doubler around the edge.
Undercarriage mount brackets:
Mat finally finished machining the complex U/C clamp brackets. I tapped and threaded for AN bolts and have lightly sanded the carbon fibre undercarriage to firm fit in the clamps. My custom made rubber bushes are rather stiff—but may loosen up.
Lateral motion limit bolt hole drilled through the undercarriage. Brackets all mounted to the fuselage after priming.
Wheel axles and boss for brake rotor:
The axles were fitted earlier to the undercarriage with 4 securing bolts and a large 4mm Ali plate to hold the brake caliper. This plate is yet to be adjusted and lightened.
Boss for rotor turned from stock free machining ali, extending 32mm from hub bolt lugs. The outer flange will receive the 3 bolts from the hub. The inner surface holds an additional bearing and the rotor, which floats on sholdered spacers, while turning on a ‘petal’ engaging lug milled by Connett Engineering.
The 'petal' matches the internal shape of the rotor. It's supposed to allow the rotor to self centre between the brake calipers, but in the end I just screwed it down to the boss. The whole arrangement is much more complicated than it looks and took ages to get the clearances correct. The boss holds an additional bearing otherwise too much twisting moment on the plastic wheel hubs.
Shimano uses 8mm fittings and olives for line ends. I made a "T" out of 12mm ali plate and made tapered holes (3) with a ground drill, then tapped the fine 8mm thread. First one leaked hydraulic fluid, but the second attempt was successful.
First fitting of the canopy was a minor disaster. Fortunately I didn't cut it, just sat it on the frame. It was too low for me to fit comfortable, so pulled off the turtledeck, added 50mm to each upright of the rear turtledeck, and the canopy frame, then trimmed and assembled and fitted. Now snug, but comfortable. Didn't take many photos at this point as it was frustrating going backwards. Canopy still needs some fine adjusting and rubber seals.
Noseleg and rudder bungees are difficult to adust inside the nose of the fuselage.
I used a wooded block with holes for each bungee after passing through its own loop. The two exiting bungees are then clamped with a hose clamp. The exiting bungees have little tension and are easy to adjust. The noseleg needs to be really tight if the nose leg isn't to depress when pilot climbs in. From advise, the cricri needs to sit as nose high as possible for easy takeoff, otherwise nose down attitude needs considerable elevator input to break from ground, with risk of "zoom". Good T/O characteristics verified later in flight tests.
Polystyrene shaped to the top of the vertical tail, with clearance for the elevator, glassed with 3 layers of 125gm cloth, then polystyrene dissolved with petrol. Still lots of work and filling to obtain a satisfactory finish.
Undercarriage also fared, using klegecell for the shape and hydraulic lines run down an internal tunnel in the Klegecell. 2 layers of glassed cloth.
At this stage I remade several components, including the right engine mount bracket to frame 1 as the holes at the top were too close to the edge. Also remade engine mounts, this time water cut rather than plasma cut, whcih is apparently a no no in the aviation industry. The heat embrittles the aluminium. Also, the bottom hole after a change in bolt size, was too close to the edge. I don't take photos when the progress is backwards!!
Weight and balance
Use this excellent Zenair web site to calculate balance, written by the nemesis of Cricri design! CRI weighs in at 88.7Kg, with CofG (empty) at 6.52% of chord (desired 11% +/- 6). I later moved the batteries rearward of the seat - to get closer to the 11% CofG empty.
Flying config weights are also with in normal range (24 - 40 % of chord). With full tank(23 Litres = 16 kg) , pilot and 2kg of luggage wt is 176 kg. Empty weight of the Cricri with all fittings on is 89.4Kg - heavier than desired but OK to work with.
Measured with a borrowed electronic inclinometer. Both wings have identical incidence at the tip (0 degrees +/- 0.1 degree) . At the roots the left is +0.9, the right wing root is +1.5 degrees.Slightly less than desired washout on the left. Can't do anything about it but probably acceptable anyway.
Same inclinometer to check excursions of flaperons, elevator and rudder. All set to correct limits by adjusting whatever is adjustable. For example the clamps on the rudder control reducing bar had to be moved ourwards to achieve a rudder throw of 24 degrees right and left.
Sounds simple - takes ages.
Engine farings and spinner mount system
Tried complex but settled for simple. The engines are so elegant it seemed a pity to cover them. So a simply round cover just to hide the electronics and mounting frame.
Lots of work starting with a borrowed 1/2 half mold - 4 weeks fabricating and fitting. See photos in gallery.
Polishing with cyclo polisher
Perfect - it gleams.
A simple flat bed design, tilted by unhitching from the towing vehicle and raising with an extending jockey wheel. The trailer has been clad with water proof Trans-tex - plywood with phenolic coating (brown). No paint.
Numerous adjustments required but with plane now residing at airport this part of the project has taken 3 months. First, there was the obvious flaws to repair and correct. A faulty radio from the outset was a setback requiring a return to manufacturer, but no problem since its return. Then, the RPM not reading correctly, eventually resolved by using schielded two core wires from a magnet and sender assembly. This was complex to set up and took maybe 20 hours to solve.
The exhaust joiners have been a trial, but beavering away with these braided, heavy type has done the trick. Initial rivets to sucure was insufficient. Needed steel pins screwed in and secured with lockwire. Maybe i should have just welded them up! I machined stepped spacers for a tight fit at each junction, to avoid welding. At least I can still dismantle.
New Stainless steel braided exhaust joiners fitted
Weight and Balance
The first test W & B gave weight forward of ideal. This was resolved by relocating the batteries (x2 Yuasa 12 volt gel/ acid ) to rear fuselage - held by a light bracket 300 mm behind the seat. New cables fabricated and installed. Empty balance point is now 9.5% chord (ideal 11% +/- 6).
Preflight LAME check
The preflight LAME check threw up a few more issues - all minor and corrected with a few weekends of work.
1. Replace fuel line filters (Debis in filters)
2. Shim nosewheel to prevent all chance of shimmy
3. Increase tension on engine mounts to minimise outboard deviation of engines under power (ie maintain airflow over tail)
4. Lubricate flaperon bearings (slightly tight)
Altered throttle cable system to minimise chance of cable parting from
final pulley. This was achieved by placing a light spring in the "close
throttle" circuit. The "push / pull" throttle circuit is now under
constant slight load to resolve this concern.
Have replaced EGT with CHTs as decided the CHT gives slightly more useful information.
So close to first flight you can almost feel the tension!
END OF BUILDING DIARY - Proceed to flight blog