Cricri aerobatic aircraft and other complex projects Page
The builder will - in the 2000 to 3000 hrs of building - encounter numerous problems. Here are some tricks and tips I have gleened that might help other builders. 2.Venting of the fuel pumps atmospheric vent to the
carburetor inlet. This involves sealing the external vent with Silicone
and drilling a 3mm hole through the carb body and into the carb inlet.
There is a casting nipple to guide the way. The process is described in
Michel Colomban's handbook on engine care. Photos show dismantled carb
pump, installed lever to open the fuel valve, and location of hole to
drill to divert venting of pump to carb inlet. The space around the
lever is occluded with silicone.
These are in the order they occur to me. Many more to be added.
1. Priming the carburetor
My mentor, Nev, achieves this by pressurising the fuel tank by blowing down the vent tubing, clamping the end of the tube, then release the carb pump cover screw. Fuel flows freely up the line.
Taking this further, I have dismantled the carb pump (here is a good web site with photos) Fabricate an elbowed lever to pass through the vent hole to depress the metal centrre of the diaphragm that controls fuel flow through the pump.
In essence the fabricated tab opens the fuel pump's output control valve to maximum output, and the pump can draw several cc's of fuel along the fuel line with each engine rotation. Even better, by pressurising the fuel tank (by blowing down and clipping the breath line) the fuel flows straight through the carburettor when the tab is depressed, without even turning the engine.
3. Project planning
Try to do something every day on the project, even if just sweeping the floor. Get friends round to go over progress - if they can stand it again! Their visits can help keep you inspired.
Buy a small lathe and teach yourself the basic skills. I regard a lathe as essential and one of the really enjoyable parts of the project. There are dozens of parts whose fabrication benefits from basic machining.
5.Do heat the Klegecell
You can purchase Klegecell already heat stabilised, otherwise this definitely needs to be done, as advised in the plans. I measured a shrinkage of 10% at 70 degrees C. Might be enough to debond your wings on a hot day flying! It'd be like Icarus!
6. Metric / Imperial
Mixing both systems works fine. Far better to fabricate using metric. All aluminium sheet is sized in equivalent metric and imperial. AN hardware is best for price and range. Metric bolts required a lot of cutting threads to size. For small fasteners you can buy nice stainless 4mm screws with cap head or button head and they conveniently fit 8-32 nutpates. I have used these 4mm ss screws for all cover plates, wheel pants - everywhere non structural.
7. Modifications to Cricri plans
Here is a list of the modifications I have made:
-Spherical bearing to upper and lower rudder pivot points
-Access hatch on front upper fuselage
-Undercarriage faring secured by two nutplates - removeable
-Bend in ss rods attaching fuel tank to fuselage
-Circuit breakers (x7)
-Klegecell / fibreglass frames foward and aft of canopy (instead of Ali)
-Polyurethane to seal visable klegecell stiffners in fuselage (aesthetics only)
-Split automotive conduit to convey fuel lines and wires through front fuselage
-Dynon 10A EFIS instead of conventional flight instruments
-6mm nutplates to receive 6mm the two elevator bolts (very difficult to fit conventional nuts)
-Hydraulic disc brakes and split hose from a single Shimano lever
-Embroidery in seat cover
-Nosecone flared to receive winglets of engine nacelles
-Tinted primer paint for fuselage (aesthetics only)
- canopy raised 50mm to fit pilot
Can't think of anything else but probably lots of small alterations
If you can acquire a list of sources and sizes of material used this can be hugely helpful to the project. Of course I have such a list - mostly New Zealand Suppliers - some French, some US.
I will make this spreadsheet available for a limited time, here.
9 Fettling & Fiddling
Coined from another author, this is a process I love. Continued small incremental refinements until you get the thing working just right - to your satisfaction. Failures are just learning points. Its a philosophy that gets you thru things not working. Keep "worrying" at the problem until you overcome it. Worrying in this context means beavering away at - not being worried by...
10. Pressurise the fuel system. This is a common process for model aircraft and has the advantage for two stroke engines of avoiding the need to hand prop the engine to suck fuel down the fuel line. Initially achieved with a blood pressure bulb (easy for me - there are dozens of unused ones around my institution) - then feeding a small pressure line from one exhaust pipe into the fuel tank. A non return valve (used in hospital IV lines) ensures a constant +ve pressue in the tank. A few risks, but some positives as well.