Version2:Airframe

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This page contains some brainstorming of ideas for a new version of the Albatross UAV for (very) long endurance/range flights, such as Trans-Tasman or flying the perimeter of the South Island (see Route). We have decided to aim for an all up weight of less than 5kg, using a 0.53 cubic inch four-stroke engine. Wing span will be 8 feet (2.64 m), dry weight of around 2-3 kg all up including engine, servos, autopilot, airframe etc, plus about 2-3 kg of fuel. Mainly balsa/film construction, relatively low aspect ratio. Would probably fly at about 80 km/hr (about 50 mph).

The aircraft will have a twin-boom tail with a pusher properlor, similar to the Aerosonde, but with a split V tail (one angled combination rudder/elevator on each boom) rather than an inverted V tail.

Contents

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Engine

  • Enya 0.53 cu. inch four-stroke model airplane engine converted to run on gasoline
  • electronic spark ignition
  • smaller carb (from O.S. 0.10 cu. inch 2-stroke engine)
  • gasoline fuel - premium 96 octane unleaded - with about 5% oil (we made end up needing slightly more)
  • run it at about 4500-5000 rpm, burning about 65 g/hr (density of gasoline is about 0.73 kg per liter, so about 90 mL/hr)
  • in flight mixture tuning using a servo attached to the mixture needle valve. Digital mixture control based on EGT, altitude, RPM, throttle position, and pre-recorded calibration data.
  • exhaust will be vented onto the propellor hub

Notes

  • Gasoline has about twice the volumetric energy density of methanol fuels
  • Optimal air-fuel ratio is 14.7:1 by weight (the so-called stoichiometric ratio), where "Optimal" refers to stoichiometry in the chemical equation of the combustion process, minimizing emissions and optimizing fuel consumption.
  • Can measure the fuel-air ratio using exhaust gas oxygen sensor. Two types of EGO sensors:
    • narrow-range sensor, simple called oxygen sensor. Easy to drive and measure. Simply power the heater (needs to run at 400 deg. C). If voltage is above 0.45 V its too rich, if its below 0.45 V its too lean. Heater resistance proportional to temperature. Heater needs about 2A.
    • wide-range sensor, called air/fuel ratio sensor. Outputs a voltage roughly proportional to air/fuel ratio, but needs some kind of active current nulling circuit (difficult to drive). Operates at 650 deg. C. Heater needs about 8A and it needs to be PWM to regulate temperature.
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Ignition

  • Open source EFI project with extensive documentation and C code that looks useable: MegaSquirt. Megasquirt also does ignition timing!
  • Open source very simple CDI for RC planes: http://home.online.no/~jon-mj/cdi_ignition.htm
  • Need to find a small/light spark plug, cable and ignition transformer/coil. May have to wind the coil ourselves.
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Alternator

  • brushless DC motor as alternator
  • mounted to the back of the crankcase, at the opposite end of the crankshaft to the propellor (power takeoff)
  • 3-phase (6 diode) rectifier, capacitor low pass filter
  • will need at least 15 W, will probably need 20-30 W
    • autopilot electronics, say 4 W
    • electronic ignition, e.g. C&H, say 4-5 W
    • servos, say 1 - 2 W typical, 5 W max
    • communication, possibly camera on earlier flights, say 5 W
  • rewind/rewire the motor to wye (most are normally delta except CD spindle motors) for higher voltage output
  • rewind with more turns of thinner winding wire for higher voltage
  • I think we should be generating (post rectifier) at about 10 - 14 V at nominal rotational speeds
    • better for efficiency than lower voltage
    • won't need to step voltage up for anything
    • switching step-down on PCB - lighter cabling, less ohmic losses

Electrical System

  • I think we should use one 7.2 V lithium pack (step up converters would probably not be needed), David wants to use one lithium cell (3.6V) necessitating step up converters to power everything
  • need a lithium charger, and a bunch of power supplies. See Next Version
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Launch

  • FIXME
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Old Ideas

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Other Engines

Honda Four-stroke Engines

Honda GX25

  • Specs: http://www.honda-engines.com/gx25.htm
  • 1.76 kg
  • 25 cc displacement
  • best torque (1.25 Nm) is at about 5000 rpm (power = 600 W, 0.8 hp; fuel consumption = approx. 250 g/hour)
  • best fuel consumption (approx 230 g/hour) per unit power is at about 5500 rpm (power = 1 hp, 745 W; torque = approx 1.2 Nm)
  • probably run it at 4500-5000 rpm
  • 18x8 propellor, could go larger (20x8, 20x10?), need to work out what prop corresponds to around 4500-5000 rpm, at some decided altitude
  • C&H RC electronic CDI ignition
  • No need for fuel injection, if its only burning ~215 g per hour
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Fuel Injection

Information

  • Main advantage is consistency/reliability rather than improving mileage. Used in cars mainly for reliability and to meet emissions requirements.
  • A perfectly tuned carb can theoretically do just as well as fuel injection (at least in constant air conditions - temperature, pressure etc. and consistent fuel etc.)
  • Fuel injection will add weight (possibly considerable weight), and draw quite some electrical power.
  • still need a throttle (e.g. butterfly valve) and associated servo
  • need a manifold absolute pressure (MAP) sensor
  • probably also want ambient pressure sensor (or get it from altimeter on main Albatross board), an air temperature sensor, and exhaust gas temperature (EGT) sensor, and cylinder head temperature (CHT) sensor, and need to know throttle position
  • development will be an extreme fire/explosion/safety risk
  • open source EFI project with extensive documentation and C code that looks useable: MegaSquirt. Can also do ignition timing.
  • info (service manuals) for automotive fuel injection and ignition systems: AutoShop101.com
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Inkjet based

  • injector somehow made from an inkjet printer ink jet assembly, most likely Epson piezo.
  • probably continually running, injecting into the throttle body
  • I have begun hacking an Epson C45 printer to function as an injection system. See Inkjet hacking.
  • Information links:
  • Pros:
    • the unpressurized fuel will be fine. There is some evidence to suggest that inkjet printers actually perform best with about negative 2 inches of water of pressure.
    • in theory it can atomize liquids almost perfectly (say, 6 pL droplets)
    • very fine control over the amount of fuel delivered (can count individual droplets)
    • would in theory deliver consistent sized droplets regardless of minor fuel pressure variations
  • Cons:
    • finding information
    • delivering enough fuel fast enough. Will probably be able to deliver enough in cruise, but might starve the engine during climb out.
    • material compatibility with the fuel (gasoline "melts" some plastics) - Update: seems this might not be a problem. See Inkjet hacking.
    • designed for specific ink viscosity - its unclear if it will even work and what the effects will be of operating it with a low viscosity, low surface tension liquid like fuel. Update: I don't think this will be as much of a problem with the Epson micropiezo inkjets as with thermal ink/bubble jets.
    • we would need almost perfect particulate filtering. A speck of dust will clog the injectors. Not that that is really a difficult problem to solve.
    • drive electronics... thermal bubble jet needs high currents at around 15 V, piezo needs low currents but at up to 50 V (not sure of actual voltage yet). Update: see Inkjet hacking
    • needs some fabrication work and reverse-engineering, research and development work
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Car/motorbike/scooter-injector based

  • find a very small injector from a car/bike/scooter engine (Honda make a 50 cc EFI scooter engine)
  • such injectors are basically a needle valve operated by a solenoid, turning on and off fuel flow into a atomizer spray
  • Pros:
    • can definitely deliver enough fuel (probably too much)
    • easy to drive compared to inkjet, at least at the low "amount of fuel to deliver" level (just PWM, but you still need to work out how much fuel to deliver)
    • reliable, mature technology
    • fuel-compatible materials
  • Cons:
    • need pressurized fuel system to some extent. Maybe the little pressure we have will do. Cars use about three atmospheres of fuel pressure. The concern here is that without pressure, the fuel may just dribble out rather that spray out of the injector. May need a fuel pump and fuel pressure regulator.
    • probably heavy
    • awkward connections etc for fuel input etc. because its designed to thread into the throttle body or manifold
    • draws quite a bit of power (so called "high impedance" injectors are about 12 ohms, and want 12 V so about 12 W at say 1-10% duty cycle)
    • fuel pressure variations will affect amount of fuel delivered, so we might need a fuel pressure sensor
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Full custom injector

  • some home-made custom injector specifically designed for our engine
  • perhaps using the glass jet plate from an inkjet as the nozzle, and some kind of voice-coil driven actuator, or a resonating elastomeric chamber excited acoustically by a small speaker to break the fuel streams into small droplets
  • Pros:
    • can be made to exactly meet our requirements
    • can build it from fuel compatible materials
  • Cons:
    • reliability, longevity will be poor
    • needs considerable research and development
    • needs fabricating
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