As usual I have a 'few' questions to better understand the intention of the idea.
I was wondering what the application goals are and what the expected useable payload and capacity will be for that particular use case? For example is it for loitering surveillance of a mostly stationary subject, or long range for collecting aerial photography of an area, or something else?
Am I right in thinking the engine directly drives he propellor and a electric generator to charge the batteries in flight?
Given that optimal forward propellor performance is achieved by matching the pitch to best glide slope cruise velocity of the aircraft, and only creating enough thrust to overcome drag at that velocity, on aerodynamically sleek aircraft this typically means high pitch forward props, with the prop diameter mostly determined by the thrust required at cruise speed.
The issue with this is, as you have also pointed out, that high pitch props are not good for static thrust for climb or low speed acceleration. Further, unlike a electric motor with torque at low RPM and across the RPM range, the available torque curve of a combustion engine, is fairly ineffective at swinging a high pitch prop at low airspeed. Also the engine torque and resulting minimum fuel burn are hard to align with cruise if compromises need to be made for climb/takeoff thrust. As such variable pitch offers the engine some "gearing" to better adapt engine performance to flight dynamics.
It's a common approach to design high performance/efficiency aircraft from the prop backwards, typically meaning that the engine needs to provide enough static thrust for takeoff/climbout. Typically this is at full engine load and RPM (which is also it's maximum thermodynamic efficiency), and then they throttle back for most efficient fuel burn cruise at altitude. Compounding this problem is that small engines are not very efficient at partial loads. With a QP this can largely be neglected, in that the quad components provide "a endless runway in the sky at altitude away from obstacles". However, they do not assist much in the way of providing forward momentum unless they tilt. (Which might be a viable option for the front quad motors on you setup to assist takeoff/climb as well)
Instead of a variable pitch prop on the engine that has some of it's own inadequacies, to overcome this I was wondering if you have considered using the attached electric generator as a motor to assist the engine in accelerating the aircraft to cruise, or to climb with a high pitch prop, or at a minimum to reduce the generator load dynamically depending on rate of climb? Or alternatively decouple the prop entirely from the engine and use only a electric motor for the forward propulsion.
The benefits of this "range extender" instead of "electric assist" configuration, at the cost of adding one extra electric motor and ESC would be:
1) The engines best fuel efficiency RPM could be matched to a optimised generator and load,
2) resulting from the completely optimised forward motor and prop combination determined by thrust required at Carsons,
3) regardless of thrust required by the aircraft for climb or forward cruise
4) Running a optimised engine/generator and forward electric motor propellor combination, will more than offset the electrical losses resulting from conversion of electrical generation and propulsion
5) The engine could be run at max power all the time including on the ground or in hover increasing hover times, without spinning the forward prop (running the engine at near WOT RPM results in best fuel conversion eff.)
6) The engine/generator could be downsized to only produce enough power to maintain forward cruise (I think you're currently running a 1.6kW, 1.6kg motor for forward cruise for a few hundred watts of cruise propulsion required by the aircraft, roughly what is your engine throttle setting or estimated forward cruise power required?)
7) The engine can be placed anywhere in the fuselage to optimise airflow/cooling/CoG/tanks/Payload configuration
8) Forward flight can continue without the engine running or it failing, to safely RTL on batteries, improving reliability and return of investment , or for periods where "stealth mode" is required
9) The ICE generator can be optional entirely. Assuming forward cruise at around 200W, you could install batteries instead of the tank, fuel and ICE saving some 5-6kg of weight. 5kg of 18650 batteries is 108cellsx10Wh= 1kWh of useable battery, resulting in 5hours of forward cruise without a drop of fuel used...
10) I'm not sure what your hover amps are (without downloading a 1.5GB log - Nice!) but I expect it to be around 2000W at say 7g/W? So that would be around 30min of hover time on that battery pack, plus whatever hover time the batteries you have already for the quad setup on board. (2C is easy to achieve safely on 18650's btw for hover)
11) Battery only wihtout the engine will also considerably reduce NVH
Also do you have some type of propellor stop on the quad motors to keep the props out of the airflow? They look pretty big compared to your very sleek airframe, so might impact forward drag.