Following on from discussion on aircraft suitable for electric power, and the need for a big enough battery that is light enough. If we think of it purely in the terms of The ability to carry X amount of energy that can be converted into usable propulsion during a flight. This opens up the field in terms of design. Any Source is now open be it gasoline- petrol, lpg, hydrogen, solar, biofuels and of course batteries. Ideally using a combination of 2 or more sources of energy provides a greater potential in energy output, increased redundancy and longer range. So that means hybrid systems which for us right now tends to be gas running a alternator to make power, then a electric driving the prop and batteries as well. It can get complex and expensive real quick and weigh quite a bit. For a homebuilt aircraft it may actually be doable and not cost a arm and leg. The trick will be doing it in a small enough package at the lightest possible weight. The greater the size aircraft, higher power needs, means we should only try for single seat designs. So sub 300kg for the rest of world and part 103 for the USA. If we look at what they are doing in drones whether a glider shape or helicopter or even now the large multirotor rigs they all tend to run a hybrid system for max range and max payload. Range can jump quite a bit into hours not minutes. Or for some days of range. So we should follow the leaders and just adapt the ideas. It can be as simple as a motor glider with a small hybrid stashed behind the seat and electric setup turning the prop. The magic comes in getting the right combination to suit your airframe. If your allowed 4 gallons in a part 103 then a 4 hour range could be done if you only need a small amount of power for sustained flight- so low drag ideally. With the greater 300kg limit comes much greater potential as fuel carrying can be relatively huge. But also does the power for most designs. We really just need to think of a systems approach in designing. Even better if we design the airframe to suit the power system. Even if that means we just select the best airframe to suit the abilities of a chosen power system. It also means we should look at the complete system weight of a current beloved aircraft design. So that means anything at all in the aircraft related to propulsion. So motor, fuel lines, tanks, weight of full tanks, engine mount, cooling fairings, cowl, prop, any electrics related to it. It adds up to a lot of weight esp on some designs. It becomes a large mass fraction of the total weight of the aircraft. Often half or more. This is the weight we really should be considering when we look for a solution in powering air chosen design. So if you X aircraft has a total figure of 300kg and all that stuff weighs in at 120kg then that 120kg is the design goal. The figures are made up and just to illustrate a point* The modern small UAV and model aircraft stuff is far ahead of the game in packaged power that is cheap and light. If we have a airframe that needs 4.6kw to stay in powered level flight, I have found a hybrid for the job, forget the price- thats a rip but nice. A bit heavy in some ways but nice. Dual engines for redundancy plus battery. https://www.aliexpress.com/item/330...9.0&pvid=1ab275b0-9f3b-4111-8f4d-1c0e87e3acda Or 10kw which is much better. And still would weigh almost nothing. 3.175 kg actually for 10kw . https://www.aliexpress.com/item/330...8.0&pvid=b740c193-1fdc-464c-8f7d-a700d060e69c Its all a matter of scale of course, and a twin 10kw hybrid could be as little as 7 kg or less. So you can have longer range from combining the gasolines superior energy density to the electric systems instant power for takeoff and boosted power. Using the electric systems full potential for boost power means we can do max safe drain on batteries and still get a extra 10kw at the prop when required from the gas engine. For longer range cruise we run the gas motor as needed for charging/powering the prop. Also we get redundancy of engine and battery provided power. This could mean a great deal more range on a much smaller package, the battery can be much smaller and thus cheaper and have a lower max current draw need- so less energy to have to cool off as a bonus. The tank and hybrid can be low weight as well. Now we only need a suitable engine that is efficient at 10kw but can punch 30 kw if needed. These tend to be light, cheap and reliable. Same with the controllers. Now as its a hybrid we add solar film cells on the wings and tail - the printable type from previous discussions. That can give 3kw dependant on design. So now we have a extra 3kw for power so lower fuel burning needs. Run the little devil hybrid on biofuel and its a green machine. A well designed prop that folds with engine would be a easy fit to a glider or other designs. A Facet Opal comes to mind but even a wind bag draggy machine could still benefit from a hybrid. I am sure it is possible to roll your own mini hybrid system at home without a massive cost. If we only need battery boost at takeoff of 20kw of total max power 30kw and then run the 10kw max on gas and or solar cells. So we have Small cheap battery and more reliable smaller cheaper gas engine and super low weight smaller fuel tank Smaller cheaper controllers Smaller propulsion engine or engines Everything is considerably cheaper than previously and now at the sweet spot for our sport. Because everything is now compact, its easily distributed around the airframe it can open up design a bit. You could use your battery if needed in flight as it would be getting a charge from the hybrid/solar as needed. So at times you could be silent running on electric only- still have noise though. Now I would not expect a long engine life on some of these but the idea has merit for some airframes. And they are very easy to re ring/ change a cylinder. Or completely replaced with another you have well run in etc. These are simply just examples but a much higher output version could be done. The idea of a inline version that has the gas engine connected to the prop engine can also be done. Reduces systems but concentrates weight to a single point. Any suitable gas engine could be done like a rotax 40hp or 100hp two stoke or four stroke if light enough. They do it in UAVs and they do it in cars. BMW has a small hybrid car which uses a motorbike engine.