How fast do the UAV flies?
Simply I thought, it is necessary for the UAV to bear windy disturbance of 15 m/s considering the nature of Lake Biwa. While Lake Biwa is 235 km long, it takes 4.4 hours at 15m/s. For instance of Yamaha Motor's RMAX, it is impossible for rotary wings or helicopter to fly the long duration.

 What kind of fuel is reasonable?
I ran a glow engine of 15 class before. It spent about 10 mL/min. If it runs 4.4 hours. it will spend 2.6 liters. It weighs about 2.6 kg. How about a motor driven by battery. A file shows that a motored RC aircraft flies 12 minutes using Li-po battery ( 145-3,300mAH )[6]. The full battery capacity for the 4.4 hour-flight will be 72,600 mAH. A 5,200 mAH Li-Po weights 278g, so the total battery weighs 3.9 kg. I had better not use battery for a long flight, though motor is easty to control[1].

 How much power does the UAV need?
A RC plane adopts a propeller D32xP10"[2]. The propeller feeds 10" air per rotation. So feed speed is proportional to rotation speed. If the UAV adopts the propeller, the engine of the UAV rotates 59 Hz ( 3500rpm ) at least, when the UAV flies 15 m/s. It is said that propeller works within 1/3 length of tips. So effective area of propeller decreases 5/9. The air flow rate is products by density, effective area and velocity, therfore 0.53 kg/s. By the first law of thermodynamics, work by engine becomes motion enrgy of air flow. The air flow power is 59.8 W. But the propeller has inertia, the engine has to work much dissipated by the inertia. If the propeller has 200 g, the inertia is 0.0110 kgm². So 757 W needs when the propeller rotates at 59 Hz. The total power is 816 W. It is said that efficiency of a propeller is 60 to 70 %. It increases 1.36 kW ( 1.85ps ) considering propeller efficiency. I have ignored take-off power.

Another hobbist adopts a 4-stroke 24cc engine of Fuji Robin. The propeller is D18xP6"[3]. The J-3CUB flew at 6,700rpm. Fuji or Honda do not sell an universal engine for a RC personal use in Japan. To apply it for the UAV, I am to buy a trimmer equipped with the engines[4][5]. A nearby shop sells it at ¥36,000. But the mount of the engine of a string trimmer seems difficult to apply for the UAV.

 Either tractor or pusher?
Almost of hobby-use UAVs adopt pusher type that lays out a propeller to the rear of wing. Pusher requires a reverse pitch propeller, but RC shops hardly sell such a large diameter reverse picth propeller.

I show sites full of skills manufacturing and knowledge about mechanical engines. However, written in Japanese language only.
  • Tezukuri mokei engine koubou has a lathe for working and make CDI itself
  • RC mokei engine kosakusitsu has a lathe and makes a very small engine
  • An example of a GAS engine using CDI from a glow engine
  • Clound cap provides autopilots, duo cameras gimbal etc
  • Toy Sonics sells Fuji-Imvac gas engines at low price
  • Venture Hobbies sells Zenoah gas engine
  • Range video provides wireless video systmes

    [1] Eagle Li-po battery series 5,200 mAH in Japanese
    [2] Votec322 in Japanese
    [3] Go go Robin in Japanese
    [4] Robin specifications Link decay
    [5] Honda GX25 in Japanese
    [6] RC battery hikaku

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