I'm going through the process of designing an RC airplane. I know that probably most calculations on scale models might be non-precise ones, but I'm trying to do it as correct as possible (it's a school project). I was trying to check how much shaft power I would need for the aircraft, so I did an excel table and used some pessimist assumptions in order to check if my 46 cubic inches, which deliveries 1,63HP, had enough power. These are the aircraft parameters: Chord= 0,3m Wingspan = 1,8m Wing Area= 0,54m2 AR=6 W= 3,7kg (assumed) Cp = 0,048 (assumed) Propeller efficiency= 0,6 (assumed) Air density= 1,225 kg/m3 (Sea Level - where I intend to operate)
I used W = 1/2 * rho * S * V2 * CL to identify the required CL for level flight at any given airspeed, then Cdi = CL2 / (pi * AR * e) and using Ri = 1/2 * rho * S * V2 * Cdi and Ri = 1/2 * rho * S * V2 * Cdi to find out both resistances I summed them up and multiply them by speed to get power, then I divided that power by propeller efficiency to get the needed shaft power.
For my surprise, the maximum speed that I could achieve with that 1,63HP it's between 39 and 40 m/s or 144km/h (89,4775 MPH), it seems like a lot. Because of these results, I decided to contact a friend who is into RC Planes, he told me that that engine was actually too little for a 1,8m wingspan plane, it looks like I'm doing something wrong, but I don't know what. L/Ds actually make sense.