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Need some help with an electric RC plane

#1
Hi everyone! I and some friends of mine are currently thinking about building an electric RC airplane. It will weigh around 2 kg, be about 1 meter long and run on 2 electric motors. We don't want it to fly very fast, but not 5 km/h either. Maybe a little bit slower than the Cessna 182 in this video:
. The plane will have a WCL of about 7, a high wing, conventional tail and non-moveable tricycle landing gear. Though we have decided a lot, we need help with a few things: What kv should the motors have? Are flaps and spoilers necessary, or would it just be way too much work for it to be worth adding? Is a BEC necessary so we're sure we don't overheat the receiver (especially if we have 6 or 7 servos).

Thank you so much for your time.
 

evranch

Well-known member
#2
I'd say around 1000kV for something of that size. The lower the kV the bigger the prop you can swing. You don't want tiny props flailing away with a heavier plane.

Flaps and spoilers, totally not necessary on almost any RC sized plane, especially a slow flyer. However, flaperons are basically free if you have the receiver channels.

Receivers do not generate current and will never overheat, the power for your servos comes from the ESC which has a built-in BEC. Check the BEC amperage rating of your ESCs to find out if you will need to use a separate BEC.
 
#3
It seems like most ESCs of the desired size has a BEC with a capacity of about 3A. 5-6 servos would require around 2A. Would you recommend buying a BEC just to be safe then? I think it'll be best. Also, if one ESC fails I will lose all control of the airplane if it is the one controlling the servos. What is your recommendation?
 

evranch

Well-known member
#4
Absolutely nothing wrong with running your servos off their own BEC and it's up to you if you want to do so. Dedicated BECs are more powerful and supply cleaner power. If you do, remember to lift the red wires from the ESCs or install diodes so that you don't end up with stray currents flowing between the regulators as they will likely have different setpoints.

If you want to be really safe, you can be triple-redundant. Install a diode in the red wires from each ESC and in the red wire from the BEC. That way you will have 3 times the current available as well as 3 separate power sources in case one fails. Schottky diodes are recommended for their low voltage drop.
 

Merv

Well-known member
#6
Just a word of caution, don’t use more than one BEC. You need to pull the positive wire from the second. If you use more than 1, they can end up fighting each other. One trying to increase the voltage and the other trying to lower it.
 

quorneng

Well-known member
#7
My own view is that provided electronics are correctly specified they are extremely reliable. Almost certainly whole orders more so than any pilot or weather conditions. ;)

My oldest electric plane (11 years now) is still flown regularly. Much modified, mistreated and on at least its 3rd battery but it still has its original 30A ESC (motor takes 18A at full power) with a 2A BEC that runs the rx and 5 small servos.