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Pumpkin drop event

RC Plane Propulsion Calculations

#1
So it's the first time I'm trying to build an RC plane and I'm really struggling. It would be really amazing if somebody can nicely advise me (I understand I'm stupid). So, the parameters I have are:
  • Max wingspan of 5ft (estimated wing area = 4.2 ft^2 with a chord length of about 10 inches)
  • Being able to fly at least 40 mph (at this velocity, it creates a thrust of 10lbs)
  • So I'm guessing the weight of the plane needs to be less than 10 lbs
  • I want to use two motors, one that connects to the controls system (receiver, ESC,etc..)
    • and a separate for the propeller
  • Using lipo batteries

I need to calculate the propeller-battery-motor system, but I guess I dont know where to start or what equations to use. Based on this parameters, how can I calculate what type of motor do I need (rpm, voltage & current) in order to hook it up to a propeller & lipo batteries. Am I missing any parameter?
 

buzzbomb

I know nothing!
#2
Hello and welcome! That's an ambitious project you've embarked upon. Let's take your parameters one at a time. A five-foot wingspan, 10lb plane is HUGE! Most of the FT style planes are around a three foot wingspan or less. What exactly are you trying to design?
 
#3
So that's the max wingspan. It will prob be closer to 3 or 4. And the weight closer to 5-7lb. I just don't know exactly what equations and calculations to use to calculate the power I need so I can find the motor and then match batteries & propeller
 

buzzbomb

I know nothing!
#4
FT does have bigger planes. It's just that most fall in at 3 foot or less. The way most of start out is to purchase the power pack Flite Test put together for a particular plane. Everything is matched, no calculations needed. Alternatively, if you click on the power pack for a particular plane, you will get a list with the specs of the motor, esc, etc. You can then purchase matching components from elsewhere if you desire.

If you're designing a plane, then you can find one that is the same size and style, and use the recommended power pack as a guide. Here's a link to the diy plane section of the store:

https://store.flitetest.com/diy-rc-airplane-kits/c12698

There are formulas to figure it all out, but I don't personally use them. @CarolineTyler might be able to help you a little better if you just want to do the math. I'll continue to monitor this thread and help where I can. I wish you luck!
 

Headbang

Well-known member
#6
Step one is you need a constant. Can not figure out anything if everything undefined. Typically design and build the plane either around a specific power system, or decide on the plane specs and get a power system to match. There is a lot of stuff that you can math the crap out of, but for the most part everyone works on watts per pound for motor selection and starts with the manufactures recommended prop and esc. I usually figure battery on 6min flight time with a very rough estimate. Flight time in minutes / 2 / 60 * max amp draw of motor * 1000 = rough needed battery capacity in mAh
Eg 6min/2/60*90amp draw*1000=4500mAh battery

Trainer/scale - 100-150W per lb
Sport Flying - 150-200W per lb
3D Flying - 200-300W per lb

Hope this helps
 

moret

Active member
#7
  • I want to use two motors, one that connects to the controls system (receiver, ESC,etc..)
    • and a separate for the propeller

I do not understand the two motors. Why would you have a motor and not have a propeller on it.
The correct ESC will run the motor and the receiver. Battery hooks to the ESC and the ESC powers the motor and receiver. Your servos ( controls ) connect to the receiver.

Good luck
 
Last edited:

buzzbomb

I know nothing!
#9
I took it as a new pilot, who doesn't understand the need for servos, but was hoping to use existing resources, i.e. two motors. WE know that's not going to work. We're just helping out the new guy. :)
 

moret

Active member
#10
I took it as a new pilot, who doesn't understand the need for servos, but was hoping to use existing resources, i.e. two motors. WE know that's not going to work. We're just helping out the new guy. :)
I agree, hopefully he will read your post and ask any more questions he needs answer.
Thanks for trying to help him.
I am new enough that I just try to use the recommended motors, ESC, etc for a model.
FT does help by having the power packs.