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#RC Airplane Series – 4
Now, we enter into the electronics side of the RC Airplane . In this article, we discuss about ‘How shall we exactly choose a motor for our RC Airplane ?’. This is one of the very crucial step because your electric motor and propeller combination in RC Airplane does the work similar to the fan engines in real RC aircrafts. This motor + propeller combination is responsible to provide the necessary ‘thrust’ required.
Topics Covered :
- Electronic Components in RC Airplane
- Deciding the type of flight
- Choosing the motor – Motor Terminologies & Procedure
- Choosing the appropriate Propeller
- Example for Better Understanding
Just to clarify !
I am sure, you will have a question that, if in last article I asked you to refer a plan from online sources, then why not copy their electronic components as well.
There’s a problem in that !!
Many students/hobbyists aim to participate in various aeromodelling competitions. And when there’s a competition, there are some rules/constraints which we need to follow. Basically, this is where, it’s important to know how to exactly model the aircraft (design part + electronics part) OR else if there were no rules, there are so many resources about ‘How to make an rc airplane ?’. So , everyone would just copy them !!
And apart from this, the joy and the interest which u generate in the field of aeromodelling once you try to understand these concepts is unmatchable !!
1. Electronic Components for RC Airplane
- Motor + Propeller combination
- ESC (Electronic Speed Controller)
- Battery
- Servo motors
- Receiver
- Connectors
Here, we are just naming the electronics needed to drive an RC plane. We cover the motor and propeller selection in this post. In the upcoming articles, we start discussing each one in detail. We are going to keep everything to the point but discuss the important and necessary things in detail.
2. Deciding the Type of Flight
In order to select motor, We need to first decide the type of flying which we need from our plane.
And based on that, we have a term called ‘Thrust-to-weight’ ratio. Also known as ‘TWR’ or ‘T/W’. Based on the type of flying we choose, we need to fix our TWR accordingly. To elaborate,
- So, the first step for motor selection will be to be fix the TWR for your plane.
3. Choosing the motor
We need to follow a specific procedure in order to get therequired motor. Refer to the following flowchart for that :
Step – 1 :
In the previous article, we learnt to calculate the the ‘model weight’ (i.e. only the design part). Now, we need to first assume the electronic components and calculate the ready-to-fly weight. It means that the plane is fully ready (design + electronics) to fly and the weight of the plane at that point of time is called here as ‘ready-to-fly‘ weight.
Assuming electronic components :
- Most of the components (motors, ESC, battery, etc) have their weight within a fixed range. And note that, you DON’T have to be very specific and exact for this. We need an approximate weight of aircraft.
- Refer the product’s website and check the specification section to get the weight
- This part will become more clear once you have the knowledge of all the components used in RC plane. I have attached a file below as an example to demonstrate the whole process
Note : If you are using landing gears for your plane instead of hand takeoff, you need to include that weight as well.
Step – 2 :
From previous section, we have fixed our TWR. Use this value to calculate the thrust. This will be the thrust required to achieve the required TWR for aircraft.
Step-3 :
It is advised to use BLDC (Brushless DC motors) due to :
- High Efficiency
- Longer Life span
- Better speed control
- Prevents over-heating
Q. What is RPM of motor ?
The number of revolutions (one complete circle) that the motor rotates in one minute of time is known as RRM of motor. RPM stands for Revolutions per minute. For e.g. 2500 RPM implies the motor rotates 2500 times in one minute. So basically, RPM is the unit of ‘angular velocity‘
Q. What is kV of motor ?
kV rating of a motor gives the idea of : At what RPM will the motor run when a certain voltage is applied. For e.g. If we have a motor of 1000kV and lets say the safe operating voltage range is 5V-12V. So if I am operating the motor at 5V, the motor will run at 5000 RPM , while, if I operate it at 12V, it will run at 12000 RPM.
Based on this, we can formulate kV rating as :
How to choose kV of motor ?
Recall the type of flying which you chose.
Now, once the motor kV is fixed, then go to the online electronic stores’ website and search for the motors of the calculated kV which are able to provide the required thrust. A thrust value greater than required is OK !
(Look through the specifications/description section of the product’s page for thrust value)
Step – 4 :
The propeller is another very important factor to consider since this fan like thing is the most responsible to generate the thrust required for our airplane. Check the datasheet or the recommended propeller size for the selected motor
4. Choosing Appropriate Propeller :
Working :
The propeller basically ‘pushes’ the air backwards so that the reaction force is acted on the propeller makes it move in forward direction. The working of propeller is a simple application of Newton’s third law.
Dimensions of Propeller
Diameter : The end to end length of the propeller. Mainly responsible for the rotary motion
Pitch : It is the distance covered by propeller in forward direction when one revolution is completed. Pitch is mostly responsible for the translatory motion of aircraft. Pitch is connected to the speed of the aircraft.
Notation : Example : 10×4.5 propeller implies diameter = 10 in & pitch = 4.5 in
How to select a Propeller ?
Again recall the type of flying chosen for your aircraft. Based on that, we need to fix the size of the propeller.
Consider the below example (pdf file) for better understanding
5. Example :
A problem statement has been given (Like a competition) and based on the constraints, the procedure to select a motor has been given. Go through it thoroughly to get a complete understanding. (We are assuming that we chose some plan and on calculating the model weight of airplane, it came out to be 250 g.
Enjoy Learning !!
RC Airplane Series – All Articles (You are at Part – 4 !)
- Part-5 (Next) : How to choose LiPo Battery ?
- Part-6 : ESC Calculations and Electronics Connection
- Part-3 (Previous) : Designing your Airplane !!
- Part-2 : Understanding the control surfaces !
- Part-1 : What’s in the Wings ?
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