Scratch Built RC Airplane

Hi, In this instructable, we'll be building our own RC airplane.

Why this instructable?

Well, I started in RC hobby about 2 years ago. Watching people build and fly their own RC airplane has always been inspiring for me. After reading lots of articles and watching a number of videos covering tutorials (which was the hardest part during the entire build process). Finally, I was able to build my first RC airplane.

So, this instructable covers everything you need to know in order to build your first RC airplane. This is a beginner’s guide. So, even if you have just got into RC hobby you can build and fly this model.

In this instructable, we'll be building a medium-sized RC airplane with a wing span of about 20 inches.

Why scratch build RC airplane?

• Well, it costs a lot less than building it from RC kits or Ready to fly models. And It’s fun
• Flying RC airplanes is always fun. But, when you build something and see that flying it feels incredible.

Contents:

Step 1: Part list

Step 2: Choosing electronics and parts

Step 3-4: Preparing the electronics for connection

Step 5-6: Connecting and testing electronics

Step 7-11: Building the foam airplane

Step 12-13: Installing electronics to airplane

Step 14: Flying and Troubleshooting

For building your own RC airplane you’ll need:

1. Emax MT1806 2280 KV Brushless motor
2. 5030 Propeller
3. 20 A ESC
4. 2 X 9g digital servo
5. FlySky FS-T6 2.4ghz transmitter with FlySky R6-B receiver
6. 1000mAh 30C 3s li-po battery.
7. Li-po battery balance charger
8. Li-po battery low voltage alarm
9. 2mm Depron Foam / 4mm Adam’s Readi-Board
10. 1.5mm Hobby grade plywood / Motor mount
11. XT60 connectors, Bullet connectors, Heat shrink tube
12. Music wires

Tools / Equipment:

• Soldering tools
• Hot glue gun
• X Acto knife or Box cutter
• Scotch tape
• Helping hands

Once you have decided how big your airplane is going to be, start by choosing a proportional propeller. By proportional I mean you don’t want to choose too big or small prop for your airplane.

Now, determining the size of propeller sure requires some maths. But you do not need to worry about determining prop size unless you want to design your own RC airplane. Anyways, you can always use e-calculators.
I used eCalc to determine the size of propeller.

This airplane has a wing span of 20 inches that requires a 5-6 inch prop. Now, I think 5-inch prop works best. Where 5 inch is the diameter of the propeller. Also, the airplane is for beginners and we do not want a massive amount of thrust. So, we’ll use a 5.0-inch prop with a 3.0-inch pitch.

What do the numbers on a propeller mean?

Suppose a propeller is labeled a X b. Here, ‘a’ represents the diameter of the propeller in inches and b represents pitch of the propeller in inches. Where pitch is the distance that the propeller is going to move forward during one revolution (Under perfect conditions).

So for this airplane, we will use a 5.0 X 3.0 propeller. Simply, a 5030 propeller. Combine this propeller with a powerful motor to meet the thrust to weight ratio.

Thrust to weight ratio?

Well, keeping the weight of the airplane you are building in mind, you need to supply a motor and prop powerful and big enough to pull your airplane into the air. Typically it’s 1:1 normally to 1:4 for slow-flyer. For example, if your airplane weighs 400 gm. For, 1:1 thrust to weight ratio you need a motor and prop producing a minimum thrust of 400 gm.

Once you know the average weight of your airplane, look for a motor and propeller combination that at least meets 1:1 thrust to weight ratio. If the design of airplane is efficient enough, it flies great even with a small amount of thrust. But, remember this was just a general statement to give an idea of thrust to weight ratio.

Motors

Type of motors used on RC airplanes:

We use brushless motors in RC airplanes these motors are highly efficient and are lightweight as compared to brushed motors. Brushless motors are classified on the basis of their size and rotational speed.

What does the numbers on a motor mean?

The first set of number is a 4 digit number or 2 two digit numbers separated by a dash “–“. The first two digits number is diameter (in mm) and the second two digits number is the height of the motor (in mm).

The second set of numbers represents KV. Or, at how many RPMs the motor is going to rotate when powered through 1V. So, if you have a 2280 KV motor running at 11.1 Volts your motor is running at 2280 X 11.1 = 25308 RPM.

Choosing the motor:

With the battery and electronics our airplane will weigh about 290 grams. So, we’ll be using a EMAX MT1806 2280 KV brushless motor. This motor produces 380 grams of thrust with 5030 prop at 11.1 Volts. This amount of thrust is good enough for this design. (In the build I’m using the same motor from a local manufacturer.)

Find how much thrust your motor produces:

Motor manufacturers provide datasheets along with the motors. Here you can find how much thrust your motor is going to generate with a particular propeller and battery. Refer to the datasheet of EMAX MT1806 here.

ESC

Moving on to powering the motor, Brushless motors require ESCs or, Electronic Speed Controller that converts DC current from your battery to AC current going to your motor because, Brushless motors operates on AC current.

ESC also controls the speed of motor through its servo lead and provides 5V DC power for your receiver that controls all your electronics. Now, ESCs are able to provide 5V DC power for radio receiver using a BEC or a Battery Elimination Circuit . So, that you do not need two separate batteries for your motor and your radio receiver.

Choosing ESC:

ESCs are classified on how many amps. of current they are going to supply to your motor. So, an ESC that’s rated 10 A is going to supply a maximum of 10 A current to your motor (Safely). Once you know which motor and propeller to use, refer to the datasheet of your motor and find out how much current your motor is going to draw.

Now, for safety reasons we always choose an ESC that’s rated 5-10 amperes more than the requirement of a motor at full throttle. This will make sure that the ESC is running cool and you don’t need to worry about heating inside your airplane.

By referring to the datasheet provided for EMAX MT1806, I found that the motor will be drawing 8 A of current from the ESC when working with 5030 prop and 3s li-po battery at full throttle. Considering safety for ESC and to run it cool we’ll be using a 20 A ESC.

Battery

Now, as already mentioned we’ll be using a 3s li-po battery. Also, we do not want to make our airplane heavy. So, use a 1000mAh 30C 3s li-po battery. Li-po batteries are rechargeable batteries that require a special charger for battery’s long life called a Balance Charger. Any other attempts of careless charging can not only damage your battery. But, can even lead to explosion in battery!

What does the numbers on a Li-Po battery mean?

Li-Po batteries are labeled with “mAh”, “S ratings” and “C ratings”.

• mAh stands for milliampere–hour; This represents the capacity of the battery.
• ‘S rating’ shows the number of cells that your battery has. If your battery has 3 cells then you get a voltage of 3 X 3.7 V across the terminals of a battery (under perfect circumstances).
• 'C rating’ of the battery explains how many amps of current can be drawn continuously from the battery without damaging it.

Servo

Moving onto servos, Servos moves the control surfaces on a RC airplane to control altitude or direction of the airplane. For this airplane, we obviously require 2 servos. One to control the altitude and another to control direction. I’m using 2 X 9g digital nylon geared servos.

Transmitter and Receiver

Finally, we need a Transmitter and Receiver for controlling the airplane.

Channels on the Transmitter and receiver:

Channels on the Transmitter and receiver decide how many functions or controls you get. For this airplane, we need 3 channels. That is Throttle, Rudder and Elevator. Where throttle controls the speed of the airplane, Rudder controls the direction and Elevator controls the altitude.

I’m using a FlySky FS-T6 2.4ghz transmitter with FlySky R6-B receiver.

The electronics that you buy does not come with connectors. For example, you need to connect the ESC to the motor but you are not supposed to solder them directly.

So, we will solder connectors to ESC, motor and battery. So that, if any of the parts is damaged you can easily replace it.

Start by soldering bullet connectors to the ESC. So, we’ll start by soldering female bullet connectors on the ESC.

How to solder female bullet connectors on ESC?

1. Hold the bullet connector using helping hands.
2. Apply heat to the bullet connector by touching soldering iron to surface of the connector and fill 1/2 of the shallow end with solder.
3. Keep applying the heat and dip the stripped end of the wire into it.
4. Remove the soldering iron. Do not apply heat for a long time or this could damage your ESC.

Why solder female bullet connectors on the ESC?

Suppose you solder male bullet connectors on your ESC and you connect your ESC to the battery without motors connected to its other end. The male connectors of the ESC are likely to come in contact with each other resulting in short circuiting. And you might end up burning your ESC. So generally, we connect female connectors on electronics that are giving power. While electronics that are receiving power takes male connectors.

Once done with the soldering, apply heat shrink to the bullet connectors. Now, we do not want ESC’s wire to short circuiting by touching each other. So cover it perfectly using heat shrink. Such that end of heat shrink lines with the end of bullet connectors. Use a lighter to shrink the heat shrink.

Similarly, solder male bullet connectors to wires on the motor.

In order to connect the battery to the ESC, we need to solder male XT60 to the ESC. XT60 is connected to the two wires coming from the ESC. And battery takes the female XT60.

Before soldering XT60, make sure to put in your heat shrink to the wire of ESC. Because once soldered you won’t be able to do that.

Soldering XT60 is similar to soldering bullet connectors. But, XT60 has polarity for soldering. So, make sure you get it right in the first attempt. Look for a “+” sign at the end of XT60 this is where you connect your red or positive wire from ESC. And the black wire at the “-” end of the XT60.

If you connect this wrong you can end up burning your electronics. And, trust me you never want that.

Once soldered, shrink the heat shrink tube to cover any exposed part of the wire that can lead to short-circuiting.

Repeat this process to solder female XT60 on battery. Luckily, my battery already had a female XT60. So, Yay!

Finished soldering? Well, now it’s the time to connect everything together and test run for the first time. Connect three wires coming off the ESC to your motor. Since Brushless motors run on AC current. So, it doesn’t matters which wire of the motor connects to which one on ESC. Connect the servo lead coming from your ESC to throttle channel of the receiver. The servo leads have polarity and so does the receiver. So if you are using a FlySky receiver like me, the ground is towards the extreme edge of the receiver in which the black or brown wire of the servo lead goes in. In order to know about throttle channel and polarity of your receiver refer to user’s manual of the Transmitter and Receiver.

Connect the servos to the receiver. One of the servos goes to rudder channel and the other goes to elevator channel. Again, it doesn’t matters which one of the servos goes to rudder or elevator. While connecting, make sure the polarity of servo lead is right.

Here’s the setup that I am using:

• Servos are connected to channel 1 and channel 2.
• ESC’s servo lead goes to channel 3 of the receiver.

Now before connecting battery, always remember this as a rule:

Unless you are ready to fly, never put on your propellers to the motor. I repeat NEVER!!!

Putting on propellers to the motor and running it can be extremely dangerous. You can hurt yourself or anyone around badly.

Now all the electronics are connected we can move forward to Binding Radio Transmitter and Receiver together.

To bind your transmitter to the receiver, insert the bind plug in your receiver’s bind or battery port. Again, look for bind port of receiver in receiver’s user manual. Now, making sure your radio transmitter is turned off. Plug in the battery to ESC. This will power all the electronics. Look for a flashing light on the receiver, saying it’s ready to be bound.

Now, keep holding the bind key of your transmitter and power it on. Light on the receiver is going to stop flashing, saying it’s bound. Release the bind key. Then remove the bind plug from your receiver. Finally, unplug the li-po battery and plug it again. Your transmitter and receiver are bound. For safety, always power on your transmitter before powering your receiver. (Unless you are binding your transmitter and receiver). Finally, connect all the electronics and test run just to make sure that everything is working.

Print the pdf plans for the build.

Make sure to scale the document to 100% (or as close as possible) while printing.

Once done cut the edges of the paper such that the plans align to one another. Use some scotch tape to stick the pdf plans together.

About the build plans:

So, I’ve designed this RC airplane using SketchUp. It's most basic design with a wing, a vertical and a horizontal stabilizer. With a wing span of 20 inches and wing breadth of about 9 inches, the wing area of this airplane is about 155 sq. inches. That makes it a nice slow flyer.

This design uses most of the traditional ideas for building RC airplanes. Making it super simple to build and fly.

The wing has a small but effective dihedral angle. That provides additional stability to the RC airplane. Which is good for beginners.

Dihedral?

Dihedral wings are bent upwards at a particular angle that provides additional stability to the aircraft. Lift of the airplane decreases due to dihedral wings. But, it provides additional stability, resulting in a smooth flight.

Lay out the plans for built on the foam board. Use some scotch tape on the edges to hold them in position. Also, use a straight edge or ruler along with X Acto knife to cut with precision.

Which foam board to use?

I have used 2mm depron foam board. Most people use 4mm Adam’s Readi-Board. But, that’s not available worldwide. But, you can use 4mm Adam’s Readi-Board if it’s available to you. Anyways, 2mm or 4mm depron foam is available in almost every country. So, I’ve used depron foam board.

Score cutting and cutting all the way through it:

The bold line in the build plans shows cutting all the way through it. While dashed lines shows score cutting.

For, score cutting you cut the foam board halfway only. So, that it can be folded easily to form a nice and strong hinge. This hinge is used as a control surface in order to control direction and altitude of the airplane. If you have destroyed the hinge by cutting all the way through foam board, do not worry just use a packing tape above the hinge to make it right.

For control horns and motor mounts:

Use 1.5 mm thick hobby plywood. Place the plans for the motor mount and control horns on the plywood (Or glue the plans on the plywood). Then, use a drill to make holes according to the plans. Finally, cut the motor mount and control horns.

Once you have cut all the foam parts, start by making the fuselage. Take the fuselage part that you cut earlier. Score cut as instructed on the pdf plans. Finally, break the foam board at the scored area to make a hinge. To break the foam, bend the foam board until it breaks. Make sure it breaks at the joint. Remove any excess foam at the joint.

Once done fill hot glue at the joint. Then bend foam surfaces at the joint to form a nice 90° angle. Repeat this process until you get a nice cuboidal fuselage.

Take the wing part of the foam board. Score cut as instructed in the plans. And make the dihedral shape.

How to form the dihedral shape?

• Bend the wing at the score cut. Such that the edge of the wing is elevated by 1.5 inches.
• Fill the scored area with hot glue. Such that the wing is able to maintain its shape.
• Keep holding the wing in the same orientation until the glue cools.

Now take the Rudder and Elevator and score cut to form a nice and strong hinge for the control surface. Finally, bevel the edge of control surfaces so that it can move freely in both the directions.

How to bevel control surfaces?

• Fold the control surface at 180 degrees from the hinge.
• Place it on the edge of a table.
• Now, with the help of a straight edge or ruler bevel at 45 degrees using your X-Acto or box cutter.

Use some packing tape on the control surfaces to reinforce the joint.

Using the reference marks on the plans, hot glue the wing to the fuselage. You need to glue the wing about 2 cm behind the leading edge of the fuselage.

Finally hot glue the Elevator and the Rudder. Then hot glue the control horns to the Elevator and Rudder. Make sure you haven't glued your control surfaces and they are free to move.

Now, while gluing Elevator and Rudder make sure that:

1. Elevator is centered and perfectly horizontal
2. Rudder is centered and makes a 90 degree with the elevator
3. Control surfaces are free to move

Once done, move on to installing electronics.

Place motor on your plywood motor mount and tighten it using screws. For safety use the screws that come with your motor. So that, screws do not touch the wires inside the motor. Hot glue your motor mount to the front of the fuselage making sure that it’s centered. Make sure your motor is spinning and you haven’t glued outer can of the motor.

Put on the servos in the servo port on the fuselage. Insert servo arms to the servo making sure that the servo is centered. Again, connect everything to the receiver as earlier.

For push rods, I am using thin music wire. These wires are flexible and are easy to bend. Before linking control horns with servos, make sure control surfaces are perfectly horizontal with the wing. Any deflection can produce unnecessary upward or downward force making it hard to fly. Take help of a ruler to make sure it’s straight.

Put a Z bend at one of the ends of the wire, Insert it in the servo arm and measure the length of wire such that it reaches the control horn put a Z bend at the other end and link it to the control horn.

How to make Z bends?

• Take your pliers and bend wire making a 90 degrees angle. Now, bend it again in the opposite direction to make another 90 degrees.

Repeat this process with both the servos.

Once done, all you need is to program your radio and balance the Centre of Gravity of your airplane to fly.

Once you are all done, check that your sticks on the transmitter are functioning correctly. I’m using right stick of transmitter for controlling directions. And left stick as throttle only.

Make sure your airplane is responding correctly to your commands. Here’s how to do it:

Keep the airplane facing away from you and plug in the battery. Move the sticks and watch your airplane’s response.

For right stick,

• Pull the stick down: Elevator should go up.
• Push the stick up: Elevator should come down.
• Move the stick to right: Rudder should move right.
• Move the stick to left: Rudder should move left.

For left stick,

• Push the stick up: Motor starts rotating
• Pull the stick down: Motor slows down and finally stops.

If any one of these doesn’t respond correctly go to functions setup on your transmitter and opt for reversing. Finally, select the channel that is not functioning correctly and reverse it. Now, if your elevator is responding instead of the rudder and rudder instead of the elevator. Simply switch the servo lead going to the receiver.

If you power on your motor and it’s running the opposite way switch any two of its wire going to the ESC, this will reverse the direction of rotation of the motor.

Throws:

Throws decide how much your control surfaces are going to move. If you have your throws set to 100%; your airplane is going to react a lot even for very small movement on the sticks of your transmitter. So, make sure to set your throws to about 30-35% while programming the radio. A lot of movement of the control surfaces is just not good, especially for beginners.

Balancing the Airplane:

Now all you need is to place the battery on the airplane using a velcro or double sided tape. Generally, the battery is the heaviest part on an RC airplane. So, we use the battery for balancing the airplane. Now for balancing we make sure that the entire weight of the airplane is concentrated near the Center of gravity of the airplane. This is a very crucial step and cannot be ignored.

I've added a reference line for Center of gravity (or CG) in the plans. Anyways, the Center of gravity for this airplane is about 11 cm behind the motor mount. So, move your li-po battery back and forward unless the weight of the airplane is roughly concentrated near CG. Here's a nice article on how to balance your airplane by FliteTest. Once you have balanced your airplane it's time to fly.

Here's a quick first flight check list:

• Make sure that your airplane's control surfaces are responding correctly.
• Everything is plugged in right.
• You have your airplane balanced.
• Your battery is charged.

Here's a nice article by FlightTest for first successful flight.

Li-po battery charging and safety:

Li-po batteries can prove dangerous if not used and handled with care. Always charge a lipo battery using a balance charger. Balance charger is important for a long life of lipo batteries. Balance charger makes sure that your lipo is not over charged and each cell of the lipo is at the same voltage.

Never over discharge your lipo batteries. Over discharging lipo batteries can damage your batteries. Use lipo battery voltage alarm that will beep when your battery is discharged. Here's a nice lipo safety guide by The Drone Girl.

For 3s lipo battery:

• Fully charged voltage: 12.6 V
• Fully discharged voltage: 9.0 V

Troubleshooting:

If you follow each step with precision you should not encounter any problems. Anyways If you still face problems and you are unable to get your airplane into air write in the comments below or refer to a good RC forum around the internet. I think RCGroups is a good place to get your answers.

Thanks for viewing. Further questions and suggestions are welcome.