COMPLETELY PAPER MADE RC AIRPLANE

While choosing building materials for RC plane, the major concern is the strength (or strength to weight ratio). So typically people use balsa wood, foam that has good strength per weight. To make the structure, even more, stiffer carbon fiber is used for reinforcement. It has enough strength to take big banding loads.

Among various materials, the paper is never considered to make RC planes. It can be used to make simple planes having the size below around 15-20 cm. As size increases the stiffness of plane tends to decrease and any aerodynamic force would tend to bend it that spoil it's aerodynamic. And powering it with electronic having weight of around 80-100 g is even more difficult. 

In this article, I tried to make an RC plane with typical electronic having around 140g flying weight and 80cm of wingspan.

• In step 1, I explained various design or approaches that I initially considered and tried to make a small prototype to understand structural strength.
• In step 2, I tried to briefly cover partially successful design made out of paper.
• Step 3 onwards, I covered design and procedures for making the final paper plane. So you can directly go to step 3 if you are only interested to make one plane instead of going into previous plane details.

I also tried to make a video on this plane it can be found at the link below but it is recommended to go through this text as I haven't covered complete info in the video.

1. Various Prototypes for Paper Planes:

The various concept for the paper plane I tried for good strength.

1. The first approach was to make it just making the skin plane with minor reinforcement using paper only. This approach is not very good to have strength. But the major problem with this approach is that not only aerodynamic force but also humidity ( or drying glue) would create lots of bending and twisting that make flying this plane impossible. Even in the image this twisting and bending can be seen. Here wing is made of doubled paper to improve stiffness, but it wasn't helping in this case.
2. As the last method was not very much helpful to resolve an issue, I tried to apply some reinforcement. And also made wing like typical airfoil (which was a curved plan in the previous case). In this case, the structure was enough stiff. But scaling it up seems a problem. I was considering wingspan of at least 60 cm. For this scale, stiffness was still not sufficient. As for stiffness problem increase with the increase in dimensions.
3. So next approach I preferred was to make paper sticks skeleton and covering it with paper. In this case, plane covering and skeleton, both would provide strength to the structure.

While making all this scale down the plane, my focus was on the method/approach that can sustain own shape instead of crash resistance.

Learning from this exercise:

1. Humidity or drying of glue creates lots of bending, which is not desirable.
2. Having strong skeleton/structure is must (especially for bigger size plane).
3. To make the skeleton, the triangle is (obviously!!!) best option. If the size of the triangle very small it cannot take enough bending load while keeping it higher would reduce its stiffness due to bending in-plane bending of paper surface.

2.Previous Full Size (50cm) RC Airplane With Light Weight Electronics:

These airplanes are covered in one of my old article, so I will only cover it briefly in this step.

I made two powered paper plane. out of first was failure and second was a flyable plane. These planes were powered by single cell 500 mah battery with Wltoys receiver. These WLtoys receivers are ideal for these type of project as it is just 6-8 grams of weight but contains 2 servo motor, brush ESC and obviously receiver. It has the total weight of 60 to 70 grams.

I basically tried two planes:

• First was a conventional type plane having rudder elevator control. For stability, the high wing designed was chosen with the small value of dihedral. the major problem with this plane was that is had very low movement of control surfaces (due to smaller servo motor) which was not sufficient at all. So controlling plane is not easy. It survived multiple crashes but didn't fly well. But after this plane, I was confident about paper as RC plane material.
• For next plane a made a Flying wing type design. As these designs are not very stable in nature small actuation of the control surface enough. The wingspan of Flying wing was slightly higher than the Previous plane, but the weight was relatively lesser. This plane was flyable but still, it has some problem with control surfaces and power was slightly less. One reason for bad control was also bending of the control rod. as control rod has to travel from receiver to aileron that also has a 90 degree bend. so lots of loss of force occurs within the path.

This module has low torque servo that is good enough for light duty application (here small RC plane). So it would have problems with controls. To overcome these limitations I took next project to use typical RC electronics like 2 servo motors, receiver, brushless ESC, and a BLDC motor.
I have explained this project in following article and youtube video:

Instruction video:

Making video:

Learnings:

1. Servo motor from WLtoys receiver was not enough for the heavier plane,
2. Paper plane can withstand not only aerodynamic loads but also impact of crashes.

3.Making an RC Plane With Typical Electronic Setup: Introduction

Now, I started to make a plane using typical electronics.previously all electronics including battery, motor, servo, and receiver is just 30-40 grams. But using typical electronics weight of electronics would surely increase by at least 3-4 times. 

So, these would be the major challenge for this project:

• The weight of electronics would become around 80-100 gram. That would require a bigger plane,
• As the size of plane increase problem of stiffness also increases,
• The speed of plane would also increase as an increase in weight that would require better control surface (compared to the previous plane).

From the previous experience It was clear that wing type of design has less issue with control and weight is also very less for similar wingspan. so we will use this previous learning while designing the RC planes.

Material required to make Plane:

• 2x of 9 gram (or less) servo motor,
• Micro BLDC motor,
• Propeller (as per BLDC motor),
• Brushless ESC,
• 500mah (or less) 2s LiPo battery,
• General Transmitter and receiver,
• Lots of general A4 size paper,
• Paper glue,
• And Most Importantly "Lots of Patience",

4. Making an RC Plane: Design

As I mentioned in the previous step, I have used Flying wing design due to ease of control. It has the wingspan of 80cm and weight of 140 grams. All dimensions of the overall layout are mentioned in the diagram.

There is a strong skeleton made out of paper sticks of various types. It gives it overall shape to the plane. then it is covered by the paper which not only shapes airfoil but also provides extra strength. However, Mounting of the motor is made of a small piece of balsa wood.

Just like any typical flying wing, it has ailvator control. that required two actuations. It also needs winglet to provide yaw stability and powered by a 40 watt brushless DC motor in the pusher configuration.

The overall layout can be designed by referring any similar size flying wing or you can simply take the design i used by taking the scale of the image.

5. Making Building Block of Plane:

This plane also uses triangle cross section sticks to make the structure. I also used the similar type of sticks to make my previous planes. This stick has the length as A4 size paper and sides are of 5mm. The side of these triangle sections is 5mm in size. The higher number of small-sized sticks are good for strength. But making high numbers of sticks takes more effort. Where the bigger size of the sicks cannot take load very well. So 5mm of cross-section sides seems optimum to me. If you are going to start making one plane that it is better to try various size cross-section.

This process I also covered in the previously mentioned video:

• Do marking 5mm apart on paper,
• Do crease cut on first 3 lines to fold it easily and cut it over the fourth line,
• Fold it into the triangle shape and stick it by one side.

You are going to need lots of these type of stick to properly make planes. you can refer the video That i mentioned previously.

6.Using Triangle Sectioned Stick for Various Purpose:

These sticks would be required to fulfill various purposes like as bending member, leading edge or overall skeleton making that also provides strength.

Here are the ways to make use of these sticks for the various purpose:

• 2*2 triangle (4-elements): This structure can provide good strength and used as the chord of the wing.
• Hexagon (6 elements): This section provides the good amount of strength by using the lesser amount of the sticks but also provide good strength.
• Trapazoidal (5 elements): This is nothing but just a base of 2*2 triangle, It cannot take the good amount of bending load but can be used to make the surface.
• Trapazoidal (3 elements): Similar to 5 elements but having lower strength and low area requirement.
• Curved faced (3 elements): This element is made by folding and creasing of 3 elements trapezoidal cross section and it is used for leading edge of the airplane.
• 1 -element: Simply using as a single stick here and there to create the surface of paper sticking and strength.
• 1-element Sharp: This is the creased element that can be used to make the trailing edge or other places.

Joining various cross sections:
As we are using A4 size paper maximum length for a stick that can be obtained is around 29cm. but as the plane span is 80 cm it is required to have longer sticks. So sticks need to be joined such a way that it acts as a single peace.

7. Making Structure of Plane:

Once you decide the layout there nothing much hard and fast rules to follow. You need to select among various cross section as per space and strength requirement.

For example,
for leading edge, there is no other option than using curve face section, for wing chord which takes maximum load need to have some higher strength section (It is the part of an airfoil having maximum thickness-this is also a factor while choosing cross-section).
For area near wing root which undergoes the maximum amount of load, it needs to have the denser skeleton (made of lots of sticks) as it takes lots of bending load.

Another care that is required is while making control surface. As it is made of paper only it would bend a lot while flying. So, throw of the control surfaces needs to decide accordingly. here throw needs to be set slightly higher than your previous experience.

Here, to make motor mount I have used balsa wood, that I glued to plane structure.

8.Crash Damage Resistance Techniques:

Here I have basically added two crash resistance techniques, that I tested in (in the destructed way) later stage.

So these are the method that can be used to reduce damage due to crash:

• Loose battery:
  It is very obvious and intuitive that heavier plane would have higher damage in the crash situation as it has the more kinetic energy to dissipate. In this plane, the battery has the good amount of mass (30 grams). so there is scope to enhance the design.
  
  So, I preferred to attach battery loosely, so in the case of the crash it just detaches from the plane and does not contribute to impact force. But the second thought that comes to mind that battery can also pull wire connected to it that can damage own wire, connectors or ESC wire. In the hard crash, the battery can even get lost (very bad). So I connected it to a rubber band that is connected to plane so basically plane would undergo two impacts instead of one big impact. here the first plane takes impact due to own weight (minus battery weight) that it would take impact due to the battery weight (if any).
  
  However, using this method can be bad for battery in case of nose dive type crash (which is anyways bad if you have assembled battery in the nose of plane).
• Leading edge wire:
  when plane collide with any stationary object while flying at low altitude it mostly damages the wing. Foam wing is good to take even this type of crash but not the paper plane. To protect it from any tear, I stick a wire in leading edge that can restrict the damage to just dent (instead of wing tear).

9. Fun Part: Flying the Bird

Any tutorial is not complete without flying video.
Set transmitter to the elevator-aileron mix. Set correct throw and direction. Set the CG properly.

Refer previously mentioned video the video from time: 4min 6sec

While testing It crashed multiple times. It seems to have lower strength at root area than required. But still, I managed to fly it. After few crashes, even motor get detached from the plane. But still, there wasn't any significant damage except some bending in the wing root.

Note:
Flying the wing is not very easy if you do not have RC plane flying experience. So it's better to take some help of experienced aeromodeller. Or you can even choose some other stable design.

10.Fun Part Continue...: Crash Test / Destructive Test

Again for video refer attached video at the time: 4min 44sec.

To check the strength of the plane I tried to crash it and see how damage propagates over the skeleton. I tried to crash the plane to the metal rod. I used one leg of the table as the hurdle and attached GoPro over it to shoot the damage. I throw plane by hand with sufficient speed such that I can tear the paper. But by analyzing video I found that it undergoes the high amount of bending and crushing but not tear. So, I throw plane thrice once to damage left side and twice on another side. The condition of the wing while crash you can be seen in the attached photo.

for further analysis, I stripped off the paper skin from the plane to see how skeleton has damaged. I have marked damaged piece in red color to have the understanding of damage spread. So It is easy to locate the point of contact while crash by using these red marks. There is also lots of damage found in nose section as this plane undergoes multiple crashes while flying. There are also marks which are due to bending of the wing (damage due to actual crash) from the root which is difficult to identify out of other big damage marks (due to test crash).

you can also observe that how battery gets detached from the plane during the crash, that reduces damage up to some extent.

So,

• The major learning from this study is that this plane can take big crashes as the energy of crash absorbed nicely by the structure.
• This plane needs to have denser structure near wing root (center part).
• Loose fitting battery seems to works, but still, its effectiveness needs to be check.

 11: Summary...

Why make RC plane using paper:

• It would give you feel of strength requirement of RC plane. As you are working on already a weak material you would find various ways to improve the strength of plane or reduce the crash damage.
• Building this planes would demand some sort of creativity, not only in term of aesthetics but technical too.
• And yes, It's fun.

Why not make one:

• This plane is really good as experiment and learning purpose but not a replacement for regular material.
• This plane is not as strengthy as a regular plane. It also has the higher weight for the similar size of the foam plane.
• It takes a few times more time to built than foam planes.

Hope you have enjoyed the overall article. If you have any query/suggestion let me know.