Dit werkstuk heeft in juni 2010 de KNAW Onderwijsprijs voor het beste Nederlandse N&T-profielwerkstuk gewonnen. Het origineel vind je hier.
Abstract
Main goal of this project was to make calculations and respective designs to create an electric airplane prototype, capable of powering its flight either entirely or partially using solar energy. This project intended to stimulate research on renewable energy sources for aviation. In future solar powered airplanes could be used for different types of aerial monitoring and unmanned flights.
First, research was done to investigate properties and requirements of the plane. Then, through a number of sequential steps and with consideration of substantial formulas, the aircraft’s design was proposed. This included a study on materials, equipment and feasibility. Finding a balance between mass, power, force, strength and costs proved to be particularly difficult. Eventually predictions showed a 50% profit due to the installation of solar cells. The aircraft’s mass had to be 500 grams at the most, while costs were aimed to be as low as €325.
After creating a list of materials and stipulating a series of successive tests, construction itself started. Above all, meeting the aircraft’s target mass, as well as constructing a meticulously balanced aircraft appeared to be most difficult. Weight needed to be saved on nearly any element. We replaced the battery, rearranged solar cells and adjusted controls. Though setbacks occurred frequently, we eventually met our goals with the aircraft completed whilst weighing in at 487 grams.
Testing commenced with verifying the wings’ actual lift capacities. Results were satisfying. In addition, further testing on drag and propulsion was gratifying as well. Finally, the aircraft truly took to the skies, but sadly crashed due to flaws in steering. Testing on solar cells however, was disappointing. Instead of the expected 50 %, we only managed to achieve 10 % profit. In addition, one can pose the question if leaving out the solar cells entirely would have meant saving such considerable weight, that there would have been more profit after all. Final costs of the project were €515.
Ultimately we can conclude purely solar powered flight is impossible, at least with the materials available and taking Dutch climate into account. Further developments in solar technology might create possibilities for solar planes in future. For now, in order to install solar cells, too many aspects of the plane are sacrificed to save weight. This for example resulted in a frame far too fragile.
Still, it should be noted that the plane we manufactured was a prototype only. Many adjustments can be made. During the project we have seen that there is quite some room for improvement. That is obvious, as this was the very first airplane we ever build. We gathered enormous amounts of knowledge and we hope that in future this knowledge will be used to continue working on solar powered planes. For if development continues, solar powered aircraft might truly be used in future. Bear in mind: Future starts now.
Introduction
As we both enjoy aviation and are considering a study involving the aircraft industry, it had soon become clear we wanted to do our research project about a topic related to flying. Though aviation is a complex part of technology, we thought our expertise on physics and our passion for flying would guide us through this project. Our first concern was finding a topic both interesting, challenging and future-proof. Due to current turbulence in aviation we decided on the following:
The desire to fly is nearly as old as humanity itself. Ever since we walked the earth, we longed to get airborne, just like the birds above us did. In 1783 this dream became reality . “Historians credit France's Montgolfier brothers with the first pioneering balloon flight” . Aviation’s next revolution was in 1903, when Orville and Wilbur Wright took off with their ‘Flyer 1’ and flew 36 metres with their plane . Flyer 1 was powered by a petrol engine, just like later aircraft. Nowadays, aviation accounts for three percent of all CO2-emissions produced by mankind .
This doesn’t seem much, but more important is that profit of commercial aviation strongly relies on the oil price. Due to high prices of crude oil lately, profits of commercial aviation have been diminished and aviation industry is now looking for alternative energy sources to propel modern-day aircraft. Options that are being considered are bio fuels, hydrogen and ethanol . An option which is rarely considered is solar energy, an option we wanted to investigate.
Phases
In order to minimize chances of failure, we set up the following scheme. It divides up the project in five different phases, all having their own planning, goals and requirements:
Phase Elements Goal
Introduction phase Determine main goal, research questions, requirements, planning. Provide a starting point for our project.
Design phase Research, orientation, plans, calculations, proposals. Set up a design proposal, construction proposal and test proposal.
Construction phase Description of construction itself: what succeeded, what went wrong and in which way did we alter our designs and plans. Give an accurate description of the progress of our project.
Test phase Observations and experiences during testing and test results. Provide an accurate description of occurrences during testing.
Conclusion phase Interpretation of test results, conclusion, evaluation, recommendations for future projects. Describe what can be learned from this project, why it succeeded or not and how this project can be used later.
Main goal, purpose and expectations
The main goal of this project is to make calculations and respective designs to create an electric airplane prototype, capable of powering its flight either entirely or partially using solar energy. This project intends to stimulate research on renewable energy sources for aviation. Hopefully this will result in the environment no longer suffering due to emissions of burning oil products. In future solar powered airplanes could be used for different types of aerial monitoring and unmanned flights. Due to their light weight, silent engines and infinite flying time, they might also be used as spy planes in inhospitable areas.
By doing broad research and making appropriate calculations, we hope to design an aircraft which is actually able to fly. Our first concern is composing the right electrical system to be used in our aircraft. Selecting suitable equipment and making useful drawings and schemes will provide us with the basis for our project. Extensive testing will show how effective our system functions in different circumstances. Once completed, our circuit will be combined with the aircraft’s frame.
If we manage to correctly set up our calculations, the aircraft should be able to fly. In case testing shows the aircraft is not able to take off, we will first investigate how we can improve the plane’s properties. Though, when nothing seems to help anymore, we will slightly shift our main goal and try to create a controllable, energy efficient vehicle using all equipment we gathered so far. This will only be done in case all goes wrong.
This project runs over an extensive period of time, which will provide us with sufficient opportunities to do research and create designs. We will make a strict planning and a precise proposal to make sure our project will run smoothly. Assuming we will be able to make accurate calculations, our expectations are that we will actually produce an aircraft capable of flying on solar power. Even though we try to plan and predict everything as well as possible, setbacks might occur. To prevent failure, we shall make sure there is enough room for unexpected events.
Research questions
From our main goal we can derive the main research question involved in our project:
Is it possible to create an electric airplane prototype capable of using solar power to partly or entirely power its flight?
Sub questions
In order to answer our main question completely, we need to look at several sub questions:
1. Is it possible to let the aircraft fly on solar power only?
2. How do you make sure no precious energy is spilled when the solar panels generate very little energy?
3. How do you deal with the cells’ varying output in cloudy or sunny circumstances?
4. Which extra equipment is needed to make the solar cells function properly?
5. Which circuit is needed to make the plane remote controllable?
6. Which equipment and components are needed to control the plane?
7. What will be the plane’s size, mass, speed?
8. Which airfoil do you use, how big is it and how does it work?
9. How much lift do you need?
10. Which engine and propeller do you need?
11. Which solar cells generate most electricity with fewest mass and how big are they?
12. How do you mount the solar cells on the aircraft (inside or outside, wing or body)?
13. What will be the structure’s design?
14. Which materials do you need to construct the plane?
15. Where do you place all electrical equipment, making sure the aircraft is balanced and what will the fuselage be like?
16. What will be the shape of the tail and elevator and how are they controlled?
17. In which order are you going to construct the plane?
18. Which effect does the position of the solar cells have on the efficiency of both the plane and panels?
19. How much profit do the solar cells provide?
Requirements
Before writing a design proposal, a complete set of requirements is needed. In this way we know which limitations there are during construction. Not only will these requirements be a guideline throughout our project, they will provide us with a starting point as well. After all, mass, power, size and costs of an aircraft are all related. You need to fill in one first in order to determine the others.
Element Requirements Notes
Available time At least 100 hours of which 50 are before the summer holidays, 20 after the summer holidays. Construction will be before and during summer holidays.
Available materials Nearly everything as long as we can afford it and it’s available in shops (on the internet) and not custom made. Plenty of tools are available.
Number of participants Two Experts, teachers and other sources not included.
Final product’s size Between 1.0 and 1.5 metres in width in order to not let it be too small or too big. These measurements can vary slightly if needed. Length and height depend on mass and balance, which will be determined later.
Number of products One (prototype)
Budget No more than €400 in total as long as no sponsor has been found.
Environment Outside No rainy circumstances
Media attention www.solarchallenger.tk, website with our progress, sponsors, TU Delft?
Design proposal
When designing, there are three things to be considered. First there is the main goal, secondly there are the research questions and thirdly there are the requirements. When combined, those three elements form the basis of making a design proposal. To start out, we use the requirements and see how we can implement them in our main goal and research questions.
The requirement we are currently most interested in, is the model aircraft’s size. We intend to construct an aircraft with a span measuring 1.5 metres. In this way the aircraft will not be too large to handle and at the same time it will be large enough to provide us with enough room for mounting solar cells and other equipment without space becoming too crowded. We will use this very measurement as a basis throughout the design proposal.
To design the plane in an organized way, we set up a plan consisting of a number of steps. These steps deal with several sub questions and will be filled in as we go:
Step 1: Designing the right basic electrical system (no sizes, properties, etc. included).
Step 2: Research on the size of the aircraft and probable mass.
Step 3: Feedback on the engine, solar cells, battery etc.
Step 4: Final decision on mass and thus the size of the aircraft.
Step 5: Designing the final electrical system including properties and components.
Step 6: Determining where to place the solar cells.
Step 7: Designing the wing and doing a final check on lift, mass, size, etc.
Step 8: Determining construction materials.
Step 9: Designing the aircraft itself, keeping mass, centre of gravity, etc. in mind.
Step 10: Calculating final costs
Step 1
Before continuing with calculations involving the size of the aircraft, we will look at the electrical system involved. After all, our plane would be useless without properly functioning electronics. This implies we will start out with answering sub questions 1, 2, 3, 4, 5 and 6:
1. Is it possible to let the aircraft fly on solar power only?
2. How do you make sure no precious energy is spilled when the solar panels generate very little energy?
3. How do you deal with the cells’ varying output in cloudy or sunny circumstances?
4. Which extra equipment is needed to make the solar cells function properly?
5. Which circuit is needed to make the plane remote controllable?
6. Which equipment and components are needed to control the plane?
To start, we will answer question 6. A properly controllable plane needs several pieces of equipment .
First there are a transmitter and receiver, which are used to send and receive signals from a control panel. The receiver, located inside the plane, will send signals to elements called servos. They can move the rudder, ailerons and elevator. Besides, the receiver also controls the engine throttle. We will come back to question 6 in more details later, when we know more about the mass, size and power of the aircraft.
The answer to question 1 was very obvious once we investigated the power consumed by an engine. A website showing engine characteristics listed a considerable number of engines. All of those appeared to consume quite a lot of energy. Besides the fact that our engine needs an enormous amount of power (which will unlikely be generated by our solar cells due to Dutch climate ) the engine also needs a steady energy flow. We don’t want our aircraft to crash due to one cloud and for that reason we will use a battery combined with solar cells.
Then the answer to question 2: When the solar panels are not generating lots of energy, we must prevent energy from flowing from battery to solar panels. Therefore we will install a diode between the solar panels and the battery. The diode will unfortunately consume some power, but this is the only way to get things working. In order to keep voltage loss as low as possible, we are going to use a ‘Schottky diode’. This specific type of diode only uses 0.1-0.2 volts compared to a regular diode which contributes to the loss of 0.7-1.7 volts .
Since we found a solution to question 1, the answer to question 3 has already been given. Using a battery will not only help us keeping the plane airborne when limited energy is being delivered by the solar panels, but it will also make sure that whenever the output of the solar cells drops (for example due to a cloud passing by) the engine still receives enough power.
Knowing all the information we gathered so far, we can draw a circuit basically showing the electronics inside the aircraft, which answers question 5 up to a certain extent. Since we don’t know anything about power, mass, battery size etc. yet, we are not able to draw all details. The following scheme just provides us with a basic idea of which elements are needed to control the aircraft, thus giving us a starting point for calculating the aircraft’s mass in future.
Step 2
Having completed the basics of our circuit, we can resume calculations about the aircraft’s size. When combined with the circuit, we can derive the engine’s, battery’s and solar cells’ requirements. As proposed in ‘Requirements’ (page 8), we intended to construct an aircraft with a span measuring 1.5 metres. The influence of the wingspan on the plane’s other measurements will be investigated next. Research questions involved are:
7. What will be the plane’s size, mass, speed?
8. Which airfoil do you use, how big is it and how does it work?
9. How much lift do you need?
These three research questions are related to each other as much as can be, namely lift comes with speed and determines mass and size, but it relies on the airfoil used. We will look at the aircraft’s size first.
Size
We determined the aircraft’s span should be about 1.5 metres. In order to make calculations and construction of the wings easier, we change the span into 1.6 metres. The ratio between span and chord (illustrated right) in average model aircraft is between 1:7 and 1:8 . In our airplane this would imply:
Width is 1.6 : 8 = 0.2 metres
Knowing both the wing’s width and span, we can calculate the wing’s surface area. This size will later be used in several calculations.
Surface area= 1.6 metres × 0.2 metres = 0.32 square metres
Airfoil
Not only surface area is important for a well functioning wing, but having a correct airfoil is important too. It is the airfoil’s shape that provides the aircraft’s lift. All airfoils rely on same principle , Bernoulli’s principle. It describes the relationship between velocity and pressure in a moving liquid or gas:
P + ½ρv2 + gρh = k
P = Pressure (Pa)
ρ = Density (kg / m3)
v = Velocity (m / s)
g = Acceleration due to gravity (m / s2)
h = Distance from reference, measured in opposite direction of the gravitational force (m)
k = constant (kg / ms2)
The formula proofs that if gravity, air density and position remain unchanged, pressure will drop when flow velocity increases. In other words: if you have an airflow of high velocity and low velocity under the same circumstances, the area containing the high airspeed, will show a lower pressure. This implies that if the air flowing over the wing moves faster than under the wing, the air pressure above the wing will be lower, thus sucking the wing up into the sky.
For this reason wings are shaped in such a way that air moves faster along the upper surface. This is done through making the air above the wing move a longer distance in the same time than the air underneath the wing. Therefore the wing’s upper surface is more curved than its lower surface.
Throughout the years many airfoils have been developed . Every single one of them had excellent properties for different purposes, like low or high speeds, or stability. Concorde’s wing was quite different from a Boeing 747’s for example. Still, there is no such thing as ‘the perfect airfoil’. Properties needed simply depend on the characteristics of the aircraft. Our plane just needs a simple and ergonomic airfoil. It will be ‘Clark-Y’ :
Clark-Y basically is an airfoil with a flat bottom and a curved top. Not only have there been many experiments about this type of wing, it is also easy to construct. The following table shows certain numbers defined as lift coefficient. These statistics will later be used in calculations on lift.
Angle of attack
Lift coefficient
0 0.29
1 0.36
2 0.43
3 0.50
4 0.57
5 0.64
6 0.71
7 0.78
8 0.85
9 0.92
10 0.99
11 1.06
12 1.12
Speed
Before we determine the aircraft’s mass, we will take a look at its speed. Investigation on several model aircraft shows the approximate flying speed is between 25 and 100 kilometres per hour. Taking into account the limited capacity of our solar cells, we aim to let the aircraft fly at 30 kilometres per hour.
30 kilometres per hour= 30 × 1000 metres : 60 minutes : 60 seconds = 8.3 metres per second.
Mass & lift
At this very point in our investigation there are two options. Either we determine the probable mass of the aircraft and try to design the appropriate plane in order to gain enough lift or we determine the lift we will probably gather and try to match the corresponding weight. We choose the latter, since it is easier to make a plane lighter than to increase its lift. Making a plane lighter can be done through removing several pieces of equipment or replacing them by lighter ones (saving on the battery for example). On the other hand, trying to increase lift takes a great effort, because it requires redesigning and manufacturing the entire wings.
Lift can be calculated precisely through ‘aviation’s most important formula’ :
L = ½ c × × A × v2
L = Lift in Newtons
c = Lift coefficient (no unit)
= Air density (kg / m3)
A = Wing’s surface area (m2)
v = Velocity (m / s)
This formula should not be confused with the formula for calculating drag . This formula is based on the same parameters, although when calculating drag the ‘drag coefficient’ is used and the letter A represents the frontal area of the object.
From our previous investigation we can derive the wing’s surface area and the plane’s velocity. The lift coefficient is taken from the table previously shown by assuming that the angle of attack is larger than one degree at take off. At ground level the air density is approximately 1.23 kg / m3. When filling in all numbers the results are as follows:
L = 0.5 × 0.36 × 1.23 × 0.32 × 8.32 = 4.88 N
The lift generated by our wings will be able to compensate for 4.88N of gravitational force. Since 9.81 Newtons of gravitational force correspond to one kilogram of mass , we can calculate the mass of our aircraft:
4.88 N = 4.88N : 9.81N/kg = 0.497 kg
The second calculation shows us the answers to sub questions 7 and 9. When travelling at take off speed the wings of our aircraft generate 4.88 N of lift, thus being able to lift 497 grams. We will make a slight margin concerning mass. Therefore our target mass will be 450 grams.
Reynolds number
Besides determining if our wings generate enough lift, we must also know if our wings actually generate lift at all. Namely, gently and laminar airflow over a wing will only start at a certain velocity. In addition, when flying too fast, the airflow will also become too irregular and turbulent. The speed range at which the airflow will be smooth and laminar, can be estimated using a value called the Reynolds number.
Reynolds number is a value given to the flow conditions around objects. For any object the optimal Reynolds number differs, but as a rule of thumb we can assume the Reynolds number is supposed to be between 5.0 × 104 and 2.0 × 105 for aircraft wings . The Reynolds number can be calculated through the following formula :
Re = ρ × v × L × μ-1
Re = Reynolds number (no unit)
Ρ = Density (kg / m3)
v = Velocity of fluid or gas (m / s)
L = Travelled length of fluid or gas (m)
μ = Dynamic viscosity (Pa × s)
The air density is 1.23 kg / m3 as determined previously (page 14). The travelled length of the air is just over the wing’s chord: 0.205 m. The dynamic viscosity differs according to temperature and air pressure. At room temperature it is 1.85 × 10-5 Pa×s. When filling in all numbers, the results are as follows:
Minimum speed = Re × p-1 × L-1 × μ = 5.0 × 104 × 1.23-1 × 0.205-1 × 1.85 ×10-5 = 3.8 m/s
Maximum speed = Re × p-1 × L-1 × μ = 2.0 × 105 × 1.23-1 × 0.205-1 × 1.85 × 10-5 = 14.7 m/s
Fortunately our take-off speed is within those boundaries. The airflow over the wings should be smooth and laminar.
Step 3
Having determined mass, size and the electrical system, we no longer need to estimate things, but we can calculate values precisely. The first thing to do now, is to combine mass and size with the electrical system. In this way we can start to create a list of materials and equipment needed.
Parts included in the electrical system are the battery, servo engines, controller, engine and solar cells. These elements are linked in the following way: The size of the aircraft determines the engine’s size. The engine requires certain power supply, which determines battery size and controller size. Because of this order we will look at a suitable engine first. The sub question involved is:
10. Which engine and propeller do you need?
Engine
Engines vary in certain ways . There are engines for high speed and engines for much force at lower velocity. Namely, the power of an engine is described in rotations per minute per volt. The lower this number, the more powerful the engine. A slow turning engine will be able to pull heavier aircraft at lower speeds, whereas fast turning engines will pull lighter (stunt) aircraft at higher speeds. Compared to other model aircraft, we intend to fly rather slow. Therefore we will use an engine with a low volts to turns ratio.
Initially we found an engine which met some of our requirements: a speed 400 engine. Then we discovered there are several other types of engines . On one hand there are brushed engines (e.g. speed 400), on the other hand there are brushless engines. The latter is a modern invention and consumes considerably less power than the initial one. Brushless engines come in two types : inrunner and outrunner.
An inrunner consists of a several magnets covered by a metal casing. This creates a powerful engine, which unfortunately produces lots of heat, thus loosing efficiency and requiring a cooler. As we cannot afford extra mass, we decided to go for a less powerful, but more efficient ‘brushless outrunner’. Taking mass and all other requirements into account, the following engine seems most suitable to us:
Element Value
KV (RPM / V) 1100
Voltage 6-12V
Nominal current 3-6A
Maximal current 8A (max 60 s)
Efficiency (at 4-7A) 74%
No load current (at 10V) 0.6A
Internal resistance 225 mΩ
Axle diameter 3mm
Dimension (Øxl) 27.5 x 26 mm
Mass 36 g
Recommended model mass 200-500 g
Maximal propeller 8.5 x 5 inch
2208/17 Brushless Outrunner 1100KV
Price: €14.50
Propeller
As shown in the scheme above, a recommendation has been given on the maximal propeller size, with the first number being the propeller’s maximal diameter and the second being maximal lead. A propeller with its lead being similar to its diameter is best suited for fast flying airplanes, whereas a propeller with small lead and larger diameter is excellent at providing maximal force, thus being perfect for heavy, slow flying aircraft. As our plane flies rather slow, the following propeller seems most suited:
Master Airscrew Electric Propeller 8 X 5
Price: € 3.35
Element Value
Mass Unknown
Diameter 20.3 cm
Lead 13 cm
To be absolutely sure this propeller is perfectly suited to propel our aircraft, we can apply the following formula and rule of thumb , which shows the relation between speed, rotations and lead. The propeller’s speed must be two to three times as high as the airplane’s take-off speed.
vprop = l × f
v = Velocity (m / s)
l = lead (m)
f = Rotations per second (Hz)
Our engine is rated to provide 1100 rotations per minute per volt. As our system functions at 8.5 volts (see below), we expect to operate at 9350 rotations per minutes, which is 156 rotations per second.
Propeller speed = l × f = 0.13 metres/rotation × 156 rotations/second = 20.3 m/s
Ratio propeller speed : airplane speed = 20.3 / 8.3 = 2.45
The propeller we selected should be perfectly suited, since its speed is approximately two and a half times as high as the aircraft’s speed. Therefore its torque is sufficient to propel the aircraft.
Batteries
The engine requires a voltage ranging from six to twelve volts. This voltage should be provided by both our solar cells and our battery. Another key aspect of the battery is mass.
In our quest for suitable batteries we first had a look at lithium-ion polymer batteries due to their excellent power to mass ratio. However, LiPo batteries bring notable risks with them, such as spontaneous combustion when overloaded . LiPo batteries work best at a steady power supply, but since the output of our solar cells varies heavily, we decided not to take the chance of our airplane catching fire in mid-air, but doing extra research on alternative batteries instead.
The second option is nickel-metal hydrate batteries. These batteries have the disadvantage of being much heavier than LiPo batteries, but their reliability made us favour nickel-metal hydrate batteries over LiPo batteries. We will install seven ‘high output’ batteries, producing a total of 8.4 volts, weighing in at 175 grams and having a capacity of 2500 mAh . This capacity means that the cells will be able to provide a continuous current of 2.5 A during one hour.
Solar cells (1)
The solar cells should provide enough power during flight to extend flying range drastically or even power the aircraft entirely. Just as with all materials used on our plane, mass is a crucial element, so the solar cells can’t be too heavy. The research question involved is:
11. Which solar cells generate most electricity with fewest mass and how big are they?
There are very few companies selling single solar cells. Cells are often being integrated in glass, something that would contribute far too much weight. Eventually we found a German company, Lemo-Solar , selling single cells. Fortunately their shop contained a variety of cells, including one that meets our requirements:
Single solar cell
Price: €10.60
Element Value
Dimensions 100 x 100 mm
Soldering strip Tinned, 2 mm
Polarity front Minus
Polarity back Plus
Nominal voltage 0.5 V
Nominal current 3200 mA
Maximal output 1600 mW
Mass 6.4 g
Because of the chosen batteries and the 0.1-0.2 volts lost over the diode, solar cells we use on the airplane must have a total output of 8.5 volts. This would require seventeen solar cells. Besides the financial aspect, seventeen solar cells will also weigh too much and will probably not fit in the airplane (more information in step 6). Therefore we will need smaller solar panels besides the ones mentioned above. The cells will be soldered together with flexible wiring. This will be no problem, as the cells are already equipped with soldering strips.
Solar cells (2)
Element Value
Dimensions 29 x 78 mm
Soldering strip Tinned, 2 mm
Polarity front Minus
Polarity back Plus
Nominal voltage 0.5 V
Nominal current 630 mA
Maximal output 340 mW
Mass 1.6 g
Single solar cell
Price: €3.-
In total we plan to use twelve solar cells measuring 10 by 10 centimetres and six cells measuring 29 by 78 millimetres. When adding properties of all solar cells, the results are as follows:
Mass: 12×6.4g + 6×1.6g = 87 g
Output: 12×1600mW + 6×340mW = 21240 mW ≈ 21 W
Engine power: 5A× 8.4V = 42 W
This implies that under ideal circumstances the solar cells will provide 50% of the engine’s power supply. Of course this would be an utopia, since we didn’t take power loss by the receiver, servo’s and controllers into account. Tests will eventually show how close we can get to our 50% energy reduction goal. We will go into more details about testing in the ‘Test proposal’.
Servos
The next step after investigating the right materials for energy supply and propulsion, is finding the right equipment for controlling the aircraft during flight. This equipment includes the receiver, engine controller and servos. First, we have a look at the servos, devices operating the plane’s ailerons and rudder , which control vertical and horizontal movement. These machines come in different sizes and shapes. Since our plane is lightweight and forces on the aircraft won’t be tremendous, we decided to go for the lightest servos :
Micro Servo 3.7 grams
Price: € 7.50
Element Value
Dimensions 19.7 x 17.44 x 8.37 mm
Mass 3.7 g
Speed (4.8V) 0.1 sec / deg
Torque (4.8 V) 0.5 kg / cm
Operating voltage 4.8 – 6.0 V
Every flap, rudder or aileron needs a servo to operate. A regular airplane is controlled using ailerons for rolling movements, the elevator for up and down movements and the rudder for crosswind compensations. This is illustrated by the drawings on the previous page .
We are unlikely to fly at high wind speeds due to expected fragility of our aircraft. Therefore we will eliminate the servo for the rudder. It will not only save us weight by not using a servo, but also through the removal of wiring. The total mass of our servo engines will be:
Mass: 3 × 3.7g = 11.1 grams
Engine regulator
Next up in the controlling equipment is the engine regulator. It controls throttle given by the engine. Again, this part should weigh as little as possible and should be able to cope with current and voltage in our circuit. We found the following suitable engine regulator :
Element Value
Operating voltage 7.2-12 V
Continuous load 12 A
Peak load 15 A (Max 5 s)
Dimensions 22 x 18 x 5 mm
Mass 7.1 g
Low voltage turn off Automatic
12A FLY Brushless ESC
Price: € 16.95
Receiver
Of course the airplane doesn’t control itself. We want a person on the ground to be able to remotely control the aircraft. For this reason, we need a remote controller and a receiver. The controller will send signals to the receiver, which will instruct the servo engines and the engine regulator. We found the following suitable parts :
E-Sky receiver, 6 channels, 35Mhz
Price: € 13.50
Element Value
Frequency 35 Mhz
Mass 14 g
Dimensions 45 x 23 x 13 mm
Voltage 5 V
Current 11.5 mA
Antenna Length 100 cm
Remote control
Both the sender and receiver should function at the same frequency. Therefore our options for choosing a suitable remote control were slightly reduced. Additionally there are great differences between remote controls operating at the same frequency. Some are equipped for controlling helicopters only, whereas others are best suited for controlling petrol powered planes only. This difference originates from different controls needed for different planes. A helicopter for example does not have a retractable undercarriage, while a large aircraft does. Eventually we found the following :
E-Sky remote control, EK2-0404A
Price: € 29.95
Element Value
Channels 4
Frequencies 35, 40, 72 MHz
Energy source 8 x 1.5 AA battery
Servo reverse Manually
Voltage indicator Automatic, Led
Dimensions 185 x 205 x 55 mm
Colour Black
Antenna length 100 cm
Usage Airplane, helicopter, Glider
Step 4
We can now make a decision on size and mass of the plane. At the start of our research project, the wingspan we were striving for was 1.6 metres. Looking at the dimensions of the equipment we will be putting in the airplane, there is no reason to change this plan.
Element Mass (grams)
Engine 7.1
Battery 175
Servos 11.1
Receiver 14
Propeller 5
Engine 36
Wiring 10
Solar cells 1 76.8
Solar cells 2 9.6
Frame 70
Unforeseen 10.4
Total 425
Now we’ve decided on the parts to be used, we can also give a more informed prediction of the mass of the plane. All the parts we just summed up add to a total of 335 grams, well below our target mass of 450 grams. However, we haven’t included all elements yet (such as the frame, wiring and unforeseen additional mass). In addition, we aren’t certain yet about the mass of the propeller, but we estimate it weighs 5 grams. We reserve 70 grams for the frame, a reasonable amount according to experts on internet forums . Furthermore we reserve 10 grams for wiring and 10 grams for unforeseen mass. All these numbers add up to a total of 425 grams, so we should be well under our target mass.
Step 5
The electrical system has now been completed. Knowing mass, size and properties we are able to draw the final circuit.
Step 6
12. How do you mount the solar cells on the aircraft (inside or outside, wing or body?)
With the solar cells being a critical part of the plane, it is logical to have a good thought about the solar cells’ position in the plane. The solar cells account for a large part of the total mass, so their position has to be well considered to keep the plane balanced, both on the ground and during flight. Since the plane’s fuselage will be very slim and the solar cells measure 10 square centimetres, it is most obvious to place the solar cells on the plane’s wings. With the plane being 160 centimetres in wingspan, we have plenty of space to place all solar cells. In addition, placing solar cells on the tail will not only be impossible due to the tail’s size, but the plane’s centre of gravity will also be shifted too much to the back.
We have to determine whether we will place the solar cells on top of the wings or inside the wings. The choice was made quickly: we will place our solar cells inside the wings, since it offers multiple advantages:
• The assembly of the solar cells becomes much easier. If we would place the solar cells on top of the wings, we would have to attach the solar cells to the covering foil which might pierce the foil and possibly ruin lift properties.
• Placing the solar cells inside the wings will leave aerodynamic properties of our wings intact, whereas putting solar cells outside the wings would drastically increase the drag of the wings and could reduce the lift they provide.
• Placing the solar cells inside the wings largely eliminates the risk of damaging the highly vulnerable solar cells, since they will now be protected by foil.
Step 7
13. What will be the structure’s design?
14. Which materials do you need to construct the plane?
Now that the solar cells’ placing has been determined, we can make a list of all the additional equipment we will be placing in the wings. Parts that will be put in the wings are:
• Solar cells
• Servos
• Wiring
• Foil
• Ribs & cornices
• Triplex beams
• Axle for aileron
• Ailerons
Our plane’s wings will have a wingspan of 160 centimetres, a width of 20 centimetres and an expected height of 2.5 centimetres. To prevent the wing from bending we will place two triplex beams running through the wings. The front and rear cornice have already been pre-shaped . In the diagram below you can find an overview of the wing. The numbers above the wing represent the rib that is placed there:
The ribs will be made out of balsa wood, the lightest type of wood known to men . Each rib measures three millimetres in thickness. We will cover the wings using a covering foil called Mylar. This material can be applied to the wing loosely, but will be tightened around the frame by ironing it . It weighs approximately seven grams per square metre and is strong enough to lift the aircraft.
Cross section 1
The drawing below shows one of the wing’s regular ribs. The yellow circles represent holes in the ribs for wiring. The black rectangles in the middle of the wing represent two triplex girders running through the wing. On top of these we will mount the solar cells. Besides providing a base for the solar cells, the girders also give the wing structural integrity. The black semi-circle and triangle show the pre-shaped front and rear cornice. The blue rectangle shows the position of a solar cell in between the ribs.
Our solar cells will be mounted inside the wing. This requires the ribs to be placed eleven centimetres apart, for else the solar cells won’t fit. We also need to make sure there is enough room to construct the ailerons. Besides, some space must be kept free to install a servo engine. These demands result in three alternative ribs which will be used in the aft of the wing:
Cross section 2
In the second cross section we see the servo’s placing in the wing.
Cross section 3
The third cross section represents the last rib before the aileron, the element in the wing which allows us to steer the plane. You may notice an extra beam just before the hole for the aileron. This beam provides a surface around which the covering foil can be wrapped.
Cross section 4
The fourth cross section is the rib that can be found in the part of the wing where the aileron is placed. As you can see, the rear cornice of the wing has been cut off. Instead we will place the aileron there. In addition, the large solar cell isn’t shown, because in this part of the wing we will locate the smaller cells instead. They are too small to rest on top of the triplex beams. Therefore they will be suspended on top of a little piece of rope attached to the triplex girders.
Step 8 & 9
Fuselage
The aircraft’s fuselage is among the most important parts of the plane. All electronic equipment is to be installed here. Moreover, wings and landing gear are connected to the plane’s fuselage. As weight and balance are crucial to an aircraft, it is important to make a detailed drawing showing where all equipment will be placed. To monitor the balance the plane’s fuselage will be suspended by one thread during construction. In that way a flaw in balance will be noticed quickly as the plane flips over when not properly loaded. Sub questions involved in parts 8 and 9 are:
15. Where do you place all electrical equipment, making sure the aircraft is balanced and what will the fuselage be like?
16. What will be the shape of the tail and elevator and how are they controlled?
Fuselage
First up is the fuselage. Since drag is an important factor when concerning the engine’s capacity, we should keep drag as low as possible. We intend to construct a frame measuring seven centimetres wide. In order to keep the structure both light weight and strong, we shall construct a triangular shaped fuselage. The fuselage will be 28 centimetres long, thus providing us with enough room to place all electrical equipment. However, our seven AA batteries measure five centimetres each. Therefore they can’t be installed in one line. The entire scheme of our plane’s fuselage can be found below. Due to heat produced by several pieces of equipment the back side will be left open.
As shown in the drawing, the centre of gravity is to be located at ⅓ of the wing’s depth. This is a rule of thumb for building aircraft. With the equipments’ mass being so low and exact mass of the frame yet unknown, it is impossible for us to calculate the momentum for each element in the plane. Therefore the scheme shown above will be used as a guideline to position all equipment.
Once our aircraft has been completed, its centre of gravity will be determined precisely by slightly moving the equipment inside the plane’s fuselage. Another rule of thumb is to rather position the plane’s centre of gravity slightly too far in front than too far to the rear. This is due to the plane getting in stall position too easy when the centre of gravity is located in the rear.
The fuselage’s frame will be constructed with balsa wood and covered with Mylar foil. However, due to safety reasons, we will manufacture the fuselage’s base out of a plate of balsa wood, for else the equipments’ heat will melt the Mylar foil. The frame itself will feature triangular shaped parts. The holes in between the 2nd and 3rd and between the 4th and 5th triangle are points to attach the wing to the fuselage. As shown in the diagram below, the main gear will be placed slightly in front of the centre of gravity. This will make the aircraft fall over to the back when landed. A little wheel mounted underneath the tail will prevent the aircraft from sustaining damage. Besides, when on the ground the aircraft will always stand in a tilted position, thus providing more lift at take off (see diagram page 13).
Another important element of our aircraft is the positioning of two supporting triplex beams for the wings. They can be seen in the diagram below. The triplex beams will be attached to the landing gear, to provide maximum support for our wings. The beams will measure 10 x 5 millimetres.
Tail and elevator
Up so far there are still two vital airplane parts we haven’t mentioned. They are the horizontal and vertical stabilizer, better known as tail and elevator. They provide stability for our aircraft. In addition they are vital elements in controlling the plane. Just like the fuselage, elevator and tail will be constructed with balsa wood and Mylar foil. Strength is once again provided by triangular shaped balsa frames.
The tail’s and elevator’s size are determined by the wing’s area. The ratio between elevator and wing is 4:1, whereas the ratio between tail and wing is 25:1. This implies the areas of both the tail and elevator will be as follows:
Wing area: 1.6 × 0.2 = 0.32 m²
Elevator area: 0.32 × 0.25 = 0.08 m²
Tail area: 0.32 × 0.04 = 0.0128 m²
Elevator size: 60 centimetres wide and 13 centimetres in depth.
Tail size (triangular): 18 centimetres in depth and 15 centimetres high.
The tail will be connected to the fuselage with a triplex beam. For extra structural integrity the beam will run all the way from tail to engine.
As a final check we will calculate the balsa wooden frame’s mass. Every rib will weigh half a gram. Twenty ribs weigh 10 grams. Triplex beams, front cornice and rear cornice will weigh approximately 30 grams. In addition, the fuselage and landing gear weigh 20 grams, compared to 10 grams for the tail. Mylar will come in with just several grams. This adds up to a total of 70 grams.
Step 10
As our design proposal has nearly been finished, we can now determine the costs of our project. Our budget is €400. The table below shows we should be well under that amount.
Element Cost
Solar cells 1 € 127.20
Solar cells 2 € 18.00
Engine € 14.50
Propeller € 3.35
Remote control € 29.95
Receiver € 13.50
Servos € 22.50
Engine controller € 16.95
Battery € 25.00
Frame € 35.00
Wiring € 5.00
Unforeseen € 14.05
Total € 325.00
Another crucial issue is weight. Having a clear picture of which elements we will be using, it is time to add up the masses of all individual components. The results show we are still well on track to reach our target mass.
Element Mass (grams)
Engine controller 7.1
Battery 175.0
Servos 11.1
Receiver 14.0
Propeller 5.0
Engine 36.0
Wiring 10.0
Solar cells 1 76.8
Solar cells 2 9.6
Frame 70.0
Unforeseen 10.4
Total 425
= estimated
Construction proposal
After having done thorough research on the plane’s design, it is time to start thinking about construction itself. Just as we did in the design proposal, we set up a plan dealing with the order of construction:
Step 11: Research on where to order construction materials.
Step 12: Determining the order of construction.
The sub question being answered is:
17. In which order are you going to construct the plane?
Step 11
To start with, we will order the parts of the electrical system. We try to keep mailing costs as low as possible. Therefore we must order as many items as possible at the same supplier. We composed the following list of materials needed. All supplies will be ordered on the web.
Electronics
All the electronics are to be ordered at rctechnics.eu. Their shop features a broad range of items, all good quality products according to people on internet forums. The following items will be ordered at rctechnics.eu:
• Receiver
• Engine regulator
• Servos and wiring
• Engine
• Remote control
Construction materials
Just as the electronics, the propeller and landing gear wheels will be ordered at rctechins.eu. Unfortunately this shop doesn’t sell other construction materials. In addition, finding a web shop which sells and ships balsa wood is extremely difficult. Eventually we found a supplier in Groningen, who could offer us balsa wood and beams for the ailerons and rudder: netshop.nl/shop/krikkem. Glue is available at a local DIY shop and batteries will be bought at the local HEMA. In contrast, Mylar is not available in any shop in Holland. At last we found a shop in Great Britain selling Mylar: indoorflyer.co.uk. The shopping list for construction materials is as follows:
• Propeller (rctechnics.eu)
• Landing gear wheels(rctechnics.eu)
• Balsa (netshop.nl/shop/krikke)
• Beams for ailerons and rudder (netshop.nl/shop/krikke)
• Glue (Praxis)
• Mylar (indoorflyer.co.uk)
• Batteries (HEMA)
Solar cells
As told in our design proposal, the solar cells will be ordered at lemosolar.de.
Step 12
Once we receive our supplies, we will put together the electrical system. After composing it, we will run several tests as described in the ‘Test proposal’. We will collect data we need and make amendments to the system if desired. After such possible adjustments we will make a new estimation of the plane’s mass and, if needed, adjust the wings’ size or the plane’s design.
With the electrical system being complete, it is time to focus on the plane’s body. A crucial element here is design and construction of the wing. This is the next step in our process of construction. We will construct two separate wings out of balsa wood, place solar cells on the correct places and then wrap the wings with Mylar foil to secure their shape.
Once completed, we can test the lift the wings generate. The test is described in our ‘Test proposal’. If the wings don’t produce the lift required, we will have to make adjustments.
With the wings and electrical system now completed, we will construct the rest of the aircraft, which consists of the elevator, tail and fuselage. Next, we will mount the electrical system in the plane’s fuselage, glue the landing gear onto the plane and determine the centre of gravity. The last step in construction is to fasten the wings to the fuselage. If needed, we will move items inside the plane in order to get the centre of gravity in the right position.
If all goes well, we have built a complete plane by now and we can start testing. If required, adjustments will be made during and after testing.
Test proposal
To make an organized test proposal we have to follow the steps we’ve set ourselves:
Step 13: Determining what needs to be tested and in which order.
Step 14: Set up plans on how to run tests, where and with which materials required.
Step 13 & 14
Throughout the project several tests need to be run. We will investigate the following data:
• Solar cells’ output and profit
• Lift test
• Drive test
• Flight test
The solar cells’ output and profit
18. Which effect does the position of the solar cells have on the efficiency of both the plane and panels?
19. How much profit do the solar cells provide?
To answer the two sub questions concerning the solar cells’ profit, we came up with the following test setting. After we ordered components such as solar cells and the engine, we will compose the electrical system. At this point we haven’t started construction of the plane yet. First, we will fully charge the batteries, connect them to the engine and then let the engine run at full throttle. We will record the time it takes for the batteries to run out. In order to minimize errors, we will redo the test three times.
In a second test we will add solar cells to our electrical system. We make sure the solar cells receive plenty of light and then we let the engine run at full throttle again. We will write down the time it takes for the batteries to run out and compare this to the previously clocked time. In this way we will be able to tell something about the profit of the solar cells.
A third test aims to determine the effect of Mylar foil on the solar cells’ output. We will run the second experiment again, but now, we will place Mylar foil over the solar cells. This will probably decrease their power supply. Through this experiment we will be able to say something about the influence of the Mylar foil.
Unfortunately the solar cells’ output strongly depends on weather. If time is available we will do the second and third test several times in both cloudy and sunny conditions. Unfortunately a photo meter to measure sunlight intensity is not available.
Lift test
Once construction of the wing has been completed, it is time to test lift. At this point we still have two separate wings. Before both parts are glued together, we will take one of the wings and attach it to a force meter. This composition will be mounted into a freely moveable frame. The frame, composed of K’nex parts, will be fit on a bicycle’s basket.
One of us will cycle at 30 kilometres per hour, the intended take-off speed. Speed will be measured with an odometer on the bicycle itself. If calculations have been correct, the force meter should measure a difference of approximately 2.5 Newton. Of course we will take into account influences that could disturb our test, like wind, loss of surface area due to K’nex frame, etc.
Drive test
Eventually we reach a point where the aircraft is finished. We will have determined the centre of gravity and will have made sure the controls function properly. Before it is actually time to take off, we will perform several driving tests. We will position the airplane on a smooth surface, for example a gym floor. Then we will slowly increase throttle to let the aircraft ‘taxi’.
Now it is time to make final adjustments. Drive tests might show deviation in driving direction, lack of power or other defects. Once those problems have been solved, we will do a final driving test. We will keep increasing throttle until the tail will get airborne. Namely, the tail coming off the ground is a first indication of taking off. This is because the mass of the aircraft is no longer supported by the wheels, but by the wings. The centre of gravity is slightly behind the wheels, thus making the aircraft falling over to the back. When the mass of the aircraft is no longer supported by the wheels, but by the wings, the centre of gravity is at ⅓ of the wing, thus balancing the aircraft.
Flight test
At this point in our research project we know whether or not our wings produce enough lift and the engine and propeller generate enough power to propel the plane. Still, there is one big challenge left: flying.
According to people on internet forums, flying a model aircraft is a very difficult discipline. It requires extensive expertise and experience with flying. As we don’t want to crash our aircraft during its maiden flight, we will bring in an experienced person from a flying club. He will fly our aircraft at first. With his help we will eventually learn how to control the plane.
The last problem that needs to be solved is finding a suitable runway. As our plane is probably going to be fragile, it is unlikely we can take off from any parking lot. Currently we plan to use an artificial turf field as a runway.
Media attention
Website
We intended to launch a website on the internet. Here people would be able to follow our progress and see pictures of the airplane. However, we didn’t want to spend any money on an expensive internet domain, so once the free testing period had expired, unfortunately the website was cancelled.
Sponsors
Since this project is quite costly, we had to take a look at possible sponsors. Being environmentally friendly is a major aspect in our project. We searched for companies that present themselves as environmentally friendly. Companies we ended up looking at were Triodos and ASN bank. However, Triodos only sponsors five major projects on a yearly basis and the ASN bank rejected our proposal.
Another effort we made was with the company that supplied us with solar cells. We asked them if they were willing to provide us with solar cells or give us discount on solar cells in return for some promotion. Luckily, they gave discount on the solar cells, but mailing costs proved to cancel out our ‘profit’.
Logo
Just like any plane, ours will feature a company logo on its tail.
‘JET’ is not only a synonym for airplane, it also represents our initials: J(esper) E(n) T(im). The globe shown on the logo represents JET’s internationally orientated roots.
International aspect
As part of our internationally orientated education, our research project should include a piece in which we discuss how our research project is internationally relevant. We think our research project is internationally relevant, because it is based on a world-wide hot topic.
Energy demands
The issue of meeting the worldwide energy demands is currently high on any politician’s shortlist. Right now we are able to fulfil our needs, but knowing we consume 85 million barrels of oil a day , it is clear we will run out of our energy supplies one day. That day is coming closer by the minute. Engineers worldwide are searching for alternatives to meet our demands. Their focus mainly lies on the renewable energy sources, such as wind and solar power .
Their reasoning to focus on these sources of energy is simple: such sources won’t run out for billions of years and are capable of meeting the world’s energy demand. Covering 10% of the Sahara desert area with solar cells for example would be sufficient to supply energy to the whole world and wind turbines in shallow coastal waters could account for 20% of energy demands in the USA .
Solar power thus is an interesting option. Recent years have seen solar power production climb with a staggering 40% per year , but when looking at statistics showing shares in total energy supply, we will notice that solar power production does not even account for one percent. We should realise that there still is a long way to go.
Our solar powered plane could let people come to realize just what the possibilities of renewable energy sources are, especially solar cells. If we can make a plane fly using solar power, other possibilities are nearly endless! People might consider placing solar panels on their roof or on their carport to supply part of the power for their houses. There are a lot of advantages to solar panels: four square metres of solar panels supply 10% of the annual energy consumption of an average household . In addition, solar panels could recover their costs in about ten years and solar panels can increase the value of your house, since a home becomes friendlier to the environment and has considerably lower energy costs.
Not only is solar power being looked at as an option to supply energy we use in our households, solar power has also come into picture for powering vehicles. This mostly shows through all sorts of contests being held for technical institutions.
Solar cars
The most serious option being looked at is solar energy to power cars. With hybrid and plug-in electric cars being introduced to the market, solar power becomes an interesting option to supply energy. A famous contest between solar powered cars is the World Solar Challenge, a race across the Australian desert being held every two years . This race catches worldwide attention. Technology used in cars involved can be replicated in other cars.
In recent years the race has become well-known in The Netherlands because of success of the Technical University of Delft with their NUNA solar powered car. The TU Delft has won the World Solar Challenge four times in a row , each time with a modified and upgraded car. When the team entered competition with NUNA 1, they won the race with an average speed of 92 kilometres per hour . Two editions later NUNA 3 won the World Solar Challenge with an average speed of 103 kilometres an hour . In fourth edition NUNA 4 won with an average speed of 93 kilometres an hour, even though this car had only 6 m2 of solar panels, instead of 9 m2 used in previous editions .
Cars competing in the World Solar Challenge distinguish themselves from normal cars because of their extremely low drag and low mass. The cars are very aerodynamic (up to 6 times more aerodynamic than cars driven today) and have a very low rolling resistance, thanks to thin and well lubricated wheels. Moreover, the cars are as light as a feather compared to normal cars. Namely, a regular car usually weighs around 1000 kilograms, whereas the newest version of the NUNA (NUNA 5) weighs just 160 kilograms. Solar panels used on these cars are of very high quality and have an extremely high output compared to normal solar cells. Although the NUNA 5 doesn’t have the luxurious items we might want in our cars, such as a radio or air conditioning, we can still learn a lot on how to make our cars lighter and how to lower their drag. Besides, we can learn how to incorporate solar cells in our cars. All these aspects could make present day cars more fuel efficient.
REACTIES
:name
:name
:comment
1 seconde geleden