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9/7/2009     

A  REPORT  ON   

 

AERO‐MODELING AND AERODYNAMICS 

SUBMITTED BY | DEBASISH DEVKUMAR PADHY 

INTRODUCTION  As a kid we have always dreamed of flying like a bird or becoming a pilot and flying an airplane. The  thought of flight has always thrilled human race. There are a lot of instances that prove that man was  trying hard to get wings and soar out in the sky just like birds. Ultimately the Wright brothers came up  with the real breakthrough, their Wright Flyer.   There were thousands of attempts before the real flyer came into action. Various concepts and designs  came into contrast before the Wright Brother’s Flyer. Langley and Da Vinci were among the few legends  who gave the preliminary idea about flight.  As a matter of fact, the modern day helicopter still works on  the same principle as given by Da Vinci.   A lot of companies came into action and the requirement for researching and developing a practical  commercial plane came up. And that’s when AERO‐MODELING came into existence.     WHAT EXACTLY IS AEROMODELING??  In the past Aero‐modeling was introduced for rapid R&D and prototype development. Later it was made  available to the public for hobby purpose. At present Aero‐modeling is still in its original form. It  is still  used in R&D and prototyping .  Prototype is an original type, form, or instance of something serving as a typical example, basis, or standard for the things of the same category. Or say prototyping, in a greater extent, practical attempt to simulate the final design, aesthetics, materials and functionality of the intended design. The functional prototype is generally reduced in size (scaled down) in order to reduce costs. The construction of a final and working full-scale prototype and the ultimate test of concept is the engineers' final check for flaws in design and allows last-minute improvements to be made before actual production units are ordered.

   Atlantica plug the prototype developed by NASA   (Source: NASA) 

This whole report is divided into 5  modules.  The report is focused on an introducing to Aero‐modeling and ultimately  designing an Aero‐model. 

  Module one: ­   Introduction to general aerodynamic and  Aero­modeling terms   Module two: ­   Classification of airplanes  Module three: ­   Designing an Aero­model  Module four: ­   Construction phase  Module five: ­    Flying of the Aero­model     

Module one:  Introduction to aerodynamic  and Aero‐modeling terms  Before we start off with classification or designing of airplanes, we have to have  ideas of some basic aerodynamic theory and some basic terms commonly used in  the field of aviation and aero‐modeling.   But first we will be studying about the airframe. 

Trailing edge  Leading edge 

  Fuselage:   It is the main body of an aircraft over which everything is assembled to make an  airplane. In more generalized way, it is the main body of an aircraft which holds  wings, horizontal stabilizers, vertical stabilizers, crew, etc      

Wing:   It is the main lifting surface of the airplane, it’s the surface where most of the lift  generated acts and its design is the most crucial part of a airplane, to decide its  characteristics.  Leading edge:  The front ends of the wing which is incident to the wind, i.e. which separates the  wind in two section.  Trailing edge:  It is the back end of the wing where the wind after separation meets again. It is  the place where ailerons are located.   Chord:  Chord is the shortest distance between leading edge and trailing edge.  Airfoil:  It is the cross‐sectional view of a wing. It’s shape is responsible for the lifting  action on various lifting surfaces,   Horizontal stabilizer:  A tail plane, also known as horizontal stabilizer, is a small lifting surface located  behind the main lifting surfaces of a fixed‐wing aircraft as well as other non‐fixed  wing aircraft such as helicopters and gyroplanes. The tail plane serves three  purposes: equilibrium, stability and control.   “Note: In short the movement of fuselage up and down along an axis is called pitching  controlled by the elevator attached to the tail plane, more detail later in the text”       

Vertical stabilizer:  The work of vertical stabilizer is same as the work of horizontal stabilizer, but  instead of controlling pitching it controls the yaw moment of the air plane with  the help of rudder.  The image below will show how these things are 

  Rudder:  Its main work is to control the alignment of airframe with respect to runway or  wind. It controls the yaw movement of the airplane. It helps to adjust the heading  of the airplane in case of gust and drifts.  Elevator:  Just as the name says it helps an airplane to elevate up and down and hence  control the altitude of an aircraft. It is located just behind the tail plane.  Angle of attack:  The angle, with which, the wing penetrates the air. As the angle of attack  increases, lift also increases up to a maximum point (along with drag).       

Now let’s jump into basic aerodynamics and flight theory…   Here we will be looking how lift is generated and how an airplane flies.   In today’s date we all know about the principle on which an airplane flies. It is The  Bernoulli’s principle, which in simple words states “Increase in velocity of fluid  results in decrease in pressure.” And decrease in pressure will result in decrease  in gravitational potential energy.   

 

 

  Imagine this way.  You are in a very narrow streets and the number of people  moving on the road is very large. You will observe the people will move slowly  and push each other. That means that the pressure is high. But when the number  of people is less, you will observe that the people will move fast. There will be no  pushing, meaning the pressure is less.   If we assume the people moving in the street as a fluid, then the statement  becomes, “At high speed pressure is low and at low speed pressure is high.”    Now with that we can easily understand the theory behind the lifting of an aero  plane.  All the lifting surfaces present in an aircraft have a unique cross‐sectional shape,  called an aerofoil, whose characteristic is that, air flowing above the wing is faster  than air flowing below the wing.  

  Aerofoil of the wing 

   Now when the wing is incident to wind, the air flowing above the wing has to  cover more distance than the air flowing below the wing. This compels the air  above the wing to move faster than air below the wing. Here Bernoulli’s principle  comes in action “high speed=low pressure; low speed=high pressure”.  Since the  transition of energy is from high to low, the wing generates a lift. When the speed  of the airplane is enough to generate lift equivalent or more than its weight the  airplane lifts off from the ground. 

Now that we have understood how lift is generated, we can go on with other  control surfaces and how they work.   An airplane in stable and straight flight is at dynamic equilibrium, i.e. every force  acting on the body of an aircraft is canceling each other out. The only force that  exists is the forward force by which the aircraft moves forward. Now whenever  there is any disturbance in the equilibrium the airplane responds to it. Any one  flying an airplane uses these disturbances to control all the movement of the  airplane, right from take off to landing.  For example,  for altitude control, the disturbance is made at the pitch of the  aircraft, when the elevator is raised up the wind flowing on the tail plane tries to  push it down as a result the tail goes down and the nose rise up, when nose is up  the thrust due to engine pulls the airplane high in the air. 

Elevator  movement 

 

For banking and direction control use of aileron comes in action, ailerons are  present in both the wing halves so when one aileron is raised and the other is  lowered, the wind flowing above the raised aileron side tries to push that side of  the wing down and the other side where the aileron is lowered the wind below  the wing tries to push it up ward result is a force couple, and the plane banks to  the raised aileron side. The image below illustrates it. 

  Other control are rudder and throttle which aids in skid and speed control of the  airplane.  Another aerodynamic term we should be looking forward is the dihedral of an  airplane…. What is exactly dihedral is?  Dihedral  The dihedral angle is the angle that each wing makes with the horizontal. The  purpose of dihedral is to improve lateral stability. If a disturbance causes one  wing to drop, the unbalanced force produces a sideslip in the direction of the  downgoing wing. This will, in effect, cause a flow of air in the opposite direction to  the slip. This flow of air will strike the lower wing at a greater angle of attack than  it strikes the upper wing. The lower wing will thus receive more lift and the  airplane will roll back into its proper position.  Since dihedral inclines the wing to the horizontal, so too will the lift reaction of  the wing be inclined from the vertical. Hence an excessive amount of dihedral  will, in effect, reduce the lift force opposing weight.  Some modern airplanes have a measure of negative dihedral or anhedral, on the  wings and/or stabilizer. The incorporation of this feature provides some 

advantages in overall design in certain type of airplanes. However, it does have an  effect, probably adverse, on lateral stability.  Also During the design of a fixed‐wing aircraft (or any aircraft with horizontal  surfaces), changing Dihedral Angle is usually a relatively simple way to adjust the  overall Dihedral Effect. This is to compensate for other design elements' influence  on Dihedral Effect. These other elements (such as wing sweep, vertical mount  point of the wing, etc.) may be more difficult to change than Dihedral Angle. As a  result, differing amounts of Dihedral Angle can be found on different types of  fixed‐wing aircraft. For example, the Dihedral Angle is usually greater on low‐wing  aircraft than on otherwise‐similar high‐wing aircraft. This is because "highness" of  a wing (or "lowness" of vertical center of gravity compared to the wing) creates  naturally more Dihedral Effect itself. This leaves less Dihedral Angle needed to  supplement Dihedral Effect and get the amount of Dihedral Effect needed.  How to calculate dihedral?  Well for aeromodelers the answer to this question is “it depends”, It depends on  the characteristics of airplane we want to design, wether the airplane is highly  stable or it is moderately stable, high stable model is less maneuverable and low  stable model is highly maneuverable so it is the requirement which helps us  calculating the dihedral of a airplane. With my experience a dihedral of 7‐ 8degrees is more than enough     

           

        MODULE TWO    This section deals with the general classification of airplane and airframes based on various  distinguishing factors, here the classifications are based on the powerhouse, wing style ,  alignment, purpose etc    ____________________________________ 

      • Classification based on wing styles  • Classification based on wing locations  • Classification based on wing dihedral   • Classification based on powerhouse 

   

       

  Classification: wing styles    There are various aircraft available all over the world and we need to classify them in order to make  designing phase easy and more feasible. The 1st categories is based on the wing style , That is whether  the wing is forward swept or backward swept , the geometry of the wing etc.  So lets start with a forward swept wing:  A forward‐swept wing is an aircraft configuration in which the quarter‐chord line of the wing has a forward sweep.  The configuration was first proposed by some German aircraft designers 

Aircraft with forward‐swept wings are highly maneuverable at transonic speeds because air flows over a  forward‐swept wing and toward the fuselage, rather than away from it.  An example of a forward swept wing concept  

 

  AIRPLANES having this type of wings are called canard airplanes. These are the 3rd generation or say  future style airplanes. For the time being its not in much practical usage    The second style of wing is back swept wings:  The wing angles backwards from the root. This reduces drag at transonic speeds, but can handle badly in or near a  stall, and requires high stiffness to avoid aeroelasticity at high speeds. Common for high‐subsonic and supersonic  designs.   As an aircraft enters the transonic speeds just below the speed of sound, an effect known as wave drag starts to  appear. Using conservation of momentum principles in the direction normal to surface curvature, airflow  accelerates around curved surfaces, and near the speed of sound the acceleration can cause the airflow to reach  supersonic speeds. When this occurs, an oblique shock wave is generated at the point where the flow goes  supersonic. Since this occurs on curved areas, they are normally associated with the upper surfaces of the wing,  the cockpit canopy, and the nose cone of the aircraft, areas with the highest local curvature. Using back swept  wing reduces this problem.  These are the present generation airplanes like the mig‐21,commercial passenger airplanes and some military  bombers  

   

Other wing configurations are  ƒ

ƒ

Straight - extends at right angles to the line of flight. The most efficient structurally, and common for low-speed designs. 

M-wing - the inner wing section sweeps forward, and the outer section sweeps backwards. The idea has been studied from time to time, but no example has ever been built.

ƒ

W-wing - the inner wing section sweeps back, and the outer section sweeps forwards. The reverse of the M-wing. The idea has been studied even less than the M-wing and no example has ever been built.

ƒ

Crescent - wing outer section is swept less sharply than the inner section.

ƒ

  ƒ

Swing-wing - also called "variable sweep wing". The left and right hand wings vary their sweep together, usually backwards. Seen in a few types of combat aircraft 

      These styles were important for classification and designing point of view.  Now moving on to the next section of classification I.E classification based on the location of the wing  which will be carried out in the next page 

 

CLASSIFICATION : WING LOCATION    The location of wing with respect to the main airframe is also a deciding factor for the maneuverability  of an aircraft. Typically there are four type of location possible.  ƒ ƒ ƒ ƒ

High wing  Low wing   Shoulder wing  And The parasol wing 

    High wing:  In this configuration the airframe lies below the wing, that is the weight of the whole airframe is  suspended below the wing. This configuration is suitable for stable airplanes like trainers and low  power airplanes. Since the weight of the airplane is suspended below the wing it helps in the auto  correction (for level flight) . it finds application in UAV’s(unmanned aerial vehicle ) and other  autonomous airplanes, passenger airplanes and cargo planes.  A typical UAV.  

 

   A typical high wing configuration airplane used in military and defence services.    Low wing configuration:  In this configuration the airframe lies above (over the airframe) the airframe, here the weight of the  airplane is kept over the wing and hence make a bit unstable but more maneuverable. These wing  configuration finds application where high maneuverability is required , that the aircraft is able to  perform precision aerobatics , And we think that you have guessed the probable fields of application  of these, Yes you are right , it’s the defense where fighter aircrafts are used. 

 

  Example: of few combat airplanes. (low wing configuration).    The next wing configuration is a shoulder wing airplane  Shoulder wing airplane:  This style of wing is a combination of high wing and low wing, That means some part of the weight is  above the wing and some part is suspended below the wing, meaning a fussion of stability and  maneuverability. It finds application in the fields of defense and combat.

 

The fifth type of wing configuration is parasol style:  Here the wing is mounted above the airplane body and is held in place by wing struts and dowels, its  like a parachute. This configuration is similar to high wing configuration.   

                 

             

  CLASSIFICATION: WING DIHEDRAL   AS you already know the meaning and physical significance of dihedral, the classification of  airplanes can be done according to there characteristics dihedral.  The characterictic dihedral are of three types  ƒ ƒ ƒ

POSITIVE DIHEDRAL  NEGATIVE DIHEDRAL  or ANHEDRAL  NO DIHEDRAL 

    Positive dihedral:  When the wing tips of a wing are raised to form an angle between the two halves of the wing , its  positive dihedral.  It adds the self correcting ability and stability to the airplane, it is the most  essential part of a stable airplane with predictable behavior.    Negative dihedral:  It’s the opposite of Positive dihedral, the wing tips are dipped towards the ground, to obtain  anhedral, it adds to maneuverability and reduces stability. It finds application in combat airplanes.     NO Dihedral:  As the name says , there is no dihedral present or anhedral present , the angle between the wing  halves is zero degree but still keeps the airplane predictable and a bit stable . Its most widely used  configuration. For some previous jet aircrafts and for some very recent airplanes.         

   

Classification: Power house      By saying power house we mean to say the engine used for aviation.   Commercially airplane uses turbojet, pulse jet, ram jet, etc to power themselves,   In AERO‐MODELING we use  

here IC engines are basically of three types i.e glow, petrol, & diesel. The main difference between  these engines is the fuel they use i.e mixture of “castor and methanol”,” petrol”, and mixture of  “kerosene castor and ether”. Apart from all these the difference is the way the ignition occurs, for   petrol and glow the ignition is initiated with a spark given out by the spark plug or glow plug  respectively, while in the case of diesel engine the fuel air mixture is ignited by subjecting them to  very high pressure in the compression chamber while the compression stroke occurs.     Moving off the IC engines there  is another power house and its called electric motors, which are of  basically of two types, 1st brushed electric motors and second is brushless electric motors. The main  difference between brushed motor and brushless electric motor is that brush motors are less costly  and brushless motor is more costly , apart from it the brushed motors are less efficient and noisy , 

causes more wear and tear. So while designing we have to chose wisely , wethere to chose AN IC  engine or a motor.   Failing to do so will result in poor airplane design.                                         

   

  MODULE 3: DESIGNING AN AEOPLANE       

In the upcoming pages we will be designing an airplane, doing some  calculations and drawing some plans. For this report we will be  designing A “High wing ” Trainer aircraft. Powered by a .25 size glow  engine we will not be focusing on how the formula was derived we will  be using formulas and wherever necessary we will be giving out the  derivations behind each formula, In this section we might be talking  about some advanced aerodynamics which were not discussed earlier.  The designing process will comprise of following steps    • • • •

Laying out specifications.  Analysis of requirements and solutions.   Calculating the required parameters.  Drawing the approx plan.             

LAYING OUT THE SPECIFICATIONS     

A specification allows you to take a vague concept and turn it into specifically what you want. It should detail everything that is important to include and exclude from your design. Since we are designing an aircraft which is supposed to be a trainer we will be looking on the following points.

• • • • • • • • • • • • •

Purpose of the model Style — Modern, Old Timer, Scale, Sleek, etc. Powerplant class Flight time Stability — Should the model be self-stabilizing, neutrally stable or somewhere in between? Airspeed Vertical performance: moderate Control response: predictable Stall characteristics: high speed stalls should be avoided Construction methods — Traditional wood, composite, etc. Control system Landing gear system: tricycle Break-down for transportation

The purpose of the airplane is to behave like a trainer such that a  novice or a beginner can learn how to fly a airplane, style like  modern,old timer etc is not of that importance but the point is that a  very stable aircraft generally is a highwing airplane, being a highwing  model adds the ease with which it can be constructed, Other advantage  of highwing configuration are   •

A high wing aircraft has a shorter landing distance than a low wing  aircraft because the wing stays out of the ground effect. 

•

•

And most importantly it helps in the self correcting ability of an  airplane due to the pendulum stability.

Pendulum stability  Now also since the wing stays high above the ground the  probability of wreaking the wing is highly reduced adding the  crash resistance effect 

 

Now that we have decided the wing location we can proceed to  other section of stability and self correcting ability, for a trainer  airplane the self correcting ability is a must for that we must have  dihedral on our wings. The higher the angle higher is the self  correcting ability to an extent so for our purpose we will go with a 5*  of dihedral.(for more details on dihedral see texts above)  Moving to power plant section , the choice of using a ic engine or a  electric motor depends largely on the way of construction adopted,  like a delicate construction will have to use electric motors despite of  that, purpose of the aircraft is also a deciding factor, it the model has  to be made noise free, then also we have to use electric motors. But  in this case the priority is a high strength trainer airplane, so  choosing a Ic engine powerhouse will not harm anything, also 

electric motors have their own disadvantage like we will have limited  flight time and the RPM of the motor decreases as the battery drains  also we have to carry many battery pack’s if we intend to have some  long day of flying.    The engines and motors rotates different propellers at different  rpm’s  Like   Generally the RPM range of a typical glow engine is from few  thousand to 16,000 or more. The propeller which an engine is  capable to swung for best performance is provided by the engine  manufacturer.  We will be using a .25 cu in glow plug engine for our airplane  swinging a 9x6 size propeller that means a propeller of 9” diameter  and of the pitch 6” that means if this propeller is rotated in a solid  medium of 9 inch then it will move 6” forward, it suggests that the  total weight of our airplane shud be some where between 700gms  to 1.4 kgs, lets fix it to 1kg.    Well the answer is this formula:  Lets say D is the engine size and W is the weight of the aircraft it can  pull happily then   D x 12 = W; or D=W/12  The result comes in lbs then converts it to kgs, 

For our purpose the engine we are using is a .25 cu in hence the  formula is  .25(D) x 12= 3lbs   1lb= 0.4535924 kg   Hence 3 lbs = 1.4 (approx) kg  We keep a margin of 400gm (approx) to compensate the weight  resulting due to repairs and crashes.    Till now what we have decided is       • • • •

A high wing style airplane  Dihedral of 5* initially (can be changed latter)  An engine of .25 cu in  Total weight of 700gms to 1kgs (max) upper limit is 1.4kgs   

    Before we go further we have to leap back into aerofoil section again  because after this is covered we will be designing wing, which will  decide other parameters of the aircraft.   

Back to airfoil    The various terms related to airfoils are defined below: ƒ

The mean camber line is a line drawn midway between the upper and lower surfaces.

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The chord line is a straight line connecting the leading and trailing edges of the airfoil, at the ends of the mean camber line.

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The chord is the length of the chord line and is the characteristic dimension of the airfoil section.

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The maximum thickness and the location of maximum thickness are expressed as a percentage of the chord.

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For symmetrical airfoils both mean camber line and chord line pass from centre of gravity of the airfoil and they touch at leading and trailing edge of the airfoil.

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The aerodynamic center is the chord wise length about which the pitching moment is independent of the lift coefficient and the angle of attack.

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The center of pressure is the chord wise location about which the pitching moment is zero.

  Camber is one we need to cover. To an aero engineer, an airfoil is cambered or uncambered, and perhaps reflex. Camber is determined by the mean camber line which is a line halfway between the top and surfaces of the airfoil. Let's say you have a drawing of an airfoil. At several different places, locate the spot which is halfway between the top and bottom surface of the airfoil and make a dot. After you have several dots, connect them up. This is the mean camber line.  

If we see airfoils in very broad category there are three types of airfoil,  1.flat bottom  2.semi‐symitrical. 

3. Symitrical.    FLAT BOTTOM AIRFOILS: This is the airfoil you normally see on a trainer. When you look at a trainer wing, one side of the airfoil is flat. You may think this is a thin airfoil, but if you double the thickness, you would have the corresponding symmetrical airfoil which would be pretty thick. Flat bottom airfoils tend to be very speed sensitive. When you add power and increase speed, they create a lot more lift, making the plane climb. A symmetrical airfoil which is equally curved on both sides, on the other hand, generates very extra lift with a power and speed change. The main characteristics of this type of airfoil are • At Low speed high lift possible ideal for trainer planes • Very bad aerobatics characteristics

Semi-symmetrical airfoils are seem on many "second plane" designs. These are sport planes designed for easy flying or for a person's next plane after a trainer. Most scale planes have semi-symmetrical airfoils because most full size planes have this type airfoil. The semisymmetrical airfoil is much better that the average modeler thinks. They are thinner than most corresponding fully symmetrical airfoils and have less drag, and their inverted performance is not all that bad. We rarely fly the tightest outside loop possible, so the average flier should never notice the difference in inverted flight. • Moderate lift at high speed • Good maneuverability • Good aerobatic capability

  Symmetrical airfoils are the most popular for RC modelers. We fly inverted infinitely more than any full scale pilot. Even the top level competition pilots do not fly inverted for minutes at a time. We also do more outside maneuvers than any except maybe the competition pilots. For these reasons alone, inverted flight and outside performance, symmetrical are the most popular once a person graduates from a trainer. Another reason to select a plane with a symmetrical airfoil, an a compelling one, is the plane tends to maintain its attitude with a change in speed. A symmetrical airfoil is generally thicker, and therefore stronger, from a bending or "g" force direction.

• • • •

High speed low lift  Highly maneuverable   Less fatigue to the G effect and more thicker in size  Best aerobatic capabilities 

  For commercially available database of aerofoil is over 1500 , the  best way to find the best aerofoil is to use personal experience  and peoples reviews, for our purpose the clark‐y flat bottom  aerofoil is best as it have been in practice since the 1st airplane  appeared and is widely used in trainer and other highwing stable  airplanes.  “These are the decisions I make before selecting a specific airfoil: 1. Specify desired flight characteristics (airspeed envelope, aerobatic capabilities, etc.). 2. Specify the wing-loading and power loading ranges. Be disciplined about designing to those goals.

3. Decide on a wing planform (chord(s), span, taper and sweep). 4. Determine the most appropriate airfoil family. Because all designs represent numerous compromises you'll have to use the above to decide which characteristics are more important than others. Select a specific airfoil using whatever information you have.” Almost nobody who designs model airplanes would have a clue how to pick an airfoil for their design based on real airfoil data. We learn from experience knowing that the subtleties between one airfoil and another close to the same shape will make a very small difference — one that would only be noticed by an expert pilot. These behaviors are not different enough to cause any problems in your design unless you do something like change a round leading edge to one that is razor sharp. One of the main concerns of fledgling model airplane designers is how to avoid choosing an airfoil having wicked stall characteristics. All airfoils have a stall angle. This is the angle of the chord line of the wing to the direction of flight. When this angle is at or beyond the stall angle the air breaks away from the wing and the wing stops producing lift. In other words, the aircraft isn't flying any more. It's falling from the sky. The leading edge radius takes the lead role in stall characteristics. A sharp (small radius) leading edge typically has a shallow stall angle. That means it will stall sooner than a blunt leading edge. A tip stall occurs when a wing tip stalls before the wing root. In most cases this causes the aircraft to roll over. If the plane is close to the ground it's usually a total loss. There are several ways to avoid or delay tip stalls. •

Build the wing with washout.

Washout simply means the wing is built with a twist so that the wing tips are at a lower angle of incidence than the wing root. Washout also limits aerobatic capabilities. •

Sand the leading edge such that it becomes more blunt toward the tip.

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Avoid high aspect ratio wings having a high taper ratio.(will be discussed latter) Taper ratio is the length of the tip chord divided by the length of the root chord. Aspect ratio is the wing span divided by average wing chord. High aspect ratio wings, such as sailplanes, with high taper ratios tend to be more prone to tip stalls than low aspect ratio wings, such as deltas.

Airfoil Thickness  Airfoil thickness is simply the percentage of the wing chord that the airfoil is deep at it's thickest point. For example a wing having a chord of 15" that has a 10% thick airfoil will be 11/2" (1.5") thick. How thick should the airfoil be? I find that wing thickness is a compromise between speed and lift. A thicker wing has more drag but more lift and is capable of slower flight. Thicker wings also tend to "bounce" around more in the air because they can't cut through it as easily. A thinner wing has less lift but is faster. The shape of the leading edge plays a part in this as well. One other thing to note is that as wings get thicker they also become stronger. If a wing is thick it is easy to build it strong using conventional construction techniques. If the wing is thin then more exotic techniques are required to prevent the wing from breaking in flight.

Of course there are limits to everything. I've seen airfoil listings that are thicker than 30%. The thickest wing I have built was about 20% and I didn't like anything about it in flight. From as far back as I can remember through the 1980's, most sport designs had airfoils in the range of 14% to 16% thick. These airfoils have proven to be safe with few or no bad habits at reasonable wing loadings and can slow down nicely to land. I normally use airfoils from 12% to 18% depending on the airplane. For an extremely fast model I may use an airfoil around 10% thick. In the past several things happened that changed the way we design model airplanes. Pilots came to desire aerobatic models that fly at speeds below Mach 1, four-stroke engines became widely available. A thin airfoil simply isn't going to slow down when the airplane is diving toward the ground even with the engine at idle. More drag was needed, but it had to be smooth, clean (non-turbulent) drag. In other words, airfoil shaped. The easiest way to create this drag was to build a thicker wing which also creates more lift at slower speeds. These models also had to revert to old-time, lightweight construction techniques because lighter planes maneuver better and fly slower. Drag increases exponentially with airspeed. Frontal area, drag and airspeed are inseparable so you need to have a feel for how they work together to decide how thick the wing should be. This is an area where I really can't speak scientifically. I have a good feel for how it works and do pretty well with that knowledge. Carefully match the power plant and propeller to the airframe instead of matching the propeller to the power plant alone. All airplanes have a maximum airspeed at which they will fly smoothly. If the engine has more power available after this speed is reached you won't see more speed, but the model will begin to buffet or worse - something might flutter off.

Airplane 

Engine 

Propeller  Top  Pitch  Speed 

Average weight Stik 

.45 

6"‐7" 

80 MPH

Airfoil 

Flight Characteristics 

15%  symmetrical 

Smooth flying, medium to large  aerobatics, reasonable landing speed.

4"‐5" 

50 MPH

18%  symmetrical 

Slow flight, aerobatics in small area,  very slow landing speed, buffeting at  high speeds and susceptible to gusts  at low speeds. 

.40 

4"‐5" 

16% semi‐ 45 MPH symmetrical 

Hovers in steady winds, very low  flight speeds, minimal aerobatics,  difficult or impossible inverted flight,  landing at a crawl. 

Sport‐Aerobatic Biplane  .60 

6"‐7" 

65 MPH

13% semi‐ symmetrical 

Very aerobatic in a smaller area. Tumbles well.  Requires more "down"  for inverted flight. 

8"‐9" 

100+  MPH