Unfortunately, I couldn’t find a single RC-hobbyist-oriented article on aerodynamics in forums, diaries or blogs, and there were no good references to the subject. Meanwhile, it raises a world of questions, which particularly concern novice hobbyists, and many of those who have dismissed themselves from the ranks of newbies won’t turn a hair to peep into a textbook. Well, I’m here to fix that!)))

I won’t enlarge upon it, because it takes as much as scientific research to highlight all aspects and a whole bunch of abracadabra formulas! All the more so, I won’t obliterate you with scary terms like the Reynolds Number, about which the most curious of you can read later.

Well, let’s make a vow: nothing except the minimum RC-hobbyist-oriented stuff!)))

Forces affecting a flying aircraft.

A flying airplane is affected by a number of aerodynamic forces, but all of them can be represented as a group of four main forces: gravity, lift, propeller thrust and drag (air resistance). Gravity is a constant value except for the fact that it decreases in proportion to fuel consumption. Lift counters the airplane’s weight, and it may exceed the weight or vice versa, depending on the amount of energy that generates propulsion. Thrust is countered by drag.

During a forward and horizontal flight, these forces interbalance. Propeller thrust is equal to drag, and lift is equal to the plane’s weight. No other proportion between these four forces allows a direct and horizontal flight. Any change in at least one of these forces immediately affects the flight. Lift exceeding gravity will result in the plane going up. A reversed balance would cause the plane to lose altitude. Disruption of the balance would pull the trajectory in the direction of the prevailing force.

About wings.

Wingspan is the distance between the planes that lie parallel to the plane of symmetry of the wing and adjacent points. W. s. is an important geometrical parameter of all types of aircraft, which exerts influence on its aerodynamics and flight performance, as well as one of the key dimensional characteristics of aircraft.

Wing aspect ratio is the ratio of the wingspan to mean aerodynamic chord (MAC). For a non-quadrangular wing, the aspect ratio is defined as the square of the wingspan divided by the surface square. Assuming a quadrangular wing, we get a simpler formula: the aspect ratio = wingspan/chord, i. e. with a 10m span and 1m chord, the aspect ratio = 10. The higher the aspect ratio, the lower the wing induced drag caused by airflow from the lower wing surface to the upper surface, which generates wingtip vortexes. Based on first approximation, it can be assumed that the size of such vortexes equals the chord, and an increase in the wingspan results in a decrease in vortex size in relation to the wingspan. Naturally, lower induced drag results in lower general resistance and higher aerodynamic characteristics of the whole system. This tempts designers to maximally increase aspect ratio. This is where the problem occurs, since high wing aspect ratios necessitate an increase in the wing’s strength and toughness and put the wing mass out of proportion. From the standpoint of aerodynamics, the most effective wing type is one that provides a better lift with a weaker drag. The notion of a wing’s aerodynamic quality was introduced as an evaluation criterion for this quality.

A wing’s aerodynamic quality is actually its lift-to-drag ratio. Ellipsoid wings are known to have the best aerodynamic quality, but they are very difficult to make and therefore are rarely used. Straight wings are less efficient, yet much easier to make. Tapered wings are aerodynamically better, yet harder to make. Arrow-shaped and triangular delta wings are inferior to tapered and straight wings at a subsonic speed, but gain big advantages at transonic and supersonic speeds. For this reason, transonic and supersonic aircraft feature these types of wings.

Ellipsoid wings boast top aerodynamic quality, a minimal drag and a maximal lift. Unfortunately, this type is rarely used because of its structural complexity, low technological effectiveness and poor stall characteristics. However, the drag level at high angles of attack with wings of different types is always evaluated with reference to the ellipsoid type. The brightest example of the use of such wings is the Supermarine Spitfire, a British fighter aircraft.

Straight wings have the highest drag at high angles of attack. However, such wings have a very simple structure, show high technological effectiveness and great stall characteristics.

Tapered wings are close to ellipsoid ones in terms of drag. This type was widely used in production airplanes. It shows lower technological effectiveness than a the straight wing type. It takes a few design tricks to achieve acceptable stall characteristics. However, tapered wings, if built properly, ensure minimal wing mass, all other conditions being equal. Early Bf-109 fighters featured tapered wings with straight wingtips:

Combined wings. In most cases, these wings consist of several trapeziums. They have lower technological efficiency than straight wings. In order to design a combined wing effectively, it has to be put to scores of airflow tests, and the parametrical benefit amounts to but a tiny percentage of that of a tapered wing.

Wing sweep is the angle of a wing’s deflection from the normal toward the aircraft axis within the aircraft’s reference plane. The tailward vector is referred to as positive. There is the leading-edge sweep, the trailing edge sweep and the quarter-chord sweep.

A forward-swept wing is referred to as having a negative sweep.

  • Better sensitivity at low speed;
  • Higher aerodynamic efficiency in all flight modes;
  • The swept-forward wing configuration provides optimal air-load distribution over the wing and the horizontal tail.
  • Forward-swept wings are highly prone to aerodynamic divergence (loss of static stability) upon reaching a certain speed and angles of attack.
  • Requires special kinds of materials and technologies to ensure sufficient toughness.
Su-47 Berkut is a jet fighter with forward-swept wings.

A Czech LET L-13 Blanik glider with forward-swept wings

Wing loading is a ratio between the weight of an aircraft and the wing area expressed as kg/m² (for RC models – g/dm²). Wing loading defines takeoff/landing speed, maneuverability and stall characteristics. Simply put, the smaller the loading, the lower the speed and therefore the smaller engine size is required.
Mean aerodynamic chord (MAC) is the chord of a straight wing, whose area, total aerodynamic force and center-of-pressure position, is equal to those of the wing, with equal angles of attack. Simply, the chord is the distance between the profile’s two outermost points.
The MAC value and coordinates for each aircraft are calculated at the engineering stage and are specified in the technical review. If the MAC value and location are not specified, they can be calculated.
For a quadrangular wing, the MAC is equal to the wing’s chord.
A tapered wing’s MAC is calculated by way of geometrical construction. To do so, a wing platform is drafted (at a certain scale). Part of the root chord’s extension is marked off at the length of the tip chord, and part of the tip chord’s extension (forward) is marked off at the length of the root chord. The ends of the marked-off lengths are connected with a straight line. Then a mean line of the wing profile is run across the center points of the tip chord and the root chord. The mean aerodynamic chord (MAC) runs across the intersection of these two lines.

Given the value and position of the MAC in the aircraft’s body as a baseline, it is possible to define the aircraft’s gravity center, which is expressed as a percentage of the MAC’s length.
An aircraft’s weight is comprised of the weight of an empty aircraft (glider, engines, fixed equipment), fuel’s weight, etc. The total force inflicted by the weight of all parts of the aircraft will pass through a certain point inside the aircraft, and this point is called the gravity center.
The distance between the gravity center and the MAC’s beginning, expressed as a percentage of MAC’s length, is called plane balance.

The shape of a wing in cross-section is called an airfoil. Airfoil exerts great influence on all aerodynamic characteristics of a wing in all flight modes. Therefore, choosing an appropriate airfoil is a paramount task. In fact, today nobody chooses an airfoil out of a group of existing ones, except do-it-yourself amateur guys.

An airfoil is one of the most crucial elements of any aircraft, as is a wing and all its integral elements. A group of airfoil sections makes up a whole wing, and they may be different across the wingspan. The types of airfoil used in an airplane define its designation and flight qualities. There are a handful of airfoil types, although they all have the shape of a drop. Just like a stretched-out horizontal drop. However, the outline is usually far from perfect, since each type has its own curvature of the upper and lower surfaces, and they differ in depth too. The classical form has an almost flat bottom and curved top surface, as it is required by a certain law of aerodynamics. This is true of so called non-symmetrical airfoils, and there are symmetrical ones, whose top and bottom surfaces have the same curvature.

Airfoils have been designed and engineered since the very inception of the aircraft. This work is done at specialized institutions, like the Zhukovsky Central Institute of Aerodynamics – the brightest example of such institutions in Russia. In the USA, this work is done at the NASA Langley Research Center.

This is the translated version. You can read the original Russian article here.