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Aerodynamics
On most models, the wing is the source of lift. There can be any number or arrangement of wings, but a single
wing is the most efficient. The size and shape of a wing determines performance. A large span with a relatively
short chord (called "high aspect ratio" or AR) is the most efficient (low induced drag - drag from lift, and less
power required), but is not desireable in an aerobatic plane. A short span with a long chord (low AR) is less
efficient (more induced drag, more power required). A trade off between the 2 can result in a great aerobatic
plane. A forward swept wing can have a lower stall speed than the same area in atraight wing. A rear swept wing
can have a higher top speed than the same area in atraight wing. but that drag reduction happens at speeds higher
than models operate at.
Airfoil
A wing's cross section is called the airfoil. The shape of the airfoil also determines performance characteristics. There are a thousand different airfoil sections out there, but they generally R/C airfoils fit into four categories.
An undercambered airfoil is typical of early WWI type planes. It is a high lift, high drag, low speed airfoil. The undercamber is difficult to cover.
A flat bottom airfoil is generally used in a gentle flying plane. It provides high lift at low speed, limited top speed (for a given engine) with its relatively high drag and easy stall characteristics. This airfoil is generally found on trainers, but can be used on aerobatic planes, although not the best choice.
A semi-symmetrical airfoil is used in an aerobatic plane that is intended to spend most of it's time upright. This airfoil has slightly less lift than a flat bottom and will land at a higher speed for the same weight and area. This is probably the best all around airfoil style for R/C planes.
A fully symmetrical airfoil is used in an aerobatic plane that is intended to spend a lot of time inverted. This airfoil
has slightly less lift than a semi-symmetrical airfoil and will land at a higher speed for the same weight and area.
Less down elevator will be required for inverted flight than a semi-symmetrical.
Stability and Control
Most R/C planes are conventional in layout. The engine is in the front, the wing is forward on the fuselage, and the stabalizers are on the rear. The aircraft is balanced so that approximately 1/4 to 1/3 of the area of the wing is in front of the CG. The farther forward, the more stable (a trainer), the farther aft, the more unstable (aerobatic, but a plane can also be faster with an aft CG). A more stable plane needs bigger control surfaces and more servo torque to get the same control response if it was less stable, ie aft CG.
When a wing produces lift, it is unevenly distributed along the chord. The resulting force is called center of lift, or
the aerodynamic center, or AC. The AC moves forward and aft as the angle of attack is changed. The horizontal
stabilizer serves to keep the wing stable. If the center of gravity, or CG, is forward of the AC, a downforce is
required to keep the wing stable. This is the optimum for stability, since a nose up disturbance
Aerodynamic Concerns
1. Seal all control surface gaps. This will reduce drag and improve control effectiveness.
2. Use an airfoil section tail on larger planes. The result will be less drag and less control deflection reqyired.
3. Tape hatches shut if they are not required to be accessed before each flight.
4. Use gear doors on retractable gear planes, wheel pants on fixed gear. Not a big issue, but it can help.
Fuselage Concerns
The fuselage serves to house the radio and connects the wing and tail. The fuselage shape adds to drag and can
govern performance. A thin, tall fuselage can help an aerobatic plane perform the famous "knife edge loop",
where as a fat, squat fuselaage can help generate lift like a lifting body.
Aerodynamics of Combat
Parts of this document were taken from "Model Aircraft Aerodynamics" by Martin Simons and
Aerodynamics and the Combat Model by Jeff Weiss for Pro Scale models
There are laws of motion that effect aerodynamic theory. The first of these is Equilibrium. If a body is in
equilibrium, then its tend to remain so. A model standing still on a table is in equilibrium unless something disturbs
it by accelerating it in some direction. A moving model flying straight and level in calm air, at a constant speed and
not turning is in a balanced state , or equilibrium and will have a tendency to stay that way if it is trimmed properly.
The same could be said for a model that is climbing or diving at a constant speed. Equilibrium is a condition of
steady motion or rest, in contrast to states of unsteady motion involving acceleration negative acceleration or
deceleration.
Changing the speed or direction of flight in any way, disturbs equilibrium. It order to cause this condition a force
variation is required to bring about an acceleration in the appropriate sense. The second law of motion requires
that the strength of force required for any given acceleration depends on the mass of the model. Mass is not the
same as weight. Weight is the force exerted by mass. A model of large mass would require greater force to disturb
equilibrium to any given extent than that of a small model. But the larger mass also requires a larger force to
accelerate it to flying speed, and more force to change the flight attitude. Whenever there is a disturbance of
equilibrium, an acceleration or deceleration, change in direction, this mass, called inertia, opposes the change.
When turning a model, inertia tends to cause the model to return to straight flight. Turning flight is a form of lateral
inertia. Pulling out of a dive involves a change of direction in the vertical plane, and mass resists this change. This
Inertia tends to cause the dive to continue.
The third law of motion establishes that action and reaction are equal and opposite. If a model is at equilibrium
then the action and reaction forces are set as equal. Any imbalance of these forces will produce an acceleration in
a given direction. When a aircraft starts it take off roll the plane is not at equilibrium, that is to say that thrust is
greater than drag. The model will continue to accelerate in the forward direction until the drag of the wheels on the
ground or the aerodynamic drag on the aircraft in flight causes it to fly at a constant speed. The thrust is the action
force and the drag is the reaction force. In level flight the weight force acting vertically in the downward direction
is opposed by the vertical upward reaction generated by the lift of the wing and other possible surfaces. If the
upward reaction against the weight fails, or is reduced, the model accelerates downward. To stop the acceleration
it is necessary to restore an upward reaction to equal the weight. This brings equilibrium but will not stop the
descent. To do this an additional force must bring about deceleration or acceleration in the positive direction, up.
All such acceleration and deceleration will be resisted by the mass of the model in the form of inertia.
A model under power and in level flight is under the influence of many forces acting on every part of it. These
forces may all be added and sorted out into four general forces arranged in action - reaction pairs. These pairs are
Lift (action) - Weight (reaction), Thrust (action) - Drag (reaction). In order for a model to achieve equilibrium, these
actions and reaction forces must be equal. That is to say we must have a resolution of forces to gain equilibrium
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