Aerodynamics


Aerodynamics is the study of air in motion and is used to design airplanes.

To get started, you can do the Sailplane Quest. In this activity you will build a hand-thrown glider and learn the basics of airfoil shapes.

Next you can try Cargo Airplane LTD.. In this activity you will design and construct a wing to be used on a fuselage of stock design. The airplane is flown on the Kelvin Power Pole to see who can fly the heaviest airplane.

When you get good at that, enter the Maryland Engineering Challenges Electric Powered Cargo Airplane Challenge. It is like Cargo Airplane LTD. but the planes are bigger and you only get credit for cargo weight.

Or you can try the Historic Aircraft Challenge where you design and build a model of an historic aircraft.

The principles of aerodynamics are also used in other power pole airplane challenges including the Maximum Aircraft Load challege where you try to fly with the highest wind loading and the Maximum Seale challenge where you try and fly with the highest coefficient of lift.

Model rockets are also a study in aerodynamics.


Lift v. Weight, Thrust v. Drag

Factors Affecting Lift

Coefficient of Lift

The Coefficient of Lift is used to provide efficient scaling from wind tunnel to airplane.

Lift /( V2 S) = CL

Similarly for Drag

Drag /( V2 S) = CD

Wing Geometry

Span (b) is the distance from tip to tip.

Chord (c) is the distance from nose to tail.

For Rectangle

Area (S) = b · c

Aspect Ratio (A) = b / c

For Ellipse

S = p · (b/2) · (c/2) OR (p/4) · b · c

A = b2 / S

For Trapezoid

S = b · (croot + ctip) / 2

A = b2 / S

Taper Ratio = ctip / croot

For Other Planform Shapes

Use geometry to find the area (S).

A = b2 / S

Finding Lift and Drag

CL2D is the theoretical CL per degree for an infinite airfoil and is 2 / 90 or 0.11

Then CL = CL2D / {1 + (2/A)}

And CD = CL2 / ( A)

Then L = CL V2 S

And D = CD V2 S

See properties of air for AIR.

Planform Selection

Stall

As the angle of attack is increased beyond a certain point, the flow can no longer remain attached to the top surface. The flow becomes turbulent and lift is lost. This is the stall point. Wings will stall at a CL of between 1.1 and 1.4. With a lower aspect ratio, this will occur at higher angles of attack.

Selecting a Cross-Section

Real Cross Sections have drag at zero angle of attack. Cross-sections with midlines have lift at zero angle of attack.

Control Surfaces

Balance

Balance point is at {SWING Distance of point from origin + SHS Distance of point from origin} / STOTAL.

STEPS IN DESIGNING A MODEL PLANE

  1. Determine available thrust and estimate possible speed.
  2. Decide on Trapezoidal or Elliptical wing shape.
  3. Choose a design angle of attack or CL.
  4. Shape wing to maximize first estimate of payload from available thrust.
  5. For Trapezoid
  1. Decide on thickness needed for structural strength.
  2. Decide on Symmetrical or Asymmetrical cross-section. Choose a cross-section (and mid-line).
  3. Second estimate of payload using Cd that includes fuselage and appendages.
  4. Refine planform.
  5. Build and balance.
  6. Test and adjust.

Adjustments Available Once Built

Swept Back Angle

cord Swept Back Angle Taper Ratio
-20 1.0
0 0.45
10 0.3
20 0.2
30 0.1

 

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