Airfield Models Formulas use with Flying Model Aircraft

Converting Dihedral from an Angle to a Measurement

May 05, 2015



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Airfield Models (http://www.airfieldmodels.com/)Calculating Wing Dihedral

Dihedral is an angle raising the centerline of the wing tip above the centerline of the wing root.  It can also be expressed as a measurement of length.

Wings having the tip centerline below the root centerline have anhedral.

Note that many plans show the dihedral measured from the bottom of the root rib to the bottom of the tip rib.  This is incorrect.  Dihedral is correctly measured from centerline to centerline.

Follow the plan regardless of this.  The designer should have worked out the correct measurement even if the plan does not express it correctly.  In other words, block up the wing at the location shown on the plan by the distance shown on the plan.

Why Dihedral is Important

  • The dihedral of a wing determines the amount of self-correction capability an aircraft has in the roll axis.

    More dihedral allows more self-correction at the expense of less lift, more drag and less axial rolls.  Nevertheless, dihedral is sometimes necessary and is especially important for trainer aircraft.

  • Aircraft designs having no ailerons need dihedral in order to turn using only the rudder.

    An aircraft that has no ailerons and no dihedral will tend to yaw and skid through rudder turns.  Some dihedral is always required for rudder only aircraft models.

  • It is sometimes necessary to change the dihedral of an aerobatic design to rid the aircraft of undesirable traits such as rolling when rudder is applied (roll coupling).

 
 

Converting the Dihedral Angle to a Measurement

In most cases you can disregard this section because the amount of dihedral is usually given as a ready-to-use measurement.  However, if the dihedral is given as an angle you will need to convert the angle into a number you can use.

This article discusses how to convert angles to measurements.  It does not cover how to determine the correct dihedral angle for your design.

In these examples, the Sine function is the most appropriate.  See the Trigonometry page for more details.

The Sine of an angle = the length of the opposite side of the triangle divided by the hypotenuse of the triangle.  You will either need a table that gives the values of Sine for various angles or a calculator that can determine the Sine of an angle.

For example, let's say you have a 60" wing that has 5 of dihedral.  The dihedral is per wing panel, not the total amount.  In other words, the included angle between the wing panels is 170 (a straight wing having no dihedral is 180).

Sketch a small diagram to help you out.  The hypotenuse of the triangle is half the wingspan (30" in this case).

Use Trigonometry to convert an angle to a measurement.

Sine = Dihedral Half Wing Span

For this example:

Wingspan = 60" (note that we will be using half the wingspan)
Dihedral angle = 5
Dihedral measurement = Unknown

1)  find the Sine of the angle:

= 5

Sine 5 = 0.087156

2)  Plug in the answer above to find the opposite side of the triangle.

0.087156 = Dihedral 30"

Dihedral = 30" x 0.087156

Dihedral = 2.6" (under each tip)

 
 

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Calculating the Aspect Ratio of a Wing or Flying Surface
Calculating Wing Area for Straight, Tapered or Delta Wings

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Copyright 2003 Paul K. Johnson