Determining dihedral angle?

[Rktman]

How do you determine the correct dihedral angle for a scratch built model? G. M. Gregorek in his Design Rules for Boost and Rocket /Gliders stated that "The proper dihedral angle is obtained by raising the tips of the wings 1/8 inch for every one inch of wing span". I've seen this repeated in several places but it seems too simplistic and doesn't answer some questions, like:


> If each wing is 5 inches long, your total wingspan will be 10 inches. Does his "rule" mean each wing needs to be raised at the tip by 10 x 1/8" = 1.25"? Or does it mean 5" x 1/8" = 5/8"? The first interpretation doubles the overall dihedral angle for the glider.


> How do you calculate the correct angle for each panel of a polyhedral wing?


I've seen some pretty complex math in various articles and forums, and opinions vary even in the RC/DLG/Catapault/Hand Chuck glider forums. Is there ANY reasonably accurate/acceptable rule of thumb for RGs/BGs that doesn't require a complicated equation?

 

[OverTheTop]

Since he is talking wing span, standard nomenclature would indicate this to be tip to tip. Semi-span is the term when looking at just one wing.

Probably does not matter greatly, but I suspect the greater dihedral angle the less efficient the glide (but more stable). Don't have a feel for how significant the difference is, but I suspect not great.

 

[Ez2cDave]

Is there ANY reasonably accurate/acceptable rule of thumb for RGs/BGs that doesn't require a complicated equation?

Eric,

Use 15 degrees of total dihedral angle.

Dave F.

 

[caveduck]

Hi Eric,

For free flight gliders, the guidelines given in the other Dave's diagram are just fine. Dihedral provides passive roll stability, and for f/f you need enough of that to make the glider track its steady turn. More generally and for RC, it's *very* complicated. For RC, if you're an experienced pilot, you don't really need any passive roll stability; you just need to avoid having it drop a wing and spin. For f/f you mostly don't care about the finer points, because picking good air usually totally outweighs the smaller benefits of making increasingly complicated airframes beyond a certain point. There are interactions with the vertical fin size, wing aspect ratio, and fuselage size (if that's significant). Dihedral tends to cause adverse yaw and reduces overall lift for a given wing area, so competition R/C planes usually don't have very much. Slope soarers often work fine dead flat. Large transport aircraft with big fuselages will have negative dihedral (anhedral) to make turns feasible.

 

[BEC]

Though 14 to 15 degrees of total dihedral (as Dave’s diagrams give) is a great starting point for three-channel RC (rudder/elevator/throttle) airplanes. Ken Willard used a trick in a bunch of his small rudder-only and three channel planes - put a piece of Sig pre-made trailing edge stock between the two wing panels. Instant 14-degree total dihedral (or something quite close enough).

 

[shockie]

Arcsine (H/L) = Angle

Or, if you know the length and the desired angle and need the height the formula is:

Sine(Angle) x L = H