Design Ratios...

[Arbo]

Anyone know where I can find information on general design info, in terms of ailerons should be x% of chord, tailplane should be y% of wing area, etc? I seem to recall seeing it before, but can't find anything now.

Thanks
Paul

[Ollie]

Ailerons should be about 20 to 25% of the chord in width.

Tails sizes are also dependent on the tail moment arm length. Some guidelines from Dr. Mark Drela are:

"Ch = (A_hori/A_wing) * (tail_arm/avg_wing_chord)
Cv = (A_vert/A_wing) * (tail_arm/avg_wing_span )

A well-sized tail will be in the range...
Ch = 0.35 - 0.50
Cv = 0.02 - 0.035
If the Ch and/or Cv are below the minimum values, the handling will suffer."

[Sail 'n Soar]

Ailerons should be about 20 to 25% of the chord in width.

Actually, from my understanding, the 20 to 25% cord is for conventional ailerons. Another way to look at this is for the ailerons to be about 10% of the wing area. With that, full span strip ailerons would be about 10% of the cord, conventional arrangement ~ 40% span ailerons would be about 25% of the cord, etc.

[Ollie]

Aileron area near the wing root contributes very little to roll control compared to aileron area near the wing tip. This is because of the difference in moment arm length to the centerline. The leverage near the wing root is low and the leverage near the wing tip is high.

Obviously narrow strip ailerons can be made to work but they have several disadvantages compared to wider outboard ailerons. Three of the disadvantages are higher drag for a given area, poor effectiveness near the wing root and poor torsional stiffness. Perhaps this is why they are seldom seen on full scale aircraft.

[Sail 'n Soar]

Originally posted by Ollie
Aileron area near the wing root contributes very little to roll control compared to aileron area near the wing tip. This is because of the difference in moment arm length to the centerline. The leverage near the wing root is low and the leverage near the wing tip is high.

Obviously narrow strip ailerons can be made to work but they have several disadvantages compared to wider outboard ailerons. Three of the disadvantages are higher drag for a given area, poor effectiveness near the wing root and poor torsional stiffness. Perhaps this is why they are seldom seen on full scale aircraft.

All true. Ailerons in general are a compromise. You mention the disadvantages of strip ailerons. Their simplicty is a relative advantage. Nevertheless, a good rule of thumb for aileron sizing is still ~10% of total wing area, ignoring the 3-D and other extreme maneuverability designs.

[Ollie]

Compromise of conflicting objectives is the essence of the design process. The goodness of a particular compromise can only be fairly judged in the context of the purpose and priorities of the designer.

If simplicity of construction and linkage to a single aileron servo are very high priorities, then narrow strip ailerons are a good compromise.

[j8m8l]

Ollie

Could you explain the variables in that tail area formula?

Thanks
Josh

[Ollie]

Josh,

Ch is the horizontal tail volume coefficient.
A_horiz is the area of the horizontal tail.
A_wing is the wing area.
tail_arm is the distance from 25% of the wing mean aerodynamic chord to 25% of the horizontal tail mean aerodynamic chord. To find the mean aerodynamic chord (MAC) of a flying surface see:
http://www.palosrc.com/instructors_corner.htm
avg_wing_chord is the mean aerodynamic chord of the wing. If the planform is not highly tapered or swept, then the average wing chord can be used with an acceptably small error.

The horizontal tail volume coefficient (Ch) is a measure of the effectiveness of the horizontal tail for stability and control. The term volume comes from the fact that it is a relative area times a relative length (area times length = volume). The larger the horizontal tail volume coefficient, the more aft the CG for a given amount of pitch stability.

Cv is the vertical tail volume coefficient.
A_vert is the area of the vertical tail.
Avg_wing_span is the wing span. In the case of a biplane, it is the average of the top and bottom wing spans.
The tail moment arm length is measured to the MAC of the vertical tail in the vertical tail volume coefficent formula.

When comparing designs with the same tail volume coefficients but different tail moment arm lengths, the case with the longer tail moment arm will damp oscillations in pitch and yaw better than the short tail moment arm case. The damping in pitch and yaw increases as the square of the arm length but linearly with tail area. In other words, long tail moment arm designs have groovie handling characteristics.

Another advantage of a long tail moment arm is that it can reduce tail drag by allowing a smaller tail. A disadvantage of a long tail moment arm is that the extra fuselage structure, control linkage weight, etc. require more nose weight to balance at the desired CG location.

Picking a tail volume coefficient from within the allowable range is a matter of choosing a compromise between conflicting objectives based on the purpose of the aircraft and the priorities of the designer.

[Sparky Paul]

Ollie says:
"Picking a [airplane design component] from within the allowable range is a matter of choosing a compromise between conflicting objectives based on the purpose of the aircraft and the priorities of the designer."
.
And that's THE aircraft design criteria. There are no hard and fast "can only be this way" rules.