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of each not exceed station wherekes, or spoibetut their

Free play of ailerons.—The total free play at the aileron edge of each aileron, when the other aileron is clamped to the wing should not exceed 2.5 percent of the aileron chord aft of the hinge line at the station where the free play is measured. Frequency of ailerons, dive brakes, or spoilers.—The frequency of the ailerons rotating symmetrically about their hinge lines (both ailerons moving together) should be at least 1.5 times the frequency of the fundamental symmetrical wing bending frequency. The frequency of rotation of dive brakes and/or wing spoilers about their hinge line, for any position from closed to open, should be at least 2 times the wing torsion frequency.

EMPENNAGE FLUTTER CRITERIA The following empennage flutter criteria are applicable only if the empennage configuration is conventional and consists of a single vertical fin and rudder and a fixed horizontal stabilizer and elevators.

1. Elevator balance. Each elevator should be dynamically balanced to preclude the parallel axis flutter (fuselage vertical bending-symmetric elevator rotation)as well as perpendicular axis flutter (fuselage torsion-antisymmetric elevator rotation). If, however, the antisymmetric elevator frequency is greater than 1.5 times the fuselage torsional frequency the perpendicular axis criterion need not apply. a. Parallel axis criterion.—The balance parameter y as obtained

from fig. 2-XVII should not be exceeded. In fig. 2-XVII the balance parameter y and the flutter speed parameter V, are defined as:

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Where: S8 = Elevator static balance about hinge line (ft.-lbs.)

I = Elevator mass moment of inertia about the hinge

line (lb.-ft.2) b = Semichord of the horizontal tail measured at the

midspan station (ft.) Vo=(m.p.h.)

fr = Fuselage vertical bending frequency (c.p.m.) b. Perpendicular axis criterion.—Need not apply if the antisym

metric elevator frequency is greater than 1.5 times the fuselage torsion frequency. For each elevator the balance parameter , as obtained from fig. 2–XVIII should not be exceeded. In fig. 2-XVIII the balance parameter 1 and the flutter speed parameter V, are defined as:

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Where: S =Semispan of horizontal tail (ft.)

b = Semichord of horizontal tail at midspan station (ft.) K = Elevator product of inertia referred to stabilizer

center line and elevator hinge line (lb.-ft-2) I = Elevator mass moment of inertia about the elevator

hinge (lb.-ft.-)

fa = Fuselage torsional frequency (c.p.m.) 2. Rudder balance.—The value of y as obtained from fig. 2–XVII and the value , as obtained from fig. 2-XVIII should not be exceeded; where in figs. 2–XVII and 2-XVIII, 6S, 6K and:

?r=T =SI S = Distance from fuselage torsion axis to tip of fin (ft.) b = Semichord of vertical tail measured at the 70-percent span

position (ft.) K = Product of inertia of rudder referred to the fuselage torsion

axis and the rudder hinge line (lb.-ft.) fa =Fuselage torsional frequency (c.p.m.) fo = Fuselage side bending frequency (c.p.m.) Sp =Rudder static balance about hinge line (lb.-ft.) I =Mass moment of inertia of the rudder about hinge line

(lb.-ft.).

TAB FLUTTER CRITERIA All reversible tabs should be 100 percent statically mass balanced about the tab hinge line. Tabs are considered to be irreversible and need not be mass balanced if they meet the following criteria:

1. For any position of the control surface and tab, no appreciable

deflection of the tab can be produced by means of a moment applied directly to the tåb, when the control surface is held in

a fixed position and the pilots tab controls are restrained. 2. The total free play at the tab trailing edge should be less than

2.5 percent of the tab chord aft of the hinge line, at the station

where the play is measured.
3. The tab natural frequency should be equal to or exceed the
value given by the lower of the following two criteria

(a) fı = 48V S. c.p.m.
or
(b) f =2,000 c.p.m. for gliders having a design speed of

less than 200 m.p.h. Thus for a glider with a design speed less than 200 m.p.h. if (a) above gave a value in excess of 2,000 c.p.m. it would only be necessary to show a frequency of 2,000 c.p.m. for the frequency criterion.

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Figure 2-XVIII. Elevator and rudder balance criteria perpendicular axes (Fuselage torsion-control surface).

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.14 .16 .18 FLUTTER SPEED FACTOR, VE=

Figure 2-XVII. Elevator and rudder balance criteria parallel axes (fuselage bending-control surface).

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