Compliance Suggestion DETERMINING FLIGHT ENVELOPE A sample basic flight envelope is shown in fig. 1-II. This envelope has been constructed for a high performance sailplane of aerodynamically “clean” design, having a full cantilever wing, with basic design values as follows: W/S =S = 3.5 lbs. per sq. ft. e (weight of wing) = 1.5 lbs. per sq. ft. a. In accordance with table 1-1, a value of K =61 was used so that the minimum design gliding speed which was used was 61 Vs = 114 m.p.h. (Vemn). The corresponding placard "never exceed” speed would be .9x114 = 103 m.p.h. (From fig. 1-1.) In this particular case, however, it is assumed by the designer that a somewhat higher placard "never exceed” speed is desirable so a design V, of 125 m.p.h. is selected making the corresponding placard speed 112 m.p.h. (.9x125). b. Plot the following equation to obtain line 1 of the positive portion of the V-n diagram. (See fig. 1-11.) V?CL max where n=maximum possible positive limit wing n = 391 s load factor at the speed V (m.p.h.). c. Draw a vertical line through the velocity corresponding to V, (line 2 of fig. 1-II). d. Plot the following equation to obtain line 3 of the negative portion of the V-n diagram (see fig. 1-II). V2 where n =maximum possible negative unit wing load factor at the speed V (m-p.h.). This is based on a Ci max. (dynamic) of 1.0 (negative). e. When applying the recommendations specified above, the following procedure should be followed: 1. Draw a straight line (line 4 of fig. 1-III) from the point where V =0, and n=1 to the point where V=V, and n = the load factor specified in item 6 of table 1-1. This will intersect line 2 at point E. 2. Draw a horizontal line (line 5 of fig. 1-11) through the greatest value of n specified in items 5 and 7 of table 1-1. This will intersect line 1 at point C; and it will intersect line 4 at point D, providing the positive gust load factor is greater than the maneuvering load factor. If the maneuvering load factor is greater than the positive gust load factor, line 5 will intersect witn line 2. Example ITEM 5. The specified maneuver load factor = 5.33 S =3.5) so for this example: ...685 x 24 x 125 x 4.8 n=1+2 O=1+4.90 = 5.90 575 x 3.5 n = 391 s plying the on a Cz mase speed V (m_007 n=1_ The greatest positive load factor of items 5 and 7 above is 5.33 and it therefore determines line 5 of the basic flight envelope of fig. 1-II. It should be noted that the positive portion of the basic flight envelope is represented by the points OCDEF of fig. 1-II. f: Draw a straight line (line 6 of fig. 1-II) from the point where V = 0 and n= +1 to the point where V=V, and n = the load factos specified in item 9 of table 1-1. This will intersect line 2 at point G. g. Draw a horizontal straight line (line 7 of fig. 1-II) through the negative value of n specified in item 8 of table 1-1. This will intersect line 3 at point J and will intersect line 6 at point H providing the negative gust load factor is greater than the maneuvering load factor. If the negative maneuvering load factor is greater than the negative gust factor, line 7 will intersect with line 2. Example ITEM 8. The specified maneuver load factor = -2.67 follows: -=1-4.90 = 3.90 575 x 3.5 It should be noted that the negative portion of the basic flight envelope is represented by the points FGHJO of fig. 1-11. h. The glider need not be investigated for gust loads at speeds higher than V,. For Vo higher than Vo, in high performance gliders only, but not to exceed 1.2 V, the right hand corner of the V-n diagram need only be investigated for the maneuvering load factors. This portion of the V-n diagram is established as follows: 1. Draw a vertical line at Vo max. (not to exceed 1.2 V.) (line 8 of fig. 1-II). 2. Draw a straight line (line 9 of fig. 1-II) from point E to intersect line 8 at n = 5.33 (point K) (item 5 of table 1-I). 3. Draw a straight line (line 10 of fig. 1-II) from point G to inter sect line 8 at n = 2.67 (point L) (item 8 of table 1-1). This additional portion of the V-n envelope is represented by the points EKLG. i. In general, an investigation of the following specific basic flight conditions, which correspond to points on the basic flight envelope, will insure satisfactory coverage of the critical loading conditions. 1. Condition 1.—(Positive High Angle of Attack.) This condition corresponds to point C on the basic flight envelope. The aerodynamic characteristics C, C.P. (or Čm), and Co to be used in the investigation should be determined as follows: where ny = Wing load factor corresp. to pt. C. 9,=Dynamic pressure corresp. to the velocity Vi, which in turn corresp. to point Č. C.P. or Cm = value corresp. Ce=value corresp. to to Cn, as deter Cn, as obtained mined from the air from the airfoil foil characteristics characteristics curves. curves. 2. Condition 11.—(Negative High Angle of Attack.) This condition corresponds to point J on the basic flight envelope. (6) Co= value corresp. to Cny (may be assumed equal to zero if positive). - CORRECTED MOMENT COEFFICIENT 4 -.16 - -.06 -.08 -.10 -.12 -.14 Cu - MOMENT COEFFICIENT OF BASIC WING Figure 1-III. Corrected moment coefficient for condition III 2. 3. Condition III.—(Positive Low Angle of Attack.) This condition corresponds to point E on the basic flight envelope. (a) Cy=1118 9111 (6) 4111 = 40 Co = value corresp. to Onn (may be assumed equal to zero if positive). (c) C.P. or Cm = value corresp. to Cnu 4. Condition III,.—(Modified Positive Low Angle of Attack.) In order to cover the effects of limited use of the ailerons at V, on the wings and wing bracing, such structure should be investigated for the following: (a) Cniui, = Cnjil corresp. to Cnur: Cm' need only be applied to that the remainder of the span. 5. Condition IV.—(Negative Low angle of Attack.) This condition corresponds to point G on the basic flight envelope. The aerodynamic characteristics should be determined as follows: (6) Co = value corresp. to Cny (may be equal to zero if positive). (Flaps or Auxiliary Devices in Operation) High-lift devices.—When flaps or other auxilliary high-lift devices are installed on the wings, suitable provisions should be made to account for their use in flight at the design flap speed Vs. Minimum values of the design flap speed are specified in table 1-1. These provisions should be based on the intended use of such devices. Compliance Suggestion INSTALLATIONS ON INTERNALLY BRACED WINGS For internally braced wings, the effects of trailing edge flaps on the wing structure as a whole, can, in general, be satisfactorily accounted for by modifying, when necessary, the basic flight conditions in the following manner: The average value of Cm' used in design Conditions III and IV should equal or exceed the quantity: Cm, X |