THE ANGLE OF INCIDENCE

The problem is to set the fuselage at such an angle to the flight path of the model that it creates the least possible drag. For a racer this will depend on die angle of attack of the wing when it is flying at maximum speed. For a soaring sailplane the angle of attack when flying at maximum Cl1 VCd is what counts, though the performance gain caused by

1

2

3

4

5

6

7

8

Cl

V*

Vm/sec

Assumed

Induced

Profile

Total

(4) X V2

Parasite Cd

Cdi

Cd

cL

0.4

120.12

10.96

0.010

.00679

.0120

.02879

1.2

0.6

80.08

8.95

0.010

.01528

.0098

.03508

.8

0.8

60.06

7.75

0.010

.02716

.0101

.04730

.6

1.0

48.05

6.93

0.010

.04244

.0107

.06314

.48

1.2

40.04

6.33

0.010

.06115

.0113

.08245

.40

1.4

34.32

5.86

0.010

.08318

.0125

.10568

.34

1.5

32.03

5.66

0.010

.09549

.0137

.11919

.32

1.6

30.03

5.48

0.010

.10864

.0149

.13354

.30

1.675

28.69

5.36

0.010

.11925

.0162

.14545

.29

FINAL DRAG BUDGET, Kg. FORCE

PARASITE INDUCED PROFILE

TOTAL

9

10

11

12

13

14

15

(5) X V2 (6) X V* (7) X

V* (8) X fcpS (9) X WpS (10) X HpS

(11) X ttpS

0.82

1.44

3.46

0.735

0.502

0.882

2.119

1.22

0.79

2.83

0.490

0.747

0.484

1.721

1.63

0.61

2.84

0.368

0.998

0.374

1.740

2.04

0.51

3.03

0.294

1.250

0.314

1.858

2.45

0.45

3.30

0.245

1.501

0.276

2.022

2.86

0.43

3.63

0.208

1.752

0.263

2.223

3.06

0.44

3.82

0.196

1.874

0.270

2.340

3.26

0.45

4.01

0.184

1.997

0.276

2.457

3.42

0.47

4.17

0.178

2.095

0.288

2.561

saving parasite drag at this low speed will be very small. For a ‘penetration’ sailplane, correct fuselage alignment at high speed is important, less so at low speed.

For a racer, ensure that the wing camber is such that the wing profile minimum drag is at the average operational Cl for speeds at which the model will fly (see Chapter 6). From wind tunnel test results if available, or if not, by assuming a lift curve slope of 0.11 ci per degree (from zero lift angle of attack) find the angle of attack of the wing profile at the operating c[. This angle is for a wing of‘infinite’ aspect ratio. It must be corrected for downwash effects as shown below.

For a penetrating sailplane the section angle of attack chosen will depend on the extent of the aerofoil’s low drag range or*bucket’. Flight at a lower angle of attack than this will bring a marked deterioration (steepening) of the glide due to increased profile drag. Wind tunnel results at the Re appropriate to flight at this Cl and airspeed allow this to be estimated, or, if NACA 6 series aerofoils are used, the extent of the low drag range can be judged roughly from the third digit – e. g. 643618 gives a low drag range of 0.3 c| above and below the ideal cl of 0.6, hence the low drag bucket ends at cl 0.3. From this it is possible to estimate the angle of attack (infinite a. r.).

For a soarer, the operating Cl must be found by the methods given in this appendix and the angle of attack found from the wind tunnel results.

Knowing the angle of attack of the wing at infinite aspect ratio, the correction to the angle for the real model wing, affected bv downwash. is found bv the formula:

Angle of attack increment: (18.25 x Cl) (Assuming a nearly elliptical lift distribution)

A

Thus, for the model considered earlier the Cl for minimum sink with a. r. 7.5 was found to be 1.55. The induced angle of attack for this a. r. is:

18.25 x 1.55/7.5 = 3.77 degrees

From the tunnel tests (unfortunately not at the correct Re, so this is useful only as an example of method) the aerofoil yields C) 1.55 at 8.0 degrees. The rigging angle of wing to fuselage should be 8.0 + 3.77 = 11.77 degrees.

For penetration the low drag range of the aerofoil ends at approx, ci = 0.5, which develops at -2.3 degrees.

The induced angle of attack will be 18.25 X 0.5/7.5 = 1.22.

The rigging angle should thus be – 2.3 + = -1.08.

(The aerofoil is highly cambered and not very suitable for a fast flying model sailplane. Its performance is very good at high ci, for which it was designed. The figures above illustrate the method only.)

A pylon racer with a symmetrical aerofoil, flying at a Cl of 0.05 with the same aspect ratio would require an angel of incidence as worked out below:

18.25 x 0.05 : 7.5 = 0.122.

The symmetrical profile would reach cj 0.05 at 0.11/0.05 = 0.45 degrees. The rigging angle should thus be 0.122 + 0.45 = 0.57 degrees.

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