THE POWER FACTOR AND L/D RATIO

For a soaring glider or duration model when gliding the total power factor, Cl1 5/Cp should be as high as possible. The most suitable wing profile is that with a high section power factor, Cl’ VQj. This may be calculated from the wind tunnel results.

The table opposite indicates the method. A pocket calculator should be used to speed the work. The section 1/d ratio is also worked out in the tables. This gives a general idea of the profile’s efficiency, while the minimum drag coefficient indicates the potential of the aerofoil for high speed flight A low «у at low q is essential for a speed model wing.

THE BEST ANGLE OF CLIMB

It is assumed here that a duration power model has enough power available to achieve any desired angle of climb. The problem is to know which angle of climb, at maximum power, will give the best rate of ascent

A COMPARISON OF TWO AEROFOILS AT Re 100.000

1

2

3

4

5

6

GOTTINGEN 801

Re 100000 (KRAEMER

C1

cd

‘Vcj

q3

AT

V7/cd

TEST)

l

FROM

FROM

Column (1)

(Cold))3

JCo (4)

Col (5)

TEST

TEST

Column (2)

Col (2)

0.4

0.0289

13.84

0.064

0.253

8.754

0.5

0.0241

20.75

0.125

0.3535

14.668

0.6

10.02181

27.75

0.216

0.4648

21.32

Min Cd at Cj0.6

0.7

0.0220

31.82

0.343

0.5857

26.53

0.8

0.0240

33.33

0.512

0.7156

29.82

0.9

0.0260

34.62

0.729

0.854

32.85

1.0

0.0300

33.33

1.000

1.000

33.33

1.1

0.0317

34.70

1.331

1.154

36.40

1.2

0.034

1 35.291

1.728

1.315

38.68

Max 1/d at q 1.2

1.3

0.0378

34.39

2.197

1.482

ПЙШ]

Max power factor, c j 1.3

1.4

0.0518

27.03

2.744

1.656

31.20

0.4

0.0222

18.02

0.064

0.253

11.396

107,000 G. Muessman Test

0.5

ІЙ.0221І

22.62

0.125

0.3535

15.995

Min. C^atcj 0.5

0.6

0.0222

27.03

0.216

0.4648

20.936

0.7

0.0223

31.39

0.343

0.5857

26.26

0.8

0.0224

35.71

0.512

0.7156

31.95

0.9

0.0235

138.301

0.129

0.854

36.34

Max l/d at cj 0.9

1.0

0.0265

37.74

1.000

1.000

Г37Л7І

Max

1.135

0.0400

28.37

1.462

1.209

30.23

Starting from level flight trim, the power is increased step by step. In level flight, as already seen:

Lift (Level flight) = Lo = W = *pV*SCL This may be re-arranged to give an equation for speed:

V2 (level flight) = V02 = W/V4pSCL

In the climb Lift (Climb) = Lc = W Cos в Also, if Vc = speed along the inclined flight path then

Lc = 14pVc2SCl =W Cos в

Re-arranging this in turn to obtain equation for Vc2, Vc2 = W Cos 0/WpSCL – From the foregoing:

Vc2 = (WCos flj ^ / W = WCos в ISpSCL

V02 WpSCL/ : [^pSCl/ WpSCL W

which cancels down to:

-rr-r = Cos в and so-гг— = y/CosB

*0* ’’O

(This is on the assumption that Cl remains unchanged, i. e. the model is not retrimmed.) In the small diagram Figure A3, Vc, the flight speed along the inclined path, is

ч.

 

Fig. АЗ The best angle of climb for a high-powered model

represented by a line at angle 9 to the horizontal. The length of this line is proportional to Vc = V0 x VCos 9

The rate of climb on this diagram is proportional to the length of the line marked C. From basic trigonometry,

-^-= Sin 9 or C = Vc Sin 9 *c

And therefore:

C = V0 x v/Cos 9 x Sin 9

For a particular model and trim condition, V0 is constant The factor /Cos 9 x Sin 9 may easily be worked out with the aid of standard tables of Sine and Cosine, for any value of climb angle, 9. The result may be plotted against 9, as has been done in Figure 4.6. The maximum rate of climb is then found to occur when the graphed curved reaches its maximum close to 55 degrees. The result is approximate. Departures of four or five degrees either way make little difference. The practical trimming procedure is thus to aim at achieving the desired climb angle by adjustments of trim, wing camber, flaps etc. then to ensure that the engine propeller combination yields maximum thrust at that angle.

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