# 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.