Drag Estimate for Example Helicopter

For purposes of illustration, the parasite drag of the example helicopter has been estimated using the methods and data presented in the preceding section.

Procedure Results Given: three-view drawing, drag data, reference speed Estimate: equivalent flat plate area at zero angle of attack See Figure 2 of Appendix A, Figures 4.15 to 4.26, Table 4.2 ref speed =115 knots Basic fuselage: Determine frontal area, AF Determine fineness ratio Find CDp from Figure 4.17 fF = AjCDf! From three-view, Ap = 74 ft2 l/d = 7 CDp= 0.078 fF = (74)(0.078) = 5.8 fi2 Nacelles: Determine frontal area, AN Determine distance ratio Find CDn from Figure 4.19 fN—AtfioN AN=»2 [n(1.4)2] = 12 ft2 y/DN= 1.4/2.8=.5 4 = 009 /w=(12)(0.09 ) = 1.1JP Main rotor hub and shaft: Determine frontal area of hub Determine frontal area of shaft Determine diameter of shaft From Table 4.2, estimate CDl„, MHRPM=0 Amh = 5 ft2 ^MS = 1 ft2 = 0.5 ft Чн=1Л From Figure 4.22 find drag ratio Calculate Cn„„ мнСОЇЇ a, = 0°, RPM = 100%, D. R. = 1.00/.95 = 1.05 c’»„„=u6 Імн = ЛмНСомн jnncorr fm = 5(1-16) = 5.8 ft2 Calculate R. N. of shaft (115 k) Determine CD from Figure 4.23 fm = ^ms^dms Total fM =fMH + f Determine Z/Wp (see Figure 4,24) Find Kj from Figure 4.24 їм — (1 + Щ/м R. N. = (6,400)(115)(1.69)(0.5) = 0.6 x 106 Чк = 0-3 fm — 1(^-3) / =5.8+.3 = 6.1 ft2 Jmuncorr Z/r;= 2.8/9 r= 0.3 *,= •15 (af = -5°) fM= 1.15(6.1) = 7.0 ft2 Tail rotor hub and shaft: Determine frontal area From Table 4.2, estimate Cp From Figure 4.22, find drag ratio Calculate CD_ * wcorr /г = ^jCdth incott fT = (0.6)(1.16) = 0.7 ft2 AT = .6 ft2 Чн=1Л For as= 0, RPM= 100%, D. R. = 1/.95 = 1.05 4hc„=(1-1)(105) = 1.16

Rotor-fuselage interference drag: From Figure 4.25 estimate ACD at df = 0 ACD =0.018 Л* = AM, fm. = 0.018(74) = 1.3 ft!

Exhaust drag: Ask engine manufacturer to estimate net exhaust thrust for engine installation on example helicopter /«—T.Ji Miscellaneous drag items: Estimate total drag of antennas, door handles, lights, steps, skin gaps, cooling leakage, ventilation, etc. Total equivalent flat plate area: /tot.= fp +/n +/m +/t +/mlg + fNLG +A 4v + /nt. Vc*. "^/nisc.

For hover in ground effect, no vertical drag or pseudo ground effects have been used. The ground effect has been taken from Figure 1.41 of Chapter 1 for a 5-ft wheel height.

The corresponding tail rotor power required is found by calculating the net tail rotor thrust required to balance the main rotor torque:

550 h. p.^ RM

T"’t== (SlR)M lT

or for the example helicopter:

TT = 0.69 h. p.M

act ■*

The gross tail rotor thrust due to fin interference is:

FIGURE 4.29 Main Rotor Performance in Hover as Installed on Example Helicopter

and the tail rotor power is:

h-P-T = 0.94[h. p. for rTjJ

The resultant tail rotor power corresponding to the main rotor power of Figure 4.29 is shown in Figure 4.30. A comparison of the hover values of CT/o on the main and tail rotors of the example helicopter reveals a mismatch that would probably generate a redesign effort in an actual project. This comparison, shown in

Figure 4.31, indicates that at high gross weights the tail rotor is more heavily loaded than the main rotor—especially when compared to their respective maximum capabilities, which were shown in Figure 4.28. This means that the high gross weight or altitude performance will be limited by the tail rotor rather than the main rotor. Possible redesigns include increasing tip speed, chord, radius, or some combination of these to lower the tail rotor CT/c.

The engine power measured at the torquemeters is the sum of the main rotor, tail rotors, transmission, and accessory powers. For the example heli­copter:

The engine power required in hover in and out of ground effect is shown in Figure 4.32 as a function of gross weight. The next step is illustrated by figures 4.33 and 4.34, where the power required for various density ratios is plotted. The curves for

Helicopter Gross Weight/(p/p0) lbs FIGURE 4.31 Thrust Coefficients for Main and Tail Rotors

density ratio other than unity have been simply ratioed from the basic curve. Altitudes corresponding to the density ratios have been taken from the atmospheric charts of Appendix C. The ratioing procedure is valid except for those cases in which the tip speed is so high that a decrease in temperature will start to produce significant compressibility penalties For this situation, the hover curves of Chapter 1 for various tip Mach numbers can be used. Also shown in Figures 4.33 and 4.34 are the installed power ratings, which are 98% of the ratings from Figure 4.1.

The information from Figures 4.33 and 4.34 has been cross-plotted on Figure 4.35 to give the hover ceiling as a function of gross weight in and out of ground effect. It may be seen that the example helicopter can hover OGE at sea level, standard conditions at a gross weight of 27,800 lb and has a hover ceiling of

7,0 ft. at its design gross weight of 20,000 lb on a 95° day.