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Quiet Rotorcraft ~ by W. Z. Stepniewski

Chapter 6 ~ Synchropter of the ABC Type

General;

Adaptation of the ABC principle to the synchropter configuration was quoted as one of the possible ways of achieving quiet rotorcraft, capable of fast cruise speed in excess of 220 knots. A brief look at a transport aircraft of that type, having a gross weight W=36,400 lb is presented below.

The ABC - Advancing Blade concept is defined by de Simone (Ref 2.. 0) as follow:

"The ABC rotor system is compose of two coaxial, counter-rotating, rigid rotors, resulting in aircraft with advancing blades on both sides of the rotor disk. In forward flight, lift is progressively transferred to the advancing blades where it is more efficiently generated due to higher dynamic pressures.

Since the retreating blades carry less lift, the classical problem of retreating blade stall is avoided. The rolling moment generated by one rotor is cancelled by the equal and opposite rolling moment of the other rotor. Thus, unlike a conventional single rotor, the ABC maintains a symmetrical lift distribution throughout the speed range.

In hover, the use of two counter-rotating rotors reduces the energy losses in slipstream rotation, and, with the favorable interaction of mutually induced inflows, the induced losses through the rotor are reduced. This, coupled with the absence of a tail rotor, increases flight measured hover performance by about 10% above a conventional, single rotor helicopters at the same disk loading."

Very preliminary studies seem to indicate that placing the ABC rotors in the synchropter position should result in a somewhat aerodynamically cleaner then the coaxial ABC, and definitely a much cleaner design than the considered single-rotor types. In the proposed synchropter configuration, the _two rotors, basically of the same geometry as in the coaxial configuration, are moved to the intermeshing position, so that the horizontal distance between the rotor hubs is l/R (Fig. 45). Its relative value can be expressed as l/R = l/R /R.

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Fig. 45 A sketch of the front view of the ABC Synchropter type transport.

The rotors probably would be three or four bladed. The blade lift should be so controlled around the azimuth that moments generated about the aircraft rolling axis by blades on the right (advancing) side are equal in magnitude and opposite in direction to those generated on the left (also advancing) side. At the same time the sum of lifts generated by all blades must be equal to the aircraft gross weight plus any possible aerodynamic download.

(51)

Should the ABC/Synchropter type appear as a potentially viable quiet rotorcraft configuration, than a large amount of experimental and analytical material acquired by Sikorsky through the years of development of their XH-59A flight demonstrator, would be of special value. Even in this study, investigation of the performance aspects of the ABC/Synchropter take advantage, whenever possible, of the Sikorsky data, as presented in Ref. 20. It should be emphasized at this point, that in this report nominal disc loading weight is always defined as xxxxx where nR is the number of rotors, while in Ref. 20 disc loading is based on the common projected

area of both rotors. Consequently, for instance, w = 10 psf in Ref. 20 in would be interpreted as w = 5 psf in this report.

From the point of view of this study it would be of prime interest to know what levels of the (L / De) R values can be reached through the application of the ABC principal to the synchropter. In that respect Ref. 20 could be of great help, since it is focused on the XXX, XXX conditions, which at SLS corresponds to the flight speed of 258 knots. - just in the realm of high cruise speeds desired for the quiet rotorcraft, while VT = 513 fps.

 

Ref. 2. shows that thr ugh 2 7 C Y'

Ref. 20 shows that through the application of advanced geometry airfoil sections (Fig. 46) considerable improvements (with respects to values represented by

the XH-59a) in the lift (gross weight) to equivalent drag of the rotor, (L / De)R can be obtained (Fig. 47)

 

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Fig. 46 Comparison of Airfoil Profiles

 

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Fig. 47 Effects of Airfoils on (L / De)R

 

 

Departure in the blade taper and twist from those of the XH­59A, can further improve the (L / De)R ratios, but their influence on the maximal (L / De)R values is small.

Consequently , the (L / De)R vs CT /σ represented by the dashed line in Fig. 47 will be taken as the good, realistic values achievable in the co-axial ABC configuration. The question is - what changes in the (L / De)R levels can be expected, once the ABC rotors are arranged in the synchropter geometry.

Since interest here is focused on the order of magnitude of the (L / De)R changes and not on their exact values, the following very simple analysis, based on the momentum theory is made. Rotor power (RP) is a sum of the induced power (RP)ind and "profile" power (RP)pr, which includes the true profile power, resulting from the blade profile drag, as well as that needed to overcome the hub drag. It may be assumed that change in rotors geometry from the co-axial to the intermesh would not very the (RP)pr values: (RP)prco = (RP)prsy , but only the (RP)ind levels.

For the whole rotor

(

~ \ = (~ ) + (j) -- )

w J \1 W ~ ~"'t" W ~ \nc:\

where the first term on the right side remains invariant and the second changes.

The (W / De )R levels for the synchropter configuration can be expressed as follows .

.,1

(*)R :: ru~nR + (~) + (t> D~)

Sf l' r'C6 R , .•. ace> W R' J

Where Ll'-" t "-_ (f De / 'W) '<. i_a <7 ::- (1) ~ / w ) Q, '" d ~ - (tit. /'11)) R \~.1 co

But for the isolated rotor system, where no additional power losses, as for instance for mechanical transmission, are taken into consideration when determining the equivalent drag it may be simpply taken that ) l ~ ~ c...1 'o~ ~ ~ WI. 'f\ '1 .{- ~v.. !. '" 4. '" •... \.

Using the simple momentum theory approach, induced drag for the coaxial configuration (Fig.48) can be written as:

b,,,~C~ k(a W \ .~~ V (54)

where kCO is the induced drag correction factor (assumed kCO = 1.15), VCO is the ideal velocity, in fps, and V = speed of flight in knots.

 

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Fig. 48 Assumed cross section of the slipstream in a coaxial configuration.

 

Assuming, in turn that the cross section of the slipstream effected by the rotors is as in Fig. 48, the ideal induced velocity at SLS becomes:

 

Xxxxxxxxx (55)

 

where the disc loading W is per rotor, and h~ in the relative rotor height hR:: h9./R.

From Egs. (54) and (55) <5,q) and (S5) and omitting the subscript R one obtains:

(~) ::: \ 6. C\ . c:L \ v-J 1"'.\(.0 ----~------ .., "1- \ -r Ul ~Q,) \J (56)

For the synchropter configuration, it is assumed that the cross section of the effected slipstream is a circle having a radius equal to the one half of the rotor system span Fig (49).

Making, small-angle assumptions

- \ -r Ul ~Q,) \J (57)

 

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Fig. 49 Assumed slipstream cross-section in the synchropter configuration.

Following the reasoning used in theco-axial case, but assuming that here the correction factor is somewhat higher, \<.sj-:: .. l·2, the (Dind / W) ratio for the SLS conditions will be:

- \ -r Ul ~Q,) \J (58)

By subtracting Eq (56) from Eq (55) the (06 i) / W) \vy~ can be computed. This was done for disc loadings W = 5, 7.5 and 10 psf assuming that for the co-ax relative distance between the rotors is constant ~ Q..:: 0· \'2. However, for the synchropter case, it was assumed that for the 36,400 lb gross weight aircraft (Fig. 25) lateral separation of the rotor hubs is constant ~ R :f7-r ft. Thus the TR values in Eq (58) are f~ 0.5'2) 0.63 and 0.73, when W = 5, 7.5 and 10 psf, respectively.

The so computed (ΔD /W)ind values are shown in Fig. (50).

 

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Fig. 50 Relative induced drag increments for the ABC synchropter in comparison to the coaxial ABC..

 

Consequently, at the same disc loading (interpreted as loading per disc of one rotor) relative induced drag of the ABCjsynchropter configuration should be somewhat lower than that of co-axial. However, at high cruise speeds, beginning at 200 knots, those gains will be small. For instance, assuming that for the co-axial (L/De) = 10.0, arranging those rotors in the synchropter configuration, would result in (L/De)R = 10.40, 10.73 and 11.11 for 'w=5, 7.5 and 10 pst, respectively.

For the initial co-axial ABC rotor (L/~~\~ value of 12, the induced drag gains could lead to = 12.6, 13.1 and 13.6 for w = 5, 7.5 and 10 psf.

This shows that taking lL ID.J~ ; (w I D.)"Q. values given in Ref. 20 for the co-axial ABC, would represent a conservative approach.

Parasite-Draq Aspects: The ABCj synchropter configuration offers the designer opportunities for a truly aerodynamically clean rotorcraft, with the cleanness level (expressed through the equivalent flat plate area loading ~< ) similar to those of the commercial fixed-wing transports of the same gross-weight class.

Absence of torque compensating devices permits, one to give the proper fuselage the best aerodynamic shape for the intended interior arrangements. Lack of a substantial wing, would probably lead to the necessity of using sponsons as a housing for the fuel tanks. However, some fuel may probably be located in the wing-like structure supporting the rotors. Thanks to the synchropter configuration, drag of those supporting structures can be minimized through an optimal streamlining.

It is obvious, that detailed design studies are required to find an optimal configuration and present drag estimates should be considered as very rough indications of the possible levels.

Statistical values of the s~in frictions coefficient based on the wetted area, given in Table 12.3. Ref. 21 are: civil transports: (f = 0.003, Light aircraft - twin engine (~ = 0.0045.

Assuming Cf = 0.00375, as an average of the two above values, and wetted area A~~ ':: \ t 0 () +~'t.. ; the equivalent flat plate area becomes f = 6.75 ft2

f+':L , and the corresponding Wf = 5393 psf.

Taking a move conservative valu~ of ~f = 5000 psf., the relative parasite drag value at V = 258 kn at SLS becomes: (b/W)? •... "'("= 0.045. Further, assuming that the design parameters of the rotors can be so selected that the equivalent rotor drag to the gross weight can be (D e./'W) 9. :: lL/D)~ =- \ 0 = ()-/

an inverse of the sum of \. \) / Vol ) ? "" + (D:~_ / W) ~

becomes (~ W ) :0 6. q

Taking forward thrusts propulsive efficiency as 0 = 0.8, l?Y~P and transmission efficiency (including running accessories and instrumentation) as ~ = 0.95, the weight to the equivalent i~~ drag, of the aircraft, based on the required engine power) will be (at V = 258 knots, SlS)

~ \ = 5- 57

Should, however lL/t)~)Q.. = 12. (values indicated in Ref. 20 as feasible for the coaxial system), then rotorcraft (W /D"'}SJ = 5·C\'l. would be possible.

The above values, when marked on Fig. 4-16, Ref. 5 (here Fig. 51) show that the ABC/Synchropter appears to have a potential of achieving at V = 260, S LS the weight to equivalent drag ratios slightly better than those of the V-22 tilt rotor.