Pilot/vehicle analysis of the lateral/vertical control of a twin-lift helicopter configuration

The use of two hel icopters i n a single t w i n l i f t conf igurat ion t o carry heavy o r oversized loads i s a feasible means f o r increasing the product iv i ty of r o t a r y r l n g vehlcles. A s t a b i l i t y and manual control analysis o f a t w i n l i f t conf igurat ion employing a load suspended from a r i g i d spreader bar i s undertaken. The e f f e c t of system parameter var iat ions upon open-loop s t a b i l i t y character ist ics i s outl ined. A two p i l o t manual control structure i s formulated i n which the p i l o t o f the ' ~ ~ s t e r " vehicle moves h i s hel icopter i n response t o i n t e r n a l l y generated pos i t ion conmands. The p i l o t of the "slave' vehicle i n te rp re ts master hel icopter t rans lat ional motion as a posi t ion cormnand t o h i s own vehicle. The required p i l o t equalization i n a l l the contro l loops I s established and closed-loop performance I s assessed through computer simulation. 1. Introduct ion analysis t o follow. i t i s assumed t h a t the two hel icopters are ident ica l . The hel lcopters and spreader bar are modeled as r i g i d d i s t r i bu ted masses. The slung load i s t reated as a point mss. A l l cables are t reated as massless r i g i d l i n k s w i th f r i c t i on less attachment points. Only the dominant aerodynamic forces and moments act ing upon the hel icopters i n hovering f l i g h t are included and are introduced i n the form o f f i v e l i nea r S t a b i l i t y der ivat ives. A l l r o t o r degrees of freedom except t ip-path plane or ientat ion are neglected, and t h i s or ientat ion i s assumed t o be d i r e c t l y proport ional t o l a t e r a l cyc l i c inputs by the p i l o t . Col lect ive inputs are assumed t o y i e l d instantaneous changes i n ro to r t h rus t and no t a i l r o t o r ef fects are considered. These aSSUmptlOnS are i den t i ca l t o those used i n the study o f Ref. 1. although the aerodynamics of our hel icopter model are somewhat simpler. Table 1 l i s t s the i n t e r t i a l and aerodynamic character is t ics used i n the m s t e r and slave hel icopter models as w e l l as the n m i n a l character is t ics o f the soreader bar. load. etc. The hel icoDter data i s The twin-lift helicopter system is r i p resen ta t l i e o f a vehic le the sire o f the considered an excel lent candidate fo r carrying UH-6OA. The spreader bar and load data were heavy loads o r loads of large dimension which w p l i e d by nr. LUigi Cicolani o f the F l i g h t are not appropriate fo r s ingle hel icopter Dynamics and Controls Branch Of NASA operation. e.9. telephone poles, logs, pipes. Research center. trusses, etc. [I] . Figure 1 shows one o f the most promising o f such configurations Involv ing As modeled here, the t w i n l i f t system a spreader bar betwen the tw vehic)cs. The POSseSSeS seven degrees of freedom: two pos i t i on work reported herein i s an analy t ica l extension coordinates for the centers Of Of each of the study of Ref. 1 which analyzed the planar hel icopter. r o l l a t t i t udes o f each vehic le and l a t e r a l / v e n j c a l s t a b i l i t y and control the i n c l h a t l o n Of the spreader bar. Fig. 2 character ist ics of the t w i n l i f t configuration these coordinates a number Of of 1. Ref. 1 proposed a feedforvard parameters. The system equations o f motion were cantro l techniaue t n which control movements i n Obtained from lagrangels the master heficopter were fed d i r e c t l y t o the slave vehicle. As analyzed here, the t w i n l i f t conf igurat ion u i11 employ a %arter-slave* type the .master* he l lcopter e f fect ive ly f l i e s the formation and the P i l o t (or autopi lo t ) i n the d m-?&Q+m+a(DE)= Q i 1) of control concept i n which a c m n d p i l o t i n d t aij aql asi ai "slave' he l icopter ' interprets master hel icopter t rans lat ional motion as a posi t ion comnand fo r h i s vehicle. This approach emphasizes the manual control aspects of the t w l n l i f t system with an eye toward establ ishing baslc p i l o t equalization requirements and overa l l closed-1 oop s t a b i l i l t y and control character ist ics. 11. Vehicle Equations of Motion where KE, PE and DE represent. respectively. the k ine t i c , po ten t i a l and d iss ipat ion energy of the system and q1 and Pi are. respectively, the I t h generalized coordinates and external forces which are assoclated w i th the system. fo l lowing the lead of Ref. 1. a set o f generalized coordinates were selected which d i f fered from those def in ing the degrees of freedom. above. These were: Only planar 1a te raVver t i ca l motion about ys l a t e r a l displacement o f center of mass of slave hel icopter r e l a t i v e t o earth-f ixed axes hover w i l l be considered i n t h i s study. *Professor, Div is ion o f Aeronautical Science and Engineerin Department o f Mechanical Engineering, Member A d * of slave hel icopter r e l a t i v e t o **Graduate Student I n the zs I v e r t i c a l displacement of center o f msS