Glider Airframe Noise: Comparison Between Measurement and Prediction

Abstract

_ . x This paper presents t h e r e s u l t s o f a study t o i d e n t i f y a i r f r a m e noise generation and suppression mechanisms. The measured f a r f i e l d noise l e v e l s are compared w i t h t h e l e v e l s p r e d i c t e d by some of t h e t h e o r e t i c a l modeis c u r r e n t l y a v a i lable. Attempts are made t o r e l a t e t h e measured surface pressure f l u c t u a t i o n s and t h e boundary l a y e r p r o f i l e s near the wing t r a i l i n g edge t o t h e r a d i a t e d f a r f i e l d noise spectra. The effect of t r i p p i n g t h e upper and lower surface boundary l a y e r s i s investigated. The e f f e c t o f sharpening t h e wing t r a i l i n g edge i s studied. The e f f e c t of sawtooth t r a i l i n g edge treatment i s a l s o looked i n t o . The r e s u l t s show some q u a l i t a t i v e consistency w i t h t h e c u r r e n t p r e d i c t i o n methods. However, improvements i n the p r e d i c t i o n methods would be desirable. 1.0 INTRODUCTION -With t h e use of h i g h bypass r a t i o engines w i t h advanced acoustic treatments, airframe n o i s e i s becoming a major component of t h e t o t a l noise r a d i a t e d fr.om a comnercial a i r c r a f t , during landing approach. There are many t h e o r e t i c a l and semi-empirical methods a v a i l a b l e a t present f o r e s t i m a t i n g a i r f r a m e noise (see f o r example Ref. i). One of t h e main components o f a i r f r a m e noise has so f a r been considered t o be t h e wing t r a i l i n g edge noise. There are several t h e o r e t ic a l models f o r p r e d i c t i n g t h i s component. Various concepts f o r reducing t h i s component also e x i s t . I n order t o v e r i f y these concepts, t h e t h e o r e t i c a l models and t h e i r u n d e r l y i n g assumpt i o n s , i t i s necessary t o perform small scale experiments. F l i g h t t e s t i n g of a g l i d e r seemed t o He a reasonable choice, because of t h e d i f f i c u l t i e s of measuring clean wing t r a i l i n g edge noise from small scale models i n a wind tunnel. --There are two w e l l documented papers on g l i d e r airframe noise a v a i l a b l e i n t h e l i t e r a t u r e (Refs. 2 and 3). I n Reference 2, Vlasov and Samokhin present t h e r e s u l t s of -f l i g h t t e s t s on two d i f f e r e n t g l i d e r s : KAI-12 and L-13.-The KAI-12 i s a t r a i n i n g g l i d e r w i t h a s t r a i g h t wing. The L-13 i s a s a i l p l a n e w i t h superior aerodynamic performance. The noise spectrum o f t h e KAI-12 had two c h a r a c t e r i s t i c maxima: one wideband i n t h e low frequency range (<IO0 Hz) and t h e o t h e r narrowband i n the mid-frequency range (160-200 Hz). I t was concluded t h a t t h e p r ï n c i p a l source of t h e low-frequency noise was due t o v o r t i c e s formed by f l o w past t h e wing and t h e t a i l s e c t i o n of t h e g l i d e r . The narrowband peak i n t h e midfrequencies was a t t r i b u t e d t o f l u c t u a t i o n s ...