Helicopters : calculation and design by M.L., et al Mil'

By M.L., et al Mil'

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Thus, i f a rough e r r o r i n s e l e c t i n g t h e blade c h a r a c t e r i s t i c s i s impermis­ s i b l e , v a r i a b l e stresses can be kept w i t h i n p e r m i s s i b l e limits by preventing t h e occurrence of resonance. I n this case, t h e r e i s no need t o c a l c u l a t e t h e variable stress amplitudes. Thus, t h e experimental designer can o f t e n U t him­ self t o t h e f i r s t s t a g e of blade c a l c u l a t i o n : determination of i t s natural v i ­ b r a t i o n frequencies and p l o t t i n g of t h e resonance diagram.

Blades of lo? y-int h e p-l+ne of r o t a t i o n . , t h e n t h e blade w i l l be adequate w i t h respect t o s t r e s s e s i n t h e p l a n e of r o t a t i o n . It should be noted t h a t , w i t h a n i n c r e a s e i n r i g i d i t y , resonance of t h e second tone w i t h t h e f i f t h harmonic t o t h e r o t o r ,& rpm must be prevented. PI" P r a c t i c e has shown t h a t , a t this resonance, t h e s t r e s s e s i n t h e plane of r o t a t i o n in­ c r e a s e r a t h e r strongly, which might even a f f e c t t h e i r s e r v i c e l i f e .

A s follows from t h e diagram, t h e second t o n e of v i b r a t i o n s of such a blade has s t i l l not ap­ proached t h e f i f t h harmonic, w h i l e t h e t h i r d t o n e w a s somewhere between t h e seventh and e i g h t h harmonics. Designwise, t h e s e are u s u a l l y blades w i t h a con­ t o u r ( o r c l o s e t o this shape) spar i n s c r i b e d i n t h e p r o f i l e . The s p a r can be e i t h e r s t e e l o r duralumin. It i s impossible t o o b t a i n a f u r t h e r i n c r e a s e i n r i g i d i t y without i n c r e a s i n g t h e blade weight.

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