Responder a: Frequencia Natural

Home (concurso para prático) Fóruns PNA – Propulsão Frequencia Natural Responder a: Frequencia Natural

Hugo Marins Silva

Fábio, você disse que fica mais fácil de acontecer ressonância se o hélice tiver skew.

Se ocorre ressonância, “the unsteady forces and the associated unsteady stresses” serão maiores, é isso?

Pelo menos é o que concluí da seguinte seção do PNA Vibration (p. 279):

The coincidence of the natural frequency identified with some natural mode and the exciting frequency of some excitation component corresponds to a condition of resonance. At resonance, rigidity is counterbalanced by inertia and limitless vibratory amplification by the excitation is opposed only by damping, to first order. Since in ships, as in most engineering structures, damping is small, resonance is in general a condition that would be desirable to avoid.
Unfortunately, resonances cannot be avoided.

Caso seja assim, não entendo como isso se correlaciona com a seguinte frase do PNA (p. 185):

Cumming used the method of Tsakonas to carry out calculations for a highly skewed propeller in the wake of a Series 60 ship. He concluded that a significant reduction of unsteady shaft forces could be obtained by a judicious choice of the amount of skew.

E ainda (p. 184):

(g) Skew. (…) Cumming et al (1972) mention the following advantages of properly designed highly skewed propellers:
• Decrease in propeller-induced unsteady bearing forces and moments
• Decrease in propeller-induced unsteady pressure forces

Ora, se o skew aproxima a frequência natural da blade da frequência de excitação, causando ressonância, como pode ela ao mesmo tempo reduzir a vibração no casco e as unsteady forces?