AS350B3-B3E No Strake
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AS350B3-B3E No Strake
Looking at different photos, I noticed that the B3-B3E does not have a tail boom strake like the B2 does.
Anybody know the reason why? I would have thought it to be beneficial to keep.
Anybody know the reason why? I would have thought it to be beneficial to keep.
Last edited by starflex3; 28th Jul 2020 at 13:02.
From a Eurocopter training manual, the effect of the strake on the B2 variants is described as follows:
"In sideways flight to the left, the main rotor downwash is deflected and accelerated over the RH side of the tail boom, which induces a negative pressure of approximately 1 mbar/cm2 along the entire tail boom. This reduces the effect of the tail rotor by roughly 5%. A strake added at 45° causes the main rotor downwash flow to separate and restores the pressure to the static value. The effect of the strake is thus to regain the 5% moment and to improve the tail rotor efficiency (including in hover)."
However, a feature found on B3's but not B2's is the added 'step' on the upper vertical fin. The training manual describes the effect of this as follows:
"In cruise flight, the asymmetric NACA airfoil of the dorsal fin generates an aerodynamic force F1 that opposes the main rotor's counter torque CR and thus reinforces the tail rotor thrust. This allows the tail rotor pitch to be reduced and power to be saved. In the B3 version, a step on the RH side of the upper fin improve this phenomenon."
My guess would be that the 'step' on the vertical fin leads to more desirable handling of the machine than the strake (in the manufacturer's view), and there may have been some unwanted aerodynamic effects of having both the strake and the step installed at the same time, so in the evolution of the type, Eurocopter elected to keep the step on the vertical fin and do without the strake.
All just speculation though so I'm happy to be corrected.
"In sideways flight to the left, the main rotor downwash is deflected and accelerated over the RH side of the tail boom, which induces a negative pressure of approximately 1 mbar/cm2 along the entire tail boom. This reduces the effect of the tail rotor by roughly 5%. A strake added at 45° causes the main rotor downwash flow to separate and restores the pressure to the static value. The effect of the strake is thus to regain the 5% moment and to improve the tail rotor efficiency (including in hover)."
However, a feature found on B3's but not B2's is the added 'step' on the upper vertical fin. The training manual describes the effect of this as follows:
"In cruise flight, the asymmetric NACA airfoil of the dorsal fin generates an aerodynamic force F1 that opposes the main rotor's counter torque CR and thus reinforces the tail rotor thrust. This allows the tail rotor pitch to be reduced and power to be saved. In the B3 version, a step on the RH side of the upper fin improve this phenomenon."
My guess would be that the 'step' on the vertical fin leads to more desirable handling of the machine than the strake (in the manufacturer's view), and there may have been some unwanted aerodynamic effects of having both the strake and the step installed at the same time, so in the evolution of the type, Eurocopter elected to keep the step on the vertical fin and do without the strake.
All just speculation though so I'm happy to be corrected.
Strakes are generally used to improve hover performance in unfavourable crosswinds - profiling the vertical fin usually helps in forward flight. Perhaps the different models were marketed for slightly different roles?
Also the fact that the B3 tail rotor is quite different in the area of the TE Tab probably has more to do with it.
Much larger tab than a B2.
Much larger tab than a B2.
Pretty much what Ray Prouty said regarding helicopter design - 'What you gain in the hover you lose in forward flight and what you gain in forward flight you lose in the hover.' There is always going to be a compromise somewhere along the line.
