. Environmental Impact

19.7.1 Noise

Strictly enforced noise regulations make the introduction of a long range supersonic delta wing type configuration almost impossible. The reason for this can be ‘easily’ understood.

Since the noise produced depends on the aircraft thrust, and since the thrust has to be balanced by the drag, higher drag means more thrust and thus more noise at a given airspeed. The large supersonic delta wing aircraft designs have take-off weights that arc comparable to those of the 747 and strive for the same take off field lengths. Unfortunately their wing spans arc typically only 60 b of the 747’s in order to make the wing longer for the same wing area. This reduced span more than doubles drag at the same flight speed.

If wc look at the equation for induced drag in the acrodynmics section wc sec that the designer can also increase the takeoff speed to lower the induced drag Higher takeoff speeds at the same thrust increase the takeoff field length and reduce the climbout distance to the noise flyover point. The designer may thus satisfy the sideline noise regulations, but not the flyover regulations or the field length constraints. If the designer decides to increase the thrust, the will reach rotation sooner and will climb out higher. Now he satisfies the flyover regulations, but not the sideline regulations.

So for an aircraft of the same weight and less span we need an engine that makes less noise at a given thrust. This can only be achieved by increasing the massflow through the engine Therefore a long range supersonic delta wing type configuration will have to be fitted with engines that base a much greater crosssection that B747 engines at takeoff. These
extremely wide engines also have to be very long because of the supersonic compression proc­ess. These very large engines cannot be spaced to close together because then they will start sucking in each others air. Even though they cannot be spaced to close together, they will have to be titled on a much smaller span. On top of all of this such large masstlow engines can double the weight and drag of the nacelles and limit the takcofT rotation angle.

The author sees no solution to the noise problem for large delta wing supersonic air­craft at this tunc. However, the oblique (lying wing is not caught in this catch 22 of noise and performance.

The high subsonic hft-to-drag ratio of the OFW allows the bypass engine to be throt­tled back to about 50 % of its available thrust while still maintaining the required airfield per­formance. Such a throttled back engine can meet the FAR36 stage 3 noise requirements without significant noise abatement measures. Future noise regulations will be met with similar penal­ties as those experienced by competing subsonic transports.

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