Tech Briefs

Improved Inlets for T-38 Airplane

The change in inlet design reduces takeoff distances and increases safety margins.

A change in the design of the engine inlets of the T-38 airplane significantly reduces takeoff distances while increasing safety margins. Although the newer inlet design (see Figure 1) is based on well-known engineering principles, it is unique and will prove invaluable to the NASA fleet and to other T-38 fleets; e.g., the fleets flown by the United States Air Force (USAF) and by foreign governments. The change in design was needed because in an inlet of the older design, separation of flow in the lower third of the inlet degraded efficiency, even under normal takeoff conditions. Johnson Space Center (JSC) engineers compensated for this deficiency in formulating the newer design by adopting an inlet shaped according to aerodynamical considerations; the shape was chosen to minimize separation of flow to produce greater engine thrust as the T-38 accelerates to takeoff speed.

Figure 2 depicts an aspect of the older and newer inlet shapes. The older design, developed in the late 1950s, was optimized for supersonic flight. However, both the NASA and USAF missions for the T-38 now emphasize subsonic flight, in which the older inlet design causes internal separation from incoming air. This separation starves the engines of air, thereby reducing engine efficiency. The consequences of reduced engine efficiency include increases in takeoff distances, decreases in safety margins, and engine failures that result in higher-risk, single-engine takeoffs.

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Figure 1. The Improved T-38 Engine Inlet design affords enhanced performance and safety. NASA's T-38 fleet will be modified to incorporate this design. The USAF T-38 fleet and the fleets of foreign countries can be similarly modified.
At such hot, high-altitude airports as the one at El Paso, Texas, the risk is even greater. Because of climatic conditions and the relative thinness of the atmosphere there, especially in summer, T-38s cannot take off at full weight. Fuel must be burned off, or flight crews must wait until surface temperatures fall sufficiently to permit takeoff. Such extremes produce a Category III condition; that is, a condition in which critical field length exceeds runway length, reducing the accepted measure of takeoff performance. JSC engineers addressed this condition in changing the inlet design.

The JSC design was not the only alternative considered. The older inlet might have been modified with auxiliary, moveable doors that could open for takeoff to allow more air to enter the engines. However, this modification would have (1) increased aircraft weight by 120 lb (54 kg), posing a sizeable risk at El Paso and other locations where weight is already of concern; and (2) increased the T-38 life-cycle cost, owing to required maintenance on the modified inlet.

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Figure 2. Older and Newer Inlet Cross Sections in a longitudinal plane differ in ways that translate to better subsonic performance for the newer design.
By enlarging the area of the inlet and significantly thickening the inlet, JSC engineers developed an inlet design that promotes laminar flow of incoming air, increasing aircraft efficiency and thrust. The exterior profile was customized for maximum aerodynamic performance and to maintain continuity with existing aircraft structures. Because there are no moving parts, the increase in weight associated with the modification is negligible [approximately 10 lb (~4.5 kg)]. Clearly, the JSC inlet for T-38 aircraft offers a superior alternative to both the older design and to the auxiliary-door proposal.

NASA's T-38 fleet will be modified to incorporate the newer design. The USAF T-38 fleet and the fleets of foreign countries can be similarly modified.

This work was done by Robert Ess and David Eichblatt of Johnson Space Center. No further documentation is available. MSC-22785