Tech Briefs

Developing an understanding of, and potentially controlling, pitch bifurcation of a store release from an aircraft during flight could improve weapons delivery.

Increasing the operational efficiency of weapons employed in hostile environments is a high priority of the United States Air Force (USAF). In recent history, the USAF has made a move to smaller and internally stored weapons, especially for fighter aircraft. Maintaining a low radar cross section signature, and thus a low observable air vehicle, is desirable so the aircraft is less detectable by the enemy.

Modified SUU-41 pod used for the weapons cavity acoustics store separation apparatus.

An internal weapons bay has a reduced load out but yields a low observable profile (reduced radar cross section). Also, aircraft can carry a greater number of weapons if the weapons are smaller, increasing the quantity of targets that can be engaged per sortie. The newer attack aircraft in the United States, such as the P-8A, F-22 and, most recently, the F-35, employ weapons delivery from bomb bays. The effects of this design element have not been fully explored and the impact on weapon release can sometimes be challenging to predict. Thus, it is important to understand the sensitivity of the flow field effects on the internally loaded weapon within a few feet from the carriage.

When using an internal weapons bay, several issues arise with the weapon release. The aeroacoustic environment formed by the cavity is unsteady. The mission stores are subject to this unsteady flow and strong acoustic loads. The unsteadiness originates from the presence of a self-reinforced acoustic resonance phenomenon in conjunction with a robust free shear layer instability. Pressure fluctuations and acoustic resonance that stem from this unsteady flow can impart high dynamic loads on the weapon stores while in motion. These dynamic loads can also fatigue the cavity structure as well as impact the release trajectory from aircraft bays.

Unsteady and unsuppressed pressure levels reach up to 180 dB and can lead to structural fatigue of bulkheads and even failure of weapon components. Strong acoustic resonance can, in dramatic cases, lead to instant changes in direction of normal forces from ‘in to’ to ‘out of’ the weapons bay; this is called “pitch bifurcation”. In general, it is possible that strongly time-dependent flow might affect mission store release. For smaller and lighter weapons, this could impact its safe release and effectiveness.

The time-dependency of cavity flow is a matter of concern for current data acquisition. Since typical wind tunnel data consists of time averaged store loads, common store separation analysis cannot detect a sensitivity to timing of the store release. Also, the workload and cost associated with a specific project increase dramatically for the testing of multiple configurations. Mitigation of the workload and cost can be accomplished with the implementation of dynamic wind tunnel testing which also provides time-accurate data. Previous research conducted in the Air Force Institute of Technology, AFIT, low-speed wind tunnel emphasized the importance of collecting such data in order to characterize the dynamic loads during mission store release.

The goal of this research was to support the static and dynamic characterization and the time-accurate dynamic load data acquisition in a low-speed wind tunnel. Oscillatory blowing was applied at the leading edge to emulate, to an extent, the strong time-dependent flow in a transonic environment, where Rossiter tones prevail. Both static and dynamic testing data in the AFIT low-speed wind tunnel were collected at speeds of 60, 100, and 120 mph, with different model sizes and angles of attack, to produce time-accurate force and moment measurements. Actuation of the Linear Motor Apparatus (LMA) accomplished a vertical store release trajectory. The data was analyzed to determine the nature of the effects of strong oscillatory flow of load profiles as the store progresses through the free shear layer.

This work was done by Ryan G. Saunders for the Air Force Institute of Technology. AFRL-0277

This Brief includes a Technical Support Package (TSP).

Influence of Leading-Edge Oscillatory Blowing on Time-Accurate Dynamic Store Separation (reference AFRL-0277) is currently available for download from the TSP library.

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