"Tweaking Hawkeye," Feature Article, Sept. 2002

tweaking hawkeye

Before you can change the Navy's early-warning aircraft, someone has to see how the idea will fly.

This article was prepared by staff writers in collaboration with outside contributors.

The Hawkeye twin-turboprop, the U.S. fleet's eyes and ears in the air, first went to sea on the carrier Kitty Hawk in the 1960s. Thanks to constant revision, it has remained a key part of the Navy's arsenal.

Northrop Grumman recently delivered the first production models of the latest version, the Hawkeye 2000, and is already at work on the next batch of improvements.

Advances in the Hawkeye 2000 include a mission computer upgrade and a new system called the cooperative engagement capability. The next generation will have solid-state, electronically steered UHF radar.

The aircraft is due for some exterior changes. Hamilton Sundstrand of Windsor Locks, Conn., has developed a new propeller with eight blades instead of the current four. At the same time, increased power requirements for the advanced radar will require larger liquid cooling scoops.

Warren H. Davis, a member of the Airborne Early Warning and Electronic Warfare Systems Group in Northrop Grumman's Integrated Systems Sector at Bethpage, N.Y., was asked to predict how the new propeller and larger scoops might affect the Hawkeye's performance. As the aero CFD principal engineer, he models the effects that modifications will have on aerodynamics and propulsion.

"These are very complex airplanes—not just wings on bodies," Davis said. "With older planes that must be augmented, designers ask, 'But what about this?' We needed capabilities that would dispense with the old 'back of the envelope to model to wind tunnel' cycle."

Questions about the new propeller, for example, concerned differences in the forces it creates. A propeller does more than provide thrust. The blades impart a swirling wake that affects the stability and control of the aircraft.

The next version of the U.S. Navy's Hawkeye will carry an eight-bladed propeller, and its radar cooling system will require bigger air inlets.

Since the Hawkeye uses co-rotating props, this swirl is asymmetric, adding side forces on the tail. The Hawkeye requires a constant rudder input to balance this propeller-induced side force. The forces from the new propeller could differ enough from the original prop's pattern to require changes in tail settings for adequate control.

Another scoop on an aircraft inevitably results in an increase in drag. To minimize the increase but still deliver the required airflow to the heat exchanger, the position, size, and shape of the scoop all must be considered.

Davis began with CAD models created in the Catia system from Dassault Systemes of Paris. He generated a grid of the airplane's surface by using Gridgen software from Pointwise Inc. of Fort Worth, Texas. Overgrid, a program developed at the NASA Ames Research Center, created the 3-D grid representing the envelope of atmosphere around the plane. Gridgen then was used again to smooth out any irregularities in the 3-D grid.

The CFD analysis software Davis used was Overflow, which was developed at NASA's Langley Research Center. Davis used 8.1 million grid points in his model.

Computational fluid dynamics showed that the propeller forces and swirl differences between the four- and eight-bladed props were small enough so they didn't need a change in the basic Hawkeye control system design.

The scoop is still being designed. Davis tests different shapes by creating them in Gridgen and Overgrid, and then cutting them into the model of the aircraft by using another NASA Ames program, called Pegasus.

According to Davis, there is a possibility of incorporating the new scoop with the plane's vapor cycle inlet, an environmental control feature. A common inlet with bifurcated ducting can result in two airflows and a reduction in ram drag for the integrated inlet. The design might reduce drag to the full configuration of the aircraft.