Compressibility

Flight Journal, Aug 2001 by Meyer, Corky

The Hellcat's transonic tribulations

Chuck Yeager's October 14, 1947 flight through the sound barrier in the Bell X-1 was a fabulous aviation milestone. His specially designed aircraft incorporated all of the "transonic knowledge" available for one reason: to see how far the totally unknown and uncharted transonic regime persisted.

To this day, many in aviation think that Yeager's historic accomplishment was a one-time daring flight that conquered the mysterious sound barrier once and for all.

What I relate here has been buried in the wartime history of test flying. It is the story of militar and civilian test pilots and en neers who worked alone to fath the unknown effects of an un xpected phenomenon that sho ed itself in steep dives and in lightly structured fighters: compressibility. Recognizing the phenomenon as only the prelude to passing the sound barrier. Unfortunately, too many service nd test pilots lost their lives when diving ito this dangerous experimental arena.

Just prior to WW II, American aircraft designers hadn't yet understood the phenomena the newer, faster fighters being tested were encountering in steep, high-speed dives. Very few in the aeronautics industry outside the government research facility called "NACA" (now NASA) had ever heard of Mach numbers or compressibility. Those research engineers who had tried to study compressibility in government wind tunnels in the 1920s and '30s had run into the problem of shock waves that developed on their models and choked the test sections of the tunnels at speeds between Mach 0.7 and 1.2. This completely scrambled all the data being collected, so they could describe only what they believed was happening and only in theoretical formulas that few could understand.

In December 1941, John Stack, head of the high-speed-- flight wind-tunnel section at NACA stated, "No one is likely to solve this wind-tunnel-choking problem for some years to come-if ever." Thus, many in NACA envisioned only one alternative to the tunnels: a specially instrumented, fullscale, transonic research airplane. On March 15, 1944, at Langley, a meeting between NACA and service personnel dealt with the possibility of designing such a machine. But it was wartime, and the pressures to produce military aircraft greatly outweighed the importance of this project.

Orville Wright introduces me to Dr. Ernst Mach

My boss, Bud Gillies, vice president of Grumman flight operations, and I first heard the term "Mach number" while listening to Orville Wright lecture at the Institute of Aeronautical Sciences on December 17, 1943. He used the term as if everybody would know what he was talking about.

The next day, when we asked Charley Tilgner, the chief of Grumman aerodynamics, about Mach numbers, he said he had heard of them but didn't really know much about Dr. Ernst Mach or his research work. Mach was born in 1838 and died in 1916; he was a renowned Austrian physicist working in the field of ballistics; he was the first expert in sonic airflow, so Mach is now the term used for the speed of sound. I was soon to complete my education in Dr. Mach's numbers-in the air.

Training to be a structural demonstration test pilot

Without knowing management's reasons, I was directed to stop work on Hellcat speed improvements and to begin a full dive-demonstration program in F4F-3 Wildcat Bu. No. 12249. The Wildcat was selected because it could dive vertically from its service ceiling until reaching max speed (terminal velocity) at 10,000 feet and could then be safely pulled out because of its designed 8G load factor.

Wildcats had also been in service squadrons for five years and had made hundreds of operational dives without any difficulties; in many of these dives, they pulled more than 8G.

Between March 14 and April 3, 1944, I flew 10 flights that totaled almost 6.8 hours in a program that consisted of many build-up pullouts to 8G at three speeds that covered the Wildcat's complete flight envelope. It gave me practice in accurate G pulling at many dive angles and at several altitudes. To verify the speeds achieved, this airplane had only a Navy velocity/G recorder.

To my delight, the day after my last dive in the Wildcat, I was designated the new F6F-3 Hellcat dive-demonstration pilot. I was soon to learn the old axiom, "Many times, Mother Nature gives us the final exam before she presents the lesson."

The Hellcat and I meet "Compressibility"

Connie Converse, Grumman's chief test pilot, had performed the initial lower-G, build-up pullout maneuvers required of the demonstration. In addition to those of airport manager, his extra duties of hiring, training and educating more than 50 production test pilots became so onerous that he asked to be relieved of his flight-demonstration tasks.

As the new Hellcat demonstration pilot, to start the Navy's demonstration program, I was instructed to reach the high-speed dive point of 485mph (580mph true airspeed) combined with a 2.SG pullout-a seemingly easy end to a straightforward dive.