Boost Pressure
Does Anyone Understand It?

After even a short ramble through the vast body of aero-engine literature, one thing becomes clear. If the subject of "boost pressure" comes up, every author seems to have a different idea of what it is!

Often, it’s thought of in terms of human experience of the atmosphere, i.e. pressure in excess of the ground level air we are all familiar with. Or, perhaps, boost pressure is associated only with superchargers, turbos, forward-facing air intakes or even blowing up balloons! Opinions vary.

However, as "seen" by an engine, boost pressure is far more simple...

Reno 2005

The "Bear is Back" was heard around the pits and throughout the stands all week, and once again she won in Sunday’s Unlimited Gold Final. Rare Bear’s winning speed of 466.3 mph wasn’t record breaking (Gold race record Dago Red, 487.9 mph in 2003 the year Rare Bear qualified at 497.8 mph) because the only real challengers were Dan Martin’s race modified P-51D Mustang Ridge Runner III and the Sander’s brothers R-4360-powered Sea Fury, Dreadnought. Ridge Runner burned a piston on the third lap of the race while Dreadnought cruised to a strong second. The apparent "secret" ingredient for this year’s Rare Bare is a wing shape modification that significantly improves its aerodynamics, making it unlike any previous Bearcat. Competitors following in the wake of "the Bear" reported a much smoother ride compared to configurations and propellers used in previous years, suggesting that the modification may have considerably reduced drag while making the airplane easier to handle.

Aero Engine Drawings by Frank Munger

Bentley B.R.2

The Curtiss OX-5

The Curtiss OX-5 was the most important and successful American engine of the First World War.

Before Liberty enthusiasts suffer a seizure, let’s check the facts. The only significant Liberty powered aircraft to see WW1 active service was the DH4. In 1918 at the Armistice less than 200 DH4s, flyable or not, were at the front. In contrast more than 8,000 OX-5 powered Curtiss Jennies were built, training wartime pilots and supporting postwar barnstormers.

The big mistake is to judge the OX-5 and Liberty by the same standards. The Liberty was a high technology engine, drawing on the knowledge of the U.S. automobile industry and Mercedes racing practice. The OX-5 was a low cost prewar design and not everybody loved it. "A failure looking for somewhere to happen" —James Gilbert, author The World’s Worst Aircraft. "Probably the least reliable aviation engine in widespread use"—Herschel Smith, author History of Aircraft Piston Engines. "Always unreliable—suffered from appalling quality control"—Bill Gunston, aviation guru and author World Encyclopaedia of Aero Engines. "A lovely engine—dead reliable, beautifully made and bloody quick"—Mark Walker, UK OX-5 expert and successful aero engined racing car builder and driver.