High Bypass Turbofan Engines

Readers will recall that with a low-bypass turbojet engine, some of the fan air is "bypassed" outside of the "hot" core, but within the casing of the engine before being mixed with the hot airflow at the jet efflux. The high-bypass turbofan engine may usually be recognized by the fan air being ducted outside the engine casing some one-third along the cowling. However, some of the smaller turbofans (e.g. Rolls-Royce Tay and FJ44; the Pratt & Whitney Canada PW500 series), to increase performance and reduce specific fuel consumption and noise, mix the hot and cold airstreams within the engine casing just before the efflux, through an integrated nozzle.

A Story About Two Small Old Aircraft Engines

Uncle Tom Maddock and his brother Charles had built the pontoons in 1929 and mounted an old fuselage and engine on them. They ran their creation up and down the canal in front of the farm. Tom said that cars would stop to watch them and one observer said, "It’s Nungessor and Coli, they finally made it!" (Nungessor and Coli were intrepid French airmen who took off from France for a trans Atlantic flight attempt in 1927 and were never seen again)

The idea so intrigued me that as a teenager in 1952, I asked Uncle Tom if I could have the pontoons. He was delighted that I was interested and told me to help myself. My friend, Ed Franco-Ferreira and I loaded up the pontoons and took them to my mother’s garage in nearby Titusville, New Jersey. The pontoons were wooden frames covered with fiber-board that looked like Homasote. By the time we sealed and painted them they were so heavy that there was no danger of them ever becoming airborne. We bought a used O-145 Lycoming aircraft engine and propeller in running condition through Trade-A-Plane for $50.

Aero Engine Drawings by Frank Munger

Napier Lion

vor Benzineinspritzung (before fuel injection)
The Mercedes-Benz DB 600 Carburettor System

Think of the WW II Mercedes-Benz DB 600-series engines and what springs to mind? Usually, inverted installation, engine mounted cannon, direct fuel injection and variable-speed supercharger drive come top of the list. While it is true that these distinctive features were to be found on all the DB 601, 603 and 605 engines, the predecessors to these marks were not quite so sophisticated.

A Detonation Scenario

With the onset of detonation, piston and cylinder head temperatures have started to rise dramatically. Although the pilot is keeping a careful watch on the engine instruments, he detects none of this. There is no discernable noise, vibration or smoke — at least not yet. There is also no indication of temperatures rising — the one cylinder head temperature probe that came standard on the Cessna 401 left engine is on a different cylinder, one that is running happily and normally along.

Although detonation in general aviation aircraft engines is rare, it is far from being unheard of. GA aircraft engine detonation is nearly always the result of excessive cylinder head temperatures brought about either by incorrect handling of the engine by the pilot, or by maintenance problems. In the case of this Cessna 401, the pilot is flying by the book, but deterioration of some aging components has eradicated the detonation margin provided by the engine manufacturer’s design.

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