Hey, all you turbo guys out there....

Stumbled across an interesting book the other day, and those of you who like me, are easily amused, might find it worth checking out....

"German Jet Engine and Gas Turbine Development - 1940-1945" by Antony L Kay (ISBN 1 84037 294 X)

Absolutely fascinating stuff (OK, only to nerds like me....) concerning the development and engineering challenges presented by the attempts to develop the first jet engines.....

Of particular interest is stuff about materials and vibration issues. Recall of course that computers and FFT analyzers weren't even dreamt of.....

Examples --

On some of the early BMW jets, compressor blade failures in the first and second stages plagued their prototype engines. They knew that it was a blade resonance issue, but had no way to measure....so they built in glass windows in the compressor section casings, attached tiny sharp pins to the blade ODs, ran the engines, and measured the resulting scratches on the glass windows to measure amplitudes. they then called in PIANO TUNERS to set with pianos in the test labs while blades and disks were struck with hammers and the resulting frequencies matched to piano pitch tones.......

Curiously, the book does not discuss balancing in any detail, even though many of the rotors in these engines approached 10K RPM......

Equally interesting is how many of these German WWII-era engineers ended up working in the US aerospace propulsion industry, some into the 1980's......wierd.

Thought you guys might find this stuff interesting.....

Not just working for NASA but running NASA. Same as the SS Nazi generals running the CIA and developing the cold war for their job justification; IMHO. But we did hide out some ex-nazi war criminals in SA. Records are now open.

You got it; a lot of interesting stuff surrounding 1930's+ Germany and the end of WW2.

Apart from the political aspects, I am surprised about the appearant lack of methods to measure blading vibes that you mention. I reserve a sceptical source critical approach to the book as a historically objective complete description.

The swedish ASEA (now ABB) owned steam, gas (and aero) jet engines turbine manufacturer here in Sweden early used slip rings and strain gauges in a vacuum test chamber to run these tests. The first swedish jet engine named Skuten (the local city lakes name) was developed 1944 to 1947. Many experiments were run during the last years of the 2nd WW. A swedish defense authority descision to run all airplanes with only jet engines was taken already in September 1945. Also early 1945, brittish de Haviland Goblin engines were tested in flight and later used in the jet fighter J21. It was started commercially from the airfield where I live in 1947. The J21, being a converted propeller ap, was soon developed to the first original swedish jet fighter called R1001 and later in service as J29, Tunnan ("the barrel"). You can still see it at a podest at the local interstate road outside the SAAB town Linkoping.

Saw it all for real as a kid and have many photos and some report of the measurement setups (sliprings, strain gauges, capacitive sensors and various exciting methods) that was developed over decades and was the base of development for the swedish stationary gas turbine development. (Like the GT35, a very rugged workhorse spread all over the world). It was a de facto modal analysis without a PC, with a method based on a lot of measurements and many manual setups. Still have early functional german, danish and US manually tuned filters with vacuum tubes and sensors from these ages in the museum.

Much later I have been visiting one of the BMW plants looking at many historical jet engine items and similar development tools north of Frankfurt. I am not convinced that BMW engineering was that primitive as you refer to. Maybe, I am just saying maybe, Mr Kay has made a text with less than a fully objective historical interest showing all facts with an angle to stress they where primitive. Maybe the strings you mention where just a small part of the experiments? At any rate, I have a hunch that the level of knowledge was much much better than the book hints to.

From time to time I see signs that there is not much under the sun left to invent in this area, but just that it is not a pdf report and made by hand, it is not so popular to find out. All modern engineers have to fiddle around a bit only to find that a round wheel is a good design of a wheel :-) Best regards Arne

This message has been edited. Last edited by: Arne Lindholm,

quote:

they then called in PIANO TUNERS to set with pianos in the test labs while blades and disks were struck with hammers and the resulting frequencies matched to piano pitch tones.......

This is an interesting 'aside'.... was asked to look at a "chatter" problem in an aluminum foil rolling mill once. At a certain speed, the mill would just scream, or in my mind, whistle. My gut feeling was this was not a roll chatter problem, but rather a strip resonance problem. Measured the vibration, and then the "whistle" with a $75 Radio Shack digital sound meter (with digital output). Took the foil, went back to the office, set up the sound meter, and then used the foil as a 'whistle'.... the resulting frequency was the same as the vibration and sound measured at the mill. They changed the oil additive package to change the viscosity (and thus the damping of the strip within the roll bite) and the problem went away.

Sometimes a 'primitive' method can be a rather elegant tool.

Arne:

Re your comment about modern engineers doing a lot of fiddling..etc
Couldn't agree more. I've seen a few papers on Computational Fluid Dynamics where a big deal was made of the tools, and a huge effort to solve problem. However, with sound knowledge of the fundamentals, experience, and judgement, the solution approach could be developed on the back of an envelope in ten minutes!!

Rusty:
Re simple elegant tools..
Again, couldn't agree more.
eg Once investigated possible residual tension / compression in tubes of a large condenser tube bundle. Went at some tubes with a hacksaw. If the saw really bound up - tube obviously in compresion. If tube "sproinged" apart, we used gap less blade width to estimate tensile load. Plant guy said it was the best research test he'd ever seen!!