When gyroscopic instruments were initially developed for aircraft in the 1920's there were no electrical systems in most airplanes. Engine driven pneumatic pumps (pressure or vacuum) were expensive and subject to failure. The worst case scenario would be having a pressure pump fail while in flight. When this would happen, not only would the pump have to be replaced, but pieces of the pump would be pushed into the gyros which would destroy them also. The cheapest and most reliable means of powering gyros for instrument equipped airplanes was to install a venturi outside of the airplane which would create a source of vacuum pressure while in flight. Then it became a simple matter of designing vacuum gyros to tap into the power source. You could say this is another application of the KISS design principle (Keep It Simple, Stupid).
Not all pneumatic powered aviation gyros are vacuum driven. Most pressurized piston engine aircraft designed for high altitude flight have pressure driven gyros if they do not have a dual electrical system. When you are talking about flying above 20,000 feet in such very thin air, venturis and engine-driven pumps simply cannot supply a large enough pressure differential to allow vacuum gyros to work reliably.
As electrical systems became common in small airplanes, vacuum driven gyros were supplemented by electrically driven gyros to meet redundancy requirements for IFR flight. In the 1960's and 1970's more reliable engine driven vacuum pumps became commonplace, replacing the older venturi-driven vacuum systems. However, vacuum driven gyros remained in widespread use in GA aircraft up until EFIS systems began replacing them in larger aircraft in the 1980's and 90's and glass cockpits became the norm for most GA manufacturers in this decade.
I have left out a lot of details in order to keep this short. You can find entire books that have been written on this subject if you look hard enough. Best thing to check at the library would be the history of General Jimmy Doolittle and the Sperry Corporation, who together pioneered aviation gyro systems as we know them today. General Doolittle was also credited with making the first flight solely by reference to instruments from takeoff to landing (!) in the late 1920's. That is still an amazing feat when I think about it.
I think you can still find vacuum gyros in most glass cockpit airplanes that do not have a secondary electrical system. They will be tucked away somewhere, whirring away as a backup to fall back on in the event you have a total electrical system failure.
Now back to watching "Smokey and the Bandit" for me. Hehe
RC
