I always enjoy finding new people who have a love for the old North Central Airlines and those who loved the Convair 580 propjet. The 580 was a mainstay of the fleet from 1967 to 1979. Rich Hoeg, alias Northstarnerd.org was one had an interest in the 580, in a way I had not yet heard of. He wrote in a recent blog entry:
"Sometimes it's amazing how data which has been buried in your brain for 40 years suddenly reappears. Most mornings, our company cafeteria posts a question on a bulletin board. People then write their responses on an easel ... sort of a physical Twitter or microblog!
- Today's question: What was your science fair project?
- My answer (from the 1960's): The Convair 580 turboprop engine!..."
I thought a science fair project on the 580's powerplant was very cool. I wrote Rich and he wrote me back saying, "....I have a collection of airline memorabilia dating back to the great airline strike of 1968. My collection has thousands of items, with North Central being the biggest airline in terms of stuff. I have most Northliner issues from 68 till they stopped publishing (first the employee newspaper, then the flight magazine). I was 12 years old at the time, and was supposed to fly down to Fort Dodge where my grandparents would pick me up to visit the family farm (via Ozark Airlines). I ended up ultimately making the trip, but via Braniff to Sioux City....and I flew many miles via North Central."
Rich doing a science fair project when he was young about the Convair 580's engine made me think that you might like to know what exactly is behind the prop in a turboprop or prop-jet engine.
Jet engines are the predominant powerplant for modern aircraft, but turboprops have found niche uses in both military and civilian applications. Sometimes they are called prop-jets. Many airlines, including North Central, when introducing the Convair 580 in 1967, liked to promote it as "jet-powered" to catch the attention of travellers sure that jet meant modern and fast and that anything prop was old and slow. What exactly is a turboprop or prop-jet engine?
NASA's education program at the Glenn Research Center in Cleveland Ohio explains them this way:
"To move an airplane through the air, thrust is generated with some kind of propulsion system. Many low speed transport aircraft and small commuter aircraft use turboprop propulsion. On this page we will discuss some of the fundamentals of turboprop engines. The turboprop uses a gas turbine core to turn a propeller. As mentioned on a previous page, propeller engines develop thrust by moving a large mass of air through a small change in velocity. Propellers are very efficient and can use nearly any kind of engine to turn the prop (including humans!). In the turboprop, a gas turbine core is used. How does a turboprop engine work?
There are two main parts to a turboprop propulsion system, the core engine and the propeller. The core is very similar to a basic turbojet except that instead of expanding all the hot exhaust through the nozzle to produce thrust, most of the energy of the exhaust is used to turn the turbine. There may be an additional turbine stage present, as shown in green on the diagram, which is connected to a drive shaft. The drive shaft, also shown in green, is connected to a gear box. The gear box is then connected to a propeller that produces most of the thrust. The exhaust velocity of a turboprop is low and contributes little thrust because most of the energy of the core exhaust has gone into turning the drive shaft.
Because propellers become less efficient as the speed of the aircraft increases, turboprops are used only for low speed aircraft like cargo planes. High speed transports usually use high bypass turbofans because of the high fuel efficiency and high speed capability of turbofans. A variation of the turboprop engine is the turboshaft engine. In a turboshaft engine, the gear box is not connected to a propeller but to some other drive device. Turboshaft engines are used in many helicopters, as well as tanks, boats, and even race cars in the late 1960's.
To learn more about turboprops like the thrust equation and other information:
Go to: the Glenn Research Center's beginners guide to aeronautics! I especially like this diagram. To me the end of the equation = POWER!
Here is an image of one of North Central's Convair 580s. You can see the large propblades on her starboard engine. These really impressed me as a kid. I knew these meant business. Click on image for larger version.

You can see this image and more of the prototype of the 580 at my Carr-Woolman Observation Deck 8. These images are from the San Diego Aerospace Museum archives. Some are labeled Model 240-21 which must have been the prototype designation. Basically a swooped up 240 with the Allison engines.
Also, for great information about North Central's Convair 580 in particular go to the definitive North Central history site, Heramantheduck.org by Scott Grieger.
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