# Some questions about big jet engines **URL:** \ **Category:** Factual Questions\ **Created:** [November 18, 2015, 11:54am UTC](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877 "2015-11-18T11:54:24Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [November 19, 2015, 3:45pm UTC](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877/21 "2015-11-19T15:45:10Z")
> [@LSLGuy](#): > > @ **jz78817** As a first approximation it’s a fight between aspect ratio and torsional stiffness. Aspect ratio in aeronautical jargon is span/chord. It is the thickness of the wing that is needed to add torsional stiffness…increasing the chord does little. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [November 19, 2015, 5:40pm UTC](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877/22 "2015-11-19T17:40:14Z")
> [@Kevbo](#): > > The earliest aircraft jet engines used radial compressors and turbines, but designers quickly worked out how to make axial designs work, and these package a bit better. There are still some turboshaft and APU engines that use radial compressors. My dad used to fly Fokker F27s, which have Rolls Royce Dart engines with centrifugal compressors. Those were produced through 1987 and I imagine there are still a few in service somewhere. Does anyone make a radial-compressor turboprop in that power class anymore? I remember seeing pictures of some of the early jet engine designs from World War II, and not really being able to tell what was what. Took me a while before I found out there was such a major design difference with axial-flow engines. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [November 19, 2015, 5:46pm UTC](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877/23 "2015-11-19T17:46:56Z")
> [@Robot\_Arm](#): > > My dad used to fly Fokker F27s, which have Rolls Royce Dart engines with centrifugal compressors. Those were produced through 1987 and I imagine there are still a few in service somewhere. Does anyone make a radial-compressor turboprop in that power class anymore? > > I remember seeing pictures of some of the early jet engine designs from World War II, and not really being able to tell what was what. Took me a while before I found out there was such a major design difference with axial-flow engines. The Pratt & Whitney PT6 combines the two, several axial compressor stages feed into a centrifugal compressor. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [November 19, 2015, 6:26pm UTC](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877/24 "2015-11-19T18:26:41Z")
> [@Robot\_Arm](#): > > Does anyone make a radial-compressor turboprop in that power class anymore? In that range, you’re likely to find hybrid compressors - multiple axial stages, that facilitate installation of variable guide vanes to regulate air flow at part power settings, feeding into a centrifugal section. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [November 19, 2015, 9:34pm UTC](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877/25 "2015-11-19T21:34:46Z")
> [@Kevbo](#): > > Aspect ratio in aeronautical jargon is span/chord. It is the thickness of the wing that is needed to add torsional stiffness…increasing the chord does little. Good catch. Thanks. ---
**Author:** ![TheBori](https://avatars.discourse-cdn.com/v4/letter/t/ee7513/32.png) [@TheBori](https://boards.straightdope.com/u/TheBori)\ **Post date:** [November 20, 2015, 6:10pm UTC](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877/26 "2015-11-20T18:10:48Z")
Getting back to clearance…I used to work for a company that manufactured the feed stock for plasma sprayed coatings on turbine engine blades and vanes. One of the ceramic powers I worked on was used as an abradable seal for the turbine section of the engine. The material created a porous structure, like a sponge but with much, much smaller holes. Once the blades were installed the shaft would be rotated and the blades would cut their own seal. It made a lot of crunchy noises but resulted in the best fit you could get. As was quoted above, the primary fan doesn’t seal much at all, its’s just a propeller. If you want to see fan blade movement, check out this [bird ingestion test](https://www.youtube.com/watch?v=rSafRuLB0c0)…at about 30 seconds. [Previous page](https://boards.straightdope.com/t/some-questions-about-big-jet-engines/737877.md?page=1)