On Monday, 3 September 2018 03:39:58 UTC+10, 2G wrote:
On Thursday, August 30, 2018 at 10:44:06 PM UTC-7, wrote:
Watch this.....and consider your life.

https://youtu.be/oYpG0HDcFsA

That video claims that "the model jet engine is a scaled down version of the real thing. It is essentially the same technology that lifts enormous aircraft into the sky." This couldn't be anymore further from the truth. Watch this video and tell me that they are "essentially the same technology:"
https://www.youtube.com/watch?v=VfomloUg2Gw

What struck me on the models is that the most vital components, the fan and compressor blades, are manufactured out of aluminum. Aluminum is not a high temperature metal. One pilot who had multiple start failures of these jets told me that ONE problem was the clearance between the fan blades and the housing, which must be tight to begin with for the jet to work. The shear number of failures I have personally heard of convinced me that I would not a jet sustainer.

Tom

The reason the compressor and diffuser are aluminum is because they do not get very hot. The compressor is usually a truck turbo compressor made by Honeywell or Garrett. IIRC Honeywell bought Garrett.
Yes the principles on which the small jets operate are the same as large ones although the small ones are turbojets with no fan, just a centrifugal compressor which is much less fussy about inlet flow distortion and much more robust than a small axial compressor would be to debris impact. At least one small axial compressor engine of about 27 Kg thrust has been made though.. It was intended to drive an alternator in a hybrid car to be made by Jaguar and fuel consumption was an issue hence the axial compressor to get higher pressure ratios. You could get higher pressure ratios by using a two stage centrifugal compressor (second stgae compressor needs to be steel)but nobody ever made a turbojet like this although the Rolls Royce Dart turboprop had this configuration.
The small engines are pretty much small scale models of the De Havilland Goblin engine from 1943 (it was a good engine)which shared their layout of single stage centrifugal compressor and single stage axial flow turbine, the difference is the small engines have a cannular combustion chamber instead of individual burner cans.
Modern turbofans use axial compressors for higher pressure ratios for better fuel economy. They operate at far higher temperatures in the hot end with actively cooled turbine blades by blowing cool air through them which comes out in many small holes. The turbine stage can operate at temperatures above the melting point of the blades. They also do things like blow ducted air around the outside of the turbine shroud to minimise clearances.
In the small engines the weakness is the rear bearing which is in the turbine wheel. It is the one that gets hot. Possible solution is to not run at 100%, where the turbine temperatures are much higher than at 75% to 85%.
There are now several engines in the thrust range required (around 40Kgf) made by AMT, TJ, Jetcat and BF Turbine. The BF 500 in particular is actually a 50 Kgf engine derated to 40 Kgf.
It is early in the development cycle for glider use. There are only the SharkJet with an AMT Titan, The PSR system and the M&D jet on the JS gliders.
One weakness is the kero start system on all these which is why fuel and vapor pressure and flash point of same is important and they are fussy on fuel. The M&D jet used to splatter fuel around. JS solution was to drill a nice hole in the bottom of the engine compartment. Latterly they have put the igniter in the bottom of the engine rather than the top and made a small improvement. I can see why. Kero or diesel can make a pretty good fire extinguisher in the right circumstances and can be difficult to get burning. There are possible solutions.
Chris Esselsteyn doesn't seem to be having much trouble with the 2 x Jetcat P300 engines in his HP18 jet but jetcat have an internal igniter which seems to work well. he told me he gets essentially no excess fuel on start. He was at Oshkosh with it again this year.
I suspect a large proportion of failures with the jets is poor operating procedures(dirt in fuel) and or poor/non existent inspections.
The other jet that needs mention is the TJ100 engine that Bob Carlton uses and sells. That engine has a real recirculating oil system and seems excellent. Being used in the SubSonex jet kitplane too. It started life as the APU for the MiG 29.

Mike