The fact that the time changed after 4 flights is odd but the fact that the poor engine works at all in such awful (aerodynamic wise ) installations is a tribute to our little engines and the aerodynamic tolerance of centrifugal compressors. If this and similar installations were for a fan or axial flow engine the results would be just a series of stalls or surges such is the degree of turbulence at the engine intake.

I have actually had a Wren 44 stall in a a Savex L39 (which is not TOO bad for an unducted installation,) doing tailslides. there was a sort of burping noise (from the engine !!) on throttle up and the engine would not accelerate at all. I shut it down knowing that the EGT was probably rising. The second time this happened I had enough height to idle the engine, dive the airplane to re-establish good airflow to the engine, and then slowly accelerated the engine out of the stall and after that it ran fine.

Any jet engine need a smooth or at least reasonably smooth air inlet and anything else will not help performance, in terms of thrust or operating temps. at all . The installation shown has a virtual flat wall in front of it, the rear of the tank, further compounded by a FOD screen (in itself no bad thing ) but some of these unducted installations really are awful, in fluid flow terms ! Its made worse by another flat screen former (without any holes visible to allow any air to pass through it) at the rear of the engine The turbulence of the air as it meets the engine intake must be dreadful. The temps. rise with the hatch installed are another clue that all is not well.

Please, Birdseed, dont take it personally, I am really directing my remarks at ALL such installations. "designed" totally without any real consideration for engine intake requirements , and there are a lot around but in many instances, inefficient though they are we get away with it.

Tale a look at the book "Jet Engines" by Klaus Hunecke. Chapter 3 deals with intake theory and practice , in many of the totally unducted installations used by so many jet modellers EVERYTHING is wrong !



Regards,

David Gladwin.