On Friday, September 03, 2010 01:49:46 am thorsten.i.r...@jyu.fi wrote: > In the end, there are hundreds of things you don't know - friction in the > exhaust tube for example... its geometrical shape (exhaust velocity isn't > actually a constant - there's some spatial profile to the velocity > field)... turbulence when the exhaust airstream enters ambient air... and > so on.
In fact things like this can be very difficult to quantify. With regard to tube shape early RR Merlin engines (meaning before 1943) had "ejector exhaust" stacks that were designed by RR. At around that same time that RR was doing their testing on these "ejector exhausts" NACA was doing experiments on exhaust stack shapes with Alison engines to maximize exhaust "jet" thrust. The NACA configuration was later compared to the RR configuration and there was about a 60% increase in the exhaust thrust/reduction in profile drag over the RR configuration. The Spitfire V that was used for these tests had a 6 MPH increase in top speed with the NACA stacks compared to the RR stacks. Later Merlins all had the NACA stacks because of the increased thrust/reduced drag. In the paper on these tests NACA wrote that they could not quantify how much of the speed increase was due the thrust and how much was due to profile drag if any. Another example is that similar engines (same displacement, manifold pressure and RPM) with a mechanical super charger vs. a turbo (think super charged vs. turbo'ed Alison engines - IE. P-40 vs. P-38 configurations) are likely to have significantly different exhaust thrust because the turbo has used a significant amount of the energy in the exhaust perhaps to the point where the thrust is too low to worry about. The point is that the same exact aircraft with the same engine with two different sets of exhaust stacks had a 6 MPH difference in top speed with exactly the same amount of air and fuel being pumped through the engine. Which means that this speed/thrust difference can not be accounted for by a thermal dynamic formula. This leads me to think that having a good generic formula that gives a good approx. of the shape of the exhaust thrust curve along with a constant to tune the thrust levels to the engine/exhaust system in question should work. Hal ------------------------------------------------------------------------------ This SF.net Dev2Dev email is sponsored by: Show off your parallel programming skills. Enter the Intel(R) Threading Challenge 2010. http://p.sf.net/sfu/intel-thread-sfd _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
