On Tuesday 17 August 2010 16:33:27 Hal V. Engel wrote: > This seems to be working. A few comments. > > The 70 lb thrust figure was for an early merlin spitfire (IE 1938) with > about 1200 HP war emergency power and with the pre NACA exhaust pipes. The > spitfire used in the NACA tests in 1942 had a Merlin XLV engine which would > have made it a mark V with 1470HP. So for a 150 octane V-1650-7 (with NACA > exhaust pipes) that a late model P-51D would have been using the exhaust > thrust at max power (1940 HP at 81 inHg) should be closer to 200 lbs. > > I am a little confused about why you include engine HP in the thrust > function. For super charged engines there is a considerable amount of power > used for driving the super charger (probably about 150HP on low blower and > about 350HP on high blower at 67 inHg for this engine) so this would result > in lower exhaust thrust on high blower. But the exhaust thrust (at the > same RPM) should be the same if not higher at higher altitudes for a given > manifold pressure. > > If my memory of physics is correct the thrust should be proportional to: > > V^2 x M > > Where V is the velocity of the ejecta and M is the mass of the ejecta. The > amount (or mass) of ejecta should be the same per engine revolution at a > given manifold pressure. This makes me think the function should be using > RPM instead of HP.
I didn't give the thrust calculations much thought, I pulled HP because I didn't want thrust from a non-running engine and HP is largely dependant on the mass of fuel/air mixture moving through the engine and we don't have m-dot-air available to use. The exhaust pressure from a windmilling engine should be significantly lower than for a running engine. http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node26.html fig 3.9 If ignition never happens the 2->3 pressure increase doesn't happen and 3->4 follows the same line as 1->2. I was thinking last night that this would increase the velocity of the ejecta, but on further review, perhaps it does not. My function for the magnitude of the exhaust force was done rapidly off-the-cuff as a concept model for using the external force. My feeling would not be hurt if you came up with a much better model. :) At the very least, I would suggest combining the three constants in the name of processing efficiency. > In addition I think altitude comes into play as well since it seems logical > to me that the velocity of the ejecta would be higher at higher altitudes. > That is lower exhaust back pressure should result in higher exhaust gas > velocity but I don't know if the velocity increase is enough to worry > about. But it is squared so even a small change in the ejecta velocity > could be a significant factor. Like the pressure difference between the running and non-running case, the same mass moves through the same tube in the same time so would there be a change in ejecta velocity? Jon is the rocket scientist here. :) > I think the location of the force should be about where the center of the > engine is located since this is approx. where the exhaust stacks are > located. Makes sense. > I changed my local version to use RPM instead of HP and adjusted things to > better reflect the V-1650-7. I also moved the location of the force. Thanks, Ron ------------------------------------------------------------------------------ This SF.net email is sponsored by Make an app they can't live without Enter the BlackBerry Developer Challenge http://p.sf.net/sfu/RIM-dev2dev _______________________________________________ Flightgear-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/flightgear-devel
