2011/6/18 Gary Neely <grne...@gmail.com>: > On Sat, Jun 18, 2011 at 10:38 AM, Bertrand Coconnier <bcoco...@gmail.com> > wrote: >> 2011/6/17 Gary Neely <grne...@gmail.com>: >>> I've always understood actionpt to be the location where thrust should >>> be applied with respect to the airframe. For a propeller-driven >>> engine, I use the approximate location of the main thrust bearing. For >>> a jet, I reckon it depends on the type of jet and the degree of >>> bypass. An older jet engine develops its thrust from the exhaust >>> chamber region. Modern engines with high bypass ratios develop more of >>> their thrust from the fan, so the action point would likely move >>> forward closer to where the main thrust bearings of the fan are >>> located within the engine. I'm not an engine expert by any means, but >>> these are the assumptions I've used. >> >> Moving the point of action of a force along the line of action of the >> aforementioned force does not change the moment. >> >> Since the thrust line of action is almost parallel to the >> turbine/propeller/fan shaft, moving the point of action from the fan >> bearing to the exhaust region will only marginally change the >> resulting moment. So my advice FWIW is to not bother about that. >> >> Bertrand. > > > I agree, with reservations. Some engines, for instance some > turboprops, have thrust bearings significantly offset from the > engine/prop mass. Perhaps that's trivial in most cases, but in my > opinion if the designer has good information on where an actionpt > would reside, it makes sense to use that information. Working toward > fidelity is part of the fun of this stuff. >
What is the offset of the bearing with respect to the engine center of mass ? A couple of inches (I remind you that the offset along the thrust direction has no effect whatsoever) What is the engine distance from the aircraft CG ? Several feet So you are correcting the distance and the moment by a few percent and thus you are correcting the tail incidence by a fraction of this few percent (because the other moments are not affected by the bearing offset). On the other hand, YASim does not take into the wing downwash or the propwash while computing the tail incidence (while they are first order contributors) so I would not use the word 'fidelity'. Bertrand. ------------------------------------------------------------------------------ EditLive Enterprise is the world's most technically advanced content authoring tool. Experience the power of Track Changes, Inline Image Editing and ensure content is compliant with Accessibility Checking. http://p.sf.net/sfu/ephox-dev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
