On Thursday 31 March 2011 02:13:58 Alan Teeder wrote: > -------------------------------------------------- > From: <castle@...> > Sent: Wednesday, March 30, 2011 3:57 PM > To: "FlightGear developers discussions" > <flightgear-devel@lists.sourceforge.net> > Subject: [Flightgear-devel] Calculating free stick position and forces > > > Ok, here are a couple of thoughts on improving the elevator model; as > > always comments and suggestions are welcome, flames will be ignored > > > > @ line 220 or so of FGAerodynamics add: > > > > alphat = alpha + tailincidence - downwash; // alpha fro the tail > > > > for now we'll assume no flying tails just a normal hinged elevator, so > > the incidence is fixed and specified in the aircraft xml file. To > > calculate the downwash we use the following formula > > > > downwash = 2 * CL(wing) / (pi * AR) > > > > where AR is the aspect ratio of (cbar*cbar)/Sw, but cbar is actually a > > variable dependent on the wing/flap configuration; so we need to > > recalculate cbar whenever the configuration changes and, at some point, > > we'll need to consider any delta cbar as a time dependent variable based > > on flap extension/retraction rates. > > > > Obtaining CL and pi is trivial,
CL may not be so trivial, especially to a hard-coded function. I believe it would need its own table or at least the FDM scripter would need to designate a table/function to create it. Also, this theory of downwash is only applicable to aircraft in forward flight with small angles of attack. This formula will fail for helicopters and aircraft on the ground with a tailwind. > > so the task at hand is the best way to > > calculate the aspect ratio - something that only has to happen when the > > wing configuration changes. > > > > So once we have the alpha at the tail and assuming a trim tab set to some > > value the stick free elevator (aka floating) becomes > > > > elev_float = - ((b1*alphat + b3*trim)/b2*elevator) * alphat > > > > where b1 = (partial) Ch with respect to (partial) alphat > > and > > b2 = (partial) Ch with respect to (partial) elevator > > and > > b3 = partial Ch with respect to (partial) trim > > > > All that remains is to calculate the elevator balance position where Cm = > > 0; i.e., steady state flight aka a trimmed aircraft or > > > > elev_float = elev_balance > > > > I'll stop here for now. The next set of calcs would be to determine the > > control force gradient to apply to the stick as it is moved from the > > neutral (or zero stick force location). > > > > Regards > > John > > I applaud what you are trying to do but would caution at using such a gross > simplification for downwash, which is a very complicated thing to estimate. > (see the literature) > Perhaps downwash and hinge force calculations should be the subject of a > section in the xml aerodynamics file so that they can be tailored on a > per-aircraft basis. > > Alan I have to agree with Alan here, JSBSim aerodynamics is a blank slate to be written on with the XML files. This type of estimation should not be hard-coded other than possibly adding a metric property for tail incidence, but that can be variable on some aircraft as well. Thanks, Ron ------------------------------------------------------------------------------ Create and publish websites with WebMatrix Use the most popular FREE web apps or write code yourself; WebMatrix provides all the features you need to develop and publish your website. http://p.sf.net/sfu/ms-webmatrix-sf _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
