-------------------------------------------------- From: <cas...@mminternet.com> 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, 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 ------------------------------------------------------------------------------ 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
