Wolfram Kuss > > >Spitfire Mk IIA > > Ah - surprising! > > Here is an email Rick "Fuelcock" sent me a short while ago. I > hope it helps. Sorry for the poor formating. > ------------------- snip --------------------- > Rather than send you the GBE code , I will direct you to the > site where I got > it: > > http://www.aeromech.usyd.edu.au/aero/propeller/prop1.html > > Just click on program 1 at the bottom of the page. Program 2 (right > below) > is mathlab code for the implementation without the bells and whistles. > > I have been playing around with program 1, and have obtained > very encouraging > results. > > I keyed in a "Spitfire" prop with radius 1.55 m and a blade > area of 0.98m^3. > The program only let's you design a simple blade with a > straight, symetrical > taper. Rather than complicate things, I just kept the cord > constant at .210 > the radius to give a total area of .327m^3 per blade. Not > knowing anything for > sure about the blade angle at a given radius, I just used the > default pitch > of 0.5, where: > > pitch = 2pi * r tan theta and theta is the geometric angle > of the blade at > r. The model also lets you tilt the whole blade +/- any > desired angle setting. > Assuming the max speed of the Spitfire to be 154.7 m/s, I > toyed with angle > setting until I obtained a max prop efficiency at angle > setting 19.45, J value > of 2.09, which corresponds to a true airspeed of 154.7 m/sec > for a 1.55 m > radius prop, engine running at 3000 rpm and gear ratio 0.477. > The model produced > a theoretical efficiency of about 85%, with Cq = 0.071. > > Next, I calculated the torque, using the formula Q=Cq * rho * > n^2 * D^4, > where n is prop rotation in revolutions per second ( the code > converts this to > radians) and D is prop diameter. > I assumed rho of 0.5 Kg/m^3, an altitude of about 15,000 feet. I than > multiplied the torque by angular velocity in radians per > second, to get the power > (watts) needed to counteract the torque of the prop. This > worked out to 865 KW, > which converts to 1159 HP. This is about 10% hiigher than what the > Merlin > could actually put out at the shaft, but it's pretty damn > good. Remember, the > model is known to be about 5% to 10% too optimistic in > predicting performance, > so if you take this into account, the prediction is nearly spot on! > ------------------- snip --------------------- > > >Regards > > > >Vivian > > Bye bye, > Wolfram. >
Why surprising? The choice was made with _impeccable_ logic: Eric Hoffman found the POH on the net, and I found some accurate 3d drawings in a book in my local bookshop. The math is helpful. At first glance some of the input data are a little off, but it shows the proposal, briefly made during the discussion on the performance of YASim, that 2850 was the propeller rpm, not the engine rpm at "cruise" cannot be sustained. I had already shown by some pretty simple math that at 2850 rmp the tips of a 1.65m radius propeller would be supersonic and therefore highly improbable, but we now know that the data of hp, gear ratio, rpm etc all tie together. Thanks Vivian Meazza _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
