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From: Stuart Buchanan <stuart13@gm...>  20100902 18:39:56

On Sat, Aug 28, 2010 at 9:09 PM, HBGRAL wrote: > Hi all > > I sent a small merge request to fgdata repo > http://gitorious.org/fg/fgdata/merge_requests/38 > > I was looking to the c172p directories and found some unused files and > some wrong paths coming from last commit (?). > >  effects in models had wrong paths, (re?)created a folder Effects >  removed unused graphic files in top models directory >  changed some .rgbgraphic files to .png (50 % data) > > I did not change any of the graphics or any other code. It is just a > small clean up and I checked the log well after this changes. > > Feel free to merge in, or not. This has now been merged. Thanks very much for the cleanup! Stuart 
From: <thorsten.renk@jy...>  20100902 07:40:07

A few comments to your math: > I haven't done anything (yet) with Hal's exhaust thrust function. I was > waiting for someone else to work out the math... > > The exhaust thrust function should return a force. Newton tells us: > > F=ma Indeed, but not very helpful here... force as a timederivative of the momentum is what you need, i.e. F = dp/dt = dm/dt v_E + m * d v_E/dt = dm/dt * v_E using p = m * v_E and v_E = const. (as you also use yourself below). > Hal had said something like > > F=m*v^2 That would be the kinetic energy  as you point out correctly, it has the wrong dimensions. > m_dot = m_dot_air + m_dot_fuel for our model. I am prepared to bet that for an aircraft engine (unlike a rocket) dm_air/dt >> dm_fuel/dt is always true, i.e. you can forget about the fuel content. > m_dot_air could also be > estimated as a function of RPM, engine displacement, and manifold air > density. Something like: > > m_dot_air = > (1/2)(RPM/60)(rho_manifold)(displacement)(volumetric_efficiency) Sounds very reasonable to me. > [2] gives us a formula for v_e: > v_e = sqrt( (T*R/M) (2k/(k1)) [1  (P_e/P)^((k1)/K)]) > > See the wikipedia article for a full explanation of the formula. Yes, this says that the thermal velocity is proportional to the root of the temperature. That is so because temperature (in suitable units) is a measure of the mean kinetic energy of gas molecules, so if you write T ~ E_kin = M v^2 you can solve it for v and get v ~ sqrt(T/M) which is what the formula says. The rest gathers the physics of expansion and pressure gradient. Note that units matter here! The temperature must be given in Kelvin [K] in order to agree with the interpretation as unit of energy measure. I doubt that the egt property is in K  so you need to convert to proper units before inserting into the formula. > k~= 1.4 > > for exhaust temperatures up to 1000 degrees C. > so the second term becomes: > > (2k/(k1)) ~= 7.0 > > The third term > > [1  (P_e/P)^((k1)/K)] > > Can be thought of as an efficiency factor, It should always be greater > than or > equal to zero and less than or equal to 1. Its value probably increases > with > pressure altitude as I believe that will lower P_e (pressure at the exit > end > of the exhaust manifold); P (pressure from the cylinders when the exhaust > valves open) is probably related to manifold pressure and RPM. I guess that's a good starting point  the lower pressure would be outside pressure, the higher pressure manifold pressure. I don't see how RPM come in though. > It may be > appropriate to think of this as a constant, too. Why  you have outside pressure and manifold pressure, so you can compute it dynamically. It's not going to be a big effect though... p_E/p is usually a small number. Cheers, * Thorsten 
From: Ron Jensen <wino@je...>  20100902 02:14:27

On Tuesday 31 August 2010 21:08:32 Jon S. Berndt wrote: > Ron, Hal, > > Just a reminder that this feature (arbitrary functions in engine > definitions)  I believe  works and is now in JSBSim CVS. Maybe it should > be moved to FlightGear? > > Jon I tend to keep my FlightGear uptodate with the latest JSBSim. I keep a git clone of this at http://gitorious.org/fg/jentronsflightgear. I don't have write access to the master repository. I haven't done anything (yet) with Hal's exhaust thrust function. I was waiting for someone else to work out the math... The exhaust thrust function should return a force. Newton tells us: F=ma or Newtons=kg*m/s^2 (SI) lbf=slugs * ft/s^2 (US pound as force) poundals = lb * ft/s^2 (US pound as mass) For the rest of this discusion I'm going to use the US pound as a force system. Hal had said something like F=m*v^2 but that yields slugs*ft^2/s^2. Not the correct units. F=m_dot * v_e [1] where m_dot is total mass flow per second and v_e is the velocity of the exhaust stream. lbf = (slugs/2)*(f/s) = slugs* ft/s^2 m_dot = m_dot_air + m_dot_fuel for our model. FGPiston currently calculates m_dot_air internally, but does not expose it as a property. m_dot_fuel is a property, and we could potentially use it, along with the mixture setting, to estimate m_dot_air. The problem with this approach is m_dot_fuel can be zero while m_dot_air still has a nonzero value. m_dot_air could also be estimated as a function of RPM, engine displacement, and manifold air density. Something like: m_dot_air = (1/2)(RPM/60)(rho_manifold)(displacement)(volumetric_efficiency) [2] gives us a formula for v_e: v_e = sqrt( (T*R/M) (2k/(k1)) [1  (P_e/P)^((k1)/K)]) See the wikipedia article for a full explanation of the formula. k~= 1.4 for exhaust temperatures up to 1000 degrees C. so the second term becomes: (2k/(k1)) ~= 7.0 The third term [1  (P_e/P)^((k1)/K)] Can be thought of as an efficiency factor, It should always be greater than or equal to zero and less than or equal to 1. Its value probably increases with pressure altitude as I believe that will lower P_e (pressure at the exit end of the exhaust manifold); P (pressure from the cylinders when the exhaust valves open) is probably related to manifold pressure and RPM. It may be appropriate to think of this as a constant, too. A first guess would be ~0,4. The first term: (T*R/M) contains two constants, R and M, so this value is completely controlled by T, our exhaust gas temperature, Exhaust gas temperature is available as a property in FlightGear but not JSBSim standalone. In short, I propose the following formula for v_e: v_e = sqrt( egt * k) where k = R/M * 7 * 0.4 Thanks, Ron  [1] http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Specific_Impulse [2] http://en.wikipedia.org/wiki/Rocket_engine_nozzle#1D_Analysis_of_gas_flow_in_rocket_engine_nozzles 