RE: [Flightgear-devel] Backing out Andy's p51d changes
Jim Wilson wrote P.S. Any chance someone good with nasal could write me a quick script for changing the BOOST value to 1.0 at 20,000FT ASL and then back to 0.4 at 19,999.99FT? I'd try and figure it out, but my time is very limited right now. tia If you still want this stuff - a Nasal file is attached which monitors altitude asl and changes the boost setting appropriately. I've added a 50ft dead zone to prevent hunting: seems to work OK here. Put it in P51d/models. You'll need to add this to the p51d-set.xml file: nasal p51d fileAircraft/p51d/Models/p51d.nas/file /p51d /nasal Regards Vivian p51d.nas Description: Binary data ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
RE: [Flightgear-devel] Backing out Andy's p51d changes
Jim wrote Hi Andy, On the p51d fdm configuration, it looks like the substantial change was actually increasing the turbo multiplier from 2.0 to 5.5, and not reducing the cruise speed as stated in the CVS log of March 23. The cruise speed change does have an effect, but it is fairly small. The problem with putting the turbo multiplier up in that range is the manifold pressure output is directly multiplied by that number. So full throttle produces an output of 164 inHG manifold pressure. We should be seeing about 61 inHG at sea level for this engine. Setting this multiplier lower to get the correct manifold pressure with turbo at sea level should reduce the maximum flight level for the aircraft since the second stage turbo cannot currently be modeled. On the other hand, using this lower value should NOT produce incorrect lower altitude performance since all the data I'm using is for below the 20,000 ft altitude where the second stage kicks in. The drag numbers calculated by YASim should be more or less correct up to at least up to 20,000 ft where the second stage would be kicking in. If there is a problem that setting the multiplier to 5.5 fixes, I suspect it is in the FDM design and not the P51D configuration. Any ideas how we can fix or work around this? Where do you get your numbers from for the boost? There are some contemporary figures around for the Rolls Royce built versions of the Mustang engine which indicate that the turbo multiplier should indeed be around 5: http://www.spitfireperformance.com/jf934climb.jpg Otherwise, you won't get the correct high-altitude performance. In the figure above extrapolate the boost curves back to sea-level, ignoring the effect of the Boost Controller (aka wastegate), and you will see that 5 is about right - even a bit more for the later Merlins. Don't forget that us Brits work in psi gauge, while the ex-colonies work in psi absolute: same thing +- 1 atmosphere. Although there is a bug in the current code which gives an incorrect readout of boost pressure (I forwarded a correction to Andy some weeks ago), the existing code gives pretty good results if you plug in the numbers right out of the book. I have just done it for the Merlin XX and was very impressed by the accuracy. Of course, we still have to model the gear-driven supercharger, but again I have forwarded some code to Andy which does this. We are still waiting to finalise some curves to match supercharger output. I've got some good-enough results here. Regards, Vivian ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Backing out Andy's p51d changes
Jim Wilson wrote: The problem with putting the turbo multiplier up in that range is the manifold pressure output is directly multiplied by that number. So full throttle produces an output of 164 inHG manifold pressure. We should be seeing about 61 inHG at sea level for this engine. But that's irrelevant: at sea level the wastegate setting* (or boost input, see below) should be clamping it. In order to reach the POH MP numbers at altitude, where the solution values are specified, you need this value. If you want to use a lower than real-life MP, you will need to re-specify the cruise parameters to an altitude where the engine is developing real-life power. Setting this multiplier lower to get the correct manifold pressure with turbo at sea level should reduce the maximum flight level for the aircraft since the second stage turbo cannot currently be modeled. What's wrong with the BOOST control input for this purpose? The second stage turbine is a manual level. Just map the first stage to a boost of 0.5 (or whatever is appropriate). If there is a problem that setting the multiplier to 5.5 fixes, I suspect it is in the FDM design and not the P51D configuration. Any ideas how we can fix or work around this? I'm just trying to reach the manifold pressures that the airplane is specified as reaching in its POH. What values are you using for cruise MP? Seriously: the supercharger in the Mustang *does* multiply the manifold pressure by this value. Are you sure it doesn't? Andy ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Backing out Andy's p51d changes
I wrote: at sea level the wastegate setting* Sorry, forgot to write this note to go with that asterix: * Superchargers don't have wastegates, of course. Instead, their behavior is generally an altitude-independent mapping of RPM to manifold pressure added to ambient. But the wastegate setting is a relatively sane way to get the same effect. Andy ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Backing out Andy's p51d changes
From: Andy Ross I wrote: at sea level the wastegate setting* Sorry, forgot to write this note to go with that asterix: * Superchargers don't have wastegates, of course. Instead, their behavior is generally an altitude-independent mapping of RPM to manifold pressure added to ambient. But the wastegate setting is a relatively sane way to get the same effect. Yes that makes sense, and actually this is what I remember from the original setup I did on that aircraft. But somehow I don't think it worked in terms of correctly approximating low end performance. We can try though with a 0.4 or whatever BOOST multiplier. The thing I'm wondering though is if the wastegate is working, why was the output 164 inHG at full throttle? It seems as though this used to work. Are the wastegate units inches of mercury? Thanks, Jim P.S. Any chance someone good with nasal could write me a quick script for changing the BOOST value to 1.0 at 20,000FT ASL and then back to 0.4 at 19,999.99FT? I'd try and figure it out, but my time is very limited right now. tia ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
RE: [Flightgear-devel] Backing out Andy's p51d changes
Jim Wilson From: Andy Ross I wrote: at sea level the wastegate setting* Sorry, forgot to write this note to go with that asterix: * Superchargers don't have wastegates, of course. Instead, their behavior is generally an altitude-independent mapping of RPM to manifold pressure added to ambient. But the wastegate setting is a relatively sane way to get the same effect. Yes that makes sense, and actually this is what I remember from the original setup I did on that aircraft. But somehow I don't think it worked in terms of correctly approximating low end performance. We can try though with a 0.4 or whatever BOOST multiplier. The thing I'm wondering though is if the wastegate is working, why was the output 164 inHG at full throttle? It seems as though this used to work. Are the wastegate units inches of mercury? The wastegate is working - the bug already reported is that the displayed value in the property tree is taken before the wastegate code is applied. This is quite useful when developing, because it is possible to check that the supercharger output is reasonable. Anyway, my earlier diff retained that while adding boost readouts post wastegate. Regards, Vivian ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
RE: [Flightgear-devel] Backing out Andy's p51d changes
From: Vivian Meazza Jim wrote Hi Andy, On the p51d fdm configuration, it looks like the substantial change was actually increasing the turbo multiplier from 2.0 to 5.5, and not reducing the cruise speed as stated in the CVS log of March 23. The cruise speed change does have an effect, but it is fairly small. The problem with putting the turbo multiplier up in that range is the manifold pressure output is directly multiplied by that number. So full throttle produces an output of 164 inHG manifold pressure. We should be seeing about 61 inHG at sea level for this engine. Setting this multiplier lower to get the correct manifold pressure with turbo at sea level should reduce the maximum flight level for the aircraft since the second stage turbo cannot currently be modeled. On the other hand, using this lower value should NOT produce incorrect lower altitude performance since all the data I'm using is for below the 20,000 ft altitude where the second stage kicks in. The drag numbers calculated by YASim should be more or less correct up to at least up to 20,000 ft where the second stage would be kicking in. If there is a problem that setting the multiplier to 5.5 fixes, I suspect it is in the FDM design and not the P51D configuration. Any ideas how we can fix or work around this? Where do you get your numbers from for the boost? There are some contemporary figures around for the Rolls Royce built versions of the Mustang engine which indicate that the turbo multiplier should indeed be around 5: http://www.spitfireperformance.com/jf934climb.jpg Otherwise, you won't get the correct high-altitude performance. In the figure above extrapolate the boost curves back to sea-level, ignoring the effect of the Boost Controller (aka wastegate), and you will see that 5 is about right - even a bit more for the later Merlins. Don't forget that us Brits work in psi gauge, while the ex-colonies work in psi absolute: same thing +- 1 atmosphere. Although there is a bug in the current code which gives an incorrect readout of boost pressure (I forwarded a correction to Andy some weeks ago), the existing code gives pretty good results if you plug in the numbers right out of the book. I have just done it for the Merlin XX and was very impressed by the accuracy. Of course, we still have to model the gear-driven supercharger, but again I have forwarded some code to Andy which does this. We are still waiting to finalise some curves to match supercharger output. I've got some good-enough results here. Hi Vivian, This sounds very interesting. I think I'll wait and see what Andy does with your patches before fixing the P51D again. Thanks, Jim ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
[Flightgear-devel] Backing out Andy's p51d changes
Hi Andy, On the p51d fdm configuration, it looks like the substantial change was actually increasing the turbo multiplier from 2.0 to 5.5, and not reducing the cruise speed as stated in the CVS log of March 23. The cruise speed change does have an effect, but it is fairly small. The problem with putting the turbo multiplier up in that range is the manifold pressure output is directly multiplied by that number. So full throttle produces an output of 164 inHG manifold pressure. We should be seeing about 61 inHG at sea level for this engine. Setting this multiplier lower to get the correct manifold pressure with turbo at sea level should reduce the maximum flight level for the aircraft since the second stage turbo cannot currently be modeled. On the other hand, using this lower value should NOT produce incorrect lower altitude performance since all the data I'm using is for below the 20,000 ft altitude where the second stage kicks in. The drag numbers calculated by YASim should be more or less correct up to at least up to 20,000 ft where the second stage would be kicking in. If there is a problem that setting the multiplier to 5.5 fixes, I suspect it is in the FDM design and not the P51D configuration. Any ideas how we can fix or work around this? Best, Jim ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
