Manipulating aircraft FDE parameters from an XML gauge ???

[Guest Ron Freimuth]

>Hi,>can you use afterburner? Or are you using a prop engine?>(Sorry it is not obvious from a quick scan of the thread).>There is an afterburner thrust factor in .cfg I think or>somewhere.>Ian It's a TurboProp. I don't know if 'reheat' has an effect, but that would only mess things up. Jet Thrust isn't all that high compared to prop thrust. Ian, as far as ITT goes, that's something you might be able to model (better). For either kind of turbine engine. Note ITT depends on compressor compression ratio. I don't know if it would be better to add the compression ratio heating as a function of CN to the "Ram" Compression and heating (TAT) or include both effects in one calculation. The idea is to get an idea of the form of ITT, one can then adjust it for the specific turbine. More efficient turbines have higher compression ratios, thus higher increase in T relative to SAT. One might be able to estimate the CR from the overall efficiency of the engine. There is no bypass in turboprop air flow, so I'd think adiabatic compression of a constant mass (per second) would make it easy to calculate temperature rise. Ron

[Guest Ian_K]

Hello Ron,> It's a TurboProp. I don't know if 'reheat' has an effect,>but that would only mess things up. Jet Thrust isn't all that>high compared to prop thrust. >> Ian, as far as ITT goes, that's something you might be able>to model (better). For either kind of turbine engine.What do you think MS are modelling here? Is ITT = Intermediate Turbine Temperature, Tt6 between the HP turbine and the LP turbine.If some one has a flight manual with this data we can do a regression with other data as I did for ERP and EGT on turbojets.> Note ITT depends on compressor compression ratio. I don't>know if it would be better to add the compression ratio>heating as a function of CN to the "Ram" Compression and>heating (TAT) or include both effects in one calculation.Well ITT would be ambient temp + ram heating + compressor heating including efficiency losses + burner heating - HP turbine extraction + efficiency losses. I think a manual would be better! > The idea is to get an idea of the form of ITT, one can then>adjust it for the specific turbine. More efficient turbines>have higher compression ratios, thus higher increase in T>relative to SAT. One might be able to estimate the CR from>the overall efficiency of the engine.The major variable is how does MSFS mangle the engine model and how do we match it to an engine in RL.> There is no bypass in turboprop air flow, so I'd think>adiabatic compression of a constant mass (per second) would>make it easy to calculate temperature rise. That is backwash from the props energising any incident surfaces as well an the intake. I don't think it is a very large temp increase. Thank you for reminding me of that.>RonIan

[Guest _FSAviator]

Ron said;<Yes we agree how MSFS works out of the box. The problem is that MSFS does not have a 'turboprop flight model', it only has a 'Pratt & Whitney PT6A flight model'. I cannot describe it as atypical since there really is no such thing as a 'typical turboprop'. They are all prototypical. Unfortunately even in a product branded 'A Century of Flight' the PT6A remains the only turboprop model included for fixed wing use. Turboprop engines have been in commercial service for 54 of those 100 years.Many turboprop engines do not work like the PT6A at all. A number of FDE authors, myself included, have released flight (engine) dynamics that 'bend' the PT6A dynamics to address this problem, but none solve it. After investigating gauge based solutions, in conjunction with others, I believe that first generation turboprops like the Rolls Royce Dart, (throttle controls airscrew rpm directly), and second generation turboprops like the Allison 501, (airscrew and engine rpm are constant in flight regardless of any and all cockpit inputs), can only be simulated with any significant realism by processing data in a module outside MSFS and writing over the MSFS data using FSUIPC (or similar utility).These two engines alone power many of the turboprop aircraft that are of greatest interest to the MSFS community. It is easy enough to scale the SHP and ESHP of the PT6A to replicate their power output, but without external processing it is not possible to link realistic simulation inputs to realistic output.The Allison 501 can be replicated reasonably well without a modular solution, but prototypical operation requires temp trim to be added. This will require a module that, apart from other code, must be able to vary fuel flow and torque at constant throttle%, and constant rpm. It looks as though Doug may be able and willing to provide that part of the solution. The initial goal however is to allow both wet and dry torque and turbine temperature limits without variation of N1 and airscrew rpm in engines such as the RR Dart. Subject to testing, the proposed module should be a complete solution to this problem. I am aware that other FDE authors have already implemented incremental modular solutions to the 'RR Dart' problem, each moving the simulation a step further away from 'uprated PT6A, towards 'true RR Dart'. It is not my intention to criticise what has been achieved so far.<>It is reasonable for the PT6A, but that is not the problem. Even if it could not be manipulated within the FDE, gauges could modify where the needle is pointing at display time. The problem is disassociation of throttle %, N1%, N2% and airscrew rpm, from ITT inside MSFS for those types of turboprop that require this. I have no doubt that gauges can be made which calculate ITT, EPR, EGT, etc, for any engine given the data. The problem is not the calculation and display of the values, it is imposing them inside MSFS and preventing it from recalculating them for internal use. That requires a modular solution. The wider problem is simulation of the pilot action of 'temp trimming' and its consequences. Generically and in layman's terms, it is a means by which the fuel governor varies fuel flow and hence power to keep the turbine temperature at the pilot selected ITT (or other valid reference) whilst rpm% and throttle% are constant, or when the throttle is subsequently advanced to a defined throttle gate at a defined % of the quadrant. In the MSFS flight model for this purpose we have to manipulate SHP via manipulation of max torque, where max torque is internally associated with 100% (joystick) throttle. For the TOGA case I propose to ignore the rise in thrust associated with the wet flow as the *rise* in thrust due to water/meth creates very little power when TAS < Vr. Change of ESHP not due to change of SHP is too small to worry about. It could be added to the module code though. The ability to monitor flow and exhaustion of water/meth by reference to ADI arming switch status and throttle% could be done from within a gauge, but it may as well be done from within the module since it must test for both conditions anyway. Reduction of water/meth payload, and variable CG consequence, cannot be manipulated in real time outside a module.<>As I explained that would cause the aircrew rpm to vary in many turboprop aircraft. Having appropriate airscrew rpm is important during take off. Not all the turboprop FDE that may be downloaded clone the default code. Many already emulate characteristics of the real engine to varying degrees. Telling the end user to set the wrong airscrew rpm with the throttle (to obtain the differential wet/dry torque limit) for take off is one of the things the module being discussed here will avoid. Displaying a false ITT on the panel is easy. The problem is controlling what FS9 is doing internally with the thrust during a wet and dry take off at identical N1, N2 and airscrew rpm.The usual solution has been to allocate rpm levers to the distributed panels of a wide range of aircraft which do not have them in real life so that the end user may disassociate throttle position, turbine rpm and airscrew rpm by moving non existent controls. The module will (hopefully) overcome that need. This assumes that ©N2 exists and/or can vary in flight. In engines such as the Allison 501 as used in the C-130, P-3, CV-540, L-188, E-2, etc., turbine rotation is invariant in flight. The FDE author has to ensure that turbine rpm is invariant in MSFS. Manipulation of CN2 maxima cannot solve the problem in such engines.Writing two different sets of FDE, with different (wet and dry) power limits, in two different aircraft.cfgs is not otherwise a problem, but that is one of the 'clumsy' solutions the proposed module is intended to replace. Different air files can be aliased as ui_variations, but an aircraft.cfg cannot be aliased as an ui_variation. This is one of the drawbacks of moving FDE variables from the air file to the aircraft.cfg.With luck this thread will, (as a direct spin off from the original question about how to enable temporary higher limits for catapult launching), lead to the first of these goals being achieved. As it happens I don't think the solution will need to be very sophisticated, but I too thought otherwise before asking here in the hope of finding out.The first step is construction of the 'catapulting and arresting module'. This will not use power augmentation, but will not use applied forces either.The next step will be to address temporary variation of applied force along the reference datum line (as a generic case). This would allow any kind of temporary thrust augmentation including water/meth, JATO, RATO etc. The fluids would be decremented from the designated load_station.n with appropriate CG effects, improved rate of climb due to reducing mass etc., in real time. Applied as a negative value and defining the fluid as 'ammunition' the same module provides both recoil and ammunition mass depletion in real time from load_stations n to whatever.A force, is a force, is a force. However for the moment I propose to apply a singular net force along the waterline, positive or negative. To manipulate temporary applied force in real time, without affecting the 'main engines', it is necessary to use a modular solution.Ian, thank you for joining in. Could you please explain for the benefit of all how water/meth injection does, or does not, allow thrust to be augmented when used as combat emergency power in aircraft like the P-80 Shooting Star? Specifically, what happens if w/m is invoked at high level in cold air with low fuel flow at full throttle and the engine already below turbine temp limits? Is there any combat emergency power benefit? Assuming you are familiar with the RR Spey case, can you propose an algorithm that would convert the TOGA wet and dry thrust ratings at SL to a different flight level in ISA, for varying runway altitudes?The question is how maximum wet augmentation of thrust should vary with altitude. Does the effect diminish to zero and along what curve?Request relates to real world cases, not limitations of MSFS.I can probably work out the best way to impose it on MSFS if I can understand the real world case. It does not appear to be difficult to impose temporary variation of (jet) thrust on MSFS at constant throttle% if that is required. I simply do not understand whether that is relevant.At the moment I assume that in a turbojet or turbofan extra thrust when running wet always results from a higher turbine rpm and higher fuel flow. Does the fuel flow autoregulate during water/meth injection to create the extra thrust, or are the throttles moved? To this point we discussed mainly turboprops as the most difficult case appeared to be manipulating shp and eshp via torque and thrust (in MSFS) given the very different ways that different turboprops receive their input from the cockpit during the simulation.Finally, Doug may or may not wish to take on the more complex applied force variation module that I have proposed in this thread, but I am now in e-mail contact with him concerning the detailed requirements, and I have now explained both the goals and the proposed solution here as far as I can. Ian, thanks in advance if you are able to fill in the gaps. FSAviator

[Guest Douglas K]

>What do you think MS are modelling here? Is ITT = Intermediate Turbine Temperature, Tt6 between the HP turbine and the LP turbine.http://www.pwc.ca/en/0_0/0_0_14/0_0_14_2.aspit may help to better understand my description. When you look at the PWC illustration, note the dotted vertical line that separates the power section on the left from the gas generator section on the right. This is the 'C' flange I referred to earlier; it's the mating flange where the power section and the gas generator section are bolted together. This is the approximate location of the ITT busbar. >If some one has a flight manual with this data we can do a regression with other data as I did for ERP and EGT on turbojets.

[rcbarend 37]

Hi all,I'm glad this thread has got so much feedback, which (due to my lack of knowledge) has gone a bit over my head unforfortunately :-)I'm also in contact with Doug Dawson, and it appears (if I understood his explanation correctly) he has found a way to directly set the aircraft's groundspeed via FSUIPC, which he has proven with a "catapult" testgauge he made.Which (going back to my original question) is probably the most realistic way to model takeoff/landing on a aircraft carrier: an external force applied to the aircraft (the shuttle of the catapult pushing the aircraft forward resp. the cable pulling on the aircraft after a catch), thereby emulating the aircraft's accelleration / decelleration without modifying the aircrafts characteristics itself....Will be continued ....Regards, Rob

[Guest Ron Freimuth]

>Hello Ron,>>>> Ian, as far as ITT goes, that's something you might be able>>to model (better). For either kind of turbine engine.>>What do you think MS are modelling here? Is ITT = Intermediate>Turbine Temperature, Tt6 between the HP turbine and the LP>turbine. EGT is not realistic for turbojets (though it can be set to be about right under cruise conditions) and ITT appears to be based on the same formula. Settings in the AIR file adjust ITT and EGT the same way. One common difference is the 'Rate of Change'. Thus, I doubt ITT is very accurate either. All I can say is they are better than the 'EPR' variable. ;)>If some one has a flight manual with this data we can do a>regression with other data as I did for ERP and EGT on>turbojets. I don't have much info on turboprops. It seems this is something that can be at least approximated as explained. Once one has a simple theoretical model for ITT he can enhance it, if necessary, for a specific powerplant. Note EGT and ITT are also used for reciprocating engines. I don't know why EGT is so closly related to ITT but a data from a real instrumented AC also showed the correspondence. Perhaps ITT is already based on aidiabatic compression. One scales the reading in the AIR file, perhaps that would tend to account for an intercooler.>That is backwash from the props energising any incident>surfaces as well an the intake. I don't think it is a very>large temp increase. Thank you for reminding me of that.>Ian I suspect the 'cooling factor' for Oil and CHT is related to a cowling parameter explained in McCormick. '0.65' may apply to the intake/outlet area ratio for the air cooling. Note 0.0 gives no IAS effect on cooling. IOW, no intake for air cooling. Ron

[Guest Ron Freimuth]

I don't have time to comment on everything here. I haven't done all that much with turboprops. As far as RPM vs 'throttle' goes, one can set 'Prop RPM' with a gauge which could set any desired variation relative to the 'power lever' or whatever else might change RPM. For example, one might set Prop RPM to "1800 + (Throttle * 200)". As one advanced the throttle the prop RPM would increase to a limit of 2000 RPM. Pull the throttle back and RPM drops some. Prop thrust would vary much more. Actually, RPM would drop below 1800 at low throttle, just as it does with no gauge setting. It's the RPM Command that varies as I showed. Or, command the RPM to vary some with shaft torque. If the FS model doesn't handle a realistic sag in RPM at high blade angles and partial throttle one could control it indirectly. I'd think the end result would be appropriate shaft torque, prop thrust, fuel flow, etc. In FS, the throttle actually controls engine torque. A CS prop changes blade angle as necessary to hold RPM constant as shaft torque changes. While a FP prop lets the engine speed up to the RPM where Prop Torque is in equalibrium with powerplant torque. In the FS Turboprop model, N2 = Prop RPM * Gear Ratio. One can think of N2 as N High and N1 as N low. Both turbine and compressor have high speed and low speed rotors, though FS links the gear box only to N2. In turbine jet engines CN1 is mapped from CN2 as a function of Mach number. One can set CN1 = CN2 in TBL 1502 though EGT will not be correct then. Jet Thrust is a function of CN1, TAS, and TBL 1506 entries. I assume N2 torque is a function of throttle setting and the 'Friction Torque' table. Jet Thrust isn't that significant in turboprop engines and it would seem the complicated 1506 and 1507 tables are much more than is needed. While the Throttle tables (which involve Mach and IAP) control N2, the Friction Table should allow one to model a change in shaft torque vs RPM. Conversely, if one changes Prop RPM N2 has to change, and that reflects backwards to N high torque at a given throttle setting. Some time ago I tried to find the RPM vs Torque variation by setting essentailly zero blade angle. Howver, the prop code didn't work correctly at that time and I couldn't see how fast the turbine (N2) would go when loaded only by the Friction Torque. Knowing 'no load' RPM would show how the Friction Torque table is calibrated. Though, I now think it is in ft-lbs vs 'per cent' N2 RPM. Where the friction torque is relative to the turbine, not the prop shaft. Since my test I think the turboprop model has more settings. Such as for 'min on ground beta'. It might be possible to get useful results at this time. Further, there is the newer TBL 1548, which sets torque vs ambient air density. One could reduce the 1.00 torque factor at SL (0.00277 sl-ft^2) to 0.1 and might then be able to get a low enough RPM with zero prop torque to see if the Friction Torque table works as I think it should. There is also TBL 1508 "Turboprop Torque vs CN2?". It can't relate how shaft torque varies as one loads down the turbine, rather it appears to set the available torque as a function of CN2. Higher CN2 = Higher torque. I don't know if more than three data pairs can be set in that table, regardless, the middle pair and the high end pair give some freedom in setting shaft torqe and RPM variations. Ron

[Guest Ian_K]

>Ron said;>><in MSFS.>>>Yes we agree how MSFS works out of the box.I thought that for the turbojet but FS9 seems to have simplified how some of the tables are read. Yes the TP is the worst case nightmare scenario and some people have been hinting I should work on it but so far I have stuck to the turbojet/fan as this is ONLY rocket science.> The problem is>that MSFS does not have a 'turboprop flight model', it only>has a 'Pratt & Whitney PT6A flight model'. I cannot describe>it as atypical since there really is no such thing as a>'typical turboprop'. They are all prototypical. Unfortunately>even in a product branded 'A Century of Flight' the PT6A>remains the only turboprop model included for fixed wing use.>Turboprop engines have been in commercial service for 54 of>those 100 years.It was only fashionable for a few years in the 1950s and there are not many technical books on the subject, despite the pile of piston, prop and jet books I have. Can anyone recommend the best one?>Many turboprop engines do not work like the PT6A at all. A>number of FDE authors, myself included, have released flight>(engine) dynamics that 'bend' the PT6A dynamics to address>this problem, but none solve it. After investigating gauge>based solutions, in conjunction with others, I believe that>first generation turboprops like the Rolls Royce Dart,>(throttle controls airscrew rpm directly), and second>generation turboprops like the Allison 501, (airscrew and>engine rpm are constant in flight regardless of any and all>cockpit inputs), can only be simulated with any significant>realism by processing data in a module outside MSFS and>writing over the MSFS data using FSUIPC (or similar utility).>>These two engines alone power many of the turboprop aircraft>that are of greatest interest to the MSFS community. It is>easy enough to scale the SHP and ESHP of the PT6A to replicate>their power output, but without external processing it is not>possible to link realistic simulation inputs to realistic>output.>>>I am aware that other FDE authors have already implemented>incremental modular solutions to the 'RR Dart' problem, each>moving the simulation a step further away from 'uprated PT6A,>towards 'true RR Dart'. It is not my intention to criticise>what has been achieved so far.>>>I did the original regression fit for the EPR and EGT for this engine (end of ad break).>Reduction of water/meth payload, and variable CG consequence,>cannot be manipulated in real time outside a module.><factor to make ITT display lower during injection. Then, one>could push the throttles up higher.>>>Ian, thank you for joining in. Could you please explain for>the benefit of all how water/meth injection does, or does not,>allow thrust to be augmented when used as combat emergency>power in aircraft like the P-80 Shooting Star? Why do you mention the P-80 specifically? I have a part of a manual for the T-33 Silver Star and did an engine model for a pilot owner last year. Can't remember if it did wet operation.Whittle knew about wet thrust augmentation in the 1930s. When he built the W.1 his assistant sprayed a methanol and water mixture into the intake. UNFORTUNATELY he was unable to get his attention fast enough to stop him before the engine overspeeded and blew up. I think he started to build the W.2 after that.>Specifically, what happens if w/m is invoked at high level in>cold air with low fuel flow at full throttle and the engine>already below turbine temp limits? Is there any combat>emergency power benefit? Wet augmentation works by 2 ways. 1. The methanol is a fuel so the enthalpy of combustion will add energy to the thermodynamic cycle.2. Thermodynamic efficiency is proportional to the ratio of the max temp (TET) to the min temp ambient. It you can use the enthalpy of evaporation to cool the air flow at the intake this efficiency can be increased.PS the Merlin design improvements of Hooker used the evaporation of fuel to improve the efficiency of the carburettor which was not available in the fuel injected BMW engine of the Bf.109.>Assuming you are familiar with the RR Spey case,No I don't actually have a Spey engine manual only an RB.211 and an Olympus manual. Can some one send me a copy please.> can you>propose an algorithm that would convert the TOGA wet and dry>thrust ratings at SL to a different flight level in ISA, for>varying runway altitudes?Possibly.>The question is how maximum wet augmentation of thrust should>vary with altitude. Does the effect diminish to zero and along>what curve?>>Request relates to real world cases, not limitations of MSFS.>>I can probably work out the best way to impose it on MSFS if I>can understand the real world case. It does not appear to be>difficult to impose temporary variation of (jet) thrust on>MSFS at constant throttle% if that is required. I simply do>not understand whether that is relevant.>>At the moment I assume that in a turbojet or turbofan extra>thrust when running wet always results from a higher turbine>rpm and higher fuel flow. Does the fuel flow autoregulate>during water/meth injection to create the extra thrust, or are>the throttles moved? Not necessarily.>To this point we discussed mainly turboprops as the most>difficult case appeared to be manipulating shp and eshp via>torque and thrust (in MSFS) given the very different ways that>different turboprops receive their input from the cockpit>during the simulation.The catapult would be a lot easier. :-)>Ian, thanks in advance if you are able to fill in the gaps.I would be happy to do so. Perhaps there is some useful manuals books etc people have access to. >FSAviatorIanPS ERP is my typo for EPR and thank you for the definition of ITT.

[Guest _FSAviator]

Earlier I said,80 Shooting Star?>>80 specifically? I have a part of a manual for the T-33 Silver Star..>>I consider the T-33 to be an aircraft like the P-80 so I would be pleased to receive an explanation based on the T-33. What I am really trying to grasp is the error involved in representing thrust augmentation due to water/meth injection into the turbine stage, as injection of fuel into the afterburner. This latter method has been universally adopted by FDE authors to date. There appears to be little error for the TOGA case, but I find the high level, high Mach, case less clear. I understood your reply but could not use the information to estimate the % error I mention above. Taking your reply as a whole I conclude that the complication of calculating a more realistic value for combat thrust augmentation using means other than default afterburner may be more effort than it is worth. Even the CFS community seem to have little interest in this aspect of realism so I leave it up to you whether you think this aspect of realism is worth pursuing further.I said,>To this point we discussed mainly turboprops as the most>difficult case appeared to be manipulating shp and eshp via>torque and thrust (in MSFS) given the very different ways that>different turboprops receive their input from the cockpit>during the simulation.Ian replied,<>Turboprop augmentation is probably even easier to implement. MSFS has the default ability to re-calibrate the joystick throttle for turbojets and turbofans by temporary substitution of a different MAX_TRUST.n value to simulate afterburner. The proposed turboprop module potentially does no more, but instead varies MAX_TORQUE.n to simulate wet v dry available torque at any n% throttle. MSFS will recalculate the resulting temperatures etc, without our help. The accuracy of such calculation is a separate subject. I do not expect the prospective module to address that. However it would be nice if the module also contained the code to read the available fluid mass and deplete it to zero in real time. I now realise that this is also simple for anyone who is familiar with the construction of modules linked to MSFS via FSUIPC. Rob said,<>Having now beta tested the (latest) module (written by Doug) I am happy to report that it overwrites airspeed and that the lift and drag equations use the overwritten value appropriately with logical dynamic consequences as the aircraft unsticks. Accordingly it works both as a CV catapult utility and as a convenient means for overcoming 'Microsoft sticky water', for both the taxi and take off cases. It therefore allows hydroplanes to operate from water up to their real maximum take off weights whilst using only realistic TOGA power. The FDE author will be able to recommend the augmentation value that replicates realistic acceleration in all cases. The catapult module does not achieve this by realistic means, but that is potentially an advantage as the numeric augmentation value required is a constant and neither wind vector dependent, nor engine type dependent. I have proposed some potential improvements and developments via e-mail, but it works as it is.I would like to thank Doug for providing a solution to the CV and hydroplane simulation problems inherent in MSFS. I hope he will now take up the challenge of providing realistic temporary variation of max torque at 100% throttle for the wet TOGA case in turboprop flight models.FSAviator

[Guest Ron Freimuth]

RF>IK>I did the original regression fit for the EPR and EGT for this engine (end of ad break). Hate to bring it up here, Ian. But remember I tried your EPR/EGT formulas and they didn't work! I ended up starting from a SS someone else made, noting the formula for EPR he found didn't need the TAT correction if I changed N1 to CN1. I couldn't see why EGT varied from 1X to 3X SAT, depending on EPR. But, working in an intuitive fit anyway. For reference, a start is to make EPR = a + b*CN1. A straight line fit is much better than the MS EPR. My fit involved a straight line up to some value of CN1, then a third degree polynomial to handle it up to about 2.15. However, a straight line, then a second degree polynomial would still be pretty good, and not deviate so much above EPR = 2.15 Finally, as a minimum, make EGT = c + d*EPR + e*(TAT-15). Where I think 'e' is '1' for higher values of EGT. For the JT8, EGT is about 500 C at TO CN1, SL, 15 C. That would be similar for many turbines. I modified the JT8D EPR for a JT3D. TO CN1 was higher for the JT3D so I multipiled CN1 by about 0.85 before hitting the original EPR code. To get EPR = 1.82 at CN1 ~ 107%. I then had to multiply EPR by something like 1.1 before hitting the EPR to EGT code so EGT would be come out about the same as before. I don't know how close the EPR and EGT values are for a JT3, but they look reasonable. At least I get the right values at TO. All in all, I think my original JT3D EPREGT code can be adjusted for many turbines by only adding and adjusting the two factors I added to approximate the JT3D. THRUST vs CN1 HAS TO BE CORRECT FOR EPR EGT TO COME OUT RIGHT! It would be nice if we knew how to calculate EPR from relative FS Thrust. Then, CN1 wouldn't have to be accurate as far as having an accurate EPR (and EGT). All I know is Thrust is proportional to EPR at a constant altitude and Mach number. It looks like EPR = 1.0 means zero thrust, while EPR = some higher value, such as 2.0, means maximum continuous thrust. The approximately straight line variation of EPR with CN1 or CN2 is consistent with the graphs McCormick's text displays. I did not get in the small variation in EPR due to Mach number. It is only signficant below EPR = 1.6 for the JT8D. That is important for approach EPR but not for climb and cruise. I put my EPREGT code in a separate XML file. The small file also displays the EPR and EGT digitally. The values could be used in graphical gauges easily enough. It's only a few lines of XML but hard to understand how the RPN works. I added p-code in comments so it can be converted to C as desired. I'd post it here but can't UL files; Java is busted in my IExplore. Ron

[Guest Ian_K]

Hello Ron,This is my original fit:EPR = 1.018-0.0031(EGT/Theta2)+1.0E-5*(EGT/Theta2)^2-0.2035*MN theta2 MN 0 0.2 0.4 0.6 0.8 1FL 0 1.000 1.008 1.032 1.072 1.128 1.20050 0.966 0.973 0.997 1.035 1.089 1.159100 0.931 0.939 0.961 0.998 1.051 1.118150 0.897 0.904 0.926 0.962 1.012 1.076200 0.863 0.869 0.890 0.925 0.973 1.035250 0.828 0.835 0.855 0.888 0.934 0.994300 0.794 0.800 0.819 0.851 0.896 0.953350 0.760 0.766 0.784 0.814 0.857 0.911360 0.753 0.759 0.777 0.807 0.849 0.903EPR MN 0 0.2 0.4 0.6 0.8 1FL 0 2.13 2.06 1.94 1.76 1.57 1.3650 2.27 2.20 2.07 1.88 1.67 1.45100 2.43 2.36 2.21 2.02 1.79 1.55150 2.61 2.53 2.38 2.17 1.93 1.67200 2.82 2.73 2.57 2.34 2.08 1.80250 3.05 2.96 2.79 2.54 2.26 1.95300 3.32 3.23 3.03 2.77 2.46 2.13350 3.63 3.53 3.32 3.03 2.69 2.33360-600 3.70 3.60 3.39 3.09 2.74 2.37EGT= 250

[Guest Ian_K]

Hello Aviator,>Earlier I said,>><allows thrust to be augmented when used as combat emergency>power in aircraft like the P-80 Shooting Star?>>>>80 specifically? I have a part of a>manual for the T-33 Silver Star..>>>>I consider the T-33 to be an aircraft like the P-80 so I would>be pleased to receive an explanation based on the T-33. What I>am really trying to grasp is the error involved in>representing thrust augmentation due to water/meth injection>into the turbine stage, as injection of fuel into the>afterburner. This latter method has been universally adopted>by FDE authors to date. There appears to be little error for>the TOGA case, but I find the high level, high Mach, case less>clear. I understood your reply but could not use the>information to estimate the % error I mention above. Taking>your reply as a whole I conclude that the complication of>calculating a more realistic value for combat thrust>augmentation using means other than default afterburner may be>more effort than it is worthThe P-80 was rated 4600 lbf SLS dry and 5200 lbf SLS wet so you can see the relative thrust increase.Cox covers wet augmentation:"22/17/1 Fluid InjectionSome increase in mass flow can be obtained by spraying a suitable high-latent-heat fluid into the air intake (refrigerant injection) so that evaporation reduces the air temperature and increases the density....."Fig 22-41 Pure Methanol max injection 2.6% (as it replaces the fuel burnt which has to be throttled back to zero to maintain max temp limits). This produces a 23% increase in thrust.Pure water injection doesn't have this limit and 2.5% injection increases thrust by 15%, and 5% injection by 27%.Relative to reheat these are about half of what an afterburner with a similar mass injection will produce.>FSAviatorIan

[Guest Ron Freimuth]

IK>This is my original fit: >EPR = 1.018-0.0031(EGT/Theta2)+1.0E-5*(EGT/Theta2)^2-0.2035*MN Again, I programmed your formulas in my XML test gauge and couldn't get them to work. Perhaps because the EGT formula you suppled with the above was off. I finally had to spend days learning to use Excel to fit curves; fortunately someone who asked about EPR in this Forum filled out a SS with FM data. Which got me started. I think I recently explained the general approach. Make "EPR = a + b*CN1" for a start. That will give a much better fit than the MS EPR. It would be better to have an EPR formula that was based on FS Thrust, Mach, Altitude, and TAT. Then, it would be based on thrust, and N1 wouldn't have to be calibrated closely for EPR to be good. In fact, one could then work backwards from EPR to CN1 and generate his own N1 and N2. Essentially bypassing the difficult to model Turbine tables. Of course,they would still be 'working', and would have to be approximately correct to get realistic variation in thrust. But, should no longer be critical. Ron

[Guest _FSAviator]

Greetings Ian,Thanks for the further reply. It was illuminating.There is still one thing that I cannot quite get clear in my mind.80 was rated 4600 lbf SLS dry and 5200 lbf SLS wet so you can see the relative thrust increase>>Suppose instead that the J33 had been rated at 4600 normal and 5200 with reheat. How would the maximum available thrust differ in % terms in level flight at FL370 with full throttle and max rate injection in the two cases.I do not expect a numerical answer just some grasp of the superiority margin (in thrust) of the better solution at 'high' Mach and 'high' altitude. Is there any significant advantage in thrust under those circumstances using reheat as the combat thrust solution. The advantage of having AVTUR rather than water aboard and double the augmentation from the same mass injection rate is not lost on me. It is variation of augmentation versus altitude (given the same max augmentation at sea level) that I still do not grasp.FSAviator

[Guest Ian_K]

Hello Aviator,I would think wet and reheat have the same thrust variation with altitude though you would want to stop the Me/H20 from freezing so the initial temp would make some difference.I have dry models for RR Nene 10 and Goblin 3 (for a Vampire) and data some similar engines:Name Nene 3 Nene 3 Nene 3 Nene 3 Nene 4 (101) Nene 4 (101) Nene 10 Nene 10 Nene 10 Ghost 48 Goblin 3 Goblin 35 Goblin 35 Goblin 35condition max.TO max. inter. max. cns. gnd. Idle max. cns 15 min max. Max. TO max inter? max cns max. TO max. cns. rec. cruiseN 12500 12200 11800 2500 11800rpm 100.40% 98% 94.70% 10250 10600 10750 10250 9500Thrust 5100 4660 4090 120 3940 lb st 5000 lb st 5000 3015 3350 1300 kg 1000 kgSFC 1.06 1.04 1.02 5.3 1.055 1.2? 1.17? 1.18?air flow/lb/sec 88.69589189 84.4 88 77 67.4 spec. thr. 57.4998446 46.68 56.82 39.16 49.7 TET/