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Guest Ron Freimuth

How to model OAT effect on KIAS?

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Hi,Does anyone know how to model the effect of air temperature to the aircraft performance? We currently have the airspeed tuned very close to the real performance figures in different altitudes, at ISA temperatures. The problem is, the performance does not change correctly when flying in colder/hotter conditions. Currently, if the temperature is lower than the ISA value, our airspeed will fall short of the real world figures, and vice versa if the environment is hotter.We'd like to know, if there is any way to adjust the flight model to take account this effect, and how it's done. Our plane is a turboprop, intended for use with FS2004. However, all tips regarding the matter are welcome!Best regards,-Esa

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>Hi,>>Does anyone know how to model the effect of air temperature to>the aircraft performance? We currently have the airspeed tuned>very close to the real performance figures in different>altitudes, at ISA temperatures. I have found that when I set the airframe (drags) prop (if any) and engine correctly for ISA atmosphere, then performace is close at non-standard temperatures. Now jet fuel consumption should change an indeterminate amount at different cruise temperatures at a given FL, but doesn't in FS. However the effect on PPH is quite small, perhaps 5% for ISA +10 or +20 C. The problem is, the>performance does not change correctly when flying in>colder/hotter conditions. Currently, if the temperature is>lower than the ISA value, our airspeed will fall short of the>real world figures, and vice versa if the environment is>hotter. For one thing, be sure to use TAS or Mach number, IAS is not a good variable. FM's use Mach number and/or TAS for cruise tables. >We'd like to know, if there is any way to adjust the flight>model to take account this effect, and how it's done. Our>plane is a turboprop, intended for use with FS2004. However,>all tips regarding the matter are welcome!>-Esa The FS2K+ Turboprop model is the least understood. And, has the complications of both turbine and prop tables. If prop efficiency (one of the prop tables)is changing a lot with moderate changes in cruise power I wouldn't expect performance to be as accurate as possible. AFSD displays a lot of Turboprop Data, including Prop Efficiency. Typically FS Tuboprop AC have the Intake Area set too high, and Jet Thrust drops too much at altitude. It may even go negative. So, check this and reduce "Intake Area" (in aircraft.cfg) if necessary to there is still some Jet Thrust at cruise FL's. Also check that Prop Efficiency stays around 83% in cruise. It should be near 70% in climb, but you will likely find it is higher. That means climb rate is on the high side, but that's not what you are concerned with here. If prop efficiency ranges from 78% to 84% with changes in Prop RPM and/or Throttle it is probably good enough. But, if it changes quite a bit when temperature changes (different air density) then one will find TAS is higher or lower than appropriate. Note a 3% change in Prop Efficiency (or SHP) will only change speed by 1% (in cruise), so it isn't all that critical. The greater effect is on PPH, which would vary inversely with Prop Efficiency (other conditions kept constant). A new Turboprop Table, 1548, was added for FS2K2. This lets one change maximum Torque vs Air Density. On the right is is 1.0, and corresponds to SL density of 0.002377 sl/ft^3. Near the left side I see it drops to 0.8 torqe at Density = 0.000587. That corresponds to a rather high altitude -- I think about FL 350. The curve drops rapidly at lower air densities. Note if SAT were high, then one could hit the rapid torque drop at FL 340 and find excessive decrease in TAS/Ceiling with only a moderate increase in temperature. That is likely higher than where you have problems, but gives an idea of how TBL 1548 would relate to temperature. Again, AFSD http://perso.wanadoo.fr/hsors/FS_Soft/index.html will display drags and a bunch of turboprop parameters. I'd suggest looking for things that vary too rapidly when SAT TAT change moderatly. Ron

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Hi Ron,Thanks for your reply! We managed to iron out some of the problems (like the negative jet thrust) and the prop efficiency now stays nearly constant with throttle changes during cruise. However, the TAS still stays nearly constant (+/- 7 kts), even with large (30 degrees C) temperature changes, while the real world data states an up to 50 kts change in TAS (please see the enclosed picture).http://people.jyu.fi/~ekkaihla/beta/datasample.jpg By changing the table 1548 you could get the desired effect on speed. This however would limit the ceiling to an unrealistically low altitude, as the change in air density caused by temperature is minor compared to that caused by altitude. Best regards,-Esa

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Hello Esa,I assume what you mean is the TAS should change with a constant throttle setting as OAT changes.As you as not going too fast Mach effects will not be important.The TP model produces thrust from the props and the exhaust nozzle, but that is a smaller proportion.OAT will effect the turbine cycle. As temp falls the thermodynamic cycle efficiency improves increasing the turbine output torque and the propeller thrust, and therefore speed. The lower temperature will also effect the air mass flow rate. The FS model is normalised to take into account the effects of temperature and pressure changes and so should be able to model these effects but you will need a deep understanding of the model. I think we are looking at turbine effects though I have not modelled TPs fully only turbofans which are quite similar. If I knew what the engine was I could find out the air mass flow rates etc in an attempt to produce the turbine model for it assuming the prop parameters were OK.Ian.

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Hi Ian,Thank you for the information! We are currently reworking some aspects of the flight model, thanks to the new tools Ron suggested. We've managed to get rid of some errors that we didn't know about before, such as the jet thrust going negative at times.You assumed correctly what our problem is. We have the performance tables (from which the snippet above is taken) for a range of altitudes (in 1000 feet intervals), and all of them are set for a fixed throttle setting (turbine RPM and TGT). As you can see from our example, the change in speed is pretty large across the temperature range (it varies, but can be as much as 50 to 60 knots). Our current model has the change somewhere around 10 knots or so.The engine in question is the Rolls-Royce Dart, specifically the 542-10K model. Thanks to the AFSD, we've already got the power output (SHP) about right. The prop values are a bit of a question mark for us, though. According to the data we have, the blade angle (beta) is a bit too low in cruise at the moment. We haven't looked into it yet, as what the cause might be.Best regards,-Esa

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>Hi Ian,>>Thank you for the information! We are currently reworking some>aspects of the flight model, thanks to the new tools Ron>suggested. We've managed to get rid of some errors that we>didn't know about before, such as the jet thrust going>negative at times.>>You assumed correctly what our problem is. We have the>performance tables (from which the snippet above is taken) for>a range of altitudes (in 1000 feet intervals), and all of them>are set for a fixed throttle setting (turbine RPM and TGT). .. I don't worry about the throtte setting that much. Other than full throttle not producing excessive thrust. However, the two 'Throttle Tables' let you set how CN2 varies with Inverse Atmospheric Pressure (IAP), Mach Number, and Throttle. The gearbox is on N2 in a turboprop. Of course, the governor keeps RPM constant, but imagine there is no govenor and you have to set the throttle to get enough torque to bring the RPM up to the required value. >The engine in question is the Rolls-Royce Dart, specifically>the 542-10K model. Thanks to the AFSD, we've already got the>power output (SHP) about right. The prop values are a bit of a>question mark for us, though. According to the data we have,>the blade angle (beta) is a bit too low in cruise at the>moment. We haven't looked into it yet, as what the cause might>be.>-Esa Beta isn't that important, think of it as an invisible Parameter that varies to set the two prop tables. If the blade angles at the top of the tables were shifted by 5 degrees the governor would change beta by 5 degrees to set the same Coeff of Power and corresponding Efficiency. Of course, the prop limits would also have to be set 5 degrees different. Make sure the blade angles in the prop tables cover enough range so Min RPM and Max RPM don't hit the limits. However, at high altitudes it is common not to be able to pull the RPM down to the lower limit at full thottle. Prop Diameter is critical; it combines with Cp and N^x to produce a specific power at a given Beta and J. You could change Prop Diameter slightly to adjust Beta. I put some prop formulas in Info, they are a bit like CL and CD, but depend on Diameter, N, d, TAS, rho, etc. In fact, the Prop Coefficients in AFSD are based, in part, on those equations. AFSD needs prop diameter, which it reads (now) from aircraft.cfg. Ron

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Hello,well the thrust SHOULD go negative at times (I mean just the jet nozzle, due to the ram intake drag). I hope you have fun with your Breguet Atlantique. I must get involved with a TP development some day especially with someone who has the performance tables :-)Ian

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>Hello,>well the thrust SHOULD go negative at times (I mean just the>jet nozzle, due to the ram intake drag).> ...........>Ian But, not at full throttle. At least, that's what I'd assume. I was flying the FS9 King Air last night, noted thrust was negative at even low altitudes. I ended up reducing Intake Area from 1 ft^2 to 0.3 ft^2. Even then, it was only 9 lbs at FL 350, full throttle. Reducing TBL 1507 elemets would be similar, but there are a bunch of them. The maximum is around '34.5' at Mach 0.5. This got the TAS up to about the 'high speed cruise' value of 315 kts. Mach Drag should have been zero, but had started increasing at even Mach 0.3. I didn't change it (yet). CG showed 36% MAC. I moved the wing_apex back so 25% MAC was at the AC Reference. CG moved to 21%. I didn't check the details of the payload CG. Trim was about +2.3 deg at 21% CG. Sounds a bit high, but I didn't change the 0.5 deg tail_incidence. I see the excessive Prop MoI of 120 sl-ft^2 in aircraft.cfg was reduced to a reasonable value of 24 sl-ft^2 in aircraft.cfg. Ron

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Hi Ian and Ron,Yes, YS-11 it is :) .Thanks for all the tips! We'll try to make best of them, and I'll let you know how it goes. Undoubted, there'll be more questions coming to us, but we'll need a while to check what we can do with this lot ;-).Best regards,-Esa

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Hi Esa,I found a description of the Dart in Cox's book with some performance data but you will have the manuals already I suppose. That will make matching the performance quite easy.Ian

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Hi Ian,If it isn't too much trouble, please share the data you've got. We have the manuals for the airplane, but not specifically for the engines. It's possible that some new information could be found there.Best regards,-Esa

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Hello Esa,I found the Dart 10-1 data for the YS-11 in an engine book yesterday. I have not sorted out variable pitch props on TPs yet so I need Table 1508 and prop table data to try to produce a performance prediction SS which is what I have done for a few turbojets & turbofans in the past. That was for people who had manuals or actually owned the aircraft and knew how it actually flew.What specifically do you need?I need the tables of engine settings and fuel flow etc. over a range of speeds and altitudes. Do you have those? Ian

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Hi Ian,Did you get my e-mail? I sent it through the forum's e-mail function. When I tried to reply directly to your mail, it got bounced back.Best regards,-Esa

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Um.... then I didn't get you e-mail. Try messaging as that may work. The e-mail address may have been unaliased by now as I have got stopped from getting e-mail notifications of replies to threads. I'll e-mail you with my e-mail address. I have a model for you engine at least as MSL.Ian

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