Performance Grand Duke At High Altitude
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Since I know this is being discussed in a few different places, I thought I should just let you know that I will be looking into this in depth shortly, as I did with concerns about the range of the Turbine Duke this week. I like to take my time with questions of aircraft performance, since there are so many dimensions to cover, and systems to consider. If you're curious about the Turbine Duke's performance too, you can read my narrative here.
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@Black-Square Thanks for the reply. I will keep this in mind for future updates. The turbine duke seems to be performing as the book says however the piston duke true airspeed at high altitude is the one in question.
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Using supplied performance cruise tables for Turbine Duke I'm getting much more fuel burn that specified. For example : FL250for economy cruise calls for 840lb 1800 rpm and a expected 28gph but instead I'm getting 33 to 34gph. Discrepancy seems like too high
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I have the simulator open right now, and have 29.8 GPH/Eng at 840 ft-lbs and 1,800 RPM. I get your 33.5 GPH if I don't reduce the torque back to 840 in-lbs after adjusting propeller RPM from maximum down to 1,800 RPM. I will assess the situation and see if a small increase in estimated fuel burn is warranted in that area of the performance tables.
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@Black-Square
i'm on SU14 ( no SU15 beta).I do adjust both to performance tables and correct them over time.
Here is another example max range power
I assume I should get between 23-34sh gph
Also at the moment I'm afraid to use Piston Duke because engine quit abruptly leveling at 25000 ft. I have not flown Duke IRL I did flown some turbocharged airplanes just to understand basic.
So figured I'm doing something wrong, and I dug up Duke POH online which calls for climbing power with mixture rich all the way to FL280-300 before adjusting to cruise power. I used old school "square rule": slowly reducing mp first, then rpm and then leaning. I was changing power and rp, very slowly. There were no signs of engine start running rough or anything. Before I reached for mixture lever both engine abruptly quit! I looked at POH again is there any warning placard about setting power in cruise. I found nothing except abrupt MP change may cause engine loss. While it' also true for many turbocharged aircraft in dramatic way like pulling in to idle for example, I was a bit of surprise how Duke just did it.I know in some aircraft POHs has disclaimer that performance figures are calculated with properly lean mixture. There are also recommendation is when to start leaning. However, Duke's POH doesn't stress it a bit.
Our of curiosity I looked in NTSB database regarding engine loss due to setting cruise power and found none. So there is something is odd there. Hopefully you are looking into that!
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My revised version of the Turbine Duke performance tables call for the following maximum range power values at 25,000 ft: 670 ft-lbs, 2,050 RPM, and 27 GPH/Eng. Hopefully these numbers will be more to your liking, as they appear to be right on the money in the screenshot below.
Sorry to hear that you're afraid to use the reciprocating engine version of the aircraft. I've had no problem flying well beyond the critical altitude for many hours, and I haven't heard any complaints from beta testers. I'm aware of the sharp falloff you speak of, but it can be mitigated by leaning while climbing past the critical altitude. I have been in search of a way to round out this falloff, which I finally found this week. When the aircraft is updated, you should be able to climb until the atmosphere is as thin as the operating limitations allow at full mixture setting without loss of combustion. Engine performance will degrade considerably, and combustion may cease if you reduce power from full, but you will be able to complete the climb at much higher than ideal air-to-fuel ratios. Keep in mind, though, that the falloff of turbocharged engine performance at richer than peak settings beyond the critical altitude is steeper than it otherwise be in a normally aspirated aircraft, because every reduction in engine power, and therefore exhaust gas velocity, is compounded by a resultant loss of intake manifold pressure after the turbocharger.
As for operating strategies specific to the Duke, I was speaking with the owner who contributed most to this project earlier this week about the subject. He read to me from his original Duke operating handbook that it was recommended to take the mixture out of the auto-mixture detent (a short-lived system manufactured by Bendix that is not modeled in this simulation) at 18,000ft to begin manual leaning, because the pressure sensing and control armature could not be guaranteed to follow the correct ratio beyond the critical altitude, and thereby run the risk of severely degrading engine performance. While anything I've flown with a turbocharged engine has not had an auto-mixture system, I've always been advised to begin precautionary leaning at the first indication of reduced engine performance after the critical altitude, so long as cylinder head temperatures allow.
I hope that answers some of your questions, and that you will be happier with the fuel flow numbers and performance tables and no longer be weary of operating the reciprocating engines beyond the critical altitude in the Duke update after SU15. I'm always delighted to see my users taking my aircraft so seriously. Please always feel free to write with any more questions or commentary!
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Hello All,
As promised, I have completed my review of the Piston Duke's performance, just I did with the Turbine Duke. Thank you again for taking so much interest in my aircraft, and treating them as seriously as I had hoped my most dedicated users would. As I said in the turbine thread too, when it comes to more nuanced questions like this (as opposed to simple code fixes), I like to take my time to really assess the situation on my own so that I can give you the best possible answers. This one was more difficult than the Turbine Duke to arrive at a satisfactory conclusion, but after two days of work, let's take a look:
1. Tablet Range - A bug and an oversimplification. (Same as for the Turbine Duke) There was a rounding error I created by accident on the tablet's payload page, which caused the estimated range to be less than the actual expected range. This is fixed now, but you can read the details here. For reference, the tablet will assume 44 GPH fuel burn, and 242 KTAS cruise.
2. Range Numbers - Accurate coversheet, slightly improved tables. (overall range estimates unchanged) Unlike the Turbine Duke, which had misplaced range numbers in the coversheet only, both versions of the Piston Duke have accurate range estimates in the coversheet, and the performance tables. A few of the range estimates in the performance tables have changed slightly, but the maximum "ferry" range, and normal cruise range, are unchanged.
3. Performance Tables - Improved with more data. I found a better source of cruise power settings and estimates for the factory-built Duke configuration, and used these as the basis for my revisiting of this subject. This allowed me to insert the real world values straight into my table for the Stock Duke, and then build the Grand Duke numbers from there. The Grand Duke is more difficult to find real world numbers for, since it's a modification package, and not a factory-built aircraft. I also chose to reformat the tables slightly to make them easier to read and understand, and more closely resemble the real world formatting. There were some slight changes to the performance of both aircraft, which brings them more closely in alignment with the real world numbers in the high altitude range where I have seen more users than I expected flying the Dukes. It's worth noting that most real world owners do not fly in the altitudes approaching 30,000ft, as the maximum allowable cabin pressure differential limits the time that can be spent in these altitudes without supplemental oxygen. Additionally, the oxygen masks required to stay at these altitudes longer are substantially more expensive and require additional equipment (diluter-demand vs. rebreather or cannula).
4. Interpreting Performance Tables - Weight and balance. When aircraft performance tables are not supplied for multiple different gross weights, they are almost always presented at maximum gross weight. This allows operators to always account for the worst case scenario. The weight and balance of an aircraft can make a greater difference to performance in some than others. The Duke is particularly sensitive to balance concerns. You may have gotten this impression by noticing the limit of only 70lbs of baggage that is allowed in the rear of the aircraft, compared to 500lbs in the nose baggage area. In my testing, I found that the difference in expected cruising speed between single occupancy loading and maximum gross weight could be as much as 15 KTAS in the higher altitudes where angle of attack begins to increase.
5. Interpreting Performance Tables - Leaning. In addition to weight and balance, the engine leaning can have a substantial impact on aircraft performance. Detailed leaning instructions are typically not provided by aircraft manufacturers, as they are seen as the purview of the engine companies. Prior to the advent of digital engine monitors, pilots were usually discouraged from attempting lean of peak operations by aircraft manufacturers altogether. This means that the exact leaning parameters used to determine the performance tables presented for most aircraft are obscured behind a single fuel flow value. In the case of the Duke, real world owners recommend leaning according to your digital equipment, and only using the values in the performance tables as an estimate for range calculations. In my testing, I found that the fuel flow numbers corresponded to best economy leaning (better for advertising the aircraft), but rich of peak EGT. Nonetheless, this means that you can skew the aircraft's fuel economy and power performance in either direction of the performance tables, depending on how you lean. At many of the optimal altitudes for operation (18,000-24,000ft), I found that leaning for best power could increase cruising speeds by as much as 10 KTAS, and increase fuel burn by 80 PPH/Eng.
6. Critical Altitude - The most important change As was mentioned in another thread, it seems that the primary source of the performance discrepancy at high altitudes was a higher than accurate critical altitude. Luckily, the new performance tables I was able to locate for the factory-built Duke allowed me to pinpoint the critical altitude and manifold pressure falloff. This change transposes the aircraft's high altitude performance to approximately 5,000ft higher in the performance tables, meaning that the old 25,000ft performance looks like the new 30,000ft performance. This matches the performance tables much better, as the resultant cruising speeds at high altitude are around 25 KTAS slower.
7. Turbocharger Simulation - Overly rich mixture power falloff An additional change that comes with some turbocharger improvements is a reduction in the severity of the power falloff at high altitudes when either the mixture levers was set too rich, or turbocharger RPM was not sufficient to boost the manifold pressure to suitable levels to avoid overly rich mixtures. Inadvertent engine shutdown at altitudes above the critical altitude is a real concern in turbocharged aircraft, but there is usually a noticeable degradation of performance that precedes the ceasing of combustion due to loss of boost pressure. Unfortunately, the hard-coded parameters in MSFS's engine simulation did not allow for the adjusting of this harsh falloff until Sim Update 13. The severity of this falloff did not present itself in my previous turbocharged aircraft, since they were all limited to a service ceiling of 25,000ft. Luckily, I was recently made aware of new parameters to adjust this curve, and I have already implemented them. As a result, you can expect that engine performance will now degrade slowly as you climb beyond the critical altitude with an overly rich mixture. Combustion should not cease, so long as you retain full throttle while climbing up to the service ceiling; however, a significant reduction of throttle at those altitudes may still result in a total loss of power. As a rule of thumb, be very cautious about reducing power well above the critical altitude, and make sure you are not operating with an only rich mixture before doing so.
In conclusion, The original post in this thread reports a significantly higher cruising speed than expected at the service ceiling. The primary culprit was a higher than accurate critical altitude for the turbochargers, which contributed ~25 KTAS of erroneous performance. Secondary and tertiary may have been weight and balance, and best-power leaning, which can also contribute ~25 KTAS of performance when compared to the performance table testing conditions. The performance tables have also been adjusted with new sources, and better formatted for easier reading. Lastly, I made some improvements to turbocharged operation near the service ceiling to overcome previous limitations of the MSFS engine simulation, which should make the engines less sensitive to overly rich operation
Thank you all again for coming to me with your questions about the Dukes. It's always a pleasure to hear from those who take realistic aircraft operation to heart, and seek to get the most out of their virtual flying experience. Please let me know if you have any more questions, and happy flying!
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@sdflyer hi, i have similar situation, adjusting mixture in the grand one very unstable, and if you start play with mixture after some certain altitude i have to use fuel pumps for not lost engines for good, or i will force to do it after i kill engines. best way for now, is start at full rich or if overfilled engines use low mixture or play with mixture due starting. taxing on 50% and before take off if weather good set max power by 760 gauge, in real as i get it it can fly full rich all time because of pressure you got from charger, and till 28kf or close you will got max mp, then it will go down to 29mp till 33kf. you can fly all altitude but set mixture at some certain altitudes, and be sure your physical axis of mixture has 0 noise, because same as in milviz corsair model if you will touch physical mixture in the flight at high altitude you will have a problems. i don't know why. so what to do... after take off whatever you do move your mixture slowly as you can with fuel pumps on. any way cruise alt if you fly high, you have to meet with leaned mixture. or probably, i didn't test it yet, till it will fixed, use only virtual mixture levers and don't touch axis, or use keyboards with mapped keys for reduce induce mixture slowly or something like that, or use custom lua with some 3rd party soft, that will bind your axis right the info you can found in google. or you can cheat and in engines.cfg change 0 to 1 in auto air fuel mixture line and don't touch it. in my case for now, i just leaning it all time i fly and try to get max horse power indicator in the gauge. but in any way if you will have full rich all way climbing you lost your engines somewhere at 20+ kf. and this is not normal for sure. i tried change -100 100 msfs axis to 0 100 mixture - same result. i opened ticked in thursday now suterday - nothing.
i really can live with it, just would be cool if playing with mixtures in the duke will not so crazy. if mix axis for sure no have any noise. you just can set levers some more that 60 and don't touch it till cruise, but take good speed and cowl flaps opened all time you climb. then go to level flight and with fuel pumps on found best power mixture position and don't touch it till dive close enough to the land, or just don't touch it at all just again open cowl flaps and land at cruise mixture, any way it will low power so leaned mix won't kill engine, if too hot move it rich i guess. depend looks cht. it's actually really strange, because it's really good model. as i said for 4 years only milviz corsair have similar behavior with mixture, after some certain update, because early versions used just manual standard mixture, that was unacceptable because corsair have two auto position in real... by some reason aeroplan heaven till now have manual mixture, and even gotfriends wildcat, so i set there auto mix in engines.cfg... it's not real but more real than use manual mixture adjustment in those aircraft. corsair after update set there auto themself, but problem still present, you can not touch physical mixture axis at high altitude, only virtual you can move however you want. so i just set autolean position, and ignore warning for real corsair - don't use lean above 36" mp for prevent detonation. probably we can try another way... before play with mixture in the grand, pull throttles back to 20mp... probably there no altitude problem, but smart modeling of mp, if mp too high you can not play with mixture. i need more time for get it more good. -
@einherz
I actually solved my problem but discovering that I had double assignment for mixture control axis (I use Honeycomb Bravo). I had conditioner and mixtures axis binding to the same Bravo leveler. After editing and leaving only mixture axis binding (deleted conditioner axis binding) I cureed all engine loss power problems. Now I can climb with full mixture all the way to FL250 and above with mixture full rich. Adjusting mixture afterwards just corrects fuel flow. This is consistent with real live Duke POH copy that I have -
@sdflyer aha, so i have same stuff... why the piston injector engine even react on condition lol? actually i even don't get it, why devs add condition levers as separate stuff. turbo engines hasn't mixture, piston engines hasn't condition. so they could just use one axis mixture or condition... that in the different aircraft will do the different job... nah? same for prop axis and heli throttle axis we had from fsx. who would lost if we still have integrated function in one axis for different aircraft? i will disable condition axis too, thanx:)
i really don't sure i remember what model even use this axis, milviz porter use mixture axis, i hope bronco use mixture axis too for virtual condition levers, well, will look what aircraft stop react on the axis after remove condition:) some stuff so odd in this simulator:)
btw i bought bundle so i hope turbo duke won't stop works after i'll disable condition axis:) i hate change profiles, because i forgot do this all the time hahaha -
@Black-Square Hi Nick, will this affect the Simbrief profiles at all? Navigraph added them shortly after release, presumably using performance data from the manuals or from you directly?
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i was wrong, when said in mv corsair auto rich in model. there actually working autorich and autolean. about dukes. i really hope turbine and piston will not react on mixture and condition axes respectively... because some people include me set mixture and condition in the same axis, in turbine duke mixture axis moves lever from 0 to 100 and back(not problem when you don't know it), in piston one, playing with mixture+condition axis on big alt can kill engine and will try to kill it in any way. for example azurpoly bronco have 0 react on mixture. and afaik some turbo models use or used before mixture axis for condition operation. so. would be cool, if turbine model will ignore mixture axis and piston - condition. because changing profiles where we can not to do this.... why?
