TAS/GS weirdness



  • I noticed something odd going on with TAS and GS. Arrows typically cruise around 133 knots at 7500 ft. Even with no wind, the best ground speed I've seen is 128 knots in level cruise at 7500 ft. The airspeed indicator shows 119 knots IAS which (using rule of thumb TAS = IAS + 2% for every 1,000 ft) indicates in the neighborhood of 134 knots TAS. THE EFB shows T125 KTAS. The GPS shows 125 KTS GS when flying headings 0, 90, 180, and 270 as would be expected with no wind.

    Both the TAS as displayed by the EFB and the GS as displayed by GPS are incorrect IMHO.

    Thoughts?

    -bv



  • @BernieV said in TAS/GS weirdness:

    I noticed something odd going on with TAS and GS. Arrows typically cruise around 133 knots at 7500 ft. Even with no wind, the best ground speed I've seen is 128 knots in level cruise at 7500 ft. The airspeed indicator shows 119 knots IAS which (using rule of thumb TAS = IAS + 2% for every 1,000 ft) indicates in the neighborhood of 134 knots TAS. THE EFB shows T125 KTAS. The GPS shows 125 KTS GS when flying headings 0, 90, 180, and 270 as would be expected with no wind.

    Hmm, at least the GS indicated in the GNS430/GNS530 is something that JF has no influence on and that should be fine.
    Also I checked with LittleNavMap that the TAS and GS speeds shown in the EFB are the same as the SimConnect interface shows, so I thought they are quite trustworthy as a baseline.

    In your calculation it also could be that the IAS is wrong and/or the rule of thumb is not applicable due to temperature gradient or some other reason.
    What you definitely should not rely on is the OAT gauge and also the TAS scale in the IAS gauge, I didn't get fully consistent results using them to calculate a TAS that would match the POH.

    I'll try to read out more data via SimConnect to see which of the data in the JF PA is reliable and which isn't.



  • What you definitely should not rely on is the OAT gauge and also the TAS scale in the IAS gauge

    Agreed. The OAT gauge moves, but its not accurate.

    With that said, I think I may have an explanation that fits the symptoms. I've been assuming the IAS was correct, but I just flew with 0 wind, 15C (std temp) at 300 ft MSL and the TAS was 132 and the IAS was 139. They should be the same as long as the calibrated airspeed and indicated are not too far off.
    Its as if the air speed indicator is indicating 7 knots high (at 132 knots CAS near sea level). If you accept that for a moment, this implies the JF Arrow III is too slow and the most likely culprit is parasitic drag is too high. I set the parasitic drag scalar to .9 in the aircraft config file, and I am now getting the TAS (as reported by the EFB) and GS (in no wind flight conditions) that I'd expect.

    I'm not sure if there is an easy way to tweak the way a gauge works in a config file, but my working theory at the moment is the airspeed indicator is reading 139 when it should be 132 knots.

    Just to be clear, I've never seen > 128 knots ground speed as reported by the EFB or GPS with the stock flight model in level flight. Reducing the parasitic drag scalar seems to fix that (I get to 135ish knots at 75% power in the 6-7 thousand feet rage). In addition the air speed indicator appears to indicate 7 knots too high at 132 knots.



  • It's a bit weird since if you switch to external view, you get an IAS gauge which is not specific to this addon and for me it was just about 2-3kts lower than the gauge in the cockpit.
    I would assume that whatever the gauges in the cockpit say (some of them are definitely off), the sim should produce correct data since this cannot be manipulated by the addon itself.

    That being said, I'm usually flying at around 7000ft and manage to get to ~135kts TAS according to the value shown in LittleNavMap. This matches the Power Cruise 75% setup as shown in the graph of the ODM page 13 (-20ยฐC OAT).
    So I basically am spot on on the numbers based on the data shown by the sim, not by the JF PA cockpit gauges (just to rule out that they're partially wrong like the OAT).

    I'm pretty sure you're leaning the mixture for cruise? In my case I was spot on on the numbers when flying with default load (50% fuel and default weights for pax). Could it be that you're too heavy?



  • Just checked, the IAS gauge is definitely showing wrong data.

    Power Cruise at 7000ft with 65% power setting with 3ยฐC TAT. Wind set to 0 (MSFS still generates 2kts, but that's not an issue).
    According to ODM page 14 I should get 128kts TAT.
    Let's first see what LittleNavMap shows:
    3f6babaa-91bd-4cf3-8a1d-8656ce268f79-image.png

    Pretty close! IAS 118kts, TAT and GS 130kts.

    And what about the ISA?
    Microsoft Flight Simulator 16.03.2021 20_01_49.jpg
    121kts
    The external view gauge shows 118kts as well.

    The EFB shows correct values for TAS and GS by the way, the same values as LittleNavMap.

    I just now tried to replicate the speeds above 130kts (75% power cruise, 2500rpm) and couldn't, I can't get it over 130kts TAS if I use the TAS shown in the EFB or LittleNavMap. Tried it at different temperatures and altitudes (even the example given in the ODM on page 13, 5000ft with 10ยฐC OAT.



  • Just to follow up on your questions;. I am flying with 50% fuel and a 170lb pilot (1984 pounds total vs 2750 max gross weight). I am leaning. I've tried both PEAK EGT (should be best economy) and a tick or two rich of peak EGT (should be best power). Unless I reduce drag, I can't get to book speeds. I've have two screen shots showing the panel and external view. The IAS from the panel is showing 6 or so knots above the external view (IAS) airspeed indicator.

    This is with the parasitic drag scalar set to .8.
    panel.jpg
    external.jpg



  • @copper by the way, I think you are closer to 75% power than 65%. From page 11 of the Operating data manual; pressure alt 6800ft Full Throttle and 2500 rpm is 75% power. Not sure I trust the fuel flow indicator, but based on the little I can see of your EGT in the picture you posted, i think you are just a hair shy of 75% power. You should be closer to 136ish knots at that power setting and altitude (assuming std conditions) based on the graph on page 13 (best power 75% power alt vs kts TAS).



  • @BernieV said in TAS/GS weirdness:

    I think you are closer to 75% power than 65%.

    Simply check the FuelFlow gauge ๐Ÿ™‚ It has marks for 55%, 65% and 75%, my screenshot is right in the middle at 65%. Depending on the temperature you can't get even to 75% at that altitude.
    So at the given data I'm pretty close to what the ODM says with all numbers lining up with the ODM chart for 65% power cruise. The difference of 2kts could be due to the charts showing based on max GW (2750 pounds) and I was at 50% fuel and 55kg payload.

    I reduced the temperature to ISA -15ยฐC and could get to 75% power cruise setup but the TAS did not increase above 131kts while I should be at 135-136kts.



  • Great conversation and I appreciate it. On a bit of a tangent, at 200 ft below your altitude you could have been making 75% power if you had increased fuel flow to 12.7 GPH. And TAS should have been 136ish knots. (See charts below).

    What TAS do you see at 6800 ft if you enrich the mixture to 12.7 GPH under the same parameters? Book says it should be 136ish. If your arrow is behaving like mine, you wont be able to do it. The fuel flow on my is limited for some reason to 11ish GPH, perhaps a bug.

    ad3d226d-097e-4324-a9e5-82eb3ae0e305-image.png

    62e965cd-7ee3-4cc4-aa8b-8e654240eebf-image.png



  • @BernieV said in TAS/GS weirdness:

    What TAS do you see at 6800 ft if you enrich the mixture to 12.7 GPH under the same parameters? Book says it should be 136ish. If your arrow is behaving like mine, you wont be able to do it. The fuel flow on my is limited for some reason to 11ish GPH, perhaps a bug.

    The fuel flow is limited by the temperature. If you lower it, you can get to the higher fuel flow. Not sure if that's realistic.
    But I could by n means get close to 134kts TAS at 7000ft even at 75% power cruise despite the chart showing that I should get there even fully loaded.

    Let's see what the update today/tomorrow brings. Don't have the time until then to try your suggested 6800ft test, but I doubt that it will significantly change to what I have tried already.



  • @copper said

    The fuel flow is limited by the temperature. If you lower it, you can get to the higher fuel flow. Not sure if that's realistic.

    It's not. I don't typically lean below 5000 ft. In real life my fuel flow is 15 to 16 gallons per hour in the climb to 6 or 7,000 ft. That's true whether it's in the dead of winter or the peak of summer here in New Hampshire.
    Generally speaking, you can get any fuel flow you want at any altitude within the limits of the fuel pumps. That's something that's currently not modeled correctly.

    Let's see what the update today/tomorrow brings. Don't have the time until then to try your suggested 6800ft test, but I doubt that it will significantly change to what I have tried already.

    I'll wait for the new release before doing any more testing.



  • @BernieV said in TAS/GS weirdness:

    It's not.

    I was talking about MSFS resp. this addon, not the real world ๐Ÿ™‚ I'm well aware that pumping more fuel results in more fuel flow even though the air-to-fuel ratio isn't optimal then ๐Ÿ˜ƒ



  • As far as I can tell, the new version doesn't change any of the reported behavior. The plane can't make book speeds, fuel flow looks to be artificially constrained, parasitic drag seems to be too high.

    Summary: The plane is slower than real life by roughly 7 knots. I think it might be a combination of the way power is modeled and parasitic drag being too high. This is somewhat complicated by the fact that the Indicated airspeed (on the panel instrument) seems off (high by about 6 knots).

    #1 : Airspeed indicator is indicating too high by 6 knots. Setup: 1356 ft msl, std pressure & temp, no wind. Power setting 25 squared mixture 100 ROP. From Little nav map, IAS is 119 kts. TAS=GS=121 kts according to EFB, GPS, & Little Nav Map. Panel airspeed indicator shows 125 knots. Conclusion: Airspeed indicator is indicating too high by 6 knots or so. This needs to be considered when doing further analysis on why the plane canโ€™t make book speed numbers. Another image (next page) captured at 6,400 ft the panel air speed indicator showed 121 kts and little nav map showed 115 kts indicated implying that the error seems constant with respect to altitude.
    โ€ƒ
    194fadd8-651e-405d-bb79-18dc56d8f62b-image.png

    #2 TAS Airspeed and fuel flow donโ€™t jive with altitude throttle, prop RPM, and mixture controls. At 6500 ft, std temp & pressure no wind, Little Nav map reports airspeeds (knots); indicated: 115, ground 126, true 126. Fuel flow is too low (I varied mixture between where it is shown and full and it had no effect on Fuel flow or airspeed, 12.7 g/h is what the book say we want to get 75% power and I cant get that according to the fuel flow indicator). In the real world at 6500 feet std temp & pressure, a wide-open throttle, 2500 RPM, and 100 rich of peak EGT should give me 75% power and a TAS around 135. GS should be 135 as well as no wind condition & std temp/pressure.

    7e9147f1-459f-4468-9f4c-9d8d2cd0d280-image.png

    #3 Parasitic drag seems too high. In an attempt to test drag independent of power I looked at change in speed vs descent rate. Level flight GS 127 kts. In real life, I start a decent from level flight 6500 ft at 75% power by trimming nose down for 500 ft/min. It takes 30 seconds or so to stabilize but this fairly soon results in +15-17 Knots/hr ground speed increase and indicated airspeeds just inside the yellow arch of the airspeed indicator. In the JF Arrow III 2.0 model, under these conditions I see a 10 knot GS increase about 6 knots less than expected.

    07702add-d321-4c98-bb88-70eff4ffdf9e-image.png

    If I continue to increase descent rate by trimming to 1000 ft/min descent I finally see +19kts, close to what I would expect with a 500 ft/min descent rate.

    289dfff4-ef7e-4c46-8be8-23170809b1c7-image.png



  • I agree, did very similar tests (your #1 and #2) and ended up with the same results as you did.
    The issue is still not fixed, the aircraft is slower than the ODM states and in clear skies preset can't even reach the 75% power cruise setup at all, even with 50% fuel and 1 pax while the book states the figures for max GW.

    As for the IAS gauge, I think mine is rather 4kts too high, not 6, but that's a minor.

    Also if you read the TAS from the EFB or LittleNavMap and dial the TAS in the IAS gauge (lower white scale) to see which setup for altitude and temp you should have had (in the white upper scale of the IAS gauge), it is WAY off, for me about 4000ft on that scale.
    I opened a ticket (#50575) for this TAS scale misalignment as well since it is obviously wrong.



  • I've found a work around if not the root cause of slower than expected TAS and lower than expected fuel flow. Basically the aircraft was unable to generate 75% power at altitude of 6-7000 ft due to inadequate fuel flow.

    In engines.cfg, fuel_flow_scalar was set to .51. Through trial and error, I found this decreases max fuel rate obtainable by varying the mixture control as altitude increases. So much so that I could not get the 15 or 16 g/h I would expect in a full rich climb nor the 12.1 g/h required for 75% power at altitudes between 5 and 7,000 ft.

    I set the fuel flow scalar to 1 and was see very high fuel flow rates (22+ gph) which is way to high. I suspect that's why the engine tuner set the scalar so low. Too low IMHO. I increased it to 0.73; the point were I could get 15ish g/h at 7000 feet full rich. leaning to 12 gph improved the TAS, but it was still too low 133ish.

    Next through trial and error, I settled on parasite_drag_scalar set to 0.825. This allowed me to get book speeds at 6 or 7000 feet.

    The airspeed indicator is still off, but this IMHO goes a long way towards allowing the modeled Arrow III to perform closer to real life.

    Summary of changes:

    In flight_model.cfg

    parasite_drag_scalar =0.825

    In engines.cfg

    fuel_flow_scalar =0.73

    Inflight stats with the new settings

    016f43ee-c79c-4413-a0fb-617907468f54-image.png

    And yes, I did update the related support tickets as well ๐Ÿ™‚



  • @BernieV said in TAS/GS weirdness:

    This allowed me to get book speeds at 6 or 7000 feet.

    Wow thanks for the efforts!
    Do these changes impact the climb performance? did you compare that to the book numbers?
    Also when getting the book speeds at 7000ft, did you lean it also according to the book? 100ยฐC rich of peak? Or do these changes also modify the position of the mixture to gain this fuel flow?

    Will give it a try later as well ๐Ÿ™‚ It's not the fastest aircraft out there, but having it getting at least the book speeds would be so nice.
    Unfortunately I have no idea about most of the flight model configs (opposed to lights) so whatever I'd change here, it would just cause something else to break for sure. ๐Ÿ˜ƒ

    As for the IAS gauge, I am not sure where this offset could be fixed. For the time being one could simply rotate the texture of the gauge, but it'd look weird.
    The wrong TAS vs. altitude scale can be fixed by turning the altitude texture 4000ft to the left (leaving the TAS scale at the old position) at least.
    Whoever does that, remember to also rotate the luminance texture since with panel lights on, they get overlayed.



  • Do these changes impact the climb performance? did you compare that to the book numbers?

    Climb performance continues to be better than real life. At 25 squared full rich in real life I see 700 ft/m climb initially (from sea level) dampening to around 500 ft/m between 5,000 and 7500 ft. The model happily climbs at 800-1000 ft/min with 25 squared throttle/rpm and full rich. I don't think I made that worse, but I would like to figure out how to slow the climb without impacting cruise speeds. I'll do some reading. Others may find this setting fine. I suspect in real life this is very aircraft dependent and varies with engine health, prop condition, rigging, paint, wax, etc.

    Also when getting the book speeds at 7000ft, did you lean it also according to the book? 100ยฐC rich of peak? Or do these changes also modify the position of the mixture to gain this fuel flow?

    The book gives climb performance at 2700 rpm. Thats nuts. Its redline speed rpm. Perhaps mechanics and marketeers will point out that in theory you can run that way all day, my instructors and all the Arrow owners I know set climb power at 25 squared MP/RPM full rich (below 5,000 ft).

    So I set it like I do in real life. 25 inch MP 2500 rpm full rich in the climb to 6,500. Fuel flows looked right with these settings (15-16 gph prior to leaning at altitude).

    I initially set fuel flow in cruise by looking at fuel flow reported by Little nav map (I set it to 12 gph based on book 75% power chart. The EGT read WAY low at 12 gph. I did try adjusting the fuel flow using mixture control and looking at egt, but at 100F ROP, I was seeing north of 15 gph which is way too high. So, not sure if the EGT gauge is being modeled correctly. in the end it did not seem to matter much if I used fuel flow as reported by the FF gauge or using the EGT and setting 100F ROP, the cruise speed was still in the neighborhood of 137 kts at 6500 ft 2500 rpm.

    As an aside, I tend to cruise at 2400 rpm or as low as 2100 rpm if not in a hurry. 2500 rpm is loud and produces a lot of vibration that over a 1-2 hour trip wears on you.

    Will give it a try later as well ๐Ÿ™‚ It's not the fastest aircraft out there, but having it getting at least the book speeds would be so nice.

    Agreed! You don't buy an Arrow to go fast ๐Ÿ™‚ Its a wonderfully stable IFR platform and with the gear up trips of 200 miles or so are not too painful. The one thing that was missing and motivated me to get the speed up a tad is that if you don't plan descents well, you will still end up too high or fast for precision instrument approaches. I think Its to the point now that it behaves pretty close to the real deal. I still want to slow the climb a bit without effecting cruise speeds. I'll look at that over the next few days.



  • @BernieV Another data point that the drag in the climb is too low is that I was able to climb to 20,000 ft in the 0.2.0 model with the tweaks I made to parasitic drag and fuel flow (albeit at less than gross weight). I barely made it to 13,000 on a hot day between Napa CA and Yellowstone in real life and book shows 15K as the service ceiling. So now I am looking for a way to slow the climb and reign in the service ceiling (hopefully with a single parameter!)



  • I appreciate all the research and testing you're doing, Bernie. I've noticed the Archer is running slower than I thought it should be -- a friend suggested maybe it was because they modeled the 3-blade prop instead of the 2-blade prop. I don't have the real world experience on the Archer to really add to or comment on it beyond that.



  • @piedmonitor Whatever they modeled, we all agree that the numbers should match to the ODM that is included in the addon, don't we?
    I also doubt that the 3-blade version isn't capable of a 75% power cruise no matter what the TAS is that results for that version. So there is definitely some issue with the flight model and I hope that someone from JF Staff is reading these findings and the devs are looking into it.