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SWS-AlexVletsas avatar image
SWS-AlexVletsas asked SWS-AlexVletsas edited

Turboprop thrust decrease after SU7 with legacy tables

After the release of SU7, I noticed that our aircraft can't hit the published figures any longer. Before SU7 performance on all altitudes was by-the-book, but now thrust drops very quickly as speed builds up after takeoff. I also noticed that the plane takes a while to go from 1710 to 1790ft-lbs of torque at full power as of SU7.

The plane uses legacy tables with data coming from the factory. Back when implemented (SU4) the data got the engine to work immediately by the book, without the need for adjustment -a testament to the legacy tables' quality.

Configuration: clean, 5000lbs weight, 101KIAS climb, 1670ft-lbs of torque at sea level.

---Published performance---


---Actual performance---

Test conditions: Clear skies, no winds

Rate of climb in feet per minute, airspeed 101 knots indicated

Sea level

Takeoff rolls are spot-on. We also checked our glide ratio which is correct, 12:1 as the manual says. Therefore, the clean aircraft is correct in terms of drag & balance.

It is also worth noting that the PC-6 coming with SU7 suffers from sluggish performance and it also uses the legacy tables system. Planes that use the scalars from the modern FM work fine.

I would appreciate a response on this topic as release of the aircraft is in 2-3 weeks and we want to have this sorted and avoid another delay.

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GrimPhoenix9349 avatar image
GrimPhoenix9349 answered

Just to clarify one comment there, 'planes that use the scalars from the modern FM work fine' just means that they do not appear to suffer from this particular problem. The reason we are using the legacy tables is that they are far more precise in their control and it was possible to achieve a far greater level of accuracy than with the modern scalars, so using them is not an option.

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SWS-AlexVletsas avatar image
SWS-AlexVletsas answered SWS-AlexVletsas edited

Paul above did some changes and improved the plane's lift, but climb rates still suffer. We barely gained 50fpm of climb rate and I believe it is clearly a problem with engine power.
I did some tests with the default C208 and our Kodiak and noticed that both of them are incapable of reaching top speed, staying about 10KTAS below their top speed. However, the drop of thrust on the Kodiak (legacy tables) is considerably greater than the 208. Test data taken at max power, sea level. Thrust measured in pounds.

TAS Caravan Kodiak Porter (ASB)
0 1940 2150 1930
50 1930 1850 1700
100 1860 1370 1430
135 1750 1120 1180
155 1330 1035 -

Percent change of thrust
TAS Caravan Kodiak Porter (ASB)
0 100% 100% 100%
50 99.48% 86.05% 88.08%
100 95.88% 63.72% 74.09%
135 90.21% 52.09% 61.14%
155 68.56% 48.14% -

It is worth noting that the Caravan is barely able to touch 160KTAS and the Kodiak can hardly reach 155KTAS, with its top speed at sea level being 166KTAS.

A difference of 10 knots down and the behaviour of table-based aircraft indicates that there is a bug with table implementation that wasn't there before. The difference in thrust at 100kts (30% drop compared to the 208) matches the one in our climb rate exactly.

Changing the thrust_scalar and cx don't seem make a difference, as even an increases of 60% fail to achieve climbs and top speeds. I also reduced my tables from 14x10 to 13x7 as the SDK says and still nothing changes. It is worth noting that 14x10 worked fine in the past and its values seemed not to be ignored.

We could increase thrust efficiency and get more out of the engine, but there are two problems with this:

  1. Our engine data is correct and we know it with 100% certainty
  2. The data worked before SU7 and we were able to hit our speeds quite closely. So if we adapt our engine to SU7, how do we know we won't have to do it again with SU8 or 9?
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