Captain Sim 757 FDE

[slait 169]

Hey folks,

I was hand flying the approach into SXM last night with the PW version of the Captain Sim 757. After disconnecting the autopilot it felt like the flight dynamics were very "floaty" for lack of a better term and was hard to keep steady. In comparison to something like the PMDG 737 where the plane feels very solid and is conducive to small adjustments, this needed more yoke to move at all, then when it did it almost side slipped back and forth. Kind of like a feather floating side to side as it comes down. Very odd for a big airliner.

Anyone else experience this and have any suggestions?

[Chock 17,769]

Without knowing the payload and the weather it's hard to say. These could have a bearing on what it was doing, especially at this time of year when icing conditions are common and your weather add ons might have been doing some of that stuff.

Beyond that, the 757 has a very high power to weight ratio and it shifts a lot of air, but it is aerodynamically quite clean. It's not really going to handle like a 737NG. Even though a 900 NG is sort of comparable in terms of pax capacity, the 757 is still a much bigger aeroplane and so it will have more inclination to keep going the way it is going.

[vc10man 732]

Have had no 'floating' issues hand flying her into GCXO last night.

[slait 169]

Thanks for the responses.

There was no icing, I was landing at Sint Maarten, and there was a little cross wind, but it was modest. Strange, maybe I'll give it a few more flights and see if it's a consistent thing or something unique to that flight.

[Cai Zicheng 171]

I think what you are talking about is the modeling having too high moment of inertia. The "symptoms" could include the aircraft may feel resistant to small control input. When you actually get the aircraft to start, say, banking, it would require more input to get it to stop.

Does this describe what you are seeing?

https://en.wikipedia.org/wiki/Moment_of_inertia

[Mace 1,902]

757's can be hot rods, especially when light.  For that reason a lot of airlines use them on hot-and-high routes, or shorter fields. I can see it being possible to float one on approach.

It's also possible that you're making the wrong comparison, trying to compare the PMDG 737's model with CS's 757.

[Bob Scott 7,715]

23 hours ago, slait said:

Hey folks,

I was hand flying the approach into SXM last night with the PW version of the Captain Sim 757. After disconnecting the autopilot it felt like the flight dynamics were very "floaty" for lack of a better term and was hard to keep steady. In comparison to something like the PMDG 737 where the plane feels very solid and is conducive to small adjustments, this needed more yoke to move at all, then when it did it almost side slipped back and forth. Kind of like a feather floating side to side as it comes down. Very odd for a big airliner.

Anyone else experience this and have any suggestions?

I think the low-speed (e.g. approach-speed) responsiveness is too sluggish as well.  The spoiler drag is way too high, and the aileron-spoileron mix is also too aggressive (comes in too early and too much).  In the real jet, the spoilerons don't move until 10 deg (half) aileron travel, and then they don't fully open even at full aileron deflection.  The FDE has the spoileron mix commencing after only 1.7 deg of aileron travel, and then they open to nearly full deployment by the time the ailerons get to full deflection.

My current tweak has spoiler drag per radian (aka Cd_ds) of 0.024 set in record 1101 of the air file (was nearly double that), and in the aircraft.cfg file, min_ailerons_for_spoilerons was set to 10 (was 1.7), aileron_to_spoileron gain set to 3.0 (was 2.05), and aileron_effectiveness set to 1.1 (was 1.0).  This is based on feel during flight test rather than detailed mathematical modelling, but it works for me.

Regards