I am working on the flight_model.cfg file for a Learjet 35 for MSFS 2024. I have reasonably accurate aerodynamic data from the manufacturer and other sources. One item I have set is the drag_coef_zero_lift (0.0242). With this value set, the Cd0 computation shows a value called “other_drag”, which I cannot account for from the SDK – Please see attached graphic:
The “other_drag” is adding additional drag to the drag_coef_zero_lift value that I entered, which considers all drag at zero lift. It also the value that is to be entered from the SDK documentation:
Even if I reduce the value entered into drag_coef_zero_lift, e.g., reduce it to 0.012, the “other_drag” will chance for the same flight conditions to effectively null out the changed draft_coef_zero_lift change.
Is there anything else that can be changed to reduce the “other_drag” value? For example, from the SimObjects Editor > Aerodynamics Tab, will reducing any of these values reduced the “other_drag”:
Hi Rich, I don’t have a great answer but “other drag” seems mainly control-surface related, and the low-profile entries in the flight_model.cfg for the ‘extra’ drag values (e.g. ailerons, gear, flaps) end up being pretty significant but without receiving much attention.
In your cut-paste from the Debug Aircraft Drag window you omitted the crucial next lines (see mine below) which shows the break-down of the “other drag” number. In my experience the only significant part is indeed the “side slip and control surfaces” part and in reality that just means the control surfaces.
What doesn’t help is the values are (of course) constantly changing but when you hit “Esc” to pause the sim the “other drag” instantly snaps to a nonsense high reading - you might have worked that out already as a Win-shift-S screenshot while in normal flight will freeze the screen to give you a good screenshot.
On my plane (it’s a glider, so suuuper-sensitive to the drag calculation) I don’t know exactly how to manage that control-surfaces number as it changes with the flap setting change but afaik that’s NOT the drag setting in the flaps section of the flight_model.cfg. In my case the sideslip-control-surfaces drag is very low and decreases to pretty much zero as I step through +ve flaps until it’s essentially zero when I’m in landing flap. At a total guess this is some kind of side-effect from the PITCH of the plane. Maybe, just maybe, the sideslip-drag is considering the pitch of the plane as well as the yaw.
I think there’s another controls “other drag” factor which is related to the “elevator trim” of the plane, i.e. it’s as if MSFS assumes an “all flying” tail. IF the plane speed is on the stable trim value so no elevator input is needed the “other drag” will be lower than if the same speed is maintained with a control input. This makes sense e.g. on the 737 where the entire horizontal stabilizer moves with the trim (so the elevators are not involved in setting the trim speed) but the planes I model control the trim by putting bias into the elevator offset, so there should be no drag difference between a stick input and a trim input. To test this you just have to adjust the MSFS trim to the test speed you’re flying and then look at the “other drag” breakdown.
The “other_drag” is listed right below the Cd0 data, and it provides the sources. It looks like the side_slip_and_control_surfaces is providing a significant amount of drag (0.1079). That could mean that the elevator is displaced for trim and creating drag (?). Do you have a suggestion on where to look for that in the flight_model.cfg file?
Also, the ‘slip_stream_and_prop_wash” seem high at 0.00626. Numbers a bit fuzzy.
Every time I change the drag_coef_zero_lift in the flight_model.cfg, e.g., lower it, the “other_drag” value changes. If I increase it, the “other_drag” decreases, and vice versa.
Now that I see that what draft contributes to “other_drag”, finding the cause might be easier (???).
Any help that you or anyone else on this forum could provide would be greatly appreciated!
I’m not sure how much engineering-accurate help I could offer, but I’ve worked on pretty much every glider in the sim and we do really care about the lift and drag figures at every point in the speed range. Having said that it is TOTAL drag (and lift) that matters, when you look at actual models in the sim it’s a bit of a mix where those figures come from but the end result is always the plane flies about right. Apart from rookie errors with things like the gear or control surfaces, for the lift and drag particular complexities are in the flaps and the way the lift/drag figures change with airspeed. This is less of an issue at flap 0 or with powerful engines.
So what we do is fly the plane to confirm the drag is in the right ballpark - of course the starting point is to set the parameters within what seems a sensible range but we can’t stop there. With gliders the height gained in a pull-up is a dead giveaway - it’s not calibrated but too little or too much drag seriously affects that. After that we need to concentrate on the lift/drag RATIO, because that’s what determines the glide slope which is what we most care about. We can fine tune that with either the lift or the drag and want to get that ratio within say 2% of the published figure for each airspeed which is a much finer tolerance than is needed for a satisfactory power plane where the lift+drag+engine+prop all have to add up to a happy package but there are ok trade-offs between those.
IF you have issues with residual drag coming from places at normal cruising speed, it might be worth looking at the flight_model parameters:
as those can dial-out the additional drag if the fuselage/tail isn’t ideally aligned to the flying position. If you adjust the htail_incidence to remove the drag from the elevators in cruise then you will need to shift the CofG to get back to the right zero-stick cruise speed. I don’t think there’s a elevator drag parameter (could be wrong) so your options to directly reduce it there are limited.
A subtle challenge is it can be difficult to get your lift/drag right at both ends of the speed range and the ultimate rescue is the “drag_coef_zero_lift_mach_tab” which enables you to tweak the drag curve differently at high or low speeds. We use that in all the gliders.
In the 2020 version, I tweaked the “drag_coef_zero_lift_mach_tab” to provide extra drag at cruise Mach above 0.75, the critical Mach number for the Lear 35A. It’s on my list to refine, but I need to get other data fixed first.
I am wondering if there is a horizontal tail issue. With the elevator effectiveness scalar set to zero, at cruise flight there is a significant nose-up trim required. At high speed flight, the stabilizer trim is generally in the nose-down range, not nose up. So, something is not correct and I’m wondering if that is causing excessive drag. You’ve provided me with some flight_model.cfg parameters that I have not looked at.
I am working make this version as close to real aircraft as possible using the new capabilities of MSFS 2024. I have a lot of data on the Lear 35A. It’s a matter of figuring out where it all goes. The SDK is not much help in that regard, but I am finding that MS Copilot is helpful with explaining some of these entries. I need to try ChatGPT, too.
maybe the nose-up bias is a bit of a clue, so the elevators are being used in cruise to push the nose down. You could try to dial that out adjusting the `htail_incidence` i.e. make it more positive and see if the ‘other drag’ goes down when trimmed to cruise at the same speed. Also it might be worth getting the SDK sample simconnect app SimvarWatcher up and running to display the elevator deflection, or maybe one of the Debug..Aircraft panels has that somewhere already.
