Update! MSFS2020 - Cessna 152 Improvement Mod - V.1.13.16.1


Flight model modification version 1.13.16.0, date 2021-02-18:

This flight model mod changes only a minimum set of parameters to apply the "workaround for missing propeller drag" to the Asobo Cessna 152 flight model. I copy the parameter files aircraft.cfg, engines.cfg, flight_model.cfg and some more files from the Official folder to the Community folder.
What is propeller drag? If you set full thrust, you have first a mismatch between how fast the air around the airplane moves and how fast the propeller wants to move this air. You get acceleration until this mismatch is resolved. If you set thrust to idle, you have another mismatch. The air moves faster then the propeller wants to move the air. This results in a deceleration that we call "propeller drag". MSFS 2020 simulates the acceleration, but not the deceleration. The workaround increases available drag in the flight model. The available drag is induced drag, parasitic drag, gears drag and flaps drag.
To land the airplane with the workaround you first reduce thrust to idle. The parasitic drag reduces the airspeed. Below the gears out speed you bring gears out. This adds gears drag and reduces the airspeed even faster. Below flaps out speed you use flaps, add even more drag. If necessary you can increase thrust now to get the airspeed you need. With the workaround you can "flare" the airplane to the touch-down.
The induced drag parameters are induced_drag_scalar and flap_induced_drag_scalar. The MSFS SDK documentation version 0.10.0.0 tells: "If you have this [induced drag] value, use it. If not, a value between 1.2 and 1.5 is usually fine. If this value is above 1.5, the plane will generate a lot of drag at slow speed. If the value is closer to 1, it will glide very easily at slow speeds". I choose 1.5 for the fast TBM930 and 1.2 for the slow C152.
The parasitic drag parameters are drag_coef_zero_lift and parasite_drag_scalar. I set parasite_drag_scalar to 1 and only adjust drag_coef_zero_lift. The airplane "idle thrust deceleration" is controlled by the maximum lift-drag ratio (L/D)max. The "1000 surfaces" airplane 3D model connects the zero-lift drag coefficient CD,0 to (L/D)max. That is you set a CD,0, use the MSFS 2020 build in DevMod tool Aircraft Editor, Debug, SimPolar VhVs to measure the (L/D)max and repeat the steps as needed.
You can ask the manufacturer for the (L/D)max value of the airplane. Very small propeller airplane like Cessna 152 have a (L/D)max of a little below 10. The Cessna 172 has a value of 11.6 and faster, more expensive airplanes have 13. I assume that the TBM930 has an (L/D)max value of 14.
As explained above the workaround increases available drag to compensate for the missing propeller drag. Therefore I set for the fast TBM930 the "workaround (L/D)max" to 80% of the real world (L/D)max or 11.2. For the slow C152 I set the "workaround (L/D)max" to 90% or 8.6. 
Gears drag is simulated in parameter drag_coef_gear. The C152 has 4 different gears and flaps settings. Normal without flaps, flaps position 1, flaps position 2 and landing, that is gears out and flaps position 3. For the test I set the airplane to maximum airspeed level flight at sea level and use the "active pause". In active pause I change flaps settings and note the airspeeds.
Flaps drag is set in many parameters. First there is lift_scalar, drag_scalar and pitch_scalar. For this minimum change flight model mod I keep the Asobo numbers. I adjust drag_coef_flaps to get the intended behavior.
The values for the flaps-position.X parameter are tricky. My cookbook recipe is: Set the third parameter for flaps position 0 to 0 and set the third parameter for the other flaps positions to 1. The third parameter is a flaps drag multiplier. The flaps drag depends on this multipler and on the flaps angle. You don't need to play with the drag multiplier parameter.
I adjust flaps drag until airspeed for the slow C152 in landing configuration is 60% of airspeed in normal configuration. For a fast airplane like TBM930 I adjust to 50% airspeed.
After you have changed drag, you have to adjust thrust, fuel consumption and brake power to get again the intended cruise speed. My definition of parameter cruise_speed is: the maximum sea level level flight airspeed in still air. To adjust fuel consumption I use DevMod tool Aircraft editor, Debug, Engines. I set thrust to 75% or 80% LVR and adjust parameter fuel_flow_scalar as needed.
At last I take care of elevator trim. I like that I get cruise speed level flight without elevator trim. I adjust the parameter elevator_trim_neutral for this.
Here are all changed parameters. Reference are the parameter files of MSFS 2020 version 1.13.16.0. The values behind the first ; are the Asobo values.

flaps-position.1 = 10, -1, 1 ; 10, -1, 0.25 ; 
flaps-position.2 = 20, -1, 1 ; 20, -1, 0.75 ; 

elevator_trim_neutral = 0 ; 1 ; Elevator trim max angle (absolute value) (DEGREES), for indicators only (no influence on flight model)

induced_drag_scalar = 1.2 ; 1.5
flap_induced_drag_scalar = 1.2 ; 1.5

drag_coef_zero_lift = 0.057 ; 0.03500
drag_coef_flaps = 0.075 ; 0.06020

fuel_flow_scalar = 0.48 ; 0.60 ; Fuel flow scalar

thrust_scalar = 1.7 ; 1.2 ; Propeller thrust scalar

Summary: The Asobo C152 has a measured (L/D)max of 10.2 and is already good to fly. The presented C152 flares hopefully even better. Only 9 lines of configuration parameter changes are the difference. Asobo, you have now an easy to follow cookbook recipe for realistic flight model. Can you please apply this recipe to all MSFS 2020 airplanes?

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