# Thread: aircraft speed vs drag in subsonic aircraft

1. Junior Member
Join Date
Jul 2010
Posts
23

## aircraft speed vs drag in subsonic aircraft

Most .air file record 430 settings I've seen have no drag from M0.0 to M0.6 increasing a little bit from M0.6 to M0.8 then heaps at M1.0.

The pilot's training manual I have says that if you double an aircraft's speed you get 4 times the drag, three times the speed 9 times the drag, four times the speed 16 times the drag, and five times the speed 25 times the drag.

M0.4 is double M0.2, M0.6 three times M0.2, M0.8 four times M0.2, and M1.0 five times M0.2.

The X coordinates in 430 are mach speeds in increments of M0.2 up to M3.2 and the Y coordinates are drag. Only the Y coordinates are editable and this is done using the up/down arrows keys on the keyboard.

As you can only use whole numbers for drag in 430 and you have to start somewhere then,

A. if X=M0.0 Y=0 and X=M0.2 Y=1 then X=M0.4 Y=4, X=M0.6 Y=9, X=M0.8 Y=16 and X=M1.0 Y=25

B. if X=M0.0 Y=0 and X=M0.2 Y=2 then X=M0.4 Y=8, X=M0.6 Y=18, X=M0.8 Y=32 and X=M1.0 Y=50

C. if X=M0.0 Y=0 and X=M0.2 Y=3 then X=M0.4 Y=12, X=M0.6 Y=27, X=M0.8 Y=48 and X=M1.0 Y=75

D. if X=M0.0 Y=0 and X=M0.2 Y=4 then X=M0.4 Y=16, X=M0.6 Y=36, X=M0.8 Y=64 and X=M1.0 Y=100

For subsonic aircraft I use the Y coordinate figure at M1.0 for all Y coordinates above M1.0. In other words the Y coordinates if using A. above would be 0, 1, 4, 9, 16, 25, 25, 25, 25, 25, 25, .......

These 430 settings are directly related to, and may require adjustment of, the Drag Coefficient Zero Lift (parasite Drag) setting in .air file record 1101 or of the parasite_drag_scalar in [flight_tuning] in the aircraft.cfg file.

Starting with set A. then going to set B. if required appears to be a better way of proportionally adjusting drag across the speed range than simply increasing or decreasing the Drag Coefficient Zero Lift in 1101, particularly if there was little or no drag across most of the speed range to begin with (see opening paragraph).

The difference between set A. and set B. above in a DC-3 at 8,000 feet at M0.25 is 2 knots.

2. As you can only use whole numbers for drag in 430...
Record 430 description from AAM2:
Compressibility drag or wave drag.
Typically peaks near 0.034 to 0.046 at M 1.0.
Critical at cruise mach. Set at M 0.8 & 1.0 to give correct value at cruise.
Reduce peak to reduce mach 1 "wall".

This mach data table is fixed size (17 elements) and is assumed to have inputs of Mach from 0.0 to 3.2, where each table step is 0.2 Mach. All outputs are additive to the base aerodynamic coefficient defined in section 1101 or 1539
As you can see below, the "X" value is fixed(greyed out) and the "Y" value is a very small increment. Not a whole number.
They are additive to the Record 1101(Primary Aerodynamics) Cd0{Coefficient of Drag at Zero Lift} or Record 1539(if present).
Also a fraction. Cd0 is generally in the range of 0.01 to 0.04...Don

3. There seem to be a few misapprehensions in the OP.

The Mach tables represent values to be added to the base (low speed/incompressible flow) aerodynamic coefficients to allow for compressibility effects. They therefore have no effects at low values of M.

aerodynamic forces for basic incompressible flow vary with density and the square of true air speed, all else being equal. Mach effects are additive to that but with a much smaller effect. Increasing M from 0.2 to 0.8 will only increase the additive force by about 1.5 times, not 16 times.

A number in the Mach tables is the value to be added to be base aerodynamic coefficients which is then multiplied by 2048 and then stored as a dw. For example if the value to be added is 0.25, the value stored is 0.25 * 2048 = 512.

4. For example if the value to be added is 0.25, the value stored is 0.25 * 2048 = 512.
Thanks for the info Gerry! I wondered why the increments of Cdo, Cdi, etc., were not updated exactly as entered. 1/2048!
Obviously, AAM2(my *.air editor of choice) takes my RW entries and scales them internally when producing the output for the *.air.
The sim must not have been coded to recognize floating point numbers for those and needed a scaling factor for some of the complex entries.
Verified by doing a dump with Air Update. Why some entries are available as floating point and others aren't is a mystery to me...Don

5. Junior Member
Join Date
Apr 2010
Posts
47
The basic drag equation is based dynamic air pressure, and includes the square of TRUE airspeed velocity (ft/sec in my world!). You can't use Mach as the scaling factor.

6. The use of integers and odd conversions are a legacy from the early days of FS that never got changed. It would be interesting to know if they have been changed for Flight.

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