Source: https://guthlabs.ai/docs/instruments/drag-force-drag-coefficient-calculator/mechanism/

instrument mechanism / verified-clean

# Drag Force / Drag Coefficient Calculator mechanism

The inspectable method record for Drag Force / Drag Coefficient Calculator, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The force equation is standard, but the instrument specification is incomplete for a final PASS. It must explicitly document the inverse-Cd formula, drag-power formula, valid input constraints, and lookup-table conditions. The honest label is appropriate, but should additionally state that typical Cd values are condition-dependent and may differ substantially from measured values for a specific object.

- Fd = 0.5*rho*V^2*Cd*A is the standard steady-flow drag equation when rho is fluid density, V is relative flow speed, Cd is dimensionless, and A is reference frontal area.

- The inverse calculation for measured drag force is claimed but its formula is not stated; it should be Cd = 2*Fd/(rho*V^2*A), requiring rho, V, and A to be nonzero and consistent.

- Drag power is an output but its formula is not stated; for steady translation it should be P = Fd*V, with V as the positive relative speed. This is mechanical power opposing the motion, not total vehicle propulsion power.

- Shape-based Cd lookup is under-specified: Cd depends on Reynolds number, Mach number, surface roughness, orientation, aspect ratio, and sometimes yaw angle. A single shape value must be labeled as a typical reference, not a universal constant.

## Units and conventions

- Use SI units: rho in kg/m^3, V in m/s, area in m^2, force in N, and power in W.

- Cd is dimensionless.

- A is the projected frontal/reference area normal to the relative flow.

- V is relative fluid-body speed, not necessarily ground speed.

- The tabulated air density 1.225 kg/m^3 assumes approximately 15 C, 101.325 kPa, and dry air at standard sea-level atmosphere.

- The tabulated water density 1000 kg/m^3 is the standard rounded engineering value near 4 C and atmospheric pressure.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "mode": "forward_drag",
    "shape": "sphere",
    "Cd": 0.47,
    "frontal_area_m2": 0.01,
    "velocity_m_per_s": 10,
    "fluid_density_kg_per_m3": 1.225
  },
  "expected_outputs": {
    "drag_force_N": 0.287875,
    "drag_power_W": 2.87875
  },
  "reference_data": {
    "Cd_sphere": 0.47,
    "Cd_source_convention": "widely tabulated standard value for a smooth sphere in common subcritical flow; actual value depends on Reynolds number",
    "air_density_kg_per_m3": 1.225,
    "air_density_source_convention": "ISO 2533 standard atmosphere, rounded textbook sea-level value at 15 C and 101.325 kPa"
  },
  "working": "Fd = 0.5*rho*V^2*Cd*A = 0.5*1.225*(10^2)*0.47*0.01 = 0.6125*100*0.47*0.01 = 61.25*0.47*0.01 = 28.7875*0.01 = 0.287875 N. P = Fd*V = 0.287875*10 = 2.87875 W."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The inverse-Cd calculation is undefined when frontal area is zero and must reject the input. The signed-velocity case also exposes an ambiguity: the scalar V² formula gives only drag magnitude, while force direction and signed versus magnitude-only drag power require an explicit convention.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/drag-force-drag-coefficient-calculator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/drag-force-drag-coefficient-calculator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/drag-force-drag-coefficient-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche7/tranche7-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`