Technical detail (optional bedtime reading..)
If you’re mainly interested in how to use Aerosensor, you can skip this section. The details here are for those who want to understand the physics behind the calibration process.
Aerodynamic basics
Aerodynamic force generally scales with dynamic pressure - the pressure increase you’d get if moving air was brought to rest.
The formula for this is:
p_{dyn}=\frac{1}{2}\rho V^2
Where:
- p_{dyn} = dynamic pressure (Pa)
- \rho = air density
- V = Air speed.
Since drag force is proportional to dynamic pressure, we can calculate a drag coefficient, C_dA:
CdA\ =\ \frac{Drag}{p_{dyn}}
Dynamic pressure is just the difference between total pressure (the pressure measured by the little tube in the centre of aerosensor) and the static pressure (ambient pressure):
pDyn\ =\ p_T-p_S
Measuing dynamic pressure
Aerosensor cannot measure the true static pressure because it sits close to the bike, where the flow is already slowing down. Instead it measures a slightly different static pressure, p_{aero}.
From aerodynamic theory the difference can be expressed as the pressure coefficient, Cp_{aero}.
C_{p_{aero}}=\frac{p_{aero}-p_s}{p_T-p_s}
What aerosensor actually measures is dp:
dp=p_T-p_{aero}
Expanding this and substituting in C_{p_{aero}}:
dp=(p_T-p_S)-(p_{aero}-p_S)
dp=p_{dyn}-C_{p_{aero}} p_{dyn}
dp=(1-C_{p_{aero}}) p_{dyn}
Rearranging:
p_{dyn}=\frac{dp}{1-C_{p_{aero}}}
Aero device calibration
This leads to the aero device calibration, cal:
cal=\frac{1}{1-C_{p_{aero}}}
So the corrected dynamic pressure is calculated by:
p_{dyn}=cal\times dp
How it's calculated in practice
In the real world, where there may be wind, calibration is derived from an out-back run or a single lap of a closed circuit, based on the assumption that the average wind is zero.
Aerosensor calculates:
- The average measured pressure dp.
- The average dynamic pressure inferred from wheel speed.
Dividing (2) by (1) gives you the calibration factor for that run.
Post-correcting for updated calibration factor
CdA can be post-corrected for calibration factor as follows:
{CdA}_{new}=\ {CdA}_{old}\times\frac{{cal}_{old}}{{cal}_{new}}
Where:
- CdA_{old} = CdA reported by Aerosensor.
- cal_{old} = Calibration factor entered in settings in CIQ app.
- cal_{new} = new calibration factor you want to apply.
- CdA_{new} = corrected CdA.
Both Aeroportal and Aeroworkbook do this automatically when you change the calibration factor.
However, this correction is not perfect in every case. To calculate altitude, we need accurate dynamic pressure. On velodromes, altitude is ignored, so calibration can be corrected perfectly afterwards. On out-and-backs, large errors in calibration will reduce CdA accuracy.
For example:
- On an out-back run, a 10% error in calibration, combined with a 25% variation in speed across the lap, results in about a 1.5% error in CdA.
- If speed were perfectly constant through the lap, the error would be zero.
Keeping your speed consistent between runs reduces the impact of calibration error on your results.
