Percent power

[tomcourt]

Add percent power to engine display.

This would requite a pressure altitude and OAT probe which means it would be of limited use until a custom shield frees up 4 more analog pins.

The code is tight so other code cleanups may be needed to get this to fit.

MGL has a reasonable formula for percent power in this doc, they have an excel spreadsheet for converting engine power tables to the 3 needed constants: http://www.mglavionics.co.za/Docs/MGL%20EFIS%20G2%20HP%20calculation.pdf

• STP power = ((((MAPmax - ((MAPmax - MAPnow) * K1)) / MAPmax) * RPMnow / RPMmax) ^ K2
• Altitude adjust = (1 + K3) * (ATMO_PRESSstp / ATMO_PRESSnow) - K3
• Temperature adjust = SQRT((519 – (3.58 * PRESS_ALT_FT/1000)) / (460 + OAT_F))
• K1 and K2 are very roughly .1. Default value for K3 is -.695
• Power % = 100 * STP_Power * Altitude_Adjust * Temperature_Adjust

The STP power would require an interpolation table for x^K2, another would be required for the square root. K2 being a user variable would mean that the interpolation would need to be engine specific. In theory logarithms could be used, but w/o floating point dynamic range may be a problem.

The Stratux could do the calculations and may be able to provide the true altitude, nearest airports temperature and pressure and the forecasted temperature aloft. Apply the standard lapse rate to the airport temperature if the aloft temperature isn't available. This would allow percent power with no additional sensors

Dynon has an O-320 table: http://preflight.dynonavionics.com/2014/02/did-you-know-percent-power-dynon-way.html

Another couple power spreadsheets: http://www.kilohotel.com/rv8/rvlinks/o360apwr.xls http://www.kilohotel.com/rv8/rvlinks/io360apwr.xls