Regarding aircraft emission calculation in emission.py

[arthurlli]

Hi, thank you for open-sourcing aircraft performance and the toolkit, in which I leant a lot from that.

While reading emission.py, I noticed that there seems to have a missing documentation in one function:

openap/openap/emission.py

Lines 30 to 36 in c9ee4cc

	def _sl2fl(self, tas, alt):
	M = aero.tas2mach(tas * aero.kts, alt * aero.ft)
	beta = np.exp(0.2 * (M ** 2))
	theta = (aero.temperature(alt * aero.ft) / 288.15) / beta
	delta = (1 - 0.0019812 * alt / 288.15) ** 5.255876 / np.power(beta, 3.5)
	ratio = (theta ** 3.3) / (delta ** 1.02)
	return beta, theta, delta, ratio

If it is not intended, would you mind adding few descriptions on the beta, theta, delta, and ratio? Thanks in advance.

Also, a suggestion for consistency:

- theta = (aero.temperature(alt * aero.ft) / 288.15) / beta
- delta = (1 - 0.0019812 * alt / 288.15) ** 5.255876 / np.power(beta, 3.5)
+ theta = (aero.temperature(alt * aero.ft) / aero.T0) / beta 
+ delta = (1 - 0.0019812 * alt / aero.T0) ** 5.255876 / np.power(beta, 3.5)

Furthermore, may I know what constants 3149 and 1230 mean in these functions:

openap/openap/emission.py

Lines 38 to 62 in c9ee4cc

	@ndarrayconvert
	def co2(self, ffac):
	"""Compute CO2 emission at given fuel flow.
	Args:
	ffac (float or ndarray): Fuel flow for all engines (unit: kg/s).
	Returns:
	float: CO2 emission from all engines (unit: g/s).
	"""
	return ffac * 3149
	@ndarrayconvert
	def h2o(self, ffac):
	"""Compute H2O emission at given fuel flow.
	Args:
	ffac (float or ndarray): Fuel flow for all engines (unit: kg/s).
	Returns:
	float: H2O emission from all engines (unit: g/s).
	"""
	return ffac * 1230

Thank you for your attention.

Best regards

[arthurlli]

The changes are suggested in PR #22. Sorry if it is not allow to modify the codes.