Dear user of the LiftCalculator app,
Depending on whether your iPhone has a built-in barometer your app shows two buttons or just one button to calculate the lift of your Hot Air Balloon. So you can choose between using your input of the air pressure measured at the airport of your choice or by using the measured local air pressure by your iPhone.
Two red buttons are shown with iPhone 6, 6 Plus, 6s, 6s Plus
One red button is shown with iPhone 4s, 5, 5s
The lift calculations are made by the following formula and calculations, and where the ISA environmental lapse rate is assumed:
In general:
L = V x (P/(rConstantx0,01) x ( (1/Tamb+273.15) – (1/Tenv+273.15) )
where
L = total lift of the balloon in kg
V = enveloppe volume in m3
P = air pressure in hPA
rConstant = gas constant
Tamb = Ambient temperature (in Celcius)
Tenv = Envelope temperature (in Celcius)
In detail I have used the following numbers and calculations:
gravity = 9.80665
mDryAir3 = 0.0289645
univGasConst = 8.31447
Tlapse = 0.0065
gMRL: Double = (gravity * mDryAir3) / (univGasConst * Tlapse)
risa = 8.31432
mDryAir = 28.9645
rConstant = risa/mDryAir*1000
volume: Double = volumeft * pow(0.3048,3.0)
ambTempAlt = ambTempTakeOff – (Tlapse*((flightAlt-startAlt)*0.3048))
pressureAtAlt = qnh * pow((1-(Tlapse*flightAlt*0.3048)/288.15),gMRL)
pressureAtStart = qnh * pow((1-(Tlapse*startAlt*0.3048)/288.15),gMRL)
maxLift = volume*(pressureAtAlt/(rConstant*0.01))*((1/(ambTempAlt+273.15))-(1/(envTemp+273.15)))
maxLiftStart = volume*(pressureAtStart/(rConstant*0.01))*((1/(ambTempTakeOff+273.15))-(1/(envTemp+273.15)))
Using this app is at your own risk. Be aware that you are only legally allowed to use the instruments, guidelines and instructions as mentioned in the operating manual of your hot air balloon.
Have a nice and safe balloon flight!