General question…How to calculate Lift? And what Unit to use?
biller last edited by
I’m doing some experiments with Unreal Engine 4’s default flying template
I’m trying to put some physics and aero calculation into its code.
So I’m putting atmosphere data and aerodynamic data into its .h files
Before I put "L = .5 * Cl * r * V^2 * A " into the flying pawns cpp file
However, I do not have reliable real-life aircraft data
So I have to do some experiments from DCS to BMS and check their data files
I did a check of the Unit
Then I found problems.
Take Mig-29S as an example.
It says its wing area is 38.1 m2
Spawn in mid air, empty weight 11347kg, fuel weight 3400 kg after a sprint
Therefore gravity should be 144,000N
Reach speed of 1647km/h IAS at 500 meters
AoA of 0.4 (almost zero)
The plane’s altitude remains steady, which means weight==lift
Then I look the mig29c.dat table in the data folder
I found that 0.07 is the closet value for the Cl
So I put 0.95(relative density of air) as r
38.1 square meters as A
457.8m/s as V
into the equation
and I found (ignore AoA here)
From the perspective of fundamental unit, the unit for r should be kg/m3
If I put 1.22 (approximately 500m’s air density) into the equation, the Lift I get will be even bigger…
That’s obviously wrong…
But the magnitude seems right,
I am wondering where did I go wrong…Was it Unit problem? Or wing area problem?
x2hawk last edited by
In the lift equation, r is the density of the air, not relative density. So, 500m altitude corresponds to 1.1673 kg/m^3 according to the Standard Atmosphere model. At sea level r = 1.23 kg/m^3. So right off the bat your value for r is a little bit off. But, doing the calculation with the correct density still gives something a fair bit off; I got L = 3.26e5 N
Assuming the mass and geometric properties used for the aircraft are correct (which, they seem to be), then the culprit must be in the assumed value of CL. I looked at the .dat file and I really have no idea the range of alphas these values of CL span, so I don’t know what alpha corresponds to the CL of 0.07, but it looks like you’d get quite the error if you just used one of the listed values instead of properly interpolating for the value you need. If you put the original expected value of lift back into the equation and solve for CL, I get CL = 0.031, which doesn’t sound unreasonable to me.
Easy last edited by
EDIT: forget it read the question wrongly