Computer simulation - flight physics
 

Hello,

I'm working on some new software to simulate aircraft - light C172 types.

I've found this website that describes the lift equation:

Modern Lift Equation

I'm wondering about simulating stall though. This is when the air separates along the wing. How does this work? I'm presuming as the angle of attack starts to get to steep we modulate lift.

Anybody got any equations to share?

Also how does the nose drop work? A low air speed, means lift of wings is reduced which leads to nose drop. I am presuming that the COM is behind the wings so that when the lift is removed the rotation brings the nose down? Or am I way off here?

Is Genghis still contactable? He would be the best person to speak to.

J.

'Modern' Lift Equation? Has it changed then?

Most simulators fit into 2 categories. They are either a 'virtual wind tunnel' sim like Xplane that constantly calculates lift, drag etc. based on the geometry of a model, or they use look up tables, like MSFS.

The former is basically using a simple form of computational flow dynamics to calculate all the parameters. The latter relies on a big data file that tells the simulator what happens at different conditions.

For example, if modelling the lift from a wing, you'd need a file with angle of attack arguments from -180 to 180 degrees, mach number arguments from 0 to however high it needs to be, and finally a list of Cl values corresponding to all of those angle of attack and mach numbers.

Using this, the sim can, for example, realise the wing is at an angle of attack of 21 degrees and a mach number of 0.3 and look up the Cl. Unfortunately this only gives a 2 dimensional idea of how the wing is performing, so the sim will also need a way of numerically solving the 3D effect of a wing (using something like lifting line theory and horseshoe vortices).

These tables need to be present for everything from Cl, Cd, Cm etc. to the planform and geometry of lifting surfaces and fuselage.

To answer the stall question, as the angle of attack in the sim is increased, the data table Cl value increases until the point of separation, at which point it rapidly decreases. The subsequent loss of lift means the aircraft is heavier than the lift that it's producing and falls.

Ideally, the centre of mass would be above the wing, reducing the weight/lift moment couple as much as possible. behind the wing is a very bad idea as both the lift and the mass will combine as a couple and cause the nose to pitch up, and depending on elevator authority, may be uncontrollable. This is why aircraft have published CoG limits.

Hope that kind of makes sense/answers some questions

.mic

What I think is missing is that Cl is not a constant, but a complex curve depending on AoA. When the AoA becomes too large, Cl will reduce quickly and that means lift is lost.

The article already hints at Cl not being a constant, or at least not a constant across the whole possible range of AoAs:

Buy (or borrow):
Theory of Wing Sections
by Abbot and von Doenhoff.

It has CL and CD vs AoA graphs for many aerofoils, including the NACA 2412 used in the C172 (IIRC). You could fit curves to the data.

It also includes some simple corrections, eg:
For the 2D-3D correction, CL_finite_wing = A/(A+2) x CL_wind_tunnel
where A is the aspect ratio.

It also has some "challenging" maths, if you are up to it.:ok:

Happy reading!

So far nobody's mentioned the tailplane. C of G forward of wing C of L, and tailplane producing a downforce.

Good point.

The book I mentioned also has pitch-coefficients vs AoA. The (typically nose down) pitching moment of the wing needs to be balanced by a tail-plane downforce, or some "standard" CoG position, in addition to any movable CoG issues.

But this down-force has to be balanced by extra lift at the wing, at high AoAs this leads to a lot of extra induced drag, and so it all goes on....

An aeroplane is a lot more than a wing.

'Modern' Lift Equation? Has it changed then?

The website does discuss the difference between old and new equations of lift.

Seriously [Pompeypaul], is your software project part of a student project, flight simulator, or project such as a video game involving aircraft where fidelity isn't particularly important?

Lots of clever people have worked very hard on computational fluid dynamics for simulating new aircraft designs, or on flight simulators for fun or serious training. It's a field where it's hard to contribute anything new, and meaning no disrespect it sounds as if you're starting from the beginning.

Look up 'blade element analysis' if you haven't already.

Look up the Navier-Stokes equations...
Navier

not for the faint hearted!

[QUOTE][So far nobody's mentioned the tailplane. C of G forward of wing C of L, and tailplane producing a downforce./QUOTE]

Nor Dihedral, washout. change of aerofoil along length..........