Hi, I am currently developing my own flight physics which I have done quite a few times using different methods (and basically because i love programming aerodynamics). However, this comes up a lot and every time I research around to try and find the best answer but its difficult because not all aviation experts are programmers and so either I or they find it difficult to translate.

My question / issue is the following - I currently have a well working flight simulator, but I am controlling the plane (ailerons, elevator ETC) using “deflection” only. So I am using another “airfoil” which pivots and creates enough lift positive or negative to create inertia on the rigid model.

But, in a bid to make it more real, I am sampling and applying lift/drag at many points across the main wings and in real life, the ailerons effect the overall lift on the wings (mainly at the section of wing where the aileron sits). If i apply this theory into my simulator, therefore making it so ailerons change the actual lift on the wing (by multiplying the force or changing the Cl), it works fine whilst I have a positive AOA, but if I say nose dive, the AOA goes negative and therefore the lift force goes negative so the “roll” that was happening suddenly changes direction.

So my question is, in “programming” terms or where there is a coordinate system, does the lift force go negative with negative AOA or does the lift force remain positive but the lift direction go negative?

Discussion around the net seem torn with if there is such thing as “negative” lift. It seems to depend on how people perceive it, as wings in real life don't know if they are inverted or not.

Any help would be appreciated.

Thanks

Oh, an aerobatics aircraft flies perfectly on the roof, with a symmetric profile that generates lift mostly from the aoa (and an overpowered engine). My uneducated guess would be that the transitions between the states should be smooth, abrupt changes that suddenly puts the contraption in different aerodramatic conditions like negative g would put wings (including control surfaces) out of the airflow/in partial stalls, loss of effectivemess, tear things off, structural damage … what i want to say: an aircraft flying abruptly "square" against the flow doesn't happen irl. There must be some sort of transition between the modes, before it gets chaotic and out of control. An aerobatics aircraft with a strong enough engine blows at the tail with the propeller's airstream, thus maintaining some control and pulling itself out while ailerons are uneffective.

A wing being blown at from above would loose its lift anyway. Like in a stall while on the roof. IRL that's a good way to get into the headlines. Posthumous.

my guess is that you yet have no wind representation thus going relative to wind direction doesnt always mean -v.

I think you are still st s point where you calculate Cl from AOA (with using a table) which is what people do but its really bad

imagine you dive at -15 degree pitch wind is at pitch 0 in whatever direction for the sanity aoa is 15 for now, depending on the velocity you either have no lift and form drag made out of the wind is blowing on wings, or you have enugh speed that is pointing (-15 degree pitch (down)) and you are producing lift anyway + the form drag of the wind itself.

but now going deeeper through your problem: seems that you dont reference within aircraft relative frame

what i mean is:

vec3 local_torque = inv_rot * elements_torque; where you aplly that to the

EAngAcc.x = local_torque.x / Ixx;

EAngAcc.y = local_torque.y / Iyy;

EAngAcc.z = local_torque.z / Izz;

EAngVel = EAngAcc*dt;

AngVel = AngVel + EAngVel;

vec3 vLocalAngularMoment = (AngVel*dt)*RAD_TO_DEG;

Dont bother Ixx Iyy Izz they are often calculated for one density shape unless you already wrote a better one that calculates dynamic changes of inertia

However another thing that can cause problems is **************too small/big numbers*********** call it whatever ypu like overfows underflows

you reference some crazy rotation at 0.000583436478324354623 withing timestep 0.00787436473642

with forces acting like 0.000347324623

now you get some stupid small number which should be 0 but either you get NAN or somethig else like INF, -INF, or -0.00005 etc.

So you always check if your calculations didint crack at some point (when applying forces )

damp(&vel, 10.0, 0.007);

damp(&AngVel, 10.0, 0.0005);

Simple answer - in terms of practical implementation, it's simplest to implement if you keep the lift direction the same as you change angle of attack from positive to negative, but your lift force magnitude transitions to being negative.

The reason is that you will have CL/CD vs AoA curves and for non-symmetric aerofoils these will have values for both positive and negative AoA. Indeed there will be non-zero lift for zero AoA.

The general principle is:

1. Construct a coordinate frame based on your wing. and the local airflow - so the local airflow is in the (local) x direction, and y is in the (local) “up” direction (perpendicular to x, and somewhat aligned with the aerofoil's “up” direction)
2. Make sure you calculate the airflow relative to the wing - including the linear AND angular velocity of the plane (e.g. when rolling to the right, the left wing gets an “upwards” velocity, which results in a negative angle of attack on that wing - this is what makes the plane stop rolling as soon as the stick is centred).
3. Calculate the forces relative to this frame
4. Transform the forces back to world space before applying them.

You won't get good behaviour by having flaps implemented as separate aerofoils to the main wing (etc). Instead do two things:

1. Figure out/approximate the change in the CL/CD curves depending on the flap deflection
2. Approximate the change in angle of attack of the wing as a result of flap deflection.

I wrote a pretty realistic simulator for R/C planes (which I fly) so went into this in quite a lot of detail! http://www.rowlhouse.co.uk/PicaSim/​