centre of lift for a plane

[bomber]

That's normally a fixed point defined using the property aerop...

It's done this way because 50 something years ago, someone came up with the idea that aplying forces and moments with relationship to this fixed point this would give a rough approximation of what's happening during normal flight.

But what we know in reality is that the centre of lift of a plane is aligned to it's CoG and that it's the movement away from this that makes the plane roll, dive or climb... and must not forget yaw.

So it's always bothered me that with the increased computer power there's not been a move away from the fixed aerop system to a mobile one.

Regards

Simon

[Richard]

My understanding is that the aero reference point is merely a datum point, usually the centre of gravity that was used to calculate the moment (CL,CM,CN) coefficients that are in the tables; and consequently allows JSBSim to be able to calculate the mass moments based on the difference between the current CG and the CG that was used for the aerodata. Each surface is going to have its own centre of pressure, and these can be added together to get an overall centre of pressure using vector maths; but that just seems to complicate the job of the modeller as if anything having a single point makes things easier to understand as different coordinate systems get confusing (for me) very quickly.

Maybe if you could expand on why multiple points would be useful I'd understand better.

[bomber]

Hi Richard, I'll try and explain...

Let's just take the longitudinal stability of the plane as an example.

With the existing method we already work out the wings lift using coefficient tables and that doesn't change, but also lets work out the lift of the h-stab using tables with it's angle of attack being the wings AoA minus the difference in incidence of the h-stab to the wing (ie decalage)...
knowing the planes current CoG position from the properties tree we can calculate the moment arms of both the wing and h-stab, which we can add together to get a total moment.... If we then divide this value by the planes total lift (wing + h-stab) we are left with the planes centre of lift value.

We then simply use this number as the X value of the aerop with the force being the total planes lift (wing + h-stab)

Using this method let's say we want the plane to dead stick glide at 80kts at it's best lift to drag...
Put the plane facing the wind and set the wind at 80kts and adjust the nose wheel height to determine the planes best lift to drag ratio
Then adjust the h-stab decelage until the planes centre of lift value is the same as the CoG for the planes value.

That's the planes Longitudinal stability set up.

[KL-666]

Hi Bomber,

As i am taught, the h-stab has a upside down profile, so it actually produces negative lift. At least in jetliners that should be the case. It does not make any difference for the calculations you propose. Just to be precise.

Kind regards, Vincent

[bomber]

Well you've been taught correctly as some planes h-stabs do have inverted airfoils.. however that doesn't mean that you can ever ignore in the calculation of planes total lift the -ve lift that they generate or their moment effects