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Centre of lift and centre of gravity

Ivan said:
With CFS1, you really don't have a choice about fixing the CoG if it is located incorrectly. My preference given a choice is to put the center (0,0,0) of the model at about 1/3 chord of the wing and slightly below the centerline of the engine on a low wing aircraft because the wing structure and landing gear shift the whole CoG down a bit.

Hi Sparks,
I joined late in this thread, but here goes: My understanding was that the Center of Lift really describes the the lift of the wing. In level flight, it would be balanced out by trim effects mostly from the horizontal stabilizer. Ideally, for most efficient flight, the center of lift should be slightly ahead of the CoG so that the h-stabilizer is also providing a slight amount of lift, but still set up in such a manner that the wing stalls before the h-stabilizer. Some aircraft are set up so that the CoG is slightly ahead of the Center of Lift and the tail has to provide downforce but this is less efficient because the wing has to also counter the tail's downforce.

Reasonable?
- Ivan.
Click to expand...

Correct on CFS1, the FM center is the MDL center.

The wing has a center of lift and the airframe has a center of lift, which may or may not be in the same position.

In FS, we only have one CoL, which is the sum of the lift of the wing, horizontal stab and elevator.

The purpose of the elevator is to change the moment coefficient (CM), but changing CM can also be thought of as changing the postion of the CoL.

In order for an aircraft to fly level, the CoL and the CoG have to be in the same location. If they are not, you will have pitch rotation - i.e. the nose will pitch up or down. That's why the CoL and CoG are usually located close together with the plane loaded and fueled, and the elevator and trim neutral.
 
This way of computing the pitching moment of the wing can have some interesting results. An example is a taildragger that sits on the ground at a lower angle than the stall angle. Firstly the tail is reluctant to come up till the wing is about to fly, as concentrating on the moment of the wing tends to undervalue the greater effect on the tail at low speeds, especially with prop blast. Secondly, zipping along in a three point attitude, pulling back on the stick with the tail on the ground will cause the mains to lift off the ground first! In reality with a conventional tail position this cannot happen. It is posssible to fly off in a three point with enough speed, but at a lower speed, the elevator cannot increase the AOA by dropping the tail further (it's already on the ground) we have a departure due to the way FS calculates things.

Jerry's above points are a good illustration of how FS departs from reality in some minor ways, with the stability factors being assigned seperatly. I expect that some of the FS ways of doing things are a result of a legacy of much slower computers in the past. For the designer the general slow evolution is generally beneficial in retaining our understanding of how things work.

Cheers: T.
 
Hello Sparks, Fliger747,

Sparks, From your diagram (using FDE notation):
lt = 1205,8,Horizontal Stabilizer Position
l't minus lt = 1204,22,Main Wing Center of Lift

So unless I am misunderstanding, 1204,22 represents the Center of Lift of the wing alone. Of course there are other contributing factors such as angle of incidence of wing, angle of incidence of hstab, Non-dimensional factors (1101,134,Elevator Trim Center) and perhaps a bunch more less obvious factors which contribute to the CoL of the aircraft as a whole. How much of this interpretation is incorrect?

I hesitated a bit before responding to this trail because I was wondering if anyone else would notice an interesting thing in your diagram: The Center of Gravity of the Aircraft is behind the Center of Lift of the Wing (if I am interpreting the diagram correctly). It seems somewhat odd to illustrate something which is never supposed to happen.

Fliger747, FS/CFS does have its strangeness such as using the elevator to increase AoA with a taildragger while on the ground, but some things like prop pitch effect on the tail surfaces are represented:

1101,154,Pitch Prop Effect on Elevator
1101,212,Yaw Prop Effect on Rudder

I have no idea what the proper values are here, but these variables DO seem to produce the effect that is claimed.

- Ivan.
 
Having flown a lot of real taildraggers (and owning one) I still find the FS effect quite anemic (other than P-factor etc). In fact many FS airplanes will fly quite well at aft CG location because the pitch stability is assigned a value rather than a deriveed one. Generally however we are able to do a reaonable job of modeling most of the virtues and vices of a typical aircraft.

One of the real issues (no pun intended) with realism in FS is trying to replicate the visual and dynamic realm of flight in a system that lacks many of the peripheral visual, aural and tactile clues that make flying IRL often easier than the same in FS. We all do the best we can....

Cheers: Tom
 
Hi Fliger747,
Your hands-on experience is what I am missing when tweaking FS/CFS aircraft. Questions I can't answer are the ones like: "How fast does the tail come around under full rudder deflection in the propeller slipstream?" "At what airspeed does the propeller effect on the tail surfaces become unnoticeable?" "What is the minimum speed for directional control with the rudder under full power conditions?"

No argument regarding differences between real flying and the 1G Flying. Lack of "outside" view is a pain and joysticks with a lot of buttons don't even come close to addressing the problem. I figure that with the creativity of folks like Sparks, and others from back in the days of the 714th, we can try to get a little closer.

- Ivan.
 
Progress is made by community interest and contribution! Many real advancements have been made by the gauge wizzards which can work around some of the "flat spots" in a sim engine, which as good as it is, does not fully apply directly to some situations. FSX accel has gotten a little closer.

HS: I have a track IR, but being of the age of needing progressive lenses which require much head turning for distance adjustment, it makes Track IR a bit jumpy. But it is a cool effect, making the cockpits seem almost 3D.
 
HS:

I went to contacts and readers, I might try the Track IR again. Some brave and dedicated souls have made multiple monitor setups, though I think a really big HDR TV that you can connect to your computer might be a decent solution.

Cheers: T.
 
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