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Centre of lift and centre of gravity

hairyspin

hairyspin

Administrator
Staff member
Dumb newbie question here - it's my first model. The Hawker Tempest had its maximum wing thickness at about 37% back from the leading edge. Should the centre of lift coincide with this, and then the centre of gravity be near this? I'm building for CFS3 using AirWrench with ACM to set the geometry in aircraft.cfg first.

I'd pinch Jerry Beckwith's FM for the FS9 Tempest, but I think he'd mind.... :whistle:
 
When it comes to FS/CFS, don't get hung up on the wing. The flight dynamics are for the entire airframe, and when all is said and done, it's the tail that determines where the center of lift is. When an aircraft has been trimmed to fly straight and level, the center of lift and the center of gravity are in the same location. The CoG when empty is typically 25% aft of the leading edge of the wing.

I don't recall an FS9 Tempest. There's an FS9 flight model for the David Hanvey/Paul Barry Sea Fury and a CFS3 flight model for the stock Tempest on my web site. If you want to use any of these flight models 'as-is' for a CFS3 upload, just add a note to your readme file to say where they came from. Any one of the flight models on my web site that have been built with the workbook should also work well as a starting point for AirWrench.
 
Thank you Sparks, it's the David Hanvey/Paul Barry Tempest II for FS9 I'm thinking of - a personal favourite. So far with the geometry, AirWrench is reporting the empty CoG at 37% of MAC, so if I've got my inputs correct for aircraft.cfg then I'll keep going.

I'm not saying it's taking as long as learning gMax, but this is not a job for an end-of-a-day head.
 
What 3D model are you working with? Ideally, the CoG and model center should be in the same location. 3D models aren't always built that way, but the ones that are tend to be less trouble.

BTW, the quick and dirty way to set these centers is click the 'Repair Aerodynamic Balance' button on the balance tab.
 
It's my own Tempest II - a new build from a newbie, lol - so I don't have an accurate fix on the original aircraft's CoG.....
 
For a new build in gmax, locating the model 'in center', is the best way to have a good start for the FM.

In gmax, I'd recommend (with the grid turned on), moving the entire model, vertically onto the a/c's thrust line. In other words, in line with the prop (thrust).

The model's master node MUST be at 0,0,0!

Then, longitudely (fore/aft),for the visual model itself, move it until gmax's centerline it through 1/4 MAC on the wing.

Once you get into the FM, it will simply be a balancing act, to set up the COG, and have the a/c fly correctly.
 
Thanks crossram, I've done that with the top node and it was already built with the thrust line on zero. There's just so much to get right and it takes a while to get my head around it, plus I'm still finding out what's important and what can be assumed.

The lack of visual tools once the model is built is a small hindrance; I wouldn't care to say how long it took to get the contact points, static height and pitch right via Notepad. Learned a few things in the process, mind you. And then I discovered ACM - Aircraft Container Manager: it was available some time ago but has been withdrawn and is entirely unsupported. Nevertheless you can still find it if you look hard enough and I found it a revelation for setting the geometry in aircraft.cfg.

I am making progress, just not as fast as I'd like - plus ca change!

BTW Sparks, is there somewhere to set the prop rotation in AirWrench, or do I need to set that myself?
 
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.
 
FS seems to work a bit differently than real planes, but we work with what we have. A real plane with a CG aft of the CL tends to be dynamically unstable in pitch. Yin fights yang to maintain speed stability. In this matter FS can be a bit more forgiving. Certainly minimizing the extra drag of tail downforce with a somewhat aftish (near the CL) optimizes cruise. We utilize this for instance in loading of the 747, however the autopilot is used to provide augmented stability for the preponderance of the flight.

Cheers: T.
 
Hi Fliger747,
For every aircraft, there are established forward and aft CG limits. I am not suggesting loading an aircraft to push the CG past its aft limit, but *AT* the aft limit is where the aircraft will perform best as far as speed, range, etc. with hopefully enough stability to remain controllable (by someone's view).

I believe this aft CG limit is often behind the center of lift but am still looking for a documentable example.

- Ivan.
 
The Forward and aft CG limits are established on a number of factors; The aircraft must be able to perform the flare manuver at the most forward CG and recover from a stall at the most aft CG and have satisfactory positive stability (for civil aircraft) charcteristics.

An aircraft with a CG forward of the CL utilizes a downforce of the tail to counterballance the nose down pich moment. As speed decreases, the downforce on the tail decreades, dropping the nose to increse the speed. Such a configuration is speed stable. A CG aft of the CL a drop in speed decreases the lift on the horiz stab and causes the nose to rise, further decreasing speed. Such a configuration is unstable in speed/pitch.

Cheers: T.
 
P.S:

A book I reccomend as a general primer on aerodynamics is "Aerodynamics for Naval Aviators", available as a reprint at many pilot shops.

T.
 
Hello Fliger747,

Actually the references I am using are mostly Federal Aviation Adminstration docs and NACA docs. The FAA docs agree with your statements that the CG must be kept ahead of CL and stability is reduced as the CG is moved aft. At some point, the aircraft will be neutrally stable and the CG limits should be set so that the aircraft remains positively stable. The point point behind the CL at which the aircraft becomes uncontrollable is much closer to the CL than I had realised.

Thanks.
- Ivan.
 
Yup Tom knows his stuff
he wouldnt take his dog with him if he didnt:gossip:
LOL
H
 
I have no doubt Fliger747 knows his stuff.
:applause:

....But the neutral stability point is most likely a inch or so to several inches behind the center of lift. That was the most surprising thing from this discussion. For no reason in particular, I was thinking it should be more like 3-5 inches behind the CL on a single engine fighter. In looking at various references, there was also a comment that various aircraft have limits that are set to make them easier to fly rather than to optimise flight performance. The Boeing 747 seemed to come up a lot in examples as an aircraft that was flown with the CG fairly far forward!
:wave:

Consider the REALLY general case here: The forward control / lifting surface MUST stall first or the aircraft is not controllable. It makes no difference whether it is a canard, conventional, or something looking like the Langley Aerodrome. Just food for thought.

- Ivan.
 
A possible exception to the above is the use of fly by wire technology in aircraft such as the F-16 in which control stability is artificially generated, but the advantages of manuverability so obtained are retained.

Some other aircraft, such as stealth models, are aerodyanmically unstable due to their configuration and require synthetic flight control to be flyable at all!

Cheers: T.
 
Additional complication results from the relative verticle positions of the CL and CG. Their position with regards to each other can change with attitudinal change. Transonic aircraft will usually have some migration of the CL position at increasing Mach number. In many aircraft this causes a serious nose down moment (Mach Tuck).

Another complication, is the use of high lift devices on both leading and trailing edges of the wing and of course none on the tail.

In the 747-400 a typical CG position for efficent cruise is around 24% MAC. Allowance must always be made for CG shifts as fuel is consumed in flight. This can be a significant factor, especially in swept wing aircraft.

The whole point of CG limits is to retain good controllability junder a wide vareity of conditions. This can include icing, turbulence, incorrect loading etc.

Cheers: T.
 
Regarding shifting CoL depending on use of flaps, slats, etc. What I found interesting in various flight manuals was that the net effect was typically a strong nose-down pitching moment when flaps were deployed. That suggests that although the wing has a higher coefficient of lift, the center of lift has shifted aft.

Another issue would be that the CoL might change depending on the angle of attack and thus change its relative position to the CoG. Hopefully it just shifts aft and increases rather than decreases stability.

I guess in wartime, the "rules" sometimes don't apply. A case in point would be the fuselage fuel tank on a P-51D or late P-51B. Another case in point is the Yakovlev piston-engine fighters carrying an internal bomb load aft of the wing.

I also saw a note in the Spitfire IX / XVI manual to NEVER use the aft fuselage fuel tank on a plane with a bubble canopy. Makes you wonder why the tank was installed in the first place.

With any of these aircraft, it is obvious that using any of these features means the CG must be shifted VERY far aft and make the aircraft dangerous to fly, but I guess that in war, the operational considerations outweigh safety.

- Ivan.
 
....swept wing aircraft. I take it that a lot of the fuel on a 747 is carried in the wings well aft of the CG?

- Ivan.
 
An empty airplane first fueled from the reserve (tip) and outboard main tanks will fall on it's butt... The weight and ballance form we are presented shows the change in CG throught the flight, including numerical enumeration (redundency....) of the tko and lndg CG.

An airplane can be flown with the CG and CL quite close together, even with an aft CG, but it becomes dynamically unstable. Continuous flight control imputs will be necessary, either manual or augmented. It however increases the posibilities for manuvering to some extent. In such a system artificial stick force augmentation (eg. bungees or springs) may be necessary to achieve a suitable stick force per G (for example 10# per G was considered good for a WWII fighter).

Cheers: T.
 
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