HELP on calculating MAC on a forward swept wing?

[CactusJackSlade]

I have read many posts on calculating the mean aerodynamic chord (MAC) on a (rearward) swept wing and also in reference to a FORWARD swept wing but I am still unclear as to if the calculation is the SAME for a forward or rearward swept wing.

Help please? I found this diagram for a REARWARD swept wing... is it the same for a FORWARD swept wing?

Thanks much!

CJS

[steve lewin]

AFAIK the calculation is identical, just turn your picture upside down ;). You just have to remember that the MAC has position along the wing not just a length. So if the wing has forward sweep the 25-30% MAC for CG will be further forward than that by the time it is projected on to the root.

Steve

[DICKEYBIRD]

Go to http://www.rcuniverse.com/showthread.php?s=&threadid=194547&forumid=19 and download the .xls file to load in Excel. Plug yer numbers in and you should have everyhing you need and then some.

Good luck!:)

[Takao Shimizu]

CJS,
I have same question.
My test flight results of two of forward swept wings, are different from the minus degree swept wing CG calculation.
The flight is quite stable.
I hope that anyone has the forward swept wing CG calculation experience.

Takao

http://www.rcgroups.com/forums/showthread.php?postid=1403635#post1403635

[bartje]

Hey Takao!

Neat machine! The reason why your CG is different has to do with the fact that your design is more or less a flying wing. Keep in mind that you need a serious margin between the CG and the CP (center of pressure: the point where the lift force is attached to your wing). The CP is a nasty one since it tends to move forward when the aoa increases. If your cg gets behind your cp you get an unstable airplane and that's not what you want.

Another issue with flying wings like yours is that they tend to be very problematic with pitch stability. They don't have a stabilizer far behind the wing so either you have to use a fancy airfoil with a positive pitching moment, or you have to put the CG quite far forward. This combined with the SFW (and compared to normal planes) it seems to be very far forward, but it is actually quite normal.

And when it flies ok like your one does, thumbs up for your design and happy flying! :D

bartje

[Takao Shimizu]

bartje,

Thanks for the hints.

My question is "Why do I have to put the CG quite far forward on SFW?"

The answer may be, the airflow of the most of this wing come togather to the center of wing where the wing space area is bigger on this plane design. Then, the CP moves back by SFW.

This plane never stall as

http://www.rcgroups.com/forums/showthread.php?s=&threadid=22109&highlight=alianator

Takao

[bartje]

My question is "Why do I have to put the CG quite far forward on SFW?"

Takao, you almost made me hurt my brain with that question. ;)

The first thing I learned in aerodynamics is that the CP always moves forward when the AOA increases. That counts for every type of wing, SFw, normal, ringwings, you name it.

I've been messing around with SFW's in the past and I think that someone once told me that an SFW stalls from root to tip.
That means that during a stall the MAC (MAC of the part of the wing that's still "flying") moves to the tips.
In an SFW that means that the CP moves even further forward which in turn makes it necessary to have a CG that's far enough forward to avoid the CP getting in front of it.

In reality that leads to CG ending up near the front side of the wing tip of an SFW wing.

I'm not very sure about the stall theory of my story. Any real life aerodynamicists on the forum who can help us out?

bartje

[Ollie]

While I am not a "real life aerodynamisist" I have studdied the subject informally. There is a very old system of describing the forces on a wing and a modern system. The old and new systems produce the same result but by different methods. The old system used a resultant force vector that moved with changes in angle of attack. The point of application was called the center of pressure (CP). As angle of attack decreased the CP of cambered airfoils moved aft producing a nose down pitching moment increase.

The modern system does away with a moving point of vector application. The wing is said to have a fixed aerodynamic center which is defined as the point about which the pitching moment coefficient is constant. The pitching moment equation contains the pitching moment coefficient and accounts for the nose down behavior of cambered airfoils without resorting to a movable point of force application. The lift and drag force vectors act through the fixed aerodynamic center.

It is helpful to consider stability and trim separately. The stability of a trimmed aircraft is the tendency to return to the trimmed flight condition after a disturbance. Trim is defined as constant air speed and attitude of a stable aircraft.
For stability of a wing only aircraft it is necessary for the CG to be forward of the aerodynamic center of the wing. The aerodynamic center of the wing is very near 25% of the mean aerodynamic chord of the wing. For trim it is necessary for the moments about the CG to be zero. This is normally achieved by wing twist in a swept wing, airfoil trailing edge reflex and by elevator trim position.