Mods-R-Me

My Feedback: (1)

All,

I want calculate the MAC for the stab of my SU-27. What Equation do I use? This is a picture of what I had in mind. The picture has the control horn quite a bit forward of where I though it would be. How do I compute the actual MAC to double check?

Thanks

Mods

da Rock

Since it's straight taper it is fairly easy to do. The only problem detail in the planform you show is the tip, and tips are usually ignored in area calculations and often in MAC calculations. So ignore the tip.

And since it's a straight taper, then the MAC is the chord that's halfway from the root to the tip.

All you've got to do is decide on where you figure the root chord will be and where you figure the tip chord to be drawn to fairly represent what the tip of the wing is going to do in the air.

The normal step for ignoring the tip is to draw a line on the planform view where you're going to assume the tip chord to be.
Most people simply go out the LE along it's straight line until they reach the point where it starts to turn toward the TE, or where it starts to define the rounded or oddly shaped tip. They use that point and draw a line through it that's parallel to the root chord. If that new line runs back through the straight line of the TE, good deal. If not, then they fudge their new tip chord line out or back.

In your case, you could project the TE line further out. Since your tip shape is basically symmetrical, you could assume that the tip chord is halfway out the area defined by that "tip". You could also draw a line for a virtual tip chord about halfway out that "tip" if you wish. That way, your RC, TC, LE, and TE lines would surround what would actually be your correct wing area. Would it matter that you actually were using those more "exact" measurements? No, not since the tip isn't going to act like it was square to the root. So fudge in whatever makes you feel right. Personally, I'd have my virtual tip chord line about 1/3 the way out the tip.

da Rock

To put that into a picture.............

I've ignored the tip. The two solid green (and an ugly green at that) lines are the assumed RC (root chord) and TC (tip chord). The dotted line is about where the MAC would be if those RC and TC are used.

I also drew that red line to show where I'd assume the TC to be after making a WAG of where the TC could be to better describe how the wing is probably going to operate, and the red dotted line to show the MAC for that.

I've ignored the area of the wing that's going to be somewhat hidden by the fuselage, that's actually inboard of the RC.

da Rock

BTW, a purist could/would work out the MAC of the tip first. Complex wing planforms are usually worked out "in pieces", each piece being the next oddly shaped section as you follow the plan from the root out to the tip. And then each piece's MAC is used. With as simple and straighforward a planform as you show, that'd be a waste of time. If you look at the difference in fore/aft displacement of my two MAC lines (the red dotted one and the green dotted one) you'll note that they are only slightly displaced fore/aft from each other.

The difference in the two displacements won't have any real effect on your tail volume calculation (which is about the only use of the horizontal tail MAC anyway) and to further refine the MAC won't move the result appreciably either.

DanSavage

My Feedback: (5)

As I remember, the pivot point/control horn for this model (scale location) is located at 25% MAC.

I'll post a top-view CG calc so you can see exactly where the MAC is on this stab.

Dan

DanSavage

My Feedback: (5)

Hi Mods,

Nope, I was wrong. The pivot point is located at 21% MAC. A flying stab like what's used on the Su-27 or any modern fighter doesn't really need to be balanced as long as the pivot point is located 20-25% of the MAC. You can balance it if you like, but it doesn't really gain you anything.

I've attached a top-view of the stab with the MAC located as well as the 25% MAC location.

Dan

Mods-R-Me

My Feedback: (1)

Hey, thanks you guys. Dan, I'm still pondering if I should add the L.E. flaps for my bird just for the cool factor. What type of control horn did use for the L.E. flaps?

Mods

DanSavage

My Feedback: (5)

You're welcome.

I used plywood horns which are recessed below the surface. If you go to the latest Su-27 thread on Ezone, you can see what they look like.

Dan