mr_matt

My Feedback: (10)

Hi everyone,

I am looking for a MAC calculator that will calculate an arbitrary shape? It is a fairly conventionally shaped planform but it has an unusual to shape and a trailing edge cutout on the root.

Thanks

BMatthews

If the cutoout isn't very large just ignore it. The only real need for the MAC is to find the CG location. And the CG isn't so cast in stone that it's going to make much difference if 1% or so of the wing area is fudged to make life easier.

mr_matt

My Feedback: (10)

This is being used to check the pivot location on a jet stabilator.

An existing stab design that fluttered and took down a $20,000 jet. I am trying to be as precise as I can.

Thanks

BMatthews

Yep, that would definetly be a different case.

I remembered seeing some which handled different panel tapers and sweep angles. A bit of searching and I found one presented in this link that should do what you're looking for.

http://www.rcgroups.com/forums/showthread.php?t=1106300

If you want to confirm the output from that tool there's others. My searching used "multi panel wing mac calculator" in Google. From the links it provides it's typically a few steps to follow the trail to a couple of other options.

mr_matt

My Feedback: (10)

Thanks a million I will try that tool.

alasdair

I take it that you intend to pivot the tail around the Aerodynamic Centre so that there is no pitching moment, no hinge moment? And that is why you want to find the MAC?

Remember that although we generally assume that the AC is at 25% of the MAC, it can vary by 2 or 3 percent depending on all sorts of things.

The safest thing is to pivot the tail just ahead of 25% (say 22%) so that when unpowered (servo passive failure) it will stream with the airflow. That allows the aircraft to be safely controlled by just the tail onthe other side, with luck and adequate stability. It means that the servo is overcoming a hinge moment, but it is still small. It has a high degree of aerodynamic balance.
The advantage is that the tail cannot go aerodynamically unstable, as would happen if the pivot were behind the Aerodynamic Centre.

The location of the pivot is not super-critical, but would err on the slightly-forward side.
What is more important is to mass balance the foreplane, at least partially.
A bit of lead in the furthest forward point of the foreplane, or tailplane, will help prevent flutter. And it is important to have all the control linkages strong and rigidly connected.
See my photo of my Eurosport foreplanes, one with lead in the LE, the other yet to be done.

BMatthews

Yes, a mass balance to position the balance of the all moving stabilator at the hinge line would be a good idea.

In the end though it might still be worth considering a few things that can be avoided or helped. For example if the stabililator is forced to a very high angle of attack so that it stalls will it produce such an effect? Would adding in a viscous damping system aid in damping out flutter resonance?

If the goal it to repair and fly again it could be well worth making a mockup of the tail section and part of the fuselage and mount it on a vehicle for testing. I'd suggest that you start with the original setup and try to get it to flutter in the same way. From there "fix" a few issues and try the new setup to see if the flutter is now avoided. Otherwise you won't know what caused it and if your fixes are doing the job.

dbsonic

My Feedback: (3)

I played around a bit with this for both a SM F-18 and SM F-16 using the CGCalc spreadsheet from the link above and it looks like the elevator rods are very close to, just behind, the AC, aerodynamic center. I am getting centers of 6.88" and 7.25" respectively. The elevator rods are maybe 1/4" behind that at most. So if I can trust I did it right that's looking pretty decent for a pivot position would you guys agree? Certainly these a proven designs that have stood the test of time but it is interesting to look at this in detail.

alasdair

To find an accurate MAC I too would suggest ignoring minor cutouts, in the first estimate.
In addition though, make an accurate representation of your tailplane or foreplane in cardboard.
Once you have your estimated MAC, mark it on the cardboard replica and check that it balances about the midpoint of the MAC, in two directions.

I once did this for a complex double tapered, double swept wing shape and it balanced perfectly about the calculated MAC midpoint in two directions. The calculated MAC is in red.

That gives you the 50% MAC position. Now calculate the 25% MAC position, and I would still put the pivot just ahead of that to avoid problems if the servo arm fails (or the servo itself). An all flying control surface pivoted too far aft will run to extreme travel if it gets disconnected.
Then mass balance on the pivot.

Tall Paul

The cutout works well with those lotsa panel wings.
For the pivot on a all-moving surface, having the pivot and control horn attach ahead of the m.a.c. is a flight safety thing.
It provides a moment arm to control any flutter. With the control horn and pivot on the m.a.c. flutter is very easy to get!

CloudyIFR

Sailplane Calc for MS Excel will do what you're looking for, quite easily too. As well as others that have been mentioned. The slightly forward of the 25% is a good call.
www.TailwindGliders.com see the "files" section.

Curtis

dbsonic

My Feedback: (3)

Thanks. Yeah, the spreadsheets and free tools are only able to roughly approximate the shapes. Since I was interested in getting down to the fine measurements I went with the method from Alasdair, (these stabilators have a fairly unique shape with curved roots and tips so it favors the cardboard planform balancing method) I was able to find that the pivot from the factory is spot on to the 25% MAC in the SM F-18 case. Very comforting to know. I think I will plan on mass balancing the stabs as well.

The other thing I have heard being done is get in the car, have someone drive you down the road and then put the stab out the window and see how it "handles". Kind of like a poor man's wind tunnel... but then you don't want to drop it or let it fly out of your hands!

BMatthews

If you try the car setup you'll want to rig it out on a long stick so it's a good 4 or 6 feet out from the car. Otherwise it's solidly in the massive amount of turbulence you'll have due to the shape of the car and the open window used to stick the test piece outside.

Ray Wasson

Find someone with a cad program. Draw an accurate to scale top view, then ask the program to show the X & Y position for center of gravity. Draw a vertical line at X. Trim vertical line to LE & TE. Draw a horizontal line at Y. This is 50% MAC at the intersection. 1/4 the vertical line would be 25% MAC, This will work on any shape.
Ray

dbsonic

My Feedback: (3)

Yes, that would work well. Some CAD programs these days have some nice functions for finding stuff like this.. even fluid packages for simulating flow.

One thing with these stabs since they have complex shapes and curved root ribs is finding the correct orientation if viewed separately from the fuselage(which provides the alignment reference). I.e what is vertical. Together with the fuselage plan view the CAD approach would help in this regard when drawing the chord properly.