SunnyFlyer09

Hey there ! What the correct way to find the cg on this bird? I have never had to find cg on a swept wing like this.

Also does anyone recognize what manufacturer the canopy is? Is this a Sig? (its 5" in length)

Thanks!

MJD

My Feedback: (1)

You would plot a top view of the wing, placing the root and tips in their correct position and with correct chord length (skip any tip extremity with tapers, radii, that sort of thing), in other words just plot the major section of the wing with the straight LE and TE and ignore the other 2%.

- At each tip, extend the tip ribs lines forward and rearward by the length of the ROOT chord.
- At the root, extend the root rib line forward and rearward by the length of the TIP chord.
- From the forward extension line of the ROOT rib, with a ruler draw a straight line rearwards to meet the REAR TIP RIB lines. Draw the other liner from the rearward ROOT extension line to the forward tip extensions line. It will make a big X in the middle of the wing somewhere.
- Do this again on the other side.
- Where the "X"'s cross on each wing panel is the MAC - mean aerodynamic chord. Draw a "rib", or chord line, at each location. If you extended this chord outwards to match the wing's span, you now have a rectangular wing.

Easy and normally "fine" method: assuming you have normally sized tail surfaces and moments etc., which you seem to, you will likely be fine if you locate the CG at 25-30% of the MAC along the MAC line (not the root rib!!!!!!) - capiche? IOW, pretend this new Hershey bar is your wing.

Alternately, you can go the next step which is to plot the MAC of the tail surface, work out the neutral points, then work out the overall NP and go from there forward in percentages of the MAC. This is necessary if the tail surfaces are unusally sized, or the tail moments are unusual.

MAC

planebuilder66

My Feedback: (8)

Now try that with a swept wing bipe or canard

MJD

My Feedback: (1)

Oh, now look, someone's thrown a wrench in..

Well, not using the quick and easy method. Method 1 is stricly for "TLAR" conventional layouts.

A canard is easy with method 2, no difference. Swept wing bipe is a bit more figgering but essentially more of the same with the odd bugger factor tossed in. I have a swept wing bipe on the way fairly shortly that will need this exercise. 1/2A to boot..


MJD

planebuilder66

My Feedback: (8)

Ultimate or pitts challenger? I guess you could view the canard as a extremely seperated bipe, the problem is the front wing on my qiuckie is swept and the rear wing is almost not.

SunnyFlyer09

Mjd, that info is worth a million, thank YOU !!!

MJD

My Feedback: (1)

No, a scratchbuilt 1/2A with 22" span, straight lower and swept upper, or so I think at this point having considered other options. It was in line after the last two projects and is now heading for the bench. IOW I've drawn most of the bits and need to get started.

You can do a workup on a canard exactly the same way as a conventional airplane, which is the second method I inferred before. You need to first work out the mean aerodynamic chord (MAC) and aerodynamic center (AC) of each surface - normally you use the 25% MAC point as the AC, unless you have more detailed aero info at hand in which case you wouldn't be asking in the first place I think. Then join the AC's (aerodynamic centers) of the fore and aft surfaces with a longitudinal line. The NP (neutral point) of the aircraft sits at the point where the moments, or "volumes", of the two are equal. This is a point on the line where the amount of line ahead and behind it are in proportion but reverse to the areas of the surfaces (what the..?!). Get what I mean? To picture that, the short line runs from the NP to the larger surface and the longer line runs to the smallersurface.

If you know about moment arms, you'll instantly realize that we are finding the point of equal moments, or surface "volumes".. i.e. the distance from the NP to the AC times the area of each surface are equal. You then want to locate the CG ahead of that point, where your static stability margin represents the percentage of the main wing MAC that you locate the CG ahead of the NP. So if your main wing mean average chord was 8", and you want a 15% static stability margin, then locate the CG 15% of 8", or 1.2" ahead of the NP.

In any of these calculations, remember that none of these numbers are the CG's distance from the wing's real LE - sometimes people get fooled by that since it is a figure we often refer to in instructions or whereever. You can work it out as a reference later. In the earlier example, the starting CG of "25-30% of MAC" refers to finding the point 25-30% of the MAC, from the leading edge of the MAC. This is the same as using the root chord because our drawing efforts above have turned the swept wing into an equivalent hershey bar, therefore the MAC and root chord are one and the same now.

I'll try to find a picture to scan and post it here shortly rather than drawing it again. easier to do than type, honestly, other than measuring and finding paper.

Aha found it - better yet, look here: http://www.geistware.com/rcmodeling/cg_super_calc.htm

MJD