GEO-XL initial Horizontal CG

[Ken Culbertson]

I just discovered an Ah-S*** in my Endgame IV twin.  I originally designed it around the Geo-Bolt wing that I have in Endgame III.  Bob Hunt convinced me, perhaps by accident, that the GEO-XL would be a better choice, so I went with that.  What I forgot to change was the tip design on the "plans" to account for the added wing sweep. Result is that my leadout guides are way too far back.  I make them so that I can adjust the center up to 1/4" ahead of the most forward CG to 1/4" forward of the most rearward CG.  So, likely the inboard tip is going to have surgery.  What I do not know (searching the site, even tried Microsoft AIs and found nothing) is what would be a proper CG range for the GEO-XL vs the GEO-Bolt.  A lot of other factors go into the final CG with electric, twin motors and most likely now a canard. If I know relative to the GEO-Bolt I can do the extra figuring.

I have no intention of trying to get the exact CG.  All I want is a range so that I don't need to move leadouts that are already maxed out.  Initial CG for flying is anywhere that lets the plane land without breaking the landing gear or anything else after a 1 min trim flight 

Ken

PS for reference I usually compute my C/G as a % of mean chord withont flaps and express it as the distance from the wing T/E also without flaps.

[Motorman]

I would think, unless there's something radically different about that wing, that the normal percentages would get you into the adjustment zone. Sorry not much help.

[Ken Culbertson]

I would think, unless there's something radically different about that wing, that the normal percentages would get you into the adjustment zone. Sorry not much help.

I think you are probably right.  If I don't get someone telling me that the leadouts go way back on the XL I am probably going to use 20% as my forward limit.  That is 10% over my norm but I keep forgetting that the Canard is a lifting surface.+ I have been moving the CG forward on III, 1/4z at a time.  I am now at 2oz added and each time it flies better.  At 2 1/4oz I picked up a tip stall on the last corner of the triangles, so I backed off.  I forgot to move the leadouts with the CG change and when I moved them fwd a bit the stall went away.  Pretty sure I was losing too much airspeed.   

Ken

[John Miller]

Ken, a good estimate can be made by doing the following.

Use your wing plan or diagram out your wing accurately. You need to find your M.A.C.  Measure your tip chord including flaps, draw a line that length attached in the plan view, at the wing root. Both sides, top and bottom. Next measure you root chord also including flaps, and in a similar manner, attach a line of that length, top and bottom, at the tip chord.

Draw a line from the top root chord line extension to the bottom tip chord extension.  Now do it again on the other two extension lines.  If done correctly, the lines will cross at some point in the approximate center of the wing panel. That crossing point is locating a spot on the Mean Aerodynamic Chord line.

From that intersection, draw a line fore and aft. That is your M.A.C.

You can locate the desired % for the CG by measuring that M.A.C. line and find the desired % of that line. Draw a line from the established intersection to the root, and to the tip. You know now the position of the proposed CG from root to tip. Locating your lead outs can be done giving you a good position to start.

Ted Fancher has often suggested that the initial center of the lead outs be located a thumbs width aft of the line at the tip. It gives a good safe starting place.

With your Canard upfront 20% might be too far aft, at least as you begin trim flights. If you're not using Canards, Ted also had another "Rule of thumb" thats
always been helpful for me. Use the stab and elevators % of the wing and flaps, to establish your starting cg %. A stab and elevator 24% of the wing area should be safe to start out at 24% of the M.A.C.


Hope this is helpful, John M.

[Ken Culbertson]

John that was a big help.  I was probably not getting the MAC right.  Close but not right.  I think your/Ted's formula helps confirm my thoughts that the fwd range should be around 20% given the canard.  So, using your calculations my current leadout guide is 1 1/2" to far back.  "I hate it when she does that"

Thanks - Ken

[John Miller]

Sorry that it's too far back, but I'm happy it helped.

[Ken Culbertson]

Sorry that it's too far back, but I'm happy it helped.

What would really be suck country would be to finish the plane then find out at the field that your furthest forward setting is 1/2" too far back!  I butchered the wing tip, fixed it and it is all back together just like it never happened, except for the freshly filled scars

Ken

[Page Peterson]

I have a GEO-XL foam wing in a profile with square tips, 12” nose, , 18.5” flap to elevator, 2 deg motor offset, and 28% stab/elevator. At 30.25” from body centerline, the midpoint of my leadouts are 5 5/8 forward of the TE. My CG is now about 18% of MAC. The leadouts are as far back as they can go. I wish they could go back more. When the CG was further back, I could have used more sweep. I will move it back some in the future. As a disclaimer, I am not an expert flier, so my set up could be way off!

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[Ken Culbertson]

I have a GEO-XL foam wing in a profile with square tips, 12” nose, , 18.5” flap to elevator, 2 deg motor offset, and 28% stab/elevator. At 30.25” from body centerline, the midpoint of my leadouts are 5 5/8 forward of the TE. My CG is now about 18% of MAC. The leadouts are as far back as they can go. I wish they could go back more. When the CG was further back, I could have used more sweep. I will move it back some in the future. As a disclaimer, I am not an expert flier, so my set up could be way off!

Page

That is pretty far back.  The GEO-XL center chord w/o flaps is 11" +-.  That would put your CG almost at 50% of center line the chord.  Is yours perhaps a smaller version?  Mine is 60" w/o tips.  68 with.

KEN

[Page Peterson]

Span is 60” with square tips and full span flaps. The flaps are a little wider at the tip - 1.25” to help at 4,800 ft elevation. Again, with my expertise, it could certainly be off.

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