Finishing my G-Shark with a dle-55. The instructions say 5 1/4" back from the leading edge at the middle of the wing. Does that mean from root to tip on one side of the fuselage go halfway and that is the CG. At that point the wing is only approx 7". Of of this because I didn't finish the 3rd. grade.[&o][]

Generally it means 5 1/4" back from the LE of the wing at the root chord. Or think of it this way: draw a line down the middle of the bottom of the fuselage lenthwise from the motor to the tail, then measure 5 1/4" back from the LE of the wing. That's the starting CG point, and you'll probably end up adjusting it to personal taste 1/4" either way.

Maybe in translation something was lost. Most are done at the fuse, but the last AW plane I built was at the tip. How deep is the wing at the fuse?



david

the tip including the aileron is 11" The root with the aileron is 15 1/2" Length from the fuselage to the tip is 39"

This isn't you, this a bad set of instructions! My guess is that they mean the middle of the WHOLE wing, i.e., at the fuselage. Why didn't they just say so? Beats me. But if you do it that way it puts the CG at about 1/3 of the chord back which is very typical. That will be safe, and then you can fly it and see how it does. Jim

if it were me...at the mid point of the wing id place a straight edge up and down...then Id measure from the straight edge back to the trailing edge of the wing 5 1/4 inches is the balance point

http://www.rcuniverse.com/forum/m_68.../tm.htm#684136

Thanks guys. Why didn't they say measure at the fuselage instead of middle of wing. KISS was not in effect/affect. Don't know which one is right. Remember the 3rd grade education.

don't feel too bad, in my circle a third grade education is equal to a masters degree

Maybe the middle of the wing is closer to the MAC than the fuselage?

Kurt

You may be right Bozarth . I think I will use that as the point like you and jetmech05 said. BTW- here is the graphics that I put on the wing. It is a G-Shark plane so I had to put a shark on it.

I wouldn't risk that. They probably meant "at the fuselage." It may be true that the MAC is nearer to the center of each wing half, but what reason do we have for thinking that the person writing the instructions would use that point as the place for you to measure the CG at? I've never seen a plan that asked you to do it that way. And even if that's wrong, the only risk from starting at the widest point on the wing is that your plane may be pretty sluggish on the first flight. The farther out you go on a tapered wing, the farther aft the CG will be, and if you get it too far back your first flight could be your last. Best to start with whichever choice gives you the most forward CG.

You can double check by looking up an on-line CG calculator. Just plug in some measurements for your plane and it will give you a reasonable starting point. A search on RCU will turn one up.

The MAC is somewhere between the middle of a wing half and the fuselage, but who cares? Did the plans say anything about the MAC?

Using the middle of half the wing will put the CG further back than 1/3. Why would you do that?

Charilie P already gave you the answer. Stick with it. Otherwise you could wind up with an unstable plane.

You guys need to go back and read the OP's first post!

Kurt

?? The wing at the middle of the entire wing (i.e. both halves) is going to be a lot wider than the MAC. The MAC will be at a point farther out than the joint with the fuselage. And, as a couple of us have said, why think the instructions would pick the point where the MAC is to measure the CG. If he does it half way out each wing half, he'll probably get a GG way too far back for safety. Perhaps we have misunderstood your message. If so, clarification can't hurt. Reading his first post again doesn't seem to help.

You are incorrect. The accepted convention is no more than a 1/3 back. 1/3 back of what? Ans: The MAC. You could have a cg more than 1/3 back at the root, but not at the MAC. Beginners can start to understand where some of our "rules of thumb" come from. It is more accurate to measure 1/3 back at the MAC and then relate this location in reference to the root. Obviously a constant chord non swept wing won't have a difference.

Kurt

Top_Gunn,

You are skipping step 1. Calculations used to determine where a safe cg location should be are based on the MAC. This location is then transcribed to the root, for ease of measuring. 33% back at the root tells me nothing unless I know the sweep and taper of the wing. Think about a highly tapered wing with a straight, non-swept LE.

Kurt

Kurt,

I'm not skipping anything. He's not trying to calculate the CG; the designer already did that. He wants to know where on the wing he should go to measure back 5 1/4 inches. The fact that the MAC plays an important role in calculating the CG does not tell us that the person who wrote these garbled instructions was trying to tell him to measure the 5 1/4 inches there. Have you ever seen plans or instructions that told you to do that measurement at the point where the wing width equals the MAC? I haven't.

Al,

Agreed, but understanding the basics of CG location is better than reading a sloppy translation or something printed in error. I have never used an on-line CG calculator, nor have I blindly set my cg based on what was printed in a manual. I use the "no further back than 1/3" rule of thumb at the root for basic unswept, untapered wings, typical on a trainer. But for everything else, I eyeball where the MAC is, mark a spot at 33%, and then see where this lies at the root. I think beginners can understand and do this rather than having to ask someone "where should my cg be?"

Kurt

If it has a wing tube balance it where the tube is. That should be where your measurement is.

If I remember this correctly the g shark has a one peice wing. The middle of the wing would in fact be the root. Being able to calculate out the MAC and set to a safe CG based on those calculations is something all R/C pilots should be able to do. Then again most guys set the CG once and never adjust it to optimum. Poor CG, it's so misunderstood.

Not to hijack this thread but I also have a question about CG.  I had recently finished my Kadet LT40 and it said its idea center of gravity would be at the rear of the main spar.  It gave 3 different locations for CG.  One was more nose heavy, the other more tail heavy.<div>
</div><div>When I did my check for center of gravity i set up some wood on my work bench so that it made almost a saw horse.  the part that the plane sat on was a 1x6 scrap lumber which was basically the width of my finger tip.</div><div>
</div><div>I set the planes fuselage on this and the plane balanced out perfectly where I marked from plan.  It was a bit windy the day I tried to maiden it. I was able to get it off the ground and fly a few circuits.   But this was no easy task. </div><div>
</div><div>I thought it was the wind but after reading some more about CG I'm beginning to believe that my plane was so unstable because it was tail heavy.  As I turned into the wind the tail suddenly would just drop and then I had to go full throttle to get it to climb and then straighten out.  Once that happened I was scared to lose to much airspeed in fear of losing it like that again.</div><div>
</div><div> Long story short i had made a mistake trying to control the plane and it stalled, chopped throttle and it went into the trees.  All in all the damage was minor and I will have it fixed the next day I'm off.</div><div>
</div><div>
</div><div>Question being is after reading I see that they say to point the plane towards you and use your finger tips at the CG point to see how it is.  Is my assumption that using the board on the fuselage instead of my fingers on the main spar the reason for it being off or shouldnt that have mattered?</div><div>
</div><div>Also i have read that being slightly nose heavy is better then being tail heavy.  As in you dont want to be tail heavy at all unless your well advanced and doing like 3d and stuff.  Is it true being nose heavy makes the plane more stable and i would assume desend better although making your landings faster?  How nose heavy is to much nose heavy.</div><div>
</div>

Wow great job guys!

Don't rely on coincidence...

Kurt

Since there seems to be such a variety of interpetions to the middle of the wing why don't you call the manufacturer for clarifacation.
I'm no rocket surgeon myself but if you truly have a 3 rd grade education why not go back to school