georgiateddy

My Feedback: (11)

Im new to the hobby but have successfully cg'd several planes I built and they fly well. I have a discontinued Waco SRE 60 stagger wing that has already been completed but never flown due to my skills. Last night the final touches were in place and I pulled the manual from Sig Planes to obtain cg criteria.

The cg recommended by Sig is 4 3/8 inch back from top wing face next to fusalage. I tripple checked my points. Finally 2 pounds and one ounce later it leveled. The weights were inside the forward cabin area so moving them to the engine compartment will drop several ounces......but that not much.

Sig's test model Waco SRE utilized the same engine I have mounted which is a Saito 100. Sig's testing and cg conslusion remarks were......... "no additional weight was needed to satisfy CG"

Boy I am in trouble. Sig used 0 ounces and Im at 33 ounces. Maybe Im over worried about the 2 pounds but It seems like a lot of weight to add


HELP !

Teddy

sensei

I am probably the worst person to reply to this question because I am a super weight freak from way back, anyway what is your wing span, total wing area, and all up weight ballast included?

Bob

georgiateddy

My Feedback: (11)

Bob,
69.5 756 sq inches
fly weight prediction from factory is 9.5-10 lbs

Top_Gunn

My Feedback: (6)

It's not as if you have a choice, short of rebuilding it and making the parts in back lighter. (This assumes you have things like the battery pack and servos as far forward as you can reasonably get them). You can't just decide that weight is more important than CG location.

georgiateddy

My Feedback: (11)

Al,

This is so true. The cg location is not negotiable. Im searching for an answer to something I may have overlooked. I will reposition the weight tonight from forward cabin to the front firewall. Hopefully this will drop the 33 ounce +/- to maybe 25. The most weight I added to other planes was approximately 6 ounces. As you said its not as if the cg spot can be moved.

thanks,
Teddy

sensei

Well if it comes in @ 10 lbs. you have a 30 oz. wing loading and a 13.3 cubed loading. Should fly scale if that is what you seek.

Bob

georgiateddy

My Feedback: (11)

Bob,

Wow that is Georgia Tech grade number crunching. Im not to that level with respect to all the variables but it sounds great. Tonight I will tac all the weight forward and see how far it drops.

Gray Beard

My Feedback: (-1)

May be too late now but before I cover a plane or install any of the gear I place the servos, switches battery and all gear on the outside of the plane with just tape and have the plane on the CG machine. This way I can locate the best places to hard mount the gear to get the correct CG. Then I mark those areas and install the mounts, take the plane apart then cover it.
Last summers build the plane was so tail heavy I installed a much bigger engine with the biggest/heaviest spinner I had, managed to get the CG I wanted. Sometimes you just have to do what is needed. I don't watch grams like Bob but I never want to add weight when I can get away without it.

georgiateddy

My Feedback: (11)

Gray beard,

Thats an excellent idea. I do not build kits but pre fab cg would work on arf kits to. Anything pre completion is easier than at the end.

Thanks

Teddy

LesUyeda

This is what I have used in the past on my scratchbuilts.

Les

jester_s1

Don't discount the value of moving that ballast out as far as you can. When I need nose weight, I put it on the engine mount if I can and as far forward as possible there. I did up one a while back where I actually made it part of the cowl ring to get it out that much farther. Since you're so close to the CG anyway when adding nose weight, every millimeter forward you can go makes a difference in how much weight you have to add.

Gray Beard

My Feedback: (-1)

It's just something I started doing years ago. Even on my plans builds sometimes the gear just has to go wherever it is going to fit just like an ARF. Last big Bipe I built I followed the plans to the letter, even got it to come out at the stated weight but I noticed the CG location was marked on the plans in the wrong spot. I knew it was going to be way tail heavy but my advanced instructor said the designer knew what he was doing and just CG it where marked. That went over like a fart in a space suite!! I ended up with three extra pounds in the cowl. The plane flew like a slug it was so over weight. The trick to that is to build the fuse on any plane that used a rotary engine a bit longer then CG it by moving the engine and firewall back and forth then mark and mount it. Then cut off the left over fuse flush with the firewall. Not true scale and not something you can do with an ARF but every plane I have built that used a round engine would come out tail heavy. I sent those plans to a friend and told him about the problem. He built the plane to CG where it belonged and the plane came out right on 12 pounds and flies sweet. Simple little tricks, my slug came in at almost 16 pounds. Move any gear you can forward and do like Jester said. I screw any added lead down onto the motor mount so it doesn't come loose.

sensei

That is very good advice from Gray Beard, and he is right about me, I do count grams, in fact I count them to a fault, Example: There is nothing that I glue in one of my airframes that I have not placed on a digital scale, documented it's weight and then placed on a diet reducing it's weight to at least 35-40% of its original weight and even a little higher percentages on somethings aft of the C/G, but that is just my thing and what I do...

Bob

speedracerntrixie

My Feedback: (29)

I'm betting if the OP posted some pictures of the tail end of the airplane we could come up with some ideas on how to get weight off the tail.

georgiateddy

My Feedback: (11)

The information each of you provided will absolutely help on my next plane. Bobs idea of placing each part on a diet is very smart. Good news from Dublin Georgia.........its hunting season. But even more important is a weight reduction from the aforementioned 33 ounces to 13.5. The weight transition from forward cabin to firewall made a huge contribution. Furthermore, I built the " lil 45 degree angle on the ruler trick " to use as a rule for marking cg. The oval face of the wing leaves ambiguity in the equation with exact placement on the wing to begin your mm count.My original mark was off. Therefore, the new mark coupled with weight forward trimmed 19.5 ounces off gross. Thanks to each of you,Teddy

flycatch

My Feedback: (26)

Purchase a sub "C" battery pack 6V 2000mah and build a ballast box. Attach both to firewall and I believe you can shave off a lot of ballast weight. Works for me on all my ARFs and it will work for you. I did verify your information by going to the Sig manual on this model and they balanced theirs at 29%. I would not use this balance point. Start at 27% for the maiden flight.

OldScaleGuy

My Feedback: (2)

Les, at the bottom of your drawing it notes "stagger in percent of chord". Do you know the significance of that statement? Thanks.

Gray Beard

My Feedback: (-1)

Shawn, I have been known to peel the covering and use a hole saw a time or two. I also have a couple of the 6 volt sub C packs on hand.

LesUyeda

OSG.

I don't know how else I would say it. If Chord is 8, and stagger if 4, that would be 50%

Les

OldScaleGuy

My Feedback: (2)

Les is that to say in a case of a 8" chord and a 4" stagger the gap changes to 50% instead of 57% that is shown on the drawing?

LesUyeda

I'm not sure how to answer this. The lower "gap" arrow is pointed at the wrong location, it should be pointed at the line from the center of the lower wing leading edge.

The "gap" and "stagger" is/are determined by the design of the aircraft, and point AC is calculated by using 57% of the measured "gap"

Les

OldScaleGuy

My Feedback: (2)

Thank you Les. Understand. 57% is a fixed number. The reason i am being so particular, i have a 50% scratch built Smith Miniplane that based on what little flying i have on it is too nose heavy.

LesUyeda

OK. I wouldn't worry about any of the calculations, if I were you. You have it flying, just go ahead and start changing things in small increments, until you get where you want to be.

Les

OldScaleGuy

My Feedback: (2)

Good idea, will use the calculations for reference only. I did use some similar calculations for the initial set up being sure it was on the nose heavy (safe) side.