I just finished my GSP/CMP Zero, this is the one with the 71 inch wing. I'd like to know from those of you who have flown this model where your CG is set. The manual says 145-152mm from the leading edge of the wing. Mine will balance with a little less than 2oz in the tail at the 145 limit. Your observations would be welcome.

Yes, I have checked other threads, they are all older.

Details - engine is a G26 gasser on electronic ignition. The ignition module is mounted on a shelf above the engine. Flight and ignition batteries are mounted in the recess under the gun housing fwd of the cockpit. Servos are in the stock location. Retracts are modified Century Jet P-51 units with oleo struts. I haven't weighed it yet, but it feels like about 12 lbs or so.

Thanks, I'm looking forward to seeing the replies.

If you dont get a replay check it against 25% of Maximum wing cord where wing meets the fuse

I flew mine at the suggested. I dont think its correct. Cant remember cause I retired that plane about a year ago and will be selling it at a swapmeet this weekend. It flew alright at that cg but seemed a little heavy on the approach during landing. Very touchy and had to be on top of the throttle all the way to wheels down. Not too bad. With a saito 120 I had to build a led box out of 1/8" ply over the engine to get it to balance. Think it was about 2 lbs in led. CMP arfs are notorious for bad cg in the instructions. Not all are like that, but a lot are. I found the spitfire to be on the money, cg wise.
Edwin

145 to 152 mm translates to 5 1/2" to almost 6 inches from the LE NO WAY THAT COULD be right ! no way that model would even fly ......as previously suggested go with 25 % wing cord closest to the fuse , I am sure you will find a great diffrence in the numbers , let me know I just got mine haven't really done much with it other then do an inventory of the parts , kit looks pretty good so far , post some pic of yours I would love to see it completed

I can tell you that someone once gave me the advice of 25% of max cord , it by far is the best advice I ever received on RCU ever
I start all my planes there , if anything is needed its always moveing back slightly for flight preferance but never tail heavy ever !!!!

Hiya Jetmex, how have you been? I've got one of these being "reworked" and look forward to a flight report from you.

Chad Veich

Hey Chad, doing good here! How's things in your neck of the woods? Sent you a PM, drop me a line when you get a few.

I'll keep you posted on the Zero. She balances at 138mm or so from the L/E with no additional weight added. We'll try the first flights there and adjust as needed. I'll post a few pics as soon as I get a chance...

DON'T DO IT ! 138 mm is still WAY OFF , .....look I have spoken to CMP and what they told me is that all of there 50 size Zero plans / manuals are suppose to refrence the larger 71" 120 size Zero wich you now have , and that it lists the CG at 95 to 110 from the LE , I own the 50 size wich I had to balance it at 75 to 85 from the LE , and it flies GREAT ! so you see the instructions you and i have for the 120 size zero is actually meant for the 80" wingspan model , SO the balance point for the 120 size is 95 to 110 mm from the LE , no matter what 138mm is almost 5 1/2" like i said before NO WAY that right ! ....... just my two cents worth

Had to go check the shop. Mine was set at 135mm. Flew fine like that. It could have used some adjustment but did alright at that point. I wouldnt give any info from CMP much credibility. RCU gives real experience.
Edwin

RCU does give real experience and i am very greatful for what i have learned on here , one of wich is 138mm or even 135mm is not 25% the wing cord closest to the fuse , I had one or two people on here tell me that they balanced the 50 size zero at 110 mm with a successful flight , don't know how ? but this also proved to be way way off , so on here or anywere else you have to take all information into account

I did some quick calculating in AutoCAD and 25% of the chord at the fuselage translates to a balance point of about 23% of the total area. That's definitely in the nose heavy range. A balance point of 138mm back from the leading edge at the fuselage results in a balance point of approximately 33% of the total area. That's probably pushing the rear most part of the CG envelope. Somewhere in between, around 27 or 28% of the total area, is probably where I will start with mine. The Zero has quite large tail surfaces and a decent tail moment which usually translates into an airplane that is not overly sensitive to CG in my experience. That's my .02 cents anyway!

Chad Veich

I'm not necessarily disagreeing with you. I set my planes up between 25% to 33% cord. 135mm on this wing is about 32% cord (I measured 16 1/4" this morning at the root) and I believe I calculated your measurement at 25%. This is a reasonable starting range depending on the pilots skill and preferences, 25% to 33%. I flew mine ok at 135mm, it could have been better but I was satisfied with the performance. The only reason I'm selling mine is to finance a meister corsair. I just disagreed with your term of "way off". Just my opinion, not looking for an arguement.
Edwin

Ok mabey I'm wrong as well but ...........25% of 16" = 4"
30% of 16" = 4.8 "

representing 25 % wing cord .... 135 mm translates to 5.3"
30 % wing cord .... 145 mm translates to 5.7" seems to me that at about 1 inch diffrence would justify the statement of WAY OFF , but again I might be wrong

Oh and again, I may be off here but i was taught the only time you use 30% the wing cord is if the wing is swept back let alone 33%

Zero-322, CG is not calculated by the percentage of the root chord but by the percentage of the total wing area. The only time the two are coincidental is on a constant chord wing or one with symmetrical taper. There are lots of airplanes that balance at 33% by the way. In fact 33%, or 1/3, is somewhat of a standard. Warbirds tend to balance further forward than that in my experience but it depends on each flyer's set up and preferences.

Double post.

none the less I belive and I would have to double check .....at 138 as a balance point would fall behind the main spar , not in my opinion a good starting point , I will check when I get back home , so mabey I might be the one that is way off here ,

If it was me, I would do a mean average chord calculation and set it at about 28%. If a Zero balances without extra weight in the nose, its probably tail heavy due to short nose moment. Understanding the fundamentals of how CG is calculated will save you alot of grief in the long run. good luck.

I would agree that's it is not a good starting point for test flights. Much better to start with a forward CG and move it back, if you so desire, once the airplane is trimmed and the control throws are adjusted. This, of course, is just my opinion.

Chad Veich

Hey Jetmex , did you use the gear doors , wheels and wheel wells that came with the kit ? Robart makes a nice set of gear with stand offs for the gear doors I am thinking of using , just wondering what your set up looks like , a pic or two would be nice if ya can post

Zero, I didn't buy retracts specifically for this model. I had a set of CJ P-51 retracts and struts lying around, so I modified them to fit. The kit wheels didn't seem like they would hold the weight with a gasser installed, so they went into the parts box. Since I had to open up the wells a bit to accommodate the new struts and wheels, I didn't use the wheel wells or gear doors either. The doors that came with my model look like they are about two scale feet thick!! Later on, I might box everything in, but I wasn't looking to build a contest airplane; I want to fly this one.

I'll post some pics as soon as I get some.

Does anyone know yet where the CG should be?

Jetmex , were is the main wing spar in corilation to the LE located on your zero ?

Zero, the spar on my model is 130mm from the leading edge measured at the wing root where it joins the fuselage. She'll balance just a bit tail low at 133.

Still working on pics....