sdg111
Flight lieutenant
Posts: 85
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Post by sdg111 on Sept 16, 2012 15:48:51 GMT 1
How do I check my centre of gravity? When flying my cub for the first time yesterday, on hand launching her she seemed a bit nose heavy, although I've nothing to base this on.
Just wondered the best way to check this, thanks.
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Post by mebillica on Sept 17, 2012 1:41:03 GMT 1
The way I was told, was to hold your cub at the very tips of the wing. The plane should be level. Again, this is what I was told and could be wrong.
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Post by flydiver on Sept 17, 2012 1:48:53 GMT 1
It's interesting what 'gets told', what gets translated, and what gets passed along. Men STILL think NiCads and NiMh have 'memory effect', though that info has been disproved a LONG time ago.
I don't want to discourage people from posting but I would like to encourage some due diligence with information that will make a difference to how your flights, and those you are helping, will go.
If you 'were told' by some other guy I'd warrant that the likelihood of that info being accurate is improbable at a high level.
CG is critical. If you are using a STOCK Cub, with NO modifications.....> don't worry about it. It's spot on and you shouldn't have to do anything. UNTIL > you are ready to mod it, THEN you better know where that spot is, which everyone thinks about later. Whoops!
CG: With ALL stock components installed the C.G. is 2.25" back from leading edge. You can measure with your finger tips, you can measure with a couple pencils in a wooden block, and somewhere around this forum someone bent a coat hanger to do the job.
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sdg111
Flight lieutenant
Posts: 85
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Post by sdg111 on Sept 17, 2012 8:28:16 GMT 1
Thanks gents. I'm totally stock EXCEPT for packing tape here and there and the ice lolly stick on the saddle. I'll check just to be sure. Thanks again.
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Post by renard80 on Sept 17, 2012 12:12:23 GMT 1
With ALL stock components installed the C.G. is 2.25" back from leading edge. Fly, this is something which continues to baffle me. Yes, I have always understood that CG is 2.25 back WITH STOCK COMPONENTS. But does that measurement change after mods (brushless motor etc)? If so, how to ascertain it? I get (and read) conflicting advice about testing CG. Some people say the plane should sit level, others that she should be slightly nose-down. As you say, an awful lot gets 'told'. Sorting the wheat from the chaff can be difficult for we technically-challenged mortals.
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Post by flydiver on Sept 17, 2012 15:46:49 GMT 1
CG on a plane is an important reference point but is not set in 'stone'. For a given plane the manufacturer generally will tell you where that spot is. Like all manufacturer information even THAT is wrong sometimes, but for the most part it's what you have to go on. Horizon, of course, does NOT publish that. We work backwards to find it. Take a stock plane (no mods at all) and find out where it balances......LEVEL. That's CG. CG is a function of a bunch of factors in the design, but primary is the lift, which comes from the wing. Generally that's about 28% of the wing cord. It does not change unless you change the actual shape of the wing. The 'helpful people' telling you to use slightly nose down are erring on the side of caution. Nose heavy planes tend to fly controllably. Tail heavy planes fly with increasing difficulty up to horribly. The Cub has a pretty sloppy and forgiving CG. Some planes do not. The CG point stays the same, even after mods. If it does not balance on the CG after you change things you need to make adjustments until it does. Packing tape IS a mod. Paint can be a BIG mod as it puts a LOT of mass BEHIND the CG. You are putting weight (paint, tape, bamboo, carbon, whatever) in a spot OTHER than the CG which WILL affect the balance. You need to adjust accordingly....always! Once you have the CG in the appropriate place you fly it. After that if you have enough experience you can start to TUNE it. That consists of fine adjustments to the CG and motor incidence (down and right) with appropriate adjustments to elevator and ailerons as necessary. One thing at a time. Make changes slowly and methodically and pay attention. Here's some reading on the subject: Chapter 6: Intermediate Trimming www.moneysmith.net/Soaring/soaring8.htmlBTW-if you screw up the wing 'lolly' mod and change the wing incidence (wing tilt) THAT is a serious mod that will change EVERYTHING. Changing things like wing and tail incidence are more advanced building and tuning techniques beyond the scope of most folks in this forum. OTOH, some kinds of crashes will change them for you (bends, twists, warps) and may account for a badly flying plane that is hard to sort out after you think you have checked all the proper and normal stuff. Play with it. To start make SURE your CG is spot on. Then tape a dime on the fuse right in front of the rudder. Fly again. Pay attention to the change. Now, add another. It won't suddenly go 'bad' on you. It will actually get MORE responsive, then start to fight you as it gets too tail heavy. There will come a point where you simply don't like it. Do the reverse with quarters on the bottom of the cowl. After awhile it'll just get boggy and sluggish. You won't like that either. If you like a 'light/floaty/slow' plane you may like the CG back just a touch. If you like 'em fast and furious then CG forward is more the call most likely. If it's windy then moving the CG ahead a tad (tape that quarter on front) can help settle the plane out a bit. It won't turn it into a windy day plane but it will help. I used to fly around with my Radian (sail plane) with a dime taped on the back when searching for thermals. That's a LONG lever so even a single dime made a difference. The plane was designed to fly slowly (off power) and be very responsive to thermal lift. It was also grossly overpowered so for the folks that thought it was a warm-liner (fast) they would not like that mod at all as it would climb nastily under power to the point of looping. Newbies were often confused by the HUGE difference between flying characteristics with power on and power off. That's one of the reasons I don't think of it as a trainer. So, CG is an essential and critical starting point but it can be played with IF you know what you are doing. Playing with it cautiously can help you learn more about what it does.
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Post by renard80 on Sept 17, 2012 22:32:32 GMT 1
Fly - I am sure many other members will join me in thanking you for taking the trouble to compile such a comprehensive, helpful and easily-understood post.
It's certainly taught me a great deal about the intricacies of CG.
If only our mod, Paul, would allow 'stickies' on here, this thread would surely be a prime candidate.
Thanks again for your invaluable advice, which is greatly appreciated.
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sdg111
Flight lieutenant
Posts: 85
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Post by sdg111 on Sept 18, 2012 14:02:45 GMT 1
Yep, thanks Fly. Great information there, thanks for taking the time.
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Post by flydiver on Sept 18, 2012 16:16:16 GMT 1
If only our mod, Paul, would allow 'stickies' on here, this thread would surely be a prime candidate. Thanks again for your invaluable advice, which is greatly appreciated. You are welcome. Since you can't make 'stickies' make a folder and fill it full of SC bookmarks. That's what I do. As you probably know by now after awhile it's the same questions that keep coming around. What annoys me is it's also the same misinformation that keeps being repeated endlessly.
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Post by SCC on Sept 18, 2012 20:27:52 GMT 1
Fly - I am sure many other members will join me in thanking you for taking the trouble to compile such a comprehensive, helpful and easily-understood post. It's certainly taught me a great deal about the intricacies of CG. If only our mod, Paul, would allow 'stickies' on here, this thread would surely be a prime candidate. Thanks again for your invaluable advice, which is greatly appreciated. Hi Ian. Totally agree with you mate. Big thumbs up to Fly. Thread stickied
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Post by hghost on May 22, 2013 4:26:20 GMT 1
Flydiver great advice and much needed.....I have a CG machine....getting ready to set CG and make sure correct, that last thing to do since some mods........Question , about the "memory " on NiMh and NiCads....I still see that on some sites being told. I saw that also on one of the battery warnings that actually came with a NiMh battery. That it has a memory so making sure battery is ran and charged in a consistant manner. For if you run it to low or charge it it has a memory set that will affect the battery life......Most RC cars in my case, I use the battery has a cut off that does not allow for complete drain on a battery...and I recharge it untill the charger light says it's done. I have actually charged batteries that the light has came on and it only charged a few minutes, had to unplug and replug it back up for it to charge completely.....do not know how the battery memory idea came up, but I have read that on paperwork that came with the battery. I to thought about it many times, But realized it runs till it runs down you charge it until it says it's charged....what memory can it have had to know the difference ? Strange how things get started and how they continue to be told and no fix applied !! Thanks again Fly for your expertise .
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Post by coupe1942 on Sept 14, 2013 0:32:21 GMT 1
This video is one that I found that at least helped me in putting my GreatPlanes CoG Machine together and understand a bit more about how to work it. The instructions to the CoG machine from GreatPlanes are really bad in my opinion, as they show pictures of pictures that have been copied so many times that they are difficult at best to see and understand fully. The fellow in this particular video clip didn't really review all his materials before filming (Step 101 to any good audio visual presentation), so he does not balance the CoG quite successfully by the end of the video, but he does enough for the average Joe to understand. I couldn't locate a better video, so this one at least will get a person started, along with reading all the previous good info in this thread. In reality, I was wondering if I had possibly made a mistake in purchasing one of these CoG machines, as the instruction book was very poor for my old noodle to quite understand fully. After watching this video clip though, it made a whole lot more sense to me and I actually put the CoG machine together without any extra parts being left over and nothing breaking from my oversized, bumbly-fingered hands. Yippy, yippy, yippy! :-) Hope it helps someone. youtube.com/view?v=fFvAgVO6Q6E
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luista
Flying officer
Posts: 1
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Post by luista on Oct 8, 2013 8:14:02 GMT 1
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Post by coupe1942 on Oct 8, 2013 9:09:52 GMT 1
Is this another spam alter ego for Lilly?
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Post by toff on Oct 8, 2013 14:38:56 GMT 1
Grrooaannn....yes. Please, Mr.Chinese fellow....go away!!!
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