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CG Location

Old 11-18-2014, 08:34 AM
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flycatch
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Is it unusual to find that the CG shown or listed on ARF models is incorrect?
Old 11-18-2014, 08:47 AM
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speedracerntrixie
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I have heard of it happening. I always do the calculations myself, test fly and adjust accordingly.
Old 11-18-2014, 10:44 AM
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Originally Posted by flycatch View Post
Is it unusual to find that the CG shown or listed on ARF models is incorrect?
It isn't usual.

Why do you ask?
Old 11-18-2014, 10:58 AM
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Good subject Flycatch and in my opinion I believe it is not all that unusual and I believe it tends more often than not to be the cut rate retailers and not as much as the majors and here is why: First the very largest retailers are able to purchase in large quantities therefore are able to demand exactly what is to be included with the airplane such as hardware and the materials used and even the so called manuals included.

Now there are many examples out there that we all probably have figured out are a wide variety of copies and even likely manufactured in the same factory even on the same line but are not even close in materials or hardware and written material to be included in the box. That written material is arguably the most expensive stuff in that box to develop. I mean ya gotta love the Chinese pictograph system thrown in the cheapest of the copied airplanes. And if one thinks CG information is even close in that situation then he is just fooling himself. I have run manuals that were not even for the same airplane that was in the box with them.

There is another scenario that is well even more heinous and causes even more problems for new folks and that is the situation where they may have bought airplanes that may be over their heads. This goes beyond just the fact that airplane may be a little much but concerns the published CG (and throws for that matter) while appropriate for that kind of airplane will cause problems for the excited new buyer.

Lets suppose this say 3D airplane can be turned into a puppy dog simply by using a more forward CG (and perhaps less throws) and many can do just that But what happens so often is the new pilot will insist on the CG to far aft and there goes any chance for success with that airplane.

I agree with Speedracerntrixie I much prefer to make my own decision decisions concerning CG.

John
Old 11-18-2014, 09:39 PM
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flycatch
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I recently purchased a ARF glider based upon the reviews I had been reading over at RCGroups. I'm not new to kit or ARF airframes and the recommended CG range did not look right. From reading the posts that related to the CG location it appeared that no one was having a problem with this location. I inputted the numbers into an online CG calculator and I was right. The CG shown would create a tail heavy flight condition. I did an online search to see if anyone else came to the same conclusion. A fellow in the UK who reviewed this model and another individual in the USA also found the same thing.
Old 11-19-2014, 06:22 AM
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I can tell you that I run CG on sailplanes much farther aft then on power airplanes. At times I have gone back as far as 40% MAC. The goal was to be able to fly the sailplane fast without having to hold forward stick. I set mine up so that in a dive they would have a slight arc downward. This made the speed task of F3B a bit more consistent. If set up with the same CG ( 25% to 30% ) as a power airplane it will most likely pitch it's nose up with speed.
Old 11-19-2014, 08:57 AM
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flycatch
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Originally Posted by speedracerntrixie View Post
I can tell you that I run CG on sailplanes much farther aft then on power airplanes. At times I have gone back as far as 40% MAC. The goal was to be able to fly the sailplane fast without having to hold forward stick. I set mine up so that in a dive they would have a slight arc downward. This made the speed task of F3B a bit more consistent. If set up with the same CG ( 25% to 30% ) as a power airplane it will most likely pitch it's nose up with speed.
So, what your saying is that they intentionally set the airframe up in a tail heavy condition and assumed the pilot in command would be aware of this. You could achieve the same results using camber mix.
Old 11-19-2014, 09:15 AM
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Not knowing any specifics I have really only gone into general info at this point. It is possible that they have given you a CG that will give best glide ratio. This should not be mistaken as a tail heavy condition. Then again it could very well be a mistake in the manual. My suggestion would be to do the calculation and see what percentage of MAC they are giving you. if it is between 30% and 35% I don't think I would have an issue with it. As far as camber doing the same thing, I'm afraid you have lost me there. I have never used camber to compensate for CG either way. Back in the day when the Eppler series of airfoils were popular it was common to give some negative camber to reduce the lift of the wing to fool it into thinking the wing loading was higher to get from one thermal to another faster. With the current airfoils negative camber is rarely used. Positive camber is used in launch mode or used while circling in a thermal either on a slider, mixed with elevator and/or both. IMO the best thing to do at this point is run the calculations, set the CG to what you deem as safe and then adjust to suit your personal preferences.

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