da Rock

Someone pointed out an online application to me just now. The following page appears to have the popular Center of Gravity application that is all over the internet in different color schemes and formats.

So what does this one do that makes it work looking at? If you look to the lower right on the page, you'll see it is setup to work wing planforms that have a differing taper from root to tip.

http://adamone.rchomepage.com/cg_calc.htm

I think it's worth a look. In fact, it'd be worth a trial run too.

edsplane

I have tried a 4-panel example using the referenced CG calculator and find that the answers it gives, don't agree with my hand calculations. One reason is that the wing tips are ignored. Another is that the tail efficiency doesn't appear in the neutral point calculation and has a strong influence on the results. I get a much more accurate solutions using the techniques at www.rcaeronauts.com , which can accommodate up to 7-panel wings and 4-panel tails.



Edited to add a space after the URL so the link will work.- BMatthews

iron eagel

DA, Thanks for the info looks good!
I have one of the planes that first raised this question (hybrid delta/flying wing) and the figures came out to right about where I have it set at to fly. Granted it only has four flights on it but it flies pretty good, the only issue is the elevator function vs the ailerons when using the elevon setup.

da Rock

Iron,
That's one sharp looking wing. (pun intended) Good looking too.
All my 2m gliders have two panel planforms and since they're easier to handle I did the wings I had for one of them to get a feel for how the application would compare to what I'd established over years and years of flying. It should be mentioned that all you can really pin down is where your CG happens to be and how you feel about the model's performance. No way for anyone to look at how a model flies and see what the margin is. That said....... I've always found that significantly better performance from other people's designs came when I move the CG back from their suggested CG location. So I can suggest that my CG probably should be predicted with an appropriate static margin but what that would actually be as a specific number is subjective, period.

Anyway, after that qualification, I found the application gave CGs that performed like I figure the input SM numbers should give. I've got a couple of different planform wings for one glider and the application spit out SMs that seemed right for how I know that glider flies when the different planforms are strapped on the sucker.

Looks good to me.

iron eagel

Thanks DA!
It's a bit sharper now after a modification after a "hard landing".
I am about 1/8 inch aft of what the program came up with, not bad considering your talking about a root cord of 18 3/4 inches, tip cord of 7 1/4 inches and overall length of 24 3/8 inches.

gerryndennis

Hi Edsplane,

When I change the tail efficiency the neutral point and suggested CG move so I assume it must be in the calculation? I had to push the 'click' button again to get it to recalculate.

Dave H

edsplane

I don't know what you're using for these calculations, but, just because something is in the calculations, doesn't mean that it is correct. I suggest that you get the best possible set of equations and do a hand calculation to see if the results you are getting, are close to correct. If you read through the discussion on determining the neutral point , at www.rcaeronauts.com, you will get an idea of what I mean. Good luck.

gerryndennis

Hi Ed,

I was just pointing out that the subject calculator seems to be allowing for different tail efficiencies for me. You said that it wasn't for you.

I'm pretty happy with the results I've been getting with both the Geistware and the Adamsone calculator so I won't go to the trouble of checking the calculations by hand.

I'm interested though, are you suggesting these two sites are in error? Do you use different equations on your site?

I did spend some time looking at the neutral point discussion on your site and it did raise a few questions for me. You will have to excuse me if I am incorrect about your site I'm working from memory as the site doesn't open for me at the moment.

You seem to describe the process as;

1. Determine the MAC.
2. Set the CG. It doesnt say where though. At a standard position?
3. Determine the neutral point.

I may be misunderstanding your intention but that is the order (from memory) you describe it at rcaeronaughts.

I notice that you have suggested to a couple of other people with trimming questions that they should set the CG to 20 or 25% MAC then use your calculator to determine the neutral point. That seems backwards to me.

It makes more sense to me, to establish the neutral point first then set the CG an appropriate distance (5 to 10%) ahead of that point. This is what the other calculators do (maybe it's what yours does too, your site doesn't say). This will often result in a CG way different than the 'standard' 25% MAC but it will give a 'safe' stable position to start from. Isn't this the whole point of the CG calculators?

I guess you know that vintage models with large tail volumes often fly with very aft CG's (up to 100%) sometimes, and free flight as well. The small flat foam aerobatic guys fly at or behind the neutral point. So recommending a 'standard' 25% doesn't seem right to me.

I would probably be happy to pay you $25 for your calculator if I thought it was better or more accurate than the other two but it seems to me that they work fine. The Adamsone calculator does four panel wings and canard models so would probably meet any requirements that I foresee requiring.

Dave H

edsplane

Hi Gerry, I certainly agree that finding the neutral point first is the way to go and also setting the CG initially at a conservative 20% or so.

rmh

Eyeball it -then fly it -
there is no perfect cg point - it shifts in accord with flight performance desired.

da Rock

CG really isn't a life or death point. It's a range where the airplane is perfectly safe.

There are huge numbers of us who never accurately find exactly where the CG is and fly our models safely. We have, "Two fingers on the spar." and, "25% back, halfway out the wing." And lots of others.

There are huge numbers of us who really don't know if our model is nose heavy, tail heavy, or not. And fly them until the engine wears out.

How is that possible? CG really isn't a life of death point.

However, there are a number of readily available apps that will give us a good idea where the CG should be for those of us who want something more than "two fingers on the spar."

BMatthews

There are no lack of folks around that have used the other FREE online calculators to achieve good flight. The two forums I frequent are full of success stories.

As already mentioned the actual CG location given by ANY online calculator is a starting point. In real life flying there's a fairly wide range for the CG location depending on the skill level and desires of the pilot.

I will also add that the product available at RC Aeronauts is a pay product.

smithcreek

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Apparently that's the new definition of "sharing."