BTW guys, what things on our models affect where the CG should be?

Chord, span and tail moment. That's about all we need to measure with a yardstick to get "the formula" to give us a safe, reliable CG location. OK, I oversimplified a bit. We need to find the chord and span of both the wing and the tail. And we need to find both the root chords and the tip chords. OK... still too simple... We need to measure the leading edge sweeps too. 9 measurements, but not really complex ones.

Having root chord, tip chord, span, and LE sweep is all the formula needs to figure out what the wing looks like from above, and where the area is from that view. It can work out aspect ratio for example, if it wants to. It can tell how the sweep affects where the wing area works it's magic.

We also have to get the distance the tail is aft of the wing. Measuring from the LE of the wing to the LE of the tail gives the formula enough information to work out everything else it needs.

We do have to provide those applications another mysterious number however. On the other hand, just give it 5% and tell it to compute. Then give it 15% or 20% and compute and you will have gotte the forward and aft CG locations for a safe, reliable CG range. No effort on our part beyond being careful not to poke holes in our new creation with the yardstick while we take 9 measurements.

Modern technology can be simple. And easy.

I have used (and reused) the calculator at geistware to do the math for me and it´s easy to use.
I did use designFOIL at the very beginning but have misplaced that right now so I don´t have any aerodynamic data to look back at.
So I will start of with the numbers from geistware and take it from there.

My two biggest issues right now is the weather and a tach that wont measure... [] But that has nothing to do with aerodynamics!

You can also use apps like geistware when you're "designing". Like when the span of the tail just doesn't look right and you wonder how increasing it by a couple of inches would work, especially if you'd like to shorten the chord a bit.

Hi "da Rock"
I did actually use the calculator at geistware at the early beginning, that together with (probably) hundreds of pics of different ARF´s to get a "feel" for what´s in use.
The British used the old "if it look right, it will probably fly right" principle and I tried the same (but with a little help from various software/calculators) .

Things did however change during the build, I decided to use ailerons that I had and those added some slight chord.
I also messed up the stabilator slightly (the control surface, not the mechanics) and that added some unwanted weight that forced me to move the wings slightly aft.

I don´t have any hopes for that I have found the final solution for all planes, I just want it to fly right now and then I can use what I've learned for the next plane (what ever that will be).

One thing is for sure, the more I learn the more ideas I want to try. This is a great hobby!

Close enough for a regular airplane. Its a bit more complicated for a T tail when it is out of the propeller wash. In this case reynolds number comes into the picture and CL of said horizontal stabilizer in such configuration. Then one needs weight of model, dynamic safety factor(inertia of model) one wishes to add. Neutral point... yada yada.

For first time modelers, make sure the horizontal stabilizer(elevator) is in the propeller wash for extra low speed performance.

Honest, the results you get from the online apps, T tail or not, are absolutely safe and reliable for a CG range to setup for your first flights on a new airplane. Having spent a couple of years playing with my own design 2m glider that started out with a T tail and wound up with a 'mid tail', and doing the basic formula by hand (the pc's around then were Commodore 64s and such), that basic formula proved to be magic. Mystical, magic, marvelous.... And the reynolds for the tail of 2m is right down there where reynolds gets it's knickers in a twist.

From personal observation, modelers really will be more than adequately served with the answers they get just using chords, spans, sweeps, and tail 'moment', even for T tails. If I remember correctly, adding in any of the fancy stuff merely narrowed the CG range. And any shifts were less significant than the affect on the range width.

Funny thing is how often I've maidened guys airplanes that were out of the range we'd discover after the flying proved they really hadn't balanced the sucker properly.

Hi there again, I thought I should make an update for you.

First flight attempt was about two weeks ago, that didn’t go so well. Hit something with my landing gear on takeoff and the plane turned in to some bushes. That went rather well under the circumstances, only the prop broke and I striped out the screws holding the landing gear.
So, I fixed the landing gear and changed out the prop and waited for some better weather.
This evening I thought I had waited enough, so out I went again.
This time the takeoff went well and I got to enjoy the plane in its natural habitat, but that didn’t last very long… I was flying in a slight angle towards me when I realized that I was going to get the evening sun right in my eyes. So I started to make a left turn banking hard, that’s when I realized I had a barn coming up quickly. So I hit down elevator as the plane was then in a knife edge left turn, which gave me a clear path by the other side of that barn. But then the next obstacle came up so I kept rolling over to get the plane inverted and then I just had to give it some more down elevator to do an inverted climb out. At that point something happened, not sure what, might have been too little throttle. But it drifted out of my sight and in behind some trees. Then I just gave it full up elevator and killed the throttle while I waited for a crunchy sound. [:@]

This time only the landing gear gave in, prop made it and no other scratches or dents on the plane. [X(]
Two crashes for one flight might be a new record!


Here´s a short clip of the actual flight, not the best as I just put my cellphone on top of the roof of my car so the actual takeoff is not seen. https://vimeo.com/43410342

Thank you all again for the time taken to help me out!

Next time I will have a better landing gear and plan my flight a little better...

You will find a good discussion on CG location at www.rcaeronauts.com.

You should know that the CG must be located forward of the Neutral Point. The farther forward of the NP, the more stable the plane, but the less aerobatic. So, you must also find out where the neutral point is, as well as the CG. The information at the rcaeronauts website should give you an idea of what to do.
Good luck.

Ouch..........

So the CG information at rcaeronauts is in the form of a $28.95 application.

That's pretty steep for something that is available for free all over the internet. For example, the application at http://www.geistware.com/rcmodeling/cg_super_calc.htm is free. It figures the NP, which is what every valid CG application would do, while computing all the needed areas and moments for you. All you do is plug in the necessary measurements. The measuring only takes minutes to do.

The calculator you referenced, and others I've seen, do not handle wings with more than one panel. Aside from the Trainers, most of the model aircraft are really 2 or 3 panel airplanes. Treating them as one-panel throws the accuracy of the CG and NP calculations off.

Secondly, the program is extremely user friendly and familiarizes the user with the technology behind the calculations. And, it is fast and accurate if your measurements are taken correctly. When I'm paying $26.00 for a gallon of glow fuel, I consider the CG Optimizer cheap insurance to have the CG and NP located with confidence.<br type="_moz" />

My winter project was an original design 96" span using two wing panels with double taper to wit the the Aircraft Center of Gravity Calculator listed in the sticky on this forum was used and it nailed the CG. I've tested both fore and aft and come back to the recommended point.

Cost was a note or appreciation to those who provided the reference.

There are actually two CG applications listed in the sticky at the top of this forum. They're both essentially the same application, written by the same author. They both consider the features that affect the pitch stability of an AC.