I hope someone could help a beginner out a little.
I have been scratch building an airplane (for too long) and it´s close to first flight. But after all the time spent building I´m starting to doubt my self...

The question is; When taking an existing airfoil and "making it" 3D/FunFly by adding ailerons that´s extended/larger in the aft direction of the chord, should the CG still be in the "25%" region?
(I realize there probably are other forces involved, but my vocabulary/knowledge regarding aerodynamics stops me from insulting my self by trying to evolve the question. )

That´s how I have built it, but I´m starting to doubt it all and thinking it might be too tail heavy and that I might be in for a surprise when I take it of the ground for the first time...

Hi Nitrovein
The answer is Yes. However, now the chord becomes larger, because you have to add the chord of the ailerons, to the chord of the wing without ailerons.

Thank you!
I have been measuring "as is", not "as would". So my measurements of 25% is over the total chord including the new/larger ailerons.

That's as good a starting position as any, and you can adjust the CG to suit your flying style as you become more comfortable flying the model. When in doubt always err on being nose heavy rather than being tail heavy.

Karol

Yes, I will start on the safe side and work my way up to the more interesting side of flying when I know how it handles.
Crashing is a part of this hobby, but I rather do it later then sooner.

Hi, I think that the advice you've been given above, is good as far as it goes. You can learn more on my website www.rcaeronauts.com , sinceit goesinto the location of CG in more detail.

As a beginner, you probably fly a high-wing trainer and the usual CG position is at the 25% of MAC(Mean Aerodynamic Chord) position. I would even go down to 22 or 23% of MAC to start out.Just be sure that you measure the 25% correctly. If you have a rectangular wing on the plane, it's just one-quarter of the wing chord, back from the leading edge of the wing. If your wing is not rectangular, refer tomy website for some ideas.

This is a great hobby and the best way to enjoy it is to learn correct techniques from the outset, both on and off the field.
Best of luck and happy landings.

Thank you "edsplane", I will try to make my landings happy!

This is not my first plane, it´s my third plane, just my first own design of an airplane. I have a trainer too, but never flown it yet...
Just started to question my self and if I thought of it in the right way.

Yeah, consider moving CG back...

CG location hasn't been a problem since about the time of the Wright brothers. Honestly.

It's not a hard problem at all for most of our planforms. The aero industry worked out a fairly simple formula (as aerodynamic formulas go) that works great and any idiot can easily use. All you need is a yardstick or the numbers off the model plans.

Measure your root chords, tip chords, spans, LE sweeps, and tail moment. Plug those 9 simple to do measurements into the formula, add the Static Margin and (on the internet) click COMPUTE (or whatever the app button is labeled).

If you run the numbers through an application like you find @ http://www.geistware.com/rcmodeling/cg_super_calc.htm, you'll get a reliably safe CG range if you use something like 5% and 15% Static Margins.

You're actually looking for a safe starting CG, and that's really simple.

Yes "da Rock", it´s so simple that even an idiot like me can use it! But to my defense, I do feel like a Wright brother right now...

I somewhat recalled reading somewhere that you should take aerodynamic forces into account, but don´t remember any dept/truth about that.
But as you and several others has pointed out, easiest way is to start safe and work the rest out by flying.

I don´t doubt all of those that have given us the math to work these things out, I just started to doubt my self!
I thank you all for your help!

Your concern is a natural process of self design and properly so, motivating your due diligence in the effort.

lol... I just re-read my post and I see why you feel the necessity for defense. Very sorry, my post does seem to be all over you. CG is something I'm usually crusading hard about. The majority of modelers really get uptight about it and go too far about it. And I wind up over selling the simplicity of finding a really safe range for it.

It really is something that won't be risky at all with just a couple of things. Plot a CG range with something like geistware, and be aware that's just a starting point.

Wish I could be there to see the maiden.

No worries! I´m not easy to offend and I always try to see what the person is trying to tell me, but I can´t pass up a good joke even if it´s on me.

I have started to weigh it in now and apparently I didn't have much to play with... But I will try to make it have a static margin of 10%, it´s about 5% right now.
The plane is built using a stabilator so it should have all the pitch authority that I could ever need anyway.

Yeah, I wish you could be here too! But I take it slowly and see how she handles.

Thank you for helping me sort out my nerves, they are not as balanced as my plane right now...

Our own designs are often the best building projects we ever have, no matter how they turn out. We learn so much from them, even if they don't fly great.

How about you sharing a picture of your creation with us.

Karol

Just completed and flown my only original design and understand the nerves thing. The emotional involvement was greater than experienced on any other project.

In turn... the pride of the build was greater as was the satisfaction when it flew well.

Good for you Arlyn, as I never had much luck on my first attempt which unfortunately flew like a pig with wings

Karol

Hi Karol... there were concerns about joining the pig squadron.

Thank you all for sharing your experiences!

Here´s my contribution to "The Pig Squardon"! (Slightly poor quality due to mobile phone camera, but I guess it helps in this case... )
[img][/img]

Center of lift is a function of wing shape,the highest/thickest part of the wing is center of lift.To asume that center of lift is a certian %( i.e. 25 30 33) on all wings is dangerous. Find the highest part of the wing which is usually near the main spar on properly designed wings,when the nose falls at that point ,center of gravity is correct.On sweept wings center of lift will be the average of the highest part of the wing at the root and the tip. Center of gravity is always fwd of center of lift that way the acft remains nose heavy.Kiwi4

Thank you "kiwi4", that was probably what I have read somewhere. I will take this in to considerations when setting it up for the first flight.

Thanks for trying to simplify the problem for the OP, but you've got a number of things that really aren't true. The OP is interested in having a CG that doesn't mess up his pitch stability. Pitch stability really is affected by at least 4 design considerations not just center of lift versus CG. Trying to work with just one consideration is dangerous, as you say.

The center of lift is usually found around the thickest part of a wing, but not just the highest part. You're right about shape being a player, but it's the shape of the airfoil that's the real player. The wing chord is another major player, just as major as airfoil. The wing area, wing chord, wing planform, tail moment, horizontal tail area and chord and planform all matter. Where the wing is thickest really isn't as important as all those other details.

You do have advice that works for most models that look like most other models. Especially with symmetrical airfoil models that have average looking tails about average lengths behind the wing. Balance those average looking models on the spar and they usually fly ok the first time. It's good advice when not much is different about the model and the plane looks 'normal'.

On the other hand, it's dead simple to get online apps like geistware to work out absolutely safe and secure CG ranges while taking into consideration the details that affect pitch stability the most.

Nice to read lots of reply since the last time I post here. As per OP, you changed two physical characteristics of the plane. Your concern of being tail heavy is correct for static characteristics depending on the amount of expansion. You will find it easy after you check the weight, existing CG location and the dynamic requirements based on the increase of its chord and control surfaces. Expansion might also affect other loading such as servo, etc. but it might not be a big issue since it’s your own design aircraft... Good luck on your maiden!

da Rock

You have some good points,but any plane will fly even if wing area, wing chord wing planform, tail moment, horizontal tail area and chord and planform are not correct Those are just rule of thumbi ideas and dont have to be followed,there are alot of odd aircraft out there that dont follow standard rules ,and these rules change with the desired function of the acft .it might not fly well but it will fly. The only rule that needs to be followed is the rule of CG and that is a function of center of lift. If im understanding the OP's request its was for finding the CG on a plane he modifiedKiwi4

None of those things are the reason a plane has pitch stability. Most of them combined is the reason. Way back in the past they figured out which were the important details and how they relate to the problem. They worked up a formula that gives excellent CG locations. The formula works for all manner of odd aircraft. Looking at it in it's most detailed version, it looks pretty awesome and scares off a lot of people. But they also discovered long ago a lot of the minute detail really doesn't affect the outcome much, and they boiled down a simple version.

The simple version shows you what the important details really are if you look at it and notice there aren't really many things in it. Good thing about those is they're simple too. You don't have to work out center of lift, or aspect ratios and such. That's really good, because things like the Center of Lift aren't fixed on most airfoils. It moves around in flight. And finding the few important measurements, wherever they are on most planes (even the odd ones), is all you have to do. And it's pretty easy.

If you look at the simplest CG application on the internet you'll notice all you need is wing root chord, wing tip chord, LE sweep, wing span (or half span), the same for the horizontal tail, and the distance from the wing LE to the tail LE. That's just 9 measurements. The wing can be swept forward or back. Same for the tail. The things that matter for pitch stability are few and simple.

I'm sorry but I don't know the "rule of CG as a function of center of lift." I do know that if that's all that mattered, there would be a whole lot more flying wings buzzing about our planet. The size and location of the horizontal tail is really important.

With most models like the OP pictured, and especially with "new designs" that are just altered existing ones, about all the designer needs to do is stay close to the CG that worked before the plane was modified. But if anyone wants to nail down a safe CG range, they really do need to consider the size and shape of the wing along with the size, shape, and location of the tail. And that only takes a yardstick, 10 minutes, and a simple application like are all over the internet.

BTW, the good thing about our models is that even with new designs, they usually aren't "odd". At least they aren't so odd that the simple, old formula doesn't work perfectly. And we've got internet applications that work that formula for us and ask only that we provide 9 simple measurements.