Help with CG on my EXTRA-300

[juvaknin]

i'm trying to Calculate the CG of my EXTRA-300
i know how to Calculate it.. but what is the "MAC%" i should use for start??
if i'll use about 28% of Mac, it's OK?
for "EXTRA-300" wings, 25%-30% it's good Mac to start with??
here is pics of the Airplane... mabe it will help...

[Sparky Paul]

Use this...
http://www.palosrc.com/instructors/cg.htm

[juvaknin]

Use this...
http://www.palosrc.com/instructors/cg.htm

i know how to calculate it..........
as i said in my original post, what "MAC%" Value you think i should use?
25%-30% it's good for Maiden Flight? (TEST Flight)?

[Sparky Paul]

30% is usually a good place for an Extra.
These are active airplanes, you'll probably want to aft of that after the first flights.

[juvaknin]

OK Thanks..
i already put my CG arround the 28%, hope it will be good..
(i belive it will for Test Flight)

[yoyoML]

Don't just put the cg at whatever % MAC (and therefore ignoring any effect of the horizontal stabilizer). It will work but you're not doing it the better way, nor the generally applicable way.

Calculate the neutral point:

np = (A1*x1 + A2*x2 + ....)/(A1 + A2 + ....)

Where A1 is the area of wing 1 (main wing) and A2 is the area of wing 2 (horizontal stab.)... and x1 is the position of wing 1's 25% MAC, measured from a certain reference point (nose will do), and x2 is wing 2's 25% MAC measured from the same reference point.

Then, if you put cg right on np (longitudinally), the plane will have zero pitch stability. Anywhere forward gives positive pitch stability and backward, negative. You'll always always want positive pitch stability (unless the pitch is gyro stabilized). So put the cg before the np, never after it.

This method works even with canards, biplanes or lots of wings strangely placed, provided each wing's CL slope is similar and is not affected by each other too much.

[Sparky Paul]

That's way too much effort to go thru for a plane that has already been designed, with a nominal c.g. at 30%, as most are.
If the computation process shows the optimum c.g. would be at 29.85%, so what?
It merely indicates the designer got it right, as most planes offered for sale seem to be.
With different pilots and flight preferences, the c.g. may be moved anyway.

[yoyoML]

That's way too much effort to go thru for a plane that has already been designed, with a nominal c.g. at 30%, as most are.
If the computation process shows the optimum c.g. would be at 29.85%, so what?
It merely indicates the designer got it right, as most planes offered for sale seem to be.
With different pilots and flight preferences, the c.g. may be moved anyway.

I don't think some multiplying and adding count as "way too much effort". Besides this is "Modelling Science" forum, where my cute little formula rightly sits.

And obviously you didn't read my post very well. My cumputations never give "the optimum cg", only the rearward limit to it. Read more carefully next time.

Where I gave a hard limit to the range the cg may be moved, you didn't. Although much is preference, some facts are hard.

[Sparky Paul]

\...
Where I gave a hard limit to the range the cg may be moved, you didn't. Although much is preference, some facts are hard.
.
Your "hard limit" isn't.
Gifted fliers can take the c.g. past that. And some 3D foamies don't care where it is.
There's absolutely no reason to do any computations for an airplane that has flown. i.e., a kit or plans airplane.
The designer has done the c.g. part.
As for where to place it, almost every airplane of conventional configuration will fly with a c.g. at 30%.
Then the flier can determine where he prefers the c.g.
It's unnecessary to compute the c.g. location for a known flier.

[juvaknin]

Thanks guys,
my CG is between 27%-29% i'm shure it's ok...
i'll fly it this Friday, and i'll post how it was..
Thanks.

[yoyoML]

.
Your "hard limit" isn't.
Gifted fliers can take the c.g. past that. And some 3D foamies don't care where it is.
There's absolutely no reason to do any computations for an airplane that has flown. i.e., a kit or plans airplane.
The designer has done the c.g. part.
As for where to place it, almost every airplane of conventional configuration will fly with a c.g. at 30%.
Then the flier can determine where he prefers the c.g.
It's unnecessary to compute the c.g. location for a known flier.

Yes it is hard. There's no doubt about it. All planes care where the np is. I'm afraid you don't understand too well a plane's longitudinal stability, if you're unaware of the np's importance. Too bad indeed.

[Sparky Paul]

46 years ago, when my Aero Engineering degree was as shiny new as I suspect yours is, I learned about static margins, and neutral points, and pitching moments and area distributions and all those technical things aerodynamicists who design full-scale planes must know.
Fortunately, at the same time, I designed, built and flew toy airplanes with Frank Ehling, who didn't know (or care) about static margins, neutral points, Reynolds number... etc.
He used TLAR,.. "the last airplane I designed that looked like this new one flew fine with these areas, moments and c.g., so this one will fly OK also.
One of his seminal articles in "Air Trails" (circa 1952) was 'It don't mean a thing, if it ain't got a wing".. where he built a wing and tail, and tried it on all types of fuselages... tractor, pusher, canard...
At model sizes, airflow is so erratic that controlling it to the extent possible with full-scales isn't practical.
And worrying about 1% of this or that is a waste of time.
Planes with similar planforms will fly with the c.g. at or near the same point.
That's why a c.g. at 30% is a good starting point for a new airplane, of conventional configuration.
Unless someone is inventing a new version of a plane.. (and after all these years, that's very difficult, most flyable shapes have been invented).. the 30% rule of thumb will work for just about anything.
Models generally are just not precision instruments. It's better to build and fly a workable plane lot than build a precision thing that flies really really nice, but not very often, because too much time is spent wondering about the static margins, neutral points, moments, areas...

[harveyweewax]

My two cents. How accurately can you measure the wing areas and distances? There is going to be some induced error in the calculated position of the C.G due to the accuracy of the measuring device that you are using (for example, if you are using a ruler that is graduated to 1/16 of an inch then your allowable accuracy is only ±1/16 of an inch). On a model with a chord of, say 7 inches, the difference between the calculated C.G and the C.G. at 30% is probably within the induced error.

I agree with Sparky. It's not worth it to do the calculations. Set the C.G. at 30% as a starting point and later adjust for taste.

Regards,

Sven

[yoyoML]

46 years ago, when my Aero Engineering degree was as shiny new as I suspect yours is, I learned about static margins, and neutral points, and pitching moments and area distributions and all those technical things aerodynamicists who design full-scale planes must know.
Fortunately, at the same time, I designed, built and flew toy airplanes with Frank Ehling, who didn't know (or care) about static margins, neutral points, Reynolds number... etc.
He used TLAR,.. "the last airplane I designed that looked like this new one flew fine with these areas, moments and c.g., so this one will fly OK also.
One of his seminal articles in "Air Trails" (circa 1952) was 'It don't mean a thing, if it ain't got a wing".. where he built a wing and tail, and tried it on all types of fuselages... tractor, pusher, canard...
At model sizes, airflow is so erratic that controlling it to the extent possible with full-scales isn't practical.
And worrying about 1% of this or that is a waste of time.
Planes with similar planforms will fly with the c.g. at or near the same point.
That's why a c.g. at 30% is a good starting point for a new airplane, of conventional configuration.
Unless someone is inventing a new version of a plane.. (and after all these years, that's very difficult, most flyable shapes have been invented).. the 30% rule of thumb will work for just about anything.
Models generally are just not precision instruments. It's better to build and fly a workable plane lot than build a precision thing that flies really really nice, but not very often, because too much time is spent wondering about the static margins, neutral points, moments, areas...

Oh, how 46 years turns an aero engineer into a set of conventions. And how it makes calculating a weighted average, that is the neutral point, such an unbearable work. Glad I'm not an aero engineer.

In case you forgot: the 30% rule will not work for
biplanes/triplanes, due to their main wing area being 2x/3x large;
canards, where the cg often lies outside the MAC;
flying wings, where the cg must lie before 25% MAC;
gliders, with very short chord and very long tail, the cg may easily lie outside the MAC.

None of the above is a recent invention. Really the 30% rule only works for your everyday Cessna or other copied trainers, or occassionally a monowing aerobatic. More like "just about nothing."

Inventing a new version of a plane? Tell THAT to Burt Rutan.

[yoyoML]

My two cents. How accurately can you measure the wing areas and distances? There is going to be some induced error in the calculated position of the C.G due to the accuracy of the measuring device that you are using (for example, if you are using a ruler that is graduated to 1/16 of an inch then your allowable accuracy is only ±1/16 of an inch). On a model with a chord of, say 7 inches, the difference between the calculated C.G and the C.G. at 30% is probably within the induced error.

I agree with Sparky. It's not worth it to do the calculations. Set the C.G. at 30% as a starting point and later adjust for taste.

Regards,

Sven

You just made the same mistake Sparky did. You didn't read my post carefully. I never said how to calculate any cg, only np.

But still I say something about the error you mentioned. 1/16 to 7 inchs is 1/112, or less than 1%. juvaknin settled at 27%-29% MAC. 3%-1% is obviously not within the induced error.

In fact 25% would be a much safer starting point than 30%, for all planes that have main wings in front (bi/triplanes, too) and stab. in back. If you'd like to return to my cute little formula there, you'll see that np is limited to after 25%MAC of the first wing. So starting at 25% almost guarantees you positive pitch stability, and that you only move the cg backwards from 25% subsequently. But with 30%, you know neither.

[Sparky Paul]

...
Inventing a new version of a plane? Tell THAT to Burt Rutan.
.
Santos Dumont.

[juvaknin]

Today was the Test-Flight..
i used arround 28% MAC on my CG..
it's flying great.. it's have alittle Front CG, but flying great...
Thanks guys! :)
(now i need to use my Smoke System, and it will be perfect!)