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Old Apr 15, 2006, 09:59 PM
who needs pistons?
nopistons96's Avatar
mankato,mn
Joined Oct 2004
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F-18 Super C of G?

Hey guys, i'm getting close to getting this bird in the air but before i do, i need some help from you guys. if anyone can help me double check the cg i would greatly appeciate it. i figured it to be where i have the red line in the pict. the wingspan is 24". (i dont know if that is needed). i used some of the free calculators that are online to check it for myself but i would feel better if i had a second opinion. i'm not quite sure how to calculate in the lex's. thanks in advance.
mike

it's a scratch build from depron. minifan. 16/15/2. 3s. auw=26-28oz. (no paint/decals yet)
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Old Apr 16, 2006, 04:35 AM
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birdseed's Avatar
sussex England
Joined Jun 2002
466 Posts
I cant answer, but when someone who knows does, theyll need the root and tip length, and wing diameter.
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Old Apr 16, 2006, 05:14 PM
EDF Jet Jam 2015 , May 28-31
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
6,882 Posts
The model looks good.

I can't do the CG at the moment but it looks too far forward to me.
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Old Apr 16, 2006, 05:47 PM
who needs pistons?
nopistons96's Avatar
mankato,mn
Joined Oct 2004
669 Posts
Hello Kevin.
Thanks. i know it's a pain, but any help would be a great help. i hope to graduate to something like your SH someday. that bird is absolutely gorgeous. it was part of the inspiration for this build. i just have start out a bit smaller.
thanks again guys.
mike
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Old Apr 16, 2006, 10:51 PM
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Lboogie's Avatar
Sebastian Fla
Joined Nov 2005
600 Posts
Give me the WS total, The length and width of the LEX, the cord of the wing at the intersection of the LEX and the wing and the tip. If you could illistrate a photo to show me where you took your measurements. I'll see what I can do.

Leonard
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Old Apr 17, 2006, 07:45 AM
Registered User
Wareham, Dorset, UK
Joined Dec 2003
1,912 Posts
Quote:
Originally Posted by Kevin Cox
The model looks good.I can't do the CG at the moment but it looks too far forward to me.
Hi

agreed. As the picture shows the AC alone already is behind the red line.
The influence of the LERX's is probably less destabilising than the stabilising effect of the elevators, at least at small AoA. I would start with the CG in the position of the AC. What sort of air foil did you use?

Have fun with fans

Klaus
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Old Apr 17, 2006, 09:39 AM
who needs pistons?
nopistons96's Avatar
mankato,mn
Joined Oct 2004
669 Posts
thanks

Hello Klaus,
thank you for your help. your drawing helped me out a bunch. i wasn't sure if i was supposed to include the area of the wing that is also part of the fuse in the calculations. i will start with you suggestion of using the AC as the CG.

sorry for the dum question, but what does AC stand for? just when i guy thinks that he's starting to figure some of this stuff out, he get's served a slice of humble pie.

i included an image of what the airfoil is. the orange lines represent the layered depron and the black is the shape that i sanded it to. thanks again.
mike
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Old Apr 17, 2006, 10:07 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
11,994 Posts
Why not just knock up a simple profile fuselage 'chuck' glider to work out an approx CG?... Not very scientific I know, but quite reliable.
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Old Apr 17, 2006, 12:23 PM
Registered User
Wareham, Dorset, UK
Joined Dec 2003
1,912 Posts
AC stands for Aerodynamic Centre, which for a wing only is usually around 25% back from the leading edge of the mean arodynamic chord. "Usually" because it can be more forward on cambered air foil sections: 22%. The AC of the whole plane is further back from the wing AC on planes with normal elevator arrangement and size. This results in an AC of ~30% back from the leading edge of the MAC. On canards the plane's AC is further forward. There is a lot more to it than one can put into a few sentences; best solution is to read a few books and get yourself "educated".

Have fun with fans

Klaus
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Old Apr 18, 2006, 04:12 AM
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jjantti's Avatar
Finland
Joined Sep 2004
318 Posts
Hi
When I used 'line method' that winmodels suggested to my 1:12 F18C pusher it was about 4cm rear. LEX had huge effect. And E/F's have even bigger LEX.

I've used the same CG in all my Hornet's since I found it. They all are C-versions though. Had no reason to move it.
My balancing point is in a line from hingeline of root of LEF (leading edge flap)to another side. Sorry I can't say it any aesier
Jyri
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Old Apr 18, 2006, 05:57 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,571 Posts
Quote:
Originally Posted by nopistons96
Hey guys, i'm getting close to getting this bird in the air but before i do, i need some help from you guys. if anyone can help me double check the cg i would greatly appeciate it. i figured it to be where i have the red line in the pict. the wingspan is 24". (i dont know if that is needed). i used some of the free calculators that are online to check it for myself but i would feel better if i had a second opinion. i'm not quite sure how to calculate in the lex's. thanks in advance.
mike

it's a scratch build from depron. minifan. 16/15/2. 3s. auw=26-28oz. (no paint/decals yet)
Looks pretty nose heavy to me. Depending on which planform you use, it's either 4.5% (wing + lex) or 0% (wing alone).

I did a quick CG calc and posted the results below. The dark blue is the wing + lex and the light blue is the wing alone. The vertical lines (chordwise) are the mean aerodynamic chord (MAC) and the horizontal lines (spanwise) are the CG balanced at 25% MAC. This should be good enough to get you airborne.

As you can see the difference isn't that much. I would definitely use the dark blue because the lex does make a difference in how the model flys. If you use the wing alone, your balance will be about 3% nose heavy, which is the difference you're seeing between the wing + lex and wing alone.

Dan
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Old Apr 18, 2006, 07:21 AM
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jjantti's Avatar
Finland
Joined Sep 2004
318 Posts
Seems like I had something fishy in my calculations back then.
The test flight I wrote was -98 or something.
I must have had something wrong.

But I do remember result.
Half loop from taxing even with full down elevator. Pulled back throttle when inverted about 30m of height.
Stall and perfectly vertical flat bottom drop to asphalt. Flies like a glove
Moved CG 4cm to next attempt. Tzadaa! it flies!

DanSavage, I just couldn't get it.
How did you took LEX into calculations?
dividing to two separate pieces, two MAC's, weighted mean to combine them?
Might be usefull to know.

Picture says it better what I was trying to say.
My "trusty" seems to be about 15% (wing only).
/Jyri
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Old Apr 18, 2006, 09:14 AM
who needs pistons?
nopistons96's Avatar
mankato,mn
Joined Oct 2004
669 Posts
Hello jjantti, Hello Dan,
Man, i love this place. thanks for helpin a guy out. it's appreciated. Luckily i haven't built the battery tray yet. but it is the next step. You two and Klaus helped out a ton and hopefully the next time you see this bird is in a video eating up the skies. if all goes well that should be this weekend. wish me luck.
thanks again.
mike
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Old Apr 18, 2006, 07:28 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,571 Posts
Quote:
Originally Posted by jjantti
DanSavage, I just couldn't get it.
How did you took LEX into calculations?
dividing to two separate pieces, two MAC's, weighted mean to combine them?
Might be usefull to know.
First, I import the top-view photo into my CAD program and traced the planform. Next, I used the Center of Gravity command in DesignCAD 3D MAX. (seriously)

In other CAD programs, it's called a centroid, or geometric center of mass (GCoM). In the case of a wing planform, the centroid corresponds to the MAC.

So, by drawing a line through the centroid that intersects the leading and trailing edges, we obtain the MAC. From here, it's a simple matter of finding the balance point along that line.

The tricky part is defining what should and shouldn't be included in the MAC. While this isn't perfect, it will usually get you close enough so that you can test-fly the model without crashing and destroying it on the maiden voyage.

The reason why you had to balance your F-18 at 15% MAC is because your calculations didn't take into account the lex. Had you done this, I'd bet you'd find that your CG was really close to 25% MAC.

FWIW, I'll import the drawing you posted and re-calc the CG including the lex so you can see the difference it makes.

Dan
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Old Apr 19, 2006, 05:04 AM
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jjantti's Avatar
Finland
Joined Sep 2004
318 Posts
Thanks Dan.
Now I got it. Keyword was centroid.
Didn't even dream that software mainly for drawing houses could do that...

For others that use autocad:
Draw outlines (wing and lex and stuff)in new layer. Convert them into region.
Calculate massproperties and there you have it: centroid. In cordinates only, but it's easy to draw MAC with them.

BTW, my 'trusty' was about 16,3% MAC including LEX. I'll have to test moving it backwards and see what happens...
/Jyri
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