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CG location puzzling me...

Old 09-22-2009, 05:34 AM
  #1  
martin_05
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Default CG location puzzling me...

I recently pulled one of my electric gliders out of retirement. It's about 18 years old and other than a busted horizontal stabilizer it survived being stored for that long very well. The model was called "Mini Challenger". It was sold by Astro Flight.

I originally had an Astro 05 and NiCd's in this model. I took all of that out and put in a Mega brushless motor and 3S 2100 LiPo's. I also installed new servos and rebuilt the horizontal stabilizer.

As predicted, in order to meet the CG location per the plans I had to shift everything as far forward as possible because the new stuff is so much lighter than the old '05 and NiCds. It flew the first time out, but it really didn't behave very well at all.

I took it back to the shop and checked incidence with two incidence meters at the same time (wing and h-stabilizer). Spot on. I couldn't have hoped for a better set of measurements.

I then verified that the linkages were solid and that there was no slop on any of the two control surfaces.

I also verified that the elevator center trim position was truly the 0 degree position.

Well, on a dive test the model pulled-up like crazy, indicating a nose-heavy condition. I shifted the battery a back a little and tried again. Still pulled up.

Today I taped a little weight on the fuselage, behind the wing, and started to shift it backwards until I got better dive test results. I didn't quite reach a neutral setup because the day was coming to an end, I really wanted to go fly my heli a couple of times and, more importantly, I really needed to think about where the CG was ending-up and why.

By the time I got done the CG balance point was about one inch behind what the plans are calling for. That is darn-near 50% of wing chord!

I fired-up DesignFOIL and took a look at the Eppler 193 airfoil that the plans call out. Of course, I don't expect the actual airfoil to be anything but an approximation of the real deal. For one thing, the ribs look a little too flat on the bottom and the monokote covering is sure to mess with the actual profile.

I can't say that I saw anything that would lead me to conclude that this CG location is due to the airfoil. I have another plain with an Eppler 387 airfoil and the CG is at about 25% of chord.

I realize that this isn't necessarily the most scientific of analysis. I am going to get my aero books out tonight and see if I can figure this out. Still, I'd appreciate a shove in the right direction if anyone on this list understand the topic and can explain what is happening here.

I attached a photo of the plans for reference.

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Thanks,

-Martin
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Old 09-22-2009, 06:32 AM
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JetPlaneFlyer
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Tail area and tail moment arm (called 'tail volume') needs to be considered when calculating the plane's neutral point in adition to wing geometry. Once you have the neutral point calculated then the CG needs to be placed 5-15% of wing chord ahead of that point.
For a plane with a tail then the neutral point will always be behind the Aerodynamic Centre of the wing.. which is the point you are refering to located at 25% chord (this is not the 'airfoil CG'.. airfoils dont have a 'CG' as such).

There are a few online calculators that help you do this: http://adamone.rchomepage.com/cg_calc.htm

The subject of longitudinal stability is explained in more detail here: http://en.wikipedia.org/wiki/Longitu...atic_stability

Steve

Last edited by JetPlaneFlyer; 09-22-2009 at 08:20 AM.
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Old 09-22-2009, 02:04 PM
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What is the "dive test" for detrmining COG?
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Old 09-22-2009, 03:35 PM
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Originally Posted by cbatters View Post
What is the "dive test" for detrmining COG?
Dive test:
http://www.charlesriverrc.org/articl...GMarkDrela.htm

Steve
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Old 09-22-2009, 04:19 PM
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martin_05
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Originally Posted by JetPlaneFlyer View Post
Tail area and tail moment arm (called 'tail volume') needs to be considered when calculating the plane's neutral point in adition to wing geometry. Once you have the neutral point calculated then the CG needs to be placed 5-15% of wing chord ahead of that point.
For a plane with a tail then the neutral point will always be behind the Aerodynamic Centre of the wing.. which is the point you are refering to located at 25% chord (this is not the 'airfoil CG'.. airfoils dont have a 'CG' as such).
I am starting to think that I need to look at the wing very carefully. As I mentioned, I measured the angles of incidence for both the wing and the h-stab and they are at zero with respect to each other. Being that the h-stab lives in the downwash region and it is a thin symmetrical airfoil it should be producing some downward lift to counteract the mainplane's pitching moment. So far, so good.

The question really is: How is this setup producing so much downward lift at the tail that it wants to balloon up on a dive test with the CG located so far back? Just about the only thing I can point to is the wing and its downwash production. I am now wondering if 18 years in storage has deformed it enough to cause some problems. Visual inspection doesn't reveal anything obvious. I guess I'll have to take some measurements.

This wing, BTW, only has dihedral at the wingtips, the center section is supposed to be perfectly flat.

As a sidenote, I had to enlarge the v-stab. As designed it is just too small and it caused serious dutch roll tendencies. About double the original area in the plans fixed the problem.


-Martin
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Old 09-22-2009, 05:41 PM
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JetPlaneFlyer
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Martin,
The fact that the tail is symmetrical and the wing uses a cambered (flat bottom) airfoil means that there is an aerodynamic angular difference between wing and tail as measured from the zero lift angles of each airfoil.. Even though geometrically thay are mounted at the same angle with respect to their chord lines aerodynamically there will be about 2 degrees of difference due to the different airfoild used.

Anyway none of this really makes mugh difference. The stability of the model is determined by the static margin which in turn is adjusted by moving the CG. Incidence angles make no difference to the static margin.

Providing the model goes ok with the CG set as you have got it then I'd not be concerned, just go fly it
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Old 09-22-2009, 07:20 PM
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martin_05
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Originally Posted by JetPlaneFlyer View Post
Providing the model goes ok with the CG set as you have got it then I'd not be concerned, just go fly it
I wish I were wired that way...I need to know that it is right.

So...I took some time this morning and put the wing on a flat plate for measurement. Sure enough, the LE on the left wing panel was tilted up about 4 to 5mm. The right wing panel was tilted down about 2mm. During my dive tests the plane would pitch up AND bank to the right. That was odd, but I chose to focus on the CG.

So, yes, the left wing was sitting pretty at a higer AOA than the right wing. That' would pretty much explain the behavior during the dive test (much more so than the other convoluted explanation I concocted!).

I reheated the covering and torqued it all back to normal (or a reasonable approximation). I moved the CG forward closer to what the prints stipulate (I still like my CG to be a little back with respect to what most kits recommend).

I flew it this morning and it did great even though we now have some nasty Santa Ana winds developing here in Southern Cal. The dive test produced just a mild pull-up tendency. I'll have to fly it with little or no wind to be able to fine-tune it better. I actually caught a thermal when I saw a group of Crows circling. I joined them and almost lost the plane because the wind was blowing so hard that it pushed me and the Crows pretty far downwind (and upwards) from the field. I turned around and had no problem converting altitude into speed to get me back to the field, but it was hard to tell which way I was going!

I kick myself 'cause I should have known better. The CG being that far back had to mean that something was seriously wrong. I guess I got lazy and just eyeballed the wing and didn't see the twist.

Thanks,

-Martin
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