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Jul 18, 2008, 02:00 AM
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GWS E-Starter and the correct Center of Gravity


Mr. Richard Feder, of Fort Lee, New Jersey writes;

Dear NoFlyZone,

My GWS E-Starter flies like crap, even though I diligently set the Center of Gravity to the recommended 2 3/8" from the leading edge, as per the manual. Why, pray tell, is this?


Dear Richard,

After owning three or maybe four GWS E-Starters, it was recently brought to my attention that the recommended Center of Gravity for this plane is way off base. The E-Starter comes with two indentations on the underside of the wing, where according to the manual, the Center of Gravity should be set.

In all honesty, I never really paid any attention to these marks, as I have my own way of setting a plane's center of gravity. But nonetheless, it came as a total surprise when it was recently pointed out to me that those indentation marks are way out of whack!

Click here to continue.

Chuck
Last edited by NoFlyZone; Oct 24, 2008 at 12:56 AM.
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Jul 18, 2008, 01:34 PM
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Since I am so fond of the GWS E-Starter, I thought it would be best to offer up what was pointed out to me, in the hopes that it would save a lot of heartache for newcomers who are building this wonderful plane. The E-Starter is, in my humble opinion, the king of the hill when it comes to first aileron trainers! With that said, however, it should be noted that there is a certain kind of inherent danger involved when one attempts to 'kick the sacred cow' of GWS. I'm sure the following post will invite all kinds of scorn, ridicule, and calls for my immediate tar and feathering. But in the words of some famous guy from some war a long time ago involving boats (or possibly submarines), I say "Damn the torpedoes, full speed ahead; and Remember the Alamo!"

Item #1: This is a cut and paste from my dear friend BruceA (a forum member here on RCGroups)...

"The E-Starter instructions are misleading regarding the balance point. The instructions show the location of the center of gravity ("CG") at 2 3/8" from the leading edge. The mean aerodynamic chord ("MAC") is approximately 7" (actually I measured it at 6.9375", but 7" is close enough). The location of the CG, as per the instructions, works out to be 34% of MAC! The first clue that this CG specification may be incorrect is that the instructions show the CG behind the main wing spar.

The E-Starter appears to have a Selig 6042 airfoil, which would have a center of lift (COL) at about 30% to 35% of MAC. Simply put, the E-Starter instructions show a CG that would be coincident with the COL, which is the neutral balance point and the absolutely most rearward CG that would allow the plane to be marginally controllable. I initially put the CG of my E-Starter at 1.75", which is 25% of MAC. I moved the CG back to 2" for better response in pitch after a few flights. Even at a CG of 2" the E-Starter is docile. An E-Starter with the CG set as per the instructions at 2 3/8" would be a handful."


Item #2: Not knowing what the hell Bruce was talking about with Selig 6042 airfoils and that other mumbo jumbo, I still knew from experience that there was some underlying truths to be discovered in these ravings of a madman; so I decided to actually put this startling bit of information to the test, because I found it hard to fathom that GWS could be so far off in their recommendations. So I wrote back to Bruce with my test of his theory...

"Hi Bruce,

You know, every time I go back to work on my Steven's Aero SQuiRT, I come across yet another of your posts that forces me to stop what I'm doing and check it out for myself.

I always build my planes (if possible) just a shade nose heavy, and then dial them in by adding the smallest of weights at the very end of the fuse tail. The way I balance them is to take them out and observe them through turns, and with a 45 degree glide test. Ideally, in a power off 45 degree dive, they will pull out of the dive on their own (hands off sticks) and assume the very shallowest of glide angles as possible with the elevator trimmed neutral.

This gives me exactly the kind of flight characteristics I personally like. Long flights out of my battery, and the slowest flying speeds possible. I think this is what's known as balancing a plane on it's "sweet spot". I never really measure the distance from the leading edge back to the C/G marks, as I figure as long as it flies the way I like, who cares what the C/G measurement is?

So reading your post about how GWS screwed up the C/G recommendations gave me a perfect opportunity to check out your theory. I went and got my newest E-Starter, which I have balanced perfectly for docile slow flying, and decided to see exactly where it was balanced, and to see how far off your 2" measurement it was."


The following pictures point out that Bruce was dead on the money, and that in fact, the recommended C/G for the GWS E-Starter is just plain wrong!
Last edited by NoFlyZone; Jul 21, 2008 at 02:18 AM.
Aug 15, 2008, 03:42 AM
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flieslikeabeagle's Avatar
My memory has been getting progressively less dependable over the past several years, but for what it's worth, my memory says that back in 2004 I used to fly my E-Starter with the CG where GWS suggested it be located - a little behind the wing spar. My memory also says that the E-Starter was completely docile and well behaved with the CG located there. With the stock brushed motor (EPS350) and a 2S 2000 mAh lipo pack, the E-Starter would also sustain level inverted flight - I learned to fly inverted circuits and horizontal eights of the flying field with that model.

Now, the E-Starter has a rather large horizontal stabilizer (if you look at its area as a fraction of the wing area). It turns out that the neutral point of an aeroplane - model or full size - is not in fact the center of lift of the wing, because the area and location of the horizontal stabilizer also affects it. The bigger the horizontal stab., and the longer the models fuselage (tail moment), the further aft the neutral point shifts. The E-Starter, with its large horizontal stab, is likely quite stable with the CG at 35% of the wing chord. (There is a formula to calculate the neutral point of a model, but it's late, I'm tired, and I don't have an E-Starter handy to measure).

Interestingly enough, models with large horizontal stabilizers also have lots of air damping - wave a paper fan through the air and you'll feel the air resistance on it; this same resistance tends to damp out any oscillations resulting from instability. As a result, models with large empennage area can be controlled and flown by a good pilot even if the model is in fact unstable, because the air damping prevents unstable oscillations from building up quickly, giving the pilot time to damp them out via control inputs. Lots of small 3D models seem to be designed and flown this way.

By the way, some of the classic free-flight models had huge horizontal stabilizers and flew with the CG at 50% or even 60% of the wings MAC. Without a horizontal stabilizer, or with a normal sized one, they would have been wildly unstable. With the oversized horizontal stabilizer, they were extremely stable. They had to be, they were free-flight models that had to fly themselves!

-Flieslikeabeagle

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