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Old Dec 13, 2012, 07:22 AM
Phil.T-tailer
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Quote:
Originally Posted by Gordysoar View Post
The fact is if you balance at about 45% of root chord with a full flying stab ship, its balanced close enough.
Just how misleading can you get?
CofG as % root chord will depend on the wing planform - a wing with straight LE & tapered TE the CofG is not going to be anything like 45% root chord

Phil.
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Old Dec 13, 2012, 07:46 AM
Red Merle ALES
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If you're only taking the wings MAC and balancing it within a percentage of that you're totally discounting what the tail is doing, i.e. tail moment arm, horizontal tail area etc...
It's worked for years with TD planes doing it this way only because they pretty much have the same planform shapes. If you want accuracy you MUST count the tail, it's there for a reason.

Martin Simons wonderful book called Model Aircraft Aerodynamics has some wonderful formulas on how to find a safe balance location for a first flight then afterwards the pilot can fine tune it in flight. I've taken these complex formulas and have made it very easy to use with an Excel based spreadsheet. I also have an article that I've written for RCSD which explains its use. I use the Bubble Dancer as an example and the results that Dr Drela has vs the spreadsheet you'll see are incredibly close.

I've used this spreadsheet for over 20 years and has a proven track record for me. Thanks Martin and another great friend of mine for their help in putting it together.

You can download it for FREE as always at www.TailwindGliders.com on the "Articles/Files" page.

Curtis
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Old Dec 13, 2012, 07:55 AM
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One thing that is being ignored here is airfoil pitch moment. This has a profound effect on dive test results, which is why the dive test isn't a good indicator of where the CG ought to be. Back in the 1980s, RCM published a cute little RC sailplane. It used an E214 airfoil and the author cautioned that the plane would tuck in a dive. I liked the look of it, but decided to use the then new SD7080 airfoil. That section has about half the camber of the 214, and a much lower Cm. Once trimmed, my ship had the CG way farther back than the plans, and showed no tendancy to tuck in a dive. The bottom line is that if you set up using the dive test, higher camber airfoils (higher Cm) will produce a CG that is further forward than it needs to be, which gives higher stability margins and higher trim drag.

As for where to put the CG, you can't make a blanket statement that airfoil X, or wing Y, wants the CG at a certain position. You also can't say that zero tail lift will be the most efficient trim setting. It depends on overall aircraft geometery. The smaller the TVC (smaller surface, shorter tail boom) the more forward the neutral point, so the more forward the CG, and vise versa. As long as the CG is in front of the neutral point, the airplane will be stable in pitch, no matter where the neutral point actually is.

As a thought experiment, consider a very large tail aircraft. The “stablizer” is nearly the same size as the wing. The neutral point will be way behind the wing (maybe 300% of root chord). With such a big tail there will be no flight condition where the “tail” is producing zero or negative lift. At the other extreme, consider a TVC of 0.25, which is a very small tail. The neutral point will be close to 25% MAC and the CG will have to be ahead of that. Now the tail will produce both negative and positive lift depending on the flight condition and a bunch of other factors. The model may tend to tuck in a dive with almost any “normal” CG position, especially if a highly cambered airfoil is used. If you set the CG using the dive test, you are almost guarinteed to have challenging dynamic pitch instability problems. In either case, without analyzing the overall design, you can't predict how much tail lift will be required to trim the aircraft, or in which direction it will be.

If you really want to find the optimum CG position. Go out on a dead calm morning with a highstart and a stopwatch. Launch your ship in exactly the same way every time, fly the same speed and pattern, and don't use spoilers. Time the flights and adjust the CG. Make small changes! Record your data! One CG position will produce the best flight times. Depending on your ship's layout, it may be flyable in more normal conditions at that CG. More likely, you will need to compromise and move it a little forward for normal flying (unless you fly as much as Gordy). The dive test will NOT produce the optimum CG unless it's by dumb luck.
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Old Dec 13, 2012, 08:21 AM
Pompano Hill Flyers
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Quote:
Originally Posted by Gordysoar View Post
Miami Mike, are you saying that I don't know more than you about rc sailplanes?
The only posts I've made in this thread were about what makes sense to me and what doesn't, and I reversed my position when I read a very logical and convincing point expressed by Tom Kiesling. I never wrote anything here about how much anyone knows about anything.

Quote:
I was there in Calgary when Tom K flew for our country at the F3J Worlds....You?
Speaking of logic, I'm still trying to figure this one out. You seem to be saying that, as a spectator of Tom Kiesling, you know more than he does. If you're trying to argue from authority you're not doing it right.
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Old Dec 13, 2012, 08:43 AM
Detail Freak
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The question is:
When will we see Gordy on an F3X team?
With all that knowledge and experience, he seems way overdue!

R,
Target
Team USA 2008 VR
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Old Dec 13, 2012, 08:43 AM
Detail Freak
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Ooops
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Last edited by target; Dec 13, 2012 at 09:40 AM. Reason: double post, must have been that infernal dive test!
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Old Dec 13, 2012, 08:53 AM
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Quote:
Originally Posted by AMBeck View Post
One thing that is being ignored here is airfoil pitch moment.
It's not ignored in the stability equations. But ... math is off limits in this thread ...

Quote:
This has a profound effect on dive test results, which is why the dive test isn't a good indicator of where the CG ought to be.
Errr...the dive test is an empirical way to determine the NP, because we know from science and maths that as the CG gets closer to the NP the response of the aircraft to a disturbance will act in a certain way.

The aviation community also knows that the fuselage aero center cannot be determined analytically, and that the best way to checkout static and dynamic stability is through flight test.
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Old Dec 13, 2012, 09:13 AM
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Ah yes, but unless your airfoil has a Cm of zero, the neutral point moves with speed changes. In a screaming dive, the NP will be further forward than when you are working a thermal, so the stability margin required to get a dive pullout can be too high for more normal situations. Setting the CG based on where the neutral point is in a flight condition you would never norrmally encounter doesn't mean that it will be right at the flight condition where you normally operate. It's kind of like setting the volume on your car radio when you're running 150 around Road Atlanta, then wondering why it's so loud on the way home.
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Old Dec 13, 2012, 09:16 AM
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I'd say there IS something to be said about those who've flown "some" planes and set them up their own way based on their on knowledge, and those who have flown "many" (from any new plane to different types of planes) and can successfully set up up any of them.

Eventually it all ends up on actual flight experimentation to see what truly works in the end.

This thread has gone the way of religion and politics....you just can't get folks to see your own perspective.
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Old Dec 13, 2012, 09:20 AM
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all those in favor of closing the thread, say "Aye!"
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Old Dec 13, 2012, 09:22 AM
Red Merle ALES
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United States, Mt, Helena
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This is a good read.
http://www.moneysmith.net/Soaring/soaring8.html

The last sentence says:
The inverted flight test is most suitable for semi-symmetrical airfoils such as the RG-15 flown by advanced F3B pilots.

On a side note I have had two nice posts and it's interesting I get no further comments; constructive or destructive perhaps because I haven't bashed anyone.

Happy Holiday's
Curtis

PS I can't wait till the snow is gone and I can get back to soaring!
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Old Dec 13, 2012, 09:33 AM
Phil.T-tailer
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Originally Posted by cityevader View Post
all those in favor of closing the thread, say "Aye!"
"Nay" !

only when everyone agrees that the crucial ingredient in the "dive test" - which is most often forgotten, is:
- when you have changed the CofG after the first (or subsequent) dive tests you MUST now RETRIM the Tx elevator trim so that the plane once again flies hands-off in level flight - before you do another dive test

Phil.
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Old Dec 13, 2012, 09:38 AM
Detail Freak
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Quote:
Originally Posted by Phil.Taylor View Post
"Nay" !

only when everyone agrees that the crucial ingredient in the "dive test" - which is most often forgotten, is:
- when you have changed the CofG after the first (or subsequent) dive tests you MUST now RETRIM the Tx elevator trim so that the plane once again flies hands-off in level flight - before you do another dive test

Phil.
Amen to that.
And add to it:
The further the CG goes back, the less elevator travel you should need.
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Old Dec 13, 2012, 09:44 AM
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One thing I think we are missing here, and I am guilty as the next, is that we are not seeing the forest for the trees. The bottom line is that in most cases the dive test will get you close to the right setup. For the typical flyer, flying the typical sailplane on a typical day, it will be well enough trimmed after the dive test to stay up and handle well. Will it be at it's constest winning optimum? No. Will it provide a satisfying flight for its pilot? Absolutely.

If you are a super competitive type who wants that last 1% of performance, you need to go beyond the dive test. If you want to go flying and have fun with a decent handling plane, do the dive test and call it good.
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Old Dec 13, 2012, 11:30 AM
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Originally Posted by AMBeck View Post
Ah yes, but unless your airfoil has a Cm of zero, the neutral point moves with speed changes.
Except the neutral point doesn't change position with speed ... which is not entirely true, it does change between subsonic and supersonic.

Are you flying supersonic gliders? That would be very cool. (there was a gent who in fact did this (launched from a high altitude balloon), and I wanted to post the URL ... unfortunately the link is dead)
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