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Old Nov 29, 2013, 01:16 PM
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United States, NY, Kingston
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Question
E-Gentle Lady

So I converted my GL and did all the CG balance both fore aft and side to side. She balanced per the instructions and my son and I took her out for a maiden today. Got her up and flying but she was porpoising a fair amount. On landing I crunched the firewall a bit and need to strengthen her up. But I suspected she was a bit tail heavy as the cause of the porpoising. Any other, more accurate, suppositions? Thanks. Scott
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Old Nov 29, 2013, 01:56 PM
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I'd actually be more inclined to suspect nose heavy. Most model sailplanes fly better with the CG a little bit aft of the point called for in the the factory instructions. Try taping a US Quarter to the top of the stabilizer and see if the GL handles better that way.
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Old Nov 29, 2013, 04:25 PM
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boy does that feel counter intuitive. I need to find a set of plans so I can see exactly where the wing sits as well. It is rubber banded on and I want to be sure i have it set in the exact spot. I think it is, but want to be sure. Any body have a top view of the plans around? SCott
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Old Nov 29, 2013, 04:48 PM
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Main spar center to tail/rudder hinge at fuse/bottom is 25.0 inchs per plans.
Shows CG at 1.0inch back from center spar, with a range of ~1/4inch.
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Old Nov 29, 2013, 06:48 PM
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great! thanks
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Old Nov 29, 2013, 07:19 PM
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You can download a PDF of the plans here.
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Old Nov 29, 2013, 08:11 PM
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Got it, thanks!
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Old Nov 29, 2013, 08:35 PM
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Joined Sep 2007
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Quote:
Originally Posted by Bariboy View Post
So I converted my GL and did all the CG balance both fore aft and side to side. She balanced per the instructions and my son and I took her out for a maiden today. Got her up and flying but she was porpoising a fair amount. On landing I crunched the firewall a bit and need to strengthen her up. But I suspected she was a bit tail heavy as the cause of the porpoising. Any other, more accurate, suppositions? Thanks. Scott
Quote:
Originally Posted by azdiane View Post
I'd actually be more inclined to suspect nose heavy. Most model sailplanes fly better with the CG a little bit aft of the point called for in the the factory instructions. Try taping a US Quarter to the top of the stabilizer and see if the GL handles better that way.
Quote:
Originally Posted by Bariboy View Post
boy does that feel counter intuitive. I need to find a set of plans so I can see exactly where the wing sits as well. It is rubber banded on and I want to be sure i have it set in the exact spot. I think it is, but want to be sure. Any body have a top view of the plans around? SCott
Don't feel bad, It does sound counter intuitive, until you really sit down and think about it. If the plane is nose heavy you will need to trim the elevator up (or add more decalage) in order to counter it in order to achieve level flight. This works fine until you try and fly the plane faster; as the air flow increases the elevator pushes down harder and the nose goes up.

Someone either here on RCGroups or WattFlyer posted a paper (in .pdf) on this subject and I wish I could remember who wrote it, posted it and where I could find it. I might have it downloaded on my other PC.
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Last edited by Fly Wheel; Nov 29, 2013 at 08:43 PM.
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Old Nov 29, 2013, 08:38 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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Bariboy, I suspect that part of your mistake is that you are assuming that the elevator MUST remain in line with the stabilizer. But such is not the case. If you have a model which flies in a constant series of slight stalls the cure is simply to put in one or two clicks of down trim until it stops this nonsense.

Also as you move the CG back and forth within the range suggested of roughly over 1/2 inch from front to rear you will need to alter the elevator trim for each new position of the CG. This is all perfectly normal and is nothing at all to worry about once it has been set to fly well.
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Old Nov 29, 2013, 10:17 PM
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Firewall is repaired and set, hopefully will get out tomorrow, a high of 28 planned but sunny and no wind, to try her out again. Going prepared to deal with heavy nose and see how we do. Thanks for all the help guys. Will see how it goes and hopefully report back. Scott
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Old Nov 30, 2013, 09:29 AM
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So I did some more accurate setting up and find that she balances perfectly level at the forward mark of the CG. I mean perfectly balanced. Can I go with that and take a quarter with me to add to the tail if she still porpoises? That seem to be an adequate plan? Hoping to get her once more today. SCott
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Old Nov 30, 2013, 12:28 PM
B for Bruce
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The 'Wack, BC, Canada
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Quote:
Originally Posted by Bariboy View Post
So I did some more accurate setting up and find that she balances perfectly level at the forward mark of the CG. I mean perfectly balanced. Can I go with that and take a quarter with me to add to the tail if she still porpoises? That seem to be an adequate plan? Hoping to get her once more today. SCott
You are still not understanding the trimming issues of how the CG and elevator trim relate to each other.

The gentle stalling cycle is not due to it being nose heavy. And if you have some porpoising now adding tail weight to shift the CG back a little will make it worse. Instead if it happens you need to add a click or two of down trim until it stops.

The problem is that the trim angle between the wing and tail is never perfect for the whole range of CG location. It's always a best guess sort of thing. So we need to adjust the elevator or do something like shimming the wing to adjust the angle. But adding a couple of clicks of trim is easier.
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Old Nov 30, 2013, 02:04 PM
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Funny you mention that. I started out with the above exactly and had a very good flight. But the wind really started to hamper the flights. Got two good flights in and then a nose in from a few feet and another firewall bites the dust. On the shelf for the winter I will return to her in the spring and have another go at it. Was freaking cold out and hands were just freezing. The wind just made it worse. So the plan is to add some down elevator as she was trimmed today and go for it in the spring. Thanks for all the help guys. SCott
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Old Nov 30, 2013, 03:32 PM
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And add short doublers in behind the firewall so it's better supported...... Don't rely on the glue joint itself. Especially when it's glued just to balsa. The balsa can't take the impact loads over that small an area and the wood grain just rips out. Adding even a short doubler in behind so the plywood motor mount sits into an L shaped notch makes a HUGE difference.

To spread out the loads these doublers should be 1/8 hard balsa and about 1 1/2 inches long with the grain running flat. So the end grain is against the back of the plywood mount.
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Old Nov 30, 2013, 04:55 PM
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Thanks again.
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