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Old Feb 07, 2013, 11:36 AM
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@ Rich,

attached a photo of my Delta Dagger and a chuck glider to test the right CG. In both cases the CG is at the same position. No difference like you said!



Heiner
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Old Feb 07, 2013, 02:15 PM
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Si Goodchild's Avatar
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Quote:
Originally Posted by LuvEvolution7 View Post
here's your CG

Wow! can you explain that graphical method?

Si G (degree in aeronautical engineering, yet very confused!)
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Old Feb 07, 2013, 03:31 PM
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I'd say that is the classic crossing lines to get MAC method.
tip cords into root - root cords into tip - draw crossing lines and you'll find MAC line. 25% from that is good first guess.

But I have also crashed with this method. It was with F/A-18. It has big LEX extensions and they do have effect.

/Jyri
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Old Feb 07, 2013, 08:44 PM
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Malaysia, Selangor, Kajang
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As a rule of thumb, my deltas all have CG at roughly 50% root chord. This equates to approximately 33% MAC. Not exactlly but close enough. I may end up shifting the final CG but it's usually due to not being able to shift the battery far enough in one direction or another. Deltas fly well in quite a large range of CG.

I prefer the 33% rule over the 25% rule for all my planes. But that's just me. I'm used to it. If it can't do tight loops then it's nose heavy to me.

Regardless, I've used the 50% root chord rule on lots of deltas. Some built for myself, some built for others. So far it has worked well enough for several different pilots with very different flying styles and levels of experience. It's the CG I set my son's delta trainer to when I was teaching him how to fly.
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Old Feb 07, 2013, 08:56 PM
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St.Catharines, Ontario
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Quote:
Originally Posted by jjantti View Post
I'd say that is the classic crossing lines to get MAC method.
tip cords into root - root cords into tip - draw crossing lines and you'll find MAC line. 25% from that is good first guess.

But I have also crashed with this method. It was with F/A-18. It has big LEX extensions and they do have effect.

/Jyri
yes, you can't exactly use the X method with LEX extensions. your numbers will be slightly off. you have to treat the LEX as a seperate delta wing. essentially, you have to CG based on two wing panels, which significantly affects the position of the CG. I can do it using the X method too, but gets a little bit more complicated.

Rich
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Old Feb 07, 2013, 09:08 PM
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Originally Posted by slebetman View Post
As a rule of thumb, my deltas all have CG at roughly 50% root chord. This equates to approximately 33% MAC. Not exactlly but close enough. I may end up shifting the final CG but it's usually due to not being able to shift the battery far enough in one direction or another. Deltas fly well in quite a large range of CG.

I prefer the 33% rule over the 25% rule for all my planes. But that's just me. I'm used to it. If it can't do tight loops then it's nose heavy to me.

Regardless, I've used the 50% root chord rule on lots of deltas. Some built for myself, some built for others. So far it has worked well enough for several different pilots with very different flying styles and levels of experience. It's the CG I set my son's delta trainer to when I was teaching him how to fly.
the reason why you can get away with it at 33% MAC, is because a delta doesn't stall in the traditional sense of a stall. it's mostly because lift isn't produced the same way as on a typical planform and relies mostly on LE vortex generation. tail heavy is tail heavy, but a delta is more forgiving in tail heavy trim. the problem with tail heavy trim, is that it robs top speed performance and more importantly, it bleeds speed much faster when maneuvering.

it's a little more complicated than just setting a CG at a certain percentage. to be balanced, you need to think of things such as decalage, Aerodynamic center, neautral point and CG. more importantly, you need to consider static margin and how that relates to the AC, CG and NP. a low static margin will give you greater elevator/elevon authority, but less stability. conversely, a high static margin will give you less elevator.elevon authority, but higher stability. if you add too much static margin, you end up nose heavy and too little, you end up tail heavy, regardless of where you think the CG should be. so it's something you need to know for a plane to fly well. I tend to set my static margin around 15% on a plane with flaps and a little less on planes without. never had a problem yet.
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Old Feb 07, 2013, 09:10 PM
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Malaysia, Selangor, Kajang
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Quote:
Originally Posted by LuvEvolution7 View Post
yes, you can't exactly use the X method with LEX extensions. your numbers will be slightly off. you have to treat the LEX as a seperate delta wing. essentially, you have to CG based on two wing panels, which significantly affects the position of the CG. I can do it using the X method too, but gets a little bit more complicated.
For multi panel wings you can use the online multi panel flying wing CG calculator:

http://wingcgcalc.bruder.com.br

It merely finds the MAC and draws the CG based on the %MAC you define so contrary to its name it's really a general purpose CG calculator - nothing special about flying wings except the 15% MAC suggested CG.

For multi panel wings it finds the MAC of the individual panels and averages them somehow.

The cool thing about the calculator is that it can create a link to your calculations so you can post it here for reference. It can even export the drawing as a PNG file.

Here's an example of a CG calculation for a slightly more complicated delta:

http://j.mp/WSriHG
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Old Feb 07, 2013, 09:12 PM
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I'll check it out. thanks for the link.

Rich
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Old Feb 07, 2013, 09:13 PM
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Quote:
Originally Posted by slebetman View Post
For multi panel wings you can use the online multi panel flying wing CG calculator:

http://wingcgcalc.bruder.com.br

It merely finds the MAC and draws the CG based on the %MAC you define so contrary to its name it's really a general purpose CG calculator - nothing special about flying wings except the 15% MAC suggested CG.

For multi panel wings it finds the MAC of the individual panels and averages them somehow.

The cool thing about the calculator is that it can create a link to your calculations so you can post it here for reference. It can even export the drawing as a PNG file.

Here's an example of a CG calculation for a slightly more complicated delta:

http://j.mp/WSriHG
yup, that's how you get the CG of a LEX airplane.
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Old Feb 07, 2013, 09:24 PM
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Quote:
Originally Posted by LuvEvolution7 View Post
I tend to set my static margin around 15% on a plane with flaps and a little less on planes without. never had a problem yet.
15% is fairly high. I tend to prefer 10% to 12% which usually translates to the 33% MAC rule (usually, not always). At 15% I find that there is often a tendency to nose up when throttle is applied for most planforms. Which is why a lot of people need a lot of down thrust. At 9% or 10% that doesn't happen as much. My down thrust, if any, is usually just 1 or 1.5 degrees. I also find that at 10% or so the elevator on most planforms is trimmed very close to zero which eliminates trim drag.

But it does require more concentration to fly. Any mistake at high speed at the static margin I prefer to fly at quickly escalates out of control.

With regards to stall - YES I love how deltas "stall" (which is to say they don't). One of my favorite things to do is to cut the throttle and pull the elevator up all the way and watch the plane that was screaming across the sky just seconds ago slow down to a halt. If you do it pointing into the wind you can even make it look like the plane stops in mid air. It doesn't work wit all deltas. In particular, deltas with long noses (have large side area ahead of CG) will enter a flat spin. But that is also fun and easily recoverable.
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Old Feb 07, 2013, 09:30 PM
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nose up usually means that the CG is too far forward and not really related to static margin. static margin is simply how the plane returns to its trajectory when the pilot disturbs it. if the CG is too far forward, the nose can pop up with power application. it seems like that is backwards, but that's what happens.
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Old Feb 07, 2013, 10:42 PM
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Originally Posted by LuvEvolution7 View Post
nose up usually means that the CG is too far forward and not really related to static margin. static margin is simply how the plane returns to its trajectory when the pilot disturbs it.
Well, higher static margin usually requires the CG to be far forward. They are one and the same thing. Some people prefer to fly slightly nose heavy and add down thrust - hence they're flying a plane with high static margin - really stable. I prefer lower static margin - hence less nose heavy than usual.

I do understand that actual static margin for the whole plane includes stuff other than CG like the size of the tail surfaces and the side area of the fuse. But all else being equal the usual method of increasing or reducing static margin during trimming is to shift the CG (unless that stops working in which case you should start looking at other causes like undersized stabilizers etc).
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Old Feb 08, 2013, 04:01 AM
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I would never rely on any "akademik" formulars in this hobby but after scratch building close to 100 various size and shape of "DELTAS" I found they all like to be reasonably nose heavy especially while landing. Nothing is worse then tail heavy delta! With my turbine powered ones they need to be extra nose heavy on full tank as the fuel is pushed back on partially empty tank during flares, making it slightly tail heavy and it can bite, Joe.
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Old Feb 08, 2013, 08:23 PM
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St. Peters MO.
Joined Nov 2007
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Mike,

I set mine to this point in the drawing. It was just a little nose heavy and would dig in the turns at slower speeds but it was a good starting spot. The Dart I built was a scratch so it might be slightly different than yours. Have you found out what kit it is?

Jeff
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Old Feb 10, 2013, 06:41 PM
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United States, CO, Castle Rock
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Quote:
Originally Posted by Ken Wright View Post
I use this calculator a lot. Seems to find CG dead on for any birds measurements I've thrown in it
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