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Who can help with THIS one? Guru needed :) WILL PAY - 9/28/2012 5:11 PM   
RP80


 

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Status: offline 		Ok, I have a custom designed plane that flies GREAT, except I have to add 16oz of weight to the front just to get it CLOSE to neutral (still a BIT tail heavy).

It's an aerobatic design, much like a yak/sukhoi, but custom design.  

I want to make it again but fix the CG issue.  I'm thinking that I can shorten the tail by about 2" to fix it, but was wondering if ANYONE out there can estimate the distance needed to neutralize 16oz on the nose?  You will of course need info to do this.  Here is the basic info.  Let me know if you need more.  I will even pay you (can send you paypal or gift card to horizon or something) for your knowledge.  Just let me know if you want to talk on the phone or email to get specifics.

It's a yak style (but funky made up design).  
Wingspan: 75"
length: spinner to tail tip : 71"
Length: Prop to hinge on stab: 57.5"
Distance Prop to CG (where the plane flies nuetral, ideal CG): 18.5"
CG to LE of Stab: 35"
Stab/Elevator Chord: 9.5"   ... Elevator 4" Stab 5.5"

AUW: no gas - 14lb 1oz.  This is WITH the added 16+ oz of weight added to the nose.  

It actually flies amazing, but having to add a lb of weight to the nose is making it hard for the engine to fly it well, AND I think removing some of the tail would NOT hurt it's tracking, as it's tail moment is much longer than any other plane (3d aerobatic) i've ever seen.  I think I went a little crazy with the moment in the original design.

the tail surfaces are BIG.  BUT I have heard that a bigger stab/elevator actually allows for a slightly more aft CG since there is extra lift in the rear.   I could reduce the rudder a bit without affect on flight i think.  The servos are mounted JUST in front of the stab, and are about 2+ oz each.  So 4.5 oz total.

Just looking and studying it, I'm guessing at moving it all up about 2" . But I want to get some good info before i go ordering laser cut parts again etc.

Thanks!



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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/28/2012 9:10 PM   
karolh


 

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Status: offline 		Why not try the easier things first like relocating the two rear servos forward and closer to the CG and see how much that helps as you might be surprised how much of a difference say 5 ozs. can make over a distance of approx 30".

Karol

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/28/2012 9:33 PM   
ZoomZoom-RCU



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Status: offline 		Whats the distance from the TE of the wing, to the LE of the horizontal stab. I'm asking this in case shortening the fuse is your only option. Also. a larger engine in the nose, or moving batteries around (heavier components) can also help. PM me for a more detailed convo.


ZZ.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/28/2012 9:55 PM   
da Rock



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Status: offline 		Developing a design....... ah, the joys of designing.....

Don't have much time but having done more than once the math you need done...... Off the top of my head, every ounce you remove from the empenage on a 70" long model will let you remove about 2.75 ounces from the front. If you only move the weight (as you've mentioned by moving the servos) you're looking at about 2 ounces for each ounce.

So move the two servos to the CG and you can figure on losing about 9 ounces.

Shorten the tail around 2" and you'll get about the same.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/28/2012 10:02 PM   
da Rock



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Status: offline 		Want to know if you will lose tail authority by shortening the rear of the fuse? Use an online app like geistware.com has and you can play with all the numbers they ask for.

BTW, while shortening the tail sounds like an excellent move for your present design, consider lengthening the nose a bit as well.

Also study your empenage construction. and the aft fuselage design... Lots of ways to make things lighter. The current crop of ARFs have one good feature. They have sensible sized holes in lots of places. Holes don't weight a thing.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/28/2012 10:55 PM   
R8893


 

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Chuck

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/28/2012 11:06 PM   
RP80


 

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Status: offline 		Thank you guys for the responses and help!  I didn't have my notifications turned on , so I thought I stumped EVERYONE, until logged on and was pleaseed to hear some experienced responses.

OK - to address a few things _ I've though carefully about other options.  Bigger engine just won't work, as I get good deals on DLE's, and the 55 would be too big.  BUT shrinking the whole thing may.  I also want to make the plans available possibly and build around a 30cc/35cc

Lenghting the nose:  I already did more than I wanted to.  I'm afraid it won't have the look I want if I go further with it.  

DISTANCE FROM TE of wing to LE of stab: about 20.5"


Moving the servos - I would but I feel it needs a good servo connection and don't like the messy set up of a longer push pull for the elevators.






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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/29/2012 12:46 AM   
rcflyertim



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Status: offline 		Do you have set of plans?

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/29/2012 12:47 AM   
rcflyertim



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Status: offline 		Custom aerobatic planes that are set up for 3-D will have a aft CG. To my amazement that can be quite a lot of difference setup. One option that I have done is to move weight. Don't add weight just move it (tank, servos, batteries, Rx, and engine). Buy your self and good CG balancer so that you can move things around to get the best setup. A General rule for weight is 1/3 aft 2/3 forward. In your case.....that would mean 32 oz. needs to come off the back of the plane.

Good luck,

Tim

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/29/2012 1:57 AM  1 votes
Jim Thomerson



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Status: offline 		What you are dealing with is torque around the CG. Torque can be expressed in inch x ounces. To get your CG where you want it, you need the same number of inch x oz on either side. Say you put your 16 oz 15 inches in front of your desired CG. That's 240 inch x oz. If you can take 240 inch x oz off the back, you won't need any nose weight. Just an example to show how it works.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/29/2012 3:17 AM   
2walla


 

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Status: offline 		Remove the lead. measure where the wing should be placed for it to balance correctly. It will be aft of where it is now. Build new one with wing in this position. Otherwise figure out how to get 4-6 oz out of the tail. It is a simple ratio of the distances from the cg point. Typically the nose is about 1/4 as long as the tail. An oz of weight on the tail will take 4 oz of weight on the nose to counter. Try to take some weight out of the design as 13 lbs for a 75" wing plane is still a bit portly.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/29/2012 12:39 PM   
049flyer


 

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Status: offline 		Wing design can make a huge difference on where the CG is located. Swept wing or leading edge moves the CG aft. Straight wing or leading edge moves the CG forward..

So in this case I believe that if you sweep the wing a bit you will be able to remove some nose weight.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/30/2012 2:41 PM   
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Status: offline 		You haven't said where your battery and receiver are mounted. If they aren't on the firewall, they are too far back. I'll also second moving the servos forward. A good push/pull setup made of carbon fiber rod will still have a solid linkage to it. You also haven't said what construction method you used for the fuselage aft of the wing. If it's plywood sides and solid balsa tail surfaces like many ARF's now, you can save some weight by going to a balsa truss design for the fuselage and tail.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/30/2012 4:15 PM   
AA5BY


 

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Status: offline 		Your question details a lot of what I went through with an original design a year ago. I had the drawings done and ready for the build and woke in the night thinking... most planes I've built require nose weight - seemed it would be better to error on the side of needing tail weight and likely far less of it.

Like yours, the engine would be a light weight 35cc and typical planform ratios concerned me so an appendage was taped to the plan adding two inches to the nose. Viola... it worked out perfectly. Batteries are right behind the firewall, and rudder servo is in the wing cavity using pull/pull with elevator servos aft and NO ballast weight was needed fore or aft.

Truss structure was used for aft empanage and stabs and elevator/rudder.

If the nose had not been lengthened, I may have been where you are with a pound of nose ballast.

Looks.... also like you, I was concerned with how a longer front section would look but as it was not a scale design, there was latitude. In the end, I chose to disguise the longer front section by using a wrap around large check pattern up front.

http://pages.suddenlink.net/arlyn/finished.html

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/30/2012 4:36 PM   
da Rock



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Status: offline 		Let's step back and actually look at the proportions of a Yak54 (my 60 size AeroWorks) to see if anything about your proportions might stand out.
Spinner tip to tail is 54"
Spinner tip to elevator hinge is 47"
Spinner tip to ele/rud servos is 42"
Spinner tip to engine is ~6"
Spinner tip to CG is ~16.5"

Let's compare the proportions of the 2 models.

quote:

It's a yak style (but funky made up design).
Wingspan: 75"
length: spinner to tail tip : 71"
Length: Prop to hinge on stab: 57.5"
Distance Prop to CG (where the plane flies nuetral, ideal CG): 18.5"
CG to LE of Stab: 35"
Stab/Elevator Chord: 9.5" ... Elevator 4" Stab 5.5"

AUW: no gas - 14lb 1oz. This is WITH the added 16+ oz of weight added to the nose.


From the tip of your spinner to your CG is about 18.5-20.5"
From your CG to the tip of your tail is around 52-54"
Your tail is about 2.7X longer than your nose ROUGHLY.

From the tip of the spinner to the CG is about 16.5 on my Yak.
From the CG to the tip of the tail is around 37.5".
The tail of my Yak is about 2.27X longer than it's nose. Your rear could be shortened from around 52-54" to around 44" and your airplane should have the same "tail power" as a successful Yak that handles perfectly. You have plenty of room to shorten that airplane.

Got a picture of it?

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 9/30/2012 5:34 PM   
da Rock



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Status: offline 		For newbies following this, the post immediately above is about proportions, not balance directly. If we look at the proportions of the two models, the AeroWorks Yak has 16.5" of fuselage in front of it's main spar. It would then have 37.5" of fuselage and tail sticking aft. So it'd have about 2.27 more tail than nose. Since most straight wing models have their CGs on the main spar (or close enough for gummit work), we're going to go ahead and compare that 2.27 to whatever we can assume from the model with the "long tail".

That model has an 18.5" nose and is 71" long overall. 71 - 18.5 = 52.5 so we're going to say that model's proportions are 18.5 : 52.5 or thereabouts. That's around 2.8 isn't it.

If we want to use the numbers from the long tail model and the ratio from the AW Yak to get an idea how short we could go on the long tail, we'd multiply the 2.27 ratio number from the AW to the nose length of the long tail. 2.27 x 18.5 = ~42"

The long tail could be shortened to as short as 42" and it should fly as steady as the AW Yak. From 52" to 42" is significant, so yeah, it really does have a long tail.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 10/2/2012 1:29 AM   
BMatthews



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Status: offline 		The problem with shortening the tail to use less lead in the nose is that you also lose out on the authourity of the tail over the plane so you end up with less control response and have to move the CG even MORE ahead because the NP is pushed forward by the shorter tail.

The most effective way to use less lead in a model is to simply make the tail lighter. It means modifying the plans to use less material or switch to a higher strength material which you can use more sparingly. But either way if the model flys well now then shortening the tail will make it fly less nicely.

The only reason to shorten the tail would be if it was already extremely long. But if you're modeling the look and feel of it on present aerobatic platforms then I doubt if you made it overly out of proportion in that way. So that leaves you with either adding weight to the nose by some means or making the tail lighter.

I don't see reducing the size of the overall model as viable unless you can ensure that you can reasonably build it with the original engine down to a suitable and acceptable wing loading.

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 10/2/2012 3:31 AM   
karolh


 

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Status: offline 		That's a very nice looking and nice flying model you've got there, plus knowing it's all your very own design/creation makes it all the sweeter I am sure.

Karol

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 10/2/2012 2:33 PM   
AA5BY


 

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Status: offline 		Thanks Karol... yes, there is a lot of satisfaction, intrigue, adventure and enjoyment that comes with a self design as you are now experiencing with your current effort. Have you firmed up the design? Started the build? A build log?

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 10/2/2012 6:50 PM   
karolh


 

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Status: offline 		I am about 95% complete with the design, it's just some of the little construction details that I keep changing as everyday a new idea of how to do something seems to pop into my head, but I am getting there. Should have something to share soon.

Karol

< Message edited by karolh -- 10/2/2012 7:31 PM >

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RE: Who can help with THIS one? Guru needed :) WILL PAY - 10/5/2012 2:58 PM   
rctech2k7



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Status: offline 		You've got plenty of advice here already and I want to share mine with regards to your question: "was wondering if ANYONE out there can estimate the distance needed to neutralize 16oz on the nose?"

You've mentioned that AUW including 16oz balancer is 14 lb 1oz, we need a measurement of distance from the balancer to that CG. Also which parts are you planning to move to eliminate balancer on your nose, example: battery and its weight so we can determine the distance where it should move.

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