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Fat Avid weight and balance

95 posts in this topic

Posted

 Hi guys, well things are getting closer with my fat avid and i.m just wondering if the datum line and formulas  on it, is like the avid c . I have the manual walking you threw it and have a friend thats did them before but want to make sure the extra 18" in length dont affect  using the avid c method of doing the W & B. Randy if you happen to have any of your numbers handy and get a chance to post them ,that would be appreciated, Thanks Willis

post-282-0-96945000-1383543206_thumb.jpg

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Posted

Willis,

How does the Fat Avid compare to the Magnum? I have the W&B specs for the Magnum.

EDMO

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Posted

Hi Willis!

It should be the same as model A-D, and Magnum.

Chris Bolkan

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Posted

Perfect, thanks guys

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Posted

Hi again,just wondering if any of you fat avid owners has there envelope numbers handy, my avid weighed in at 733 lbs with the soob,only got the weights done so haven't  quite finish the balance part yet, feels good to be getting closer to launch.post-282-0-58766400-1384228386_thumb.jpg

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Posted

good looking plane .I like the  paint ,that weight isn't bad either ,performance should be great with that horsepower .I have no cg info for you but mine is at the forward limit  and flies beautifully.

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Posted

Thanks Dynomike , ya really cant wait to try it out , yours looks pretty sharp there too, a big thanks too all that helped get it this far.

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Posted

Hi Ed, I dont know the difference between the avid plus and the magnum but think we pretty much got it figured out, need the battery in behind the baggage area and some survial gear and it should be good to go

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Posted

Hi Ed, I dont know the difference between the avid plus and the magnum but think we pretty much got it figured out, need the battery in behind the baggage area and some survial gear and it should be good to go

Great - Hate to hear about someone hauling weight they don't need - You need both of those - Add in a bottle of Jack D. for a firestarter and you are flying!

I have the Magnum forward and aft CGs, but should be about same as FA.

EDMO

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Posted

I don't know if this is what you are asking, but my original MKIV (gone several years ago), my Magnum and my Fat Avid were the same WRT weight and balance envelope and calculations. I can't remember the numbers exactly, but the front of wing is the datum and acceptable range is something like -11 to -16 inches. DONT TAKE THE NUMBERS I JUST SPOUTED AS CORRECT. All I mean to say is the calculations and acceptable range is the same for what ever Avid you fly so if you have a MKIV manual that tells you how to run the numbers, it is done exactly the same on a Magnum or a Fat Avid, or an A model.

 

With regard to where the weight and balance falls within the acceptable range, I love to camp so I like it to fall towards the fromt of the CG range empty so I can carry all my camping gear. Every ounce of gear (for camping, kayaking or biking) moves the CG back so it's a no brainer for me.

 

I have also found that my Avids flew very well near the forward limit when i wasn't loaded with gear, although I've heard a number of people complain about operating at the forward limit. I suppose if you could care less about camping or carrying stuff and want to eek every bit of performance out of the plane you should shoot for a rear CG. But then you can't carry much stuff without ending up too far back.

 

It's all about your mission I guess.

 

ChrisB

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Posted

the wing is the same therefore the CG should be the same.  11.185" to 16.5"  I like mine best around 18".   In a phone conversation with Dean Wilson, he stated the published aft CG limit was very conservative and for someone wanting the beast STOL 18-19" was the best number, and DO NOT stall it with a CG aft of 20" unless you have full flaperons on or you will never get the nose pushed over.

 

Mine empty is close to the forward limit, however like ChrisB, with survival gear and the typical camping gear / supply load my normal flights are right around the 18" mark.

 

:BC:

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Posted

Thanks for the numbers Leni and Chris, the numbers we came up with are a little off your guys,mine are 8.90 to 12.51,and it looks like we need 12 lbs of weight on the tail according to my buddy who built a few planes already. I like Ed' s idea of the JD for weight lol

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Posted (edited)

Thanks for the numbers Leni and Chris, the numbers we came up with are a little off your guys,mine are 8.90 to 12.51,and it looks like we need 12 lbs of weight on the tail according to my buddy who built a few planes already. I like Ed' s idea of the JD for weight lol

No matter what your engine or equipment weighs, your AC (Aerodynamic Center) of the wing will not change - The AC of the Avid / Kitfox wings are at 25% of the © chord, (But, is that figured at 43 inches, or include the flaperons and use 51 inches?), and your maximum forward and aft CGs should be about 15%C and 33%C, but I am not sure what % Avid and Kitfox uses -

so whatever the manual says should be your limits - You cant change that - Please check it out again before flying.

A gallon jug of JD should weigh about 10 pounds - Please don't put any lead in it......

EDMO

Edited by Ed In Missouri

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Posted

I am with Ed, not sure where you came up with your numbers at, but they are WAY off from the factory.  at 8" your gonna have to land FAST and without flaps to have enough elevator to pull that nose up and flare.  When you pull on the flaperons it will pitch the nose down and you are gonna be using a half a stick of up elevator to keep the nose up even at 14-16".  I would rethink the numbers you came up with and get them more in line with what the book says if it were me.

 

:BC:

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Posted

Ok thanks for the info, will recheck it ,i never did one of these before so its all a big learning curve so thanks alot for the help ,maybe we screwed up on some of the measurements

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Posted (edited)

Just for my personal curiosity, I am going to run the numbers in my manuals, and see what % and chord they used to calculate the forward and aft CGs - will post it when I get done.

Listen to Leni - use the manual - we all make mistakes - some are more serious than others!

EDMO

edit:

I ran the numbers using the wing plus flaperons, for a 51 inch chord.

My Magnum book agrees with Leni's numbers:

11.185 max Forward CG = 22% chord

17.5     max Aft CG = 34% chord

There are too many design variables to list in coming up with these figures.  Just calculating the size of the tailfeathers takes a full page, or two, of handwritten calculations.

Dean Wilson knew what he was doing, and he tested his design.

I would not try to second-guess or change his numbers, unless, as Leni says, HE advised some deviation in the Aft CG.

 

P.S.  Avid made provisions for sweeping the wings forward for heavy engines. 

My Soob with nosewheel was even heavier, so I added 4 inch Leading Edge extensions which should move the CG forward about 3 inches, and the Riblett Rib modification which eliminated some of the nose-down force of the wings.

Fly Safe!

EDMO

Edited by Ed In Missouri

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Posted

It sounds to me like your numbers might not be that far off. You did say you have a subaru conversion, right? Depending on which conversion that can be a heavy engine / prop combo. If the battery to start that big engine isn't placed about as far back as you can put it, it is very likely that your empty CG will fall too far forward. I would level the plane to flying attitude shimming under or on top of the scales as necessary taking account the weight of any shims on top of the scales (per manual instructions) and carefully weigh and calculate again. (measure twice, cut once :unsure: )

The extended tail of the Fat Avid will accomodate a bigger engine than the original two strokes, but even the MKIV was way nose heavy with just a 582. I think the original Avid ran a Cayuna! So it may be that you need to move the battery WAY back if you haven't already, or even add weight to the tail. I've run the numbers on my Fat Avid with 912 and it comes out very close to the forward limit, so it wouldn't surprise me that the Sub engine which is heavier pushes the plane past the forward limit if you haven't accounted for that. In a perfect world you've made a simple measurement or calculation error and everything is fine. In the real world often and unfortunately big engines need to be offset with additional weight near the tail.

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Posted

I think I may have mistaken what he posted.. I too can see it as being in the 8-12" range with the subie, but I took it as he had run some numbers and determined his CG range was 8-12"  thats what I was talking about when I said the numbers were off from the book and I would not want to fly it.

 

:BC:

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Posted (edited)

I think I may have mistaken what he posted.. I too can see it as being in the 8-12" range with the subie, but I took it as he had run some numbers and determined his CG range was 8-12"  thats what I was talking about when I said the numbers were off from the book and I would not want to fly it.

 

:BC:

Still, unless he changes the wing, he needs to be in the CG Range as set by the manual.  Load it real heavy with baggage, or put an anvil in the tail - whatever it takes, it has to balance on the CG while flying.  I agree with you "He aint going to get the nose up......"  His rear CG is barely in the forward CG by the manual.

Hope he comes up with better numbers, or changes his wings.

EDMO

P.S.  My old Kitfox 1 was flown with a Soob and had a 25 lb car battery just in front of the tail, and the wings were swept forward -  there was not enough tailfeathers - They had to be enlarged too.

Still, when the engine quit - It got nose heavy real quick and flew like a rock!

Edited by Ed In Missouri

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Posted

Right now the battery cable are long enough to reach the tail section just really don't want to cut an access port in it ,this weekend going to sit the battery as far back threw the turtle deck open as far back as I can mount it and re weigh it and if that still not enough some lead in the tail will be needed. It does have the soob coversion, 72" 3 blade warp drive prop,just hope its not -28 c   again

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Posted

-28 ©???  Not (F)?  Better put in about 5 Gal of JD in the baggage - and maybe drink the other gallon!

Let us know how the numbers go.

EDMO  45F low tonight

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Posted

Ed your right much to cold for beer, i'll let you know what we come up with

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Posted

Measure measure measure before you cut anything! You might be fine. I have heard that the Fat Avid was actually designed with the soob in mind. I am not an expert on Fat Avids for sure even tho I now own one! The builder of my plane used a pretty big battery with the 912 but it wasn't located too far back, so maybe carefully re measurung and moving the batt way back will allow you not to use any lead. Even if you do need a bit of additional weight along with the batt, make it a tool kit or survival gear. Don't use lead....it's so..wrong :-)

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Posted

Ed your right much to cold for beer, i'll let you know what we come up with

Either C or F, much too cold for beer - I got a little more educated last night after you posted the temp, and I went and checked my dual thermometer - I knew that F temps above 0 went up faster than C temps - What I had not realized, was that C temps go down faster than F temps below 0 - and -28C is about the same as -20F - guess never too old to learn something -

Now, if I could just think in sillymeters instead of English!  :lol:

EDMO

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Posted

I agree Chris Im going to do my best not to put any lead on it, glad to here your battery location isnt that far back so maybe I have a shot ,my battery weighs 20 lbs so its pretty heavy,Ed im pretty mixed up on the sillymeters too,

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