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Old Sep 11, 2012, 09:36 PM
Vintage wood is the best!
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Quote:
Originally Posted by AllanK1 View Post
I will size a single tapered spruce spar to match you flight loads. That will help those wanting to respar the whole wing.

Allan
Excellent.....as stated earlier my plan is to join the two panels together and make one wing panel per side.
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Old Sep 12, 2012, 12:06 AM
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Quote:
Originally Posted by DEAN GRADWELL View Post
Spar configuration of my 40% CNC Modellbau K6.

Regards Dean
This is good basic engineering and should, with the "D" tube, give more than adequate strength for the normal flight loads with a good safety margin.

The multi layer spar is a very good approach: simple, easy to do and very efficient. It could be improved (if necessary and for an added peace of mind) by putting a Kevlar or "S" glass (light UD) layer to the bonding interface between the spar elements for a minimal mass increase.

By doing so it should be easier to justify the requirements for the heavy model to the authorities when needed.
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Old Sep 12, 2012, 03:34 PM
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Quote:
Originally Posted by DEAN GRADWELL View Post
Spar configuration of my 40% CNC Modellbau K6.

Regards Dean
Sensible looking structure Dean and a far cry from the aircraft we are concerned about.

Allan
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Old Sep 13, 2012, 08:43 PM
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Hi Allan a group of us now have these in NZ, 6 I think.Looking at the posts I decided to peel the covering off,and I will place a tapered carbon tube between the two main spars.This is just quick fix for peace of mind.

Woo
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Old Sep 13, 2012, 08:51 PM
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Ka 8s in NZ

Quote:
Originally Posted by WOO NZ View Post
Hi Allan a group of us now have these in NZ, 6 I think.Looking at the posts I decided to peel the covering off,and I will place a tapered carbon tube between the two main spars.This is just quick fix for peace of mind.

Woo
Good to hear you are doing the right thing Woo.
Mate, let me do just a little more before you do that. It is even uglier than we first thought. Looks like the beef up will need to do out a bit further.
You need to treat this model under LMA too I'm afraid as it is likely to be over the 15Kg limit. I know you won't like that idea but this think is just bloody dangerous as you buy it.

Allan
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Old Sep 13, 2012, 08:55 PM
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span position chord Cap width Cap thickness
0.00 21.80 1.25 0.976
23.30 19.29 1 0.755
46.60 16.78 0.75 0.547
70.00 14.27 0.5 0.351
86.00 12.30 0.42 0.182
102.00 10.32 0.33 0.124
118.00 8.35 0.25 0.124
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Old Sep 13, 2012, 08:57 PM
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Ooooppps

Quote:
Originally Posted by AllanK1 View Post
span position chord Cap width Cap thickness
0.00 21.80 1.25 0.976
23.30 19.29 1 0.755
46.60 16.78 0.75 0.547
70.00 14.27 0.5 0.351
86.00 12.30 0.42 0.182
102.00 10.32 0.33 0.124
118.00 8.35 0.25 0.124
Sorry that was scramble please disregard!
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Old Sep 13, 2012, 09:25 PM
WGTN NZ
WELLINGTON NEW ZEALAND
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Allan flight weight is 11.5 kg which is light for the size,I guess that is why only a few have failed.I think if not looped and gently flown they might survive a little longer.I am also going to beef up the airbrake well.
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Old Sep 13, 2012, 09:32 PM
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Required spar sizes

Hi, I hope you are all sitting down, the spar size is a bit of a shock
This is for a 32 pound Ka-8 at plus minus 5g. Spar cap will fail in compression above that load.
This is the sort of load you get if you fly at twice stall speed then pull up at maximium wing lift. Full up elevator into a tight loop.
The spar is aircraft grade spruce.
Span is the distance in inches out along the wing from the root
Chord in chord width
Width is width of spar caps in inches
Thickness is vertical depth of the spar cap in inches.
The spar is made up of upper and lower caps with webs to connect them

Span --- chord--- width--- thick
0.00--- - 21.80--- 1.25--- 0.976
23.30--- 19.29--- 1------- 0.755
46.60--- 16.78---- 0.75-- 0.547
70.00--- 14.27--- 0.5--- 0.351
86.00--- 12.30--- 0.42--- 0.182
102.00--- 10.32--- 0.33--- 0.124
118.00--- 8.35--- 0.25--- 0.124---

These sizes don't looks so wrong when you think how big this thing is The wing is 3.125 inches thick and has a 22 inch root chord. These cap could be webbed both sides with .125 ply or you could build an I (eye) beam with end grain balsa between the caps. Balsa thickness should be at least 1/2 the cap width.

Now you have probably figured out this lot needs a man sized joiner too and that needs to fit in between the caps and be boxed in.
If you make a sold carbon rod then 0.675 diameter is OK or if you went to 4230 steel tube condition N as used a lot in light aircraft you would be OK with a 0.875 diameter tube with .063 wall. This stuff is available. You could just take it down to the muffler shop and get it bent for dihedral angle in a tube bender. All you need then is receiver tubes to match to build into the wing. Joiner should go 9 inches into each wing.

Comments, apart from Darn!

I'm fine if anyone want to check my math. Just drop me a PM.

Allan

I will now work on a carbon tube spar system so you don't have to wreck the wing to fix it.
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Old Sep 13, 2012, 10:15 PM
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Wing flex

Oh by the way for those of you interested in wing flex, the tips come up 25 inches each side under 5 G load.
If you were to use the spar as sized above it would bend in a lovely constant radius with a little flattening out toward the tips. It would look very scale.

Allan
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Old Sep 13, 2012, 10:24 PM
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.

Now you have probably figured out this lot needs a man sized joiner too and that needs to fit in between the caps and be boxed in.
If you make a sold carbon rod then 0.675 diameter is OK or if you went to 4230 steel tube condition N as used a lot in light aircraft you would be OK with a 0.875 diameter tube with .063 wall. This stuff is available. You could just take it down to the muffler shop and get it bent for dihedral angle in a tube bender. All you need then is receiver tubes to match to build into the wing. Joiner should go 9 inches into each wing.
I will now work on a carbon tube spar system so you don't have to wreck the wing. Allan, Are you saying that the original 4 aluminum and carbon joiner tubes are not sufficient to hold the wings with the new spar, so an additional joiner tube and housing will also have to be installed?
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Old Sep 13, 2012, 10:26 PM
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A carbon tube spar system, or any type of tube spar system, will put a lot of stress on the ribs and the external structure as the bracing between the tube and the remaining structure becomes critical.

I don't see a lot of effort involved to just put a normal laminate pine spar as mentioned earlier. Brace the wing properly on a building board (well supported: critical). With a small cutting disc cut the structure along the outer line of the spar you want to put in. If you want a three laminate spar you will need to cut at the three different depths along the span. Then use a router bit and clean the channel for the spar. Build your spar from the bottom up: first the shorter root element, than the medium one and the full span one last. One evening. Let cure.

"De-mold" the wing and repeat operation on the other side. Before you bond in place the spar you will need to work on the shear web. Nothing fancy, but make sure that you tie up the two spars and the wing joiner system. One evening (maybe two if it is pretty involved). Put in the spar system. One evening. Let cure. General cleaning and sanding: ready for covering.

Repeat for the other wing... In this process you can decide if you want each wing in one piece (easier) or two pieces (more involved)... For ease of process I would start with working on the wing bottom: better control of alignment with tabs under the TE and better control of the wash out.
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Old Sep 13, 2012, 10:29 PM
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. Allan, Are you saying that the original 4 aluminum and carbon joiner tubes are not sufficient to hold the wings with the new spar, so an additional joiner tube and housing will also have to be installed?
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Old Sep 13, 2012, 11:05 PM
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Joiners

Quote:
Originally Posted by flyingfever View Post
. Allan, Are you saying that the original 4 aluminum and carbon joiner tubes are not sufficient to hold the wings with the new spar, so an additional joiner tube and housing will also have to be installed?
Yes, we need to get that bending load directly and symetrically from the spar to the joiner.
The existing could be retained for alignment I expect althongh you could fit alignemnt pegs front and rear instead in the usual way the new joiner needs to tie into the fuslage structure where it has to transfer 21 times 5G = 105 pounds from the joiner to the fuse structure. Some thought needs to be given to this too to get it right.
Big aeroplane = scary loads huh?
Beleive me, we are not over doing this when you consider we can generate this sort of load by diving to twice stall speed, say 40 mph then pulling up into a tight loop so aerofoil is up at max Cl. There is no safety factor on this. This is what is enought to break it.
For perspective, we would put a safety factor of 1.5 of this ultimate load if it were a real aeroplane and come up with what we call allowable load. In this case plus minus 3.5G.
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