comparative weights

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StRaNgEdAyS

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That's some good food for thought there Billski.
I agree with your reasoning on the longerons, I have incorporated them into the foam sandwich construction to a: hold it all in place while I lay it up, b: give me a place to mount the canopy and latches to and c: to give me easy to use places to secure the internal pieces to, like pedals, instrument panels, seats and the like as the sandwich is sufficiently strong enough to do the structural work and at 5/8" they will not encroach too much into the cockpit space.
The reason I opted to use the AS5048/5 tapered airfoil, is that it was specifically researched and developed for this aircraft (the KR2S). There is an constant AS5046 foil option, but the reports from UIUC gave the AS5048/5 (albeit marginally)better overall performance. The outboard wings, where the airfoil tapers from the 18% to the 15% section will not have fuel tanks in them, as the stub wings (a constant 18% section) should hold sufficient fuel on their own.
The airfoil section of the empennage is I believe 9% and changing this to a 12% foil would not pose too much of a drama, and it would allow more room for balancing.
The winglets I am retaining simply because the don't appear to detract from the wing's performance, and it give the craft a singularly distinctive look that will stand out a mile from the other KR's on the flightline at future gatherings I'll be attending. If at a later stage I decide they have to go, I'll just take them off. Simple as that. :D
I'll not be building the wings until much later in the year, if not next year, so i have a bit of time up my sleeve to mull it over.;)
 
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A lot of people think composites are some magic solution to a lighter and stronger airframe as if by the default. Just because the outcome is possible it doesn’t mean its likely, the only way you end up with a lighter airframe is that you are a better designer then the other guy not because you changed the material from wood to composite. You will be spending o lot more money, building a lot longer and in the end you will end up with heavier airplane then the wooden one not lighter.

George
 

StRaNgEdAyS

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Interesting point there George.
It has given me cause to go back over and start doing somoe hard number crunching. I'm working out the total mass of the parts I am replacing (drawing them all out in CAD and using the analysis tools to give me a total material volume and multiplying by the material weights) and doing a full comparative analysis.
I must admit I DID just assume that such a construction method could be lighter, and after asking a few questions and taking a SWAG from a couple of sketches, decided that it probably would be lighter.
So now the jury's out until I get this more detailed analysis finished. One thing that drawing the entire structure as constrcted in wood has done for me already is point out a couple of areas where my dimensional changes needed adjusting in order to make them fit properly. :D So it's all good!:ban:
 

AVI

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StrangeDays

When I started my project, my thought was that if the completed composite design resulted in an overall weight equal to or even slightly heavier than the comparable aluminum structure, my overall goal would have been achieved. Why? Well, I wanted to build an airplane based on a certain existing design but I could not see myself building in wood. Nor was the tooling available with which to scratch-build a complicated design in aluminum. Thus, composites.

As Bill/Orion would say, "Airplane design is all about compromise."

My questions would be: What are you trying to achieve? What are you trying to accomplish?

By the way, your 3D CAD is cool. What program are you using?
Myself, I'm just starting to learn how to use Rhino and my 2D has so far been done in AutoCAD. Any advice from any of the 3D users?
 
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StRaNgEdAyS

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Agreed it is all about compromise, the trick as I see it is to come to the best compromise to suit the individual situation without compromising the saftey of the aircraft.
I could say I am trying to cut the costs of purchasing expensine lightweight woods, but I will still need to purchase a significant quantity for spars and other sections of the aircraft to effectively make that an inaccurate assumtion. What little money I save on the purchase of the extra mahogany ply and spruce to construct the fuselage out of wood would be spent on the extra cloth and materials to construct the same fuselage out of composites.
If I was shooting for ease of construction, I would stick with the wood. So that leaves weight saving. So therefore, if I can make a significant reduction of the overall weight of the fuselage by building it out of composites, then that is the way I will go. I am quite willing to put in the extra finishing time to build a composite fuselage if it nets me enough of a bonus.
 

AVI

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StrangeDays

Check out Dan Raymer's "Simplified Aricraft Design For Homebuilders".

In the chapter about Preliminary Structural Sizing, he describes Peter Garrison's new four-place Melmoth 2.

"...... is typical of a modern all-composite homebuilt, made largely from foam core sandwiches with glass-epoxy and carbon-epoxy face sheets (skins). The fuselage skin sandwich typically has 2 plies of bidirectional cloth, totaling about .020" thick, on each side of a half-inch-thick polyester foam core.

However, the weight of this sandwich construction is equivalent to that of 0.036" thick aluminum sheet. The weight savings we hope to get from composites don't seem to apply to small structures such as homebuilts (and Melmoth 2 is a large homebuilt).

The wing skins of Melmoth 2 are unusual in having a bidirectional carbon inner skin (0.016" thick) and glass outer skin (0.018" thick) over a 1/4" thick foam core. All torsional loads are carried by the inner skin, which is also the liner of and integral fuel tank occupying almost the entire wing. All of the wing and empennage spars are of carbon-epoxy construction.

The tail surfaces are solid foam cores skinned with two or three plies of unidirectional E-glass. The ailerons, flaps and trim tabs are all carbon. The trim tab skins are only .005" thick - hands off!

The Melmoth 2 canopy is made of three different thicknesses of acrylic: 1/4" for the windshield, 3/16" for the side windows, and 1/8" for the rear window.

Garrison's first Melmoth was of all-metal construction, making him one of the few people to design and build both a metal and a compostie homebuilt. Comparing the two experiences, he says, " When I started the second airplane, I thought it would be quick because Burt Rutan seemed to be able to turn out airplanes in a few months. Instead, I found composite construction of a large, sophisticated airplane to be far more complicated and time-consuming than metal. The weights of the two airplanes are about the same. The main advantage of composites was the ability to form compound curves easily - but Im not sure that in the 20 years that I spent on the second airplane, I couldn't have learned to use a planishing hammer and and English wheel just as well. Viewed aesthetically, even a perfect composite airplane (and mine is not perfect? is less satisfying than a metal one because its perfection is superficial and can be added later. There's no way to fake good metalwork."

But this author (Raymer) has seen Melmoth 2 and it is truly beautiful! "

from Simplified Aircraft Design For Homebuilders
Dan Raymer
 

StRaNgEdAyS

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I have one of Mr Raymer's other aircraft design books and it has been a great help to me.
I do agree that the weight benefits of composite construction do get smaller as the size of the project decreases, but even if I end up staying with the wooden construction, it has been an interesting and informative experiment on my part, and that's what I'm in it for as much as the enjoyment of building and flying the finished plane.
I'll be hunting down some density figures for the spruce and mahogany ply and I'll then I'll know what's what.
The fuselage is really the only area where I am considering deviating from the plans method of construction, apart from maybe using full depth cores in the wings.
This is what the stretched and widened fuse boat would look like made as per plans, now I can work out it's weight.
 

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AVI

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Banff, AB
Yes, we're all in it for the experiment, the enjoyment, the self-satisfaction and sense of accomplishment associated with building an original design or modification of an existing design. Otherwise, we'd either purchase an airplane or build a kitplane.

I mean, what is sanity?

Who in their right mind without formal education (in aerospace) would set out to design and build an airplane and then hope to fly it? Sounds crazy to me.

But isn't that what we're all trying to do?

Fun, isn't it? So, go for it. Do it.

Oh, by the way, what's wrong with the Mark Langford wings? I thought that the way he did his KR2 wings was pretty cool! Fast and simple.
 
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