Sunday, September 16, 2007

AV - Chugger's Tail

Back in May I mentioned I was using Pete Bower's Fly Baby as the basis for Chugger's tail. At that time (ie, 16 May 2007) I uploaded the drawings for the rudder and vertical stabilizer box-spars, which use 1/8" plywood shear-webs and 3/8" square cap-strips (as opposed to the 1/2" square cap-strips used in the original design). Chugger's tail is smaller in both height and span but I've tried to retain the elegant curves Pete used.

To ensure the four shear-webs would be identical, after rough-cutting them 1/8" door-skins I tacked them together on their center-lines and sanded them to final size as a single 1/2" thick slab. Since door-skins have an inner and outer face, when stacking them I made sure the rough-sawn blanks were oriented with the inner faces against each other.

The horizontal stabilizers use a built-up C-section for both their spars and diagonals whereas the elevators use box spars. As you can see in the drawings the diagonals are of a different size than the spars and are best handled as a matched pair. The six shear-webs for the spars are identical in taper but the elevator shear-webs are shorter. I treated these as matched sets of three, cutting the elevator shear-webs to length after sanding the stack to the proper taper.

Saturday, September 15, 2007

Design by Concensus

Back when the world was young and I still had hair the Navy hired a gaggle of eggheads to contribute to the design of what eventually became the Spruance-class of destroyers (i.e., DD-963 class). At that time I was the Leading Chief of the computer shop for Pac Fleet's cruiser-destroyer force. I was told to give the eggheads access to anything they wanted in the way of maintenance and repair data, which I did with a cheery aye-aye, sir.

Marvelous stuff, watching those eggheads at work, doing their computerized statistical analysis of equipment failures, tracking everything back to the manufacturer on one hand and the Navy schools on the other.

The product of their work was a list of recommended equipment to go into the new ships; only the best stuff as determined by its failure rate, required maintenance man-hours, mean time to repair and so forth.

Which was all bullshit, unfortunately.

At that time (early 1970's) ComCruDesPac had about 137 ships. The analysis covered such things as electric motors, pumps, air compressors, ammo hoists and so forth, the ancillary systems that are the glue of a modern-day warship. (The hull design and the turbine powerplants were determined by other groups.) The objective of the study was to determine the best of that equipment and on the surface, their methods of analysis appeared valid. But in providing them with data I noticed that while all destroyers had high-pressure air compressors (for example) some of them had never failed. (Not many... four, I think.) Same thing for the other components. All of the ships used a certain type of gear-head motors but a few ships had never reported any problems with them. Which brings up a point worthy of mention.

Even though built to the same plan, vessels within a given class are not identical. The ships are built at different yards and while their specs were identical their equipment came from a variety of manufacturers. In the case of electric motors for example, while most of the ships used motors from General Electric or Westinghouse a few of them had motors from manufacturers I'd never heard of. The key point here is that some ships had never reported any form of failure for certain pieces of equipment.

The bottom line is that the study failed to consider the possibility that some equipment had never failed. Their final report identified only equipment that had failed, giving high marks for designs and manufacturers that failed the least often.

Which completely ignored the Really Good Stuff.

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So what's all that got to do with airplanes? Quite a bit, when it comes to home-builts.

A fairly common thread on various aviation-related mailing lists and newsgroups is someone polling the subscribers in hopes of determining the ‘best' ...whatever. The best way to paint a spam can; the best brand of tire; the best vacuum pump and so on. Which gets down-right scary at times. (One such poll decided that the ‘best' aluminum was 6061 :-)

Polls and surveys, and the methods of statistical analysis that supports them, are valid tools. But only when your sample is an accurate reflection of the population being polled. Ask a room-full of pre-schoolers to define a balanced diet, don't be surprised if the answer is graham crackers and milk. In a similar vein, wood makes the best fuselage (according to builders of Pietenpol ‘Air Campers'), welding is easy (according to experienced weldors) and flying is inexpensive (according to people earning $100k p/a or more). In the case of the New Ship Design Study Group they failed to include the entire population of ancillary equipment, inadvertently limiting their investigation to equipment having a history of failure. (They were aware of the others but deemed them ‘statistically insignificant.’)


The Internet offers unprecedented access to information but does not provide any means of determining if that information is valid. Indeed, within the field of home-built aviation only a small percentage - - probably less than five percent - - of the available information is valid and even then, only in a particular case. The remainder is either skewed by commercial interest or is a reflection of ‘conventional wisdom,' wherein the poster is simply parroting something they have heard.

Common sense has become remarkably uncommon stuff in modern-day America. Given the risk inherent in rising above the ground on wings I believe the wiser course is to treat all information on the internet as invalid until you can test it yourself. Fortunately, with a technical subject such as aviation the required tests are fundamental and well defined. For the homebuilder, especially those lacking an engineering background, the tricky bit is devising methods of applying such tests to their particular situation.


PS - - So what happened with regard to selecting failure-prone equipment? I've no idea. By the time the first of the new class slid down the ways I'd been retired for a number of years. But it's interesting to note that several of the Spruance-class have been scraped after barely twenty years service. (Navy ships are designed for a minimum service life of thirty years.)

I identified the Really Good Stuff aboard our own ships and submitted a report on the matter, producing a minor controversy with regard to maintenance. Sailors know what I'm talking about and it really doesn't apply to anyone else.


(The above was originally posted to RAH in 2004. Recent posts to this blog [Chugger's Rib, 4 July 2007 and Chuggers Progress - III on 3 Sept 2007] have generated quite a bit of mail from folks who are upset by my failure to use Sitka Spruce, aviation-grade plywood and T-88 glue. The whole purpose of the Chugger files is to explore the use of less expensive materials that are commonly available. This isn't really a new idea. During World War II aviation was forced to use other woods, different glues and so on. History shows the planes (and gliders) flew just as well. Unfortunately, there isn't a lot of quantified data for those alternative materials. )

Monday, September 3, 2007

AV - Chugger's Rib

Labor Day, 2007.

Weather has been hot. A few miles north of us a mountain lion was seen drinking from a swimming pool and the coyotes are staying close to the few creeks that still have water. To hot to work in the shop, even with both fans going. Even the breezeway is pretty warm.

Too hot for welding and too sweaty for working on the fittings, I turned my attention to Chugger's wing. I have tentatively settled on the 4415 airfoil and needed some ribs for testing. I converted the 4415 coordinates into a rib drawing of the required chord, laid a few lines across it for alignment and printed it out. You'll find it in the Wing folder in the Chuggers Group, along with a pattern for the nose rib.

I didn't have a suitable piece of 3/4" plywood for the rib jig but did have some particle board that was wide enough. I don't like to use particle board for jigs because it's nothing more than thick paper and warps like a bitch but I glued a couple of stringers across the bottom and after the glue had cured, soaked it good with dilute varnish. That was a couple of days ago.

The several sheets that made up the pattern were trimmed along one edge using a straight-edge and razor. The jig board was given a coat of un-thinned varnish and each page of the pattern was painted with varnish on its back-side. The pages were then stuck to the varnished jig-board and aligned. Bubbles were chased to the edge of the sheet with my thumb and the whole thing was left to dry. But if you've never used this method, don't. It happens to be a quick & dirty method but varnish isn't a very good adhesive when applied to typing paper. It will hold the paper in position long enough to install the bits & pieces that will hold the rib's sticks in place. In doing so it will also fasten the pattern to the jig board. The whole thing will then get a coat of Deft Satin Finish Wax, which I understand is no longer available in the USA due to the tree-huggers. (The advantage of a wax finish is that nothing sticks to it.)

At this stage the thing looks like hell but it should work okay and only took a few minutes, if you don't count the clean-up :-)

I'm going to try using an Ison-type wing with wooden drag/anti-drag struts instead of wires or rods. The red hatch-mark is where the diagonal struts will pass through the rib. I'm also going to try building the ailerons in situ following the lead of Leonard Mulholland. I've not yet decided how to do the leading edge. I'd like to use 1/16" (1.5mm) birch ply but it is fairly expensive. Unfortunately the less expensive foam & fiberglas alternative, of which I've already built several samples, is about 3X heavier than the plywood.

I'm still tinkering with the leading edge but it looks as if I'm going to have to bite the bullet and run up to Corona (ie, Aircraft Spruce) for a couple of sheets of 1/16" ply.

After posting an article about building stick ribs in which I used 1/8" doorskin gussets attached with 1/4" aircraft nails and Weldwood 'Plastic Resin' glue I got several messages from people who found it impossible to use such small nails, having found they couldn't hold them with their fingers. The secret is to not hold them at all but to use the magnetic end of your tack hammer to pick them up and drive them into place. Unfortunately, that takes a bit of practice and since most of you are first-time builders I'll try using staples and/or pneumatically-driven 23 ga. wire brads.

The wing span will be a tad more than 28 feet, dictated by the available work-space (ie, about 15'). Chord is 56" so the wing's area will be approximately 125 square feet for a gross weight of 850 lbs, giving a 1-g loading of about 7 lbs per square foot. At 3.3-g that's about 22 lbs. With a rib spacing of 12" that's about 100 lbs per rib. Given the lift distribution of the NACA 4415 at its maximum angle of attack that means the portion of the rib between the spars will see about 80 lbs, the trailing edge will see almost no load at all and the remainder will be concentrated near the leading edge. One reason for cobbling-up a rib jig at this stage is that I want make and then break a few ribs to ensure they'll be strong enough.

According to the classic design formulas as published by Raoul J. Hoffmann (and others) in the 1930's, the aileron should be about 40% of the semi-span in length and 20% of the chord in width. As with the leading edge structure, this is another area I'm still tinkering with. If everything works out I'll post the required patterns in the Chuggers file archive.


PS -- Be sure to read Chugger's Progress - III posted on 4 July 2007. This post (ie, Chugger's Rib) produced a couple of comments that made it pretty clear their authors were not aware of what has gone before.