Wednesday, September 17, 2008

TWO-SPAR WING TRUSSES

The wire-braced truss is probably the original method used to fabricate a ladder-like truss. The only jig required is a pair of saw-horses, the only tools a carpenter's framing square and a level. The method is also extremely versatile in that the compression members, shown here as a strengthened rib, may be completely independent of the ribs. The Volksplane uses a heavy dowel-rod, for example, while the Fly Baby uses a steel tube. The turn-buckles may be located on either end of the tensioning cable.

Following World War II new turn-buckles, cable eyes and Nicopress sleeves were available as surplus, often priced at their metal value. This allowed homebuilders to utilize this type of wing structure which would otherwise have been too expensive.

By comparison, the Pitts-type of truss makes use of rods as the tension elements with re-enforced ribs as the compression members. The rods, typically of .156" to about .185" in diameter were threaded. The fixed end was fitted with a T-nut or other fitting that prevented the wire from turning. The other end of the wire was fitted with a coupling nut, secured with an elastic stop-nut. Tightening the coupling nut provided the required tension. The assembly was then locked in place by the elastic stop-nut. The
key factor in this method was the use of filler blocks that presented a perpendicular face to the tensioning wire. For lightly loaded wings, mild steel rods and cut threads (vs rolled) provided more than enough strength.


I've called the third type of truss the Ison Truss because I first saw it on a set of drawings from Wayne. In this truss a wooden spar serves as both the tension and compression member. The key to success with this truss is to ensure adequate gluing area between the spars, the ribs and the diagonal tension/compression member. This is accomplished through the use of plywood gussets having a generous surface area.

This truss is specifically designed for use with a C-type built-up spar, in which the plywood gussets are glued directly to the spar caps. To increase the load-carrying capacity you need only increase the area of the glued surface between the spar and the diagonal strut. Indeed, the advantages of this method are almost too numerous to mention but first among them would be low cost, followed by ease of fabrication and light weight.

This method is popular among ultralights and may be found on Leonard Milholland's 'Eagle' series of VW-powered airplanes. When fitted with a D-cell leading edge of adequate depth, the wing proves remarkably rigid making it suitable for use in a Primary Glider.

Friday, September 5, 2008

Big Red, your basic flasher


A basic chore associated with airplanes is making something flash. Sometimes you want them to flash fast, other times you want it to go slow. What's getting flashed may demand a couple of amps whereas other circuits may need only a fraction of an ampere. This can lead to the use of half a dozen different flashers, resulting in a heavy and complicated circuit.

The circuit shown here is a basic One-Size Fits All.

The basic idea is to use an integrated circuit (ie, the Ne-555 chip) to toggle the circuit at a given rate. The output of the flasher is then fed to a small RELAY capable of handling about an amp. If you need to flash a higher amperage you simply wire the existing relay to one having a higher rating.

This isn't a new circuit. If you dig around you can probably find a circuit-board mask for it in one of the archives. The advantage of this circuit is that it's very inexpensive and may be sized to handle anything from a 1A nav light to a 10A strobe by simply selecting a suitable relay.

NOTE (17 SEPT 2008)
One of the comments suggests replacing the fixed resistors R1 & R2 with variable resistors. In fact, that is what I did when bread-boarding this circuit. Once I'd found a setting that gave the approximate flash rate & duration needed for an automotive turn signal, I lifted one leg of the variable pots and measured their resistance. This was matched to the nearest standard value FIXED resistor. The purpose was to make the module easy to fabricate by guys who weren't born with a soldering iron in their hand. Fabricated from all fixed or sealed components, the finished circuit could then be potted with epoxy or similar sealant, rendering it weather-proof. The relay of course may be mounted almost anywhere. -- rsh

Sunday, August 10, 2008

Chugger's Spar


Chugger's spars are built-up C-sections. The shear web is 1/8” aviation grade birch plywood with the face-grain oriented vertically. The par caps or booms are made of hemlock or Douglas Fir. The drawing shows the profile of the spars. Please note that the 144” dimension shown in the drawing is not correct. I know some of you have been following Chugger's progress and that my recent medical problems have left you in the lurch. This posting should provide enough information for you to fabricate your shear webs and spar caps, which must be scarfed.


Scarfing is a standard woodworking procedure used in all wooden aircraft. Solid members, such as the spar caps are scarfed at about 15:1 whereas plywood uses 12:1. You'll note that this is a much flatter angle than is used by boat builders. Mark Langford's web site offers what has to be the best explanation of scarfing you'll find on the internet. Please go there... http://home.hiwaay.net/~langford/kspars.html and see what he has to say before continuing here.

It will take two 48" x 48" sheets of plywood to provide the material needed for the two front and two rear spars. We must also provide for aileron spars, gussets and various doublers, meaning we'll be using a lot more 1/8" plywood. But these two sheets are all we'll need for the spars.

The drawing of the spar (below) is not complete. I've posted it to give you some idea of our goal. I am still working on the best method of transferring the load into the lift-strut and wing root fittings. I would like to use the bay adjacent to the wing root for the fuel tank but this too is still under development.

Your basic guide to aviation woodworking is AC-43.13, the manual showing acceptable methods for the repair of aircraft structures. You can buy a printed copy or download the manual from the FAA's library (see

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99C827DB9BAAC81B86256B4500596C4E?OpenDocument

Wood and fabric is covered by the first three chapters.


The Department of Agriculture, which 'owns' the U.S.Forest Service, got out of the aviation wood inspection business in the 1950's when someone noticed that, since airplanes were now built of metal rather than wood, at our present rate of usage the government had about a three hundred year supply of aviation-certified wood on hand, stacked in warehouses all over the country.

Nowadays, if you buy 'aviation-certified' wood, what you're getting is a promise from the seller that the wood appears to meet those government specs from days of yore. (And when it doesn't? Well... tough darts, Charlie. Maybe they replace it. Or maybe not.)

Another interesting slice of reality for the newbies is to read AC-43.13 (or any of several other references) and see that Sitka Spruce has no magical aeronautical properties. Indeed, there are several commonly available woods that are superior. The reason for the Spruce Myth is buried within the historical context of aviation... and of sailing ships. At the turn of the century and for forty years thereafter, masts, booms and spars were a common item at any lumberyard, not only near sea ports but at any city having water-borne transportation. When those pioneers of aviation needed wood they simply visited the nearest lumber yard. If it didn't happen to have something suitable in stock it was never more than a few days away, thanks to Railway Express.

You can find all of the wood you need to build an airplane inside the wood at your nearest lumber yard or box store. To get at it you will have to re-saw the wood that is there but this isn't as great a disadvantage as it might appear. Using Chugger as an example, the most critically needed pieces are the four main spar caps, about fourteen feet long, three-quarters of an inch thick by an inch and an eighth in height. While the three-quarter inch dimension remains fixed, all other pieces used in the airplane are shorter or shallower than the spar caps. If our Donor Timber was a two by ten joist, for example, ripping it into 3/4 x 1-1/2 laths gives us ten chances to find the perfect stick. And if we don't find it, we can cut out any imperfection and splice around it.


Saturday, August 9, 2008

Giving Credit Where Due

It doth often trouble me to Think
That in this Business we are all to Learne
and none to Teach...
-Robert Cushman, 1619

The Chugger Project is an on-going series of experiments using
inexpensive, commonly available materials to build a simple
single-place airplane. As the work progresses, drawings and photos
are placed in the Files archive of the 'chuggers' Group on Yahoo.
Text and periodic progress reports are posted to my blog.

It's important to note that the primary purpose of these experiments
has to do with materials and methods. For the structure I've simply
scaled up (or down) from proven designs. In doing so, I've tried to
give credit where due, although that isn't as easy as you might think.
Bernard Pietenpol used parallel wing struts on his Aircamper ...but
so did Claude Ryan on the NYP. The scaling is necessary because the
inexpensive, commonly available materials I'm using do not enjoy the
same ratio of strength-to-weight as for aviation-grade materials.

Pete Bowers' Fly Baby has a beautiful empennage. By adapting Pete's
empennage for the 'chugger' project, I'm paying homage to a past
master but in doing so I'm taking advantage of an invaluable
training-aid. Pete's tail-feathers incorporate no less than nine
built-up spars of box- and C-section designs, as well as curved
laminations. For the novice builder, the empennage is their Trade
School. Fabricating the tail-feathers provides a No-Fault opportunity
to acquire the skills they will need to build the wings and fuselage.

Clearly, the Fly Baby's empennage is more complex than the relatively
simple structures found on a Volksplane or Jo-Del but when scaled for
the Chugger , none of the components are especially large, reducing
these training materials to table-top dimensions. If fabricated from
locally available materials such as door skins and Box Store lumber,
the cost of this training exercise is only a few dollars. In fact, if
the goal of the novice is merely to learn how to build a wooden
airplane, there is no reason to build the entire tail, making the cost
even less.

While the parts-count of the Fly Baby tail makes the structure fairly
complex the required skill-level is delightfully low. Most of the
parts are duplicates, allowing you to take advantage of stack-sawing.
For example, the six shear-webs needed to produce the spars for the
elevators and horizontal stabilizer may be cut-out at one go. The
same holds true with the four shear-webs needed to produce the
stern-post and rudder spar. The diagonals in the horizontal
stabilizer are identical, left to right, so they too may be cut-out as
a stack. The only singleton is the shear-web for the diagonal brace
in the vertical stabilizer. That means all thirteen shear-webs can be
produced from only four patterns.

Once the shear-webs have been stack-sawn, Pete recommended attaching
the spar-caps and filler-blocks to them. In effect, the shear-webs
become your patterns. Since you're dealing with straight edges here
you need only apply a bit of glue (to both surfaces, please), tack a
piece of scrap to your bench-top to act as a back-stop, press the
pieces firmly against the bench (don't forget the waxed paper) and
tack them together with a pneumatic pin-nailer. Thanks to the use of
the pin-nailer the work took only a couple of hours Then comes
fitting the filler blocks, which takes longer – a couple of months
longer in my case, thanks to some health problems that had me lolling
around various doctor's offices instead of working in the shop.

In October 2007 I received a comment from Mr. Corrie Bergeron who is
building a Fly Baby. Corrie pointed out that there were other,
equally accurate methods of fabricating the empennage spars and
diagonals than the one advocated by Mr. Bowers. Rather than make the
shear-webs first – and use them as patterns – Corrie fabricated the
guts of the spars first – and used the guts as the pattern for the
shear-webs. Since I'd already tackled the project using Pete's method
I attached Corrie's comments to the appropriate article in my blog
http://www.itwtr.com/2007/09/av-chuggers-tail.html

Once back on my feet I was anxious to finish the tail surfaces, hoping
to carry the job right through to covering. But before doing so I
recalled the words of Robert Cushman and thought it only fair to give
Corrie's method a try, allowing readers of the blog to draw their own
conclusions. Accordingly, I made up a simple jig for the spars of the
elevators and horizontal stabilizer.

This may verge on heresy but I found Corrie's method offers several
advantages for a novice builder – or for any builder without a shop
full of tools. Pete's method of stack-sawing is dead-simple and
superbly accurate... if you happen to have a band-saw and a big belt
sander. But for the boxed spars, after attaching the spar-caps and
filler blocks in the recommended manner you're faced with the chore of
figuring out where not to varnish on the other shear-web. Corrie's
method offers greater latitude for the novice builder.

I've not posted any photos of the two methods as yet; I'm trying to
learn how to embed video in the blog. When I do, it may appear that
I'm changing horses in mid-stream when in fact I'm merely showing that
even an old dog is capable of learning a new trick – and of giving
credit where due.

-R.S.Hoover

PS – Robert Cushman was one of the Pilgrims

NOTE: This article was originally uploaded to the Fly Baby Group about two weeks before I was diagnosed with cancer.

Thursday, August 7, 2008

How ya' doing?

How ya' doing?” tumbles out of my in-box a dozen times a day. Surprisingly, most of the queries are from people I've never met. When they provide a valid e-mail address I tell them I'm doing fine and thank them for asking but most aren't meant to be a medical report; most are a simple show of solidarity from one airman to another. And as I've said to them, it is warmly appreciated.

As for the purely medical aspects of 'How ya' doing?' I've completed the radiation therapy phase and have started on chemotherapy. The pain is pretty much under control, my weight-loss appears to be flattening out and I'm getting more sleep. Overall, I think you can say I'm one lucky fellow.

Some say we make our own luck. I've got good evidence that much of it is a shared commodity, transferred from one individual to another by something as simple as asking: 'How ya' doing?'


Just fine, thanks. But a lot of that is because of you.


-R.S.Hoover

Wednesday, August 6, 2008

Cancer Report 01

You keep filling my in-box asking how I'm doing. Your interest is warmly appreciated but I'm afraid my answers -- doing fine, thanks. Doing okay... leave a lot to be desired. So let me use this Report to try and answer the essence of your question.

On Tuesday, the 5th of August 2008, I completed the radiation therapy. The x-rays have supposedly chopped up the tumor, killing most of it. In doing so, the PAIN has been reduced to a magnitude that is relatively easy to manage. And it's really all about controlling the pain.

After being destroyed by the x-rays the tumor doesn't just vanish. Apparently it is blasted into a soupy residue that your body must now eliminate. I'm told this will take about four weeks and will be the most debilitating phase of my treatment, in that I will be extremely weak. There is already some evidence of this. Even with two canes to ensure my stability, walking about forty feet left me too exhausted to return until I'd taken a rest break.

To facilitate the removal of the tumor residue I must force myself to drink about twice the normal amount of fluids. Doing so also serves to dilute any pain-killers you may have taken so that you are forced to keep track of what you've taken and when. Failure to do so gives the pain an opportunity to sneak up on you. If it gets you at the wrong time, you may find yourself immobilized, separated from your pills.

Chemotherapy is the hand-maiden of radiation therapy. Where the x-rays attacked the tumor in a macro fashion, chemotherapy goes after the cancerous cells in a micro-manner, seeking out each individual cell, which it either destroys of prevents from binding to a healthy cell and reproducing. The chemicals used to attack the tumor are toxic and there are a number of side-effects, such as nausea, hair loss and so forth. The chemotherapy began about two weeks ago and will continue for several months at a minimum.

Multiple myeloma destroys bone. Once destroyed, it can not be replaced except through surgery, which is successful in only a few particular cases. The damaged bone is quite fragile. There is the possibility that performing some accustomed chore such as dressing or bathing can over-stress the weakened bone causing it to fracture. There are chemicals that can bind to the damaged bone and provide some re-enforcement and I will begin taking those chemicals as soon as blood tests say the bone is ready to accept them.

So the messages ask: "How are you doing?" And I respond: "Fine; thanks for asking." But as you can see, there's a bit more to it than that. Such as the rash, a nasty side-effect of the chemo. Or the edema in my lower legs, an artifact of the tumor residues. And a dozen other little things that taken on the whole make it impossible to offer a comprehensive answer as to my condition.

Which is why I'd rather look at it from a slightly different perspective.

I'm a pretty lucky guy. I came within an ace of dying from an unsuspected tumor, recognized -- and properly treated by a superbly skilled physician who just happened to be in the ER when I was brought in. I'm doubly lucky in finding myself surrounded by people -- many whom I've never met -- who have provided support and encouragement that has served to level the often difficult path I have been forced to follow.

-R.S.Hoover

Saturday, July 19, 2008

The Orphaned Engine

Most Experts Aren’t. That's something the late Smokey Yunick said back when I was a seaman deuce. Every month my mail delivers one or two messages saying it's still true. The messages usually come from some superbly experienced fellow who has literally spent his life working on cars or trucks. He is the Local Guru when it comes to engines for homebuilts and he's taking the time to let me know that the automotive engineers I like to cite in my articles aren't quite as bright as I seem to think they are, offering an experience-based example to prove his point.

Unfortunately, the offered example invariably deals with cars or trucks, things in which the fellow has a life-time of experience, whereas automotive in the sense used here, does not, although it could include them. To an engineer, automotive means something that can move under its own power. Like an oil tanker, the Space Shuttle, or a gold dredger.

My usual reaction is to hit the delete key. I get more mail than I want, most from people with real problem, some of whom I can help. But it's always sad to hear smart people say dumb things. And on the whole, these are smart people, even though a life-time of experience hasn't tipped him off that we're taking about two different meanings for automotive.

We all start out pretty dumb. As we age we gather information and gain experience and, assuming a fair share of native wit, we end up a bit smarter than when we began. Mebbe all this guy needs is a nudge in the right direction. So you say hello and the odds are the fellow is having the same problems as everyone else except he was a bit too proud to say so.

With this type of Expert you'll often discover his life-time of experience has been with just one type of engine or perhaps one type of car and he has been trying to transfer that experience to a Corvair or a Volkswagen and isn't having much luck. I mean, who ever heard of a head torqued to only eighteen foot-pounds! That has to be wrong... right?

If the fellow hasn't figured out the meaning of automotive there's a good chance he won't have any idea in the blue-eyed world about Class of Service but a good understanding here is the real key to a successful conversion so you give it a shot.

A car or light truck uses a variable speed, high-rpm, low-torque engine whose nominal output approximates 25% of its peak output. Nominal output is defined as the amount of power the engine was designed to deliver for approximately 98% of its service life. The only time it’s expected to produce more… that wayward 2%… is when accelerating or climbing a hill. Once on the flats -- once you've reached a Stable State of cruise -- the figures are a good match. For hilly regions vehicle manufacturers offer different ratios for the rear-ends. Economy takes a hit but over-all, the figures match up. Respect an engine's Class of Service and you'll be rewarded with 2,000 to 5,000 hours between overhauls.

You can always demand more output from either type of engine but doing so will reduce it's service life. With a converted VW, for example, your Mean Time Before Failure will typically fall from about 2,000 hours in vehicular service to about 200 hours when powering a plane.

By comparison, an aircraft engine is a single-speed, low-rpm, high-torque engine whose nominal output approximates 75% of its peak output. Peak output may be defined further as maximum sustainable output, and as Peak-sub I, meaning an instantaneous value or dyno blip, something you might use to impress the newbies.

Since our goal is to produce thrust throuigh the rotation of a propeller, our primary interest is in the amount of torque that appears in the crank, and in the propeller's efficiency at a given rpm. The measurement of thrust is quite simple and articles describing different types of homebuilt thrust stands have appeared in the literature and on the internet. You will note that horsepower, which serves no useful function at this stage, has not been mentioned.

It usually takes an exchange of half a dozen messages or thereabouts to arrive at this point, if in fact we arrive at all. In the overwhelming majority of cases the Local Expert simply vanishes. Which is doubly unfortunate because the best is yet to come.

When we convert an auto engine for use in an airplane we are trying to convert it from one Class of Service to another to make it more suitable, usually in the area of mechanical reliability. By comparison, the typical flying Volkswagen starts out as a marginally suitable auto engine that is then made even less suitable for aircraft use by turning it into a hot-rod enigine. Why? Usually because the person doing the conversion has little understanding of an aircraft powerplant. Indeed, most such experts are merely the local Guru grown old, selling dune buggy engines to the kiddies. And after all, it does fly the plane, right?

So why even bother.

Well.... because we should. A properly built engine is more efficient. It produces the required torque at a lower rpm and wear increases exponentially with rpm. That means a properly built engine uses less fuel to deliver the same thrust and last longer, too.

But a properly built engine is also a lot less expensive to build and nowdays that's becoming a critical factor.

See that chart down there? The one title BORE VS STROKE? (It's embedded in the article in the blog; you guys on r.a.h. will have to go dig it out and print yourself a copy.) The chart shows the bore & stroke combinations for most common conversions and for everything using 88mm jugs or larger, or a 78 mm or longer crank, is going to have to machine the crankcase & heads to match. What they'll end up with is a dune-buggy combination -- a high-rpm engine that produces most of its torque up high. Itty-bitty toothpick of a prop. Not very efficient at all. Lots of machining to do. Lots of tricky bits to go wrong during assembly... which is why some folks don't even offer the thing assembled.

But it's all a bit of a joke because no matter HOW BIG the engine, it's MAXIMUM SUSTAINABLE OUTPUT is going to be between 35hp and 45hp.

Yeah, I know -- everybody is selling 80hp and up. Which is a dyno blip, not a steady output. Lotsa cubes is going to get you out of the weeds quicker but once you get the puppy cleaned up you're flying behind your basic 40hp engine, depending on the local atmosphere.

The limitation has to do with the heads, not the displacement. The cylinder heads only provide enough fin-area to manage the waste-heat from about 40hp. Unless its nice and cold or you are nice & high. But the dune crowd only knows how to build big-bore strokers.

Now go take anohter look at that chart. Limit your jugs to the stock 85.5mm.s and your cranshaft to a 78mm. At those sizes there's NO MACHINING REQUIRED. Your displacement is 1791cc, your maximum SUSTAINABLE output is about 45hp and your peak torque is going to come in at about 2800rpm.

Did I mention that no machining is required?

You've altered your cam timing but you're running a stock cam or a Schneider 'chugger,' the one used in the orchard-blower engine. You're running SINGLE PORT HEADS... because you're now an airplane engine, not a hot-rod. Your Volumetric Efficiency is pushing 70% and you're about a $1000 dollars ahead of the game because you haven't had to buy all that machining and you're using a higher percentage of stock, off the shelf parts. You're also running a longer, more efficient prop -- hopefully one you've carved yourself.

The thing starts on the first flip because it has an efficient ignition system, one that automatically adjusts itself to the load and a 20A. electrical system. But no starter, please. As it is, it weighs about twenty pounds less than any engine offered by anyone else.

But of course, it's not a dune-buggy engine. And the Instant Experts will stand in line to damn it with faint praise for that fact alone even while it flys circles around them and is still going strong when they're doing their second valve job of the year.

In my opinion, this is the perfect engine for an aerodynamically clean single-seater, like Bruce Kings little beauty. Had fate dealt me a different hand, that's what it would be going into. It would also be a good match for a KR-1, the early Jodel, Druine, the Teenie Two and similar designs.

Kill the parent, you got orphans. And that applies to engines, too.

-R.S.Hoover
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