Notes & Comments

From "Construction Notes" Falco Builders Letter, March 1992

Hans Sonntag from Germany stopped by recently. It was interesting to hear him tell of his difficulties of trying to find out the meaning of a few words. It's so easy for us to lapse into jargon that we forget that there are plenty of people, particularyly those in other countries, who haven't a clue what the words mean. What tripped Dr. Sonntag up was "dry flox" and "reamers".

If you're similarly confused, a reamer is a cutting tool that's very much like a drill bit, and it's used to enlarge a hole just a little. A drill doesn't produce a hole of high quality, so if you want a very accurate hole in metal, you drill it slightly undersized with a normal drill bit, and then use a reamer to finish off the hole.

But it was 'dry flox' that really drove Dr. Sonntag up the wall. He consulted with innumerable aerospace engineers, pilots and mechanics, and no one knew the term. Finally, he stumbled across a Varieze builder who knew all about the stuff and explained it to him.

Sorry about that. Dry flox is one of a number of terms that grew out of the Rutan series of fiberglass airplanes; there's also wet flox, dry micro, and wet micro. What we're talking about here are various mixtures of epoxy and a filler material-'micro' stands for microballoons, and 'flox' stands for flocked cotton.

Microballoons are microscopic glass or phenolic bubbles, originally developed as ping-pong balls for sports-crazed ants, but California weirdos started using them as a very light filler to mix with epoxy. When the mixture hardens, you have something similar in weight to styrofoam. Great stuff for filling low points on a wing.

And flocked cotton is just a bunch of loose cotton fibers, rather like what you'd get if you shaved a couple thousand white rats and then put all the hair in a bag-that's probably what they do. You mix this stuff with epoxy when you want to have a little strength to the goop.

The 'dry' and 'wet' part of the phrases simply means how much epoxy is mixed in. Wet micro is a soupy mixture while dry micro will stand up on its own like cake icing. Same precise engineering standard is used for wet and dry flox.

From "Construction Notes" Falco Builders Letter, March 1992

I was surprised to have a builder ask me a question the other day about something that had long since been covered in a revision. I hope he is the only one, but somehow he had missed out on how our revision list is handled. Because he's an engineer, he thought our drawings followed the standard practice of noting all revisions on the drawings, and that our revision list was just a record of all that.

The way we handle revisions is very non-standard, but with the number of builders we have out there, there is no other way I know of that's practical. Our revision list shows revisions that you are expected to note on the drawing, and they are not a history of what we have done. They are extremely important, and every builder should keep up with them.

In fact the other day George Barrett called with a question. Something about a screw in the electrical kit. "Have you checked the revisions, George?"

"Oh, yes!" said George, and as I explained the situation I flipped through the revisions and came up with the appropriate entry. George was very embarrassed to have this pointed out to him, and a few days later I received a check from him for $5.00, his self-imposed penalty for asking such a question. Damned if I know what I'm going to do with that check, but so far I'm just enjoying that fact that his checkbook doesn't balance!

From "Construction Notes" Falco Builders Letter, March 1991

Steve asks if anyone who has been through the final FAA inspection process has worked up a check list of all the things that you need to do before the final inspection. Things like ELT, name plate, aircraft logbook, whether the inspection panels should be on or off, etc. That sort of thing. Anybody got anything on that one?


Jeff Morris reports that the specified welding rod Lindy 65 is no longer available because Lindy was purchased by another company, Esab. Jeff says the equivalent to Lindy 65 welding rod is AWS# ER70S-2.

From "Construction Notes" Falco Builders Letter, September 1991

At the McCall Fly-In, Richard Clements mentioned that he found a bean bag to be a great addition to a shop. When working on the bottom of the airplane, he just shoves the bag under there, and then scrinches himself around on it until he gets comfortable. When working in the cockpit, he just throws the bag into the plane and does the same thing.

From "Construction Notes" Falco Builders Letter, December 1991

Highland Hardware also has a number of other potions and goops that are worth taking a peek at. Dri-Cote is a spray lubricant that you put on saw blades to slow the accumulation of resins and carbons that builds up on blades. I've tried it, and I like the stuff on a saw blade. It dries to a slightly greasy-feeling surface-not the hard-dry surface that you imagine from its name. I use it on saw blades but not on my router bits because I use a diamond hone to sharpen the router bits.

There are three other interesting products that I haven't tried yet. Top-Cote is a hard, dry teflon-like coating for cast-iron tablesaw tops and tools. It reportedly lowers friction dramatically. There's also something called Renaissance Wax with is a synthetic wax which can be used on tools and tablesaw tops, and which was originally developed for the British Museum for cleaning and restoring fine art works.

Finally, there's Boeshield T-9, a corrosion-protection product developed by Boeing for long-term protection of metal components. It dries to a waxy, waterproof film, and this is something you might want to spray on metal parts in the Falco, over top the zinc chromate primer. But you will want to be careful, because any wax you spray will prevent glue adhesion if you don't have all the woodwork finished.

Boeshield T-9 is hardly an experimental product. It was developed by Boeing after exhaustive testing of all the popular penetrating lubrications. Boeing's engineers were unsatisfied with the performance of all of the brands they tried, particularly in the area of long-term corrosion protection. Some protected well but were messy and thick, while others penetrated well but didn't last long. What they needed was a penetrating oil that dried to a clean protective film, and everything they used came up short in one area or the other.

Boeshield T-9 is a combination of thirteen solvents, oils and most importantly, a wax that remains as a barrier film after the other components have penetrated, lubricated, displaced moisture and evaporated. Boeshield T-9 has received rave reviews for its ability to protect engines, electrical systems, tools, machinery, firearms, trailers, bicycles, etc. It's used all over the world, particularly in marine applications where a saltwater environment is exceptionally tough on metal.

From "Construction Notes" Falco Builders Letter, March 1989

If anyone is looking for small "tee"-type blind nuts, your local hobby shop will have them in sizes ranging from 2-56 to 8-32 (usually by Du-Bro), cadmium plated. We modelers use these all the time for permanent inaccessible installations such as motor mounts, landing gear blocks, tec. I used some 4-40's to make a flush 2"x2" opening under the trim tab cable clamp, with a 1/16" Lexan cover. When it comes time to fly, I'll try to scare up some stainless steel 4-40 flat-head screws to keep the molecules happy and the rust away. These tee-nuts would also work well on the tail light fixture.

Craig Bransfield

November 1998

One of the items on page 24-26 of the construction manual calls for the usage of a 'bedding compound' to prevent moisture from getting under metal fittings. Can you give me reference to a specific product that you can recommend. I hope its something I can get at my local auto supply store.


We mentioned a number of possible moisture-excluding substances. They are not important, but generally you want something that will stay somewhat rubbery. Silicon rubber compound, caulking compound, polysulfide rubber, zinc chromate paste... all would do.


From "Construction Notes" Falco Builders Letter, December 1989

If anybody wonders-as I did-where on earth to acquire fine brass screening to use as anti-mud-dauber barriers inside the underwing fuel-vent outlets, go buy a $1.89 kitchen-faucet aerator. Inside the kind my hardware store carries is a fine-mesh brass screen-actually, and propitiously, two such screens, one for each fuel vent-that's about 3/4" in diameter and works just fine when carefully bent, cut and crimped to shape around the shank of a drill-bit that fits loosely inside the vent tube.

Steve Wilkinson

From "Construction Notes" Falco Builders Letter, December 1989

Our fax machine has been busy with messages to and from Stuart Gane and Neville Langrick in England concerning an alloy of aluminum extrusions that Doncaster Sailplanes supplied. The alloys supplied have roughly the same strength as 2024-T4 aluminum and are acceptable for use as hinges and many other parts of the airplane. Some of these extrusions do not have the usual fillet radius in the angle; instead, there's a sharp corner. This is not a good thing at all, since a short corner creates what engineers call a "stress riser"-a concentration of stress at the sharp corner. While the part may be of roughly the same strength of a part that has the fillet radius, the fatigue life of the part will be dramatically lower.

For many parts, such as the aileron and flap hinges, the extrusions have been used only at the base of the hinge. Because the base of the hinge is so wide, the loads are spread over a large area and these sharp-cornered extrusions do not give us great concern-although a regular inspection for cracks is a good idea. We do have great concern about using such extrusions for the P/N 720 fittings that support the engine mount. These extrusions have a substantial bending load and the fittings are extremely critical to the integrity of the airplane. Please do not use these no-fillet extrusions for the P/N 720 fittings. Neville Langrick has such fittings on his Falco and is in the process of replacing them.

From "Construction Notes" Falco Builders Letter, March 1988

The Environmental Protection Agency is cracking down hard on metal plating companies, particularly on cadmium plating. You may be told by your platers that the EPA has required them to use zinc plating now in its place. Don't believe it. What is happening is that the EPA is requiring the plating companies to clean the liquids that they discharge. In the case of cadmium plating, this requires million-dollar equipment. The result is that only the largest companies are still in the cadmium plating business.

(In Fort Worth, the EPA actually poured concrete into the drains of one of the largest platers because of the presence of cadmium plating effluent in the sewer system. It turned out to be the fault of another smaller company in the next block, so the EPA has to dig it all out.)

You really do have to use cadmium plating. The reason is that cadmium is very close to aluminum on the electrolytic scale. Zinc would just cause problems. I should explain that zinc chromate primer uses the properties of the electrolytic scale to a different purpose. Zinc has a corrosion death-wish, so when corrosion is going to occur, it happens to zinc first, and not to the other metals that are around. Zinc protects by volunteering to die. That's why we use it in paint and why boat owners attach pieces of zinc to their hulls.

From "Construction Notes" Falco Builders Letter, March 1988

Now that everyone is using the aluminum inserts for the screws for access panels, we have a new problem-how to insure that the screws don't rattle out. My suggestion is to dip the threads of the screws in silicone rubber compound before you install them. The rubber hardens enough to keep the screws flying in the same formation with your plane, but you can still get them out with ease. My friend Parke Smith had nothing but trouble with the spinner screws on his CAP10 until he tried silicone rubber.

From "Construction Notes" Falco Builders Letter, December 1988

A Falco builder asked me a question the other day about a small detail of reading the plans. I realized that I had never explained this point and so will do so now. Some of our drawings have part numbers and assembly numbers shown inside circles-or "balloons" as they are sometimes called. A good example of this is Drawing No. 864-sheet GG48b. The flat plate is the "-2" part, and the tubes are the "-3" part.

Balloons for part numbers are used only for the part numbers that originate on that specific drawing. Thus the -2 part has the full part number description of P/N 864-2. This is a convention that is used on almost all drawings in the U.S. and one that I have adopted wherever it was appropriate. On the wing and tail ribs I use "dash numbers" to identify the part numbers which apply to individual ribs.

Sometimes it is necessary to show an assembly number, which is just a part number that applies to a part made from a number of other parts. An assembly number is indicated by surrounding the dash number with a double circle-see sheet E2e for an example. For the most part, I have avoided that sort of thing since our drawings are very clear to our builders and showing the every last piece as a part number might be technically correct, but you would find it confusing. For example, a rib is made up of lots of pieces of wood. To be correct, every capstrip, brace and gusset should have a separate part number, and the rib should have an assembly number.

From "Construction Notes" Falco Builders Letter, December 1988

I also hear Falco builders worrying about the weight and balance before they fly the airplane for the first time. Inevitably the center of gravity ends up rather far forward. As it turns out, the Flight Manual does not specify the empty weight center of gravity, only the center of gravity in flight. Obviously that it all that is important. It is your responsibility to load the aircraft with pilot, passenger and fuel so that the center of gravity is within the specified limits.

Almost all Falcos end up with their center of gravity such that in certain loading conditions, the center of gravity will exceed the forward limit. The forward center of gravity is determined by the ability of the airplane to do a full stall in ground effect with the gear and flaps down. It is a measure of elevator power, and it is not desirable to have an airplane that will nose down on you while you are landing. You should always be able to hold the nose off while landing.

When I was in Italy in 1982, I made a point to ask Mr. Frati about the center of gravity limits. The Falco was originally built with a smaller engine and a lightweight wooden propeller. The Series I and II Falcos also had slightly smaller tail surfaces-although I don't know the measurements. With the Series III Falcos, the tail areas were increased and then on the Series IV, a 160 hp engine and constant speed propeller were installed.

On all other aircraft that I was familiar with, such a growth in engine and propeller weight always required the addition of some weight to the tail of the plane. I had seen nothing in the Falco drawings or flight manual to indicate any adjustment. I asked Mr. Frati about this. He said that they had originally thought that they would have to add some weight to the tail, but found after flying the airplane that it was not necessary.

I've never spent any time exploring the forward CG of the Falco, in part because my Falco has the lighter fixed-pitch propeller. But I have a suspicion that the forward CG limit might be moved forward slightly since the Falco has a very powerful elevator. This can only be determined by flight testing, but I doubt that anyone will bother. My experience is that Falco builders worry themselves sick about this until they fly the plane. Then they get a feel for the handling of the plane and just go fly it.

But if any of you out there have any opinions on this, or have done any testing, I'd love to hear what you have to say. Has anyone had the nose fall through on landing with a forward CG?

From "Construction Notes" Falco Builders Letter, March 1987

It seems almost too obvious to mention, but I've seen some pretty complex glued, screwed and tattooed devices suggested for use in the balancing of control surfaces. Yet the simplest things to use-since they're cheap, can be erected and disassembled any number of times in seconds, take virtually no work to make and can even be applied to use as shelf brackets when you're done building your Falco-are exactly that: shelf brackets.

Buy four 89-cent sheetmetal shelf brackets in a hardware store-the eight-inch size is just right to get the aileron high enough to insert a typical postal scale under the trailing edge-and drill out the end-most screwholes to 1/4 inch. Clamp two facing pairs to the edge of your workbench with C-clamps, one pair at each end of the control surface with a gap of about an inch between them, and suspend the aileron from its own mounting bolts run through each pair of shelf brackets. You can align them nearly perfectly by eye, and though there might be an extra gram of drag hear or there if you have bad eyes, we are dealing with a ±2.5-ounce criterion here, after all.

Steve Wilkinson

From "Cecil's Speed Mod" Falco Builders Letter, September 1997

In addition to this go-fast item, you may be interested in a conversation I had recently with a Glasair owner. Seems he has installed a mylar tape, that sailplane owners use, on his flap gap, stabilizer-elevator gap and fin-rudder gap. He claimed a 7-8 knot increase in speed as a result. Of course, we can't use this type of seal for the flaps on the Falco, but seals for the other gaps should be feasible.

The mylar tape is in two parts. One is a longitudinally curved piece (adhesive backed) that is attached to the stabilizer and fin and overlaps the elevator and rudder. The other piece attaches to the elevator and rudder on which the first piece slides as the control surface is moved.

The tape is available from Knauff and Grove Soaring Supplies, 3523 South Eagle Valley Road, Julian, PA 16844. It's not cheap-a 22mm x 15 meter roll sells for $45.00 and the 38mm x 15 meter is $85.00. However, 15 meters should be enough to do four Falcos. As far as I know, it only comes in white. So, too bad for those of you that own one of "them red Eyetalian things".

From "Mailbox" Falco Builders Letter, December 1999

The Forest Products Lab has published a new book in 1999 called the "Wood handbook--Wood as an Engineering Material". The whole thing is availble to be downloaded chapter by chapter in PDF format from their web page at Chapter 9, "Adhesive Bonding of Wood Materials" has some interesting information on adhesives, preparing surfaces, and bonding wood.

Ron Strong

From "Construction Notes" Falco Builders Letter, December 2001

Neil Aitkenhead asks, "The various bronze bushes in the brackets are specified in the drawings as SAE 660. To date the various engineers I have spoken to cannot identify this grade of bronze. Could you please advise an alternative spec. or the composition of the metal recommended?

SAE 660 bronze is a very common bronze alloy here, probably the most commonly-used commercial type. I did a search for SAE 660 at, and within seconds found my way to a page described it as a leaded bronze particularly suited to the manufacture of bearings and suitable for all free running medium to heavy load bearings. It also gave the composition as well.-Scoti

From "Construction Notes" Falco Builders Letter, March/June 2002

I am considering making most of the simple aluminum fittings. I have easier access to 7075 T0 material. Can I substitute this and following machining, heat treat to T3?

Mike Aherne

It's okay to substitute any alloy that's stronger than the specified alloy. We use 7075 for many of our parts. However, I don't know a thing about heat-treating 7075 since we buy it only in the heat-treated condition. This is something your heat-treater can explain to you.