Trim blues

I suspect I might be genetically unable to trim model gliders.

I tried and tried with the little homebrew FF glider (see previous post). I moved the CG forward then experimented with all possible variations of wing and tail incidences. I was not able to get two consecutive consistent flights: with what seemed like the exact same settings, one flight would be lovely, floating down to a nice pretty flare, and the next twenty would either climb and stall, or dive.

Perhaps it's my launching method. I mean, I can't really say I was consistent in my (hand) launches, and the six feet from my hand to the ground does not give the glider lots of room to settle into a nice trimmed attitude and speed. But still.

I suspect I should just buy some RTF (better than a Guillows toy, but not necessarily the fanciest) and follow the instructions (especially about CG settings) religiously, hoping to build experience in how a properly constructed glider should feel. Or maybe I should join the AMA and get someone to teach me. Or something.

Quick FF model glider

This weekend, my son Aden and I made this quick free flight model glider:

We started with the wings and tail of a HobbyZone Firebird Phantom park flyer. The fuselage is made of two strips of 1/8 x 1/2 balsa stick sheeted with 1/32 on either side. We didn't bother to draw a detailed design. First, we laid the top and bottom strips in the curved shape we wanted with some guide blocks on a building board, and added some cross pieces in between. Then we added one side sheet, removed from the board, trimmed the sheet, and cut and inserted a noseblock. Finally, we added the other side and trimmed, and sanded the corners.

(I learned this fuselage construction method from an issue of what must have been Radio Modeler magazine back in the 1970s. And it's been years since I've done any model airplane stuff. Let's see how far we get this time....)

When first test flown, the CG was too far back: it had little or no tendency to adjust its pitch attitude as a function of its speed, and would either dive and crash, or climb and stall. Part of the problem is that we used steel ball bearings, rather than lead shot, for the nose weight, and we ran out of space in the nose compartment.

We are still testing. Stay tuned for more details.

By the way: notice the price of the whole Firebird Phantom is only about $50, while the wings and tail alone cost a whopping $17.50. Whassapwiddat? If they can afford to sell the whole thing -- motor, RC, fuselage and everything -- for fifty bucks, then these two parts can't be costing them more than a buck each to make.

So, note to self: next time, build up the tail surfaces from balsa and Monokote. The hinged elevators on the store-bought foam surfaces are a useless pain anyway, and the tail surfaces don't need to be cambered so they should be pretty easy to make.

B&M IQ Fly Senso Plus dissection

I've wanted a Sturmey Archer Dynohub on my Raleigh Chopper since the mid 1970s. Now, I've finally decided to treat myself to a hub dynamo.

After some research -- and much resistance of my lust for the multi-centibuck machined aluminum alternatives -- I bought a B&M Lumotec IQ Fly Plus Senso headlight. Here are a couple of exterior photos:


In the rear view, note the white spot beside the switch: that is the opening for the light sensor. Note also the two pairs of spade terminals, and the built-in power wire. Here is one more shot showing how the emitter faces backward and downward against the reflector:


After some messing around, I decided that this must be the effective circuit for the thing:


Specifically: the switch is single-pole on the power side; the standlight capacitor does not appear to drain back into the spade terminals when the power is turned off; and the spade terminals bypass the switch.

This last fact is highly annoying. It means that, to use a hub dynamo, you must use their supplied wire. Which happens to be too short to work with my recumbent bicycle. But the wire is built in and cannot easily be replaced -- either to make it longer or, as is likely down the line, to fix it if it starts to wear from road vibration.

What I really wanted to do was to cut the provided cord short and attach connectors of some sort, then use these to attach my own wiring. But I was worried about what would happen if I messed up and needed to redo the connection. Should I cut the cord longer than I would like just in case? Even more annoying!

It's usually easy to convince me to take something apart; this was the excuse I was waiting for.

To open the thing, I pried off the front cap (which holds the reflector) with a flat screwdriver. This does mar the finish a bit and also slightly damages the clips that hold the cap in place, and will I'm sure Void Your Warranty™, so proceed at your own risk:


Here is a view of the opened unit looking at the circuit board from the front:


The circuit board is retained inside the back case by the two small screws in the picture. Note that B&M did not bother to put strain relief in the power cord. Tsk tsk. One fortunate fact, though, is that the points where the cord is soldered onto the circuit board are pretty easily accessible should the cord need to be replaced. Happiness. But while we're having fun taking things apart, why stop here...?

This is the back of the circuit board, revealing the standlight capacitor:


As you can see, the light sensor on the board is the little square white dot to the left of the switch. Finally, here are two shots of the innards of the actual beam unit:


In the second image, you can see the emitter; the formed aluminum heatsink; the reflector unit; and a retainer spring that holds the heatsink against the emitter when assembled. I expected thermal grease between emitter and heatsink, but there was none.

Simple bending brake

This is a simple 2' bending brake I built recently. The goal is to validate the overall design approach in preparation for building a 4' version.


The leaf and bed are 1x4 oak from Home Depot. The bending bar is made of two pieces of 1x2 hard maple from a local lumberyard, topped with a piece of 1x4 pine, and a 1/8" radius is created with a beading router bit. The bed is screwed down to a large 2x6 for stability; the latter is clamped my workbench during use. (The reason the bending bar is in several pieces is because I didn't want to buy a wide -- read, expensive -- single piece of maple. I'll splurge for the next brake.)

The construction is pretty obvious. I ensured proper alignment of the edges of the leaf and bed with the edge of the piano hinge by "match drilling" each side separately while fixtured to a base, like this:


For the #8 wood screws I used, I match drilled using an 11/64" drill for the unthreaded portion, ensuring reproducible alignment. I also drilled a deeper, pilot hole for the threaded portion. (Note that, in so doing, I ignored the pre-drilled holes in the hinge.)

One problem I encountered -- perhaps because my pilot holes were too small, or not perfectly centered, or whatever -- is that the (admittedly, small and questionable quality) wood screws would torque off at the junction between the threaded and unthreaded portions.

I added setback stops to provide a repeatable location for the bending bar:


I aligned the stops by putting some scrap into the brake, raising the leaf to the angle I expected I would need to get a 90 degree bend taking springback into account, then snugging the bending bar forward evenly:


This is the bend line being set up for making a 2" wide channel. I figured out from some experimentation that my parts came out 1/32" undersize, so I needed to "steal" 1/64" or so from each flange, which is why my alignment is just a smidge to the left of the line here:


After bending, this is as far as I could go towards 90 degrees. This is due to inadequate leverage in my leaf -- I need to attach a handle:


But a few taps of a mallet put me all the way to a right angle. Note that the long 3/8" dia. lag bolts that you see pointing upwards and engaging the wingnuts are just about to get in the way of the channel if I bend it far enough. Also, you can't see this, but it's really pretty painful to tighten the wingnuts without mechanical assistance (hence the pliers you see), so one might as well just use regular nuts. In any case, the result is that, at one end, my channel is pretty exactly 2" wide:


But it's about 1/32" too small on the other end:


This is within spec for my (forgiving) uses, so I'm happy, though I will run some more metal through this to see how reproducible (or improveable) these results are.

The following are the inspirations for this brake:

1. Dave Clay's brake, made of steel angle sections; and
2. Murray Johnson's "Home Depot" (wood) bending brake.

The following are the things I would/will do differently next time:

1. More leverage for the bending leaf;
2. Make all 3 working surfaces (leaf, bed and bar) out of maple;
3. Use larger and more durable wood screws that won't torque off;
4. Make the bending bar out of one wider piece of maple;
5. Secure the bending bar with bolts tightened from the top, as with Dave Clay's brake (above).

Of Recreational Vehicles and Army Navy Hardware

Last Sunday, I visited my friend Paul Eastham, builder of an RV-9A aircraft, at his hangar at South County airport. We chatted about riveted aluminum, and went on a short trip to Watsonville for lunch. He very generously let me take the controls and boy, I tell ya, that was a blast! He is building a camera mount, so he also took the opportunity to teach me how to drive solid rivets, and I learned about rivet smileys. :) All in all, I had a great time.

As a parting gift, he gave me some leftover hardware (mostly AN3) to experiment with for my own projects.

Now, meet my son, Aden. He is a nut (so to speak) for AN hardware. It was like showing a bag of diamonds to a jewel thief. He had to have an RV-9A. It had to be made of "real, lightweight" aluminum just like the real thing, and it had to be made with AN bolts. These were, so to speak, the design constraints. Here is the result:



You may notice that it ended up being an RV-9 instead of an RV-9A. That's life, I guess. You start out trying to build one airplane, and you end up building the other. It just happens.

Curta calculator

Yesterday, at the Harvest Festival at Ardenwood Historic Farm, I saw an exhibit of antique surveying instruments. Among those was a Curta mechanical calculator. Fascinating little thing. I subsequently found out that there's a Curta simulator in Flash; that these things go for about a kilobuck on eBay; and that someone out there had the chutzpah to disassemble theirs.

Back in the early 1960s, my dad was studying at the École Nationale Supérieure de l'Aéronautique in Paris. My mother told me stories about him doing his homework late into the night, while she listened to the clicking of one of these things.

Fragment of bracket detail

I recently put together a fragment of this design just to get a feel for how things go together. This is also the first time I'm using the Tempo zinc oxide rattle can primer.


Notice that the bracket, made of 1/8" thick material, is not a complete "T" shape. This is because I just happened to have a thin strip of the stuff, so I cut whatever I could and worked with what I had. This Is Only A Test.

Notice also that I had trouble getting the primer to go on uniformly. It was scratch resistant on the sheet material, but seemed to easily de-bond from the 1/8" plate. I think the latter was because I didn't slap it on thick enough. Surface prep was to scuff with brown Scotch-Brite, wash with warm water and Dawn dish soap, dry, then apply the coating.

You might think these random pieces of stuff I make are useless. Not so! I'll have you know that this latest creation of mine was used as a scoop to rescue a crawfish from the neighborhood street. My wife tells me that the handle on the side was helpful.