SMALLnet post321

Just a short note to let all the E-power fliers on the list who live in the Northern Virginia, Maryland area know that the Loudoun County Aeromodelers (LCAA) will be having an Electric Fly In at our club field south of Leesburg, Virginia on July 22, 2000.

This is strictly a low-key, come-out-and-fly or swap-stories kind of gathering. No competition (except perhaps for the biggest story!) -- just a gathering of the local E-power fliers to fly and have fun. There will be a modest entry fee of $3 which will include lunch. We are also planning on having a small raffle of some e-power items. If you have the day free, please plan to attend. Please also spread the word to all your E-flier buddies who may be interested. Please contact me at < > for information. Thanks all, and hope to see you!

This is a response (alas a tad late!!) to Charlie Fries' dilemma in SMALLnet # 317. For Charlie, and others like myself seeking some compass points in the electric mire:

Roy Clough's friend with the Gee Bee Z take-off problems should contact the plane's designer. (I think that's me.) If it is my design, I would be happy to help him get the thing into the air.

BTW, Joe's answer is correct: keep the tail down throughout the takeoff. The airplane won't do a half loop; it will just fly away when it is ready. Land the same way - 3 point.

Since I'm here, I would like to mention that I have a new web address: < http://www.adrianpage.com >. Adrian

I was awaiting the arrival of another Ace Simple CAP 21 that I managed to source in the USA. Excited at the prospect of upgrading from a trusty 2-year-old .061 Norvel to a new .074 Revlite, I called the local shop (and then many more) to find one of these.

The answer from most was no, and those who knew more muttered of no more Norvel distribution in the U.K. Will I have to order the engine from the USA as well? Does the .074 Revlite have a more efficient exhaust than the .061? (A gasket would be nice.)

The joining of an .074 Revlite to an Ace Simple CAP 21 should make an already spectacular plane even more so.

I will confirm that the original all-balsa Dakota will thermal! I bought one after I saw it at one of the old hobby shows at the Los Angeles Shrine Auditorium. They had a small kid build it and fly it as U-control. As I remember I had a C*x .020 in the nose with an eye-dropper tank. Flew it several times at the Rose Bowl parking lot. Funny plane, lots of left thrust, but Joe, you knew what you were doing. Last I saw of it was still climbing, engine off in one of those big thermals they grow around the Arroyo Seco where the Rose Bowl is located.

Someplace in Pasadena is a Dakota rotting away, or maybe some kid found it. Hope so and that it got him into model building.

To satisfy the needs of clients and company requirements I have used all the recent incarnations of AutoCad, though I thought that program was overkill for my model design needs.

Jerry < > inquires (with respect to the quoted state- ment from a previous Posting that follows): "... most of the time I pre-assemble the wing spars in Buzzard Bombshell style...doing that guarantees that the dihedral in the completed wing will be as designed. [JW]"

Could you elaborate on this a little for those of us who have never built a Buzzard Bombshell? TIA Jerry

...Glad to: the Buzzard featured a polyhedral wing whose spars were pre-assembled over full-span layouts on the plan. There were two spars, both made from 3/16" balsa. I think the main spar was some- thing like 3/16" X 1" and the secondary (rear) spar 3/16" X 1/2".

Anyway, though modern "replica kits" MAY differ, the original Buzzard Bombshell spar junctures were tapered "scarf joints" with no reinforcements.

After the spars were pre-assembled, the wing went together the usual way, except that when each panel was being constructed the other parts of the wing stuck up above the building board. One had to be careful not to snag his sleeves on those while working on the ribs and leading & trailing edges of the panel "on the board"...

Another popular line of model kits that featured pre-assembled wing spars was the Jasco series of Thermic towline gliders. Even the mighty "100" called for the spars (8 1/2 feet long!) to be pre-built, with their three dihedral junctions carefully scarf-jointed together over the spar construction layout on the plan -- before adding the ribs and LE & TE pieces to complete the wing.

Building model wings that way was unconventional -- but it worked so well for me, I've used similar methods in my own model designs ever since. [JW]

To SMALLnet flyers complaining about a shortage of small wetengine starters: I have one that I picked up at a garage sale a few years ago. It is the Mini starter for planes, cars, etc. sold by ASTRO FLIGHT, God only knows how long ago. It is small, plugs into your flight box, and works great on small engines. I contacted ASTRO about this starter and they said they did not market it any more.

I had to replace the motor with a 550 electric motor, but it still works fine. I suggest we encourage ASTRO to reconsider making them available again, as I believe there is a market for them.

Re Joe Wagner's submission on wing spars, I agree 100%. Plywood dihedral braces are slovenly and inefficient. I, too, like to use hard balsa or small pieces of hardwood for joiners. Coffee sticks can be sanded thinner if necessary, and are A-1. Spar joiner strips should be straight-line tapered in thickness from their center to both ends. This cuts their weight in half, with zero increase in stress.

Tapering both sides of the spar to nearly a knife edge at the joint, and fitting matching tapered hard-balsa splicers comes a step closer to minimal-weight perfection. The glue also sticks much better since the end grain is partly exposed.

The grim reality is that any spar joint, no matter how efficient, increases weight. I particularly dislike placing a joint at the point of maximum spar load, the wing centerline. I prefer to assemble the wing panels without spars, and join them. Then, I install a tapered spar that runs, unbroken, right through the joint. If dihedral is needed, I simply steam and bend the spars, using a hardwood jig and C-clamps, if necessary.

A lot of weight can be saved by using a wing spar that is tapered so that it has constant stress along its length. The bending moment at only 1/4 of the way from root to tip is just over half that at the root, for a constant chord wing. Halfway out, it is only 1/4 of that at the center, so only 1/4 as much spar cross-section is needed. As you go further out on the panel, bending moment on the wing decreases very rapidly. At 3/4 of the way out toward the tip, the bending moment is only 1/16 that at the center, but wing-tip ground contact accidents are likely to break a spar that is slimmed down that much.

A wise modeler uses a larger section spar out near the tip than would be indicated by consideration of aerodynamic load alone.

An ideally tapered spar can come out at about 1/3rd the weight of a constant-section spar. You do have to be careful to use enough shear web between the upper and lower members of the spar. Use a stronger web nearer the center where shear load is heaviest.

For a further refinement, a very hefty weight reduction can be achieved by using tapered wings. 40% taper will reduce spar weight by another 25% or so, and I find that tapered wings just fly nicer, with less aileron yaw.

Incidentally, our MicroLite iron-on Mylar covering is now available in 16 colors. It is also now available in 3 micron thickness, but in fewer colors. The 3 micron version weighs about 0.02 ounces/square foot (6 grams/square meter). I am also testing a fuel-proof version of the 5 micron material. Carl Risteen

...Two other SMALLsters sent in comments on the stress properties of tapered vs. constant-size wing spars. They questioned my statement that a tapered spar was "twice as strong". I should have gone into more detail in my brief explanation, making it clear that I was talking about load-carrying capacity under FLIGHT loads, rather than just static-load conditions.

What causes catastrophic failure at a wing root is a combination of static and dynamic stresses; primarily tension in the "extreme fiber" of the spar (from "bending" loads) and shear across the spar's cross-section.

With a spar of uniform tip-to-tip cross-section in a cantilever wing, this combination of stresses maximizes at the "fixed anchor". In an optimally-tapered spar, the stresses are distributed along its length. No "stress concentration" exists at the root, and IN PRACTICE a flexible wing can withstand a much greater load without failure than a rigid one made from the same materials.

I haven't time to provide a detailed analysis; but bear in mind that loads on a wing spar sufficiently large to cause failure in flight are DYNAMIC loads.

As a rough comparison: each time a Dutchman wearing wooden shoes takes a step, the sole of one shoe is subjected to an instantaneous dynamic compression load (i.e. impact) equal to double the Dutchman's weight. That load consists of his weight applied to the top of the sole PLUS the reactive counter-pressure exerted by the sidewalk.

But put that same Dutchman in a pair of cushion-sole running shoes, and the dynamic loading on the bottom of his feet is greatly reduced. The "instantaneous" load- (and stress-)doubling element of rigid shoes is effectively eliminated by the energy-absorbing capacity of soft soles. Something much like that occurs in flexing "mechanical structures", such as fishing poles, diving boards -- and wing spars.

One further point: When I say "double" (or "twice", or "half" etc.) I do NOT mean a factor of 2.00000. I mean "somewhere in the neighborhood of two". E.g. to me a model engine that costs $47 (including sales tax) is half the price of one that retails for $105. [JW]

I did a similar thing last winter but I used the plans to make an all-foam model. Not having to glue all those pieces of wood together, my foam model went together very fast. The fuselage was Styrofoam and the wing was made from blue foam.

The only wood in the wing was 1/8-inch square stringer across the top of the wing to handle compressive loads. A narrow strip of fiber tape was used under the wing. Epoxy and shipping tape were used to bond the wings together; otherwise no reinforcement was used. The leading and trailing edges used only heavy tissue paper with Elmer's glue. The wing handled abuse very well.

The only wood in the foam fuselage was 1/16 balsa for the elevator and rudder and some thin plywood for the firewall. To smooth out the fuselage, it was covered with tissue paper and Elmer's glue. In hindsight I should have used some stringers in the fuselage because it tended to snap in half in a crash from a bad launch.

Watered-down Latex house paint was used to finish the BF-109. The paint was air-brushed just enough to color the airframe. Colored copies of the decals were glued on.

I used an FMA receiver and S-80 servos, with a 110 mAh battery pack. To save weight on the power switch I used a jumper wire. The servos and receiver were just stuck in hollowed out sections of foam. I only used two channels instead of Steve's three. I tried to cut corners on weight everywhere I could. This allowed my airplane to weigh only 7 ounces.

My BF-109 was powered with a Cox TD .020. With the TD .020, it seemed over-powered. A throttle would have helped. Flight times were around 2 minutes. I was working on a larger fuel tank, but the model had a bad crash and I put it on the shelf.

Overall it was a cheap project and I still have the Guillow BF-109 kit. I had successful flights and some bad crashes on launch. I guess with Steve's success I will have to dig it out and rebuild it. I do have a sleeve-throttle Pee Wee .020 engine that needs an airplane.

...Over the weekend I re-read Frank Zaic's compendium of old-time model airplane material published in "American Boy" magazine between WW1 and 1934. I was surprised to note, in reading the advertisements included, that RTF model airplanes are far from being a recent devel- opment. As early as 1930 several companies offered pre-built, ready-to-fly models. They couldn't have been truly popular, though: I started building model airplanes myself only 5 years later, and can't recall ever seeing any of these early R-T-F's, either for sale or being flown.

No, I'm not referring to craft such as the American Junior all-balsa gliders and rubber-powered planes; nor to similar products issued by "Ace Whitman" etc. I mean built-up-and-covered-with-tissue full- fuselage model airplanes, with wingspans from 18" on up.

I hope to see you at Maumelle! [JW]

SMALLnet Posting post321