Author: Bob Angel
Edition: Model Aviation - 2010/06
Page Numbers: 128,129
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Old-Timers - 2010/06

Bob Angel — [email protected]

Loss of undercamber

Aerodynamic explanation

In the February column I requested comments from aerodynamicists regarding the apparent sudden tail‑heaviness and control difficulties that occurred when a model's covering released from the undercambered portion of the wing. Thanks to Edmund (Ned) Smith, Alan Brown, Basil Cooper, and Dick Fischer for their responses.

The four were in basic agreement, and their answers differed only in the amount of detail. I had a rough idea of what had happened, but I wanted to keep it simple and accurate while sounding like an aerodynamicist myself by tossing in a proper aerodynamic term or two. Following is a condensed version of their composite answer.

A line drawn from the leading edge (LE) to the trailing edge (TE) of an airfoil and centered between the top and bottom surfaces is referred to as a "mean camber line." An undercambered airfoil is one with a high degree of camber, or downward curvature in that line.

Most airfoils have a "pitching moment," or tendency to pitch up or down, pivoting at roughly the one‑quarter chord point (about 25% back from the LE). Large or highly cambered airfoils (big downward curvature) have a stronger tendency to pitch down. Symmetrical airfoils (flat camber) have a zero‑pitch tendency.

An aircraft nose's down‑pitching tendency is normally kept in balance by downward pressure from the stabilizer/elevator assembly, which is set at a slight negative incidence relative to the wing. In this case, when the airplane's undercamber covering came loose, the airfoil became essentially a flat‑bottomed wing, having less camber and therefore less downward pitching tendency.

That allowed the nose to pitch up and stall. Stalls and their recovery cause oscillation, resulting in control difficulty.

Also, in that case, the pitch up during each stall probably pressed the loose covering back into at least a partial undercamber. That resulted in an even more pronounced dropping of the nose during stall recovery. A free‑flight (FF) model would continue to pitch up and down all the way to the ground. An RC airplane could fare either better or worse, depending on pilot skill and the ability to time elevator movement with the pitching. By trying to correct, the RC pilot could easily get out of phase with the oscillations, making the pitching more severe.

SAM hot rods and performance

The hot rods of SAM (Society of Antique Modelers) are the RC designs—and in particular, those competing in the limited‑engine‑run events. They use either the strongest‑running old spark‑ignition engines or strong‑running modern glow (nitro) engines. Those are basically racing engines, which are usually run unmuffled and without RC carburetors at top speed. The high power‑to‑weight ratio results in takeoff rolls that vary from roughly 1 inch to 3 feet, followed by a nearly vertical climb.

The models can climb vertically, but a climb angle of slightly less than 90° allows the wing to do a little work. Otherwise, the wing would be trying to "lift" the airplane off of its vertical course.

This type of performance was uncharacteristic of the original designs, which greatly disturbs some of the purists in the Old‑Timer (OT) movement. But there's a positive side. Many of the younger people (or young at heart) who come into OT flying are at first drawn in by the hot‑rod aspect of the spectacular performances. Once they have built and flown a few OT models, they may branch out to add some of the more "civilized" classes such as Texaco, which emphasizes fuel economy. Those events more closely re‑create the original sights, sounds, and gentle climbout of the majestic old airplanes.

We each enjoy reliving the past on our own terms. I've heard many older FF fliers describe, in nostalgic terms, an aircraft "that climbed straight up." Today that's a repeatable performance. And some of us can imagine what heroes we might have been in the 1930s with such a flight.

Fast or slow, I enjoy both types of flying. But the best part comes after engine shutdown when you seek out and quietly "dance with the thermals."

An accompanying photo is of Pete Samuelsen's Anderson Pylon design, which fits into the hot‑rod category. It's a class C model powered by a Nelson .60 engine burning 60% nitro. It can usually perform nine‑minute max flights with an engine run limited to just 18 seconds.

Balsa wood is still the material of choice for those of us who build our models

But we're in competition with heavy industry for the best and lightest grades.

During World War II, large life rafts were made from balsa. Today, oil tankers contain balsa as insulation between double‑walled hulls. And huge wind‑turbine blades are now using balsa in their cores. Modelers might have been the dominant users at one time, but not today.

As a result, much of our sheet balsa is no longer one piece; it is made from smoothly joined smaller sections. Commercial suppliers do an excellent job of connecting the pieces, and you have to look closely to see the joints.

However, many 3‑foot by 36‑inch sheets seem to have at least one joint, and it runs parallel to the grain. This is easier to notice when you try to cut across that grain with a hobby knife. The glue leaves a hard spot in the grain.

When cutting diagonally to the grain, the slight shift in grain direction can throw your knife off course. So it pays to develop an "eye" for those joints and plan your cuts accordingly.

Bob Holman's Plans

Bob Holman's Plans has recently released a simple, inexpensive little aid for those who build with balsa. Bob is well known for supplying scale and other plans to the modeling community. Perhaps less known is the fact that he and others in his family are active OT model builders and fliers.

Bob laser‑cuts precision 30°‑60°‑90° plywood building triangles. They have several uses, but the most obvious is for aligning fuselage sides. These tools can be stood vertically alongside a fuselage during construction to encourage making square assemblies.

An interlocking crosspiece at the base allows a triangle to either stand alone or be pinned to a building board or held in place with magnets. Bob sells these 5‑1/2‑inch‑long triangles in a packet of 10 for $5, plus $2 shipping.

In the December 2009 column I mentioned the Frank Zaic Year Books as a great source of OT information. Later I received a nice note from Bill Hannan, who wrote the discontinued Model Builder magazine's "Hannan's Hangar" column for almost 25 years.

Bill and Joan Hannan run a small business called Hannan's Runway, which sells an assortment of books aimed primarily at those who still construct their own models. The Hannans were friends with the Zaics and still keep in touch with Frank's wife, Carmen.

Hannan's Runway stocks all of the Zaic books (roughly 10) that are still in print. The company also sells several other authors' collections that cover a variety of specialized subjects about building models. This includes 10 books that Bill has written throughout the years, and they are modestly priced at approximately $12 each.

Postage is reasonable, starting at $3.75 for any size order (media rate), if you're not in a hurry. If you're a builder, you might find a visit to the Hannans' website interesting. It includes a photo gallery of quirky little models that Bill has enjoyed.

SSIGNCO transistorized ignition circuit

In the April column I mentioned Larry Davidson's new transistorized ignition circuit, which he designated "SSIGNCO."

When an engine stops with the ignition points closed, a coil can be burned out or batteries damaged if the current isn't shut off within a reasonable time. Larry's unit automatically cuts off current after two seconds of inactivity with the points closed. It reactivates immediately when the points are reopened.

The engine‑run timer in most FF models shuts off the circuit, so burnout isn't a problem unless the timer fails. But for Texaco designs, both FF and RC, the power plant normally stops on its own after exhausting the fuel supply.

This can be a problem for the FF airplane, since it is seldom recovered quickly after the engine quits. But it can also be a problem for an RC aircraft if the pilot forgets to switch off ignition after the Texaco run, or in case of an early flameout during a timed‑engine‑run event. (It happens!)

I wrote the report on Larry's unit from his press release, and unfortunately I made an untrue assumption. So an apology and a correction are in order.

I mentioned that the unit would replace a servo and microswitch, which it does not. Later I received a test unit and found that an auxiliary arming or on/off switch of some kind is needed for either FF or RC. A radio‑operated switch would be unnecessary for the FF flier.

The unit I received for testing did everything as specified. It provided a hot, continuous spark; it shut down after close to two seconds of point closure; and it fired back up immediately upon reactivating point movement.

The on/off switch for RC use requires either a servo/microswitch combination or an electronic (E) switch of some type.

Since E switches have sometimes caused RF (radio frequency) interference when used with spark ignition, I tested for that possibility using an old 72 MHz AM radio. I wired one of Marvin Stern's ES‑1 switching units between the receiver's throttle port and Larry's unit.

The spark was steady, and the unit switched off and on as it should have with point position. Two servos also plugged into the receiver didn't twitch, despite the batch of loose hookup wires.

One should always do an engine‑running range check before flying, to make sure there isn't some gremlin in a particular aircraft installation.

MA

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.