Twin Boom (oz2953)

About this Plan

Twin Boom. Free flight gas model. Air Trails, Dec 1943. Gas powered 96in span FF.

Quote: "Twin Boom by AR Bertill.

This unusual Class C gassie, developed from a series of simiar designs, has averaged over three minutes on a fifteen second motor run for six consecutive non-thermal flights.

Here's a streamlined soarer of unique design that's tops in all kinds of competition. It is the fourth in a series of boom jobs deisgned and built by the author in the last three years.

The original design, first conceived in the winter of 1940, incorporated a straight gull-wing centre section and used an upright Dennymite for power. This model had exceptionally fine performance qualities and was flown in several contests around Minneapolis. This first model, however, did not have a chance to prove itself a contest winner as it was demolished when it crashed into some high tension wires and plunged into a concrete highway.

The second in the series was built in the sumer of 1941, and with minor changes mounted two Ohlsson 23's one in the front of each boom. This model was tested and flown in several contests that summer and proved quite successful. This model was discontinued because of the high cost of losing both motors in case the model should be lost.

The third in the series had several minor revisions, including a curved gull center section and a longer span. It mounted an inverted Ohlsson 60 in the streamlined pod. An attempt was made to incorporate flaps as de-thermalizers. This, however, proved unsuccessful because when they extended, the lift coefficient of the wing was changed, which resulted in a very violent stall.

This model is the one shown in the pictures. It had an exceptionally flat glide which has been measured several times at 16.1. This is undoubtedly due to the high aspect ratio of the wing, the aspect ratio being 11.6 to 1. This is exceptionally high for a contest ship, and at first some difficulty was encountered in building the joints between the outboard and the center-section wing panels strong enough. However, the method shown in the plans has proved highly successful and has never failed the author yet.

The latest of the ships in this line was designed last spring, and the only change from the previous one was that a Super Cyclone was used in place of the Ohlsson 60. The same tail surfaces and out-board wing panels were used on all four ships.

The design of the boom job is unique in that it incorporates a very high-lift tail section with the center of gravity located quite far back on the chord of the wing. The stabilizer has built-in dihedral to raise it out of the downwash of the wing. The airfoil used in the stabilizer is a Grant X-8 mounted at a negative angle of incidence. The airfoil used in the wing is the conventional Goldberg airfoil which has proved itself so successful in the Zipper (oz387) and other ships of that line.

The idea behind the utilization of these two airfoils was that the Goldberg, being a high-lift airfoil at relatively high speeds, would lose its lift when the model approached a stalled condition sooner than the Grant X-8, which is a high-lift airfoil at relatively slow speeds. This would eliminate extreme stalls, because when the model nosed up sharply and slowed down, the wing would lose most of its lift, while the stabilizer would not. This design angle was proved in all four of the ships in this series, as none of them have ever had a tendency to stall with the ensuing steep dive following. the stall. Instead, if the model begins a stall because of faulty adjustment, the nose slowly drops until the glide angle and speed have been restored. This happens with but little loss in altitude.

The thrust line and the chord line of the wing are co-incident. The center of gravity is located two inches forward of the trailing edge of the wing. The model should be test glided and adjustments made until a fairly flat glide is obtained. Power flights should then be tried with the motor idling and final adjustments on the glide made. The power should be increased slowly on succeeding flights until full power is reached. The desirable climb is a wide right circle with the ship going up at an angle of about forty degrees. The glide should he flat, with the. booms in a horizontal position. The glide circle is about three hundred feet in diameter.

The weight of the completed model should be about sixty-four ounces, which gives it a wing loading' of 11.6 ounces per square foot. The author's model averaged three minutes and fifteen seconds on a fifteen-second motor run for six successive flights with no thermals. "

Quote: "The article specifies the Goldberg Zipper and Grant X-8 airfoils. I have included these airfoils taken from other plans. They are not drawn to scale."

Supplementary file notes

Article pages, text and pics, thanks to BillW.

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