Vintage RC Helicopters - Dan Johansson

Dan Johansson

Has a large collection of helicopters and below is his note concerning a purchase in 2009 of a Du-Bro Tristar.

Another example of DU-BRO helicopters is the Tri-Star; so named because it was available in three versions:

    * Scorpion (based on the full-size original from Rotorway)
    * Hughes 300 (Chris Joiner's refurbished items painted in colors Columbus police district)
    * Enstrom F-28 Shark (here a full size image of 1960)

A helicopters most important part is the rotor head and the Tri-Star is fixed pitch, rotates anti clockwise and is controlled only by the paddles (Hiller).  Because you cannot change the blades angle of attack, you are totally relying on the engine's ability to quickly change speed to keep or change altitude.  Because it is fixed pitch and also lacking a one way bearings, auto-rotation is not possible.
The rotor blades are bolted without rubber damper, so they can not flex in all directions only move sideways. The head is held in position by springs which dampens the rotor blades (teeter) though it is difficult to say if it actually helps, but guess it is better than nothing.  The rotor blades are sharply profiled with a flat bottom (lifting section) which limits the rotor speed. The attacking angle is fixed to approximately 8° which is kind is about right for a fixed pitch head.
The paddles are fairly broad and the paddle bar is very long and 4 mm thick; more suitable for a modern 60 helicopter.  Like the main blades they are designed as an airfoil with a flat bottom which contributes to the lift at slow head speeds.  he material paddle material is aluminium and the weight plus the dimension was required to ensure good stability which was the main requirement back then.

Tri-Star is a very thin helicopter. Probably due to the tail boom that is only 1/2" in diameter. The helicopter has no actual chassis. Everything is bolted to a one piece kind of central aluminium frame. Tri-Star is not a big helicopter even though it is as tall as modern 30 helicopters. It is slightly over one meter and despite its slender appearance, it feels to be relatively heavy.

The cabin is a separate unit and is quite unique looking more like the front of a pedal car with a futuristic touch from its time.  However, it is actually copied from the world's most popular home-built two-seater helicopter, the  Rotorway Scorpion - Model 133.  The canopy material is ABS and painted in a purple hue typical of vehicles in the 70's.

Our helicopter surfaced as an advertisement at Blocket in January 2009.  A very young girl's voice answered, which turned out to belong to an Emma Brantsjö that wanted to sell her and her sister Sophie's memory of their father Hans Anders Åberg, who died very early in 1982 at 40 years old with the girls only being babies.
He was a skilled mechanic from Kungsholmen in Stockholm, and had as a hobby to tinker with and re-build everything he could lay his hands on, especially models. The sisters did not know much about the helicopter other than their father built in the 70's. Unfortunately, it has become scruffy through the years and was equipped with Futaba IC servos and a Futaba 27 MHz receiver for frequency 1 (red flag).
 

The tail rudder linkage consists of a thin wire and the mechanism is spring loaded to pull back the rudder arm.  In contrast to the main rotor, the tail rotor blades are rigidly fastened with double screws. For some unknown reason, they rotate down wash. Eg, the tail rotor blades do not meet the air flow from the main rotor, but rotates with the down wash. It remains to see whether it means anything for the functionality?
Of course there is no gyro as they were not in general use at that time however, with the Tri-Star lacking both vertical and horizontal fins, a gyro would have helped a lot. To fly around without anything to help the overall stability was probably an adventure and no doubt contributed to Du-Bro's poor reputation as helicopter designers (still to be investigated - JWR).

Power transmission is via a plastic gear directly fitted vertically on the tail boom which determines the tail rotor rpm with angles brass gears for the main rotor.
Hans Anders had made a larger heat sink to deal with overheating as the cooling fan spins freely in the air. Perhaps it was only meant to cool the crankcase while the top would have been kept cool by wind speed? How it is supposed to function in hover we can't really figure out.
The helicopter is powered by a conventional side-mounted OS Max 40 aircraft engine and according to consistent information from that time, the Tri-Star was miserably underpowered. Another thing that caused problems was the clutch that rarely functioned properly for more than a couple of flights.
The link arms to the servos must also be replaced as the white plastic from that time is not age-resistant. They are extremely fragile and break at the slightest stress. Probably we must also fit more modern servos if we want to avoid unnecessary problems and a small gyro will be fitted.  It would be a shame to ruin a rarity, even though Tri-Star is not considered as a 'classic' of its time.  In addition, like American models of that time , Tri-Star allows you to really get use of your Allen keys in inches.

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