Twinstar Modifications


The modifications described here were an attempt to make the Twinstar II more capable at carrying a higher payload - in particular a digital camera for taking aerial photographs.

The modifications were not made for an increase in flying speed or aerobatic ability.

 

 

 

 

 

 

 

 

 

 

 

 

 

The very first flight with the 220g camera was made with the Twinstar in its standard state. It was a success, because the plane still flew (despite the addition of 45g of ballast on the tail) and the photos were good. However it highlighted the very low margin of flight which the Twinstar had with the extra weight.

Weight is an aeroplane worst enemy especially if you want to achieve slow or scale flying speeds. The more weight that is added to the plane the higher the stall speed becomes. If you add enough weight, you get the situation where the stall speed is higher than the plane's maximum speed, and it will not fly.

The standard Twinstar with the camera on board would still fly but the margin was very small. During takeoff, the plane needed a very shallow climb angle any more than slightly above horizontal, the plane's speed would drop below the stall speed and it would begin to loose height. This meant that a very long climbing area was needed and if anything else happened, such as a change in wind speed, problems could arise.


The first modification, was a larger battery. The standard battery was an 8.4 v unit and a 9.6v unit was purchased to replace it. The idea was that an increase in the motor speed would give a higher airspeed and hence more lift from the standard wing area.

Performance results were inconclusive. The motor tone with the new battery suggested that the motors were turning faster but the airspeed of the plane didn't seem much different. It is possible that the propellers were not shifting more air despite their higher speed or perhaps increased performance was offset by the extra weight of the larger battery. 

 

 

 

 

 

 

 

 

To try and generate more lift, 2 different propellers were tested.

The standard propeller was a 5"x5". This meant that both the diameter and pitch were 5".

In very basic terms an increase in pitch would give a higher top speed and an increase in diameter would give more lift.

The image on the right shows the original prop in white and 2 larger diameter propellers which were tested.

The 2 new propellers were 6" and 7" in diameter.

 

 

 

 

 

 

 

 

Each propeller was placed on a test jig which recorded the thrust, rpm, current and temperature of the motor. The thrust was measured using some domestic scales, the temperature by hand and the rpm, using the digital tachometer. However the tachometer was unable to keep up with the high speeds, so this data was not available.

(Click to enlarge)

 

 

The results for each propeller are shown on the right. As expected the larger diameter propeller produced more thrust but at the expense of more current. This would lead to a reduced flight time but also possibly (and more seriously) a burnt-out motor.

 

 

 

 

In the end the 6" propeller was selected. The 7" one made the motor too hot to touch and by the end of the test it was smoking.

Even the 6" one was drawing more than the recommended current but the intention was to only use full power for the initial climb, perhaps 2 minutes in duration. Then the throttle would be backed off for cruising and photography.

This new propeller with the original motor was used successfully for many photographic flights but in the end the motors did burn out. The resulting low climbing rate meant the Twinstar crashed into a tree and got stuck. During the recovery the fuselage was broken.

A replacement Twinstar was purchased but the old wing was kept as a spare and was used for testing modifications. The new wing was built up as standard for leisure flights and the old wing modified for reconnaissance flights.

 

 

 

 

 

 

 

 

The solution to turning a larger propeller without drawing too much current was to add a gearbox to the system.

This new system consisted of a 3:1 reduction gearbox turning a 10" propeller. The motors were also upgraded to 7.2V ball raced designs.

The upper photograph on the right shows the other small parts in the kit including the screws, motor pinion, prop adapter and capacitor. The original propeller is shown at the bottom for comparison.

Obviously 2 of these systems were used as the Twinstar was a twin engine plane. 

The performance of the new drive system was tested without the camera payload and performance was worrying slow and lethargic. So before the camera was added the 9.6v battery was added to the system and this made a big improvement.

 

The images below show one of the motors fitted to the old Twinstar wing. An aluminium adapter plate had to be made up to fit the new assembly to the existing engine mounts.

 

Conclusion

There is no doubt that with all the modifications described above, the Twinstar was very heavy. There was a larger battery, heavier motors with gearboxes and larger props, ballast weights on the tail and wing, plus the camera weight itself.

With more money this weight could be reduced, for example by using a lithium battery, or a smaller more expensive camera. However the set-up used here did work and it worked well enough to give both a controllable flights and decent photographs.

 

Image showing fully modified Twinstar with Camera Fitted

 

 

 

 

 

 

 

 

 

 

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