I ‘needed’ to experience a tricopter, and I chose a Hobbyking X900 frame kit.
I tried to glean some useful information from G3TXQ’s measurements of windowed ladder line loss at Windowed ladder line loss – G3TXQ.
In reviewing his article today (05/02/14), there is new information on a further series of measurements of the same line.
The shape and position of the two lines does not reconcile with the formulas stated, so I digitised the data points and analysed the data set to try to find the most appropriate model for the reported measurements. Note that although the chart above is in imperial units, my work is usually in ISO metric units, and usually basic units.
The digitised data points were converted to loss in dB/m, and fitted to the model MLL=k0+k1*f^0.5+k2*f using regression techniques. Note that the digitisation process introduces some noise, but it is estimated to be small compared to the noise in the underlying measurement data.
The coefficients k0, k1, k2 were reviewed to test that there was sufficient data to support the hypothesis that they were not zero, and all three passed that test, the standard error of the coefficient was significantly less than the coefficient. Note that k0 is not derived from a DC measurement of resistance as done by some modellers, but from the measurement data over the range of 3.6 to 48MHz in this case, and extrapolation beyond that frequency range increases uncertainty.
The above chart shows G3TXQ’s measurements as digitised from his published graph, and it shows the components of loss indicated from the model I built (the k0 component is allocated as conductor loss).
The “G3TXQ model” line is equivalent to his MLL=0.063+0.063*f^0.5 dB/100′ converted to dB/m, and as you can see it is not a good fit to the measurement data points, nor does MLL=0.063+0,063*f^0.5 dB/100′ reconcile with the blue line on G3TXQ’s chart earlier in this article.
G3TXQ’s measurement points (as digitised) are quite a good fit to the model MLL=0.001456+1.499e-6*f+5.631e-11*f dB/m where f is in Hz, and provide a good predictor of MLL over 3.6 to 48MHz.
The 1245SF props arrived and replacement HK922 servo, both were fitted.
The tricopter is very hard to fly with 1245SF despite lots of time tuning pitch and roll, and level PID parameters. It appears that the propellers are just too much and that control loop stability is hard to achieve at lower motor speed.
So, the 1145 propellers were refitted and fine tuning of the saved PID parameters commenced.
The replacement servo works fine, stable stand alone with and without load, and mechanically smooth through its entire range (no sign of any stiff spots in the gears as in the previous one).
The project is coming to an end, the tail mechanism is an obvious vulnerability in this implementation though it seems to work quite well… more in the final report.
I had a contact with a newcomer recently, and struggling to hear him, I learned that he was using the 3030 antenna described in the WIA’s “Your entry to amateur radio” 2nd ed, the Foundation licence training manual.
My article Foundation watts explained triggered some discussion on the thorny issue of compliance with power limits of the LCD.
One correspondent was confident that the Foundation candidates are properly trained, which leads to examining the training materials.
Tests on the tricopter using a data logger to capture motor speed, current and pack voltage gives an insight into platform operation.
Above, data capture from the tricopter on 3S. The datalogger adds 50g mass.
The plot shows the device hovering at about 5000rpm and 10.8A at 11V. Two WOT ascents were made to check acceleration and motor recovery during closed throttle descents. From idle, it takes about 500ms to accelerate to hover rpm. This test was repeated and the motor reliably restarted every time and the tumbling craft stabilised quickly. A set of 12×4.7SF props has been ordered to try at 3S to more fully load the motors.
Similar tests were conducted also on 4S, and peak current on WOT was 80A on 15.5V for 8500rpm. Operation on 4S would provide sufficient lift to carry a camera payload, but WOT operation exceeds the motor’s continuous current rating.
The tricopter project lumbers on, delayed by faulty parts.
The recommended tail servo, a Hobbyking HK 922 Metal Gear Digital Servo, failed. It had a rough spot at about 70% of its range from new and which became worse quite quickly until the motor had insufficient torque to overcome it.
From time to time, correspondents have asked how the Cobwebb antenna works, and particularly how the impedance matching scheme works.
Firstly, what is the Cobwebb?
It is an innovative antenna for small spaces, quite compact and as I recall originally intended to cover five amateur bands from 20-10m.