IntroductionSo why the Fox Cub..?, well I was looking for the cheapest possible model to convert to High Power EDF as a quick fill project between all the other projects that I have either as work in progress or waiting to be electrocuted.
The usual criteria was applied less than a 2 meter square from the ground and light to get the best performance, several models were on the short list but I came across the Fox Cub in one of the local Singapore LHS. As always some homework was performed and quick check in the pile of RCJI magazines that I have I found a review of the model using a Wren 44 which it was designed for.
Several minor problems were reported but it seemed to fit the criteria and had I duly gave the model the name of HP EDF Stick as it resembles the famous Ugly Stick models but is a little fatter..!
One nice feature but not exactly pleasing to the eye is the ability to change the fan quickly, undo the jubilee clip disconnect the motor and bingo slip in a different fan. Great news as I can now try different fans and pack combinations and see the results.
Specifications and PerformanceWingspan: 64” 1630mm
Length: 67” 1700mm
Weight: 13lb 6kg
Engine thrust 12-14lb 5-6.5kg
I have as some of you know a selection of big fans and the TAM Jets TJ-100 from tests with a 85% of FSA exhaust area would give me 11.36 lbs thrust on 11S 5,000mA packs.
A consideration when doing an electrocution of a turbine models is the infamous weight factor, the figures usually quoted are without any fuel on board (Dry weight), and with an EDF model we always have the pack weight to consider on top of the dry weight. At the same time we can as we less bits and bobs to install that are required by the turbine model achieve a lower dry weight. So a quick weigh of all the parts in the box plus the EDF parts ESC, BEC and switches and wiring and I had a calculated dry weight of 11.43lbs and with a 11S pack combination this would increase the weight to 14.99 lbs which is less than the turbine version with 2 litres of fuel on board. One last thing to calculate was the thrust to weight ratio anticipated which worked out at 0.79 so this one should fly well.
I have a spreadsheet to do al the sums as my brain is getting lazy as I get older..!, another consideration is that one needs to remember is the wing loading. Turbine just do not have the wing loading of your regular trainer, in the case of the Fox Cub we are in the 44 oz./sq. ft regime. This needless to say will mean a longer take of run than your usual model. Using NASA provided equations we require 60 feet if we use full power from let go at 30 degrees C and I know that the CTP DT always uses graduated power for take off’s we can double that figure to get past the calculated stall speed of 30mph.
The predicted in theory maximum speed is 105 mph (lower than was achieved in both flights) and a rate of climb of 39.43 ft/sec or 2,366 ft/min .... umm this model is going to perform..!
Here are the Fox Cub predictions
You will note I can change the fan type, cell combinations and exhaust size dynamically and see the flight performance all rather nice before I get the tools out to build the kit.
The KitHere is what you get when lay the box of bits out on the lounge floor...!
There is loads of room in this model for the conversion with plenty of space for the packs, radio gear and other essential items.
Centre bay (avionics area)
Front bay (packs area)
Rear bay (air tank and radio pack)
So now it is time to start the build..., stay tuned for this section.