Part 1 – Introduction
I’ve been considering a flying wing for some time primarily as a way to extend the range of my FPV flights. Multicopters are hampered by the need to generate sufficient thrust to support their entire weight, which consumes a large amount of power. Wings on the other hand are supported by aerodynamic lift, with the motor only needing to supply sufficient thrust to counteract the drag, which is typically only a fraction of the weight of the aircraft at cruising speeds, so power consumption is lower resulting in longer duration and range.
Yesterday I took advantage of the Team Legit Black Friday sale to purchase a Fohdy 40″ wing to build during the December holidays. This is designed more as a cruiser wing than a racer, with some inherent stability and good slow speed flight characteristics. I haven’t seen details of the design but I expect it will have a thick asymmetric aerofoil for good low speed handling, with washout to eliminate tip stalls and some dihedral for lateral stability. That, plus the size – just small enough to fit in the boot (trunk) of my car, makes it ideal for my purposes. I was sorely tempted by the new Legit Wing 5D, but that would have to travel in the back seat of my car, which would rule it out on family excursions.
Since I plan to undertake some longer flights I decided to go with name-brand components that I hope will prove reliable. With assistance from Ian at Legit-Wing tech support (that would be Ian Jenkins, the 2016 US Drone Nationals Wing Champion), I configured my Fohdy with a Cobra 2221-12 1250 kV motor, Cobra 60A ESC, and Spektrum A3040 servos.
The parts list is shown below (prices in US$ excluding discount):
Description | Qty | Price | Total |
---|---|---|---|
Team Legit FOHDY | 1 | 135.00 | 135.00 |
Cobra CM-2221/12 1250kV | 1 | 2.99 | 32.99 |
Cobra 60A Wing ESC 6A SBEC | 1 | 47.99 | 47.99 |
Spektrum A3040 MG Digital Servo | 2 | 24.99 | 49.98 |
APC 8×6 Prop | 1 | 3.00 | 3.00 |
Foxeer Arrow Mini V2 (2.5mm IR block). | 1 | 21.90 | 21.90 |
Foxeer TM25 Switcher 25-600 mW VTX | 1 | 33.00 | 33.00 |
SMA right angle adapter | 1 | 2.99 | 2.99 |
RCP Omni Antenna | 1 | 9.99 | 9.99 |
Amazing Goop 2 Fl. Oz. | 1 | 6.49 | 6.49 |
TOTAL (US$) |
343.33 |
The intial configuration uses 5.8 GHz video, but I plan to upgrade to 1.2 GHz video for longer range.
I haven’t finalized the battery choice yet. I first want to see what the weight is without battery. I’m expecting this to be around 650g, in which case the flying weight would be between 986g and 1162g for 4S 35C batteries ranging from 3300 mAh to 5000 mAh. Based on a wing area of 380 sq. inches that I estimated from photographs, I expect the cubic wing loading will be in the range 8.1 to 9.5, which puts it in the “nice flying planes” category of this list from East Bay RC.
ECalc predicts flight times at 60 km/h (37 mph) from about 45 minutes with a 3300 mAh battery, to an hour with a 5000 mAh battery! I’m not sure I quite believe these times, but I’ll be satisfied even if I get only half of the predicted endurance.
The eCalc performance predictions are shown below for a 5000mAh 4S battery. You’ll see that it still manages a power/weight ratio of over 150 W/lb, even if the motor current is getting a bit close to the limit – I guess that’s what you should expect when you ask a champion wing racer to help with your configuration! I think it will be fine as long as I only use full throttle for short periods when the prop is unloaded; but may end up propping down to an 8×5, which would reduce the motor current and eliminate prop stall, while still having a power/weight ratio of 150 W/lb and a top speed of 100 km/h (62 mph).
Click here for a high res image of the eCalc results.
I’m conceptualizing this ‘plane as a traditional Gran Turismo like the Ferrari F12berlinetta that is capable of covering long distances at speed and in comfort. I’ll post the next part when I receive my Fohdy parts… I can’t wait!