I'd like to fly my plane 100km. Partly to earn the cool badge, but mostly to see if I can.
The airframe is a Bormatec Maja airframe which is made from EPP, has a wingspan of 2.2m and an empty weight of 2kg.
I've been tuning it over the last several months to get the best efficiency I can. I've tested dozens of different setups such as props, motors, PID settings, C of G positions etc by flying a circuit in auto mode. By post-analyzing the telemetry data in Excel I can look at the wattage (and speed, pitch etc) for each lap to see what effect the particular setup had on efficiency. I found the AVERAGEIFS function invaluable to pull data from the telemetry csv file and calculate the average values per lap.
The two biggest breakthroughs were finding the best C of G (20% improvement) and best motor and prop combo (50% improvement in several steps, thanks to Reto Buettner and his expertise for helping achieve the last 20%).
Some of the smaller details I've been working on include:
- Fairings (seen below in front and behind the wing mount).
- Swapping the protruding telemetry antenna for a smaller monopole one (rfdesign.com.au ).
- Filling gaps, like the hole left by removing the antenna, the flaps hinge gap, around the aileron servos etc.
To calculate the maximum range I used a formula that calculates how adding weight affects speed, drag and power consumption. The manufacturer says the Maja can carry a payload of 1.5kg which allows me 4 x 5000 mAh (1.6kg of batteries, AUW of 3.6kg). I calculated a maximum range of 85 km if I run the batteries down to 20% reserve. (I don't want to wreck my batteries but the range becomes 102km if I run them down to 10%.)
I've done one round of flight testing with 4 x 5000 mAh on board and the results were slightly better than my calculations. At an airspeed of 13 m/s it consumed 89 W (8.2 A @ 10.9 V). Extrapolating that out gives a maximum range of 89km. Just short of the ton, but still worth a crack.
Both the manufacturer and another experienced Maja pilot expressed concern about making it any heavier, but I'm tempted because my calculation for 5 batteries (AUW 4.0kg) is a range of 103km. My experience with 4 batteries was good so 5 doesn't seem out of the question. I’m still undecided whether I should go for 5 or look for further improvements with 4.
I've got three more small things to try.
- The EPP surface isn't very smooth so I want to cover the wings (I've got some CP Laminating Film - 1.7 Mil on the way).
- Doing something about the white plastic rods used to hold the wings on. They're very convenient, but I'll find something that doesn't stick up like that. I'm not sure that I'm brave enough to trim the coroplast trim to the wing.
- Moving the Pitot tube to the wing so the tubes and brackets don't cause drag.
I think I’ll make an attempt with 4 batteries anyway. I'll happily land with 20% reserve if it looks like my 89km calculation is correct. But if conditions are very good I might get lucky.
Any other suggestions for how to improve the range appreciated.
Comments
I would definitely replace 5000mAh Lithium batteries with these wonders: http://www.fasttech.com/product/1141100-panasonic-ncr18650b-recharg... to reduce the battery weight by some 20%-30% (buy non-protected ones to go couple % further).
Depending on 5000mAh battery manufacturer, inside you'll fine cheaper cells, like 2400mAh or 2700mAh.
Maja is just not a nice clean airframe, reminds of a C-130 cargo plane. On the positive side it should have lots of room for batteries. You might even to be able to strap one under each wing next to the fuselage, I've seen this done on EZ stars for long range flight. I think with the right motor/prop it can do it.
My X8 broke the 100km barrier with 5x5000mAh 3S batteries on a 900kv motor and 12x8 prop which gave it an AUW of about 4kg. It flew at about 70km/h average speed.
My Tlog file is on droneshare : http://www.droneshare.com/view/bi47v7s
...and you can spin that very important big efficient prop.
There are quite a few 300-400kv multicopter motors around now. T-motor have a few and even Hobbyking has a couple (http://www.hobbyking.com/hobbyking/store/__26963__Turnigy_Multistar...)
Hi Graham
I follow your calculations, but I can't see the advantage from what you've said. Doesn't a 6 cell 5000 mAh battery contain the same energy and weigh the same as two 3 cell 5000 mAh batteries (it's the same cells, just wired differently).
If you double the V and halve the I you've still got the same P, and weight.
If I may: we all know power is volts x amps (P=VxI), so hypothetically if you need 400W to fly you can use a 3 cell (11.1v) which will draw 36A or you can use 6 cell (22.2V) drawing 18A. The 6 cell while heavier will last much longer than the 3 cell as it's supplying half the current. It's weight increase is only a relatively small % of the total aircraft weight. (Note: These values are only illustrative not what you actually get going from a 3 cell to a 6 cell.)
Also bigger props are more efficient as is reducing the number of blades (two is the lowest practical on our models). So the secrets are high voltage and largest prop which is only achievable by low kv motor (or reduction gearbox).
Magic
I knew of Wayne and Reto, but not u4eake. These guys are the royalty of fixed wing flying.
@ Wayne, Reto helped me with my final motor and prop selection, he said in effect the same as you. I dropped my kVa from 1150 to 700, and increased prop from 11 inch to 14.
What is the advantage of higher voltage? Transmission is more efficient at higher voltages but does that make any difference over the distances we're talking about? Or is there something more than that?
(A small weight advantage because of two less battery cables cables and one less Y harness).
There is my badge.i have plans to take the 200k badge this spring. I definitely think you may see greater range with the laminate material. Also try a higher voltage battery, low Kv motor, setup with as big a prop as feasible. For reference I did my 140 km flight with 10,000 mah 6cells. About the same amount of energy of you 20,000 mah 3 cell set up. Other than making room for the batteries that was a completely unmodified techpod.
http://diydrones.com/profile/u4eake
http://diydrones.com/profile/RetoBuettner
http://diydrones.com/profile/waynegarris
I haven't seen any 100km badges in this discussion yet. If there are any reading this blog I'd love to hear from you.
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