Hi,
I am considering a shift to electrics, it'll be my first. I fly pattern-ships and would call myself a good Sunday flyer, desperate to improve. Currently I am flying a self designed aircraft to practice pattern style maneuvers. The reason I design and build my planes is to get them as light as possible with just the right amount of power for the type of flying. The one that I fly is 74” WS, 1100 Sq. in W Area, 76” Fuse, has a BME .90 Gas engine. The all up weight (minus the 8oz fuel for12 mins) is 3600 grams (7.92 lbs). She flies quite well, great verticals and has survived a few minor mishaps.

My electric project is for similar specs. I expect the final weight to be 3500g-3600g (max 7.92lbs). From preliminary investigations, seems to me a AXI 4130 outrunner (rated 40A cont) with a 70 Amp controller and BEC is what I need. Sure bigger would be better, but I am looking only for 950 watts (under load with 16 or 17” prop). My concern is to correctly estimate the 8S lipo pack capacity for 12 min flying with either 16 x 10 or 17 x 10 prop (part throttle flying). I would like not to have too much margin due to the weight and cost penalty. Hoping to have at least 2 batt pack sets, so that I can field charge for multiple flights.

Looking for your inputs and thanks in advance.

Gyri

I think you will have a problem combining good performance with 12 minutes of flying.
12 minutes is a long time, and would therefore require large batteries, which again would increase the weight, requiring even more power etc.
Most electric pattern planes fly only about 8-9 minutes, and I think this is a better target.
Considering that it takes at least one hour to fully charge most Li-po packs, having only two packs will seriously limit your flying. This wil also depend a lot on how you charge the batteries, what kind of charger(s) and power source you are using etc.
I have had anything from 1 to 5 battery packs. I feel limited with anything less than 4. This is my opinion only, others may feel differently. (Particularly those who don't fly electric, but have read about it.)
For F3A performance, it is normal to use 10s packs, with a capacity of about 5000 mAh. Models weighing close to 5 kg. With a model weight of 3600g, using 8s, you would probably be looking at 3700-4000 mAh for reasonable (sport) performance. 1100 sq. in. is a large wing area for a 3800g pattern model. This area is actually larger than most full size (2m) pattern models weighing 4900-5000g.

Magne

The model I fly has parallel chord and hence the WArea adds up a lot (1030sq.in to be precise). The tapered design I have planned also has similar WA. When you mention 8-9 mins, did you mean for part throttle flying or full?
Thanks,
Gyri

Gyri.
I meant 8-9 minutes of aerobatic flying, which is various throttle settings. Full power is only used for a few seconds at a time.
In my F3A model I pull about 80 Amp static on full throttle. However, average current flying the P07 F3A programme is only 24-25A. (This is equivalent to using 3750 mAh in 9 minutes)
It would typically take 8-9 minutes to fly that programme. The new P09 programme is shorter, and should take about 7 minutes.
Around the world there are many variations of easier, less demanding programmes, but I don't think many of these would take longer, or require more power than the F3A programmes. Here in Scandinavia we also fly an intermediate programme (the Nordic class), the maneuovers are simpler, but the energy consumption proved to be very similar to the F3A class, when flying the same model.
If you fly competitions, then your battery size is dictated by the programmes you fly, i.e. you need enough power to fly the individual maneuovers, while the capacity has to be large enough to allow you to complete the whole programme, pluss a safety margin. With Li-po's, you should not discharge more than about 80% of the total battery capacity, or you will damage the packs.
I noticed that you are aiming for 950Watt max, this is quite low, even for a 3600gram model. I found that increasing my max. current from 60A to 80A did not significantly alter the total battery consumption, as the extra power is only used for a few seconds during the flight.
If you don't fly competitions, then of course you can set up your model to fly however you want, trading power and performance for flight duration. It's all a question of balancing the pros and cons.

Magne

Thanks for that reply. This is interesting. Ok, I am one of those typical sunday sport flyer, with a leaning towards pattern style flying. I don't plan on competing in any serious or judged event. So I may not look for a very strict set of sequence or for that matter time. The true F3A birds and their equipment are in a different league altogether. I only want something like that.
Now, from various web pages on glow to electric conversions, I understood that you need 100 watts per pound for aerobatic models and 150 w/lb for 3D. I took 120watts/lb. Assumed weight being 7.92lbs, arrived at 950watts. Please sort me out if I am wrong.
Could be that I always draw refrence from my current model, weighing 7.92lbs, does very good verticals when spinning a 14 x 6 APC wood at 9500RPM. Most of my aerobatic flying is done with 1/2 to 3/4 throttle, say 7000RPM average.
During my research on the forum I came across http://www.rcuniverse.com/forum/m_5889882/tm.htm and it's about experimenting with lesser capacity packs to save weight on a 2x 2 and the individual posting decided that his 5000mah could be replaced for 3700mah (they were flying a schedule, i think 9mins). In my case, it's not just weight, but cost as well.
Gyri

Gyri.
The Dutch importer of MVVS engines, Mr. Pe Reivers, has a very good prop-power calculator on his internet site. Go to http://www.mvvs.nl/ and look under "utilities" on the left hand side. This is an excel spreadsheet, where you can compare different propellers, revs, loads etc.
According to this sheet, the APC 14x6, at 9500 rpm, develop exactly 950W and gives a static thrust of 4170 grams. So your calculation should be quite good.
But also remember that having more power available, does not mean that you have to use it all the time. As I said earlier, in my F3A model the average current is about 25A, while max. is 80. If I propped it to max. 60A, I would still use about 25A on average, but I would not have the same vertical performance.
You will also find that by going to electric, you are not likely to operate at 9500 rpm, but maybe 6500 rpm. You therefore have to increase the propeller pitch, typically to 10/12/14" pitch. Using 3700 mAh packs may be a good way of keeping costs down, just make sure that you don't over-discharge the batteries, or the lifetime will suffer.
Please note that what I write here is my opinion. If you ask 10 people, you would guaranteed get 10 different answers.
(As you can see from the thread you are referring to, there are different opinions and ways of doing things. There is no right or wrong way, just different ways. I posted my opinion in that thread as well.)
Hopefully somebody else will comment with their experience and opinions.

Magne

I understand that opinions vary but thanks a lot for taking the time to give yours. It helps! This being my first electric, i would love to hear on many different opinions, set-up, etc. If whatever reason I find that it would not suit my requirement, then these opinions (after averaging) would help me stay away or atleast invest right.

I also noticed that going electric is not just about getting the max watt requirement, it's also about correct prop selection that suits the flying while also making energy consumption efficient. Like some people pick more volts vs amp or vice versa, not sure why, but i guess all those are considerations also.