i agree

What a useful article it is!

For a very easy to use calculator that will help you choose specs for your power system, have a look at FliesLikeABeagle's homepage.
http://flbeagle.rchomepage.com/
Click on the "Software" tab on the right hand side of the page, and then on "WebOCalc". I had a quick go and it looks quite good! Give it a go and see what you think.

Chris, thanks for the mention! I'll add that WebOCalc is Free Software and is Open Source.

For anyone interested, here's an example screenshot of WebOCalc at work, predicting the performance of my Haikong Models 48" Spitfire. The model weighs about 42 ounces, and is powered by a 3S A123 battery pack (9 volts under load), powering an inexpensive Turnigy outrunner motor from Hobby City. The motor has a Kv of 1100 rpm/V and turns an APC 11x8.5 "E" prop at a measured current of 33 amps.

In real life, the model has short takeoffs, fast steep climbs, and is aerobatic. Since it is heavier than most parkfliers of similar size, it flies a little hotter and heavier than they do, which is quite appropriate for a fighter aircraft like the Spitfire.

To use WebOCalc to predict the performance of a model, you enter the models weight, wingspan, wing chord, and a few other details such as the models "flight mission", i.e. what sort of flying you expect to do with the model. You also need to pick the generic motor type you're using (such as "cheap brushless outrunner"). Finally, you enter the battery voltage (I use 3.5 volts for each lipo cell) and desired motor current, click "Calculate", and WebOCalc finds suitable props, gear ratios, and predicts the models performance.

As you can see, WebOCalc's predictions are very close to the real-world observed numbers and behaviour. WebOCalc even found the exact same propeller, and predicted almost exactly the same current draw.

-Flieslikeabeagle

Hi Guys,

Don't know if you've seen it yet, but we've created our own simple system for matching up electric power systems for planes. It's called EZ Match, and basically all you'll need to know is how much your plane weighs and how you want to fly it. With those two bits of info, you'll be able to find the right motor, ESC, battery and prop for most any plane.
We tested out each of the different setups listed and verified that they'll work. We've got all the data laid out in charts, or you can use our motor wizard to spit out a list of systems that are suited to your plane. There's also a motor equivalent chart in case you've already got somebody else's motor in mind. You can check it out at this link - http://www.commonsenserc.com/page.ph...atch_html.html

Best of luck to everyone,
Bruce
www.CommonSenseRC.com
The "Go To" Guys in Electric Power

Is there any standard method to derate a power system for high altitude (6000'), or is it just determined experimentally?

[b] Peter, You Saved My Day. This Type Article Is What Forums Should Be About. Hope To Get To Read (print) Some More Of Your Good Stuff.
Latecomer In Homestead, Fl.

This is a great thread...thanks for all the info.

Would someone like to write a quiz?

Haha...I know...sounds ridiculous but I've been a full-scale instructor for years and I like quizzes I need to check my understanding...I 'think' I "get it" but then I start thinking about it and get confused again so maybe I'm over thinking it or maybe I still don't get it

I do have an end goal...a 48" span high-wing trainer (tail dragger) set-up for slow stable flying (no aerobatics) but near vertical climb performance...think Alaskan bush plane Oh...and I'd like to use 3s 2200mah li-pos rated for 25C. Not sure of the weight yet...figure an 'average' balsa 48" trainer.

Taking a rough 'stab' at it...lower Kv outrunner with a short, high pitch prop? Or an inrunner with a gearbox?

Thanks 500. You got the low kv outrunner or the inrunner with a gearbox bit right, but go for a big low pitch prop which will give you lots of satic thrust but not much speed i.e good for vertical and slow flight.

What would be considered a "low pitch" prop? 4? 6?

A prop with a 2:1 ratio would be considered as a medium or "normal" pitch (and most efficient). Below would be a low pitch, above high pitch.

A low pitch like 13x4 (3D prop) would give you a lot of thrust but you would probably need a lot of throttle to get airspeed if you would like to cruise around.

I'm asuming your wing load is in the 15-20 oz/ft² range.

Peter

Originally Posted by 500Driver

What would be considered a "low pitch" prop? 4? 6?

Pitch doesn't tell you much by itself, it is the relationship between the diameter and the pitch of a prop that counts. For example a prop with a pitch of 6 and a diameter of 8 inches would be considered a high pitch prop, if it had a diameter of 12 inches it would have a ratio of 2:1 and have a medium pitch. If it had a diameter of 14 inches it would be a low pitch. I prefer to stay around the 2:1 ratio (i.e. the diameter is double the pitch) and use a more powerful motor/battery/esc if I want vertical performance, but this increases weight so wouldn't be good if I wanted to do 3D. Peter is right about pitch speed. If you don't have enough pitch you could be flying dangerously slow even at full throttle. Make sure you have a watt meter of some sort, get a few different prop sizes and try them out. Start with a medium pitch prop and then increase the diameter and see how you go. You will be suprised the difference that prop pitch and diameter make, but make sure your prop doesn't overload your power system.

Interesting! thanks again.

I've always flown helicopters and they pretty much have the 'ultimate' variable pitch prop My recent foray into the fixed wing world has been very educational.

Originally Posted by 500Driver

Interesting! thanks again.

I've always flown helicopters and they pretty much have the 'ultimate' variable pitch prop My recent foray into the fixed wing world has been very educational.

Of course, now, you can fit variable pitch props to aircraft if you have a hollow shaft motor!

Originally Posted by 500Driver

I do have an end goal...a 48" span high-wing trainer (tail dragger) set-up for slow stable flying (no aerobatics) but near vertical climb performance...think Alaskan bush plane Oh...and I'd like to use 3s 2200mah li-pos rated for 25C. Not sure of the weight yet...figure an 'average' balsa 48" trainer.

If you provide the wing chord (or wing area), and a best-guess weight, I can help you choose a power system that will do what you want, using WebOCalc, which is a free and open-source calc program I created to do exactly this sort of thing.

The attached screenshot shows WebOCalc predictions based on my guesses as to your models wingspan and weight. As you can see, an E-flite Park 480 910 kv motor with an APC 12x6 "E" prop and a 35 A or thereabouts ESC will do the job with your 3S 2200 mAh (at a little over 10 C current draw, so you'll have 10 - 12 minute flights with a little throttle management, and the battery should last for many cycles since you won't be pushing it to its limits.)

I'm sure there are cheaper motors that will also do the job for you, if you look for them. One can never be sure if the specs advertised by United Hobbies/ Hobby City/ Hobby King/ have any bearing on reality, but assuming they do, here is one interesting candidate: http://www.hobbycity.com/hobbycity/s...6_750Kv_/_262W

If the Kv really is 750 rpm/V, this will spin a 13x8 APC "E" instead of the 12x6 "E" that the Park 480 is limited to. The bigger prop will gain you performance because it will be more efficient. (And it doesn't hurt that this Turnigy motor costs about a third as much as the E-Flite's advertised price.)

-Flieslikeabeagle