December 2007 Issue Electric Flight Column by Greg Gimlick
Email: rctraveler@gimlick.com
Merry Christmas:
It’s hard to think of Christmas when it’s the end of August and the temps here in NC have been hovering in the high 90’s or 100 for the last couple of weeks. Then again, it’s refreshing to think it will be a lot cooler by the time you read this. I hope everyone has a great holiday season and the New Year brings you all the toys that make life so much fun. Thanks for your support this year, I hope we have another great one.
Farewell to a Friend:
A while back, I mentioned working on
an Approach Engineering Cobra conversion kit for my Swift helicopter. James
Linder was Approach Engineering in
He and I were collaborating on adapting his new Apache kit to fit the Quick QJ-10 airframe I had just sent him. He was a talented designer and incredibly smart engineer by trade. I’m at a loss how to express my feelings to his family and the rest of his friends. I hope someone will buy his kit line and continue it, but there is no way to make up for the loss of James and his friendship.
On the Soapbox:
I want to take just a minute to remind everyone about the purpose of our last few columns and the lessons within. These are intended to be guidelines showing how to plan a power system. I’ve taken some heat over the August issue where I showed how to figure out amp minutes and divide by current, etc to make some decisions regarding duration. Some readers took me to task for planning on discharging lipo batteries to their full capacity. These were exercises to show you how to do the calculations without regard to efficiency, battery type, etc. I thought I made that clear, but apparently not. I am not advocating complete discharge. It’s all relative in planning though, if you get more duration from one setup than another it will still be so when you figure in efficiency or reduce the discharge to 70% of capacity. The purpose it to understand the steps involved.
Electric Airplane Performance Simulation
Programs:
One thing is without question in my mind – owning one of the simulation programs will save you more money than they cost in the long run and teach you more than you will learn in hours surfing the net. Are they perfect? No, but they are a lot more accurate than you will be with your pencil, paper, and calculator. These programs contain databases and algorhythms to consider all the parameters and can be updated with ease. I believe the top two programs are ElectriCalc and MotoCalc. I’m going to briefly touch on each of them this month and next, but won’t pretend to do them justice since that would take a full feature article for each.
MotoCalc:
This gem of a program is the brainchild of Stefan Vorkoetter, Capable Computing, Inc. http://www.motocalc.com Stefan has been around the electric flight world for a long time and can be found on many of the RC Groups forums. Right off the bat you’ll appreciate the web support this company provides for this simulation. You can download a full featured version for a trial period before you buy it so there’s no risk at all.
MotoCalc (Mcalc) has often been accused of being difficult to learn and it probably does have a bit of a steeper learning curve than ElectriCalc for a beginner. Don’t let that influence your decision though until you’ve gone thru the tutorials, both online and off. These tutorials will carry you step by step thru the whole planning process. Most questions I’ve gotten over the years about MotoCalc could have been answered had the user taken some time to read the manual or try the tutorials. These tutorials will teach you more than any book on the market right now.
When you open the program you’re greeted with a “tip window” that’s wise to keep activated while you learn your way. The next screen is the MotoWizard that
will walk
you thru each step of a plane from start to finish. 
In my example, I plugged in a Sig LT-25 and just made choices along the way as it worked across the screen. Here are the steps and choices I made:
Performance – sport
Model – just plug in wingspan, area, and weight without motor, battery and ESC
Airfoil Type – basic info like flat-bottomed, semi-symmetrical, etc.
Thickness – they give you diagrams and you pick thin, fat, etc.
Field – plug in your elevation, I left it at sea level.
Motor – plug in your choice; brushless, brushed, either. I chose brushless
Gear & Prop – I chose direct or geared and a max prop size of 14x10
Results – you will be given a list of motors to select from with ratings to let you know Mcalc’s best choice. I chose the recommendation of a Neu 2215/1.5Y
Once you’ve completed these steps and hit “accept” it will take you to the main window.
From there you can select “compute report” and you’ve be presented with a window full of information.
This is where many folks get confused and discouraged, but there is a not so hidden gem that you’ll learn to love – the “Opinion” button. When you hit this you’re presented with a very easy to read opinion of your setup. Here it is:
MotOpinion - Sig LT-25
Sea Level, 29.92inHg, 59°F
Motor: Neu 2215/1.5Y BAM; 1020rpm/V; 1.6A
no-load; 0.004 Ohms.
Speed Control: Castle Creations
Drive System: Generic 14x9in Prop w/2.5:1 Gearbox;
14x9 (Pconst=1.17; Tconst=1) geared 2.5:1 (Eff=92%).
Airframe: Sig LT-25; 724sq.in; 104.8oz RTF;
20.8oz/sq.ft; Cd=0.059; Cl=0.6; Clopt=0.67; Clmax=1.24.
Stats: 83 W/lb in; 72 W/lb out; 21mph stall;
29mph opt @ 57% (32:06, 66°F); 30mph level @ 59% (31:10, 66°F); 1450ft/min @
35.1°; -244ft/min @ -5.5°.
Possible Power System Problems:
·The
full-throttle motor current at the best lift-to-drag ratio airspeed (31.6A) is
lower than the motor's maximum efficiency current (81.8A). A higher current
level would improve system efficiency.
·Current can
be increased by using more cells, a larger diameter or higher pitched
propeller, a lower gear ratio, or some combination of these methods.
Power System Notes:
·The voltage
(16.6V) exceeds 12V. Be sure the speed control is rated for at least the number
of cells specified above.
Aerodynamic Notes:
·The static
pitch speed (57mph) is within the range of approximately 2.5 to 3 times the
model's stall speed (21mph), which is considered ideal for good performance.
·With a wing
loading of 20.8oz/sq.ft, a model of this size will have trainer-like flying
characteristics. It would make an ideal trainer, for use in calm to light wind
conditions.
·The static
thrust (94.7oz) to weight (104.8oz) ratio is 0.9:1, which will result in very
short take-off runs, no difficulty taking off from grass surfaces (assuming
sufficiently large wheels), and steep climb-outs.
·At the best
lift-to-drag ratio airspeed, the excess-thrust (58.6oz) to weight (104.8oz)
ratio is 0.56:1, which will give steep climbs and excellent acceleration. This
model should be able to do consecutive loops, and has sufficient in-flight
thrust for almost any aerobatic maneuver.
General Notes:
·This analysis
is based on calculations that take motor heating effects into account.
·These
calculations are based on mathematical models that may not account for all
limitations of the components used. Always consult the power system component
manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
This should be enough to get you over the initial hump of looking at the opening screens and wondering what to do next. From this point, you can tweak the setups by changing all the parameters as much as you please. You can set up filters to only assess props within a certain range or gear ratios, etc. It’s extremely versatile, you can hit the “Opinion” button any time you don’t fully understand the results, and it will spit out a report as shown above along with recommendations.
Data Gathering:
I’ve mentioned before that data gathering tools are essential if you want to get into tweaking and experimenting with electrics. I’ve been using the Micro Power from Eagle Tree and it does everything most modelers will want to do, but I just recently got the Seagull system from them complete with wireless data transfer.
This is the pro level unit and I can now monitor the two motors I have ganged to an Inner Demon Gearbox in my giant scale Pitts. Look for a full article later about this incredible system!
Final Approach:
I hate that the space has run out, but next time will look at ElectriCalc and how to get going with that one. I’ll say again – I think all electric flyers should own at least one of these programs, especially as they’re learning. Remember this – if you don’t want to play engineer you don’t have to; just copy someone else and have fun at the field.
