Intro to EDF  - Jan 2013

As an avid pilot and builder of EDF jets for the past 5 years, I thought I would share some insights into the world of high performance electric aircraft.  I’ll try to keep this in terms that the everyday guy or girl will be able to comprehend, just be forewarned that there is a lot of information to be passed along.  I got into this discipline after LiPo batteries were already in the production stages, so I never got to experience the days of 45 watts per pound.  By comparison we are flying jets capable of 300+ watts per pound.  My A-10, even at 18.5 lbs still turns in 302 watts per.  More on this later.

The first order of business is that jets are HARD on equipment.  That means motors running past their stated limits, ESC’s right at their’s, batteries that need to be up to snuff, etc.  Guys who are familiar with my stuff know that I use Castle Creations ESCs.  Several reasons for this: They are the most bulletproof, can be run up to 30% over their capacity, will be replaced with new should one encounter a problem, and they have data logging.  Customer support is second to none.  You pay for them, but the VALUE is there too.  As for motors, just about any brand will work, you can overdrive them to a fair amount.  No manufacturer states the actual rated performance, for example the HET series that I use have a max stated wattage of 1600 to 2000.  I routinely push 3000+ thru several of them every time I fly.  If you have seen my Hog, MiG or Viperjet fly, each motor is over 3000 watts at full tilt.

The key to longevity here is heat control.  You have to keep the electronics cool enough to function.  Part of why I buy CC ESCs is that they tell you exactly what temp you are running them at.  As the most expensive single component, I want to keep it happy.  Motors on the other hand require a bit more investigating to see how they are running.  Nothing expensive here, just a finger or two after a run to see how much heat it is shedding.  Some guys see me doing a cool down run after a flight.  The reason is to keep the hot motor from soaking all that heat into itself and the fan.

On to batteries… the best you can here.  I don’t mean the most expensive.  My Thunderpowers have proven to be the flakiest I have.  Cost too much for what you have to deal with.  There are plenty of reputable dealers with good solid lipos to choose from and they are all getting better every day.  Ignore the “C” rating, they aren’t accurate.  So how do you choose one for cell count and capacity?  As I said, EDFs are HARD on stuff.  They draw the most current of any RC gear except boats.  Why?  Because you are trying to get a small tube of air moving fast enough to do some good.  This isn’t the model of efficiency in a powertrain.  EDFs are notorious for their lack of ability to turn a large number of watts into useful work.  So pick a battery(s) that have enough capacity to handle the expected current draw, regardless of their “C” rating.  More to follow.

Right along with ‘hard on the gear’ is the landing gear.  Who wants a jet with fixed gear?  Exactly.  Jets by their nature have smaller wings, no prop wash, smaller control deflections, heavier weights for their size, etc.  All of these variables can and will add up to a beautiful jet sliding down the runway after landing, with the gear stuck in the approach end of the runway.  Jets have to be flown onto the runway, no Senior Kadet flops here.  If you have access to a paved runway, so much the better.  But many of us have just the club field.  I plan every jet to be able to fly off the grass.  This leads to some compromises, but you can usually find something you want to build and make it work at the field.

Now, this article just being an intro, let’s discuss what some of these electric terms mean.  
Watts:  this is a relative measure of the amount of power one is consuming or generating.  Your jet consumes watts, your chargers generate watts.  Both need to be up to the task.  Watts are a measure  of voltage times amperage.  Example: 12v times 30A equals 360 watts.  Real example:  A 3s 2200 lipo powering a small handlaunch jet running 30 amps wide open, weighing up to 24 oz, will fly nice.

Volts:  Electrical capacity of your battery.  Each cell in a lipo makes 4.2 volts.  This is a resting value.  As you put more of a load on your lipo, the voltage drops.  If you have a 3s lipo, you have a 12.6 volt battery, resting.  Under the above 30 amp load, it’ll drop to 3.5 to 3.7 volts per (10.5 to 11.1 for the pack).

Amps:  Measure of current flow.  How fast one is taking the little electrons out of the battery.  More is not always better.  Well, more amps will make your jet faster, but generate more heat too.  Heat is bad.

MaH:  Measure of how much your lipo will hold when fully charged.  A 2200 MaH (or 2.2aH) will hold enough juice to run your 30 amp jet for a decent 6-7 min flight.

Now lets try and put some of this together.  You get your little foamy F-16 and need to power it.  Start with the lipo size that can fit into it.  For this one, anything from a 1s 1000 to a 3s 3000 will fit.  It’s approx flying weight will be from 14 oz up to 25 oz.  For jet like performance you need at least 200 watts per pound.  If you choose a 1s 1000 (the lightest), take the 200 watts you need (let’s assume a 16oz weight) and divide that by the loaded voltage of the lipo.  200/3.5= 57 amps.  For a 1000 MaH pack, that is a 57C discharge rate.  Even a top of the line lipo isn’t going to provide that.  Plus your RX will be in a constant state of reboot, Futaba or Spektrum.  So knowing that you must lower the amp draw, increase the voltage.  A 2s 1000 pack will give you 7 volts under load.  200/7=28.5 amps, not bad but still a hefty discharge rate.  It would be useable except for one thing, you’d get a minute and a half of flight out of it.  Not much fun.  For flight time you need to either increase capacity in the form of MaH, or decrease amp draw.  No try a 3s pack.  You get 10.5v at 19 amps.  Now as you lower the amp draw, the loaded voltage will rise.  At 19 amps the pack will probably hold 11 volts or so.  You still have the 200 watts to fly your 16 oz jet.  But for how long?  Depends on throttle useage.  At full noise your 19 amps will eat your lipo in a hair over 3 min.  Double that for mixed flying.  So 5.5 to 6 min flight times for a nice, lightweight jet with decent speed and performance.  You’ll have so much fun with it, you’ll leave the throttle wide open longer than you think.  And the lipo is going to get warmer than you might like, as it’s resultant discharge rate will be up there.  A more realistic compromise would be to gain 2 oz and use a 2000-2200 MaH pack and double your fun time.  The watts per lb go down slightly, but because you are effectively letting the lipo only work half as hard, it will generate less heat, and last longer.  Also, should you choose to go with a bit more motor or fan, there is room to grow without hurting the lipo.

Ok, so that was all basic electric stuff, not particular to jets.  Why type it?  Because it IS for all electrics.  If you don’t want to try jets just yet, you might have learned a bit more about electrics in general.  If you still want to try an EDF, then keep in mind, you are going to be running towards the high side on everything mentioned above.  Even on something like an airliner or a cargo transport, with lots of wing area, the numbers are going to be pretty startling.  Especially if you have to try to get off of a grass field.  The Habu when stock runs at 1600 watts.  Mine has over 3000, and still takes a bit of time to get in the air.  Once in the air it boogies, but you have to get off the ground safely first.

A word about this whole watts-per-pound thing.  As a general rule for electric powered models, 100 will get you into the air, a cub, powered glider etc.  200 will bring scale type flying, mild aerobatics.  300 with a prop will take care of a lot of the rest.  With an EDF, lean towards another 50 to 60 percent to get the same performance, ie 150 will get you there 250 will spice it up, and 350 will start to put a smile on your face.  Add a few more for grass.  

With that bit of prep and warning, we’ll delve into some more specifics as we go.  My goal is to have a good deal of what I know, and my experiences with EDFs out in the newsletter by mid summer.  Each month or so we’ll take a look more in depth on each topic and try to tie it all together.  Next issue we’ll look at the actual jets, what is out there and what to expect when you open the box.