Large Electrics

David Theunissen

 

I have built some fairly large electric-powered aircraft which fly very well so I thought I would share my experiences with this size of model.  However, it occurred to me that owning three aircraft hardly makes me an expert.  Furthermore, these models only just qualify as ‘large’ in LMA terms.  So, after trolling the Internet and wading through dozens of Electric Flight magazines, I now have a more meaningful list of large aircraft against which to relate my experiences.

 

This list is too long and too detailed to present here.  So, I have summarised my findings in the table below and will continue to add to the full list which you can see on my web site at www.flyelectric.ukgateway/largesum.htm.  The on-line list contains more details than are presented below, and identifies each model separately together with photos where I can.

 

To explain the table, I will describe the two largest models.  These are a half-scale Klemm L-25 and a 1/7th scale Hercules C-130.  Their wingspans are 22.3 and 19.7 feet respectively.  Both weigh 44lbs (20kg) which was the maximum to avoid the registration process in Germany (being increased to 25kg this year apparently).  Wing areas are 43 and 36 sq ft which average 39.6 (rounded to 40 in the table below).  The Klemm has a single 32x21 inch prop driven by two motors through a belt-drive mechanism.  The C-130 has four 20x10 props with individual motors and gearboxes, hence the average of 3 motors for this grouping.  The Klemm was built using fairly traditional built-up techniques but will only be covered, painted and more detail added when the weight limit referred to above is increased.  The C-130 is mainly of foam construction with carbon-fibre reinforcement and Silkspan covering.  They would both prefer to use 96 cells for this size model, but even on 48 cells they fly between 7 and 10 minutes.

 

Span

(feet)

Models

(number)

Weight

(lbs)

Area

(sq ft)

Props

(inches)

Cells

Motors

Monoplanes (Prop)

19.7-22.3

2

44

40

26

48

3

15-19.6

6

23

?

21

42

3

10-14.9

25

22

15

14

38

3

8-9.9

50

15

11

16

29

2

<8

87

10

7

15

23

2

Bi- and Tri-planes

7-9.9

5

17

?

23

31

1

<6.9

8

13

?

18

27

1

Ducted Fans

10-13

3

24

22

N/A

48

7

7.9-9.9

2

13

11

N/A

28

3

<7.9

6

15

?

N/A

32

2

 

Not all large models reach the magazines or appear on the internet.  However, this list hopefully contains most of the new large models publicised over the past three years or so, worldwide.  It includes every model about which I can find stats which has a wingspan of over six feet.  So, as you can see ‘large’ in LMA terms, is a fairly scarce commodity in the field of electric power.  No surprise there, I guess.  The greatest number of large planes on the list come from Germany, followed by North America and then the UK.  Interestingly, most of the largest Ducted Fan models come from the hangers of Chris Golds in the UK.

 

Note that the weight of electric models is usually given in flying condition (ie: including batteries) and is typically referred to as AUW (all up weight).  By contrast, ‘rule books’ (eg: BMFA rules) usually specify weights excluding fuel or batteries (eg: the 7kg rule).  I wonder if this is wise given that the weight of 48 cells would typically exceed 6lbs.

 

Motors and Controllers

 

The next fact which emerges from this analysis is the number of multi-engined models.  Many are scale replicas which had more than one motor such as the C-130.  Electrics are particularly good for multi’s due their inherent reliability and ease of synchronisation.  You can easily change the direction of rotation of motors if you wish, but torque and side thrust seem to be less of an issue with electrics.

 

As demonstrated by the Klemm above, multiple motors are also useful for large single-engined prototypes.  The main reasons for this are probably cost and the relative scarcity of large motors.  The more powerful electric motors are rated at about 1400-1600 watts which equates to about 2 hp.  One such motor in my 1/5th scale Stearman develops about 10.5 lbs of static thrust with a 16x8 prop turning 7000 RPM at 40 amps.  More thrust could be obtained with a larger prop.  Performance is good with 17lb AUW but might not be enough for a larger model or greater performance.  Larger motors are available but start becoming disproportionately expensive.

 

The efficiency of an electric motor is an important consideration.  ‘Traditional’ motors have brushes to transfer the current to the spinning armature.  Construction of these has long been mastered and ESC’s (Electronic Speed Controls) are reliable and affordable.  Prices for large brushed motors typically range between £150 and £250.  30 cell speed controls can be obtained for less than £30-40.  ESC’s for higher cell counts are available but are much more expensive and harder to find.  It is easy and common, therefore, for a large model to use two or more relatively inexpensive motors and controllers.

 

‘Brushless’ motors are a newer innovation and as the name suggests, do not have brushes.  Instead, they are wired directly to the ESC and as a result are typically 10% more efficient.  However, this design requires the magnets to spin which poses some manufacturing challenges (due to the centrifugal force of heavy magnets).  As a result, although these motors are simpler, they cost a similar amount to brushed motors of similar size.  However, their ESC’s are very expensive, often costing as much as the motor themselves and also handle up to about 30 cells.  Fortunately, however, the technologies are improving and the new ‘sensorless’ controllers offer brand independence so competition and volumes are bringing prices down.  As you might now expect, many of the large single-engined models use brushless motors while most of the multi’s use brushed motors.

 

It’s not my intention to explore every aspect of electric flight or the technologies involved.  However, as you may be able to discern from the above discussion, part of the attraction of electric-powered flight is the technical innovation.  The easiest way to configure a new model is to copy another.  However, the challenge in determining your own preferred motor, gear ratio, prop size and cell count can be great.

 

Gearboxes or belt-drive mechanisms are common as they allow the use of larger propellers which are usually more efficient.  They have the added benefit of allowing an electric motor to ‘unload’.  This allows you to run a smaller motor at a higher voltage than would be possible on direct drive.  Higher voltages need less current to yield the same power (Power = Watts = Voltage x Current).  Lower current results in longer flights.  Smaller motors also weigh less which results in lighter and better flying aircraft.

 

Power and Duration

 

The power of a motor comes in part from the quality of the magnets and partly from the product of Volts and Amps (Watts).  Watts is a very handy measure because it is relatively easy to determine.  I won’t try to cover the concept fully, but if you relate watts to weight you end up with a reliable ‘power to weight’ ratio (usually expressed as ‘watts per pound’).  This is used extensively to select the power chain for electric models and is a measure IC flyers don’t really have.  For example, my 54” sport model, ‘Bubbles’, gives exciting performance with 300W of power and 61oz AUW, ie: 78 w/lb.  At these power levels, it can (just) hang on the prop and will loop from near stall airspeed.  By comparison, my 17lb Stearman has 1300W and it too has similar performance which indicates that the measure is scalable.  100 w/lb is generally sought for full aerobatic performance, whereas scale bomber type performance can be as low as 50 w/lb.

 

The most practical cell for large models is the 2400 mAh Nicad (although 3000 mAh Nicads and Nickel Metal Hydride cells (NiMHs) can be popular).  This cell will deliver 2400 milliamps for 60 minutes, or in other words, has a 144 amp/minute capacity.  This means that if you draw 40A to run your motor, the cell will be flat after 3.6 minutes (144/40).  So, wherever possible, a pilot needs to conserve power in order to seek duration.  As indicated above, gearboxes can help reduce power utilisation if higher cell counts are used.  Here are some specific examples of flight durations:

 

·        Ducted Fan models usually require full throttle to achieve exciting performance.  I think I’m right in saying that Chris Golds’ 28lb 12 foot B-52 flies for only 4 minutes to leave a little margin for safety.

·        My 17lb 1/5th scale Stearman draws quite a high current (40A) and gives me about 6 minutes of good performance.  With 2400 cells, this means my average throttle is about 60%.

·        My 54” Bubbles with similar performance flies consistently for 9 minutes with extensive aerobatics.  It only draws 25A at full throttle which is seldom needed.

·        Similarly, my 9’ wingspan 1/3rd scale Fly Baby flies for 9 minutes and with some authority even in strong winds.

·        Franz Schmid’s Klemm referred to above flies for 10 minutes and Jorg Golembek’s C-130 for 8 minutes.

 

Some people are able to achieve longer flight durations with electric models, usually by overpowering the model.  In practice I find that 9 minutes is normally longer than most IC flight times although clearly it is much easier to achieve longer flights with fuel than with batteries.  9 minutes also allows me to fly a full FAI scale schedule by following a turn-around pattern.  It’s worth mentioning that FAI scale competition rules limit electric-powered models to 42v, I assume due to the risk of electrical shock from higher voltages.  42v is interpreted as 30 cells but I don’t believe it limits anyone to multiple circuits or more than one 30 cell pack in parallel to increase capacity.  Note that this limit applies only to formal competitions not to general flying.

 

Construction Techniques

 

Electric-powered models are generally regarded as being H E A V Y !  However, I have a view that purpose built models can be LIGHTER than their IC counterparts.  For instance, my Bubbles is the same size as any glow ‘40’ but at under 4lbs is a pound lighter.  One reason for this is that the batteries represent the greatest single weight.  Given their small size they require very little structure to keep them in place.  For instance, I usually hold my batteries to a 1/8th liteply base with just two Velcro straps.  And the liteply usually has lightening holes in it!  On a conventional aircraft, the batteries are also usually very close to the CG so linking the supporting/mounting battery structure to the wing spar and undercarriage to carry flight and landing loads is relatively easy.  Furthermore, with most of the weight concentrated at the aerodynamic centre of the model, the rest of the airframe can generally be much lighter which in turn only needs smaller control mechanisms (servos, batteries, etc.).

 

Electric motors also have far less vibration than IC engines, and starting loads are basically non-existent.  Consider the strength we typically build into an IC model to cope with a heavy hand or starter motor.  So, all in all, the airframe becomes quite simple and can be very light which in turn must lead to better flying characteristics???

 

Ancillary Equipment

 

Internet ‘chat’ groups can be an interesting source of information, but also generate some heated debate on the subject of large electric models.  I think most electric flyers still favour smaller models although this is changing as they step up to well-powered ‘40’ and ‘60’ sized models with higher cells counts (eg: 20-30).  Fortunately I don’t have to explain the attraction of large models to an LMA audience!

 

Of course there are some adverse consequences of flying large electric models.  Large numbers of cells have to be purchased which is akin to buying a few years’ supply of fuel before your first flight.  Given such a large investment (eg: about £100 for one 30 cell pack), it can be helpful to design models which use the same packs.  Batteries can also be spilt into smaller sizes which allows them to be used in even more models.  For instance, I use 12 cells in Bubbles and 20 in my 9’ Optica (photo on page 38 of Journal 80).  Together they power my Stearman at 32 cells.  If you want to fly many times in one day, two packs for each plane will be required eventually although I think many people find that the time between flights with just one battery is not too restrictive.  While I would always recommend at least 2 packs for a sport model, 1 can suffice for a large model until one’s battery stocks increase.

 

You will need a high quality microprocessor-controlled charger which will probably cost you £140-180 for 30 cell capability although you may choose to buy a more expensive one with a higher spec.  Most chargers cut-off when the 12v source battery reaches about 11.6v.  I use one which can be set to continue charging down to almost 10v and this is a major advantage to extracting the most from your equipment (don’t try this on your car battery or you won’t be driving home!).  Two chargers may be required if you convert entirely to electric and want to fly a lot, or need to charge more than the rated cell count.  The top-end chargers (Schulze and Orbit) can sustain 4 or 5 amp charge rates into 30 cells so charge times are typically less than 30 minutes.  A small 12v computer fan is a useful accessory in summer to cool the battery after flight (you can typically start charging within 5 to 10 minutes).  One pack and a good charger will therefore allow you to fly about 40 minutes after landing.  Two electric models, the odd spare battery and 1 charger will allow most modellers to fly more frequently than they would want.

 

I have found that a 90ah 12v leisure battery makes a good source for charging and generally keeps me flying at all day events.  However, this assumes you arrive with freshly charged packs in the morning, and have another source battery on charge at home to recharge before the next day.  My second battery is a 44ah car battery which is adequate for this purpose.  I fly mainly with 12, 20 and 30 cell packs.  Naturally you will need more source battery capacity if you fly with larger cell counts.  I use an automatic 11 amp charger which can fill either of these batteries overnight.

 

Conclusions

 

Well, I hope I have given you a flavour of electric flight at the larger end of the scale.  The greatest constraints are probably the up-front investment and the need to ration power consumption in flight.  Essentially you can have as much power as you want but this has to be traded off against flight duration.  I have found that 9 minute flight times are sustainable and can be accompanied by exciting performance.  Electrics definitely suit multi-engined models.  Furthermore, their ease of operation, cleanliness and low sound levels easily surpass IC engines and these characteristics have converted me.  Most converted electric flyers will tell you they fly more than ever before and the experience is more enjoyable.  In short, we have more fun!  Those that fly large electric models will also tell you their models are larger than any IC model they have ever owned.  I think you will also see more industry and individual modeller innovation with electrics.

 

I have talked a little about my 12 cell sport model because most experienced electric flyers will recommend that you start with a sport model before trying something too ambitious.  However, that said, there is no reason why you cannot start with a large model if you follow a proven package.  So, please feel free to contact me if you want more information or advice.  Email is the easiest. Please visit my web site if you are connected (www.flyelectric.ukgateway.net).  Now go Fly Electric (in a BIG way!).