Dedicated to the Promotion of Electric Propulsion in all types of Aeromodeling

The Twin Star II Brushless Revisited, Pt.1


One huge advantage of the Twin Star II,  with that long nose and high wing protecting the motors it is the only front motored airplane with the crash resistance of an Easy Star.  But the most lasting importance of this article may be the realization that by substituting an $18- motor for a $45- one that you can write off a third of the thrust and half the duration.

Follow along the adventure of what works well,  how to keep it flying for a long time,  and what doesn’t.  I’ll be making use of the Virtual-Dyno where possible.

With so many Magazine and Internet articles,  why another one?
Why would you believe the opinion of somebody that just finished assembling a foam airplane according to the manufacturer’s instructions and only made a couple of flights,  that the published report represented the truth,  the whole truth and nothing but the truth?  Fellow club members,  there’s more to it.

On my desk is a nation magazine with a seemingly factual coverage of towing small gliders into the air with medium i.e. Twin Star II size electric airplanes.  There is no mention whatsoever that one of four gliders is made of foam so soft that it’s very difficult to use,  and that one of them is an awful flier.

Censorship by omission,  the articles supported by manufacturer’s advertising can’t report on anything bad.  Although in this rare case,  there is almost nothing bad to be left out of a report on a Twin Star II!  The Twin Star II is among the most pleasant,  general purpose RC airplanes ever made available.  Other then maybe,  just the way it comes from the factory isn’t necessarily the end of it’s development.  Although the build ups with pictures on the Internet are good starting places,  they report right after the first flights,  before things get bent,  scraped,  broken and worn out.  Rarely does an author try more then one set of propulsion components!  With the motors,  propellers,  speed controls and batteries so easy to change that indicates a severe lack of genuine interest in determining what works,  or censorship.

I made over (600) flights with Twin Star IIs,  just because a power combination worked at all doesn’t make it satisfactory.  Nearly all of the other publishing authors participate,  knowingly or not,  in the miss-representation by omission that watts-in equates to watts-out since they omitted watts-out entirely.

Even though seemingly slight changes can sometimes make bigger then expected differences,  I use the P-Calk (available at Fly-DMA and NeuMotors Web sites,  among others) as a starting place.  If an article doesn’t explicitly note the efficiency of the propulsion equipment,  either the author (often at the publishers direction) is deliberately concealing something ugly,  or too ignorant to believe.

NeuMotors and Scorpion proudly report their equipment’s parameters!  Even the limited P-Calk available from the Multiplex USA’s website explicitly notes power to the prop and efficiency.  Let me repeat that.  Watts-out is ALWAYS less then watts-in. If the supplier is not willing to define the efficiency of a power combination they are hiding low performance,  lousy durability and inconsistent quality.  You can confirm that yourself,  the cheap stuff heats up in use,  even mediocre stuff doesn’t.

If you have found a more all encompassing virtual-dyno that includes the airframe,  please notify the Editor as I would be interested in using it too.

Speed costs money,  since a Twin Star II can’t go all that fast
A Twin Star IIs speed varies from comfortably slow to the lower end of medium.  Although we Under Igstadt bei Wiesbaden am Rhein to date lack a way to quantify velocity,  other published reports citing a stock brushed motor Twin Star IIs maximum speed as about (36) km/h ((22) mph) subjectively seem about right.  I doubt that any of these goes much faster then (50) km/h,  even straight down,  Twin Star IIs are a high drag,  medium wing loading,  slow,  sport airplane.

I once installed in the nose of a Twin Star II a combination that,  thanks to the virtual-dyno at Fly-DMA could be estimated,  of an Aveox Cusinart motor geared by an Astro Flight transmission (even if P-Calk may not be including losses within the transmission or increased in internal friction as the thrust goes up) that put an estimated (450) watts-out to the propeller at (40) amps on 4S LiPos.  At seven times what the Twin Star II was originally laid out for,  it was more then there was any reason for.  My airplanes glide a lot of the time,  similar to the Fun Jet Lite,  as soon as the amps of that high powered Twin Star II were shut off,  no matter how much power it had with the motor on,  the glide was a function of weight only.

Although I enjoy sending my Reinforced Mini Mags and Fun Jet Lites vertical,  that’s not what a Twin Star II is for.  They aren’t for going fast,  an first estimate of where increasing power against that continuously increasing logarithmatic curve of drag goes up with weight and speed goes pointless is about (100) watts-out per motor for a Twin Star II.  That’s just a personal observation,  Twin Star IIs start being reasonable performers at (50) watts-out per motor,  somewhere beyond (150) watts-in per motor,  maybe you need a different airplane and a reasonable place to land.

But,  you don’t get that shoulder wing twin that puts reasonable sized props and all but the wing tips out of danger without a cost.  In this case:  Two propellers,  two hubs,  two motors and two motor controllers,  all identical.  Some setups even use two separate batteries!  But it’s worth it,  something hard to set to words.

Old history
As noted in my book “So You Want to Fly RC” I’ve had three Twin Star IIs with which I made over six hundred cumulative flights,  nearly all with brushed motors.  Lots of brushed motors as 6 Volt Speed 400 motors wore out quickly,  like (30-40) flights on (8) NiMh cells,  sometimes way less.  Then I went to the last of the ball bearing,  brushed motors that fit in the stock motor mounts for as many as a hundred consecutive flights (at ten to twenty minutes a flight) for as many as a hundred flights without maintenance.  Today you can’t give that combination away,  not even for gas money and a case of wine.  See the ancient history section of my updated for 2012 book for beginners for brushed details,  this article centers on brushless.  But the admonition to not fly with a hangover still applies.

Let’s try it again
Similar to my revisiting the Mini Mag with modern brushless power,  a friend just had to try one,  and the adventure began again.  Among the nice things about the Twin Star II,  the wing is semi symmetrical,  it flies most of the dynamic,  traditional aerobatics,  program,  and being big and slow you can study what you are doing.  With enough power Twin Star IIs are surprisingly agile,   accurate handling airplanes.  They can be demonstrated in a fifty meter wide circle,  or one a kilometer long.  The stock from the manufacturer HiTeck 82 like Multiplex Tiny servos provide all the control,  precision  and durability anybody needs.  With their under-cambered wings Mini Mags imprecisely skitter around,  Twin Star IIs actually track well.  Twin Star IIs also make great trainers.   And they look kind of like a little Aero-Commander,  the likes of which are hanging around general aviation airports all over the world.  That looking like a “real” airplane is important to many middle aged beginners.

Part of the motivation for the purchase was a desire for a slow,  aerobatic, astoundingly durable airplane which could be landed on the farm fields we fly on here in Western Germany where I am waiting out the recession.  The Dogfighter was a step in the wrong direction for  that pilot.  After seeing me wander all over the place on windy days one of my friends has begun to realize the joy of playing with/against the wind,  IF you have an airplane that can withstand it.  My whole life,  no mater what the activity,  I have been accused of being a bad example.

His Multiplex Gemini,  with the better positioning HS65 servos and the Multiplex Tuning propulsion flies wonderfully,  but it really needs either a decent runway (which we don’t have access to),  or some work to the nose for a fully folding propeller.  Even if the Gemini’s prop folded flat,  the lower wing is even with the bottom of the fuselage, and the control horns stick out below that.  With winter on the way,  and with it’s slow flying speed (even with the tuning combination) the Gemini might be useless for about half the time from November to March around Western Germany,  except if it were fitted for floats so he could fly off of snow (ick).

That,  and,  a new radio ready Twin Star II came up used,  new in box,  for half what it originally cost.  Such enthusiasm by a,  now out of the beginner stage,  new pilot is not typical.  And maybe,  for his income (Euro100/$140 just isn’t an issue for him,  to me it’s (2/3)s  of the whole months discretionary money),  buying another airplane while bored in a dull,  dreary office waiting for something to do is interesting as he knows that it’s going to get flown.  As a,  sadly (by necessity) master scrounger,  at least it provides an adventure in RC flying for both of us.  Maybe if the original owner had been clued into what a great airplane a Multiplex Twin Star II can be he would have kept it.

Now if I can just get him to wait to fly his Twin Star II until I beef it up as I am tired of flying airplanes with dirt and grass stains embed in the foam under the fiberglass.  My Fun Jet Lite has the usual scars of my development program all down the belly between layers of fiberglass.

All those Internet reports I read about Fun Jets,  nobody even mentioned that the “heel” of the fuselage gets beat up, or that when the nose flexes on even slightly adventurous landings (without breaking off),  that the canopy gets torn and bent,  or how easily the wings leading edge gets pushed in.  I added fiberglass and carbon fiber reinforcement four times.  Irritated by the oscillations around neutral (and that long pointed nose hanging up in the grass),  I took out that odd casting flange along the sides of the fuselage forward of the wing and shortened the nose (6) cm,  now it tracks even in pitch and glides better.

Unreinforced Twin Star IIs develop a hinge just past the wing spar at around fifty flights.  About the same time the control horns start slowly tearing out,  by then if you haven’t added some tape to the leading edges performance is down some.  And you wonder why I lost faith in the other publishing authors?

So you bought the motors/controllers/props that the Hobby store had to sell you
The following in an excerpt out of my Fun Jet Lite Flight Book.  Although a Mini Mag would be a better representation test bed for comparison with the Twin Star II (just about one to one for the propulsion,  even if they fly differently),  The Fun Jet Lite is what I’m flying lately.  It is a direct attempt to quantify the selection of potential power for a Twin Star II,  except that in an airplane with the props pulling,  noise is hardly a factor.  With that pusher prop in a Fun Jet,  noise is a bad issue.  One thing I always liked about the Twin Star II,  that harmony of the two props that audibly stated “something big is coming through”.

For the start of December in Germany 2011 it wasn’t actually cold,  but it felt like it.  I am going to freeze when the real cold gets here.  The farmer has given up his pick it and pay flower fields.  Before the wife and inherited the lease on a garden I used to enjoy selecting and buying flowers for her here.  On this nearly still,  overcast moment the freeway background roar from two kilometers away is annoying.

Using the 3S 1100 mAh battery “Dick” after five seconds of warm up the count of ten to glide down runs ran:  Forty three seconds,  fifty seconds,  fifty three seconds and forty eight seconds.  Subjectively the HP Motor with the Graupner CAM Speed 5X5 climbed about as high at a count of ten as the Graupner with the Aero-Naut 6X5.5 at a count of six,  or the Typhoon with the APC 5.5X4.5 setup in a count of six,  or the Hype with the Aero-Naut 6X5 at a count of five,.  The other two (originally) identical LiPos (Tom and Hairy) gave similar results.  At least the noise went from “that’s unacceptable” of the Hype and Typhoon to merely annoying.  As best I can remember about the same frequency of a Speed 400 and only a little louder.  At least the performance and duration was about double what I remember from brushed days.  The flights seemed really short,  like ten to twelve minutes,  instead of the something over twenty minutes I’ve been getting.  Even if getting the weight down improved the handling,   this motor just isn’t very effective at converting electrical energy to flight energy.

There might be some better output efficiency by fitting a motor controler with more lead,  but it doesn’t seem worth the while.

You can tell when a motor has been run hot,  the normally nearly clear,  paint like insulation on the wires,  turns dark.  When the windings short and the motor dies,  the paint is on the verge of burning,  it has gone brown.  The windings of this individual motor still have clear insulation,  so it has never been overheated.

With that HP Motor hanging out the back in near freezing weather there was no residual heat in the motor at the end of the flight.  Since the battery’s weren’t warm either,  I wonder where the missing energy went as it didn’t go into moving the airplane.

Good quality motors (the fifty dollar range on up in this size) have magnets that will withstand nearly the temperature of boiling water and continue to be magnetic.  As the quality of a motor drops,  so too their magnets ability to maintain their magnetism with increasing temperatures.  Bottom line,  you can run a decent quality motor hot to the touch now and then,  do that with an inexpensive or cheap one,  and they half quit.  Wore yet,  just some of the magnets partly give out resulting in vibration and ok low amps performance (as the bell functions like a flywheel) to go awful at the amps and rpm increase leaving you to wonder what happened.

With their low hysterics motor plates and high performance magnets the competition motors from NeuMotors,  Hacker and Scorpion “index” between winding/magnets.  What is happening is that as the magnets are rotated past the coils they generate an opposing force.  Even sport motors at half to a third of their cost will stay in position between magnets.  Some clue how inexpensive this HP motor is,  you can hardly feel the poles when you flip the prop through.  That hasn’t changed since the motor was new (23) flights ago.

As been the case with other low end inexpensive motors,  what felt like the bearings being gritty turned out to be the dust shields,  this HP motor is now turning nearly smooth,  so maybe the bearings aren’t as junkie as first thought.  At least the paint has stayed on,  the aluminum prop carrier runs true and it hasn’t rusted.  It can’t be all bad,  at least it runs smooth,  after replacing the first shaft (it howled pitifully for the first ten flights until the shaft broke) there is no harmonic vibration anywhere in the rpm range.

The transition between inexpensive and cheap is that this motor does continue to function reasonably,  a cheap one doesn’t.  So a functional evaluation of that HP Motor is that it is at the bottom end of inexpensive.

If you had walked out of a reputable Hobby store on Convoy Street in San Diego California USA (or bought the same through the Internet) with this HP motor,  a Castle Creations (17) amp 2-3S LiPo motor controller,  a Fun Jet kit,  a container of glue,  the necessary radio equipment and a 3S 2200 mAh battery at (200) grams that HP 2208/09 would fly the Fun Jet,  or two of them a Twin Star II,  even if only for relatively short duration at low speed.  In comparison to a “clean”  or stock,  Fun Jet,  half of the extra weight of the bigger battery is represented by the added reinforcement in my personal Fun Jet Lite,  so a stock Fun Jet would only be about a fifty grams heavier or just about (630) grams with a (200) gram battery.  But at that weight a standard Fun Jet would be dinged up and torn in two after just a few flights.  Even my slightly better equipment (the Graupner,  Hype and Typhoon motors cost about twice as much as the HP) just overwhelm this HP motor’s performance.

This winding would not be strictly suitable for a Twin Star II.  There just isn’t any point to flying an airplane this size and weight on 2S LiPos any more,  a slower winding in kV 1300 on 3S would turn about a seven or eight inch prop.

Fly with motors just above the level of cheap and I will either fly rings around you,  or be in the air long after you have landed and are bragging how much you saved by mail ordering from King Hobby.  That’s if those cheap brand $4- servos haven’t quit and crashed your airplane first.

In an otherwise identical airplane,  the count of ten maximum climb to glide back down (average) for the four nearly the same size and weight motors,  all at just about the same amps and volts was:

HP average  (10/50)  typical flight duration ten to twelve minutes 1000 mAh on 3S LiPo
Graupner     (10/70)                twenty five         1000 mAh on 3S
Hype          (10/80)                twenty two          950   mAh on 3S
Typhoon      (10/65)                twenty               1000 mAh on 3S

As I noted in my book,  nobody flies dynos,  you have to try it.  Ten to twelve minutes in still air with an $18- motor when for just $30- more per motor you could have been flying for twice as long with otherwise exactly the same combination.  And that’s just twelve minutes with no wind.  Again from my book,  how much of the time and how hard the wind blows determines the importance of the propulsion.  It’s an X squared thing,  if the minimal flying speed goes up by fifteen miles an hour (because you are flying in the wind),  the HP motor can keep up,  but it would have to be operated at full amps for the nearly the entire flight to stay in the air at all.  The other motors have enough reserve that they will still be used only a part of the time to fling the airplane in the air and glide back down.  What fun!  I like flying in the wind,  it turns out that all three of my favorite airplanes,  the Twin Star II included,  fly great in the wind.  With it’s larger size to a Twin Star II gusts are a challenge and not a threat.  But that flying in the wind requires you have enough thrust and duration.  Two HP 2208/09 motors would only be satisfactory in a Twin Star II on calm or only three times walking speed wind days.

The Fun Jet Lite is sensitive to weight (390/395/410) grams,  although the other two motors put the airplane far higher,  it glided slightly better at the lower weights.  The one that was the most fun was the Graupner.  About half again louder then the HP,  but half that of the Hype (some of which was due to vibration that would not be present in an undamaged motor),  flights just went on and on and on.   Although the HP clearly loses in comparison,  if you didn’t know any better,  it DID function.

I’ll run into somebody with either no funds to afford equipment and wants to fly,  or a beginner that just needs to get started and the motor won’t last long before a crash does it in,  and give this HP motor away for two and a half dollars and a sandwich.  With Easy Star I kits going for Euro40/$50,  this HP motor in an Easy Star might make somebody a pleasant flier.

Even if it howled in the USA Fun Jet until the shaft broke on the tenth flight,  and even if it was too high a turner with a 6X3 Aero-Naut folding prop in the German Mini Mag on 3S LiPos,  it was there in the drawer.  And,  the HP 2208/09 is representative of a typical purchase by a sport flier in San Diego California trying to get by with a minimal outlay from a trustworthy source.

So,  the $15.99 plus tax Hobby People KMI LIKE 2208/09 was installed with the included output carrier hung off of the back of the motor plate using an inexpensive Robbe motor controller.  Hung off the back as a single mention by an German Internet supplier,  that their budget Fun Jet systems costs half as much and goes almost as fast as the Multiplex setup,  that by just running the motor off the back of a Fun Jet instead of inside that the top speed in level flight went from (214) km/h to (224) km/h.  A good first order approximation of the thrust increase for an additional increment of speed is X squared,  so that’s a ten percent improvement in thrust by just a simplest possible change!  As dull as reading seemingly the same stuff in successive Internet reports gets,  sometimes there’s a worthwhile tip in there.

My own climb/glide testing confirmed that running the motor hung off the back was an improvement with no observed additional risk to the motor.  Another reason for trying this motor a third time was curiosity,  if this HP motor might be usable in a Twin Star II as it fits in the stock Multiplex motor mount.  The Graupner and Hype motors don’t,  their diameter is too big.

That motor is still available from Hobby People, from the Internet copy of the manufacturer’s specifications:
Weight    (44) grams    Graupner (45),  Hype (60),  Typhoon (50)

Io    1 Amp    Check with Steve,  I think this has something to do with the minimum current required to get the motor to turn.  That also reflects my attitude about designing motors,  let somebody else do it. 

9T    nine turns of wire around each coil  How the coils are connected makes up part of the kV too,  see Delta verses Y variations.

kV    1920    That’s two thousand rpm per applied volt,  optimum for a whole bunch of Multiplex airplanes including a Fun Jet Lite on 3S,  or Mini Mag and Twin Star II on 2S LiPo.  But let’s be done with 2S LiPos in airplanes this size,  get a motor in the kV 1100 to 1300 range for your Twin Star II if you go with 3S LiPos.  Better yet,  go to 4S LiPos.

Amps    12/16  That’s twelve amps continuous with reasonable cooling at shirt sleeve temperatures.  The sixteen amps is for a short burst,  like ten seconds for inexpensive motors as the excess heat is stored in raising the temperature of the motor itself.

Eff.    80%    That is if everything were at optimal.  Only rarely are we going to be running an electric motor anywhere near it’s theoretical efficiency.  As a relative value though it’s good for a basic comparison.

Resistance 78mOhm    This is for the virtual-dyno.  It gives some idea of the relationship between how much resistance the wire alone gives. 

Inductance?      These pulsed DC i.e. alternating current at a variable frequency motors work on magnetizing metal plates,  then reversing the charge.  One way to save manufacturing costs so it can be sold for less is to cut quality here,  resulting in higher loses.  They left that specification out.

Poles    14    Effectively this is the “gearing” of the motor.  If also figures into the kV,  for the desired speed range at this diameter in a Fun Jet on 3S it’s just about right.  By just adding turns per coil and how the coils are connected the same basic components could produce a motor (visually identical) that turns a bigger propeller at lower rpm.  If you insist in sticking in the mud and flying on 2S (or NiMh car batteries) this is a correct combination for a Twin Star II,  but on 2S with this HP motor it’s going to be a slug and you will crash more due to lack of power. 

Esc    20-30 degrees  The rest of you need to pay more attention to this.  The inexpensive Castle Creations general purpose motor controllers,  are,  as are virtually all motor controllers that are not specifically settable (Multiplex included),  set at about 5 degrees lead.  But,  since we want to analyze what you get for what you likely spent,  all four motors mentioned here (HP,  Graupner,  Hype,  Typhoon) used fixed motor controllers at (probably) 5 degrees lead.

RPM?    Good bearing cost more then cheap ones.  Sport Scorpions and Hackers in this size are rated at (25,000) rpm,  which is plenty enough for sport flying.  See my translation of an article where a very experienced fuel burner pilot made the dumb mistake of giving an electric motor maximum amps under no load to check the transmission,  and nearly wrecked his expensive combination by turning it over at speeds that ruin the bearings.  The inexpensive stuff burns up even easier.  NeuMotors bearings are good for (60,000) rpm.  Since if you bought just the bearings they would cost more then a decent lunch each,  it makes you wonder how they can sell their motors for that low a price.  HP omitted any bearing information.  A reasonable mail order source of replacement bearings is Boca Bearing out of Florida.  They list ones about this size for $16 a pair,  plus shipping.  Why not just buy a decent motor in the first place.

If we take the manufacturer’s rated (12) amps at maximum continuous current,  of a 2S LiPos (7.0) volts (measured under load,  before the motor controller,  after ten seconds to get to the (nearly) flat part of the discharge curve) that gives (84) watts-in.  Although the manufacturer gives an efficiency of (80)%,  from running other similar motors (Sport Scorpions at (84)% maximum efficiency) on the virtual-dyno that would be running at about (59)% efficiency,  so let’s estimate the HP at (.55) efficiency.  That gives (46) watts-out,  or about half again more then 6 Volt Speed 400s deliver,  at much lower motor(s) weight.  But Twin Star IIs aren’t that sensitive to (100-35)x(2) = (150) grams.  So the gain in thrust would be relatively big in percent,  but still small.  See my work up of testing brushed motors verses modern brushless in “So You Want to Fly RC”  for 2011,  The Motors,  starting page 8.

Far more effective would be (12) amps at the (11) volts under load of a 3S LiPo for (132) watts-in and at (.68)% efficiency (90) watts-out,  or three times the original brushed motors.  I can’t put to words the effect of tripling the power of an airplane while dropping the weight,  it dramatically changes how an airplane flies.  The target is about (100) watts-out per motor on 7X4 to 8X5 propellers for a Twin Star II.

Since the latest Multiplex,  and other,  motor controllers can regulate 4S LiPos directly,  let’s start thinking fourteen volts,  even if acquiring the Multiplex Twin Star II package as by just fitting a 4S battery and smaller props it would function well,  very well indeed.

Weight,  ready to fly,  without the battery was a surprising low (390) grams in the Fun Jet Lite.  That’s (5) less then the Graupner and (20) less then the Hype, or just the difference in the weight of motors/propeller/collet.  Equipped with a  Graupner Speed CAM 5X5 (if you can’t source Graupner or Aero-Naut,  an APC in 5.5X4.5 works about as well) it drew the following:  Note that the first value is “burst” i.e. maximum values for dimensioning components,  after ten seconds the amps (and watts) would drop about ten percent.  Watt-out are based on the lower value.

Four year old Kokam 2S 2200 mAh    8.3 Amps at 6.9 Volts    (57) watts-in at an efficiency of (55)%      (31) watts-out  At least theoretically the same as a 6 Volt Speed 400  In reality,  this minimal output would still be an improvement.
Three year old Neu1 2S 2200 mAh    7.4        6.1        45      25
Three year old Neu2 2S 2200 mAh    7.0        6.2         43      24
Auto  3S 3200 mAh crashed             9.1        7.4        67
This battery might still go good in a Twin Star II.  It is also the reverence for a healthy 2S LiPo. And,  it could be converted to 3S LiPo.

One of the Neus hung in a tree for two days,  none of the three 2S batteries has been used for six months.  Trial use of this motor combination would require a larger diameter propeller as (20) watts-out as the battery draws down would be insufficient for fun flight.  Even then,  from tests in the German Mini Mag,  I don’t think it would be worth the bother.  That hanging an airplane on absolute minimum thrust is a stunt best left to really experienced fliers.

Lemon,  failing 3S 1300 mAh        12.3        11.1    136/125  max/run
It’s time to discard this one.
Neu    3S 2600 mAh            15.0        10.5    160/144
too big and heavy to fit,  but at room temperature,  used for reference
At an efficiency of 68%    98 watts-out
Neu   3S 1100 mAh at near freezing    15.0        10.4    156/140
A surprise that the low temperature didn’t throw it off.
Even if this was a “inexpensive” motor,  efficiencies for the far better quali