Daily Archives: May 24, 2013

 

 

This is an original year 2013 article for The Silent Electric Fliers of San Diego.

 

I am appalled at the omissions from the rest of the material available in print. Be that as it may,  if you feel the need for pictures about how an airplane goes together,  or want to enjoy the creativity of others,  the Internet is where you start.  Keep in mind that almost all modifications (even paint,  heavier then most pilots realize) increase both the weight and drag of the Twin Star II;  Then go for it!

 

The pilot may still be the determining factor

 

As noted by a fellow SEFSD Member;  When we first met you were lying on your back,  no shirt,  no shoes,  flying a Zaggi.  Since then I have reformed myself,  down at Mission Bay I now fly strictly by the rules.  I even wear shoes.  

 

That club member has been flying a Twin Star II on two parallel BECs for years.  At first running both motor controllers with their built in BECs in parallel went ok in ours,  sort of,  for about twenty flights.  Even though the motors had an odd harmony as they pulsed back and forth.  But after a while the radio kept cutting out increasingly often,  I pulled the red leads from both motor controllers and added in a (3) ampere auxiliary BEC for Euro8/$11-,  that took care of the pulsing and radio cut outs both.  But now we had a problem with room in that narrow trough down the middle,  not enough for everything in there.

 

Sometimes it’s ok to run two motor controllers in parallel with both Battery Eliminator Circuits connected.  You could luck out and get two BECs that electronically really match,  and,  as the internal components change values over time,  stay matched.

 

So my deference to that club member,  a talented RC pilot/builder and somebody I’ve always enjoyed hanging out at the field with,  who’s Twin Star II brushless is a “light” version,  no reinforcement or aeromodeling customizing at all,   that he uses for just drifting around the flying patch. What a graceful vision his flying is,  like watching a sea gull soar around,  he too can land right at his feet.  At no load all four servos and the radio draw something on the order of (0.2) amps.  You should check your servos,  first on the bench,  then in the airframe,  with a watt meter.  Then check everything again for the first few flights with you fingers,  make sure nothing overheats.

 

But,  I fly the stuffing out of my Twin Star IIs,  more like a crow trying to get to a road kill first.  If a single servo is blocked (like the hinge isn’t right) it can draw an amp.  Although I doubt maximum in the air loads on all four servos ever happen simultaneously,  if there is somebody that goes for it in a Twin Star II,  which drives the power draw of the servos way up,  I’m it.  From now on I’m adding an auxiliary BEC to power the radio gear on any and all my brushless twins.  The economics of adding in a separate BEC becomes trivial compared to the overall expense of a Twin Star II.

 

The hardest to describe
factor may be the batteries.  Holding the battery labeled capacity constant at 3S 2200 mAh LiPos:  
Just moving them back to correctly balance made a big difference.  
Then by just switching from the lowest cost to a little better quality we picked up about ten percent in performance.  
Advancing the timing from five degrees to twenty and then twenty five degrees picked up another fifteen to twenty percent.  
Pre heating the batteries added yet another little more.  

 

Flights went from twelve minutes to twenty five.  What is more important,  every improvement made the RC flying experience more fun.  All with the same motors,  the same motor controllers,  the same propellers,  the same Twin Star II airframe and the same pilot.

 

From the gallery
When I fly my foam airplanes among other experienced pilots I always get some commentary.  It looks beat up.  How do you get it to stay up there so long?  Why does yours fly better then anybody else’s?  Last time I heard that at Mission Bay the flight log written right on the foam wing had reached flight (318).

 

So think about it next time you read a glowing report where they fitted it with just one combination and are so delighted they published after just a couple of flying sessions,  before wear and tear became a factor,  without anything to compare it too.  Is that the truth,  the whole truth and nothing but the truth?

 

What I fly are mostly low net cost per flight,  
almost no maintenance,  carefree RC airplanes which may be landed almost anywhere.  

 

Although I always enjoy being down at Mission Bay,  sometimes I need to see out and get away from urbanity.  For which,  when EPP first became available around 1995,  I started flying the hills and mountains of San Diego County.  Unlike the Austrian,  who’s all balsa flying wings were detailed in a different article,  who watches hawks for information on flying conditions,  I watch crows.

 

Generalists,  omnivores and urban pests,  crows are dependent on their wits to get by as they are only average fliers,  abet maneuverable and good at landing,  even in the wind.  Crows have to be opportunistic,  but after they have taken care of the mundane task of getting enough to eat (other birds babies,  garbage,  road kill etc.) that turns to amusing themselves.  Nothing else that flies is so inquisitive about the world around them.  When my out of control flying wing flopped to the ground,  way out of my sight,  after hours of searching Cowls Mountain,  I realized that half a mile away the crows were all excited about something.  From the crow point of view something flying poorly that hits the ground and holds still is important.  Even if it was fluorescent green and they had never seen anything like it before.

 

Hawks and gulls are experts at riding updrafts,  but for life’s necessities,  it if doesn’t get them on their way,  they don’t,  if it isn’t what they usually eat,  they aren’t interested.  Crows though can be observed entertaining themselves for hours with unexpected air currents.  Just the kind of thing I’m looking for too.  It was sometimes too dangerous with gliders,  a modern Twin Star II with power to climb out of nearly anything is just the equipment for daring where we never would have before.

 

Somehow translating
an extensive article from German into English about a transmitter that costs Euro2500/$3375- didn’t seem revalent,  even more so since there are already English language reviews for the USA version available.  Like you don’t need sixteen servos,  with provisions to double that,  in a Twin Star II.  My most expensive radio to date is a Spektrum Ds6i, the individual servo offset at the ailerons comes in handy for tuning aileron differential and even throws at the rudder.  If you are going to fly like the pros you need exponential.  For the rest of us,  a Twin Star II flies just fine with the simplest of radios,  four, or even three,  channels.  

 

As for those neat programmable servos from Graupner,  the applicable to the Twin Star II size cost Euro28,60/$37- (each,  plus the programmer) they are for a different airframe and clientele.  Such as our F3A contingent running a separate servo for the stearable nose wheel in addition to the rudder.  Being able to set individual limits probable wouldn’t make any worthwhile difference to flying a Twin Star II,  but it they would have made my Acromaster more usable.

 

Neither did it seem worthwhile to translate an article about using hardware store stuff to create a warm briefcase to transport LiPo batteries in,  for San Diego,  although we start heating our batteries here at November through April in Germany.  Even in San Diego there would be some advantage to preheating LiPos,  but not much.

 

Our thanks from The Somewhere Out There Fliers (SOTF) to The SEFSD Newsletter including FPV as we are interested in it too.

Heaven has yet to deliver us a ready made Champion
Zu Deutsch;  Es ist noch kein Meister von Himmel gefallen.  Among the SEFSD are at least two world ranking racing pilots,  a bunch of really good aerobatics competitors and a whole lot of skilled pilots and builders.  Although some of us learned faster then others,  and some of us fly and build better,  we all flew a lot,  which also meant learning how to acquire and use the equipment (a clue,  it doesn’t all work as advertised) to get where we are.  That includes the several military,  airline and private pilots.  My Grandfather,  who made his living estimating credit risks,  opinioned;  A mans opinion is no better then his information.  We also have the manufacturer of the worlds best RC electric motors and at least two hobby dealers.  One of at least a dozen engineers myself,  the club is loaded with technical nerds who love to fly.

 

From my book for beginners “So,  You Want to Fly RC” flying is only learned by flying.  And,  determining what functions,  and continues to function,  is only learned by actually using,  and comparing,  the stuff.  It’s only recently I started keeping detailed records,  I made at least two thousand flights with,  among others,  various Multiplex foam airframed airplanes the last eight years.   

 

As for a carefree,  does some of anything RC airplane,  nothing with the motors in front that can withstand a crash beats a Twin Star II brushless.  At what our competition pilots do best,  I’m no match for them.  I have benefited from our experts flying and the technical knowledge they shared with me.  After getting the equipment to function really right,  it’s up to you to use it.  I fly,  and land,  under conditions and places that nobody but a fool would.  It might have something to do with having learned to fly sitting next to my father in Alaska’s bush,  our lives dependent on getting back down able to walk away from the landing.

 

Any Economic analysis
would need to include how much the item originally costs,  maintenance,  how long the device is going to be in use,  consumption and any residual value.

 

Let’s start with the expected service life of the Twin Star II’s airframe.  From watching down at Mission Bay,  figure three flights at fifteen minutes to half an hour each time you are there.  As much as I like the Twin Star II,  most of us bring a couple of other airplanes with us.  With some nominal additions including some strapping tape under the wing and down the belly plus improving the servo arm to the foam attachment,  that could well go on over the course of a couple of years for one hundred to two hundred flights.  If you don’t reinforce the wing it will start bending out beyond the spar at fifty flights,  that’s when the elevator horns start tearing out too.  The servos may go five hundred flights.  Dings and tears at the hinges are easily repaired.   About all that wears out is the motors,  props and batteries.

 

Each and every flight will contribute to you having enjoyed your limited stay in this life.  Even if occasionally changes go away from being an improvement,  as long as you didn’t crash and ruin the airplane you learned a little,  even that is still an important part of the journey.

 

Let’s figure you get the kit at $110-.  To that you need servo extensions and glue for another twenty bucks.  Servos can be either the (recommended) HiTeck 82s or 85s at $25- each times four (plus tax),  or the house brand at $12- each for four.  I figure minimal paint as five bucks.  That results in airframe versions from $240-.  plus wiring harness,  to $190- plus wiring harness.  

 

There are two ways to get a motors wiring harness.  Either just buy the whole setup from Multiplex (good stuff and worth every bit of the one and a half times the price of a bare kit if you don’t really know what you are doing) and add a ten dollar (3) ampere (minimum) Battery Eliminator Circuit,  or make your own wiring harness for about twenty bucks plus BEC.  Although on 2S LiPos the linear (non switching,  excess voltage is converted to heat) USA Multiplex motor controllers can supply (2) amps to the electronics,  and probably not much less on 3S.  I wouldn’t trust just one motor controller’s BEC when ten bucks just made the problems of a harmonic between the motors and ever increasing radio failure,  even on 3S LiPos,  ends.  

 

I haven’t hooked up an oscilloscope since about 1988 (when a developing sensitive to solvents ended my days in factories) and you as a hobbyist don’t need the details,  even in a single motor setup there is an issue of feedback (which is why the all out equipment uses opto coupler motor controllers to completely eliminate any influence from the constantly varying flight load of the motor feeding back to the input electronics) which,  for ten bucks,  I just didn’t want to learn about in a twin.  That’s why I didn’t disconnect just one motor controller’s red lead,  even though the latest Multiplex 4S motor controllers include switching BECs so that increasing the input voltage doesn’t reduce the BEC’s output.  That and (2) amps for the electronics,  even a switching one,  so the input voltage is less of an issue as in the latest from Germany Multiplex,  is just a little to little for a Twin Star II being flown hard.  

 

Mid 2005,  Looking for a lost Easy Star,  I taped a digital camera set for video to that Twin Star II with brushed motors.  Encouraged by spotting the lost Easy Star in once sequence,  I sent it back up,  not realizing that although rated for up to (11) volts,  the BEC was then too weak to power the electronics,  and stood there helpless on the bluff over the Santa Ana river as it nosed over and crashed after the radio lost all contact.  The difference between the previous twenty five flights with the same combination was that with the additional weight and drag,  that instead of coasting motors off some of the time and not needing much control surface inputs,  the brushed motors were now being run at continuously at maximum and the flight surfaces constantly being manipulated.  About to give up after two hours of following the few trails through the otherwise impenetrable scrub in the forest,  I heard servos chattering and found the wreckage,  less camera,  about ten feet from a trail.  After cooling down,  everything still functioned.  So,  the parts were installed in another Twin Star II,  with a different single brushed motor control.  The servos went over six hundred flights,  other then the constantly wearing out motors,  props and nickel based batteries.  

 

However,  since Sureflite checked out,  cruising the Internet for USA prices is harder.  Hobby People carries the Twin Star and other Multiplex stuff in their Convoy Store,  but not currently on the internet.  I didn’t find a USA reference for the Radio Ready version,  which,  if you aren’t a competent builder,  I recommend.

 

Although the airframe is identical,  propulsion wise the German kit is currently different then the USA one.  The one we get here in Germany has two (55) gram Permax (the Multiplex house brand) motors,  aluminum motor plates,  really good Multiplex 8X5 props and notably worthwhile Multiplex motor controllers that go to 4S LiPos and have adjustable timing and switching BECs,  so the output to the electronics isn’t a variable of the input voltage of the battery.  Although we determined that our twin still had to have a separate power supply for the electronics,  until the motors bearings started giving out around a seventy five flights,  and the owner gained enough experience on other RC airplanes to understand the justification for better (more expensive) equipment,  we thought it was great.  

 

Let me repeat that,  the latest Multiplex 8X5 propellers are so good you should plan the rest of the Twin Star II propulsion around them,  and the latest Multiplex settable timing motor controllers with their switching BECs are what inexpensive motor controllers should be,  even though for a twin you need a separate single BEC.

 

The USA propulsion kit has two (65) gram HiMax motors and APC 9X6 propellers,  that may be the second version of brushless offered by Multiplex for the Twin Star II.  By way of Tower Hobbies it’s about $160- plus tax and shipment.  The motors are Multiplexes specification for a moderately performing,  easy to use,  medium priced item.  You can take their pricing as about the lowest price level that will consistently perform as advertised over a reasonable duration.  Our experience with two (80) gram HiMax motors (intended for the Park Master and low powered Gemini) was that the bearings are giving out at about a hundred flights,  and the motors magnets are fading out.  What we are using in Germany are Permaxes (the Multiplex house brand) for which a durability comparison to the HiMaxes is neither valid,  or completed.  

 

Replacements bearings for the HiMaxes are available from Boca Bearing out of Florida at $17- a pair plus shipment,  they are probably better quality bearings then the originals.  That’s if you can get the set screw to back out and take the motor apart.  For an average pilot,  when you fly a Twin Star II you need to configure for continuous maximum output,  which for the (80) gram motors is about (150) watts-in.  That note from Multiplex about the higher short term limit being for (15) seconds,  they weren’t joking.  For the Twin Star II the equipment needs to be dimensioned for a (100)% duty cycle (the lower i.e. continuous rating),  even though we are often flying at part amps,  and glide some,  we may just as well be doing endless loops and careening around at full amps for a good long while too.

 

Multiplex USA offers through their web site what appears to be a simplified virtual dyno of the same as DMA used to make available and is,  in the full version,  for NeuMotors motors,  also at the NeuMotors website.

 

Anything that flies may crash,  the resistance to which of the Twin Star II is unmatched by anything else that flies full house.  Even if you manage to damage the airframe beyond reasonable repair,  you can (usually) remove the motors,   and the servos to go into another airplane.  You can actually hit the ground so hard the electronics quit,  and still usually reuse everything but the foam.  We lost sight of one Twin Star II,  it landed in a vineyard,  the airframe sliced up.  We cut the components out and put them in another Twin Star II.  If you wear out the motors,  or decide you want something else,  they are easily replaced in an hour or two.  

 

Unlike a car,  or man carrying airplane,  there appears to be little in the Twin Star II airframe (provided that you reinforced the wing and control horn attachment) which sets an upper service life limit.  Time,  as the foam goes brittle,  wear on the servos,  maybe the foam giving out at hinges and nacelles,  rusting of the control wires,  but probably crashing sets the service life.  

 

Maintenance consists of replacing propellers,  motors and batteries,  and patching up the occasional ding and rip.  Although at seventy flights on ours it isn’t proven,  it appears that those Multiplex 8X5 propellers may last at least long as our goal of one hundred to two hundred flights.  APC propellers last until you hit something with them.  I’m going to put propellers in the same category as batteries,  part of operating costs.  We are getting about fifty flights out of the $20- house brand Hobby People 3S 2200 mAh LiPos,  flown starting at room temperature and run nearly all the way down at thirty five amps before the performance starts really falling off.  Preheating LiPos improves both the performance and durability of any LiPo battery.

 

Residual value of the airframe is going to be taken as zero.  Oh,  the control horns and a few other parts might be reusable,  but for accountancy at the end we write the whole thing off.

 

Let’s chose a weight and speed
Although no other airplane is so often customized,  that gets into Aero-modeling,  which is beyond the scope of this article.  The Twin Star II already looks a lot like an Aerocomander,  a twin still common at General Aviation airports.  Corona California had five awaiting complete restoration,  until the most recent 2010 flood.

 

My recommendations for tape,  fiberglass and recognition paint add about thirty to forty grams above a bare airframe,  they extend the life of the airframe from starting to fly odd,  and then fall out of the sky,  out beyond about fifty flights into the hundreds.  Twin Star IIs aren’t all that sensitive to weight,  and the balance can be anywhere within half an inch of the wing spar,  they still fly well.  The stock from Multiplex decals are great stuff (heat shrink them on and they outlast the airframe),  but I like my own paint.  You must do something to be able to discriminate top from bottom of this otherwise all white airplane with a range limited only by the pilots sight.

 

The airplane weighs just about the same no matter what servos or radio equipment we fit it with,  the differences are mostly the weights of the differing batteries,  motors and propeller assemblies.  For my fourth personal Twin Star II I substituted a carbon fiber spar for the fiberglass one in the kit.  It’s not that much of a weight savings at (20) grams,  the supplied fiberglass one functions perfectly,  but then it’s a one time fifteen buck investment that makes every single flight,  and the stubs fit my latest Fun Cub,  where weight really does make a big difference.  It’s also an introduction to how great an airframe it is for how little money,  improvements will cost ya.

 

Propulsion Combinations:
This article concentrates on currently available equipment,  it stays with cost effective direct drive outrunner motors in the one hundred to two hundred watts-out range that will bolt right in with at most a little grinding of the mounts for clearance.  Although there are some exotic combinations available,  like geared inline (20)mm diameter motors (a hundred and thirty bucks each,  on up, although you can save a little on the motor controllers with their correct for inrunner five degree lead) on custom mounts,  most people wouldn’t try them,  and I’m not sure I would either.  Even though you need to complete the evaluation of the motors by including the motor controllers and any lead.  

 

A historic relic,  a majority of motor controllers in the Twin Star II price group (down near the bottom) had,  until recently,  fixed timing correct for inrunners,  that sort of functions for outrunners.

 

Although I enjoyed them in other airframes (see my Mini Mag article),  high rpm turning direct drive (no transmission) combinations using inrunner motors have been omitted for the high drag,  low to medium speed Twin Star II as 3S and 4S LiPos make them,  in a Twin Star II,  inefficient.  Inrunner motors put out plenty of power for their weight,  more then outrunners in fact,  but lately they aren’t making short (28) mm diameter inrunners,  at least not in any kV a Twin Star II can use without a transmission to get the output shaft rpm down and turn a reasonable sized prop.  It is the small diameter fast turning props that aren’t optimum where the inrunners lose out and you really don’t need (200) watts-out (each) Typhoons,  or (300) watts-out (each) NeuMotors,  in a Twin Star II.  The market just isn’t providing inline motors in sizes reasonable for the Twin Star II,  at the moment.

 

If I wanted to win a Twin Star II pylon race,  where class rules required using the stock motor mounts,   I’d use my (80) gram Typhoon brand inrunners with their 2500 kV on 7X4 props with 3S LiPos.  Typhoon as with their lower kV they can swing a bigger prop then the NeuMotors of the same case size at their kV of 3000.  NewMotors weigh ten grams less,  they are at least ten percent more efficiency,  far longer lasting bearings,  they have maximum quality control,  but with that higher kV on 3S they need to swing just too small a prop.  

 

When we flew with seven and eight cell nickel batteries (about the same voltage as 2S LiPos,  although much heavier) and had fewer choices in outrunners,  it was clear cut,  inrunners ruled.  It is mentioned here as inrunners like that were one of the first offerings from Multiplex for brushless in the Twin Star II.  

 

Something you have to watch as you research the Internet,  without a date you may be reading something as out of date as fourteen years.

 

That too much of a good thing applies to even the highest of electric racing classes F5B and F5D.  Before they started leveling the field (some) by limiting the amount of electrical energy,  with our own Steve Neu providing the motors for a third of the whole international field.  Of our three USA pilots,  his personal setup drew two hundred amps on NiMh batteries,  the other pilots,  at their request,  used two hundred and fifty amps combinations.  At two hundred amps the batteries held,  at two hundred and fifty it was a (expletative deleted) shot,  sometimes the batteries exploded.  Sometimes less power,  and getting the whole system really matched is the key to best performance.

 

Since it was easy,  I once put a Aveox Cuisinart motor,  geared down with an Astroflight gearbox in the nose of a Twin Star II for a virtual dyno estimated (450) watts-out on 4S LiPos.  It was too much of a good thing,  other then climb nothing was better and the whole airplane shook.  Any thoughts of a single motor version with a cleaned up wing ended.  With a five blade prop and just a little trim here and there though it would have looked like a neighbors airplane at Corona Airport.  

 

A recommendation about higher power;  Above three hundred watts-out from the pair of motors,  get a different airplane.  It’s an X squared thing,  above some speed the requirements to go faster are all out of proportion to the required power.  Above about (300) watts-out all you get is more climb,  and the whole Twin Star II starts to shake in the air (due to turbulent buffeting) even if the motors run smooth on the bench.  That estimated four hundred watts-in works out to about (20) amps/motor on a 3S LiPo’s under load voltage of ten volts at a likely seventy percent (0.7) efficiency in the air.  Matching the propellers to what your motors can do and your flying style are more important then just more watts-in.  

 

Let’s be airplane designers here,  with all the choices in motors now days,  you should start by selecting your propellers first.  

 

To complete the reminder that you are getting censored information in the vast majority of publications,  now that we have airborne monitoring,  that’s three hundred watts-out in the air!  Figure on 3S LiPos about twenty five amps maximum per motor on 3S on the bench after ten seconds of full amps as even in a Twin Star II the current draw goes down in the air as the props are swinging in moving air.  Did you ever wonder why our competition pilots spend so much time experimenting with different equipment?  And they don’t base their evaluation on just one flight either.

 

Before you over prop and over load an inexpensive motor I’d consider upgrading the whole assembly (controller/motor/props) rather then just more amps.  If you have a vibration problem and know that your props are balanced,  then check the prop collets.  I pay $14- each for Aero-Nauts collets in the (3.2)mm size,  they often eliminate the off centering that causes the vibration.

 

As for why you can’t put even more power to a Twin Star II,  the foam at the motors and then,  even if you beef the motor mounts and whole nacelles up,  the wing attachment can’t sustain it.  

 

If you are using the Twin Star II as a glider tug,  you may need to open up the battery compartment (before you glue it together) for a bigger battery,  I use top quality sport motors (Hackers) in the (80) gram size swinging 8X6 Graupner propellers.  Those props are rigid and strong enough to break plastic motor plates if they snag something on landing.  As an easily available alternative two of the HiMax (80) gram motors sold for the Park Master and Gemini would work well.  Curses,  we have two,  but we wore the bearings out.  Experience in a Fun Cub (with improved motor air ducting) and Gemini and Acromaster was that over (150) watts-in continuos the HiMax (80) gram (28) mm diameter motors over heat.  See that note from Multiplex,  maximum output for (15) seconds,  they weren’t kidding.

 

It’s taken over six months to get a replacement wing.  I finally just ordered another whole kit.  The other one will be sold to somebody someday.

 

Cheap motor controllers aren’t going to get the best output either.  Other then Scorpions,  if you read the fine print most outrunners need twenty to twenty five degrees of lead,  which requires a more elaborate motor controller and you having the skill to use it.  Get the lead set right and subjectively,  you get twenty percent more duration out of the same combination.  As you advance the lead from five degrees (standard for motor controllers that can’t be set,  just about right for inrunners,  which are now less