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

The Acromaster Pt.1 & 2

This is an original year 2012 for the SEFSD extended article covering the whole experience of acquisition,  modification,  fiddling with different propulsion components and flying an Acromaster.  If you are looking for a shilling,  magazine style,  I just put it together and everything is great,  article that  glosses over anything bad,  doesn’t at least try other equipment,  lands their RC airplane at flying fields only,  and doesn’t fly it long enough to uncover weaknesses,  go check the Internet.  There are many worthwhile articles with pictures covering the assembly available there.  Why the rest haven’t caught on to improving the weaknesses,  or trying different combinations,  is a mystery to me.


I usually don’t trust a report unless the pilot has made at least fifty flights.  I stopped my fiddling with my Acromaster at forty five flights because of it’s failure to satisfy.  It’s a fun airplane,  but with an airframe and airfoil prioritized on older,  heavier components,  it’s really for close in ( a hundred yards) and slower flying,  maybe modern airframes have passed it by.  The Acromaster is durable,  but no match as a flier to the modern,  if far more fragile,  similarly equipped ARFs,  or even the latest from Multiplex itself.

If you just want to cruise around the flying patch,  this isn’t the right airplane either.

Although it can buck wind better then a Parkmaster,  and flies slower then a Gemini (it was flown next to stock,  if reinforced,  ones often) either of them,  depending on wind,   are more fun (a lot more fun) to fly.  One nice thing about the Acromaster,  even without modern gyroscope stabilization,  it can buck wind better then the flat plate equivalents.  If you accept it’s inherent lack of self righting it makes a good general purpose sport airplane too.  Those thick trailing edges are a poor man’s constant speed control (and restricts the oh no speeds for pilots of moderate skill),  at low speeds they don’t matter,  at speeds go up they slow the airframe down,  something which the recommended large diameter very flat pitch propeller does too.

You have to use good servos to really make use of the Acromaster.  As with any airframe cheap motors,  controllers and batteries will cut into performance more then any magazine article will admit to.  I’ve been at it long enough to get by with a standard four channel radio,  this one benefits from separating the ailerons to different,  tunable channels and needs exponential if you are going to hover it.

With large control throws though it could hover and pretty much do outdoor what the indoor crowd enjoys,  but we have easily affordable Depron flat plate fliers that outperform the Acromaster for that type of flying at Mission Bay,  out doors,  as long as there is nearly no wind,  at a lower price.

Even good quality servos don’t always match,  which was an issue with this specific airplane,  check the throws on the ailerons to make sure they are the same!

An attempt to use an inexpensive motor,  which  proved to be just plain cheap,  was frustrating,   get at least the Multiplex recommended one.  Attempts to use a lighter Park Master motor ((80) grams verses (135)) on 4S LiPos didn’t work out.  The airframe needed both the weight forward and the occasional burst of torque for worthwhile flying,  the smaller motor overheated if flown constantly at (225) watts-in.  The best choice was a high end (105) gram outrunner (Plettenburg) on a high end (Hacker) motor controler on 4S LiPos at about (350) watts-in.

It would come down to what and where you expect to fly,  for flying the full aerobatic program(s) at Mission Bay you might want a modern plywood/balsa ARF or one of the newer Multiplex offerings instead.  If your reflexes and eyesight either never were all that good,  or are falling off,  the Acromaster could be a great choice.  The next best alternative for the slow of reaction group who want to try acrobatics might be a Twin Star II with modern equipment at about 2/3s the cost.

The Acromaster is a Slow to Zero Ground Speed Airframe

Although there is no exact visual match in Real Flight 5.5,  that isn’t much of a disadvantage as there is a whole range of similar virtual airplanes in there.  In contrast to some other Multiplex airframes,  the Acromaster isn’t dramatically different from the norm for it’s type,  just the opposite actually.  It is in durable,  affordable foam with a four foot wingspan an F3A type RC flying machine with the neutral flying to be expected of that type.  Conventional aerobatics airplanes expected to fly a standard pattern of aerobatic maneuvers are more shades of the same thing then different from each other,  going too far outside the performance envelope would be a disadvantage.  With it’s moderate wing loading and huge,  low speed control surfaces it can be stable for traditional aerobatics,  or wild for the newest ones.

Depending on the adjustments at the control surfaces throw and power the Acromaster can be tuned to the skill level of the pilot,  who should be well beyond the beginner stage.  The Acromaster can be a wonderful second aerobatics RC airplane capable of about any maneuver an RC airplane can perform,  and cause to smile for pilots nearing the end of their life’s journey too.

One thing the Acromaster isn’t,  with no self-righting characteristic,  it goes where it’s pointed and keeps going so until directed differently,  the pilot must regularly exercise his will to keep it flying.  With it’s moderate wing loading,  puffs of wind can change it’s direction.  If what you really want is something for relaxed extended just give a direction now and then cruising around the flying patch,  get something else.  It’s not that this bigger flies better airplane isn’t,  for it’s type,  stable,  even in the wind.  But,  if you get distracted for just a couple of seconds,  any F3A like airplane is already into the ground.

The range of satisfactory propulsion  combinations far exceed the specified ones.  Due to the short landing gear it needs a decently flat,  even,  wide,  place to put down on (or long grass) although,  with nothing more then the addition of some packing tape to the wings leading edges (and an acceptance of breaking large diameter APC props),  it can tolerate the weathering shredded wood,  brought to a stop by a small bush,  of a just off the runway landing many times.

Let’s Separate the Durable from the Competition Stuff

Although no longer under contract to Multiplex,  the German ace RC pilot Martin Müller reportedly had a hand in laying out the Acromaster out.  I saw his brother fly last summer demonstrating the Multiplex line up,  it reminded me of landing on the mat looking up bewildered at martial arts experts.  My average reflexes and eyesight set a strict upper limit even back then.

But I’m not going to be torque rolling between trees,  or breaking the laws of physics with my Acromaster.  Nor is anybody at the national or international level going to be entering an Acromaster in competition.  Thanks to it’s size,  medium wing loading,  and consequently lower speed in both airborne maneuvers and landings,  even marginal reflexes and eye sight are entirely adequate for sustained use of the Acromaster.  A pilot with below average flying skills can enjoy flying,  and look good,  with an Acromaster.

Although somebody can screw up the assembly of anything,  it looks like it would take some real fumbling to get the Acromaster together so wrong it couldn’t be used after somebody that knew what they were doing set it up.  As always,  one of the benefits of the SEFSD is a collection of other pilots ready willing and able,  if you just ask for help,  to verify the serviceability your equipment.

I’d like to remind our expensive and hand built contingents of a one sided offer. For the few minutes when somebody has a three thousand dollar airplane in the air,  or something special with dozens and dozens of hours of building invested,  I want them to be able to concentrate on flying without the distraction of my careening around.  If it’s just a couple of us in the air and I can land off field,  I’ll stay clear of a box around the runway.  If I need the field to land on,  before you send your airplane up,  I’ll land and watch as I don’t want to risk a collision.

Just tell me when you want to fly,  I’ll get my foam and fiberglass buzz bomb out of the way.  I can take the wings off of an Acromaster and toss the thing in the back seat,  they can’t.  I may be a flying fool,  but I’m a courteous fool who will enjoy watching them fly.

I did a rethink about how to best share the common airspace a while back.  That being hit from underneath by an expensive motor sail plane on one of it’s first flights while it’s owner was fully focused on just his airplane,  and a scale pilot landing off field because I left my flying wing on the runway a couple of minutes,  changed my attitude. Among many other problems of brain damage during 1997 through 2000 my sense of time was screwed up along with a inability to maintain attention of what my right eye registered.  Although we always did  clear the sky when the F5B contingent showed up,  it was the at appearance of the F3A members when I decided I just didn’t feel right about mixing my “lowest net cost per flight” stuff in with their fragility and investment,  since they too tended to fly after the Mission Bay wind tunnel set in around ten am. At first the expensive/fragile stuff pilots had to be convinced I was genuine about the willingness to make room for them that I consider to be courtesy.  They fly for seven minutes at a time,  I may be blasting around for hours.

Furthermore,  if the night before,  you consumed your commission from a local vintner for assembling his airplane,  or,  just had dental surgery,  don’t fly RC until you recover.

How to Get an Aerobatic RC Airplane for a Minimal of Cash?

Being almost,  but not quite,  completely broke,  makes justifying a new airframe kit and all new parts difficult.  Wanting something really aerobatic,  the choices were;  A Gemini,  which a friend already has (landing in Norco or Jamul would chew it up),  a Park Master,  for which there is already one in the group (I’m going to get one during my visit the USA trip the spring of 2012,  it won’t be landable stock with a fixed propeller and landing gear at Norco or Jamul either),  or an Acromaster,  which I haven’t seen one really fly yet.

Sitting out late January and the first half of February 2012 because of the worst cold in Germany and Europe since 1956 (and a recession in the USA that seems to have no end),  the two of us elected to go flying right near home instead of getting up early on a Saturday morning for the hundred mile each way drive before dawn over a frozen autobahn to the Sinsheim RC swap meet.  We’d have enjoyed looking at all the fancy/big/expensive stuff,  blown some budget on small stuff and ohed and ahed at the Depron indoor airplanes flight show.  But it’s a cost verses benefits thing,  would the savings on any purchase and the entertainment of the only RC event during the “dark” months of the year justify the drive?  Our Editor asked for some coverage of events in Germany and Europe,  from October through April there aren’t any!  At least not outside.

So we just went flying instead.  My Depron biplane wallowed around,  I soon afterwards junked it.  Before I try that again I’m getting myself some help,  see DW Models.  ´

The friend alternately flew his Park Master and Gemini so we could directly compare them.  No,  the motors don’t put out as well with the batteries that cold,  nor do the pilots perform as well,  but it was still worthwhile flying with two great airplanes!  From past experience that kind of cold can slow down cheap servos,  our minimal to better quality ones did fine.

That springy single wire landing gear of the Gemini,  which is also common to the Acromaster,  Fun Cub and Mini Mag,  is holding up sort of ok (broke the first plastic mount at twenty landings,  the second is holding at thirty more) landing in long grass,  it was later replaced with a carbon fiber one.  The Park Master’s landing gear at the fuselage,  after fifteen flights isn’t as durable.  The folding prop in the Gemini (had to clearance some and toughen things up with fiberglass) ignores the headstands.  The lighter Park Master with it’s slightly flexible APC slow flier prop,  at least the nose is holding up,  no prop has broken yet.  In flight the Park Master covers the slow for Indoor like shenanigans (zu Deutsch Strolchen) to medium speed dynamic aerobatics,  the Gemini the medium on up to a little faster speeds for classical aerobatic flight.  The Acromaster is about in between.  Unexpectedly,  the flight characteristics of all three over lap a lot.  We get way longer flights then the magazine authors report using just ordinary equipment although,  being the biggest,  the Acromaster flights are the shortest.

Although it might seem that the asphalted farm access fields at three paces wide and km long might function as a landing strip,  our experience has been that even puffs of air at the airplane,  which we can’t feel from where we stand,  are enough to move the airplane sideways into either the rougher sloped grass or mud faster then we can respond.  My friends that learned RC partly on simulators all have difficulty placing their landings,  on a computer I can’t do any better.  As a former balsa RC airplane slope soarer,  I am an expert at landing in the real world.  None of us can reliably hit a ten foot wide road if there is any wind or thermal at all.  Our favorite place to fly is down in a bowl,  the wind overhead tumbles through in an unpredictable manner.  It’s so unanticipateable that once,  twenty minutes apart,  at exactly the same place sixty paces from where we were standing,  we both tumble stalled Mini Mags clear to the ground from straight and level ten foot up.  Unknown to us,  as the headwind,  swirling backwards from the clouds overhead,  passed over the airplanes,  they quit flying at the abrupt reduction in airspeed.  Our usual two hour flying session that clear,  sunny,  windless,  otherwise perfect day was cut short by frozen hands in the 20 F cold.

A week later I crashed my latest project,  a Dog Fighter Lite (what a squirmy airplane to fly,  rocked by the wind a hundred and fifty meters out,  unable to determine which way it rolled due to flying near the sun,  two seconds later it was junk),  after getting enough experience with it to conclude it was a wrong direction.  One useful result though,  attempts to lighten it up with a smaller motor and moving the battery all the way forward didn’t really get what we wanted,  a useful speed reduction,  since the weight reduction was minimal.  Even if the tuning motor in the Gemini can put out more power then desired,  both the Dog Fighter and Gemini airframes still need the weight up front for balance.  I’ll rather carry a bigger motor or battery then lead.

Of Servos and Quality Control,  of Cabbages and Kings

Since the propulsion of Multiplex airframes can be radically changed in under an hour with just hand tools,  whereas the servos are glued in,  the servos are one of the bigger choices in building up a new foam airframe. An Acromaster is capable of both worthwhile semi-precision aerobatics,  and the wildest of the new stuff.  So if you can’t afford decent servos,  you need a different airplane. Try a Fun Cub,  with it’s also twenty to forty dollar less expensive airframe that takes the same size servos (and many of the same propulsion combinations) if fifty bucks more at the servos means that much to you.  Just don’t expect nice balanced aerobatics for the seventy five flights or so until the hinge at the wings just past the spar becomes apparent.

We found out that the Gemini really benefits from servos with first rate resolution i.e. HS-65s.  With the better precision it turns into a first rate aerobatic combination with which the pilot knows anything happening is a result of him at the controls,  not random wandering of the servos.  If we wanted to improve the precision of the aerobatics the next step would be gyroscopic stabilization,  not better servos.  Then we’d need to get the overall weight down,  a (100) gram high performance motor instead of (135) grams HiMax,  4S instead of 3S LiPos moved forward,  and a three bladed prop for more ground clearance.  After that pilot skill and the flexibility of foam set the limits.

In my Blizzard,  I really didn’t see much benefit in flight from better (8-11) gram size servos,  except maybe that the Karbonite gears can withstand a rated four times more landing impact then the standard plastic ones.  But the Blizzard is a semi hotliner for drawn out,  fast,  graceful maneuvers,  not precision aerobatics.  Since at the landing the Blizzard neither does a headstand or spin,  unless you hit so hard the airplane is a write off anyway,  and it needs less aileron throw then Multiplex recommends,  at my budget I’d go standard servos in both wing and tail for a Blizzard.

In my fifty first flying wing,  a little bitty thing from Hacker with great propulsion and the best cut EPP I ever handled (that’s odd,  it was a little more expensive then the usual stuff,  about a quarter more,  and yet it flew a lot better and half to twice again longer with the same batteries as compared to others that looked just about like it),  better servos DID make for an improvement.

Where a manufacturer offers a range of gear material/strength,  the stronger material alone,  as least as reported by the manufacturers,  who don’t as a rule quantify free play in the servo’s,  doesn’t always benefit precision positioning,  the servo’s electronics have to be upgraded too.  In practice,  the HiTeck/Multiplex Karbonite gear train has less slop then the standard plastic one,  in addition to the servo’s electronics having better resolution and centering.

For the Park Master,  it’s a question of budget,  both HS-55s and HS-65s can function,  the better servos make a better flying airplane.  But the lessors aren’t at that much of a disadvantage particularly at the lower speed range,  except for hovering.  I can’t put it to words,  just think about the fifty bucks additional for four first rate servos over the couple of hundred flights you and your Park Master might make,  verses putting gas in the tank to get to the flying field.

There is a bewildering plethora of equipment offerings.  For which I can only remind you,  when you are buying things new,  you get what you pay for.  If you are trying to compare otherwise similar servos/motors/radios/airframes ect,  the higher price will determine which one is the better quality.  Quality isn’t linear with price,  economy of manufacture and marketing can make some difference,  sometimes the higher price is complexity you don’t need,  often better materials don’t show their advantage until the cheap stuff gets some use.  I’ve been at this RC bit a while,  over time,  on the average,  if the items are otherwise seemingly identical,  the more expensive stuff always functions better.

But,  if you spent too much on some parts and not enough on others,  don’t forget that all it takes is a single under performing component to wreak everything. Lately for us in Under Igstadt bei Wiesbaden am Rhein,  it’s the transmitter! If you don’t have enough experience to know what works and what doesn’t,  ask for some help down at the field.  Bored,  I sometimes read Internet threads,  half of all of them are people wondering why things don’t work right,  which when you recognize the cheap components that don’t work as advertised they are using,  that’s the answer.

Sometimes it’s about quality control.  I have had four samples of three $4- (8) gram servos from four different cheap manufacturers.  In two groups one out of three quit,  one of them intermittently.  One group all of the teeth of the output shaft at the control arm just disappeared over two hours of flight. Some of the gear trains were full of slop.  All of them the internal variable resistance they use for positioning corroded during storage in a year.  They intermittently have double neutrals (sometimes they come to a neutral stop at different positions),  and even when they wear in they don’t usually have matching neutrals to each other. That means the length of the control rods,  and with it the geometry of the control surface travel (a continuously varying relationship between transmitter lever and throw at the control surface set by the geometry of the parts),  doesn’t match at paired (aileron) servos unless you can separately zero them at the transmitter.  There is noticeable slop in the gear train.  Over a year the housings go brittle as the cheap plastic finishes curing.  So much for any repeatability or precision as part of a flying machine.

Still,  in a hand built Depron Event 3S,   cheap servos are part of a very entertaining combination.  Partly because it’s a great design and built straight,  and maybe at that low a speed a little imprecision at the servos isn’t a deciding factor.

Having bought and used twenty inexpensive (8) gram servos at Euro5/$7-,  at a time when I couldn’t afford anything else,  I was glad to have them.  They performed fine for slewing Easy Stars and Sturmoviks  and Depron Ugly Sticks around.  Some of them lasted past a hundred of my “flown hard and put down wet” flights,  I haven’t worn any of them out.  They were even an ok choice in a “lets use what I have laying around” Mini Mag and two meter,  one speed only (slow) motor sail plane.  At that moment the difference in price between “free” (I had them already) and having to spend fifty bucks to get new ones justified the lesser controllability.  If I have to chose between fitting a friends Mini Mag or Merlin out with on hand $7- servos that I know work,  and better cash outlay at $12- each for better quality new ones,  or having the gasoline in the tank to go see him and fly,  the cheaper ones will get used.

And sometimes another pilot just clearing out his accumulation by selling me his slightly used RC airplane brings me into contact with “Brand X” servos,  many of which do function satisfactory.  At around the (18) gram HS-82 size on up a lot of the cheap servos start functioning ok,  it’s the small stuff that seems to be the worst problem.

Back in 1998 when Wayne Walker offered to sell me some JR servos,  at a time when the (8) gram size for use in a Speed 400 pylon racer was uncommon,  I wish he could have put the difference to words.  Thanks for putting up with me during those brain damaged four years.

Just my luck,  left over from more flying wings then the Harbor Soaring Society  allowed meI have half a dozen decade old HiTeck HS-81MGs.  In case you were wondering,  my Fun Cub HS-81s are in their fourth airframe,  they have (600) flights on them.  Although flights then in foam flying wings were much shorter,  and the landings were often violent,  some of my HS-81MGs have a thousand flights on then.  My pair of Mini Mags with their HS-55s are doing fine at (350) flights,  including more then a few “why didn’t it break into pieces?” impacts.

Considering the precision aerobatics an Acromaster,  Gemini or Park Master can perform,  and that the choice of servos must be decided on before gluing them in during assembly as swapping them out involves cutting,  patching,  using expletatives the Editor deletes from this publication and so on and so forth,  cheap servos glued in would be false economy.   I can swap out a crummy performing motor,  or controller,  or prop assembly anytime in a Multiplex airplane…

The Acromaster Adventure Begins

Encouraged by the friends success at E-Bay Internet auctions (a new in the box Twin Star II that arrived by mail for half what they cost new,  a worthwhile used Blizzard he drove over to pick up for a third of new) I was the top E-Bay bidder on a,  reportedly,  slightly used Acromaster at,  with shipment,  Euro120/$162-.  Included were two HS-82mg (Metal Gear) fuselage servos (including extended arms for wild minimal zero speed maneuvers like hovering),  two of the much better then HS-55s resolution with stronger Karbonite gears HS-65 wing servos,  an unknown ( to me) (350-450) watts-in Extreme Flight 2818/900 outrunner and an ok Robbe 2-6S (40) amp 2-6S tacked BEC motor controller.  I had five 3S 2600 mAh NeuMotors batteries that should work with it,  plus some 3,  4 and 5S 2200 mAh ones.  Although there are two motor mount circle size motor plates,  I have both and a selection of propulsion already on hand.

To go to the end of the adventure,  an average,  modern “building,  what’s that?” RC pilot would have been better off just buying average (or better) quality components new and having our F3A contingent check them down at the field.  We didn’t do all that well with the used Blizzards either.  Too many hidden things wrong,  like crash damage,  parts that were too inexpensive,  poor initial assembly ect.

I used a Ds6i computer transmitter and AR6200 receiver w/satellite receiver.  According to our advanced pilots,  a computer radio with it’s exponential curves for coordinating control throws is a big help with this type of airplane,  they are right.  I flew mine first with standard settings,  then moved to the more advanced control throws tuning. An Acromaster flies fine