Daily Archives: April 26, 2012

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The Acromaster page 2

The Acromaster continued:


It Started in a Mould
From the add description where the valuable servos (a hundred and fifty bucks new) have only five flights on them,  and oversize control arms,  the few dents on the leading edges and the tension crack in the paint on the fuselage right behind the main landing gear mount.  I wish I could have flow next to the previous owner.  He could have pulled the motor and controller in one minute,  and chopped the servos out for use in another project in five minutes more.  Either just the servos or motor and controller would cost new as much as I paid for the whole airplane,  which,  if it has the same number of flights on it as claimed for the servos,  only flew five times.  I wonder why he sold it,  but a thanks to him.  It has the older issue plastic motor mount.  It looks like about the same tail wheel mount that proved too fragile for extended use in a Fun Cub.  But,  I’m beginning to realize that my hundred flights airplanes aren’t typical.

The bottom of the nose was slightly cracked and repaired,  most likely from hitting the ground on a nose over.  It was decently painted in two tones.  Either the original builder used a primer,  or the paint reacted with the Elapor as sanding both the blue and orange resulted in an icky green mess that irritated my lungs and stank.  The wings leading edges were slightly dented up,  for no more then five flights the outer edges abraded.  Had the airplane had been used in a  crime (contributing to the delinquency of an adult wouldn’t count) there were debris embedded in it which could be analyzed. The right wings lower foam section didn’t evenly match the upper.  After flying with typical “minimal” level servos it’s always a surprise to see how much smoother and exacter even slightly better servos function.  The original assembly and painting was plenty competent,  if I was putting one together stock I would have done no better.  Things like not getting a perfect match between the Elapor components are determined by the staggeringly economical net cost of the kit and that’s just the way it goes gluing together a material that is inherently flexible and compressible.  Initial examination and sanding took three hours.

For reference sanding,  spachtel,  a layer of heat shrunk on packing tape at the leading and outside edges and fiberglass with resin reinforcement at the control arm and adjacent foam of the aileron took the right wing from (120) grams to (121) grams. That working over the wings after cleaning them up took an hour.

I had been paying Euro8/$11- for worthwhile name brand cans of spray paint that gave better results then any consumer level spray paint I ever used in the USA.  Just a thin coat covered well,  it stuck great to Elapor and the cans coverage went a long way.  I took the Euro6/$8- cans of the house brand back to the hardware store for a refund.  I wanted paint with pigment,  not clear with an accent of icky color.  The local artists spraying graffiti murals on approved places use Euro3- a can paint that’s better.  A Tau of Poo event,  how else do you determine the level of financial expenditure that provides satisfactory results?  The new paint, only a third of the area,  added (8) grams.  The wings are now clean and smooth,  however the mix of sanded off blue/green and orange/green with the new (expletative deleted) yellow and red looks awful,  even by my low standards.  It looks kind of like the airplane is healing up from acne,  skin grafts after being burned all over,  and leprosy.  I couldn’t make the surface treatment any uglier if I tried.

But fly it some first is my method.  Usually that determines what really needs improvement rather then just trying to anticipate everything first.  I often build two of a type for that reason.

So a tried and true Multiplex Acromaster airframe well assembled with what could be considered best choice servos and a decent paint job.  The motor is on the cheap side, I don’t expect it to either perform all that well,  or for very long.  The controller,  downrated by the latest expectations as of 2012 of being able to advance the timing to correct for an outrunner,  is only ok quality,  but it will go directly to 6S and can reliably provide sufficient power to the servos.  As shipped the APC 13X4 propeller verses a better performing and stronger Graupner in the same size is a toss up.  Sometimes I’d rather the propeller break then the motor mount,  motor or nose.

From experiments with the Fun Cub,  which weighs just about the same as an Acromaster;  4S 2200 mAh and a Graupner fixed 9X6 provided more thrust for flights twice as long when compared to the APC 13X4 on 3S 2200 mAh,  both using the same controller and HiMax 1130 kV (135) gram motor.  That big flat pitch propeller,  specified back when 4S motor controllers were exotic and expensive,  provides great low speed controllability,  braking in the downward sections and in general a poor mans constant speed flight profile.  It also hangs up and breaks at every possible opportunity.  In the Fun Cub the controllability on 4S was even better,  the constant speed aspect will have to be explored with the Acromaster.  The slightly higher weight of the 4S 2200 mAh battery verses the 3S 2200 mAh was just buried by the higher thrust and duration.  Think going straight up to two or three times the height restrictions at San Diego’s Mission Bay at a count of ten and taking three minutes to glide back down.  On 3S that was more like a minute and a half.

This Acromaster will be slightly reshaped to accept a folding propeller.

Just what is this for a Motor?
In every speed shop in the whole world there is a standard question;  Speed costs money,  how fast do you want to go?

It’s subjective,  I had no idea three years ago that foam airplanes could last hundreds of flights,  or use propulsion components so far off from what everybody else was using.  But all it takes is one time taking off with the computer radio set for a different airplane to trash the whole thing.  Even in my reinforced foamies the motor can get broken.

There wasn’t much info from the web site of the American manufacturer (importer really) brand Extreme,  about the motor that came used with my Acromaster,  the most important parameters are (35) amps max,  (105) grams and 3 or 4S LiPos at (900) kV with an allowable of (350-450) watts-in.

You have to be suspicious of that kV number.  If you are buying NewMotors,  or competition level ones like Hackers,  you may expect that to be accurate to two significant figures.  That means if Steve and Jeff report their world class racing motor as having a kV of (3100) it may be expected to be between (3150) and (3250).  Check with them personally as maybe they can hit even closer!  It takes top quality materials and manufacturing to be that accurate,  real measurements and a willingness as a company to be honest.  NeuMotors even offered a meter to verify kV for a while.  Anybody else’s motors,  figure more like one digit accuracy,  or off by up to (15)%,  maybe deliberately to con the customer into thinking they have more performance then they paid for.  Keep that in mind when your results don’t match P-Calk,  that any calculations or program’s accuracy is limited by the inputs.

If you ever wonder why my stuff puts out more power then yours,  some of it’s because I didn’t believe what the package the motor came with was marked as and fiddled with prop sizes using a watt meter and my fingers as a thermometer until the motor was running on it’s limit AND matched to the airframe.  The results are still sometimes a surprise.  That’s part of why I have destine reports where all they tried was one combination.  Then too,  I like a “burst and glide” flight style reminiscent of slope soaring antics,  and holding my breath hunting lobster.  That may explain why my average flight times are about double what the magazines report.

So all I really may expect is that the kV of this motor is likely between (800) and (1000).  At this price level you must expect that individual motors of a series will not match.  There is insufficient information to model it on the P-Calk virtual dyno.  Luckily,  I have a watt meter,  like to fiddle and have been using motors in this size and kV for a while.  Test and tune will decide what works.

The 4mm shaft (interestingly with the brand name Axi etched on) was hack-sawed off shortened.  Turning it over in my hands the bearings feel smooth.  Weird,  the front bearing,  that can be seen,  is about average size for this weight of motor,  but the rear one,  inside the bell where it can’t be easily seen,  is very thin.  The inward rear bearing of an inrunner is usually one size smaller then the front one,  but this is way smaller,  like three sizes down.  That “mini” size bearing may have kept the initial price down.  Maybe it allowed larger coils for higher magnetic flux density,  getting an improvement in power.  It brings suspicions of poor durability as lately it is the bearings that give out ending the service life of brushless motors.  Something you won’t read about in reports made after just a few flights.  I own this thing,  I’m going to use it.

My similar Graupner (125) gram motor,  run at the much higher shaft RPM on 3S LiPos instead of the 2S LiPos Graupner rated if for,  held up for (75),  each and every one was a worthwhile life’s event,  flights.  I’d spend the twenty bucks for new bearings,  but the magnets came lose and I already have better…  One clue with bearings that small as the Extreem’s,  keep the RPM down.  In particular don’t screw up and over rev it with no load (the prop off) as you can wreck the bearings faster then you can pull the amp stick back.  Me,  I don’t ever run up an unknown combination without an amp meter connected.  Not even changing the prop diameter by half an inch or the pitch by half an inch.

The cheap motors,  I reran the measurements every twenty five flights or so as the increased drag from the bearings causes increased wattage draw burning them up.

Now days I just buy better motors in the first place. That,  was two years ago sixty dollar,  now eighty five dollars new,  (80) gram Typhoon motor puts out about (85)% of what the same size (70) gram NeuMotors one does for about (90)% of the duration,  at half the purchase price.  After a hundred flights (proped for the manufacturers (350) watts-in rating,  the same size NeuMotors go to (400) watts-in,  although now obsolete,  the same size HiMax was rated at (300) watts-in) the bearings of the Typhoon are just starting to show some wear in the form of slightly higher current draw,  the Neumotor is still as perfect and smooth running as when it came out of the box and the dust shields wore in.

That’s something to watch for,  although electric motors as such don’t have any break in period,  the dust shields of the bearings need a little time to clearance themselves.  What’s happening is the metal plates rub past each other,  after a little while the high spots get worn off.  My $18- HP (45) gram motor it took twenty (flown hard and put up hot) flights for the dust shields to quit dragging.  It was still a dog of a performer,  flights with the same batteries lasted half as long as even slightly better stuff.  Cheap bearings with their lose clearances wear out,  at which the shields dragging again.  If you are running inexpensive motors at 2S LiPo Slow-Flier speeds it isn’t much of a factor,  run a smaller prop on 4S and it quickly is clear that you get what you pay for.

Even the “pros” from Multiplex,  paid to demonstrate the companies products,  were amazed at what a seemingly ordinary Mini Mag can do with that kind of quality components and careful matching.  Know anybody else’s Mini Mag that can take off straight up,  slow down to walking speed and stay up for three quarters of an hour on a single (120) gram battery?  Think a ten inch folding prop and 4S LiPos.  Another shock to the “we’ve seen it Alls” the durability available from a nominal fiberglassing.  Think of a logged three or four hundred flights by just adding five percent to the overall weight with fiberglass.  Just a nice paint job could weigh just as much.

There is some cog turning it over,  nothing like a racing motor,  but not “flip right through” either.  A NewMotor in this size you have to hold it firmly to turn it over,  even sport Hackers and Plettenburgs index between magnets.

The clear insulation on the copper wire of the motor windings is as clear as the day it was made,  this motor has never been run hot,  not even warm.

Hum,  they spent enough on the surface treatment that at least it looks nice.  Almost as good as motors costing two or three times as much.  From W.C. Fields “You can fool all of the people some of the time.  And you can fool some of the people all of the time.  But you can’t fool all of the people all of the time!”

Other Bits and Pieces
That old production white plastic motor mount plate (Multiplex is now issuing aluminum motor mounts) with the “big” bolt circle is prone to warping,  it was sanded flat.  That not having full contact between motor and mount was determined to be a cause of vibration in another airplane.  Left over from a Gemini I also have a “small” bolt circle motor mount plate.

The Robbe controler is a decent one,  with a setting for brake,  but not for timing.  Typical for this now three or four old design.  It appears that the dramatic increase in the cost of materials for magnets,  requiring either a drastic increase in price (like a third) or a hidden but equally big cut in quality,  finally motivated the electronics manufacturers to introduce advanced timing in the controllers as the difference between a fixed (5) degrees lead and a more suitable (20-25) degrees for the outrunners which now represent virtually all sport level electric RC motors is about a fifth of the output.

Those cool looking wheel pants were removed.  I don’t have them installed on my Piper Pacer either,  for the same reason.  I am going to be landing the thing on rough terrain.  The main landing gear is the same wire landing gear/plastic plate at the fuselage that has proven so usable in Mini Mags,  Gemini s,  Fun Cubs and the like at our grass as a crop or gravel landing spots.  If it’s rusted and a little bent,  I can bend up another wire in minutes.  But how the Elapor wheels could be so dented up from presumably just five flights is a wonder.  That little bitty tail wheel could be a problem,  except that as awkward looking as the,  every single landing,  of the Gemini’s nose over is,  we have accepted that it’s an effective way to finish a flight on the grass field we have available,  provided that the nose is clearanced for a folding propeller and reinforced.  We keep takeoff runs as short as possible Under Igstadt bei Wiesbaden am Rhein,  so if that little wheel isn’t enough for taxing,  we won’t know it.

Lets Make it Usable where I Land
So,  the nose was sanded down and remodeled so that the folding propeller can fold.  Then the whole fuselage forward of the wing and all down the belly was treated to fiberglass held on with resin.

Argh,  the thaw has set in,  I could put the Acromaster together and go flying the next day if the epoxy fiberglass resin was cured,  but at the basements 50F temperature it takes a day and a half for the man carrying resin sold to the hobby trade here in Germany to cure.

Worse yet,  the ground is frozen for two feet down,  and it’s raining.  I’m afraid of any unnecessary driving as the rain can instantly freeze turning to clear ice.  I experienced it once (33) years ago,  I can remember it like it was last week.  You can’t even stand still let alone walk when that happens.  An ice skating rink is at least level.  A kid skidded off a nearly level,  nearly straight stretch of road last night that I sometimes ride my bicycle on,  before even getting to the steep part down to the Rhine river (at the legal limit for a public road,  I’ve melted mediocre bicycle brake pads going down it) about thirty miles away.  He wound up dead,  down fifty feet of steep slope,  crushed in his car bent around a tree.  The local live TV coverage showed four fire trucks parked on the road,  because the first fire truck sent out,  trying to rescue him,  skidded off the straight level section of road about a hundred yards before getting there to slide down the hill be brought to a halt before landing in a stream by being bent around a tree too.

So I spent the evening siting in an easy chair,  drinking Bier,  snacking on hard French sausage,  while watching britisch Snooker on TV.  I’m glad I went and melted a hole in the ice for my pond fish last night.  With the nighttime temperatures down around Zero Fahrenheit the last couple of weeks even the styrofoam insulated breathing holes would ice over in minutes.  I was worried the whole ponds,  suffocated fish included,  would turn into one solid block of ice.  So much for going off to Italy for a couple of weeks,  the sort of warm wind (like,  it is above freezing) is out of the north and won’t melt all that snow down south of the Alps.  Even if I could get there it just wouldn’t be worthwhile in visiting Venice when the canals are frozen over.  Kind of like my last trip to Yosemite,  with low clouds so I couldn’t see a thing.  I made at least twenty trips wandering down to the French Mediterranean Coast as a twenty something,  I really want to go one more time before enjoying the San Diego sunshine puts it out of reach,  but not when it’s too cold to function.  So if you were wondering about no coverage lately of RC outdoor flying events…

There are great days here during the summer and nice ones in the spring and fall where thoughts about San Diego don’t even occur,  and ones like this where even sitting in traffic might be justified if I could just support myself and my wife there again.

Then too,  for a second coat of fiberglass,  the first one must be sanded to take off the high points.  As awful as what’s left of that blue/green left of the original paint job looks through the nearly clear layer of fiberglass,  it usually takes a couple of fiberglass/sand events to get it right.  Things like after the outside of the nose is strengthened up I will most likely have to clearance for airflow inside around the motor,  and then fiberglass the inside.  To get fiberglass to follow compound curves and keep the weight down means using thin cloth,  sneeze,  or catch a single strand in the paint brush,  and you start over.  You can’t always get it the way it needs to be on the first try.  My airplane(s) may not look much different then a stock one,  but it/they is/are.  An attractive paint scheme will have to wait.

I’m not all that sociable.  From my twenty years as an adult in the USA the only club I was ever a member of was the SEFSD.  I can’t afford it to be a member of an Aeroclub in Germany.

Knowing that I suffer from greasy fingers,  the black paint was sanded off of the canopy,  at least it wasn’t green underneath.  It was then layered over the top with (20) grams per meter squared (that’s five ounces per square yard) fiberglass held on with water based hardwood flooring paint,  the nose “tongue” and under the finger grips got a layer of fiberglass with resin.  Weight went from (17) grams to (before paint) ss.

Weights before and After
Wings assembly             (290) grams
Fuselage assembly         (370) grams
Motor,  controller and collet     (175) grams
Receiver,  prop assembly ect    (030) grams
It looked like with a 3S 2600 mAh (220) gram battery that the all up weight would have been about (1100) grams.  Just about the same as a Fun Cub.

have This one has.

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