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

Das Fun Cub pt.2


Since the battery is at room temperature the Fun cub was flown at part amps on and off for about a minute.  As always although as the pilot I don’t find the Fun Cub (repaired,  kind of beat up,  no landing gear) all that interesting to fly,  with it’s radical agility and low speed people passing by are always entertained.  Down thrust is now enough that on application of full power that it just pulls forward.  Despite the higher voltage it still takes a little for the motor to come up to power.  And it still climbs on it’s own at maximum amps,  but for maximum climb some up elevator is required.  Count of ten maximum climb and glide back down with this weaker of the two available 5S 2200 mAh batteries is 100 to 110.  It was more of an improvement then expected.  

Although wing flex and warp makes precision impossible,  inverted flight (zu Deutsch Rückenflug) in particular is difficult to control,  just the same it’s still an amazingly agile RC airplane.   On landing the battery had cooled to freezing.  I’m going to block off the useless chin inlet next time I do fiberglass,  but at these temperatures and flight times (half an hour?) the batteries are going to cool down.

Repeating the measurements with freshly charged (on a balancer charger),  if now nearly three year old,   NeuMotors 5S 2200 mAh batteries and Graupner 3mm hinge folding propellers.  The following are “burst” measurements taken at room temperature with the as it came delivered Turnigy 45 amp motor controllers.  Although I usually quote the power and voltage after getting past that ten of fifteen second initial peak,  this time it’s overloading the motor I’m concerned about,  and it’s too loud to run for more then a few seconds in the basement.  As a guess net efficiency was taken as .80.  Increasing the voltage too much beyond what an electric motor is optimized for and the net efficiency goes back down.  One of the batteries is not near as good as the other.  Well,  they were old stock bought at a discount and have just sat around for years…

7.5X4 folding propeller
12.7 amps    18.6 volts    236 (peak) watts-in        190 watts-out (estimated)
13.8    19.2        267    1/8 more            214

8X4.5 folding propeller
16.0    18.1        290    plus 23%            232
17.0        18.8        320    plus 18%    1/10 more    256

Loud on the bench,  just a trace of some hum at the two or three amp level,  the motor ran smooth enough on the bench to justify trying it.  

Since outside temperatures are just below freezing,  and there was no evidence of overheating yesterday,  the decision was to try the bigger blades.  If this combination works it might be a good one for the Gemini.

Flights 104 and 105  Wiesbaden Waldacker  07 December 2012 (Fr.)  Although snow far and wide is predicted,  right now it’s just below freezing and completely still.  So still that the far away freeway and airport noises are irritating.

This would be the first use of one of the just bought pair of new Turnigy 6S motor controllers,  despite the useless setting instructions.  And the second use of the now nearly 3 year old NeuMotors 5S 2200 mAh LiPos.

Well,  starting at the long field,  it went right on up,  about the same as the 4S combination the day before.  Outside the noise is only just enough to notice.  It won’t due to compare the somewhat breezy yesterdays late afternoon to this stillness,  and the propeller folds,  subjectively there seems to be no useful increase in performance.  Climb to glide was seventy with the first LiPo.  The flight seemed kind of short,  the battery was losing power when landed.  On application of full amps it gains speed a little slow,  pulling down just a little,  the motor angle seems about right.  Since it only took 1600 mAh on recharging maybe it wasn’t peaked to start.

Aerobatics weren’t as good as with the 4S setup.  And there is the weight gain,  plus 25 grams for the heavier,  dimensionally much larger controller and the additional LiPo cell.  Duration seemed about the same.  On recharging the battery took 1650 mAh.

These things always glide a lot better when the prop folds,  with the change of motor controller it now does.  It doesn’t seem to improve the time to glide back down so much as the handling.

Use of flaps flares the airplane more then with heavier combinations,  and less then some lighter ones of the past.  In this complete wind stillness they were used to put the airplane down in a mix of crop and snow right next to me.

The second identical battery was louder.  Nothing was loose,  maybe it’s just a better battery as there was more power.  The second flight from the field overlooking the Rhine reminded me of the view from the parking lot over the Santa Ana in Norco.  Clear out to the horizon,  if increasingly gray.  Climb to glide improved just a little.  Deliberately the plane was run hard for about a minute and then landed with the flaps long before the battery was run down.  The battery,  which had cooled down in my pocket,  was neither warm nor quite as cold as the otherwise a little below freezing surroundings,  the motor just barely warm.  The motor turned free,  no evidence of the bearings dragging.  On recharging it took 950 mAh.

Although I would have liked to change blades for 340 watts-in and go right back up,  three quarters of an hour after the five minute drive back the apartment,  as predicted,  the wind picked up and the horror of snow showed up.  The van was parked where it could be easily backed out.  I made the best use of the last daylight I could,  despite Susan having the next four days off and the cash on hand to go somewhere,  it’s time to stay put.

With new
Graupner 3mm hinge 8X4.5 folding blades on
5S 2200 mAh the
55 gram Hacker drew
18.5 amps on 18.5 burst volts.  

Since Hacker rates the motor as 200 watts-in maximum (15 seconds),  on 2S to 3S,  which works out to  18 amps,  that seemed like a little too much.  So,  

Graupner 3mm hinge 7.5X4 blades were mounted for
13 amps at 18.6 volts.  

250 watts-in verses 340-watts-in. The vibration about ended when the smaller blades were mounted.  That 250 is about all I trust the 3mm hinges to for extended use,  for tests the 340 will be ok if they run smooth.

On changing the motor controllers noted is that all four adjustment screws are bottomed (the top ones are longer) and the plate is warped at the outside ends,  again.  
The whole airplane is a little beat up,  the wing is showing it’s use and that big crash.  If it hadn’t been five months and waiting for a replacement Twin Star II wing I’d order a replacement Fun Cub wing and tail wheel for a Mentor to build this one up again.  
The Fun Cub is still useful for testing things.  
Next time I fiberglass that nose scope gets blocked off.

Flights 102 and 103  same place,  same day  by now the wind had fallen off  The sun is so low in the sky that it feels like darkness at 3:30.  Quieter and a little better thrust.  Depending on 4S 1100 glide with the windmilling prop was on the order of a count of 60,  with 4S 2200 maybe a little more due to the higher climb although the extra 120 grams really shows up.  On a 3S 1600 (warmed up in the van) LiPo Nano glide went to a count of 30,  it was landed early.

After trying to reduce the down thrust,  all the screws are bottomed.  Fitted with 3mm hub 9X5 Graupner folding blades it draws just about the same as the previous combination i.e. 12 amps on 4S LiPos.  On 3S it draws 10 amps.

Flights 100 and 101  Under Igstadt  same day  Noisy,  but ok power.  Less thrust down low,  so not as good for aerobatics,  but better climb.  First use of the new 4S 2200 mAh inexpensive Turnigies.

Fitted with a bought years ago used but never in service old style gray Graupner 8X6 folding propeller after I remembered I have one 3.0mm folding hub for 3mm hinge blades.  In the van it drew 12 amps.

End,  truncated from the original log