LMFC Articles - "The Graupner Spark"

The “Graupner Spark”

- an electric model designed for “Increased Penetration”

- review by Mick Saxton

This is the story of my most crashed electric model ever - it is a tribute to the design that I still have anything left to mend. So far I have achieved several holes greater than six inches deep, stuck it into the river bed under 3 feet of water and most recently achieved a full speed tree arrested take-off.

My only previous experience of electric flight was a Graupner Chip which I inherited in a part built state from a friend. Due to the numerous instances of hangar damage I decided to build a complete new wing so it was not really a real “Chip”. It came with a Speed 600 (8.4V) to which I attached an 8 cell 1700SCRC pack, this was very successful (mainly I believe due to using 8 rather than 7 cells). This whetted my appetite for electric flight, I could now envisage owning a very fast aeroplane without the noise-meter bandits ganging up on me.

A visit to “West London Models” saw me coming away with a Graupner Spark, a Speed 480, ESC with BEC and 8 * 800AR nicad cells, it seemed alot of dosh for such a small aeroplane and with hindsight perhaps it would have been cheaper had I gone out and bought a helicopter instead.

The basic electric flight philosophy of build light is seriously being questioned - the heavier the model got the less it broke - flight characteristics were about the same and it is nearly impossible to land without breaking the prop anyway!

My recent efforts at a full spec pylon racing model have shown me how pedestrian this model was but I felt it was best to continue my practise with it before I doubled the cost!

Hardware Used

Graupner 480 Race Motor , Cam 4.7 x 4.7 Propeller

8 x 800AR Sanyo Cells - Graupner connectors

2 x Hitec HS80 Servos

Microstar 20 with BEC (later changed to Kontronics Easy 1000)

Futaba 113 mini 4 Chan Rx (later changed to Multiplex Pico 4/5)

Flying weight with the above came out at 685 grams (24 oz) - flight “experiences” have gradually increased this to over 27 oz. As you will see a Speed 400 with smaller batteries could well have been the more sensible option.

Building (the easy bit)

The kit is supposed to be “quick build” and this is borne out by the fact that I had mine ready for the weekend after a Monday purchase. It is a very complete kit with a very light GRP fuselage with moulded in fin, a single piece wing and all the hardware necessary to install the motor and radio.

The most difficult part of the construction was installing the motor, this was made even more difficult on my model due to a some spurious glass cloth which had set in a hard lump just behind bulkhead which was very difficult to remove. I eventually glued the motor to the plywood mounts using white glue so that I could get all the pieces in place and the screw holes lined up. I have since found that a bicycle spoke can be used to pull the motor into the correct position.

It seemed like the inside of the fuselage was coated with some type of release agent my first attempts with epoxy peeled straight back off, to be fair this is mentioned in the instructions and after cleaning with acetone and keying the surface no further problems were encountered.

With hindsight I believe it would be a good idea for the manufacturer to strengthen inside the wing seat with a few lengths of carbon fibres as my Spark cracked here in several places - any strength added here during the lay-up stage can be achieved for a fraction of the weight that will be required for later repairs .

The wing construction is very straight forward my only problem here was that the ailerons, these are cut out from the wing but had warped by the time they were installed and covered?

The wings are attached by a single 3mm nylon bolt and strips of plastic tape to keep it straight, I am still not sure how tight the designer contemplated doing up the bolt and how much he intended to rely on the tape - it would be nice to know? The 3mm wing bolts are about the right size - they break when the plane crashes and haven’t failed in flight - compare that to the size you are using.

Total building time including solarfilming the flying surfaces was achieved in just four evenings, I did not paint the fuselage as it originally looked quite nice in white – a good decision - it would have been a total waste of effort.

As you will see from the “flying” details the layout whereby the spinner is very close to the front of the fuselage to which the motor is bolted and a battery pack just behind the motor (separated by a layer of polystyrene and masking tape) is about optimum for a model of this type and results in very good penetration with a minimum of damage during vertical arrivals.

Flying (if you can call it that)

Launching this model was its first big problem, it has a tendency to nose dive as it is launched. I believe this was due to the squashy feel of the fuselage which prevents one from holding it tightly enough. On one occasion I felt that my new plane had just been thrown straight at the ground, it didn’t go much more than 10 feet - luckily at the time our flying field resembled a swamp and not much damage was done.

I have now mastered launching the plane myself, it seems fine if it is thrown very hard and fast at upwards trajectory of about 10-15 degrees. The plane is certainly fast but it is difficult to say how fast due to its small size, I believe this creates an optical illusion of even greater speed.

Using the 800AR cells flight time at full power is only about three and a half minutes. Given the short flight times it is a shame that it is necessary to remove the wing in order to recharge as this is quite fiddly and it is easy to lose the wing bolt, change the wing alignment or jam something in the servo linkages.

Within the first three flights, I had broken two Props and broken/lost three wing bolts, I really think that Graupner should include a packet of spare wing bolts as this size were difficult to obtain. The prop breakage rate seems to be a real and ongoing problem with this type of model.

Having equipped with some more props and more wing-bolts further flights were then blighted by premature motor shutdown. One does not expect an electric motor to flame out during take-off after the battery pack has been fully charged and what’s more one cannot hear it stopping - this resulted in several more hard landings and broken props. In desperation I had my battery pack cycled and charged on a different charger without any improvement often only getting flights of about 30 seconds and then purchased a new (pre-built) battery pack - still no improvement in reliability.

At this point the plane went in very hard at the Newdigate site and “penetrated” so far that I had difficulty in pulling it out without breaking the cracked fuselage. This crash was caused by a sudden flick roll when the plane was otherwise flying perfectly. I put this crash down to good old interference and sent the Rx back to Futaba for checking.

Two weeks later the Rx came back and two more flights were aborted by premature motor shutdown - two more props were broken even when landings did not result in a broken plane. I then measured the static rpm and found it was down to 16,000 at this point, I had previously recorded 18,600. From this I decided that perhaps I had bent the motor very slightly, burnt the windings or overheated the magnets. A brand new motor was purchased and fitted - I now measured 19,600 rpm but the ESC still cutout about 30 seconds into the flight.

I had already ordered a Kontronics Easy 1000 ESC for another plane but in desperation this was fitted to the Spark. With the new ESC flight times increased immediately to 3 minutes plus with the knock-on advantage that the plane was up long enough to get it trimmed properly before landing - landings were improved such that 4 flights were made using the same prop.

Even with this setup I believe I was getting some glitches when I turned the plane so that it was flying directly towards me ie. the motor is between me and the aerial. Just when I thought I had it sorted the total control failures started again!

Flight (Crash?) Log for Mick’s Spark

30/12/97	*	2 short flights, two launches that did not get away due to bad throwing.
31/12/97		1 flight - fast smooth landing still broke wing bolt
10/1/98	*	motor cut shortly after launch - broke tail
	*	mended tail - rolled in shortly after takeoff
11/1/98	*	total loss of control after 1 minutes flying - unusual normally happens sooner!
24/1/98	*	good flight/landing
25/1/98	*	non-control shortly after take-off
		Receiver checked by Futaba
7/3/98	**	3 flights marred by early motor cutoff
		Replaced Motor due to excessive commutator marking
14/3/98		2 flights very short - plenty of power left if ESC reset
		Replaced ESC with Kontronics 1000
15/3/98		3 flights (approx. 3 mins) - same prop
8/4/98	*	good flight - cross wind landing
14/4/98	*	climbed out after takeoff followed by total control loss (Black Batt)
25/4/98		good flight
26/4/98	*	good flight
2/5/98		good flight
3/5/98	*	2 good flights but hard landing
10/5/98	*	total loss of control after climb - into river tailplane floated away - model eventually recovered from 4 feet of water
17/5/98	*	total loss of control - totally burnt ESC & fried motor - (Red Batt) - could be ESC was affected by dunking?
		Replaced battery pack
		Replaced Receiver with Multiplex Pico New - Kontronics Easy 1000 ESC
14/8/98		(BBQ) - good flight weight now up to 26oz
16/8/98	*	good flight - broke prop on runway
20/9/98		good flight - almost lost it in sun!
10/10/98		good flight
14/11/98		good flight
15/11/98		good flight
18/11/98		good flight
22/11/98	*	too fast x-wind landing
16/12/98		good flight
27/12/98		good flight
30/12/98		good flight
10/01/99		good flight
20/02/99		good flight - but motor slowing - replaced 480
21/03/99	*	faster again
24/03/99		bad throw 3 linked recoveries then hit tree!
26/03/99	*	2 flights
27/03/99	**	good first flight,
		then suffered aileron failure on 2nd launch - 5 inch deep hole in runway.

Flight Log Summary (see log above)

Out of a Total of 42 flights 28 landings &14 crashes (>20 props!)

As one can imagine the above flight record had me severely confused in the early stages - good flights were intermingled with total control failures - what was the cause of the problems?

BEC failure ?

I had read reports of the voltage regulator chip shutting down due to overheating.

The ESC was changed - from Microstar 2000 - to - Kontronics Easy 1000 - this fixed the premature cut-off problem but did not cure the crashes. (The premature cut-off has subsequently recurred with this ESC far less dramatically in an electric glider).

Rx failure / interference?

The Futaba 113 Rx was replaced with a Multiplex - PICO 4/5 - this was changed at the same time as the 3rd battery pack and the total loss of control was cured at this point. (The Futaba Rx has worked without problem in my son’s Flutterbug electric glider).

Battery Pack Failure

Three battery packs were used with this plane - the first “home built” the other from “Gliders” both consisted of 8 x Sanyo 800AR cells, unfortunately details as to which Battery pack was in use was omitted from my flight logs. The dunking in the river ruined one battery pack. A third pack was built to eliminate this possibility.

Servo jamming / failure

I was worried about the proximity of the elevator and aileron linkages- this possibility was designed out – but failures still occured.

Broken HS80 servos were replaced as required - suspect servos were assigned to throttle and retract duties in other models, where they have behaved perfectly.

Had the model got some undesirable Kamikaze aerodynamic feature?

Nose weight was added to reduce flick roll tendencies and my javelin throwing ability has improved.

The model is now on its fourth Speed 480 motor at £15 each, its third battery pack about £30 each, third ESC (one destroyed, one discarded ) and twenty-something props at £4.00 each, the initial receiver has now been reused elsewhere where it working fine - Glow fuel must be cheaper!

At about this point I took the plunge and built a new model using a brushless motor.