Electric Setup for a Fun Fly

[Nigel Heather]

Hi,

 

For Christmas I got a Fun Fly kit (Bills Big Fun).  It is an old design originally aimed at a 25-size IC engine (typically one bored out to 32/35/37), has a 47" wingspan, 700 sq in wing area, a flying weight around 3lb.

 

I want to use an electric setup but the instructions don't cover this.  I've spoken to the manufacturer (Bill) and he says many do convert them to electric but he doesn't have any details and he only uses IC.

 

I've looked at the 4-Max site at their recommended setups - they don't cover this exact kit but they do for the AviCraft Fun 'E' as follows

 

Motor - 3547-800

ESC - 60A

Battery - 4S

Prop - 13x4

 

The advice I am seeking is what capacity battery to use - and this is tied in with balancing and thrust to weight ratio.

 

Firstly is there a rough guide of what Power To Weight ratio I should have for a fun fly aircraft - 3D, prop hanging etc?

 

Secondly - I've sized up the weight of an IC power system and it comes in around 500-550g, the electric system excluding battery weighs 210g

 

I have two battery approaches - a smaller capacity (1600mAh) or a larger capacity (3300mAh).

 

The larger battery weighs 300g so the total weight will be about the same as the IC system it was originally designed for - I also assume that this will make it easier to achieve the correct CoG

 

The smaller battery weighs 170g so the plane will be lighter and the thrust to weight will be better.  It may be more difficult to achieve the CoG though.

 

Personally, I like the idea of the bigger battery because which ever 4S I go for I will have to buy new and the 3300mAh would be a much more versatile battery that I can use in other models.

 

These are my thoughts - interested to hear your thoughts.

 

Cheers,

 

Nigel

 

[Masher]

Hi Nigel

 

I've PM'd you a useful electric power setup guide which originated from 4-Max, this may help answer the basic questions.

For some reason 4-Max seem to have excluded this document from their 'useful info' section?

[Shaun Walsh]

A quick look with eCalc on that combination.

1600 mAh battery gives a mixed flight time of just under 5 mins 

3300 mAh battery gives a mixed flight time of just under 10 mins with a thrust to weight ratio of approx 1.3.

I would go with the larger battery.

Edited January 5, 2023 by Shaun Walsh

[leccyflyer]

The 1600mah batteries are too small for this size and weight of model and, from your description, you'd end up adding lead to get it to balance. The larger batteries are definitely the better choice.

[George Worley - 4-Max]

11 hours ago, Nigel Heather said:

Hi,

 

For Christmas I got a Fun Fly kit (Bills Big Fun).  It is an old design originally aimed at a 25-size IC engine (typically one bored out to 32/35/37), has a 47" wingspan, 700 sq in wing area, a flying weight around 3lb.

 

I want to use an electric setup but the instructions don't cover this.  I've spoken to the manufacturer (Bill) and he says many do convert them to electric but he doesn't have any details and he only uses IC.

 

I've looked at the 4-Max site at their recommended setups - they don't cover this exact kit but they do for the AviCraft Fun 'E' as follows

 

Motor - 3547-800

ESC - 60A

Battery - 4S

Prop - 13x4

 

The advice I am seeking is what capacity battery to use - and this is tied in with balancing and thrust to weight ratio.

 

Firstly is there a rough guide of what Power To Weight ratio I should have for a fun fly aircraft - 3D, prop hanging etc?

 

Secondly - I've sized up the weight of an IC power system and it comes in around 500-550g, the electric system excluding battery weighs 210g

 

I have two battery approaches - a smaller capacity (1600mAh) or a larger capacity (3300mAh).

 

The larger battery weighs 300g so the total weight will be about the same as the IC system it was originally designed for - I also assume that this will make it easier to achieve the correct CoG

 

The smaller battery weighs 170g so the plane will be lighter and the thrust to weight will be better.  It may be more difficult to achieve the CoG though.

 

Personally, I like the idea of the bigger battery because which ever 4S I go for I will have to buy new and the 3300mAh would be a much more versatile battery that I can use in other models.

 

These are my thoughts - interested to hear your thoughts.

 

Cheers,

 

Nigel

 

 

Hi Nigel

For good funfly/3D flying you should be looking at a 1.75 to 2.00 to 1 power to weight ratio

So if your model weighs for example 500g you should be looking for 875-1000g of static thrust

We no longer refere to watts and watts per pound as from experience we find this not a good indication of performance.

 

If you used our products then the nearest test results we have are as follows

We haven't tested it with a 13x4, the top speed of a 13x4 may be too low at around 36MPH

 

13x5 JXF High performance Wooden prop

14.8V under load

33.2A at 14.8V

8955RPM

Measured static thrust 2987g/6.59lbs

Estimated top speed 42MPH

 

To get the best flight performance you need the lightest possible setup.

Funflys generally have short noses so you need to get the motor as far forward as possible,

then work out how much weight you need to get the model to balance.

based upon that weight choose a battery near that weight.

 

We recommend the smallest battery for a funfly/3d model to be around 15 x max Amps

In this case 33.2/15 = 2.21 so Look at the weight of a 4S 2200mAh LiPo

 

Cheap LiPo's don't last and they don't perform even if the "specification" is the same.

Try one of our 60C LiPo's you won't be dissapoinrted.

https://www.4-max.co.uk/lipos.htm

 

George

4-Max

 

 

 

[Nigel Heather]

Thanks for all the useful information.

 

@George Worley - 4-Max

 

I've just sent an email because I'm a little puzzled by the motor.

 

I've used the 4-Max guide that @Masher pointed me towards.

 

I worked through that and concluded that I would need 600W and it was best to use a 4S which would deliver 45A at 13.3V

 

I came to same conclusion on battery as you - 4S 2200mAh.

 

But I'm puzzled by the motor, PO-3547-800 - it is clearly listed in your 600W-1200W section but when I look at the specifications its states

 

PO-3547-800 Professional outrunner 35mm diameter from 4-Max

 

3S or 4S, continuous current = 29A, max current (10 Sec) = 36A - that is only 386W on 4S - not even close to 600W

 

Are those specifications correct - or am I misunderstanding something?

 

 

Edited January 5, 2023 by Nigel Heather

[Colin Low 2]

Amps x Volts = Watts

36A x 16.6V = 604Watts

[john stones 1 - Moderator]

4 cell 14.8V, 5328W.

[Nigel Heather]

Also you surely can’t use the max current that can only be safely sustained for 10 secs.

 

Continuous current is 29A.

 

My understanding is that the nominal voltage for a 4S LiPo is 14.8V.  The 4-Max guide says to use 13.3V when the battery is under heavy load.

 

So

 

29 x 14.8 = 429W

29 x 13.3 = 386W

 

 

[leccyflyer]

The voltage of your pack, in flight, should be somewhere between 3.8v/cell and 4.2v/cell. The nominal voltage. typically shown on the pack, is based on 3.7v/cell and is really the voltage when you would be landing - not taking off. For a 3s1p pack, for instance, when you put the pack in the model to take off, it'll be at 12.6v, not 11.1v.

[leccyflyer]

Those are the off-load voltages of course.

 

[Geoff S]

Before I electrified it, my Limbo Dancer flew well with a ST34 and a 13x4 prop.  I chose a fine pitch prop because funflies need acceleration at very low airspeed.  I electrified it with a 4S LiPo and used the same size prop.  Can't recall the motor I used (the model no longer exists) but the main snag was the need to remove the banded on wings to replace the battery.  I mounted the esc under the fuselage between the undercarriage legs which allowed me to connect the battery after the wings were re-fitted.

 

For rough approximations when choosing an electric power train I usually assume the cell voltage to be 4v - so 12v for a 3S and 16v for a 4S

[Nigel Heather]

2 hours ago, Geoff S said:

but the main snag was the need to remove the banded on wings to replace the battery.  I mounted the esc under the fuselage between the undercarriage legs which allowed me to connect the battery after the wings were re-fitted.

This is one advantage of the Bill’s Kits Big Fun - it has a traditional nose rather than the short snub - it doesn’t look as good (in my opinion, others make think differently) but it does mean that it should be possible to make a battery hatch so the wings and undercarriage can remain on.

 

https://www.billkits.com/big-fun/
 

I did originally want a Limbo Dancer, I almost bought one at Wings and Wheels when they were £80 but regretfully I didn’t and now they are really silly money.  But given the battery issue, maybe it has worked out right in the end.

Edited January 5, 2023 by Nigel Heather

[Geoff S]

Limbo Dancers are an easy scratch build from the drawings.  All the ribs are identical and the wing can be built as one as there's no dihedral.  The fuselage is a simple balsa box with triangle stock in the corners to allow lots of rounding off.  I wrecked my first one and scratch built the second (now gone 🙂 ) but I think the wing's recoverable so I might build another.  I extended the nose about 25/30 mm as well as making it a bit higher to get battery under the fuel tank because it was difficult to get the CoG right.  The tail has to be as light as possible.  I used standard 148 servos as on the drawing but I'd use 12gm metal geared mini servos if I built another to save weight ... and you need one fewer for electric.

 

There's so little space for a fuel tank that, even with a small engine, like the ST34, the endurance isn't great (6/7 minutes IIRC).  My electrified version had much better endurance.

[Rokitron]

Bought a limbo dancer extended nose 

50mm for electric then 4s battery fitted in from top behind motor no weight for balancing required flies well

[SIMON CRAGG]

Nine of these models were built as last years club model.

 

My set up was:

 

BIG FUN    4S 3300    LB 4.1    FMS 3948 - 760    13X5.5    60A ESC    AEROSTAR 49A / 709W / 177 W PER LB

 

Foam / veneer wing version.
 

Hope that helps.

Edited March 8 by SIMON CRAGG

[kc]

Rokitron - welcome to the forum.

There is a forum section just started about the Limbo Dancer - perhaps you could put your info there and maybe a photo showing how you arranged the hatch