Prangster Electric help!

[Airpaddler]

Objective is to convert surplus old Prangster 3D, to mild hack.

It was flying with a 25cc IC and so I supposed surplus G25 Turnigy @ 870 kv would probably be useable. However!! I tried to do some calculations from web gleanings and I fear I have something awry?

I would appreciate a learned check my suppositions and calculations please :-

Model + Turnigy + 2200mAh 3 cell weighs in at around 4 lbs. So added ~40% for flying weight of 5.6 lbs ?. Going for 150W/lb = 840W?

Usually use the 3 cell lipos so :- Given Amps = Watts/Volts. ( I used 10V nominal for the batteryl) so 840W /10 = 8.4A call off.

Calculating battery capacity with Time x Current /60. :- I want say an 8min. flight. Current is 85A peak so go for 70% in use = 59.5A. So 8 x59.5 /60 = 7.9Ah or 7900mAh. So my three cells will not cut it by these calculations.

Does not seem right! Be gentle, bit of a newbie at this.

[Steve Hargreaves - Moderator]

OK...lets take a look through this....first off...25cc sounds like a big engine for a prangster? do you mean a "25" which is 0.25cubic inches or 4.5cc??

AUW...where has the extra 40% come from?? 5.6lbs sounds like a lot for a prangster....especially if you are flying it on a 25....if you have the model all ready to go in IC form then just stick it on the scales.....that will be near enough.

Lets get that bit sorted first & then we can talk about a few calculations......my initial thinking is that your Turnigy motor might have too low a kv to be successfully used on 3S.....you would probably need a prop around the 15" mark to make any power. It looks more like a 4S motor to me....

But we'll see......motors are pretty cheap these days so we should be able to get you up & running....

[Steve Hargreaves - Moderator]

PS I've moved your thread into the Power System Selection area Airpaddler...it will get more attention here....

[Airpaddler]

Steve - Yes thanks for info. I did indeed mean .25cc

I put the G25 + an extension onto the Prangster. Slipped the 3 cell 2200 in just to see if the recipe would give me a CoG. After little lead forward, it was balanced. That setup was a touch under 4 lbs. - I only added the 40% following a crib sheet off the net. So may have misunderstood, possibly meant just the airframe + 40%??

[Steve Hargreaves - Moderator]

OK...so lets take 4lbs as AUW.....thats pretty heavy for a 25 glow motor......how did it perform? Lacking in the verticals I would think......

OK....150w/lb will give it pretty good performance & should take it vertical.....4 x 150 is 600watts & here we hit a snag.....your motor is only rated to 32A which is 32 x 10(volts) is 320 watts.....erm...not enough... That said it will probably match the performance of the 25 glow motor.

So your options are.....

1) Use the motor/battery you have & settle for less than sparkling performance....

2) use that motor on a 4S battery 32A x 13 (volts) will give you 416watts...better but still a way off your target....

3) Find another motor that will give you 600watts......but you will need about 60A out of a 3S set up to get 600watts & this will certainly punish a 2200mAh battery as this is just under 30C current draw which is quite high.....you could use 2 batteries in parallel to get a 4400mAh battery which would make the current draw under 15C which is much better....is there room for 2 batteries?

4) start from scratch with a 4S set up (& a bigger motor)

As they say.... Speed is just a question of money......how fast do you want to go?

[Airpaddler]

Steve - Hm! Thanks for guidance. Will go for the 600 W / 4S option.

Would you be able to make any recommendations please? i.e. Motor, ESC, Prop.

Hope I am not asking too much? But as said, bit of newbie. But learning at pace.

[Steve Hargreaves - Moderator]

OK lets have a think.....600watts off 4S is about 46A so I would suggest a 60A ESC for starters & probably a 4S battery around the 3200mAh mark....should give you reasonable duration.

For the motor we need it to take arounf 50A....a slight concern I have here is ground clearance....a 25 glow motor will probably turn a 9" prop....could we use an 11" maybe even a 12" prop & not mow the grass??

[Airpaddler]

The prop axis foreward point is about 9" above ground. So 12" prop would be no problem.

[Steve Hargreaves - Moderator]

OK.....maybe this motor would do a good job...quite a high kv though so you'd probably start on a 9x6 prop & see what power that takes....maybe a 10x6 would be good.....I've no direct experience with NTM motors though so can't vouch for their quality....

Alternatively this motor on similar props......might be a bit big though.....

Or closer to home this motor on an 11 or 12" prop would work well....

Or this perhaps.....again 11-12" prop

You will need to check the numbers with a wattmeter to make sure everything is in spec & possibly experiment with a few props to get the power you want but I think any of these motors will put a real smile on its face......enjoy...

[Steve Hargreaves - Moderator]

You might even get away with a slightly smaller battery...a 3000mAh perhaps as I don't think you'll be using full throttle very much....

[Tim Mackey]

I had a prangster a few years back, and it flew fine on a .25 engine.

I would say 500 Watts would be ample - after all these rubbery bouncy thingies are not renowned for their performance

That way, you may get away with a good highC 3s pack, which would reduce weight too.

I think for this model I would go slightly higher revving motor than I would normally use.... so maybe 1100 - 1200 kV and a 10" prop??

[Steve Hargreaves - Moderator]

Thanks for the input Tim....I was a bit concered that 600watts seemed rather a lot for a 25 powered model....

A bit like putting a 40 in one....