Modern day equivalent for electric power source help please

[Andy C]

So I am part way through building my DSC Fly Baby and need some help in selecting power source and battery please.  Its quite an old design and information I can find on its plan, build notes box and also the web is that the following were suggested power sources. 

 

.049 to .074 engine

035 cobalt or 05/075 ferrite electric motor such as the DSC 075 Hyperthrust motor

.05 to .075 electric motor with 6 or 7 cells battery

Wing area is shown as 275 sq inches

 

I have also found on outerzone:

 

reference to a more recent build using Firepower 450 Sport motor (1100kv) and 1800mAh battery on a 9x4.5 prop but it has not follow up info on whether it actually flew.

 

And a link to an old RCME article written by the designer Ted Davey which gives more details, weight ready to fly as 34ozs and a wing loading of 16.8oz per square foot and he uses an 8x4 prop.

 

Many Thanks

Edited April 5, 2021 by Andy C

[Dad_flyer]

I don't know the model, but in one of my models the 049 replacement is a 1900kv Overlander 2826 on 2s with an 8x4. Gives about 70W and a 1000 to 1300 mAh is a long flight.

You want something more than that. The same motor on a 9" prop would take the motor to its limit at 100W ish.

[Jonathan M]

You could always ask George at 4-Max for his suggestion - he really knows his beans... and his volts!

[PatMc]

Andy, I converted my "Henry" motor glider design from a Cox 049 to a 400 can brushed motor running on 7 cells nicads driving a 5x3 folder or 6x4 fixed prop. Later that was upgraded to a 57 gm 1022kv BL motor on 3s 2200 (1st generation circa 12c) lipo battery that actually weighed the same as the nicads. This combo drove a 9x5 folder at 9000rpm pulling 11.3A for 115W & transformed the model performance.  This is Henry with the brushed motor. 

The BL motor mentioned is no longer available so I was going to suggest a larger more powerful 76g Keda 2217/16 1050kv which I use in my Amigo 2 but that also seems out of production now. However this DYS motor has similar weight & kv so should give similar performance. In the Amigo the Keda turns a 9x5 folder at 10000rpm, 14A for 165W using a 3s lipo. Battery size in the Amigo is from 700 to 1400 mAH (depending on the wind speed) but I only power on for about 25 -30 secs per launch. I suggest that a 3s 1300 - 1500mAH might be adequate capacity for your Fly Baby. 

 

[Andy C]

Thanks guys for the replies.

[Andy C]

@PatMc - Out of interest, is there science behind your suggestions?  What calculations are required to arrive at the power source being suitable please?

Thanks

[PatMc]

@Andy C - my rationale goes as follows : in the OP suggested power for the design included .049 to .074 engine. 
My Henry design initially flew quite well on an .049  & as mentioned the subsequent conversion to brushed electric /nicads gave similar performance using a 5x3 or 6x4 prop. Converting to a 57gm brushless with very low "C" rated lipos driving a 9x5 prop transformed the performance at the same time increasing the duration (over an hour per charge was normal but it is a motor glider).
This BL/lipo combo using a 9" prop was now delivering power at a level similar to a pre schnurle .09 engine driving a 7" prop.  
The Keda motor using modern 3s lipos in the Amigo gives about a 40% increase in power using the same 9x5 prop, no doubt due to it's increased dimensions & weight. The DYS motor linked is similar in size, weight & Kv to the Keda therefore I would expect similar power output from it using the same prop & lipos.
As for duration,  I normally reckon for a typical sports model  to use a battery capacity that would give, theoretically at least, about 5 - 6 minutes of full power. From experience I've found this actually allows a much longer flight time without depleting the battery overmuch.

Edited April 11, 2021 by PatMc

[Andy C]

Excellent,  Thanks for the response.