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View Full Version : Bit confused with Motorcalc (7)


Real_Finn
11-29-2005, 02:59 PM
Hi all,

My 1st post here...

I bought trainer from Raidentec http://www.raidentech.com/steviinigasr.html, which I am electrifying.

I used motor calc:
Motor: Model Motors AXI AC2820/12; 920rpm/V; 2A no-load; 0.063 Ohms.
Battery: Thunder Power TP1950; 3 series x 3 parallel cells; 1950mAh @ 3.7V; 0.015 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: Generic 10x6in Prop; 10x6 (Pconst=1.25; Tconst=0.956) direct drive.
Airframe: VNLines Trainer; 532sq.in; 71.4oz; 19.3oz/sq.ft; Cd=0.053; Cl=0.56; Clopt=0.67; Clmax=1.38.
Stats: 47 W/lb in; 36 W/lb out; 20mph stall; 28mph opt @ 74% (59:52, 108F); 31mph level @ 79% (54:04, 111F); 462ft/min @ 10.6; -228ft/min @ -5.2.


I am bit confused about battery selection. It says TP1950, 3 series and 3 parallel, 1950mAh

So, it sounds I need 1950mAh battery ? But it also says 3 parallel, which would mean ~6000mAh. Eh, or ? :confused: (3 series means ~11V)


Real_Finn(Flyer) lol

Mike Parsons
11-29-2005, 03:34 PM
Welcome to Wattflyer Real_Finn.

The TP 1950 cells are a generation1 cell if I recall. There are newer spec's available from Motocalc that have the TP 2000mah gen2 and Pro-lite cells.
However, I usually only use Lipo's in motocalc to get an average weight and use the appropriate number of round cells for performance. Motocalc is usually a bit off base when it comes to using Lithiums and especially when using outrunners.

In todays world of high discharge batteries and increased C ratings, our systems dont require as many cells in parallel to handle the current. With that motor, you will only be in the 35-40 amp range at max, so a 3S2P 4000mah pack would do fine. The new TP pro-lites can handle 12C continuous discharge, 15c burst for 10 seconds and 18c burst for 5 seconds. Plenty of headroom with that pack.

You will most likely be swinging a 12X8 prop and not a 10X6 for ~350 watts. To really liven it up at 400-450 watts you could go with an AXI 2826/8 swinging a 12X8 on 3S.

-Mike

Matt Kirsch
11-29-2005, 03:41 PM
Well, the point is really moot since TP1950s are no longer available AFAIK, but what they mean is a 3 series, 3 parallel (3S3P) pack "MADE USING" TP1950 cells. The pack would have a total capacity of ~6000mAh as you suspect.

There have been a couple of generations of cells since the TP1950s, which were good in their day... Now, you could get away with a 3S2P pack of the latest TP2100 cells. 4000mAh, but ~6oz lighter, and more usable capacity because you're not stressing the cells as much. Even with 3P, the 6C rated 1950s would be straining while the 15C rated 2100s will just laugh in a 2P configuration. You could also go with a 3S1P configuration of Kokam 3200 cells, either in Cellpro or BalanceProHD flavors.

Real_Finn
11-29-2005, 04:07 PM
Thanks for replies!

Now it is all clear. I haven't seen lipos for sale as "cells" here, but instead only as ready made packs.

Since I already have the motor mentioned above, I will go forward with it. At the moment I think I will go for 4000mAh (TP4000-3S2P "Pro Lite") pack. That one should give 12mins full gas flying time and something like 1h with half power (taken from motor calc). Let's see in real life.

I still need to look where from I take the pack. In this country they seem to be made of gold.... :(

Above the lipo I also need to build skies before I can go for first flight.

Mike Parsons
11-29-2005, 04:13 PM
I think your real life numbers will be different, but it will still perform.

Contact Cliff @ www.Atlantahobby.com. He ships international and has the entire line of TP's in stock.

-Mike

Real_Finn
11-29-2005, 04:24 PM
:eek: ...

Atlantahobby has really nice lipo prices... Hehe, now I know where from I take the lipos.

Thanks Mike!

Mike Parsons
11-29-2005, 04:33 PM
Your more than welcome RF. Tell Cliff I sent you.

starcad
12-04-2005, 05:30 AM
Hey Matt, just change it from 3 series, 3 parallel to 3series, 1 parallel and you should be in the ball park. I had the same problem awhile back and had to search deep to find an answer.

Ben
12-06-2005, 10:05 PM
Is there a table showing wattage out to mimic the rated power of wet engines? Is there ANY way of reading watts instead of meaningless class descriptors of electrics? I want to have a more general idea of what to think of as a replacement for wet power in a model that has the flying characteristics I want! Ben

hoppy
12-06-2005, 10:17 PM
Here are some glow to electric conversions - that should give yu a start
http://www.hobby-lobby.com/glow2econ.htm

Ben
12-07-2005, 12:27 AM
Thanks, Hoppy - - guess my note was ambiguous - try this: Which brushless would come closest to replacing a "40" two-cycle? or more generally, how many watts?, then, what is the secret to finding the output power (watts) of a "300" motor, or more obscurely, an AXI 2620/12? I need to be able to find a dry drive that matches a wet drive!

Matt Kirsch
12-07-2005, 02:07 PM
Ben, quite simply put, you don't want to replace a glow engine. You want to power the plane based on how it should fly and how you want to fly it. Glow engines are a one-size-fits-all that don't always fit all that well.. Take a ".40-size" Piper Cub for example. You never get past 1/4 throttle. Take a ".40-size" Extra 300 for contrast. You're wishing you crowbarred a .61 in there... Don't match the drive, power the plane.

Motors in and of themselves have ZERO output power. The motor's just the crankshaft of the electric "engine." To get power, you need a battery, prop and sometimes a gearbox. The power output varies with the battery voltage, prop size, and gearbox ratio (if applicable).

Now before you run away screaming, remember that these are just basic concepts that I'm explaining to better help you understand how electric works. Back to your questions:

Remember I said to power the plane? How many Watts depends on how much the plane weighs and how you want it to fly. Piper Cub? 75 Watts per pound. 3D Extra 300 profile? 150 Watts per pound. For your typical sport plane, including semi-scale warbirds, I like 100 Watts per pound so the plane's a bit overpowered and flies as if it's glow powered. A typical 5 pound .40-size would do well on 500 Watts of INPUT power. We use input power because output power is near-impossible for mere mortals to calculate, and because these figures I've quoted take motor efficiency into accout, and then some.

If you're looking at AXis, all you need is that Watts figure and your prop clearance. They have extensive, accurate application charts for most of their motors at www.modelmotors.cz (http://www.modelmotors.cz). Your "300" motor example is actually more obscure than the AXi as far as figuring out

Real_Finn
03-17-2006, 08:13 AM
It is still winter here and the local rc-club's field is covered with snow (20-30cm) so there is no action with wheeled planes.

But, yesterday I sneaked out of office and I went to nearby lake with this trainer. There was small spot (read it: airfeald) more or less cleaned from snow. The only issue I was not that happy that there were too many people skiing or walking around etc.

The day was absolutely windless with blue sky.... :D

Take-off with blade 11x8 3 propellar was bit "sticky", but once in the air the plane flew just nicely. After 6 mins flight with each 4000mAh battery I charged to them 1485mAh and 1235mAh. So, it looks I should be able to fly with them something like 15mins. Or just limit the time to 12mins and have lots of margin.

I guess I will order wheels with bigger diameter and 3 blade 12x8 to get little bit more power (wheels due to ground clearance).

I was so happy that I ordered the Halair MIG 29 EDF kit from www.rbckits.com (http://www.rbckits.com).... :p

Real_Finn
06-30-2006, 10:46 AM
I have ended up using Master Airscrew 12x8 Electric Only on this trainer. Setup otherwise as in this thread. Some minor modifications were needed to landing gear to allow this one to turn....

This setup draws slightly over 40A at full power (on ground) resulting bit over 400W on 3 Cells. In practise I make 6-8min flights and usually top up the 4000mAh lipos with 2300-2600mAh. I guess extending flying time to 10mins would be fully possible.

I regularly hear the glow guys commenting about this plane having power... :p