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Old Jun 22, 2011, 08:22 PM
'FPV'er...not a "LOS'er
Vantasstic's Avatar
Las Vegas, NV
Joined Sep 2003
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Quote:
Originally Posted by Cralis View Post
Thanks man! I'll keep that formula in mind. I'm not sure how it works out though. To me, the mah is the mount of petrol the tank can hold... So how does a tank with more capacity, effect the C (Which I am not sure what that represents, but I thought it was how tough a battery is...).

Wait, this is way off topic, so let me add ... Anyone else flying 500mah packs in their Skywalkers ?
C=Current rating. A 20C 1000mAh pack means it's rated to deliver 20x1000mA (or 20x1A) for 20A of constant current while holding voltage. A 20C 5000mAh pack is cable of 20x5A for 100A of constant current. That doesn't mean your power system will draw more amps though. Your power system will only use what it requires. A higher capacity and higher C rated battery is able to supply the current easier than a smaller capacity and smaller C rated battery.

Okay, I said a higher C rated battery doesn't mean your power system will draw more amps...sorta. In most cases it actually will. That is because of the voltage drop under load. A lower capacity/C rated battery will have a reduced voltage under load. A higher capacity/C rated battery will hold a higher voltage under load. Amps=Voltage/Resistance. Resistance is pretty much fixed via the windings in the motor and design of the ESC. Consider resistance a fixed value that doesn't change. Let's say a low capacity/C rated battery only holds 10v under load and a larger capacity/C rated battery holds 11v. Both batteries are connected to a power system measured at 2ohms. The 10v battery will draw 5A (10v/2ohms). The 11v battery will draw 5.5A (11v/2ohms). So the larger capacity/C rated battery will actually allow the system to draw more amps, which equates to more power. All very confusing at times. Just consider larger capacity and higher C rating means easier ability to provide the amps the power system wants at a higher voltage.

Lastly (on the C rating stuff). Don't take C rating as gospel. It is often exaggerated as part of the marketing. I just knock of a couple of C's for a rough guesstimate of what it can actually do. So a 20C battery if probably more truthfully like an honest 15C, A 30C like a 25C, etc. Either way, the larger the pack and the higher the C rating the easier the battery can provide amps while holding a higher voltage level.

As for the second part of your question. Yes, I fly my SkyWalker with a 5000mAh, 3S, 40-50C Turnigy battery. The SW handles the weight with no issues. I cruise around in the 6A-8A range at ~24mph/38kph.
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Old Jun 22, 2011, 08:27 PM
OSUFPV - KF7VFT
Corvallis, OR
Joined Apr 2010
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Quote:
Originally Posted by Vantasstic View Post
So the larger capacity/C rated battery will actually allow the system to draw more amps, which equates to more power.
Actually, in your example it equates to a constant power. (P=IV)
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Old Jun 22, 2011, 08:39 PM
'FPV'er...not a "LOS'er
Vantasstic's Avatar
Las Vegas, NV
Joined Sep 2003
18,243 Posts
Quote:
Originally Posted by Blueshy View Post
Actually, in your example it equates to a constant power. (P=IV)
Well, maybe more to 'apparent power' (P=IV), where 'real power' is P=I2R. All mute really as the higher the current and voltage the higher the power in Watts. A higher C pack should provide higher amps since it's holding a higher voltage under load so the overall power is also increased.
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Old Jun 22, 2011, 08:41 PM
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Daemon's Avatar
Lakewood, Colorado
Joined Aug 2002
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Quote:
Originally Posted by Blueshy View Post
Actually, in your example it equates to a constant power. (P=IV)
Actually no. That was the whole point of the example. Read it again.
The current draw is more or less fixed by the demand of the motor, but the voltage of a
lower C battery may drop under load so P=IV means less total power delivered.
A higher C battery is less likely to drop the voltage under high load, so
given a fixed current draw, and higher voltage, it delivers more power.

Before one jumps to the conclusion that higher C is always better, remember the SW
and most other FPV planes only need 30-40Amps at most, so a 20C 5Ah battery (capable
of delivering close to 100A draw) won't sag at all, and a 25C or 30C battery offers no
benefit at all. It'd be a different story if we were talking 2000mAh batteries at 15-20C rating
for a small fast plane like a Funjet with a hot motor.

ian
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Old Jun 22, 2011, 08:57 PM
OSUFPV - KF7VFT
Corvallis, OR
Joined Apr 2010
1,763 Posts
Quote:
Originally Posted by Daemon View Post
Actually no. That was the whole point of the example. Read it again.
The current draw is more or less fixed by the demand of the motor, but the voltage of a
lower C battery may drop under load so P=IV means less total power delivered.
A higher C battery is less likely to drop the voltage under high load, so
given a fixed current draw, and higher voltage, it delivers more power.
Oops, you're all right. I totally missed the point that he was trying to make.
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Old Jun 22, 2011, 09:14 PM
SLO Flyer
Jaybee's Avatar
San Luis Obispo, CA
Joined Dec 2003
98 Posts
Beautiful video, Tom. Favorited.

Quote:
Originally Posted by tom223866 View Post
little flight with my skywalker, lawmate 2,4 500 and scherrer uhf

http://vimeo.com/25464739
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Old Jun 22, 2011, 09:36 PM
Fly Chitty Chitty Bang Bang!
EarthOrbiter's Avatar
Central California USA
Joined Feb 2011
379 Posts
Has anyone vertical flown the SW?

I am thinking about doing a vertical strait lift for about a min on the SW.
If moto calc calculates right This configuration will do 2419ft(737m)/min @ 90.

Here is the analysis.

In-flight Analysis - Test4blade
1000ft above Sea Level, 29.92inHg, 55F

Motor: Scorpion SII-3026-1190; 1190rpm/V; 3.26A no-load; 0.008 Ohms.
Battery: 11600 LIPO (25C); 3 series x 2 parallel cells; 11600mAh @ 3.7V; 0.0015 Ohms/cell.
Speed Control: Scorpion Commander 110A; 0.0008 Ohms; High rate.
Drive System: APC 10"; 10x6 4-bladed (Pconst=1.11; Tconst=1) direct drive.
Airframe: Skywalker; 550sq.in; 92.5oz RTF; 24.2oz/sq.ft; Cd=0.068; Cl=0.87; Clopt=0.95; Clmax=1.72.
Stats: 156 W/lb in; 136 W/lb out; 19mph stall; 26mph opt @ 47% (255:04, 87F); 27mph level @ 48% (264:21, 87F); 2419ft(737m)/min @ 90; -177ft/min @ -4.4.

AirSpd Drag Lift Batt Motor Motor Input Loss MGbOut MotGb Shaft Prop Thrust PSpd Prop Total Time
(mph) (oz) (oz) Amps Amps Volts (W) (W) (W) Ef(%) Ef(%) RPM (oz) (mph) Ef(%) Ef(%) (m:s)

17.0 2.8 35.9 83.9 83.9 10.8 909.9 119.7 790.2 86.8 84.9 11662 106.7 49.3 28.5 24.2 16:36

I think the stiffening of the tail will need to be done. The plane oscillates with speed.
All opinions accepted.
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Old Jun 22, 2011, 10:46 PM
I love FPV!
Cralis's Avatar
Eastern side of Australia
Joined Jan 2007
4,650 Posts
Elevator throws... Are you all using maximum deflection, or are you dialling it down a bit? I am going to maiden mine soon, and wondering if I will battle with full deflection.
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Old Jun 22, 2011, 11:08 PM
FPV junkie
m_beeson's Avatar
United States, UT
Joined Jan 2011
3,379 Posts
Quote:
Originally Posted by Cralis View Post
Elevator throws... Are you all using maximum deflection, or are you dialling it down a bit? I am going to maiden mine soon, and wondering if I will battle with full deflection.
I put mine into the ground for not having enough deflection.

I have it at full deflection.


& I am using the mechanical Anti reverse yaw technique that is described in previous posts.

essentially offset the servo horns one notch to the back, so you get more upward deflection than down.

When you go into a bank, it helps to pull it around, instead of the lower one pushing it in the wrong direction.


Mike
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Old Jun 22, 2011, 11:12 PM
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Lakewood, Colorado
Joined Aug 2002
28,488 Posts
My SW does loops and dodges between trees with +/- 5 to 7 mm of elevator deflection
and the CG somewhere beyond 30mm behind the servo wire slot. The further ahead of
that your CG, the more elevator throw you'll need.

ian
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Old Jun 22, 2011, 11:14 PM
FPV junkie
m_beeson's Avatar
United States, UT
Joined Jan 2011
3,379 Posts
Earthorbiter,

I have seen some steep climbs in youtube. search Skywalker FPV, and Clouds.

I think that will pull up what you are looking for.
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Old Jun 22, 2011, 11:15 PM
I love FPV!
Cralis's Avatar
Eastern side of Australia
Joined Jan 2007
4,650 Posts
I just put in my 5000mah pack, and find I am still tail heavy. So I added an extra 2200mah pack (dead weight - not functional), and it's perfect. So, might have to fly with dead weight... OR ... remove my power for my camera/vtx from my harness, and power it off the spare 2200mah pack... Maybe that would be a good idea? Lots of work though...
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Old Jun 22, 2011, 11:24 PM
Registered User
Daemon's Avatar
Lakewood, Colorado
Joined Aug 2002
28,488 Posts
Define "tail heavy"? Where is your CG? If it's anywhere near the wiring slot,
then it's actually severely nose heavy. 25-35mm behind the wiring slot is a better range.
Where I run the CG, I can't put a 4 or 5Ah battery anywhere near the nose.
If you need 7A worth of battery to balance it, something is very wrong.

ian
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Old Jun 22, 2011, 11:29 PM
Fly Chitty Chitty Bang Bang!
EarthOrbiter's Avatar
Central California USA
Joined Feb 2011
379 Posts
Quote:
Originally Posted by m_beeson View Post
Earthorbiter,

I have seen some steep climbs in youtube. search Skywalker FPV, and Clouds.

I think that will pull up what you are looking for.
Thanks, M B.
I will look for a 1 min climb there.
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Old Jun 23, 2011, 01:22 AM
Registered User
MO, USA
Joined Jun 2010
686 Posts
Short of reading all 500+ pages, is there a synopsis of the "must-do" mods for the SW? Mine arrived yesterday, and I'd like to know what's in front of me, assembly-wise.
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