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Old Dec 03, 2007, 12:11 PM
Peter Mar is offline
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EMFSO, CRS, MAAC
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Toronto, Ontario
Joined Oct 1999
363 Posts
Siren Lite

Stats of my 2nd Siren should help some of you trying to figure out power systems and servos etc,

Powered by $25 UH/HC HXT 35-36C https://www.unitedhobbies.com/UNITED...idproduct=3886

TP W30A Brushless Speed Controller $13 https://www.unitedhobbies.com/UNITED...?idProduct=658

Esprit sourced Turbo 38mm AL spinner pictured above by rcplanefubar.

Grapupner 12x6 folder, pulls 33A HOC

Batteries = 2 packs, in line, CSRC 3s1p 2000 mah 8c or 10c (155g-160g per pack), making it a 3s2p system. The now puffy packs are about 80mm x 34mm x 25-28mm thick depending upon puffiness of the individual packs (25-30 cycles on them, 8c packs are puffy the 10c are ok).

Servos HS-65HB all around.

Rx = Old reliable FMA Fortress 8 DC

*** CG = 80mm from leading edge!!!! ***

This second bird came with much more wrinkled covering and the stab did not fit level. It seems the QAQC is getting poorer. Overall it is still worth the street price.

I wanted this bird to be lighter than my first one so I could improve its thermalling ability.

The way far back CG is accomplished with everything moved as far forward as possible. The cheapie drive system works like a charm. It does not climb nearly as fast or steep as my previous geared setup (reported earlier in this thread). Once again, I don't care about that. It takes 20 seconds to get to climb and dive height (150m) and about 40 seconds to get to serious thermal hunting height. With two packs on board I can climb til the cows come home.

The wires on the outrunner rub against the case. I have yet to butcher the fuse to let the wires hang out so to speak.

The real treat is the glide. With a rearward cg of about 80mm and the low AUW, the Siren can be a nice floater in calm conditions (no not like a sailplane with a 5 oz/sq ft loading). It can actually thermal very well in winds greater than 15 kph (10 mph). I have flown it in 30-40kph winds (just trim for faster glide to prevent stalling due to turbulence). Under those conditions it lands at slow ground speeds without spoilers. However, it does get bounced around dangerously.

*** Warning the 80mm CG may not work for you. ***
It is a full inch behind the recommended CG. So if you are new to the Siren or have a new one to maiden, I suggest you set the CG at least to 65mm-70mm as a start. Depending upon the decalage of your particular bird it may be too far back to be controllable. Now that I am accustomed to it (really touchy controls and easy to snap roll) I am confident in all aspects except for slooow flight. I fly very slowly only when I am way up there in calm conditions. There is a real danger of stalling with vicious snap rolling. Test this behaviour at 150m or higher as you'll need some diving space to get the speed up before you pull up. Otherwise you will get a second and third snap as you plow it in. Don't ask how I know...

Lastly, make sure the lipos are well anchored. A good hand launch may shift them back and at 80mm there is no CG margin left to play with. My packs are wedged in with copious wiring holding them forward. They do shift a bit and I can tell when they move back a few mm. Once again, if the CG shifts too far back you are toast right at the worse time (right after launch).

My first Siren core sampled due to Rx lockout: http://www.rcgroups.com/forums/showp...10&postcount=3

***CG UPDATE***

Others in this thread have mentioned CGs as far back as 100mm. I have tried 100-110mm and would not recommend this to Siren newbies. I am only able to use these rearward CGs when flying in light to no wind and when I want to float around. Because of possible tolerance diffs between each batch, the decalage of your bird may not allow a CG that far back.

The best way to test this is to do the usual dive test. Then do the same dive test inverted! Then you will know if the CG is too far back. For me at 110mm the bird will pull out inverted and that tells me my CG is way too far back. However, this is also the float CG I use when I have one or two flights at sunset when the boundary layer has set up and the air is completely dead. That's about the only time the Siren looks like a floater. Of course it'll snap stall on a blink so be very careful with the amount of up elevator you feed for turns etc.
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Last edited by Peter Mar; Apr 28, 2008 at 03:56 PM.
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