Sunday, March 4, 2012

SBach 342 30e

Picked up this 1323mm wingspan SBach 342 from Hobby King this weekend. I'm loving how my Duramax Corsair and Mustang is showing in the air (once I got the engine thrust aligned for better tracking in the air with the 2 blade props). And this SBach 342 is one of my favorite flyers right now on Real Flight 6 flight sim. Now this 53" wingspan version will only be about 50% of the size of the featured plane there (so call it 17% scale), and likely be about 4.5 lbs AUW (compared to close to 27 lbs for that 35% scale with almost 9' wingspan gasser), but cost is about 1/5th to fly this versus that, how do you ever fly it when you have to bring it to work with you in a trailer?

Will put a 620kV Turnigy 4240 in it, probably running around 800W on ~3300mAh 4S lipo's (maybe 2650mAh ones too for short light flights), drawing around 55A. Initial guess is a 14x8 or 13x10 prop. Again, 150w for sport/acro planes per lb, is a rough rule of thumb. Go with 175-200W/lb for extreme 3D player. If it gets over 5lbs AUW, I'm going to be a bit under powered...

Getting a 2.5" red spinner for it matches perfectly (2.25" is too small). Also, going to probably mate up a Turnigy 60A Plush with this guy.

Update (4/6): Well, it FINALLY got here today! Going to start the build tomorrow. No instructions, so, uh...Final build ended up at 1718g (3.79 lbs) without LIPO. 2650mAh are 10.6oz.

Quick Reveiw: 4 stars out of 5. Mostly because they only include hardware for pull-pull rudder and it would have been VERY simple for them to include another control horn blank, push rod and have indicated better the spinner size needs. Also, the hardware was very cheap (mismatched in many cases and two of the blind nuts for the firewall were stripped/ruined) and the horizontal stab was covered in RED instead of the rest of the plane's "black" matra, plus lack of instructions. Also, the hardware for the wheel pants was...uh just inadequate, to say the least and one wheel pants was chipped. Other than that, the covering seems pretty good, and most of it looked pretty clean, and HK always packs these planes pretty well.

Specs for my build:
  • Hobby King 1323mm 30e Balsa ARF
  • Turnigy Plush 60A ESC
  • Turnigy SK3-4240 620kV motor (830W)
  • Nano-tech 4S 2650-3300mAh LIPO
  • Hitec HS-65HB Servos (4x) I just didn't trust the Corona 939MG's...too much noise/slop/jitter
  • Great Planes 2.5" Red Spinner
  • APC 14x8E (think 15" is too big) and I wouldn't mind trying a 14x10E for higher speed...
AUW is really close to 4.5lbs with 3300 mAh LIPO.

Notes from build: They glued one of the magnets in the canopy backward, so it was "pushing" instead of latching. Also, both the rear magnets pulled out of the hole, so only the front magnet was holding. You should be sure to carefully check these. I also messed up and didn't get the tail wheel assembly put in during the rudder CA attachement. Last, without any instructions, there was no CoG indication. I put a 10.6oz 2650mAh 4S in the battery tray as far forward as I could go (be sure to protect the lipo from the front firewall screws), and it was balancing about 4.5" back from LE. On a 13.2" wing root. Seemed too far back. That is about 1" behind the main spar, so I figured it was a bit tail heavy, but with plenty of power should be ok and not stall unrecoverably. It was VERY tail heavy, actually. I'm not sure how much the rudder servo mounted back there contributes, but it can't help, and likely you will need >3300mAh 4S or more to get it to balance well. I also don't particularly like the flex in the elevator with the small U-rod connector, but it wasn't too bad. Motor timing seems wrong, and will grind when going from a decreasing idle back to full throttle (or going full throttle from stopped).

Maiden: Taxied the plane around the long asphat drive way and culdesac at my home, and this plane looks phenomenal. Sleek long lines, turns well and runs nice. Maiden run up on grass was a bit draggy, but no tendency to flip up, and plenty of power. No real nasty veer left like the corsair or mustang. Nice straight run out, and a pretty steep climb (need to use less up elevator, unlike mustang to keep straight tail dragger track and just let it rotate up naturally, but oh well...was more worried about a nose over in the thick fresh spring clump grass). Rolls are EXTREMELY authoritative. Pretty. Lines are straight and just need a little right aileron and rudder trim. It was windy (15-25 MPH gusts), so hard to trim it consistently, but I have it close. Slow speed and about 5-7 MPH above stall, it really drops the tail, and then begins to die. CoG is way too far aft. Ok, but add some speed, and it is nice and straight. Inverted is good with the symetric wings. Tight loops have very severe snap roll tendency, so I probably have too much elevator throw...I can change that later. Rudder coupling to ailerons is set at 11% to do a flat slide turn with no banking (from my YAK 54 foamie). It is either reversed because servos are backwards from the Yak or way too much...either way, Knife Edges (plus the wind) aren't very good and roll out fast. Can't really try elevators to see how it balances in a stall drop because of the CoG, but snap rolls are very responsive (almost too fast). I figure top speed on this is around 60 mph with the 14x7, which is a little high, but maybe with the wind and on a full LIPO.

Then, tragedy. Either something snapped loose (the aileron was completely pulled from the CA hinges on the right side with no obvious damage or issue with the wing, which seems strange, post mortem) or I got blasted with interfereance. I'm not real sold on any of those explainations, and most likely (reviewing it in my mind over and over), I got into a high alpha dive/loop, and it kept snap rolling all the way into the ground. Pavement, to be exact. Straight in. I don't believe in coincidences much, and I've been trying to be honest with myself and figure out what happened. I fly a lot of foamie 3D, and really try to use a lot of elevator authority. It hurts me on my warplanes, because I always have to go back and limit the servo travel to avoid snap rolling in aggressive loops. In this case, I've been thinking about how CoG may affect you in a high alpha tight loop where the wing begins to stall out (this is what causes most snap rolls in tight elevator loops). If you have a very aft centered CoG, the tail will naturally want to swing down/out more...putting it into even MORE of a tight loop/angle of attack. Which with eventually just feed back on itself until it flips over (counter clockwise on these planes), which is exactly what it seems like I remember happening. And the more you yank and bank on the sticks, the worse the drag and more it wants to stall. So, I think I just learned a very valuable (expensive?) lesson. I've always tried to maximize the throws on my planes (and this one seemed not huge compared to my YAK foamie), but the surfaces are pretty big. I believe I simply had really bad CoG (was almost ready to land and try to address that...sigh, but two passes in high had made it slow and the tail to settle very badly, so I throttled up out of it and was playing a little more before trying a fairly fast speed landing to avoid stalling). Not fixing that sooner likely lead to my demise.

Update (4/9/2012): This thread seems extremely applicable. Pretty much supports what I was running through in my mind. In retrospect, I think (hard to remember everything that happened...need flight recorders...grin), but I *think* I even pulled off the throttle as it went into the spin/stall, as full power always seems to tighten these things into oblivion, and I'm almost certain I was probably trying to naturally correct for the roll with aileron...which is a bad no-no in spin/stall...but in this case, the reduced power probably aggrivated the control surface and wing stall and the rest is morbid history. I'm just shocked how much the aft CoG affected this model here. Granted, it was pretty severe (very nasty pigish wallowing and immediate nose up as I checked high altitude slow speed stall response)...oh well. Live and learn...I grow ever more wary of that effect, and have pushed aft CoG for hovers in my 3D planes, but they have insane wing loading and control surfaces. Always thought that aft CoG was dangerous, primarily only in planes with low thrust/weight this case, I could easily pull out of a dropped tail stall with power...the ever tightening spin/spiral/snap hasn't slapped me in the head hard enough before, I suppose. Build thread

1 comment:

  1. Got my replacement airframe, and the Turnigy SK3-4250-500kv 1350W motor (running on 4S or 5S 3300mAh LIPO). Probably will run 15x8 or 15x10 on this.


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