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Old Jan 07, 2013, 07:11 PM
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Sbach 342 3D - Finding COG with different batteries?

Hiyas,
first post, I have defected from heli world and bought myself an SBach 342 3D. Maidened yesterday, durable little bugger, and quick!

One question, how do I determine the correct COG? The manual states the COG when using the suggested 180mAh battery, but I am using 325mAh batteries from my 130x so I expect they weigh a fair bit more. I guess this means the COG in the manual will not be correct. What is the correct way to work out the best battery placement for different batteries?

Thanks
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Old Jan 07, 2013, 07:26 PM
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Whatever you determine to be the best cg for you will be at a location on the wing that the plane will balance at. The factory recommended cg location is most always a good place to start. When you use different battery weights, you will have to determine where any particular battery must be placed to make the plane balance at the cg. Once the preferred cg of a plane is established, it's just a matter of placing different weight batteries in different positions to maintain the same cg location.
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Old Jan 07, 2013, 07:48 PM
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Originally Posted by bobly View Post
When you use different battery weights, you will have to determine where any particular battery must be placed to make the plane balance at the cg.
Thanks, I guess that's my question - what's a good method to determine that the COG is good? Is it just flying it straight and level, and seeing if it needs any elevator to stay level?
Does this need to be tested while inverted too?

This is probably obvious, but I've only had 1 day of experience with planes =)

cheers
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Old Jan 08, 2013, 12:26 PM
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If an Sbach 3D is your first airplane, this ought to be wild.

Go buy a T-28 or Polecat and fly that for a few weeks, before attempting a 3D Sbach, or it will be a smoking hole in the ground on the first, white knuckle flight.

As for the CG, a beginner is going to want the CG @ 25% of the wing root. Measure the wing root, divide by 4, and this distance back from the LE is where the CG should be (and not more than a couple mm more aft than this).

This method works for pretty much any of the monoplane, unswept wing micro planes.

An advanced flier (someone who has months of aerobatic plane flying under their belt) can move the CG back to the Servo wire channels (and maybe a little aft of this).

The Sbach is a fantastic plane for Intermediate fliers up, but I don't think a beginner will enjoy it more than a T-28 or Polecat, because it requires quick, instinctive reflexes, rudder & throttle managment and advanced skills to keep it alive, and these are things beginners don't have.
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Old Jan 08, 2013, 02:12 PM
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+1 on this probably being the wrong plane to start on.

That said the CG isn't a big issue from what I have seen. Generally speaking you push the battery back as far as you can. The plane is very forgiving as far as CG is concerned. It isn't going to be unflyable no matter where you put the battery.
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Old Jan 08, 2013, 04:33 PM
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Great, thanks for the concise descriptions.

I've put a few batts thru it on 50% rates and it flies beautifully.. couple of minor crashes but its pretty strong. Didn't want to buy a t28 only to need a new plane after 2 weeks, i made that mistake with rc helis. Now if I can only work out how to land hehe..
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Old Jan 08, 2013, 08:41 PM
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You didn't say where you had the cg or the battery, but it's easier to land if the cg is towards the front of the range. You might try with the battery about 1/2 in front and 1/2 in back of the split where the cover meets the rest of the fuselage. On your question of flying upright and inverted to test cg, that is normal to do with some experience, but maybe you're ready. The idea is that what is usually referred to as neutral cg will have the plane fly upright or inverted and stay level without elevator input. Keeping the cg a bit forward of that point where only slight down elevator is required when inverted to maintain level flight gives a bit more stability to the plane and it's not as touchy on the controls.

And congratulations on the natural ability you have. If you are flying the sbach on 50% throws on your first day with a rc plane, you have a gift for sure.
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Old Jan 08, 2013, 10:24 PM
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Originally Posted by Rattler888 View Post
Great, thanks for the concise descriptions.

I've put a few batts thru it on 50% rates and it flies beautifully.. couple of minor crashes but its pretty strong. Didn't want to buy a t28 only to need a new plane after 2 weeks, i made that mistake with rc helis. Now if I can only work out how to land hehe..
The T-28 and Polecat land themselves.. just point them toward the runway and reduce power to get a glideslope and they fly right in to a perfect landing...If you try to land an Sbach like this the nosecone will be crushed after your first few attempts, the prop or prob adapter broken by the 10th attempt, and the nose will likely be cracked by the 20th attempt.

The only way to land an Sbach is a high alpha harrier, touch the tailwheel, then reduce power until the mains settle. This results in a slower landing than a T-28.
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Old Jan 09, 2013, 04:57 AM
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bobly, thx for the compliment, but its just because I have been flying helicopters for the last year. fast forward flight and orientations in a heli is pretty damn similar to flying a plane

UNGN, thanks for the description on landing - I have indeed somewhat crushed the nose cone hehe.. I fly on grass so today I mastered the 'fly into the wind, stall and belly flop then tip gently onto the nose'.. not ideal, but doesn't seem to break anything so far! hopefully one day I can get a proper touchdown like you described.. does this need to be done on perfectly flat ground, like a cricket pitch?
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Old Jan 09, 2013, 05:38 AM
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You have the right idea you just need to control the stall with throttle until you are close enough to belly it in. When I fly off grass (not often) I do without the gear and that stops the nose over when the gear gets caught in the grass the wheels are just too small to roll on grass. I would recommend some servo covers if you take the gear off because the last thing you want is dirt and damp in them. I would venture to say out of all my UM planes this one is the most difficult to land and even more so with an aft CG & some wind.
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Old Jan 09, 2013, 06:38 AM
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Servo covers would be a must if not using gear. I've milled out the wing pockets on mine, so the servos are 90% below the wing surface... this is a mod that I was surprised HH didn't do on the V2 and moulded pockets would have been stronger than my milled pockets.
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Old Jan 09, 2013, 06:47 AM
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A plus one for taking the gear off to fly over grass. But this thing can be flown in for a main wheel or belly landing as long as the cg is just right, but maybe not as far back as a lot of folks want it to be.
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Old Jan 09, 2013, 06:51 AM
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I would venture to say out of all my UM planes this one is the most difficult to land and even more so with an aft CG & some wind.
With the harrier method, this is one of the easiest planes for me to land. I have a very small strip with curbs on the sides, a 3 foot rock face wall below the appoach and a barrier/guardrail 25 feet back from the threshold, so I get one shot at a landing.

Champ/polecat/Flyzone WW1 are easy and land themselves (10 of 10 landings are good), T-28 it a little fast and I have to fly it down so I make about 7 out of 10, Mustang is a little faster/harder still (5 out of 10 are good), sukhoi is impossible and is more of a controlled crash (2 of 10 are good)

With the harrier method 10 out of 10 Sbach 3D landings are good, maybe 1 of 20 are "a little hard" and have a squealing prop strike.

The Sbach 3D is so stable in high alpha that landing in high alpha is easy.

If I fly without gear, I plop it onto a dense bush.
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Old Jan 09, 2013, 09:53 AM
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While you guys are discussing landings I recommend removing the gear from the rudder and attaching it to the fuse. You can still steer the plane with the rudder only now the rudder will be less likely to get damaged. I actually attached a bigger wheel to fuse. UM T-28 nose gear that I bent and attached with CA and tape.
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