Hi all, i just got a 2.5m artf kit 'Mythus 2500 s' a cz made kit, the instructions are very very poor , a photocopied 7 page affair that is at best awfull! hard to read splattery print and very bad english used to describe the build. Ok the prob is the c'of'g here is how they describe it.....

'The models gravity centre can move 50-70mm after sliding the drive inside the acumulator from wing edge.
By moving the acumulators on a carrige arrange the gravity centre according to the allowence.'

Ok above was taken from the instructions, as my model is the 's' version not the 'e' (elec) i have no motor or batery pack, they give no other reference to c'of'g at all, so is my c'o'g supposed to be 50-70mm from the front of the wing or the rear measured foward?

I did alot of searching on net and not alot of info on this kit, any links/help/reviews would be great!!..

Thanks in advance James.

What exactly is the model of plane?

hi wings, its a 'MYTHUS S 2500' made by 'TELTECH' Ostrava cz. (non elec version).

Click [link=http://www.rc-dymond.com/Electric/ElectricGliders/60716Mytus22.htm]HERE[/link] to see the 220 elec version (looks exact same as my 2500 s).

thanks , Jim.

CG is always measured from the front of the wing. Should end up around 1/3 of the wing chord.

James, you can go to this sight and "plug in" your wing info and figure the CG from here http://www.palosrc.com/instructors/cg.htm

I'd say the recommended range for CG is from 50 to 70mm back from the leading edge.
Dennis-

ok i think i have it nailed now, i will list a few stats here for wing, if anyone could run the math for me just to check i have it right.

Root chord = 200mm
Tip chord 150mm
Half span(wing length) 1120mm

Average chord 560mm
Mac 178mm
mac distance 106mm

Total glider length is 1120mm
Weight 980 grams

Ok i think my c'of'g should be at about 70-75mm from the leading edge.


I just finished the radio install and batterers ect and the c'of'g is 120mm from the leading edge, to get it to balance at the 70-75 mm mark i need to add about 1000 grams to the nose! that seems alot to me!, ok as you can see i still need help!!

thanks in advance , Jim.

Its common to need to add a bunch of lead to the nose of a glider. relatively short nose long tail and no engine. The Goldberg Gentle Lady for example, typcally needs 3 oz lead in the nose (if you hollow out the nose block and cast the lead insde it.. 3. 5 oz behind the block)

1000 grams... 1 KG appx 2.2 lbs... thats a heck of a lot of lead...The 2.24 meter span, I would expect more in the line of 5 oz (under 1/8 what you are estimating.) Get the lead as far forward as possible. Get the battery right behind the lead.... then the servos, then the RX. No part of the radio should be behind the CG.

Don't know that plane, but sure seems like a lot of weight. Did you put the battery pack (and whatever else will fit) ahead of the CG?

Like FH, I wouldn't expect to have to add nearly that much weight.
Dennis-

Everthing is in front of c'o'g ,tomorrow i will take a few pics to show you.
What ii realy need is ur help to verify the actual c'o'g from the stats in my last post.

Thanks, Jim.

See post below...

Here's a little graphic representation that shows how to determine MAC, then 25% of MAC is a good starting point for CG.

If you plot it out to scale, then you can easily determine the MAC for your wing, but I'd still say that the 50 to 70 mm is the CG range.
Dennis-

Thanks for that Cherry, when i get a chance to rig it i will let you know, maybe tomorrow, all i know right now is that it balances at 120mm back from leading edge :P.../jim hunts for lead items to smelt /..

ok got it all together today, 365 grams in the nose makes it balance at 65mm from leading edge.
This is over 1/3 the total weight of the glider i presume this is ok....

Ok thanks all. Jim

365 gram... a bit over 12 oz. thats still double what I would have expected based on the aircraft size. Does it have a heavy, long tailboom and heavy tailplanes? Short nose?

Still a heck of a lot better than 1 kg

yes,yes and yes

Look at pic above , i spoke with the guy in shop today and he said that seemed an ok amount of lead to add as this model can either be elec or just glide, as i opted not to convert it to elec power (firewall ect was in kit but optional) this would maybe account for the weight. would a motor wit folding prop and relevant batt pack add up to my 365 grams..??

The battery pack would be at LEAST 6.5 oz... might be 10 or 12 oz... (depends on the power system... could be heavier) So yes, the electric drive probably would add more than your 365 grams (12.1 oz appx... 355 gram = 12 oz)

due to the placement of the electric motor... even with the motor and battery, you might have ended up adding weight. The motor is about wher you probably put the lead. the motor would be nowhere near as heavy, and the battery would be furhter back... so you'd need more total weight to balance it.

****

Yes... that is a LONG tailboom and a short nose... that always makes you need a lot of weight to balance it. Proportionately... the Goldberg Gentle Lady has twice the fuselage length ahead of the wing. (and that typically needs appx 3 oz in the nose... with the battery and servos further forward than the front of your aircraft.)

ok here are a few pics of the nose end [8D], all is up front of the cg, my bat pack is on top of receiver with all that lead up front behind the foamy firewall i made...She's just about set now for a few test flights , about 20-30 hand lobs and let it float so i can trim it out and make sure all that fat up front was correct[:-]..will post and let ya know how it goes..if u see anything in pics that makes you scream let me know!! if weather is good tomorrow i will be out there..




Ok more to come soon ...Jim.

Well, the pictures are too small to really see any detail, but if the control surfaces were centered when you took the pics of the servos, you need to center the servos when the controls are at neutral.

In that pic looking down the inside of the fuse, is that a control rod in the lower center? If it is, I'd also make sure there's no "flex" there. An unsuppported pushrod like that can be a problem.

Hope the maiden goes (went?) well.
Dennis-

Hi,

lines are epox'd in , what u can see in that pic is only a few inch's where the lines exit there support/guide block, then it goes into the long fuse(another 23 inch past wher u can see) wher it lays flat and epox'd down.

The (black) ariel is also running past there maybe its that u can see [:-] lol.

The servo is inline, in one of the pics it wasnt conected, in the other it is and straight

thanks for ur comments ...Jim