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Model Forum / Radio Controlled / Air Models / April 2004Suggestions needed for major CG problem (long)
Man, am I frustrated. A while back I was passing Hobbies and Helis (Giant Scale Planes) in Coopersberg, PA and I stopped in. They had just gotten in the 72" Yak54 and I just had to have it. So, I bought it. A couple of weeks later I came across a nice Saito 180 at a swap meet so I purchased that also so I then had an engine for the new Yak. This engine was larger than the recommended 140 4 stroke but I figured what the heck, I'll mount the elevator servos in the tail instead of using the wooden pushrod setup that came with the plane. I figured that the elevator servos were behind the CG in the stock setup so how much could it upset things. The building went well and I am actually impressed with the quality of the model considering some of the negative feedback about GSP and Irwin, the owner. Anyway, the model is finished except for the balancing. Even with the Saito 180 mounted as far front as possible, it would take 42.5 ounces of weight as far front as possible to get this thing to balance! Thats over 2.6 pounds! Does anyone have any suggestion as what to do? I have the battery and the reciever mounted near the fuel tank and can't move anything else around. Should I just add the 42.5 ounces and fly it? Regards to all, Jim >Does anyone have any suggestion as what to do? You could always hang a second gas engine under the first! Honestly though, I would definately double check the balance point and make sure it is correctly stated either from the plans or from the manufacturer or calculate it yourself. Sounds like the nose is too short - maybe you can build a motor mount box and shove the motor out some more, would affect the cowl but you might get it to balance. Also bring those servos back to the cg as well. Hard problem. Good luck! FredD >>Does anyone have any suggestion as what to do? >You could always hang a second gas engine under the first! >Honestly though, I would definately double check the balance point and make >sure it is correctly stated either from the plans or from the manufacturer or >calculate it yourself. Sounds like the nose is too short - maybe you can build >a motor mount box and shove the motor out some more, would affect the cowl but >you might get it to balance. Also bring those servos back to the cg as well. >Hard problem. >Good luck! Dittoes to what Fred said. Definitely fly it first with a safe CG. Nose-heavy airplanes do OK, as a general rule. Tail-heavy airplanes can be really, really hard to handle. Don't ask me how I know. :o( Marty Did ou put the srvos in the tail? I bet you don't want to redo that but give it serious thought. mk > >Does anyone have any suggestion as what to do? > [quoted text clipped - 11 lines] > > FredD > Did ou put the srvos in the tail? I bet you don't want to redo that but > give it serious thought. 2 oz at 20" from teh CG = 20 oz at 1" from the CG. Balance wise. I build as light electric planes as I can. Often short nose WWI and WII fighters where the really heavy power plants of the originals lead to very short noses. The key is NOTHING that adds weight to the tails. These are built of empty space and covering film with the arest skeleton of structire to hold it in place. Servos as far forward as possible, even snakes go out in favour of carbon pushrods or pull-pull lines. Once at the CG I stop worrying. Noses are ultra strong ply structures like as not. with nose to tail ratios as high as 10:1 an oz of weight at the tail needs nearly a pound of weight up front to balance it. Battey backs located at the firewall or even further forward. Big heavy can motors that are run at or near maxiumum efficiency, rahter than max power - since the weight is needed anywy, why cane a smaller motor? In your case a big gasser up front plus taking the servos forward and dremelling out anything that isn't needed up the tail will get it all on line. Just take those rear mounted servos and cables out of the model, and see how much less noseweight you actually need. Then put them in the fuselage and use lightweight CF tubes as pushrods. That should get you almost on line. > mk > [quoted text clipped - 27 lines] >> >>FredD > Man, am I frustrated. A while back I was passing Hobbies and Helis (Giant > Scale Planes) in Coopersberg, PA and I stopped in. They had just gotten in [quoted text clipped - 14 lines] > reciever mounted near the fuel tank and can't move anything else around. > Should I just add the 42.5 ounces and fly it? Holy Cow, that's worse than a DR1. I can only suggest switch to a gas engine and at least get something productive out of the weight. It will cut your operating costs, too. (That last lines for the wife, btw). Might space out the mount a little farther forward if you haven't trimmed all the overlap off the cowl yet. You should also switch out your servo installation and mount them as far forward in the fuse as possible. You can open up the lightening holes in the bottom of the fuse to gain access to your pushrod install, they'll re-cover easily. I'd also let the kit maker know my disappointment. They might send you another cowl if you've already cut yours down, if you play it right. Thanks for the help folks! Does anyone know the formula for finding/checking the COG on a tapered wing so I can verify the instructions are correct? Jim > > Man, am I frustrated. A while back I was passing Hobbies and Helis (Giant > > Scale Planes) in Coopersberg, PA and I stopped in. They had just gotten in [quoted text clipped - 28 lines] > also let the kit maker know my disappointment. They might send you > another cowl if you've already cut yours down, if you play it right. >Thanks for the help folks! Does anyone know the formula for >finding/checking the COG on a tapered wing so I can verify the instructions >are correct? <http://moleski.net/rc/WingMAC.htm> Let us know how it turns out. :o) Marty >Thanks for the help folks! Does anyone know the formula for >finding/checking the COG on a tapered wing so I can verify the instructions >are correct? > >Jim http://homepages.ihug.co.nz/~atong/ Go to this site and look under the CG calculator - there are several ways available. FredD Boy, are you ever in denial. Take the servos out of the tail and stick them where the instructions told you to put them, tape the pushrods to the fuse in roughly the correct fore-and-aft position, then recheck the CG. CR > Man, am I frustrated. A while back I was passing Hobbies and Helis (Giant > Scale Planes) in Coopersberg, PA and I stopped in. They had just gotten in [quoted text clipped - 17 lines] > Regards to all, > Jim Hey CR, wake up on the wrong side of the bed did ya? Jim > Boy, are you ever in denial. Take the servos out of the tail and > stick them where the instructions told you to put them, tape the [quoted text clipped - 24 lines] > > Regards to all, > > Jim |
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