Hi everybody, I made my first plane and have a succeasfull first flight... Just take of, make two turn and then go down... All in all was ok, but I have a feel that the plane is too heavy on bottom side, don't know exsactly how to explain... But my question is this: Is there some way to find where must be the VERTICAL center of gravity of the plane???
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Yes, it's fairly easy to do. Tie a string to the nose and the tail of the airplane and hang it by the string. Hang it with the nose up 30-45 degrees (does not need to be exact). From the same point the string is suspended hang a plumb bob. It will point at the cg, so mark that line. Then hang the plane with the nose down 30-45 degrees. Again add the plumb bob from the same point of suspension. Where it crosses the first line is the CG.
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Ok, I understand, but I don't need to FIND where is my vertical CG, I need to know where MUST BE the vertical CG, so I can move to the right position the CG, because, I think now it's wrong (too heavy on the bottom)...
Thanks!
< Message edited by neuroza -- 3/16/2008 12:20:22 PM >
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A low vertical cg will make the plane relatively stable in roll and not tend to fly as you said yours did. I suspect the problem is something else. Like underpowered, flying near stall climbing after takeoff, bank to turn, stall while banked and fall . You may have too much weight, too little power, fly too slowly, etc. The fore and aft cg may be too far aft. Did it act pitch sensitive? More detail of the plane would help .
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quote:
ORIGINAL: Jack Hyde
A low vertical cg will make the plane relatively stable in roll and not tend to fly as you said yours did. I suspect the problem is something else. Like underpowered, flying near stall climbing after takeoff, bank to turn, stall while banked and fall . You may have too much weight, too little power, fly too slowly, etc. The fore and aft cg may be too far aft. Did it act pitch sensitive? More detail of the plane would help .
I fly a lotof flat foam aerobatic stuff I designed and some arfs I always thot getting th e CG on the rotating axiz ( the spinner) was a good idea- so on these little buggers I tried to prove it - it was easy to do - just bolted the prop shaft to a board and suspended the entire model, horizontally, from the shaft - then shifted stuff till it balanced in rotation. it does help - it ain't a fix for other things but it can help. Foamies can teach you more in a few experiments than much of the "theory " you can read and try to interpret. best yet you can make drastic changes - screw up -crash repair and be backinthe air in minutes. Where were these things when I was slowly trying to sort out the "facts" years ago.
< Message edited by dick Hanson -- 3/16/2008 4:26:20 PM >
Hi neuroza You may not get the information you are asking for because it is not something that is studied or recieves a lot of attention. Relative to the center of the wing, a High wing plane has a low veritcal CG, a low wing plane has a high vertical CG, and a center wing plane has a vertical CG near the center of the wing. I know of no "rule" that dictates the location of the vertical CG. If you will describe the configuration of your plane and what manuever you want to do, and what happens when you do it, you may get a suggestion from someone that may help you.
I have a Canard that I designed. I wanted it to fly in knife edge as perfect as possible without a lot of correction. I therefore made the area of the rudder that was above the vertical CG the same as the area of the rudder that was below the vertical CG. It flies in knife edge very well.
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quote:
ORIGINAL: neuroza
Hi everybody, I made my first plane and have a succeasfull first flight...
Congratulations on your maiden. If I remember right this was also your first flight (other then simulator) of an RC airplane. The fact you kept it in the air would indicate nothing is too far off. I doubt the vertical CG is an issue.
Thanks! Jep! this was my first plane & fly... But I work a lot on simulator and try several times just to take off for 10 meters, then go down... Anyway, the first fly was pretty much ok... I think it's a problem with vertical CG because the wheels are pretty heavy... Maybe I will try to put some more light wheels.
That's a SPAD model? It looks pretty much like almost any Ugly Stik related sort of design. In light of that the vertical part of the center of gravity will be fine. If it's doing funny things it will be due to some other cause. Although your stabilizer size is a little bit small at 15% combined with a rather short tail moment arm. Because of that I would balance your model at around 25% to 27% back from the leading edge. Since this is your first model before you change the wheels balance it so it is level on your fingertips with one on each side and see where it balances. If it is much further back than 80 to 90 mm's from the leading edge I think it will be unstable.
You should check this before you change the wheels so you know where the balance point is right now.
It also looks like it may have a full flying stabilizer. If so then you want to start out with only about 6 to 8 mm of movement up and down at the trailing edge for a total of 12 to 16 mm.
Also since this is your first model airplane ever you need to know that a design of this sort is not what I would call a good trainer. Simulators are good but they cannot simulate the troubles you will have in a model that has stability troubles. And also this model will be a hand full to fly. Good luck with it.
Anyway, check your balance point. I can tell you that it's not the wheels.
< Message edited by BMatthews -- 3/16/2008 9:37:01 PM >
I made this plane from some kind of plastic table (that I us for my work) it's pretty hard and really light. (the complete plane with engine, servos, fuel, etc. it's weight about 2 kg).
Because this is my first plane (and I need it just to learn - and likely break it) the design was absolutely at the bottom of priority. The main goal was to make it fly (and the goas was done! ). Maybe my next plane will have much better design...
< Message edited by neuroza -- 3/16/2008 10:16:37 PM >
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I agree with Bruce, there is another problem lurking elsewhere if the plane isn't able to be trimmed for hands off flight, or if it has some over nasty habits. If the problem is related to how well the plane handles inverted flight VS right-side-up flight, then the vertical CG is a consideration. Many very aerobatic models have been proven over the years to fly very well with high or low CGs. I've seen a high wing float plane that could fly circles around your average sport model with wheels [in spite of the floats] because it was built very light and had a superior power to weight ratio. A plane the size of yours could be built to weigh at least half a kg less and the overall handling of the model would put a big smile on your face.
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The only planes I've had which showed any effect from a vertical c.g. change were the "lifter" types, which empty would weigh about 8 pounds, and loaded over 30 pounds, with the added weight moving the vertical c.g. down several inches. As these planes didn't have to manuver anyway, the added stability was a plus.
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