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Building Tips and technical articles. => 1/2 A building. => Topic started by: Motorman on March 09, 2017, 11:45:24 AM
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I don't know the math ,nor did I try to figure it out,but I'm thinking you want about (give or take a little) 200Sq inches for a 1/2A Stunter.
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The Brodak .049 isn't all that peppy. Something around 150'ish squares is probably appropriate. With the high aspect ratio wing on the Thundergazer, it still ends up with a pretty big "little" plane.
158÷630 = .250
√.250 = .500
= 50% reduction of the full size plans.
.500 x 60.75 = 30.375"
30.375" span = 158 sq.in
If you want a smaller 125sq.in. version you would ask for a set of plans sized at 44%.
125÷630 = .1984
√.1984 = .445
.44 x 60.75 = 26.73 wingspan
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My buddy had a Goldberg Jumping Bean with a brodak .049. Man it was lame, wouldn't even loop. It ran good, but didn't have enough power to pull a greasy string out of a goose's patoot. He put a Medallion .049 on it and it sparkled y1
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Too small! Aim for 190 sq.in. And 8 to 9 ounce weight.
Scaling models is NOT linear. Scale by:
weight = k * wingspan * wing area
where k is a factor you determine by running this calculation in reverse with your base airplane.
k = weight / (wingspan * wing area)
Then figure the weight your new engine can pull and run the calculations. Typically a stunter needs
thrust = 1.5 weight.
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After a bit of thought, I realized that what is really needed is the WDITK number. That is "What Do I Tell Kinko's". That's the factor by which to scale down your original plans.
After a session of rusty algebraic thrashing around, the number is quick to calculate, and you only need two data numbers to do it!
WDITK =(Scaled Down Weight/Original Model Weight)^.333 (that's 1/3 power or cube root, not a multiplier)
So, an example! A Ringmaster has 381 sq.in, 42" span and weighs 30 ounces (mine is 28 oz., so there! VD~ ) We want to use a mild .049 engine that probably generates 12 ounces of static thrust. Guessing that we need a 1.5 thrust to weight ratio, we have a weight of 8 ounces targeted.
WDITK = (8/30)^.333 = .644
That comes out to a 27" span and a resulting wing area of 158 sq.in. H^^
This is all based on my article on cubic wing loading as published in Model Airplane News when it was still worth reading. I can provide a copy of the article for those who are interested, along with my derivation of WDITK. y1
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Scaling, as Larry points out is not only not linear, but is affected by thrust, area, volume, weight, speed, etc... not simple. So experience with engines, areas, and weights of actual similarly-sized stunters is probably more valuable.
I posted a lot on SSW about scaling a few years ago. A search might still find that material. It included things like the volume of a model varying as the cube of the linear dimension and the area as the square. Since weight is determined by volume of parts, rather than just plane volume of the model, even this is not as helpful as might otherwise be. That's why, unless you are really into computational stuff, what I'm going to attach is only useful in predicting trends as you scale a model up or down. It's probably just an FWIW here. Although my total file size for attachments is only 851 Kb, I can't post them all because a message of "too large" comes up. So although I don't even know whether the text is legible, I'll go ahead and make two posts. Edit: It seems to me that I disagreed with one of these authors on something, but I don't have the time to see where that might have been now. 'sorry!
For Larry: Here on Robert's forum, there actually are subscripts and superscripts available. So "A squared" is "A2". Otherwise, I have always used the upward-pointing "^" for exponents/powers. So "A cubed" would be typed as "A^3," for instance. The square root of A could be typed "A^.5" (A to the halfth power). That allows you to use "*" or "x" for multiplication.
SK
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Here's the rest - from Model Builder.
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The formula I gave is based on square/cube wing loading factors. It works very well for scaling up or down a particular design, and fairly well for going from one design to another. it is derived from my article on 3-D Wingloading in Model Airplane News.
Of course, comparison to models in the target size regime is important, and once a test model is built, tuning in the size and structure to a final design is the thing to do.
I am attaching my original article.
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Hello, I don't know when this was first explored, but I do remember it being covered
by Ron St Jean, in a MAN article published in 1959. It was entitled:
"Wing loading is three dimensional". By then I had been flying "Ramrod 250"s for
about 3 years. I was just transitioning to mid-thrust(Phil Kraft's Hi-Fli) and high thrust
designs. I could go on with this, but the article does reveal the failings of the Ramrod 250
compared to the Ramrod 750. If any of you built Berkeley's Ramrod 250, the production
modifications took a wide detour from the original design :o.
Tony
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I did credit Ron StJean in my article. He was my inspiration! The Starduster was my favorite FF. Holland Hornet power.
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Is it worth putting moving flaps on it?
MM
Hi MM, simply put...NO... Your 1/2A sized model will turn tighter without the flaps.
You will need to trim it for this, and practice a lot, popping straight out of the turn
avoiding any bouncing. ;)
Tony
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I did credit Ron StJean in my article. He was my inspiration! The Starduster was my favorite FF. Holland Hornet power.
Hi Larry, please expect some kind of communication from me, discussing the
Starduster design. The 'Duster was the basis of my favorite 1/2A competition
ship; which I jokingly lettered "Starduster". I just didn't want to talk about
FF competition stuff on a control line forum.
Tony G