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Old 10-07-2013, 11:11 AM   #1
solentlife
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Default Scratchbuild ratios ...

My recent scratchbuild NiJet has brought fwd one aspect that became obvious once construction started ... size of components in relation to each other.

I drew out on the foam sheet the outline of the model and all looked fine. I reckon the old adage ' If it looks right ... it likely is right ' is quite true. But in this case once pieced together - the stabs were far too small and elevators way out of relation to overall.

So what I'm looking for is not formulae ... but ratios that would work for various styles of craft ...

If we split the types to :

General trainer high-wingers - the Easystar / Yamamoto style
General sport straight / limited taper wings - Cessna, Piper, etc. style
Conventional wing / tail intermediates / performance - incl. biplanes,
High speed swept wing / conventional wing / tail
Delta's
Modern swept wing jets - F15, F18, F16 sort of stuff
Early jets - Mig15, Panther etc.

Examples : The ratio of distance wing to tail ... size of tail feathers to wing ... size of elevators .... length of nose ... length overall vs wingspan .... wing chord... taper of wing / tail .... width of fuselage .... general rule of thumb to produce a basic foundation that new guys coming into scratchbuild can use to bring their first scratchbuilds ...

I am not talking aeronautical / physics degree stuff ... this is a hobby and simple relations is enough ...

An example of the model with the quotes would be excellent so it can be graded as to what class it sits in and more relevant for people needing to start scratchbuilding.

I'm sorry that I cannot quote figures to get it started now ... I'm sitting 10mls offshore at present !!

Nigel

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Old 10-07-2013, 11:53 AM   #2
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http://adamone.rchomepage.com/design.htm

not sure if this is the kind of thing your looking for. Used this page to design my very first plane, flew great.
I'm one of the odd balls, only been flying for about 8 months, all my planes have been scratch built from my own designs.
Over the weekend just designed my next project, 60" span motor glider, built up fuselage, V-tail on a carbon boom and a KFM Dollar Store foam wing, 20" flat centre section, 11.5 degrees dihedral on the 20" tip panels.
Just trying out as many styles of plane as I can until; I find what I really like.
So far built my trainer, a stick, a 32" wing and now this glider. All have been KFM wings.
The 32" wing was just a guessing game, drew it out until it looked right, plugged it all into the online wing cg calculator, built it and it flew.
I do preliminary design using RCCad then transfer it over to a real cad program for the templates etc.
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Old 10-08-2013, 04:43 AM   #3
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Seems no-one else is interested to post info.....

Once I get home later ... I shall start to measure and detail various models (I have 17 in the hangar + Helis now ... so a good spread).

I'm sure someone will use / appreciate the data ...

Nigel

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Old 10-08-2013, 04:47 AM   #4
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Originally Posted by Siberianhusky View Post
http://adamone.rchomepage.com/design.htm

not sure if this is the kind of thing your looking for. Used this page to design my very first plane, flew great.
I'm one of the odd balls, only been flying for about 8 months, all my planes have been scratch built from my own designs.
Over the weekend just designed my next project, 60" span motor glider, built up fuselage, V-tail on a carbon boom and a KFM Dollar Store foam wing, 20" flat centre section, 11.5 degrees dihedral on the 20" tip panels.
Just trying out as many styles of plane as I can until; I find what I really like.
So far built my trainer, a stick, a 32" wing and now this glider. All have been KFM wings.
The 32" wing was just a guessing game, drew it out until it looked right, plugged it all into the online wing cg calculator, built it and it flew.
I do preliminary design using RCCad then transfer it over to a real cad program for the templates etc.
The page is very good .... the first part is interesting but outside my intention. The diagrams later though of ratios of measurements is spot on.

Nigel

222kph PKJ,EDF Concorde, Mini4,Mig3,T45,PKJ twin,ME109,Edge540,Cessna182,Skymaster Biplane,F15,F16,Badius,Ultimate,SE5,Qbee10,450 Heli,V911,J3 Cub Founder 9x forum: http://groups.yahoo.com/group/Flysky_RC_radio/
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Old 10-08-2013, 04:50 AM   #5
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Why the thread ... I think more people would design and scratchbuild own if they had basics to work from ... I'm hoping that from foundation data as I ask for here can then develop into materials ... then structures / methods of build ... creating a How To Scratchbuild ....

But first lets put together the ratios ... other stuff can wait till later ...

Nigel

222kph PKJ,EDF Concorde, Mini4,Mig3,T45,PKJ twin,ME109,Edge540,Cessna182,Skymaster Biplane,F15,F16,Badius,Ultimate,SE5,Qbee10,450 Heli,V911,J3 Cub Founder 9x forum: http://groups.yahoo.com/group/Flysky_RC_radio/
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Old 10-08-2013, 05:50 AM   #6
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Nigel,

I'm not sure it's quite that simple. All things are dependant on other variables. While it's easy enough to come up with rules based on models that are known to fly that doesn't mean that all other 'shapes' of model wont fly.

For instance tail size.... You can come up with a rule for how big the tail needs to be but then how do explain the fact that tailless planes can fly just fine The reason why tailess fly fine is down to aerofoil, reflex and/or sweep and wing twist. If you use the right combination of airfoil/reflex/sweep/twist you don't need a tail at all. If you use other airfoils you need a large tail because of the high Cm. How would you factor those complex and inter-related variables into simple rules of thumb?

Every other aspect of design has similar dependancies on multitudes of veriables. Short of writing a pretty thick book IMHO there are too many variables and inter-dependencies to come up with meaningful rules especially once you depart from the conventional layout.


If you want a simple guide to scratch building with a guarantee of success, I'd say this:

Find an existing design that most closely resembles the type of plane you have in mind and that flies the way you want your model to fly. Shamlessly copy the key design aspects (airfoil, approximate wing planform shape and position, moment arms, tail size etc). Make your own cosmetic changes. Scale up or down as required.

This is a pretty foolproof approach to scratch building and will work for any airplane type as long as something similar has been done before. If on the other hand you want to design something trully unique then you really need a decent understanding of aerodynamic principals.
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Old 10-08-2013, 12:15 PM   #7
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Nigel, I applaud you for wanting to share information that may/will help new builders, or those "On the fence" about building their own planes. I too was once a wanna-be builder, now I am loving the inventing/building side almost as much as flying the planes themselves!

That said, I have seen some plans that seem to defy physics in regards to traditional aeromodeling design. For instance, in the SPAD world (Simple Plastic Airplane Designs) one of the more popular 3D plans is for what's called a "Pizza box flyer" which is simply a powered flat piece of coroplast. hey, they fly!

So while having the basic physical laws of design is great, you can sometimes "Cheat" the same laws with a little creativity and a lot of thrust lol!

Happy Building!

Mark

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Old 10-08-2013, 06:42 PM   #8
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Guys,

I think what Nigel is aiming at is a basic design plan, not some advanced tinkering as you describe.

Sort of like starting in Calculus or any other course, you start with Calc 101 (or maybe just basic Algebra) not Calc 305.

The link given by Siberianhusky in post #2 links to a pretty good sight for a beginner wishing to design a simple airframe that more or less guarantees (if there is such a thing in this hobby...lol) success on his/her first build.

I applaud your efforts Nigel....hope this thread draws a few more hesitant potential builders into the flock....

Don
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Old 10-08-2013, 07:24 PM   #9
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Originally Posted by DEG View Post
Guys,

I think what Nigel is aiming at is a basic design plan, not some advanced tinkering as you describe.

Sort of like starting in Calculus or any other course, you start with Calc 101 (or maybe just basic Algebra) not Calc 305.

The link given by Siberianhusky in post #2 links to a pretty good sight for a beginner wishing to design a simple airframe that more or less guarantees (if there is such a thing in this hobby...lol) success on his/her first build.

I applaud your efforts Nigel....hope this thread draws a few more hesitant potential builders into the flock....

Don
That's why I wrote what I did, I totally love the fact that he is helping to clear up some of the myths surrounding designing and building your own plane from scratch! We need more designer/builders! The ARF selection nowadays is BORING!!! Same planes every week at the field, never anything new..."OH look, a (Insert ARF plane here) just like so-and-so's!" *Yawn....I remember when there were a lot of different planes showing up every week it seemed, and a lot of them were one-off, new designs.

Ah, the good old days...I miss them..

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Old 10-08-2013, 08:38 PM   #10
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Thanks PE and Deg .... you have understood my idea as I put fwd.

I agree that there are strange designs out there that fly ... I used to floy flat sheet balsa stuff .. and now fly flat sheet foamies as well as full form.

Tailess .. of course - but that's down to design ...

The idea is to get people on the road to scratchbuilding ... to shake of the worries and concerns ...

Have simple ratios for various styles so a person can set-to and be sure he's on the road to a successful flyer. If he develops of that path into the more advanced designs - then I for one would be proud to think I may have helped by this thread ...

Most models DO conform to general ratios ... its only when you get to specialised that the data gets more complicated ...

Let me now throw a curve ball in ... there's also what we call in UK 'Mix and Match' ... where you can take various of one idea and add to another creating a new set-up ... it works here as well !

So put aside the aerodynamics ... the polynomials ... the slide rules ... lets get to basics ... (Oh Dear just reminded myself of all the years of Integration and Differentiation mathematics I had to do !! )....

Nigel

222kph PKJ,EDF Concorde, Mini4,Mig3,T45,PKJ twin,ME109,Edge540,Cessna182,Skymaster Biplane,F15,F16,Badius,Ultimate,SE5,Qbee10,450 Heli,V911,J3 Cub Founder 9x forum: http://groups.yahoo.com/group/Flysky_RC_radio/
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Old 10-15-2013, 05:02 PM   #11
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After playing with a few, I found a couple of rules of thumb. When blowing up a plane from scale, say a Harvard for instance, the tail surfaces need to be about 10 to 20% larger, mostly due to Reynolds Numbers, where to put it simple, the air molecules are fewer traveling over a flying surface. Same with the aileron. Make them about 20% more area than stock to be affective.

Gord.
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Old 10-15-2013, 09:05 PM   #12
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I think we may have the same thinking here Nigel, although I usually hear it as "bashing" a kit or parts of. I did a plane a couple yrs ago I called a "Bitsa"........I had salvaged the wings and horiz tail from a built up model and I designed a box fuse to use them on....."bitsa this, bitsa that" hehehe......I also did a GWS Tiger Moth into a low wing trainer ala PT19 style. I designed and made the wing and tail to mount to the GWS fuse'........this is the main point in "looks right, flys right".
TLAR = That Looks About Right

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Old 10-15-2013, 10:30 PM   #13
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Originally Posted by Wrench66 View Post
I think we may have the same thinking here Nigel, although I usually hear it as "bashing" a kit or parts of. I did a plane a couple yrs ago I called a "Bitsa"........I had salvaged the wings and horiz tail from a built up model and I designed a box fuse to use them on....."bitsa this, bitsa that" hehehe......I also did a GWS Tiger Moth into a low wing trainer ala PT19 style. I designed and made the wing and tail to mount to the GWS fuse'........this is the main point in "looks right, flys right".
TLAR = That Looks About Right

best,
--Ray
Yep- you got it ...

I took a QB10 Pilot model .. cut the straight wing and swept it back ... swapped out the 10FSR engine for a 20FSR ... became a real fun machine ... based on simple ratios gleaned from various other models.

I had a Pavel Bosak Mig17 ... threw out the weakl wing when it snapped its main spar ... took a slope soarer wing ... cut and re-joined it ... which made for a new wing for the Mig ...

I've taken many a bitsa here and bitsa there ...

I was hoping here that more people would contribute a bit though ... sadly I feel that the older ways are dieing out and the 'feel' for putting together own models vs out of a box ...
I am just trying to revive a bit of it ...

Nigel

222kph PKJ,EDF Concorde, Mini4,Mig3,T45,PKJ twin,ME109,Edge540,Cessna182,Skymaster Biplane,F15,F16,Badius,Ultimate,SE5,Qbee10,450 Heli,V911,J3 Cub Founder 9x forum: http://groups.yahoo.com/group/Flysky_RC_radio/
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Old 10-15-2013, 10:41 PM   #14
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Nigel it sounds like WE are cut from nearly the same cloth, my friend.......check post #13 as it describes a build I have going right now. http://www.wattflyer.com/forums/showthread.php?t=71815
Half of my fleet is made up of self designed planes

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