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Old Oct 17, 2010, 01:35 PM
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Herndon, VA
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OPD (Other People's Designs)

I've noticed that there are a lot of people who, as soon as they buy a kit or ARF, start modifying it without every having even built one as specified. Yet, they are intimidated by the prospect of designing their own model from scratch. How can it be less intimidating to spend money on somebody else's design, which you know little about, and introduce changes that could cause all sorts of unforeseen problems, than to just build your own plane, about which you will have complete knowledge and can confidently modify as your heart sees fit?
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Old Oct 17, 2010, 02:33 PM
Fly lower!
aerolite's Avatar
West Texas
Joined Jan 2005
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I think the type of modification makes a difference. I build and design flat foamies, and have specific ways I like to brace the planes, as well as mount my servos, motor, etc. I rarely brace the plane the way the kits show. An example would be the Clik! F3P pattern plane. This is an excellent design, and kit, but instead of using carbon fiber bracing on the fuse, they use gediplac strips. These work fine, but I personally prefer CF truss bracing, so this is one modification I'll always make on the plane.

Another example would be landing gear. Almost all indoor foamies come with landing gear skids, but I always convert mine to small wheels.

If someone is changing the shape of the design, which would affect aerodynamics, then you have a great point. So it seems to me that some kit bashing is OK, but whenever modifications that affect flight characteristics are done, you would probably be better off just building your own.

interesting question!
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Old Oct 17, 2010, 02:42 PM
...design-build-fly-publish...
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Ware, herts. U.K.
Joined Sep 2008
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I now design all my own models but it was not always so.
I built several kits and part kits first, during which I learned how a model aircraft goes together. In the process it becomes obvious that there are better ways of doing things - or perhaps just different ways that happen to suit our particular skills.
I started designing - more of a redesign really - based on a part kit of Skyway Models' Mini Spit. This was originally supplied as a pair of foam core wings with veneer covering and a cockpit canopy. The fuselage was slab sided, which looked far less like a 'proper' Spitfire for my tastes at the time, so I gave it a make-over to a shaped fuselage and brought it up to a Mk14 while I was at it.
The scare factor was outweighed by the hopes during building - and ameliorated by the fact that I already knew that the original design flew very well.
(It was right back then that I learned the value of having large amounts of washout to the wings as it was a model which, quite uncharacteristically for a Spitfire, was capable of buing slowed down so much for landing that the tail touched down first).

I would always encourage a modeller to do a bit of redesign work when they think they could do better - and even if the alteration is later proved unsatisfactory - because each modification brings in its train another bit of experience or a lesson learned. Even on my own designs! - Although I know why I did it that way in the first place (even if the reason was laziness!).

One place where I would be almost emphatic that modification SHOULD be done is on a type such as the DH Mosquito which for a model absolutely HAS to have a liberal dose of washout on the wings outer panels if you want a model that will fly (and land) at a slowish pace. For an RTF it is worth slitting open the trailing edge and twisting the tip to give it a better chance of longevity.

So while I agree with you sir, I would have to add a rider that not all of us seem able to work a design through from start to finish without we first build other peoples designs (and I do mean 'build' rather than just stick a few pre-manufactured bits together). Not all of us understand the structural requirements from the word 'go' - but can learn. Bit by bit is good - and less scary.
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Old Oct 17, 2010, 04:03 PM
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Easy. My mechanical aptitude is good, but does not cover all requisite aspects of aerodynamics. I trust myself to do mods that don't involve the basic airplan. I don't trust myself to design an airplan from scratch. With so many to buy or build, I just don't have the motivation to go down that road.
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Old Oct 17, 2010, 06:13 PM
Visitor from Reality
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It's what does it for you. My first OD - of sorts - was done when I was around 12 or 13. 'Aeromodeller' magazine published a plan of a 35 powered CL stunter by the world champion. Liked the model, couldn't afford one, so drew up my own plans from the tiny repro in the mag, to match one of my engines (I think I had two!).

Halfway through building, got bored/fed up/sidetracked and converted the wing to a 'combat wing'. Even after all that, it flew too.

Many years later, when I got to where I could afford RC, I got up to tricks like designing a 48" scale biplane as my first true OD, on the principle that the full size flew, just make a smaller one. That it had a biplane tailplane just made it more interesting. My first really aerobatic model went through three fuselages before I stopped it refusing to come out of flat spins - I suppose I could have build a proven design to learn pattern flying on, but that's for commoners

Got bored with kits too. Built one of the old 'Pilot' kits - the distant ancestor of the modern day fully laser'd kits and the BARF too - on flight one, I took off and rolled inverted right off the deck. Not how one is supposed to test fly, but I figured it's a kit, should do it.

The ultimate result is that I haven't built off another designer's plan in long enough and seldom build a kit without some modification. This varies - my sixth and present Lazy Bee has a slightly modified nose - built to take a specific motor/gearbox - and a cowling. My electocuted Great Planes 46 sized CAP 232 ended up with only the cowling and rudder left unchanged.

At least I left it the same shape as the original design - but I knew the designer.

Any model with an unknown designer - which pretty much covers the BARF market - is treat with far more respect. Like, won't waste my money on the whole.

I think I've designed around 35 to 40 models now, most of which have been published in those strange stone tablet like things called 'magazines'. The aerodynamic aspects are not hard to research up on - try 'Chuck Cunningham' from RCM for about as good a series of design numbers as you'll find.

Okay, that's about a month's output for Peter Rake, but I don't have the spare time or contract labour

Structurally, you can do better on your own as you are responsible for QC of all elements of your model's framework.

Example - have seen kits with wing spars of massive dimensions and weight, which I've halved in weight by personally selecting the wood I used.

A kit manufacturer will just buy hundreds of whatever size and toss them in the boxes, using a larger size to compensate for any oddities in grain, etc, in that big purchase. I'll go through however many pieces it takes to get what I need with the best grain and quality available.

Designing your own models is a natural aeromodeller's progression - every model has to be better than its predecessor, it's successor will be better. Yes, it takes effort - skills learned from building simple kits grow with model's complexity, then on to plans building. Own design is pretty much the pinnacle of personal satisfaction - for a while, you have the only one.

Will admit it's old-fashioned. The mantra now is 'buy what I'm told' or just 'buy new' and no model is a progression - it's just another shrink-wrapped product.

Time for dinner Go do your own thing, the satisfaction is immense.

D
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Old Oct 18, 2010, 07:46 PM
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Kit-bashing is a great way to get into designing. Start by making small changes, then work your way up. Keep the basic alignments and areas, but change outlines, add a turtledeck, change the LG from trike to taildragger, etc.

Even when you do push off further from shore, one way to have a confident start is by turning the plans over and tracing the important parts from the other side. One of my first designs was Loening M-8, done by tracing a Little Stik wing and tail, but with a deeper fuse and a taildragger LG.

Anyone who builds a model strictly per the plans, with no changes, modifications, or improvements probably also eats plain vanilla ice cream, with no chocolate syrup, cherries, sprinkles. Boring!

Gotta learn to try some funky flavors and toppings, even if they wind up giving a bad taste in your mouth.

CD
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Old Oct 18, 2010, 08:36 PM
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Originally Posted by Captain Dunsel View Post
Anyone who builds a model strictly per the plans, with no changes, modifications, or improvements probably also eats plain vanilla ice cream, with no chocolate syrup, cherries, sprinkles. Boring!
I'm a sinner, then; I own a few Parkzone and E-Flite PnPs that have almost no intentional mods among them. (Other than a small wooden block to shore up the nose wheel gear on the T-28.)

Even so, the vast majority of my models, both kits and ARFs, have needed modifications for any number of reasons. Weak landing gear, motor mounts, servo mounts, or wing-attachment schemes. Warped tail surfaces. Cracked longerons. Conversion from single to dual-aileron servos. Many mods are just to make the model more usable, eg. futzing with hatches, canopies and battery trays.

I suspect that's generally true for most models that enjoy a long life. They get banged up and fixed, and often improve with age as built-in weak spots are identified and addressed.
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Old Oct 18, 2010, 08:39 PM
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Originally Posted by Edmonds View Post
I've noticed that there are a lot of people who, as soon as they buy a kit or ARF, start modifying it without every having even built one as specified. Yet, they are intimidated by the prospect of designing their own model from scratch. How can it be less intimidating to spend money on somebody else's design, which you know little about, and introduce changes that could cause all sorts of unforeseen problems, than to just build your own plane, about which you will have complete knowledge and can confidently modify as your heart sees fit?
RE
It was a good enough principle for Orville and Wilbur. They were hardly "original thinkers" when it came to aviation.

Stepwise refinement is what engineering's all about. Make it just a little better than somebody else's.

And then there's always the learning aspect. Nothing like making a wrong choice to help you remember next time!

Finally, the analog world is for the most part one in which changes happen along a generally-linear curve. Rarely are there points with large-magnitude reflexes, so it is generally safe to make small incremental changes. I think that's why people modify what others have done.

Oh, and then there's the pleasure of saying, "I changed X because it could be better."

Hopefully, it gets them to thinking that they can do something totally off-the-wall, like one of your sans-elevator models!

Andy
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Old Oct 18, 2010, 09:00 PM
Electric Coolhunter
Thomas B's Avatar
United States, TX, Fort Worth
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Originally Posted by Edmonds View Post
I've noticed that there are a lot of people who, as soon as they buy a kit or ARF, start modifying it without every having even built one as specified. Yet, they are intimidated by the prospect of designing their own model from scratch. How can it be less intimidating to spend money on somebody else's design, which you know little about, and introduce changes that could cause all sorts of unforeseen problems, than to just build your own plane, about which you will have complete knowledge and can confidently modify as your heart sees fit?
RE
As others have mentioned, not all changes in an OPD are created equal. There are minor cosmetic changes that do not affct the structure or aerodynamics of a design, at all, or so very little that it does not matter even a little bit.

There there are a range of structural changes, some of which might be a good idea, some of which might not be a good idea. A lot depends on a person's level of experience. A lot of people that think they are improving a model by adding more structure to make it stronger are misguided.

Another way to change a design is to change the media....convert a balsa design to foam, or to foam and fiberglass, or even vice versa. Substituting a foam wing for those that hate to cut their own wing ribs is a time honored change that most carry out with no ill effect.

Typically, a lot of older glow R/C designs were pretty overbuilt, based on standard practice when they came out. A large number of people have built lighter versions of older designs using more modern design techniques and the nice things that come through using CAD and laser cutting.

If you change the moments and areas and airfoils and decalage and other reasonably important aspects of a design, then you are making a change of some greater importance.
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Old Oct 18, 2010, 09:31 PM
Rehab is for quitters
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West Middlesex, PA, US
Joined Jun 2001
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The one thing I mod on every balsa/covering aircraft is the landing gear.
For the reason being is that the designer/engineer etc. thinks all flyers will land and take off on a paved runway. Hence, puny wheels and weak landing gear especially where the mains attach.
I can't tell ya how many times the gear has been ripped from the braces. And this is after a smooth landing. I will even add some strength to foam planes with gear.
I usually go with at 2 1/4 inch wheels minimum. Our field aint the smoothest and sometimes, they forget to cut the grass. Nothing worse than greasing your landing and have the mains break off.

Lucky...
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Old Oct 20, 2010, 02:49 PM
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Oh man, this is the most thoughtful thread I've ever seen, and you guys are making me a believer. There seem to be plenty of legitimate reasons for almost every level of departure from the designer's intent, from isolated part replacements to general design overhauls. This thread was originally about people who just open a box and assume that everything in there was done wrong, without every having tried just following directions for a change and seeing how it turns out. I have a unique problem in my model rocketry product line (EdmondsAerospace.com), in which I have gone to extreme lengths to make the product suitable for beginners. This means leaving out a lot of things that are done by contest-oriented modelers to maximize performance, but aren't necessary for satisfactory fun-flying. Rocketeers with a little more experience that attempt the products are constantly asking me if they should do things that they think were left out of the instructions. They have enough experience to have heard of these techniques, but not enough to realize that they are only performance enhancers, and in some cases could actually hurt models not designed to use them. After a while it sort of makes me want to say, "Why don't you believe what I wrote?" But, this type of thing is clearly not what you all are talking about.

Also, I tend to think of airplanes as being "fully integrated systems" to the point where every component is completely dependent of every other one. In other words, once the design is frozen and tested, you can't just take this one element and make it stiffer, because now that carries more load through to this other piece which will fail now and plus it makes this end heavier, which increases this moment of inertia and ruins the dynamic behavior.... (have you ever read the story about the spars in the WWI German monoplane where the thicker they kept making the rear spar the more in flight failures they got, because the stiffer it was, the more the wing tended to twist around it). In fact, with a modern, overpowered model airplane there is so much margin everywhere (structurally, aerodynamically, in stability and in performance), that kind of thing doesn't really happen. You can do practically anything to a foam ARF and it will still fly. So, really, people probably shouldn't be that afraid to make mods. Respect your designers, understand that chances are they have thought of things you haven't, give them a few builds the way they intended when you're starting out, but don't be afraid to try some things later. Don't be afraid to do anything.
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Old Oct 20, 2010, 03:14 PM
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Originally Posted by Captain Dunsel View Post
Anyone who builds a model strictly per the plans, with no changes, modifications, or improvements probably also eats plain vanilla ice cream, with no chocolate syrup, cherries, sprinkles. Boring!

Gotta learn to try some funky flavors and toppings, even if they wind up giving a bad taste in your mouth.
This is very interesting to me. I have always felt this way: if you have a constant need to supplement your ice cream, maybe it means you don't really like ice cream. I mean really like it. I always felt this way about people never seeming to be satisfied with the things they know or the things they are offered. For example, people that always just have to eat some kind of exotic foreign food. Do they really like food?

Personally, when I do get ice cream, I almost always get vanilla. I like the taste of dairy products and I like the taste of sugar. If the ice cream is any good at all, I'm already completely satisfied. Actually I don't even really need the Vanilla flavor, just the cream and the sugar really give me all the flavor I'm looking for. My brain only has so much capacity to interpret what I'm tasting, if I add a whole bunch of supplements to it, it's only inhibiting my ability to appreciate the actual dairy flavor. In other words, I'm really getting a sprinkles and fruits experience rather than an ice cream experience. The concept of "boring" doesn't even apply, how can I be bored if I'm experiencing the taste that I like? An even better example is chocolate milk. If you like milk, you don't want chocolate milk.

I don't think it's necessarily the natural state of human beings to have a need to be trying new things all the time, a need that seems to be constantly increasing in this era. A lot of the time I think it's because they haven't found what they really do like.
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Old Oct 20, 2010, 03:27 PM
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I have built several copies of the Blitz airplane from steelheadproducts.com. I mention it in this context because it's a very interesting "kit." For $20 you get three pieces of foam and a set of instructions. The three foam bits are: a fuselage and two wings, each around 25 inches long, hot-wire cut with a nice semi-symmetrical airfoil.

In other words, it's a great base for folks who like doing things their own way, but may not feel up to designing a complete airplane.

On the Blitz, you get to do a lot of building and choosing, but you are almost 100% assured of a well-flying plane, assuming you don't make it too darned heavy.

You get to cut the tailfeathers yourself; the vendor supplies a rough dimensional sketch. You get to cut and hinge all of the control surfaces yourself. You can make the wings as long or short as you like, and set them at any dihedral angle you like. You can build it with or without a rudder. Motor mount, landing gear, servo placement, etc. are all left entirely up to the kit builder.

It's kind of a Rorschach plane... it can be whatever you want it to be, within the limits of your imagination and building skills.
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Old Oct 20, 2010, 06:21 PM
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what are the two black "things" on the LE of your Skeeter?
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Old Oct 20, 2010, 06:41 PM
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what are the two black "things" on the LE of your Skeeter?
Just electrical tape to hold down the servo wires.
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