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Author Topic: Modifying an existing design  (Read 970 times)

Offline James Mills

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Modifying an existing design
« on: August 16, 2007, 09:33:04 PM »
If you take a given set of plans and take a blown up drawing of a different profile (ie a Mustang, Extra 300, or whatever) and are interested in a semi scale look (but not as scale as say a Rabe design) and keep the moments the same, will it still keep the same flying characteristics?

James
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Offline L0U CRANE

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Re: Modifying an existing design
« Reply #1 on: August 16, 2007, 09:53:50 PM »
James,

None of us will ever know or be able to predict....

However, if you can build as well as the guy whose successful design you start from, and don't really botch the basic proportions, you should have a pretty good flying model - and remember, the guy who made your inspiration a winner had certain flying and trimming talents that he might not even be aware of...

A reasonable approximation of the 'prototype' that inspired you should be a decent flyer, but we do have those stray factors to deal with. Don't let a "master's" techniques, necessarily, change you to trying a structural procedure you're not sure of, for example. Go with what you know, as far as possible, to reduce the range of unknowns when you get into dialing the end result.

However well your end product flies, it is a great kick! - that's YOUR airplane out there, no one else's! MAN, is that ever worth it...!
\BEST\LOU

Offline Bill Little

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Re: Modifying an existing design
« Reply #2 on: August 17, 2007, 07:25:03 AM »
Hi James,

Bob Hunt refers to it as "changing the clothes on the mannequin".  That's how my Geo Juno came to be! ;D 

As long as you stay within the numbers of the original, you will come out pretty much the same performance wise, all other factors being equal.
Big Bear <><

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Offline Bill Gruby

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Re: Modifying an existing design
« Reply #3 on: August 17, 2007, 08:12:33 AM »
James;

 This is an interesting thread, and it really hasn't gotten off the ground yet. Both replies so far are dead on. Lou says it perfect in the first sentance, "None of us will ever know or be able to predict ....  I have done what you wish to do, I have created a "Ringmaster" that has never been seen before. Again Lou is right, although one or two more may be built, this one is MINE, it is one of a kind, what a rush this is. New construction and old school were combined with great success. Most of the numbers are identical, the wing loading is just a tad higher. The "Aspect Ratio" is the same. We have high hopes for this one, but only time will tell. You do this with no promise of success, but you do it anyway. It is your baby from start to finish, go for it, I promise you, you will not be sorry you did.

  "Billy G"   (PE**)
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Offline Bill Little

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Re: Modifying an existing design
« Reply #4 on: August 17, 2007, 08:29:00 AM »
I guess the most common, well known, example of this theory is all the myriad "original" planes that were built after Top Flite kitted the green Box Nobler in 1957.  Even World Championship planes came from this one! ;D
Big Bear <><

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Offline Mark Scarborough

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Re: Modifying an existing design
« Reply #5 on: August 17, 2007, 10:13:24 AM »
James,
IMHO, I have done what you speak of. The Brodak P-40 profile really gave me confidence as I am learning, still have one. I like the way it flies so well I am designing around it. So far, using the exact P-40 wing, I have built a Gee Bee replica that if anything flys better than my P-40.(Its ok Pat Johnston flew my GB and he is the one that said it, and the P-40 is NO slouch) I also have a bf 109 based upon the same wing, but I have taken a step back and am only flying shrink wrapped airplanes the rest of this year, somethihg about the hours finishing being lost when I make a rookie mistake. The 109 is hanging on the ceiling waiting for paint to come this winter. As has been said, wrapping new clothes on a proven platform is the highest form of flattery for the original designer,, (right Pat) and gives you a pretty good chance at hooking your lines to a good design when you are done. It also gives you a chance to tweak, for example I lengthened the nose moment about 3/4 since both my p-40s were tail heavy. worked out perfect,, ok so the second time cause I stuffed it, my fault. Its flying again and I am a happy camper cause when I set MINE down next to the cardinals and P-40s, and Vectors, its different. Truth is it only looks different, though,,
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
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Steve Kientz

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Re: Modifying an existing design
« Reply #6 on: August 18, 2007, 05:42:49 AM »
 Do you keep the same side area of the fuselage,or is the rudder area the only critical dimension?

Steve

Offline Randy Powell

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Re: Modifying an existing design
« Reply #7 on: August 18, 2007, 01:29:04 PM »
There is a difference in making cosmetic changes to get a different look and making substantial aerodynamic changes. In the first case, knock your self out. As long as you keep the basic areas within shouting distance of the original, the plane should fly pretty much like the original. If you go to make substantial aerodynamic changes, just know why you are making those changes and what you hope to accomplish. Then let her rip. Chances are, as long as you aren't going too far off the res, it should fly fine and you may improve the original.
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Offline Bill Little

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Re: Modifying an existing design
« Reply #8 on: October 08, 2007, 05:12:42 AM »
Getting back to this one, I had some further thoughts.  As I openly admit, I am the #1 thief of ideas in this event.  I don't know enough to design my own, but I DO know enough to copy those that are very successful!

I am going to build a Classic plane in the very near future that is a great example of the "changing the clothes on the mannequin" theory.  Mr. Louis Grondal of Belgium was the first "Official" FAI World Champion in 1960, and he backed it up ni 1962.  We (erroneously) call it the Grondal Nobler.  He called it the AMA Special in defference to having one flight (out of the three) at the Worlds using the AMA pattern instead of the FAI one.  In several phone conversations with Mr. Grondal before his passing, he told me he built the firat one from a '57 Green Box Nobler kit.  It has a descending top blick, bubble canopy and "T-Birdish" wing-tips.  The moment arms, etc., stayed fairly unchanged.

I DO have a Semi-Scale Stunter on the drawing board that I have received much help from Ron "F-4-F Guy" Burns.  It is, however loosely based on a set of R/C scale planes.  Just making the necessary changes to it to make it a dedicated CLPA plane.

To make my Geo Juno, I called Mr. Werwage and told him of my plans, along with a call to Mr Robin Hunt.  I took vellum, traced the fuse sides of the Saturn, then laid it over the Juno plans and traced the areas that gave it the Juno *look*, including the fin/rudder, wing tips and stab/elev. tips.  The turtle deck was very easy.  All in all, I came up with a Saturn that looks like a Juno for all intents and purposes.  Just slightly bigger and a straight TE.  It is a great flying plane!

Big Bear <><

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James Hylton Motorsports/NASCAR/ARCA

AMA 95351 (got one of my old numbers back! ;D )

Trying to get by

Offline Dennis Adamisin

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Re: Modifying an existing design
« Reply #9 on: October 16, 2007, 08:30:59 PM »
I'm surprised there have not been more posts on this thread too - but I'm guilty too, until now!

Dave Thornburg of RC Sailplane fame wrote a series of articles on sailplane design, with one comment that stood out for me when I read it and I think applies here.  He said that the wing IS the design and everything else is pretty much cosmetics.  I think it applies to CLPA; the wing will determine whether the airplane flies well fast or slow (or at all) it will determine how much weight the design will carry, how much power it requires to fly well and so on.  Moment arms, stabilizer size, etc all make a contribution but frankly it you stay anywhere near what looks like a stunt model and you will probably be OK.

As has been pointed out in the Classic era a lot of "original designs were base on Nobler wings (for a straight taper), or T-bird wings (for an elliptical look), or other "safe" starting points.

Of course, if/when you try to change an existing design you should have a REASON in mind - ie what do you want to achieve - then go for it!
Denny Adamisin
Fort Wayne, IN

As I've grown older, I've learned that pleasing everyone is impossible, but pissing everyone off is a piece of cake!


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