Re: Maybe tracing is easier but ...


David J. Gall
 

Ben,

That's pretty much exactly what I've been saying; To use degree-two single-span (degenerate) splines is, mathematically, exactly the same as to use conic Bezier curves. It's just a choice of terminology that reflects the user interface of certain software packages. As for getting tangency at the joins of piecewise conic Bezier curves, that, too, is a matter of the tools available in the user interface of the CAD software and not of the particulars of the type or mathematics of the curves being joined.

Vacuum bagging could have its place in the manufacture of a Q2 or Q-200 if one chose to do so. Certainly, bulkheads and formers and console pieces could be vacuum bagged. One might also consider the Cozy Girls' so-called "LoVac" low-intensity vacuum bagging method for wings and tail and control surfaces, but it's likely to be more bother than it's worth and it will always run the risk of compressing the blue foam cores if you get just a bit too much vacuum.

Carbon fiber? Nah. Except for spar caps, there's no part where carbon fiber could significantly contribute structurally anywhere on such light airplanes as these. And carbon fiber brings it's own set of problems with it: it's difficult for amateur builders to control wet-out and get good fiber-to-matrix ratios with hand laid-up carbon fiber cloth; carbon fiber sanding dust is more corrosive, abrasive, and toxic than glass fiber sanding dust; and carbon fiber must be isolated from contact with metals, typically achieved by adding a layer of glass fiber that likely has sufficient strength that the carbon could be omitted. And you couldn't get the carbon fiber thin enough to grant a weight benefit because you still need it to be thick enough to resist secondary loads like keeping people from poking holes through it with a bare finger, which is about where we are already with the fiberglass. If you want to start over with the full DarkAero treatment you might save some weight but at what cost and complexity? These are supposed to be inexpensive and quick to build.

In the immortal words of "Jimmeh" (Jim Masal), follow the plans except where the plans have proved to be wrong.


David J. Gall

-----Original Message-----
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Ben Wilson
Sent: Thursday, October 6, 2022 1:17 PM
To: main@Q-List.groups.io
Subject: Re: [Q-List] Maybe tracing is easier but ...

Johannes and David,
I have had good results for this kind of problem using Bezier curves instead of splines. Believe it will be easier to get tangent at joints.

And now to throw a "hand grenade" into the group -- just four words, "vacuum bag?" and "carbon fiber?"
Sorry,
Ben







-----Original Message-----
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of David J. Gall
Sent: Thursday, October 6, 2022 7:35 AM
To: main@Q-List.groups.io
Subject: Re: [Q-List] Maybe tracing is easier but ...

Johannes,

Try to stick to degree-two curves (conic sections) as much as possible, and fit them piecewise with tangent continuity at the joins. Use the control point "weight" on the middle control point to adjust the amount of curve for each degree-two conic curve. This is how the P-51 was designed and (except for the possible occasional use of a French Curve) is likely how the Q2 molded fuselage was formed. (Also much TLAR freehand drawing....)


David J. Gall

-----Original Message-----
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Johannes Weissmann
Sent: Thursday, October 6, 2022 8:43 AM
To: main@Q-List.groups.io
Subject: Re: [Q-List] Maybe tracing is easier but ...

At the current stage I simply tried to reconstruct the existing shape based on the available plans and templates.

Basically, I used the fuselage jig and bulkhead templates as reference and in addition traced the fuselage profiles from the three-view of the plans. The templates are to scale, the three-view needed some scaling.

Based on that, I tried to make the templates and bulkheads match with as little error as possible. The main difficulty is to exactly locate WL0 and WL15 as I could not find any references.

Currently, there is no limit on the degree I used to generate the curves. Some curvatures from the templates just can't be fitted with three-degree splines.


Here is what I am not sure of:

What I would *like to do* now, is to construct a design that matches as close as possible with simple mathematical forms.

What I don't know because I am not an aeronautical engineer (I am a
physicist) is how much I am getting into the realm of aircraft design or if the deviations from the original are small enough to not affect the resulting aircraft performance and specs.


// Johannes


On 06/10/2022 14.28, David J. Gall wrote:
Robert,

If your intent is to duplicate the existing QAC shape I would caution you that, although "CAD is fun" it can also lead you astray. The state-of-the-art in the early 1980s was compass and protractor and a wooden batten as a spline; the CAD equivalents are *not* degree-three curvature-continuous NURBS curves and surfaces. Stick to degree-two conics or arcs of circles joined tangent-continuous for cross sections and nothing more than degree-three single-span curves in the longitudinal direction. If more than one degree-three span is needed longitudinally, join degree-three splines at their ends using nothing more than tangent-continuous joins.

If, on the other hand, your intent is to make it "better," first, define "better," then go for it! Coolio!

(If you're surface modeling in SolidWorks, well, there's no hope for
the world.... ;P)


David J. Gall

-----Original Message-----
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of
Johannes Weissmann
Sent: Thursday, October 6, 2022 3:41 AM
To: main@Q-List.groups.io
Subject: Re: [Q-List] Maybe tracing is easier but ...

Great!

CAD is indeed a lot of fun. I finished the first surface model of the Q2 fuselage yesterday. Not perfect yet, but at least it is spanning the whole fuselage now.

On 06/10/2022 06.27, Robert Schmid wrote:
... this CAD stuff is fun too. First version of panel test cut is
done,

CAD still needs some fine tuning but not bad for a start.
--
Robert "TheFrisco" Schmid
(408) 805-5450

www.facebook.com/TheFlyingFriscos (
https://www.facebook.com/TheFlyingFriscos ) www.theflyingfriscos.com
( http://www.theflyingfriscos.com )

Love building planes almost as much as flying. Latest completed build is "Loki", a Chinook Plus 2 bush plane.





--
Johannes Weissmann










--

Johannes Weissmann

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