thanks for this great tool

Airshipworld Gertler Series 58 Generator – Version 0.6 (27.03.2009)
(c) A. Grunewald & J. Eissing
http://www.airshipworld.info/software
Helpfile by Johannes Eißing, 18.08.2009

General

The Series 58 hullforms were developed by Morton Gertler and Louis Landweber for the David Taylor Model Basin DTMB in 1950 [1], [2]. These shapes are described by five parameters such as slenderness ratio, prismatic coefficient, location of maximum thickness, bow- and sternradius. The series 58 shapes are well covered in literature because of their parametrics and reproducibility. Their value for research and development is comparable to the well known NACA four- and five digit profiles.

User Interface

- Render Profile
pretty much self explaining, klicking this button updates the plot window.

- Copy Values to Clipbord with TAB/semicolon
Points shown in the plot window are copied th the clipbord as x/y point coordinates. Only the coordinates for the upper shape are exported to the clipbord. The number of Points “n” is set in the dialogue box “Input Parameters”, see below. Coordinates are given nondimensionally, referred to the length of the body. Points are allocated in a full cosine distribution. The delimiting character can be chosen as TAB or semicolon. Chosing the TAB Character facilitates import to e.g. EXCEL by CNTRL+V

- Calculated Parameters:
- Cs
Surface Coefficient as introduced by Gertler [1]
Cs = Swet/(L*pi*D) where
Swet = wetted surface area,
L = Length of the body
pi is the ratio of perimeter to diameter of a circle with approximately 3.1416
D is the diameter of the body.
Reversely, wetted surface area is computed by
Swet = Cs*L*pi*D
Cs is calculated numerically, meaning it’s accuracy increases with the
number of points n (see Input Parameters). Already with 20 points the
error is less than 0.5%, with 50 points about 0.05%.

- CB
Centre of buoyancy. This is the volumetric center of the body referred to the body’s length.

- a1 to a6
are the computed polynomial coefficients for the shape function
y(x)=D*sqrt(a1*(x/L)+a2*(x/L)^2+a3*(x/L)^3+a4*(x/L)^4+a5*(x/L)^5+a6*(x/L)^6)
where
y(x) is the local radius (ordinate)
x is the abscissa
D is the maximum diameter
L is the length of the body.

- Input Parameters
- n – Number of steps
Here, the number of points to be computed for the shape can be set. For standard use, 20 to 50 points will do.

- m – Point of maximum thickness
This is the position of the maximum section, referred to body length. A typical value is 0.40.

- r0 – Dimensionless bow radius
Bow and stern radii are nondimensionalized by the following relationship:
r = R*L/D^2=(R/D)*(L/D) where
r is the nondimensional radius
R is the dimensional radius
L is the Length and
D is the diameter of the body.
A typical value for the bow radius is 0.5, being the bow radius of any prolate spheroid. A pointed bow would show a value of 0.0.

- rl – Dimensionless stern radius
See above r0. A typical value for a tail radius is 0.1. A pointed tail would show a value of 0.0.

- Cp – Prismatic coefficient
The overall prismatic coefficient is a measure of how good a slender body fits an enveloping prism, built from the maximum crossection extruded for the length of the body. In case of a body of revolution, this prism is a cylinder. An arbitrary ellipsoid shows a prismatic coefficient of 2/3. Typical values for airships and submersibles are in the range of 0.60 to 0.70.

- L2D -Length to Diameter ratio
This is the slenderness ratio of the body, typical values being in the range between four and ten. A typical value for airships is five.

References
[1] “Resistance Experiments on a Systematic Series of Streamlined Bodies of Revolution – For Application to the Design of High-Speed Submarines”
DAVID TAYLOR MODEL BASIN WASHINGTON DC
Gertler, Morton, APR 1950
[2] “Mathematical Formulation of Bodies of Revolution”
DAVID TAYLOR MODEL BASIN WASHINGTON DC
Landweber,L. ; Gertler,M., SEP 1950

1. Well, go ahead then.
2. see 1.
3. There are no menues.
4. Actually this program is a tool to establish basic shapes.
5. see 1.
6. This program is a basic tool, not a toy for senselessly clicking around.
7. see 1.
8. Did you notice the “download the Preview” link?

I for my part would have even been happy with a command line interface. The way Andreas put the algorythms together and gave it to the public is just great in my honest oppinion, no bell and whistels needed.

Concerning double bubble envelopes:

1. For hogging and sagging loads, the section modulus of a flat section is lesser than for a round section of the same cross sectional area. So how could a flat envelope be more stiff than a round one?
2. Shorter height is an issue concerning hangar space.
For a given volume, length is a question of crossectional area and not of crossectional shape.
3. Why this? Zeppelins did “land” on water. Only problem being waves.

Regards, Johannes

1. Clearly establishing the needed elements in the software workspace in a logical manner (a list, maybe).

2. Initially designing the software workspace simiar to a common photo-editing or program (to make it easier to learn).

3. Using tiny picture icons grouped cose together, and avoid too many GD menus and sub-menus.

4. Providing lots of basic shape templates; lets not have to re-invent all this stuff.

5. Provide lots of automated functions, like before and after dual views, etc.

6. Always have UNDO (to 30x at least) as a toolbar command, not a command from a GD dropdown menu.

7. Windows and dialog boxes need to always have last-position memory as the default (somebody at Corel and Ulead needs a few shouts bout this).

8. Make it affordable for others to use !

Now a word about dual-envelope designs. I hope it is now evident to most (if not all) that for multi-passenger airships this is superior to the long traditional and nearly obsolete single-envelope designs because it:

1. Provides greated structural rigidity.
2. Provides shorter lengths and heights, while having greater lift volume.
3. Provides for designs that can land on water(with passenger compartments at the sides).

But youre off to a good start. now howsbout adding to the above ?

Thats all for now. Any questions /comments ? – paul

thanks again for implementing the tool in java and bringing it to the public. I think the Gertler Series 58 is a big deal, comparable to the NACA four and five digit profile sections.

Let me suggest further improvements for the tool, not wanting to be pushy, just if you agree.

- polynomial coefficients
It would be nice to have the polynomial coefficients a1 to a6 as output too. The format should include 10 significant digits.

- CB
The centre of buoyancy or centre of volume, should be computable (if I did my homework correct) by
(a1/3+a2/4+a3/5+a4/6+a5/7+a6/8)/(a1/2+a2/3+a3/4+a4/5+a5/6+a6/7)

- Cs
A surface area coefficient is a little more challenging. Analytically this might be fun for mathemaniacs, but I’d rather prefere either a numerical or a statistical approximation. WRT the next point, CG, it should be done numerically.

Since the point coordinates x and r are already computed, we can compute the distance between the points ds by
ds=sqrt(dx^2+dr^2)
where
dx(i)=x(i+1)-x(i) and
dr=r(i+1)-r(i)
with i from 1 to n-1, n being the number of points.
Then we need the mean radius rm between x(i) and x(i+1) with
rm(i)=r(i)+dr(i)/2
The surface of a slice between x(i) and x(i+1) is then approximately
dS=pi*rm^2*ds
The surface area S should then be
S=sum(dS(i)) for i=1 to n-1
where n is the number of points.
There are several approaches around to build a surface area coefficient. I’d propose to use, at least in this context, the Gertler variant with
Cs = S/(L*pi*D)
Here, the referencs is the surface of a cylinder of the same length and diameter. Obviously, L is 1 here and D is L2D.

- CG: The centre of gravity of the surface
This is the ratio of the moments of dS times xm and the surface area S with
CG=sum(dS(i)*xm(i))/S for i=1 to n-1
where
xm(i)=x(i)+dx(i)/2

- Clipboard format
How about a TAB as delimiting chacacter between x and r? With OpenOffice CALC a semicolon is not a problem, but a TAB would facilitate the import to MS EXCEL via clipboard.

- n
Would be great, if the number of points could be chosen by the user.

Later we could even add approximations for added masses and the destabilising Munk moment, but this is a different story )

Again, thanks a lot, and dont feel pushed. These are just suggestions.

Best, Johannes

PS.: Try as input the following:
m = 0.5
r0 = 0.5
rl = 0.5
Cp = 0.667
L/D = 1
;o)