Need some help

Hello , am sorry if am violating the forum rules but i donno where to post this

I am currently a member of a project to construct an autonomous blimp

about the preliminary design , i have found few data from the old projects (Fineness ratio , weight , volume .. etc)

and i was advised to use datcom to get the aerodynamic characteristics 

so i want to ask if someone has some data similar to this project , also what method anyone used to obtain initial shpe and size ? has anyone used datcom before ? or there is a better choice ?

sorry again if my thread is wrong

Hi student person,

Nice to see you here.  We'd love to have a regular student member. Actually we think a more univ departments should be making airships, they're gaining respect and are the future of aviation.

A few of us use spreadsheets to help solve aerodynamic problems but no one has mentioned using a real air design tool like Datcom.

What we normally do is survey airplane and airship pictures and fall in love with a particular shape and make physical study models of it and/or analyze it with math or with wind. Myself, I hung my model in front of a 1/2 hp drum fan and watched the response of my 15 foot model to an artificial headwind.

In short, it seems like an inventor has to intuit the first step; pick a shape that excites your imagination and then it'll be more fun to manifpulate it with science.

"DATCOM was written with 1970's technology and as a result some aspects of the program are outdated and/or archaic. A result of it's age the only input accepted by the Digital DATCOM is a tediously-formatted text file (for005.dat) that the user must generate according to FORTRAN syntax. In its purest form, the DATCOM format is time consuming to compile, troubleshoot, or even read. In addition, the only interaction a user has with the application is through the inherently difficult to use command prompt. 
OpenDatcom is being developed to combat these (and many unmentioned) outdated features. OpenDatcom is written completely in Sun Microsystems Java SE and uses the Java Virtual Machine (JVM) to interface with USAF DATCOM. The interface is completely coded into OpenDatcom; as a result, no modification to the original DATCOM code is needed."

http://openae.org/software

Good luck!

http://smallblimps.lefora.com/2010/08/26/hull-shape-and-drag-estimation/0/

In my opinion, a blended spheroid cone is the best shape to reduce drag. It has been shown to perform well in computer analysis if one assumes turbulence across the majority of the airship body. It hasn't be tested much in real life though. But there is a LOT more to airship design than hull shape, as we have learned. 

Is this a competition?

Blended spheroid cone? Do you have a picture of this? Are you refering to a Piriform ( teardrop shape)?

I logged in but the system is treating me as a guest .

Swampie777

Attachment: teardrop.bmp (576.0KB)

Now I'm in. The above teardrop shape is the most "low drag' shape. With that advantage comes a huge disavantage, it's hard to build.

Also check out where the center of lift vs the center of gravity would be.

  It's most interesting to see the efficiency of the teardrop shape, It certainly is popular in submarines. The difference is that subs have a steel hull, capable of withstanding the force on the bow. Early zeps found that an un-reinforced bow would cave in if speed was attempted. This reinforcing added weight, but also provided a handy nose mount for future mooring.
  My design was to reduce weight and improve performance in a high speed airship. Moving the fan inside would make for easier and lighter motor mounts, hoping that the weight and drag of the tunnel would not negate my efforts. The central force from this drive would not have the usual porpoising that many airships suffer. The drive would be at an easy access location for in-flight maintenance, this high speed ship also being a long distance ship.
  It was my hope that a singular reinforcing ring would suffice to strengthen the bow to prevent it from caving in under the force of high speed flight.

Blended spheroid cone? Do you have a picture of this? Are you refering to a Piriform ( teardrop shape)?

Pretty much. Take a sphere, put a cone on it and blend the shape smooth. 

Thanks for responding everybody 

@Nav : The thing is , it is hard for us to build many shapes for testing .. we want to start off with the theoretical study before marching to the building process so i want to use something like Datcom as initial prediction to the lift and drag etc.. and hence we can have a guiding line to where to start from also it will help with the dynamic modelling and choice of the control surfaces

@swampie : yea i know that datcom is hard and alot of pain so thats why i was asking if anyone knows a better solution , i never tried the open datcom tho 
but my biggest concern isn't which to use , its which is more applicable to my case .. if am to use the datcom i'll choose the body alone configuration which is used to calculate the aerodynamic forces on a fuselage so i guess it won't be sufficient 
do you have any idea if the open datcom is more suitable for blimp shape ?

@dude : thanks for the papers , i'll read them and see .. but the picture you posted here i guess would be hard to use a gandola with that ?? also the controls ? or maybe am wrong 

and no its not origianlly a competition , its a graduation project .. but if we found a competition to enter with that we sure will :D

Hi,  as we talked about on this night's conference call (a hum...) here is my musing on a high speed airship. I got tired of futzing with SketchUp and other 3D programs and just scanned my scribbles.

Its a semi-rigid that uses a lower keel of a triangulated truss to connect a forward pilot house (can't really call it a gondola) and the rear fin and propulsion section. It also is the primary connection for the envelope and lift bags.

The front section is a hard composite conical "capsule" that gives the ship a rigid nose for air resistance. It contains the pilot, controls, a lateral vectoring fan and perhaps a canard elevon for controlling pitch if its needed. 

The rear section has the vertical and side control surfaces, the horizontal ones are dihedral 45 degrees and form the landing legs and prop guard.  I drew it with a tip-jet propulsion but I guess you could use a regular motor and prop. I like that big rotor blade looking one.  The propane (or H2O2, or engine or whatever) fuel tank and any other machinery is stuffed into the rear section between the stabilizers both to make room for the rear most lift bag and also to maximize the counter weight for the pilot house at the other end of the ship.

One problem readily apparent with this design is when the pilot leaves the cockpit, the ship will get very light in the nose. to the point where it would probably try to stand on its tail!  To counter this and to give the ship an ability to trim its CoG for flight, it would have a pair of small (fire extinguisher type) gas cylinders for mantaining and adjusting lift balloutte and envelope equilibrum, that could slide back and forth on the keel to adjust the trim.  Before the pilot steps out, the tanks run all the way forward to hold the nose down, etc.

It would be a quite advanced and complicated craft. Its beyond the scope of the project here. But I thought I would share this in case it gave anyone any ideas to work with.

Cheers,