Bladder tanks

I recently weighed a 3 oz hayes clunk tank .8 oz and a 3 oz square wedge 1.4 oz. A bladder tank would be lighter yet. Works well in combat but does quit without warning. What has been the success or failure of bladder tanks in stunt. Thinking forward CG and nose heavy may be a problem I have yet to see them used during the past season and this year.

Can you describe the benefits and liabilities of such a situation?

Bob Smiley
A Lancair 360 builder/flyer trying to be a modeler

LAST EDITED ON Jul-16-03 AT 11:18 PM (CDT)
 
I'm relatively certain that the archives here should include a couple of old message threads on the subject. Somewhat more recently, it was the subject of a thread over at Flight Lines.

Added via edit: (Wrong. The most recent one was this one:

http://clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=5154&forum=DCForumID1

.. and that thread was right here.)

.
Kiwi

They don't quit without warning, unless you use a pressure regulator, as Paul Walker did on the B-17.

I've had them quit without warning when somebody flew through my airplane.

I've been trying a bladder in a stunt application for awhile. Here are some solid observations.

I have the best luck running the bladder in a compartment that supports it when filled. Usually a 1.25 in. tube and a 1/4 in. id bladder. The tube and bladder weigh about half an ounce complete(18gr.) If the bladder is filled smoothly from one end it will run just about perfectly and consistently. Leave at least half an inch of the bladder unfilled to help prevent pressure inconsistencies. About 5-6 laps before the motor quits it will richen up. then it cleans out and runs a lap or so lean and quits. Couldn't ask for a nicer stop.

It is a royal XXX to fill a bladder consistently when the tube is hidden away in a plane. I can only get the fill right about one try in three. For best results make sure the tube is easily viewed while filling the bladder. If you make a nice, neat window in the wing complete with lots of fussy details that show how much time you spent on it, it shouldn't hurt you appearance points much. Make the access hole easily accessible, just like on a combat plane. Trying to run the fuel line through the fuse etc. is just asking for trouble.

I've been running a stock LA 40 with the head relieved slightly to lower compression and peak speed. Bladder-fueled motors do tend to run better when run 2 cycle, just below peak rpm. the LA does do pretty well though 4 cycling. that's what got me on this. It tried one for speed limit combat and it would 4 cycle for a whole flight easily. Don't overload it though. a 10/4 prop is about the maximum. 9/4 works even better for a good run.

Thanks for the response. I read the long thread previously referenced. Very educational.

Where can you acquire the 1 1/4 inch tubes? I saw 1/4 inch id surgical tubing at the local sporting goods store. Should it be encapsulated in a baloon? or make the tube fuel proof and sealed and dump the tube if a bladder bursts?

I forgot the problems of shutoff access prior to starting ie problems of a fuselage hidden fuel line. Hmmm needs some thought here.

My overall thoughts when posting the first question were: Constant pressure systems could have tanks located at or close to CG, kinetic energy could be reduced in turning, bladder tanks are lighter moving CG aft, and one could build lighter birds, keeping outside dimensions the same for Vintage and Classic and have reduced wing loadings.

Another question I have is how can we calculate the effects of a heavy vs light paint job on CG? That is a decision point in building and often times too much paint requires nose weight. How can we plan and calculate a paint job and come out reasonably balanced? I hate to add weight and would rather design and build to a final weight.

Another question again. Is there some general relationship between CG, center of lift, Mean Aerodynamic Chord, and center of lift which must be followed for a good flying aircraft? What are the general theories and relationship to these four terms? Knowledge of this might allow one to better understand what makes a good vs lousy flying machine.

Thanks for your replies. You see, I really wanted to be an aeronautical engineer when I was a kid but dad said "be an architect and make money". Now I want to learn something about flight.

This is a very general answer;
The center of gravity MUST be in front of the center of lift to have stability, unless you have an F-16's onboard computer.
How far in front is a matter of controlability, to far forward results in sluggish controls, to far back extreme sensitivity.

The MAC is a theoretical position on the wing usually used as the reference point of CG.

The center of lift is very complicated, it is a combination of wing lift, fuselage lift, negative lift of the tail, and the propeller also influences the exact position of the center of lift.

On full scale (real) formula one racers the CG MUST be in front of 25% MAC to insure stability. They are also known to have very small tails.
Andrew R

Thank you Andrew. I will reference these definitions for later use.

good bladder tubes are T-08 fluorescent light bulb covers from Home Depot or Lowes.

Bob,

The Dirt has been doing relaxed bladder tanks for a long while. Actually uses one on his Old Time Stunter. A Zilch, I think. You might want to talk to him about it.

Bob,

Randy is close. Kinda. As mentioned in the thread Kiwi suggested, I actually flew in competition with my Zilch X fitted with a bladder, containment tube pretty much centered astride the balance point.

It gets worse. The idea was to have no outside indications that the model packed anything other than a hard tank. So there were ball check valves all over the place, weird plumbing in other places. While I did make it work, particularly for something like an OTS model it was just way too much hassle. Again with the application there was no discernible benefit. Other than head games...

The relaxed bladder tanks Randy refers to are Tettra tanks which operate like standard suction tanks, except for some mildly strange plumbing and filling procedures. These actually work quite well, I have been using a couple different units in my Impact since conversion to pond-scum power.

HOWEVER, and thanks to Godzilla primarily, a month or so ago I switched to a 4-ounce Sullivan tank. Run quality appears to be the same. But less fiddles in fueling, better and cleaner shut off at end of run. Plus they are a LOT cheaper.

I do think the Tettra tanks would be sweet on a profile model such as your Primary Force. Yep, they are available in a 3-ounce size. Going to Vancouver, B.C. this weekend? I'll toss in a spare piece and we can try it in a foreign country.

Dan,

I'll take a look at that tank. The Force however is flying quite well so I do not want to mess up that setup.

Bob

The right lexan flourescent light protector tubes can be hard to find. The usual ones you find at Lowe's, etc. are the bigger size (about 1.5"). I've been able to find the smaller size (T-8) at lighting contractors.

To seal the ends, cut 1 1/4" discs out of a foam food tray using a tool made from a sharpened piece of 1 1/4" drainpipe. Scuff the insides of the tube with sandpaper, and then glue the discs in with polyurethane glue. Putting a couple drops of water inside the tube before doing this helps the glue set quickly.

How did Paul Walker do it in the B-17? Those were the longest bladder tanks I've ever seen.