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Tip Tanks: Design and Fabrication

For Designers and Homebuilders, Revisited

By Michael C. “Mick” Myal, EAA 7978

Tip tanks
A two-place Cavalier experimental airplane using Cessna-style tip tanks 
Photo: Karl Walter

In the November issue of EAA’s Experimenter e-newsletter, we published a short description of a new book, Tip Tanks: Design – Fabrication. Several readers e-mailed to say they were upset that the lead-in to the article seemed misleading. They anticipated finding an article and what we presented was an announcement of the availability of a new book. So we contacted the author and asked him if he wouldn’t mind writing an article on the subject. He agreed. Read more

I must confess. I’m a believer in tip tanks. An EAA member since 1959, I saw the adaptation of this World War II technology in the 1970s. Larry Burton’s cool-looking Cavalier won the 1st of Class – All Wood Construction award at the EAA annual convention in 1975. Joe Alveraz came in with the Polliwagen, an advanced composite two-place design with a pollywog-shaped fuselage. And Pazmany’s all-metal creations rounded out the application to wood, plastic, and metal, respectively.

Then came the canard craze preaching quick build, which left little market room for conventional designs. Nevertheless, metal workers had their say about “compost” airplanes. Canards and tip tanks as factors in CG management didn’t mix then and never will.

I concluded that tip tank technology was a dying design feature, overcome by numerical success of the RV series and new composite offerings. Why would a designer want to incorporate this feature and add complexity to his design when not demanded by homebuilders?

Let’s argue the advantages of tip tanks. Replacing in-wing tanks with containers at the wingtips should require less homebuilder hours. Consider the extra effort needed to seal wing structures and the time needed for repair. Roll stability can be expected to be better. (Picture the circus high-wire act.) Placing fuel away from the cockpit is always a good idea.

My mission was clear; reintroduce tip tanks to the movement. My initial plan was to piece together a story on how to build a tip tank plug. The goal of the story would be an attempt to convince designers and builders that the process was indeed simple and inexpensive. At that point the article’s impact seemed weak; there wasn’t enough meat to make my point clear. And that tale was already told many times over, like those that tell of the production of a fiberglass engine cowling. My focus changed from a boring how-to article to providing actual patterns and hardware references.

Ideas Just Happen
In my case, publishing full-size patterns as a means of inducing people to take a close look at the tip tank has other advantages. The book format is unconventional; pages are legal sized (11 inches wide by 8-1/2 inches tall) which produces a booklet fitting nicely in U.S. Postal Service flat-rate envelopes. At this early stage it’s too early to know if my work will generate a fresh interest in tip tanks. I’m proud of the end product, but time will tell if my mission was successful.

What Makes a Tank a Tip Tank?
Essentially, it must be a closed container at certain times and its contents accessible on demand at other times. It must be located in close proximity to the longitudinal CG of the airplane so that flight balance is ensured at all times. That location falls approximately at the 25 percent of the design mean aerodynamic chord (MAC).

I consider tank hardware to be a duplicate of existing conventional tank systems and leave the selection to the builder. A large variety of filler caps, fuel senders, vents, strainers, quick drains, and other related items are available from supply houses, and almost any one of them can be made to work. Installation of these parts should follow standard practice as ably covered in the Tony Bingelis books. If a builder is at the point of beginning wing construction, it’s time to study the plan drawings for these called-out parts and the specified fuel lines.

The volume of the tip tank is a key variable and needs to be addressed early in the process. My project tank was based on eyeball measurements of a Beechcraft Bonanza 15-gallon tank. Since I felt this tank shape had an elliptical look, I estimated the major axis at the tallest point to be about 13 inches and its minor axis to be 8 inches. The tank chord or length was about 72 inches.

To contain the shape, I decided utilizing the shape of a circle segment. I found an obscure formula that provided the essential data points:

R = (4 x T2 + C2) / (8 x T) where T = template and C = chord.

Two radii controlling the major and minor design lines were imputed into a CAD program to produce the spreadsheet published in the booklet. Our first showing at COPPERSTATE 2011 was a hit. A customer went home and devised an automated version of the original spreadsheet illustration (as shown on page 9).

You’ll save a lot of time using this tool. It’s free—just send me an e-mail at mick22@cox.net and ask for Tank Volume from me. I’ll send it right out.

The rest of my work was the CAD drafting of full-size patterns that were used to produce the male plug in the photos in the book, the female mold, and tank shells. The use of a water jet was investigated as a time/money tradeoff option for cutting the form bulkheads. One caveat: Seek the finest orifice to minimize the effect of cut growth taper. I forgot my earlier experience and nestled duplicate parts. Guess what—the bottom piece was smaller than the top.

Other than locating the tank midpoint at 25 percent of tip chord, not much more can be suggested for structural attachment. Simply put, each type of wing construction (wood, composite, metal) will have its unique subsets of differing structure arrangements. A simple load diagram can be drawn to show the tank load moments acting on the tip spar. Attachments will be in shear, the number dependent on materials, techniques, or both. Fittings such as seatbelt restraints are stressed to 9g. I would use that number.

I trust this narrative will prompt designers and kit manufacturers to evaluate the tip tank as a viable alternative. Homebuilders, too, can find this tank an intriguing possibility.

Feel free to contact me at mick22@cox.net with any questions you might have.

Mick Myal graduated from the General Motors Institute in 1953 with a Bachelor of Mechanical Engineering degree and is a U.S. Army veteran. His 35 years at GM included assignments in body production and styling among other tasks. An EAA member since 1959, his aviation hobby includes consulting on two-engine STCs, the building of a Minicab, a 2/3 Mustang, and a VariEze. He’s the author of several Sport Aviation articles, three Alternative Engines books for homebuilt aircraft, and is the founding publisher of CONTACT! Magazine.

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