# Letters

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## Mold Making

I have two questions about making rubber parts [“Basic Mold Making,” December 2015]. What is used as a mold release on the mold and inner rod? Also, is there a simple method of determining the approximate Shore hardness of the part we are going to duplicate?

–Ray Henrie

Bob Hadley responds: Any type of furniture paste wax will work for mold release on a rubber part. I used Meguiar’s M8 Mold Release Wax. It’s available from Aircraft Spruce. Wicks also sells a paste wax product called Partall Paste #2.

You can make a very rough guess on Shore hardness by comparing materials of known value. Before they started labeling skateboard wheels with the Shore hardness, the “street method” comparison test was to bite the wheels. It may seem crude, but it’s a pretty effective way to compare similar samples. If you plan to do a lot of testing or comparing of samples, you can buy an inexpensive Shore A scale hardness tester for about \$30 on eBay. It may not be super accurate, but it’s probably better than putting old rubber parts in your mouth.

“Learning How to Drag Your Tail” [December 2015] is the best article I have ever read about taildraggers. I have checked out numerous people, and they use exactly the same technique. I had a friend train me many years ago, and I learned the same way.

–Ray Tows

We agree—learning to fly with the little wheel in the back is fun and not that hard if taught properly. There are so many taildragger designs in the Experimental world that you limit your choices if you don’t do some transition training.—Ed.

In “Stressing Structure, Load Distributions, Part 2” [September 2015] on page 67, David Paule gives the equation for the moment distribution for a constant load as taken, he says, from ASM section B4.0.0, Table 4.1.1-2, load case 13. If you look carefully, ASM and David’s equation are not the same. After integrating the differential equations, I can say that David’s equation is the correct one. In my opinion, David failed to let us know that there is an error in the ASM table that he has amended.

–Mauro Darida

David Paule responds: The equations are actually the same. I used somewhat different letters identifying the terms than ASM. With those substitutions, they are the same, or at least they sure looked the same when I checked.

But Mr. Darida is right; I should have pointed out that the ASM figures differ from the ones in the article. A bit of caution is always needed with stress analysis.

## Becoming a Test Pilot

Just a quick note to acknowledge an excellent article [“Becoming a Test Pilot,” September 2015], written with humility, accuracy and insight. I learned a great deal. It is one of the best I have read on the subject. Please pass my compliments to the author.

–John TenHave

Glad you enjoyed it. Elliot Seguin’s new ongoing column, High-Desert Tales, can be found on page 52 of this issue. —Ed.

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