Design Group, March 1994

Vernon Blomquist

Originally published March 1994

Some time ago there was a letter in one of those aviation magazines complaining that the low priced end of the homebuilt market was being ignored. I'd have to agree that there isn't much to fill the gap between the ultralights at one end and the Lancairs, Glasairs, etc. at the other end. This month in this newsletter and after the chapter meeting, I want to present my idea for a design and get some feedback from all of you on what you think. I'll take comments on the general idea, the specific design, building methods, materials, hardware, engine, and anything else you can think of. I would be especially interested to hear from any of you who might like to build this plane.

First the general design goals. The primary concerns are cost and building time. Usually, there is a trade off between these two. You can get an expensive kit that's quick to build or an inexpensive design that takes thousands of hours to put together. We need something that is inexpensive and quick and easy to build. The specific goals here are: under $2000 for the structure (excludes engine and instruments) and under 200 hours of building time for a single seat airplane.

Next are the performance goals. This is a general purpose fun machine so it needs to be aerobatic. I think +6 and -4 g's (+9 and -6 ultimate) would do. It also needs to be useable for cross-country. To me, that means at least 150 mph cruise speed at 7500 MSL. Two-hundred mph would be better, but more than doubles the horsepower required and would drive the cost to an unacceptable level. Finally, the plane should have a gentle, low stall speed (50 mph max) and good handling characteristics to make it easy for a low time pilot to fly.

I don't have room to discuss all the tradeoffs (Vern wrote this on an Air-Mail envelope from Australia--ed.), so I'll go straight to the design. The plane I propose is just over 14' long, 18' span with just over a 2' wing chord. It's a low wing, tractor layout with fixed tricycle landing gear and a bubble canopy. The fuselage is plywood but instead of the typical built-up framework with a thin skin, this design uses 1/4" plywood at the primary structure. The entire fuselage is cut out of two pieces of 4' x 8' plywood and should be able to be drawn out, cut, and assembled in a couple of weekends. The wings use a solid spruce spar with the typical foam and fiberglass skins or could also be plywood covered. They are sized so a single sheet of 4' x 8' plywood wraps around each of them with no cutting required. These straight, rectangular wings also mean only one size rib is needed and the stall should be gentle and predictable.

I'd like to keep the empty weight under 250 pounds to keep the stall speed down and the performance up. A light weight 40-60 HP engine, possibly a two cylinder motorcycle engine or even the 1.0 liter Geo Metro engine would be about right for this design. You gotta have a name for this thing too, so I'm going to call it the Puffin. Finally, the idea could be extrapolated into a 2 or 4 place aircraft as well. I'll bring my drawings to the meeting and I look forward to seeing all of you again. --Vernon


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