Building and Flying the Hirsch HU-15 Flying Flea: A Practical Walkthrough

I picked up a copy of

The Flying Flea Callie And Me Bill Wallace

a few years ago when I was deep into my second homebuilt project. It is not a modern, glossy how-to with exploded diagrams and step-by-step photos. It is a written account of Bill Wallace's experience building and flying his own Flying Flea, which he named Callie. What makes it useful is that it covers the things nobody thinks to write down until it goes wrong. The Flying Flea design comes from Maurice Hirsch, a French engineer who created a very lightweight, fabric-covered, tractor-conversion biplane in the 1940s. It uses a simple tube-and-wood framework, fabric covering, and typically a small Lycoming or Continental engine in the 40-to-65 horsepower range. The original designs were distributed as kits and plans, and thousands were built over the decades. The beauty of it is in how cheap and light it is. The problem is that being cheap and light means every decision matters more than it would on a heavier aircraft. What you actually get from the book

Wallace does not rehash the official plans. He went through his own build and tells you where the plans were vague or where real-world construction deviated from the published specs. For example, he talks about wing rib alignment and how slight misalignments compound into control issues that show up first at low speed. He describes how he ended up truing the wings by adjusting the cabane strut attachments rather than trying to bend the ribs into place, which is the kind of detail you find only through experience. He also covers the engine selection process and why he settled on a Lycoming O-235 rather than pursuing a lighter option. The reasoning is practical. He had access to an O-235, and while it pushes the weight toward the upper end of acceptable, it gives you enough reserve power for rough field operations. That matters more than the theoretical weight savings of a smaller engine you might struggle to keep running. Key construction points that catch people out

One thing that trips people up repeatedly is the firewall mounting. The plans assume you have a very precisely located engine mount already. If your engine mounts are off by even a quarter inch, the propeller clearance becomes unpredictable and the vibration pattern changes in a way that will loosen fasteners over time. I learned this myself on my first build when I discovered that my firewall bracket alignment was slightly skewed after I had already bolted the engine down. The fix was not to move the engine. I shims between the mount pads and rechecked everything with a straightedge and a dial indicator before re-torquing. Another area is the fabric application. The original Hirsch recommendations call for cellulose nitrate dope, which shrinks aggressively as it cures. You need to apply multiple thin coats rather than one thick coat, and you need to let each layer dry completely before the next. Rushing this process leads to wrinkles that are nearly impossible to remove later. Wallace mentions this briefly but the practical implication is that fabric work on a Flying Flea takes about twice as long as you think it will if you are doing it right. Flight characteristics and what to expect

Once the build is done and you get it airborne, the Flying Flea behaves like what it is: a very light, low-wing-loaded aircraft with short wingspan and high maneuverability. The stall speed is genuinely low, often in the low forties depending on weight and configuration. Takeoff and landing distances are short. The aircraft responds quickly to control inputs, which is engaging but requires attention at low altitude during the initial approach phase. One thing worth noting is the crosswind handling. The aircraft is light enough that gusty conditions can make the final approach feel restless. You need to be comfortable making small, prompt corrections rather than trying to hold a rigid glidepath. I found that using a slightly higher approach speed than you would in a heavier aircraft and then flaring early gave me more consistency on landing. Limitations and when this design is not the right choice

The Flying Flea is not suitable if you need cross-country speed or baggage capacity. The fuel capacity is limited, and the payload is modest. If your primary goal is to carry two adults comfortably over any distance, you are better off looking at a different design altogether. A Rutan Quickie or even a standard J-3 Cub will serve you better for those purposes. The Flying Flea is for someone who wants a simple, affordable, fun-to-fly aircraft for local flights and field operation. There is also the matter of parts availability. While the design is well documented, some of the original components like specific strut fittings or fabric types are harder to source now than they were thirty years ago. You may need to fabricate replacements or adapt components from other light aircraft projects. Where to find the plans and related materials

The original Flying Flea plans are in the public domain through various archive sources. The EAA and experimental aircraft communities often have copies or reproductions available. Bill Wallace's book is available through used book channels and occasionally through specialty aviation literature sellers. It is not widely stocked new, so expect to order it or find a used copy. If you are serious about building one, combine the plans with Wallace's account and the community resources. The forum threads at Homebuilt Aircraft and similar sites have builders who have worked through the same issues he describes. Reading his experience alongside those discussions gives you a much clearer picture than either source alone.