The HM-143 is an outgrowth of the original HM-14, with the following goals:
To accomplish this, the HM-143 borrows features from the Mignet HM-293, and to a lesser extent the HM-360 and the Wood Sky Pup. Some of these borrowings include:
Additional modifications may be made as detailed in future additions to these notes.
In short, the requirements of FAR Part 103 that not all Fleas meet are:
The empty weight has been achieved by some 1934-model HM-14s, even with the heavy engines available then. The maximum speed limitations are not likely to be a problem with a small engine and draggy airframe, and though stall-speed calculations are difficult because of the heavier-loaded front wing stalling first, the worksheet the FAA uses to determine compliance says the Flea fits just fine, and I'm not going to argue with them.
After the accidents concerning the original 1934 HM-14, it was revised in several ways. It received
These changes resolved the diving tendency that out-of-balance Fleas of the old design had displayed. The new airfoil reduced the tendency to dive; the counter-rotating rear wing increased pitch authority to combat the dive, and the longer fuselage improved longitudinal stability. The final point addressed the root cause of the problem, however: even the original Flea could have been flown safely at reasonable speeds rigged correctly and with the right center of gravity.
For that reason, I am using a much better airfoil than even the revised HM-14, the NACA 23112 section, which has a nearly constant small coefficient of moment. With care to keep the trailing edge of the front wing from overlapping the leading edge of the rear wing, and caution to maintain correct balance, there should be no diving tendency at speeds appropriate to low power and FAR Part 103's 55-knot upper limit. Therefore, I can dispense with the lengthened fuselage and save the weight that the extra foot of length would add.
Mignet's plywood-skinned fuselage has some disadvantages for our purpose: it is not the lightest that could be built, and its heavy aft section contributes to tail-heaviness, which requires a longer engine mount, which is already too heavy because of the excess material required to achieve the necessary thrust line, far above the nose of the fuselage. All of these points could be rectified with a fabric-covered fuselage similar to a shortened Kimbrel Banty, with a bed-mounted engine and fabric-covered truss behind the cockpit.
A truss fuselage for an ultralight Flea is probably not necessary, and it would require more real engineering than I am comfortable with.
The HM-14 wing structure of 1934 is inappropriate for use with a modern airfoil, because it has no plywood leading-edge covering to maintain the correct shape; it is also barely strong enough for safety. The wing of 1936 is stronger and better-shaped at the leading edge, but it is unnecessarily heavy and trades speed of construction for use of a lot of (now very expensive) plywood. While the usual solution is to use the HM-360 wing, which is solid and appropriately-sized, it is too heavy for an ultralight. My solution is to resize the chord only of the HM-293, but keep the spanwise structure unmodified; since the span-loading of the HM-143 is less than a similarly-loaded HM-293, and the airspeed is lower, the structure should be more than sufficiently strong. This allows us to have a folding wing for easy transportation that is still light and strong.
The NACA 23112 airfoil is not only the one used on the HM-293, but has other advantages. It produces sufficient lift, has a low coefficient of moment, is easily derived by simple and broadly-available formulae, it was developed and has been studied by knowledgeable, unbiased people, and it has been used on Flea-formula aircraft for decades. It is also easy to construct compared to the theoretically slightly-better performing Fraser airfoils that have more history in conversation than flight. In any case, the airfoil of a fabric-covered wing in motion is only an approximation of the one the ribs define, and Fleas have flown successfully even with the much less well-suited NACA 23012; it would be easy to overthink this decision.
There is a difference between the mounting of the wings between the HM-14 and the HM-293: the front wings of the HM-14 are mounted on a three-legged pylon, partly to allow the front wing to rotate to a lengthwise position for towing, and the rear wing is mounted by bolts through a reinforced fitting at the center. The HM-293's front wing is mounted on two inverted v-struts, and the rear wing by bolts through the reinforced longerons. Since we have folding wings, there is no need to rotate the front wing, and the inverted v-struts are more stable and allow better visibility. If the rear wing bolts are through the longeron near a bulkhead, the load is better transferred to the structure of the fuselage. In both cases, the HM-293 mounting is to be preferred.
The landing gear of the 1934 and 1936 Fleas is not great: it is heavy, labor-intensive to build, too short, and liable to nose-overs. For this reason, the HM-143 adopts a landing gear adapted from the Wood Sky Pup, which is lighter and simpler, even though it does nothing about the practical flaws of the Mignet design. In this system, the bungee- and steel-tube forward gear is replaced with a tapered "spring" of maple, making it lighter and wider, more resistant to ground-looping, or what Mignet calls a "wooden horse" in reference to a carousel. A pad of spruce pieces, like the one below the seat, distributes the landing forces among the fuselage members.
Mignet's tail gear, consisting of dual tailwheels on a heavy steel post, is also far too heavy. It is replaced in the HM-143 by a single tailwheel fixed to the rudder, as in the latest versions of the HM-293.
While the main wheels are too far forward in relation to the horizontal center of gravity, worsening the craft's ground-looping tendency, that cannot be remedied without lowering the vertical center of gravity, or the nose-over tendency would be worsened. The balance between these two problems is difficult, but at least the improved tailwheel should make ground-loops slightly easier to control.
The HM-293 rudder is lighter and more stable in structure than the HM-14 rudder. It can easily be built in an appropriate shape and with the right dimensions for the HM-143. There are really no downsides to this choice.