Weight and Weight/Balance
FAA Part 103 limits the empty weights of an ultralight as 254# or 278# with a parachute. A review of history will provide some insight into how the FAA arrived at these figures.
The Beginning
Federal Register / Vol. 46. No. 143 / Monday. July 27, 1981 / Proposed Rules
In July of 1981, the FAA's Notice for Proposed Rulemaking regarding ultralights aka powered hang gliders was published in the Federal Register. Due to the growing popularity of hang gliders in the 1970's, the FAA had published Advisory Circular 60-10 in 1974 with recommendations for "safe flying" of hang gliders. Hang Gliding activity continued to grow with flights exceeding 500 feet AGL and with the addition of engines and other control surfaces making some powered "hang gliders" approaching the definition and performance of certificated aircraft. There were a few instances of near miss encounters between ultralights and airliners. It is estimated by 1982, there were more than 10,000 ultralights flying in the US.
Given the FAA's mission to protect the safety of air carrier operations and generally the safety of all airspace users, the FAA determined that additional regulations were needed to define an "ultralight vehicle" and differentiate from regulated "aircraft". The result was Part 103. One major definition of an "ultralight vehicle" is it's weight.
Unpowered "hang gliders" were set at 150#. Powered ultralights without an engine and fuel tank weighed more---in the range of 165-168#.
So where did the 254/278# weights come from? Federal regulators always seek to find examples of successful real life examples that rationalize the regulations. They found their best example in the most popular ultralight at that time: the Quicksilver MX weighed 254# empty and 278# with a BRS Canister Parachute.
In some countries, ultralight weight limits are defined by maximum takeoff weight. (e.g. UK max weight limit for unregulated microlight is 300kg=660# which equates to 380# empty weight carrying 250# pilot and 5 gallons of fuel.) The FAA like some countries (e.g. Germany) chose the vehicle's empty weight. (Germany defined an ultralight as 120kg with a rescue parachute. (264#=14# less than the USA weight) Using maximum takeoff weight makes more sense if the purpose of limiting weight is to limit performance so as to enhance safety.
The 254# Quicksilver was equipped with a Rotax 447 two cylinder engine. The 447 was heavy as two stroke engines go, as it was equipped with fan cooling and had cast iron cylinder liners. More than 15000 Quicksilver "vehicles" have been sold; by far the most popular ultralight.
Changes
Rotax stopped production of the 447 model, and Quicksilver adapted a highly modified version of the Hirth F23 engine as it's replacement. The F23 allowed the empty weight to be reduced to 250#. Modifications to the engine are not done in the Hirth factory or by Hirth personnel.
The Aerolite 103, aka the Aerolite 120 in Germany was originally designed in 1996 and first flown in 1997. It was more streamlined than the Quicksilver and weighed slightly more with it's dual surface wings, strut rather than wire bracing, front wind screen and compliant flexible "landing gear". With less drag, safety was enhanced since there was more time for a pilot to react in an engine out situation. With a flexible suspension, airframe is more resistant to "hidden" damage from hard landings with impacts at 4-5fpm. The Aerolite 103 with a Rotax 503 engine weighed in at 275#. (Weight of the 503 was about 8# more than the 447. The empty weight without engine of the Quicksilver came in at 165-168#. The Aerolite coming in at 173-178#--i.e. abut 8-10# heavier than a engineless Quicksilver.)
Clearly the Aerolite could not meet Part 103 with the Rotax 2 cylinder engines, so "legal" Aerolites were equipped with one cylinder engines in the US and Germany. (e.g. the Hirth F33)
But, with the introduction of more modern "all aluminum" engines with Nikasil cylinders not requiring fan cooling, being more than 20 pounds lighter than the Rotax engines, the Aerolite 103 could meet the 254# limit with a two cylinder. Just as the F23 Hirth replaced the Rotax in the Quicksilver, the MZ201 engine became a viable engine choice for the Aerolite 103. The MZ201 installed is lighter than the Hirth F23. The F23 has horizontally opposed cylinders requiring two mufflers and two carburetors. The MZ201 has inline cylinders requiring only one carb and one muffler. The Kawasaki 440 is slightly heavier than the MZ201 but enough lighter than the Rotax 447 and the F23 that it does meet the 254# limit using a recoil starter. A flight by 14 year old Scott Henry from Virginia to Oshkosh demonstrated this as shown in the video shared in the above “Transition Training” post.
The greatest advantage of the 2 cylinder engines in ultralight vehicles is the climb rate. The Quicksilver with Rotax 447 and the Hirth F23 advertised a climb rate of 850 fpm. The Aerolite 103/120 with a one cylinder engine advertises a climb rate of only 500 fpm. (Less at altitudes above sea level and/or on a hot day) With the two cylinder, this jumps to over 900 feet per minute. Higher Rate of Climb is important in regards to safety-- faster takeoffs on grass strips, easier to avoid obstacles, better able to recover from downdrafts or stalls, easier to avoid wake turbulence during takeoffs, and less time running engines at full power. On a hot day, at 1000’ MSL, a single cylinder ultralight might only climb at 5 feet per second and would only clear 80 feet on a 1000 ft runway. A twin engine craft would probably clear the 150 foot obstacle with 1000 foot of runway. Although some 1940 vintage general aviation planes climb about the same as a single cylinder ultralight, the planes generally operate with longer runways. An underpowered ultralight flying off a short private field might attempt to clear an obstacle with too steep a climb and risk an unintended stall.
Weight and Balance
It is important to establish a datum for weight and balance--often it is simple to establish this as a component of the vehicle--in this case, the datum will be the center of the tire. The CG can be established as slight behind the tires (empty) and somewhere forward of the tires with fuel and pilot. The Aerolite 103 will lean back on the tail wheel when empty, but will land as a tricycle gear plane using the front tire for steering when pilot over 100 pounds is on board. Pilots weighing below 150# may require some ballast in nose to insure level flight without excess elevator input. (180-220# pilot weight is ideal). Too light load of fuel and pilot will increase risk of unintended stall and would require excess stick/yoke forward position. Too heavy a load may require excess stick/yoke pull interfering in reaching desired climb. Weight on front tire when craft is loaded and pilot on board should be determined before flight.
Plane should be able to achieve Vne with power off and stick/yoke forward, and stall speed with stick/yoke at max pull. Level flight with little force on stick/yoke is considered ideal--especially for a plane that does not have adjustable trim.
The following video will show a lifting of a Aerolite 120 with a single cylinder engine with the center of gravity being just slightly to the rear of the tires.
Here are the actual results of my weighing my Blue and Green Aerolite 103 S/N 210713, nickname “BG” on 8/31/2022 with new Accuteck scale. 119#2oz+119#9oz+11#10oz =250#5oz without wheel pants. Almost exactly as expected from data in literature. CG calculations per manual show CG at 75” from datum (nose) full of fuel with 215# pilot—CG moves rearward 1 inch as fuel is depleted.
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