AC 103-7, 20,c.,(1)

c.  Acceptable Methods of Determining the Maximum Level Flight 
    Airspeed of an Ultralight.

   (1)  A calculation, using the information in Appendix 1, is 
        an acceptable method for making this determination.

       NOTE:  The engine manufacturers maximum horsepower rating 
       will be used for all computations associated with maximum 
       level flight speeds (unless the operator can provide 
       documentation from the engine manufacturer that a method of 
       derating an engine will result in a predictable reduction 
       in horsepower)

AC 103-7, 20,d.,(1)&(2)

d.  Use of an Artificial Means to Limit the Maximum Level Flight Airspeed.

    (1)  An artificial means of restricting the total power output 
         of an engine in order to lower the maximum level flight 
         speed at full power would be acceptable if the method 
         used to restrict the power available is one which cannot be 
         modified, bypassed, or overridden in flight and the pilot or 
         operator can provide the FAA, on request, satisfactory evidence 
         that the vehicle meets the requirement of Section 103.1(e) (3).

    (2)  As a general guideline, a method is unacceptable if it can be 
         modified, bypassed, or overridden in any way while sitting in 
         the pilot seat so as to further increase the power. There may 
         be some ultralights which could be operated as ultralight 
         vehicles if such restrictions are employed to meet the  
         requirements of Section 103.1(e) (3).  If you change or modify 
         the restricting elements, your vehicle may be ineligible for 
         use under Part 103.

e.  Use of a Less Efficient Propeller/Shaft Arrangement. 
    The use of a less efficient propeller/shaft arrangement to lower the 
    maximum level flight speed at full power is acceptable, if the 
    operator or pilot can provide the FAA, on request, satisfactory 
    evidence that the vehicle meets the requirements of Section 103.1(e) 
    (3).  If you change or modify that arrangement to increase the 
    efficiency, your vehicle may be ineligible for use under Part 103.

One can see that for a stall speed of 24 knots the maximum wing loading for a wing profile with lift factor of two is 3.90 pounds for square foot. But there is a little wiggle room in:

AC 103-7, 21

21. MAXIMUM POWER-OFF STALL SPEED OF A POWERED ULTRALIGHT VEHICLE. The maximum power-off stall speed of an ultralight vehicle cannot exceed 24 knots (28 mph).

Converting 28 mph to knots gives 28/1.15 = 24.3 knots. And so, may be I just squeek in with a calculated 24.1 knot stall speed.

This condition is the most difficult to meet. The wing area is the wing chord times the wing span, and the load is determined by the ultralight empty weight plus the weight of the standard pilot (170 pounds) and the weight of five gallons of fuel. Because the FireFly is complete, one is not going to add wing area. I don't want to give up the brakes, strobes, some extra instruments, or trim for safety reasons. The only way to get more into compliance is to install a lighter engine and/or to reduce fuel tank capacity.

1. Empty weight. The dry empty weight of the vehicle (excluding floats and/or parachutes), as established through some form of evidence satisfactory to the inspector, should be used.

2. Pilot weight. The pilot's weight will be assumed to be 170 pounds. In the event that an ultralight is being operated under exemption allowing two-occupant operations, the pilots' weight will be assumed to be 340 pounds.

3. Fuel weight. The weight of the fuel (6 pounds per gallon) is included and for the purpose of this computation will be assumed to be filled to capacity.

4. Wing area. The total wing area (square feet) should be determined. Ailerons and flaps may be included, but canards (which generally have a higher stall speed than the main wing) are excluded.

STEP ONE - Add the weight factors.  

Example:       Empty weight            240 lbs.
               Pilot weight            170 lbs.
               Fuel weight              30 lbs.
                                       440 lbs.

FireFly:       Empty weight            247 lbs.
               Pilot weight            170 lbs.
               Fuel weight              30 lbs.
                                       447 lbs.

STEP TWO - Divide the total weight by the total wing area to
           obtain the wing loading of the vehicle.

Example: Weight    = 440
         Wing Area = 151 = 2.9 (Wing Loading)

FireFly Wing Area = Chord x Length = 5.13 x 22 = 112.9 ft2

FireFly:  Weight    = 447
          Wing Area = 113 = 3.96 (Wing Loading)

STEP THREE - Select, from the wing profiles provided below, 
             the lift factor which applies to the ultralight 
             in question.

LIFT FACTOR	                  WING PROFILE



WING DESCRIPTION: Single/double surface with camber of less 
than 7 percent (see Appendix 3) and all symmetrical and semi-
symmetrical airfoils without flaps, regardless of camber.

LIFT FACTOR	                  WING PROFILE



WING DESCRIPTION:  Relatively flat-bottom, double surface 
wings with camber of 7 percent or more.

LIFT FACTOR	                  WING PROFILE



WING DESCRIPTION: Single surface with camber of 7 percent or 
more or double surface with flaps extending up to 50 percent 
of the total wingspan.

LIFT FACTOR

2.0

WING DESCRIPTION:  Double surface with flaps extending more 
than 50 percent of total wingspan.

This lift factor was picked for the Firefly.  The FireFly has a 
high camber wing with flaperons that extend over more than 50 
percent of the wing.

INSTRUCTIONS:

1. Enter the bottom of the chart at the computed wing loading of the ultralight (Example: 2.9) (FireFly: 3.96).

2. Proceed straight up the wing-loading line to the point where it intersects the applicable lift factor curve (Example: 1.6) (FireFly: 2.0).

3. Note horizontal line which also intersects at that point, proceed to the left side of the chart via that line. Read power-off stall speed (Example: 23.2 knots) (FireFly: 24.1 knots).

Relevant considerations for this computation are:

1. Chord or chord line - an imaginary straight line joining the extremities of the leading edge and the trailing edge. For the purpose of this computation, this imaginary line will be measured midway between the tip of the wing and the wing root.

2. Camber - the curvature of a wing. For the purpose of this computation, only the camber of the upper surface (above the chord line) of the wing is considered.

STEP ONE - Determine the mid-point of the wing and measure the mean chord line.

FireFly wing chord = 61.5"

STEP TWO - At the mid-point of the wing, determine the chord line horizontal position on the airfoil profile (assuming straight line from the extreme forward point on the leading edge to the extreme rearward point of the trailing edge).

STEP THREE - Measure the distance from the chord line to the highest point of camber on the upper surface.

Firefly measurement

STEP FOUR - Divide the measurement obtained in step one into the measurement obtained in step three.

Example:      Upper surface camber =   8 inches
              Mean chord line length = 100 inches = 8% camber

FireFly:      Upper surface camber =   6.5 inches
              Mean chord line length = 61.5 inches = 10.6% camber

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