HP-14 Weight and Balance Report

(Article courtesy of Alex Upchurch, who is co-owner of HP-14 #12, C-FWHZ)

 

Calculating HP-14 Weight and Balance

1. Place a 12 x 12 x 3/4" piece of plywood across two bathroom scales.

2. Place sailplane main wheel in center of plywood and block fore and aft to prevent ship from rolling off scales.

3. Place third scale on a box which is high enough to level the rear fuselage frame reference line. A level on the top center of the main wing fittings will indicate the proper attitude.

4. Record sum of both front wheel scales.

5. Record weight on tail scales.

6. Measure horizontal distance between main and tail wheel centerlines.

7. Remove ship from scales.

8. Record tare weight readings on scales.

9. Determine net weights on both wheels by deducting the tare from gross weights.

10. Enter weights in following table and perform the operations indicated.

 

Datum line = Leading edge of Mean Aerodynamic Chord = Aft side of seat back steel tube.

 

Item

Weight (lbs)

Arm (in)

Moment (in-lbs)

Main Wheel

 

-1

 

Tail Wheel

 

178

 

Parachute

 

-12

 

Pilot

 

-20

 

Baggage Compartment

 

19

 

Side Compartments

 

0

 

Totals

 

C.G.

 

 

C.G. (in) = Total Moment ¸ Total Weight = ___ in 

MAC (in) = Area ¸ Span = (138.3 x 144) ¸ (54.6 x 12) = 30.4 in

C.G. (% MAC) = C.G. (in) ¸ MAC (in) = __ ¸ 30.4 =___% MAC

Preliminary allowable C.G. range = 25 to 40% MAC

NOTE:

1. The moment arms for the main and tail wheel should be measured from the actual ship being weighed. The numbers in the table were measured on the prototype.

2. The pilot moment arm assumes you are upright in the seat. If you "submarine" at all it can have a noticeable effect on the C.G. location. The best solution is to place a larger scale (or 3 bathroom scales) under the main wheel and weight the ship with the pilot aboard.

Dick Schreder’s comments from the original W&B sheet dated 4/25/67.

"Stalls in the prototype loaded at 30.2% MAC are very docile. If forced into a spin, recovery is normal. Ailerons are unusually effective in checking rotation. Spin characteristics at more aft C.G. locations have not been investigated at this date. Although problems are not anticipated up to 40% MAC, it is recommended that light weight pilots flying near this limit check spin recovery by carrying ballast in the nose and dropable ballast attached to the rear fuselage."