Vs = 64
Normal category load limit factor
sq rt 3.8 = 1.949
1.949 x 64 = 124
Utility category load limit factor
sq rt 4.4 = 2.0976
2.0976 x 64 = 134.247
134 + 124 = 258.
258 / 2 = 129 mph
See what it looks as though they did? Right, simply average the utility and normal category calculated maneuvering speeds to arrive a a single published Va applicable to BOTH categories.
Interesting trick. I've never noticed this. But I think the numbers working out this way is just a coincidence. Or perhaps the manufacturer chose to do this for some reason, but it still leaves Va exceeding the regulatory requirement.
Remember that the stall speed changes with weight. So your calculated utility Va should be adjusted accordingly. Correcting for weight, the utility stall speed is 61 MPH, and the utility Va should be 128 MPH (rounded off). I think a more likely explanation is that the manufacturer identified the correct utility category Va and used it as the design speed (128 MPH v. 129 MPH could be simply a result of rounding error somewhere).
Try this for the dual category certified C-172 and see if you don't find a similar result!
Not in the models I've flown. The Va for the normal category that I've seen is the correct minimum-required Va. However, if you correct the Va for both new stall speed and new limit load factor at the max utility weight, you get an unchanged value for Va. Although if I recall correctly, the published Va is adjusted downward for weight (probably because you can still be in the normal category if the CG is outside utility limits, but that's just an educated guess).
In the Grob G-120A, the numbers are as follows:
Utility:
- Vs = 67 KCAS;
- LLF = 4.4 g;
- Minimum Va = 141 KCAS;
- Published Va = 145 KCAS.
Aerobatic:
- Vs = 66 KCAS;
- LLF = 6 g;
- Minimum Va = 162 KCAS;
- Published Va = 165 KCAS.
Interestingly, if you use the utility stall speed and the aerobatic LLF, you get to within rounding error limits of the aerobatic Va.