Power Loading

Power loading is the figure that tells us how many pounds of weight each horsepower the engine produces has to drag around (lb/hp). It can be a little counterintuitive because an extremely powerful airplane like Tsunami (above) has a much smaller number for power loading (7,200 lb / 3,500 hp = 2.1 lb/hp) than a Cessna 172S (2,550 lb / 180 hp = 14.2 lb/hp).

	gross weight	power	power loading	max speed
Cessna 172S	2,550 lb	180 hp	14.2 lb/hp	143 mph
Beech S35	3,300	285 hp	11.6 lb/hp	212 mph
RV-7A	1,800 lb	160 hp	11.3 lb/hp	199 mph
AR-5	661 lb	65 hp	10.2 lb/hp	213 mph
RV-14A	2,050 lb	215 hp	9.5 lb/hp	218 mph
DarkAero 1	1,500 lb	200 hp	7.5 lb/hp	275+ mph
SX 300	2,200 lb	300 hp	7.3 lb/hp	288 mph
Lancair Legacy	2,200 lb	310 hp	7.1 lb/hp	275+ mph
Tsunami	7,200 lb	3,500 hp	2.1 lb/hp	500+ mph

Both maximum speed (as seen in the table above) and rate of climb are correlated with power loading. Raymer suggests that for a fixed-gear smooth design such as we are working on,

$$ \scriptsize \frac{W}{P}=215V_{max}^{-0.61} $$

where is in knots. Since power required is a function of speed cubed and our desired cruise speed is 200 mph at 75% power, we’ll speculate that the top speed might be 220 mph, or 191 kt.

$$ \scriptsize \frac{W}{P}=215(191)^{-0.61} =8.73 \text{ lb/hp}$$

Because we have in mind a 200 hp engine, this suggests that we should limit gross weight to the neighborhood of 1,750 lb. But, of course, the above equation is not set in stone (you can see the variation in the table above) so we’ll run through the initial sizing process and check performance more accurately at the end.

$$ \scriptsize 8.73 \text{ lb/hp} \cdot 200 \text{ hp} = 1,746 \text{ lb}$$

Next up is the lift-to-drag ratio, which is the last thing we’ll need to estimate before moving on to weight estimation.