If you've ever flown an airplane at max gross weight, you've definitely noticed a decrease in climb performance compared to when you're light. Here's why it happens...
Several Factors Affect Climb Performance
For propeller-driven aircraft, climb performance is directly dependent upon your plane's ability to produce excess power.
For the purposes of this article, excess power means any reserve power your engine has above what's required to maintain level flight. Excess power can also mean converting kinetic energy (airspeed) into potential energy (altitude), but we'll save that for another article.
Excess power is affected by weight, altitude, and aircraft configuration. Depending on how you plan your flight, all of these factors work in combination, resulting in an increase or decrease in climb performance.
According to the FAA, "an increase in weight, an increase in altitude, lowering the landing gear, or lowering the flaps all decrease both excess thrust and excess power for all aircraft. Therefore, maximum Angle-of-Climb (Vx) and maximum Rate-of-Climb (Vy) performance decrease under any of these conditions" (FAA PHAK 11-8).
Weight Makes a Big Difference
Aircraft weight is one of the most significant factors for decreased performance, but the reason isn't as simple as "the airplane is heavier, so to overcome gravity you need more power."
Increasing an aircraft's weight affects its climb performance in two ways:
1) A change in weight changes the drag and power required.
2) A heavier aircraft needs to fly at a faster speed to achieve Vy.
Higher Weight = Higher AOA
When weight is increased on an aircraft, it needs to fly at a higher angle-of-attack to produce more lift, opposing the aircraft's increase in weight. This increases both the induced drag created by the wings and the overall parasite drag on the aircraft.
It's the same principle as increasing the AOA through flap extension, as pictured below.
So how does that increased drag affect climb performance?
Power is needed to overcome any increase in drag. This trade-off means there's less reserve power available for climbing in a heavy aircraft.
The Heavier You Get, The Faster You Need To Fly For Vy
Vy is your best rate of climb speed, and it changes with weight. Most aircraft POHs only publish Vy for max gross weight, and admittedly, Vy doesn't change significantly with weight in most aircraft.
That being said, if you look at the diagram below, you can see that as weight increases, the horsepower required curve (gray curve) moves up and slightly to the right.
The heavier your plane is, the faster you need to fly to truly achieve Vy. Keep in mind though, your POH most likely published Vy for max gross weight. So if you're under max gross, you actually need to fly slower than your published Vy to achieve your weight-adjusted Vy speed.
How many knots does Vy change with weight? It takes some relatively complex math to figure that out, but a general rule-of-thumb is that for every 100 pounds under max gross weight, decrease Vy by 1 knot.
Weight And Climb Performance
When aircraft weight increases, climb performance decreases. And it decreases because you have less reserve power to climb.
Have you ever tested the performance difference across different flights? Tell us in the comments below.
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