Explain Gross Weight for Engine Choice

[skelrad]

What drives a gross weight changing with an engine size? I guess I expected the recommended gross weight to be the same for an airframe regardless of the engine in it, but the 9A has a different gross weight recommendation depending on the engine.

[RV8JD]

The engine horsepower would make a difference in the airplane's performance depending on maximum gross weight. That's maybe why Van's has the lower gross weight limits with smaller engines, in order to maintain their performance goals for the airplane, e.g., takeoff distance, rate of climb, etc.

However, the Van's RV-9/-9A Weight and Balance Data Quick Reference documents only show the Recommended Gross Weight of 1750 lbs (and the Maximum Utility Category Weight of 1600 pounds), with no mention of any horsepower-dependent recommended gross weight limits.

RV-9:

https://www.vansaircraft.com/wp-cont...9/01/RV9wb.pdf

RV-9A:

https://www.vansaircraft.com/wp-cont.../01/RV9Awb.pdf

[airguy]

My DAR granted me 100 pounds over based on my IO-360 and constant speed prop. He said anything more than that, and he wanted to see some engineering and performance test data, but he would grant 100 pounds for the engine/prop combo.

Of course, when I'm loaded close to gross, I'm carrying that excess weight as fuel in the wings very near the CG line too.

[rocketman1988]

Interesting because as the manufacturer of the aircraft, you can assign any weight you want as long as it is tested during phase 1. Wasn't there a guy that put a 3600 lb gross on an rv-10? I had not heard of a DAR that requires engineering analysis for modifications...until now. The other question is if provided with said analysis, would the DAR have the credentials to verify the analysis?

[skelrad]

Huh. I hadn't given it a ton of thought. I just assumed the gross weight was what the airframe could physically "handle," as opposed to also making considerations for performance.

[Ironflight]

Originally Posted by skelrad

Huh. I hadn't given it a ton of thought. I just assumed the gross weight was what the airframe could physically "handle," as opposed to also making considerations for performance.

You’re not alone! Many pilots are told that gross weight is based solely on what the structure can handle, but in fact, many aircraft over history have their gross weights based on minimum climb performance.

Here in the RV world, with power-to-weight ratios that would be considered ludicrously generous in the Cessna or Piper world, it can be hard to see that - but check out the limiting performance on a C-150 on a hot day in Denver and you’ll see that they can barely climb.

So yeah….give thanks for the overpowered Rv’s, and have compassion for the folks that have to worry very day about density altitude…..