gross weights

[papakeith]

I've noticed that there are a broad range of gross wieghts listed for the same models. As an example I see RV 6A's listed anywhere from 1650 to 1950.



Why/how is this possible? Aren't the gross weights for a particular design determined by the kit designer?

[avaviat]

The builder is the manufacturer. The builder writes the POH. The builder can placard any limitations they want, within limits. A DAR must sign off on the limits as I understand it? Read FAR part 23 Subpart C for what certified planes are allowed. Specifically 23.337. I doubt anyone would give you an airworthiness certificate if you grossly violated those limits.

You need a load factor of 3.8 to make "utility" category. 1950lbs (from your question) * 3.8 (utility load factor) = 7410 lbs load on the wings. 1400 lbs (acrobatic max for a 6a) * 6 (acrobatic load factor) = 8400 lbs load on the wing. 7410 is less than 8400, ergo the builder declares it safe. In fact, they could've declared it to be 2210lbs and been within that utility on takeoff.

One catch is that the landing gear and other systems might not be designed to handle the loads of a 1950lb landing, or even taxiing at 2210lbs. The stall speeds will also be higher. That's where you get "max takeoff" vs. "max landing". I've flown a plane that at a 1692lb max takeoff weight and a 1620lb max landing weight. If you filled it up to max gross on takeoff, you had to cruise 2.5 hours (4.8GPH burn) before you could land!

Anyway... that's what I know... I'm sure others will fill in the gaps.

[Spike]

If your looking at the Van's website, its typically because of engine size. Many times max gross weight is affected by the power of the engine. Certificated aircraft also have performance criteria for determining max gross, not just structural strength. Take for instance the Cessna 172. The max gross weight is increased when putting on a 180HP engine because the aircrafts performance (bulked landing) is better, not because the airframe is stronger.


-- John

[avaviat]

See... others will fill in.

Dan's w&b database shows -6As w/ 160HP at grosses from 1650 to 1950 and 180HP from 1800 to 2000lbs. There is a 200HP w/ a gross of 2100. So the general trend is exactly as Spike suggested, but there is still quite a bit of fudging.

[Dan A]

The late Bill Bennett told me, when he demoed the RV6 with me, Van set the gross weight to get the performance figures. and the real gross weight was much higher. I guess it depends on what the builder wants to do with his / her plane and what limits you test it to.
Dan