How Fat can you get it.
pierre smith said:
How do I go about getting a higher gross weight listed for my airplane? I've read a story or two here but do I fly it at increasingly heavier weights and document that, or do I also have to weigh the airplane and calculate the CG to back up the flights?
Great question, get this, put down what ever you want!
Just kidding, but I was a structural engineer for Boeing, so I do have respect for airframe strength and loads. My comment about what ever you want is not too far off . You can write down the limits you want as the manufacture, even if it is more than Van's spec. However I also have a lot of respect for Van and his limits. They are there for a reason, a margin of safety.
With that said, to answer your question, 1800 lbs (heavy) max gross is not unreasonable for a RV-6, but with some caveats, cautions, disclaimers, explanations and limitations. Since you are the manufacture you can write down what you want, within reason, not that I think it is a good idea or saying you should do it. However here is what to consider.
Higher gross means you are affecting:
strength,
controllability and
performance.
As far as strength from Van's spec's you have an aerobatic airframe (+6/-3Gs) at 1375 lbs. If you look at the standard FAR 23 normal/utility load factor +4.4/-2 Gs, you could ratio up the aerobatic gross to determine the a "normal" gross.
1375 lbs X 6G/4.4G = 1875 lbs. (17% above standard 1600 lb gross)
The FAA allows aircraft in Alaska to exceed gross by 15%, FAR 91.323(b)(2). Why or How can they do that? They are waiving or reducing the margin of safety built into the structure.
When you are increasing the gross over Van's spec you are reducing the margin of safety. You must recognize this and respect it. That means flying more conservatively. Remember you are sacrificing structural margin for more payload weight, but done with intelligence the extra risk is small if done with caution. Also what about landing weight? Those 5x5 tires are small and the model "A" nose tire is real small. I would be careful about taking off a soft field with fwd CG and heavy weights.
The other parts of the equation is control and performance. As a pro-pilot you know the difference between having the hopper full and empty.
As far as control never exceed Van's CG limit. Period. In fact stay well with in the CG and not near the limits at the "extend" gross would be advised. Also when you fly avoid the corners of the speed, performance and maneuver envelope. Also in turbulence it would be advised to fly at or below Vno, 180 mph (structural cruise). Consider reducing Vno by 1 mph for every 20 lbs over 1600 lbs gross.
As far as performance the RV has excess performance and can stand more weight, however you are correct you should document calculations or flight test results of at least the following parameters at the new gross: Stall speed, take-off/landing distance, climb rate, cruise speed/fuel burn and best glide speed. The RV is great but a RV-6 fixed pitch, high/hot airfield at gross is going to suck wind, so don't over estimate the capabilities either.
The Key is to build a light RV. The lighter the plane the better it performs, the more efficient and the better it handles.
A side note: I have only talked to max gross for normal flying, not aerobatic gross. That is the problem of building a heavy RV, you end up with a solo only aerobatic plane. Of course some justify flying above the "designed" weight by saying I will fly "light aerobatics". That is cool, since all typical aerobatics can be done at 3-3.5 Gs if you know what you are doing. However you can easily screw up and go right to 6 Gs if you are not careful or know what you are doing. A RV is an OK aerobatic plane but builds speed fast. Be careful. As I said build you RV light, especially if you want to do dual aerobatics.
Regards George (Former structures engineer for Boeing)