[ChiefPilot]

(subtitle: I'm even less smart than I thought I was)

One of the cool things about building an RV is, as others have noted, the people you meet along the way. One such individual I have come to know over the last couple of years was trained as an aerospace engineer but wound up driving an (Air)bus to pay the bills.

I've always wondered about the details behind gross weight increases on RVs. The general consensus, for an RV-6A anyway, has been multiply the aerobatic gross by 6 then divide by the desired gross to see what the max load factor at that weight would be. Van's has said, via the RVator I believe, that which there is some vague substance to this theory it isn't really accurate or complete or words to that effect. This weekend I learned why.

The "aha!" moment for me came while discussing this topic with my friend Tom. Looking at the -6/6A spar plans and doing some basic math for various loadings (1375lbs @ +6gs, 1650lbs @ +4.4gs, 1800lbs @ +3.8gs) suggests that the wing spar is not overstressed at the 1800lbs gross weight that is commonly used with the -6/6A aircraft. In fact, using this math suggests that at 1800lbs a load factor of slightly more than 4.4gs could be applied. So what gives?

Tom opened my eyes to several possibilities as to why 1800lbs might not be a great idea. While I've seen discussions here around strength of the landing gear, conservative margins, etc. I hadn't read anything regarding control & stability issues.

While the A/C might be just fine structurally at 1800lbs when flown at +3.8gs or less, what happens to control stability as 3.8gs is approached? As the load factor increases, the angle of attack increases, and as the AOA increases, the CoP moves forward on the airfoil closer to the center of gravity (CoG). If the CoP moves forward of the CoG, the aircraft becomes unstable in pitch.

Tom asked if I had actually flown the aircraft at 1800lbs in phase one. Yes, I had, and I done with with the CoG in two locations. What were stick forces like he asked? Much lighter in pitch than at lower weights, but still manageable and the aircraft still demonstrated positive stability in pitch but with a much longer recovery period/reduced damping action. Did I actually fly to 3.8gs in that configuration, he asked? No, I hadn't - it didn't occur to me that doing so would yield any useful information. My eyes were opened.

Net result is that I will develop a test card (or two) to explore this area a little further, and until I complete those tests again in my phase one area I'm temporarily limiting gross weight to Van's recommendation pending outcome of stability tests at the higher weight.

I'm a little embarrassed at my lack of knowledge in this area, but I'm really happy to have had the opportunity to learn more about this topic to fill in the void.

 

[Mike S]

Ahhhhhh the things you can learn on VAF

 

[Snowflake]

Another consideration worth thinking about is, where will the extra 200lb go? Remember, you're not distributing that 200lb uniformly across the aircraft. With the light glass panels these days, and the small variation in empty weight of the basic airframe from build to build, you'll more likely than not put all of that extra 200lb into the passenger or baggage compartments.

Let's assume you have a light RV-6 at 950 lb empty, and you've got full fuel of 228 lb. That leaves 192lb for the pilot at aerobatic gross. That means your seat pan is supporting 192*6 = 1152lb at max operating G.

At 1800lb, assuming you started with the same plane and pilot, where will you put your extra weight? Adding a 200lb passenger more than doubles your seat pan load (192 lb becomes 392 lb) but you're only limiting your max G load by about 30%. So at your max G load, you're applying 392*4.4 = 1724lb to your seat pan.

Of course, that's not the whole story either... You could be 300lb and fly with 1/3 tanks to do aerobatics. The important thing to take away is that the spar isn't the only component that you need to analyze structurally.

This might be a good time to ask once again: People here on VAF have reported that Van used to issue letters to RV-6 builders authorizing gross weight increases (they don't anymore). I would be very interested to see what the wording of that letter is, whether it imposes any restrictions or makes any recommendations. In 10 years I have not been able to find anyone who holds such a letter.

 

[ChiefPilot]

Another consideration worth thinking about is, where will the extra 200lb go? Remember, you're not distributing that 200lb uniformly across the aircraft.

Excellent point.

I think it would almost certainly need to go into the seat pans - the baggage area has its own 100lb limitation and making the fuel tanks bigger, while not unheard of, isn't super common and for sure isn't anything I've done.

The interesting thing to me about the discussion was that stability can be a factor - stall speeds, climb performance, etc. was obvious to me but stability wasn't.

 

[Bagarre]

You also have to consider performance in a go-around with full flaps at gross and a 'drop test' on the gear (fairly sure tail draggers must clear by 9 inches at 1.5G deflection).

I don't think the go-around test will be an issue in an RV but that was one of the reasons the Cessna 172's flaps were reduced to 30 degrees from the earlier (and much nicer) 40 degres. They couldn't get the needed rate of climb at 40 degrees flaps at the new gross weight.

Now since the RV is experimental, NONE of this NEEDS to apply but I'm pretty sure Vans was taking it all into consideration when calculating gross weights.

It's much more than just the strength of the spars.