Best CG location ?

So the design range for a 6 is between 68.7 and 76.8 from datum.

With fuel at 70', a CG near 70 would mean you don't get any shift with fuel burn. That's good.

But 70 is awfully close to the 68.7 and everything else you add (pilot, passenger, baggage) is all aft of 70 and we know we don't want to be flying around with an aft heavy plane.

So, what's your opinion of the best empty CG location ?

and we know we don't want to be flying around with an aft heavy plane

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Why?

An aft CG gives you a more pleasant handling experience, lighter control forces, less trim required. Agree, as the CG moves further aft, these characteristics eventually become undesirable and then unsafe. That's what the Aft CG limit is for.

I might have misunderstood your line of thinking, but in an RV I would prefer to be in the aft portion of the CG envelope - it's what an RV is all about

The CG on my RV8 is calculated from a datum point out in front of the spinner. So adding passengers bagggage and fuel does not necessarily move the CG aft. So it depends on the location of the datum. If the RV6 is also at an imaginary point in front of the spinner, adding passengers may move CG forward.

The datum is a reference point only.

The location of the datum has nothing at all to do with CG. It is simply a common point from which to measure.

Personally I use the leading edge of the wing as datum.

The center of the CG range falls forward of the pilot/passenger seats in an RV-6. Therefore any loading of people and baggage will move the CG aft.
The only thing forward on the -6 is fuel. Therefore as you burn off fuel, the CG will move aft.

cg effects

I don't agree that an aft cg makes for better flight characteristics. It does have some positive effects but overall I would prefer to be closer to the fwd limit than the aft. It will have better stall characteristics, it will be more stable, it will handle better in a crosswind landing...

I just installed a really light prop on my RV4 which moved the cg 2 inches farther aft and I can't wait until I can find a solution to return it to where it was with my heavier prop.

I used the example in the builders manual which placed the datum 50 inches fwd of the leading edge ( fwd of the spinner ). I think the advantage is that all of your moment arms will be positive numbers.

cm

Christopher Murphy said:

I used the example in the builders manual which placed the datum 50 inches fwd of the leading edge ( fwd of the spinner ). I think the advantage is that all of your moment arms will be positive numbers.
cm

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But the advantage of using the leading edge is that, my landing gear is exactly in line with the leading edge. That puts a lot of zeros in the equation. Zeros are VERY easy to deal with.
And there is no disadvantage in dealing with negative numbers. It's simple algebra.
There is no problem either way as long as you know what you are doing.

Christopher Murphy said:

I don't agree that an aft cg makes for better flight characteristics. It does have some positive effects but overall I would prefer to be closer to the fwd limit than the aft. It will have better stall characteristics, it will be more stable, it will handle better in a crosswind landing...

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The negative side to a far fwd CG is elevator effectiveness.
Full flaps at slow speed during an engine out forced landing (no prop blast on tail) can result in a situation where there is not enough elevator effectiveness to fully flare.
An extreme example is RV's with very heavy engines (as in 6 cyl). Depending on where the compromise was made in the final empty C.G. position, it could literally be impossible to make a forced landing at a slow (RV typical anyway) airspeed.

it will be more stable

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and this is where we differ I spend my day job flying boring stable S&L machines. The delight of the RV for me (and I think many) is the light handling given by reduced stability:

Elevator control is positive and quick, varying from light at aft CG to moderate at a forward CG....
Directional and pitch stability are positive for all loading conditions. Roll stability is neutral, not uncommon in short span airplanes with low dihedral angles. Rudder turn and side-slip qualities are quite normal, but rarely needed except in crosswind landings, which RVs handle nicely.

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[Vans] and with neutral roll stability, control harmonisation is better matched with the light elevator feel - for me anyway.

Just stay within the cg enveloped provided by Vans. The best position within that limit is a pilot preference based on experience, perception, and familiarity. Could be filed under never ending debate.

Italiancowboy said:

So the design range for a 6 is between 68.7 and 76.8 from datum.

With fuel at 70', a CG near 70 would mean you don't get any shift with fuel burn. That's good.

But 70 is awfully close to the 68.7 and everything else you add (pilot, passenger, baggage) is all aft of 70 and we know we don't want to be flying around with an aft heavy plane.

So, what's your opinion of the best empty CG location ?

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Looking back at your original question I feel nobody really answered it as it drifted to flying qualities and not loading.
My opinion is that you want the empty CG as far aft as possible and still be able to put 100 lbs in the baggage area behind the seats (max Van recommends) plus two adults in the seats and not be behind the 76.8 limit with zero fuel. That means any fuel state will move the CG slightly forward since for my RV-6A the empty CG is at 70.3.
I am not a big believer in getting the Empty CG as far forward as possible (which usually is done by big engine and heavy prop) because it tends you to believe you can add lots of weight in baggage or people weight that tends you to exceed Vans recommended max gross weight. I think the RVs flight best and funnest when light (that gets us full circle to flying quality preference). YMMV

cg preference

Who ever said preference within the limits is a good answer. I have flown my airplane from the fwd limit to the aft. I prefer it closer to the fwd...I have a light airplane with a light engine and I have never been out of elevator control in a full stall power off landing. The same may not be true for airplanes with the bigger engines.
Cm