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C of G and load envelope

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C of G and load envelope

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Old 11th Jan 2019, 18:20
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C of G and load envelope

I have been playing with the load/balance figures in the C172M POH - I made a simple spreadsheet so I could easily try different combinations of weights and positions.

There is no mention of how handling and performance deteriorates/improves as the intersection of moment and weight moves around within the polygon of the envelope. It appears that we go from 'ok' to 'not ok' instantly when leave the envelope. I am sure that is not true, so I guess my question is, what does the performance gradient look like within the envelope? If it was surface on the z axis, would it be gently sloping? and where would the peak of optimal performance handling be?
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Old 11th Jan 2019, 20:41
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I think the (left and right) edges of the envelope are the places where you run out of elevator authority at slow speed: The elevator hits the stops before the required (certified) aerodynamic effect is reached. Those stops are literally that - hard stops, so there's no real gradient at the edges. You either hit the stops trying to achieve what you want to achieve, or you don't. Although there will be a safety margin, of course.

As far as flying characteristics is concerned within the envelope, obviously with higher weight comes lower performance (take-off distance, rate of climb and landing distance mostly, although there's also an effect on cruise efficiency, and the speed at which the best glide is found).

Move the weight fore and aft and you'll find that with a forward CofG the aircraft is very speed stable (it recovers quicker from speed excursions), and with an aft CofG the aircraft is more efficient: It cruises faster for a given power setting. The latter is explained because the tailplane doesn't have to work as hard keeping the tail down, which means less induced drag from the tailplane.

And then there's the effect on morale from rear-seat passengers. Depending on who you bring, this can be positive or negative.
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Old 11th Jan 2019, 21:29
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The weight of the airplane has the predictable effect on performance.

If your C of G is too far forward, you may stall the elevator as you apply nose up control to raise the nose, particularly when heavy. Form the factory, GA planes are usually fairly safe from this. However, once a number of mods are installed (larger engine and amphibious floats, for example) the C of G can move forward (and the plane gets heavier too). I've had a few planes, which during testing were not adequately able to raise the nose to flare power off. One example was a 182 amphibian (search Youtube for "Rompene 2014"), which was right at the forward limit if you flew it solo. And, it ran out of up elevator in the flare, particularly on the water, where you want the nose really high. This was in part because of the mods on the airplane, and in part due to the installation of wing extensions and a STOL kit, but no corresponding changes to the tail to compensate for an increased pitching moment of the wing. The solution I developed was a vortex generator installation under the horizontal stabilizer which increased the effect of up elevator by preventing it stalling. That, and 50 pounds of ballast carried in the furthest back float compartments, and the plane flew well. The ballast was carries as tools, spares and emergency supplies to be useful. If used, they were to be replaced with rocks or such for the flight home.

If the C of G is too far aft, the effect of the natural [designed in] characteristic to lower the nose in a stall is reduced. More seriously, too far aft C of G will dramatically effect spin recovery. During spin testing of a Cessna Grand Caravan I flew, I was required to demonstrate the spin recovery at both C of G limits. The forward limit test was straight forward. Very nose down, and a high rate of descent, with high speed and G recovery, but predictable and recoverable (I put a video clip on Youtube a while back, "C 208 spin" or something like that). The same aircraft at the aft C of G limit would not lower the nose well when I moved the controls forward. I held the control wheel hard forward against the stop, as the aircraft spun around nearly level, though going down flat and vertically. The aircraft did recover, but very differently than the forward C of G. I sensed that there was not much more control available to me to recover the stall.

So what I learned was that in "normal" flying, you will have less sense of how the plane is loaded, or misloaded, but, during more extreme maneuvering, you may find shockingly that the plane will not do what you expect it too.
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Old 12th Jan 2019, 03:38
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Another consideration: One of the certification requirements is that the plane must be flyable by a pilot of average skill. That is one of the things test pilots assess during testing. You don't know how rapidly any deterioration occurs from that 'limit', once you exceed the envelope. And what if you're at the...um...lower end of the skill curve? You will find the limits of your skill sooner than, say, a test pilot.
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