CofG and Longitudinal Stability

[Timothy]

May I split my previous thread - keep that on landing technique and start again on Dinga's point about rearward CofG?

My first question regards efficiency. How much difference is it going to make to a draggy beast like the Aztec to operate at the rear limit (let's say for the time being)? Obviously the less negative lift provided by the tailplane the less drag it will create and also the less positive lift and therefore drag from the wings, so I would expect the difference to be marked, or am I tipping at windmills?

Secondly, how alarmist is dinga being about the rear limit, in reality? You guys obviously test aircraft and set a rear limit, but presumably there is a fudge factor to allow for rigging problems, pilot technique, turbulence, icing etc? If all is well, and I am in a position to move things forward if necessary (we are two crew in freighter configuration) surely I can push it a bit?

If you all feel obliged to tut in public, but reckon in private that there is room for manoeuvre perhaps you would PM me.

Thank you.

[richatom]

Having played about a fair bit on DA42s I have found that flying at CG fully to the rear increases cruise speed at a given power setting by about 1.5%. You could probably take that as a ball-park figure for your Aztec. Even if you did all the calculations or did some controlled flight testing, I don't reckon the end result would be all that much different.

If you fly outside of the CG limits your plane doesn't suddenly fall out of the sky (well except in extreme circumstances). All that happens is that stability goes outside of limits of what a pilot (or autopilot) is expected to cope with in normal flight, but it is nothing dramatic. But don't plan to do it deliberately under any circumstances - the slight gains in speed and efficiency you might get from a slightly rearwards CG are vastly outweighed by the grief you will get at the enquiry or inquest if you crash..

[Timothy]

The idea of my being bothered by what happens at the inquest is somewhat metaphysical, but I take the point, thank you! :p