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As a general rule, must the cg of a tandem ultralight ( Re 500k-1,000k) be greater than 25% mean aerodynamic chord?

I understand the centre of pressure of a classic wing is at it's quarter point. It also produces a nose down moment, so that the cg and/or and horizontal tail moment need to offset this, or the plane will dive. To provide a nose up moment, the horizontal tail must produce a tail down force.

If you want a tandem airplane, you want the exact opposite.

So the to provide a nose up moment, I would imagine you want the cg always behind the Cp ( 25% MAC) of the wing, but in front of the plane neutral point, as you want to the tail to provide an offsetting tail up force,(nose down moment)

Is this correct?

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you want the cg always behind the Cp (25% MAC) of the wing, but in front of the plane's neutral point, as you want the tail to provide an offsetting tail up force

Yes, that is correct. The cg should be between the forward wing's quarter point and the common neutral point (NP) of both wings. However, if you want some agility when maneuvering the plane, it should be much closer to the neutral point. If both wings are of equal size, you definitely want the rear wing to contribute some lift. Since it flies in the downwash of the forward wing, it will see a smaller change in effective angle of attack and have a smaller lift curve slope, which already supplies some stall margin by helping to let the forward wing stall first. By shifting the cg a bit forward of the NP (10% to 25% of MAC should be sufficient), you will reduce lift on the rear wing further and increase stability.

I understand the centre of pressure of a classic wing is at it's quarter point.

Only if the wing has a symmetric or reflex airfoil. With regular cambered airfoils the cp is further aft and moves forward to the quarter point with increasing angle of attack.

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  • $\begingroup$ I don't think it's correct to say that downwash "supplies some stability". It supplies "safety", but by itself it reduces the pitch moment slope and hence stability. $\endgroup$ – Zeus Jul 5 '19 at 0:57
  • $\begingroup$ @Zeus: Agreed. But it does not reduce stability - the downwash only shifts the neutral point forward, away from the geometric mean of the quarter points of both wings. $\endgroup$ – Peter Kämpf Jul 5 '19 at 1:22
  • $\begingroup$ Technically yes; some prefer to say "pitch stiffness" for this. $\endgroup$ – Zeus Jul 5 '19 at 1:27
  • $\begingroup$ @Zeus: Interesting. For me, aerodynamic stiffness is proportional to and caused by dynamic pressure. $\endgroup$ – Peter Kämpf Jul 5 '19 at 1:30
  • $\begingroup$ That's apparently not standard, but I found this usage in Etkin, and this is a popular textbook. He argued (correctly) that this term (the pitch moment slope) is not sufficient to describe stability per se, so he called it "stiffness", similarly to any other similar first-order term (like for a spring). $\endgroup$ – Zeus Jul 5 '19 at 1:50

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