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I understand that in a tandem layout( 1 wing at front of plane, 1 wing at rear, both have positive lift) , the Cp has to be ahead of the Cg, and the Np behind the Cg.

Is there a rule of thumb how far forward the Cp has to be in front of Cg?, like 17% to 33% of MAC, etc.

Should Cm vs alpha, still have a negative slope?

Is it possible to have a 50% lift split between rear and front wings, just have a higher incidence in front wing, so it stalls first.

Any help would be appreciated

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    $\begingroup$ Cp of the forward wing will be ahead, Cp of the read wing will be behind and overall Cp will coincide with Cg, otherwise the aircraft will pitch up or down (because Cp is, by definition, the point around which the pitch moment is zero—if it does not coincide with Cg, then the net moment is non-zero and the aircraft accelerates in pitch). $\endgroup$ – Jan Hudec Jun 12 '19 at 20:29
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I understand that in a tandem layout, the Cp has to be ahead of the Cg, and the Np behind the Cg.

No. Yes.

For a true tandem (without horizontal tail), it makes little sense to talk about an isolated wing. The combined CP will always coincide with CG (in a static flight). Otherwise, as Jan already noted, the airplane will pitch up.

For NP (neutral point), see below.

Is there a rule of thumb how far forward the Cp has to be in front of Cg?, like 17% to 33% of MAC, etc.

Forget about CP. It's useful in aerodynamics (and somewhat in structural design), but for the dynamics and stability analysis only NP matters. CP is very inconvenient, it moves all the time.

All these 'rules of thumb' apply to NP, not CP. In particular, the distance from CG to NP (CG being ahead) in terms of MAC is your 'static stability margin', and is the main contributor to static stability.

The rule is roughly the same regardless of the aerodynamic configuration. Some minor effects will be different (for example, damping will probably be lower), which may require some minor adjustments.

Should Cm vs alpha, still have a negative slope?

For static stability, yes. This is essentially the definition of static stability; how you achieve this slope is another matter.

Is it possible to have a 50% lift split between rear and front wings, just have a higher incidence in front wing, so it stalls first.

Yes. You'll normally need a somewhat smaller front wing for that. Or you can have a swept front wing and straight rear. Note that if you want a stable aircraft, at a higher angle of attack by definition the lift on the rear wing must rise more than on the front one. If you want to keep this AoA (e.g. at a lower speed), you'll need to dump this extra gain via re-trimming.

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  • $\begingroup$ The lift curves should "cross" before stall at higher AOA, at lower AOA you want more lift in front to "pull up". This all of course remains dependent on torque around CG as well. $\endgroup$ – Robert DiGiovanni Jun 13 '19 at 13:11
  • $\begingroup$ "[...] simply by pitching the fore wing to a higher AOA so it stalls first" And what will happen when the aircraft pitches down and the front surface finds itself less loaded than the aft one? Sounds like a poor solution. $\endgroup$ – AEhere supports Monica Jun 13 '19 at 14:08
  • $\begingroup$ Start with a "conventional" plane. Make the Hstab flat and as large as you like. Set the decalage to optimal wing AOA. With proper CG position the plane flies entirely on its fore wing. If weight is moved back to make a "tandem", then rear wing must generate lift. But the fore wing must stall first. One can progress all the way to XB-70, or stop at tandem. $\endgroup$ – Robert DiGiovanni Jun 13 '19 at 16:35
  • $\begingroup$ The downwash from the front wing already ensures a lower lift curve slope on the rear wing, and sweeping the front wing will make it stall at higher angles of attack - not really what you want. the lift on the rear wing must rise more than on the front one please add "relative" to lift. Otherwise this would imply a higher lift curve slope on the rear wing (by using flaps?) which will not help in terms of stability. $\endgroup$ – Peter Kämpf Jun 13 '19 at 16:44
  • $\begingroup$ @AEhere, it's a bad wording on Robert's part—he apparently meant by setting it at higher angle of incidence. $\endgroup$ – Jan Hudec Jun 13 '19 at 19:12

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