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Does a tandem ultralight need flaps on the rear wing if the front wing has flaps?

I understand that most Rutan canards don't have flaps as I was told if the main wing has flaps, the canards need one too.

Rutan did make one with flaps on both the canard and the main wing. I think it was called the Grizzly.

So... what about a tandem aircraft (with positive lift on the horizontal tail) with flaps on the front wing ? Does the rear wing need flaps too?

Can't you just use the horizontal trim tab as a flap if you need more rear lift to offset extra lift of the main wing with flaps extended?

After all, wouldn't most conventional aircraft with rear cg and slow flight ( low Cm) in a dive with a symmetrical horizontal tail, with elevator down, be generating positive tail lift anyways, so they are acting like a tandem aircraft, no?

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  • $\begingroup$ The horizontal tail on a normal airplane never generates positive lift in a static condition, only in transient ones, like a hard push over. Once stabilized in a dive, the tail is back to making down force. The aft limit of the CG must always be forward of the neutral point so when not maneuvering there is a positive pitching moment and the tail is still generating down force whether level, diving or climbing. Otherwise, the airplane becomes unstable in pitch. $\endgroup$
    – John K
    Jul 6, 2019 at 16:02
  • $\begingroup$ Little off topic, but I think there is a difference between descending and diving. Descending is cutting power, so less lift, so the plane descends. I defend a dive with forward stick, so a foil like a naca012 would have a positive aoa, therefore it would generate positive lift. Is this correct? $\endgroup$
    – Fred
    Jul 6, 2019 at 16:42
  • $\begingroup$ When you push the stick forward you are reducing tail downforce, the airplane pitches over because its pitching moment wants to do that, and it accelerates until the original tail downforce is restored with the new forward stick position you set. There is still a net downward lifting component, making the same downforce as before, but you are going a lot faster so you overall negative AOA needed is way less. The horizontal tail is like a wing with a flap (the elevator) where the flap is reflexed up (but flipped over). The reflexed wing is still making lift, but it needs to go faster to do it. $\endgroup$
    – John K
    Jul 6, 2019 at 17:22
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    $\begingroup$ @JohnK: No, the tail doesn't need to fly at a negative angle of attack (generate downforce) in order to have static stability in pitch. It merely needs to fly at a lower angle of attack than the main wing. The OP's intuition is correct on that point. $\endgroup$
    – Vikki
    Jul 6, 2019 at 21:54
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    $\begingroup$ I thought the whole reason why classic config will always be more efficient than a canard or tandem is that they fly with zero lift on the tail, which is when the Cm is o. So in a straight and level flight at the optimum point, there is no downforce of the tail, no? $\endgroup$
    – Fred
    Jul 8, 2019 at 20:16

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As the love of tandems continues I will answer. Because you are balancing lifting duties between two wings and (hopefully) still have the one in front stalling before the one in the rear, any configuration change in the front should be proportionately matched in the back so that the stall characteristics remain safe when flaps are deployed.

Deploying flaps increases the Angle of Attack of the wing and may cause a shift in the center of pressure. So the design needs to be approached with care.

You could also put slats and flaps on both wings of a biplane too, but the whole point is to have one (smaller/less drag) wing for faster cruising flight and a larger, higher lifting wing for low speed flight (take off/landing).

For a GA personal use recreational aircraft, you may not need flaps at all.

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