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

Question

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?

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. — John K, Jul 06 '19 at 16:02
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? — Fred, Jul 06 '19 at 16:42
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. — John K, Jul 06 '19 at 17:22
@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. — Vikki, Jul 06 '19 at 21:54
@JohnK: So all Airbus aircraft since the A310 are not normal. What then must they be called? Abnormal? — Peter Kämpf, Jul 07 '19 at 01:45
@PeterKämpf are you saying Airbuses can be loaded with the CG aft of the neutral point so the tail has to lift? — John K, Jul 07 '19 at 02:18
@JohnK Are you still believing in that nonsense? The tail may very well have lift while the CG is still ahead of the NP. Just less per area than the wing. — Peter Kämpf, Jul 07 '19 at 03:21
A word of caution to small GA plane designers, an aircraft that is not STATICLY stable will very easily stall (not good) on approach to landing if the pilot takes eye off of airspeed even for a few seconds. This is why static stability should not be lightly discarded (before at least understanding it), even though it is theoretically possible to have the rear "wing" positively lifting. The high wing Piper Cub, Fiesler Storch, Cessna 172 designs represent the hard earned lessons of the first 40 YEARS of aviation. Study of them is Aviation 101. — Robert DiGiovanni, Jul 07 '19 at 11:37
@PeterKämpf a tail lifting UP while in steady state trimmed flight? What opposes the ND pitching moment of the CG acting about the neutral point then? I'm not talking about gust or maneuvering cases where loads on the tail are momentarily reversed. — John K, Jul 08 '19 at 01:27
@JohnK: Are you confusing neutral point (NP) and center of pressure (CP)? All you need is to have the CP in the same station as the CG - this is very well possible with lift on the tail. The NP is still further aft, so the plane is stable. — Peter Kämpf, Jul 08 '19 at 03:46
@PeterKämpf IIRC, NP is the longitudinal point at which pitching moments of the entire body is zero. I think of NP as the "effective" CP, or, CP adjusted for other pitching moments that have influence, like the fuselage surface area, offset thrust line etc. Everything I've ever read indicates that static margin requires the CG to always be forward of the NP, so there is always a net nose down pitching moment, outside of transient maneuvering or gust cases.This means the tail has to be providing down lift in steady state conditions to provide good static stability. — John K, Jul 08 '19 at 16:21
@JohnK -- Re "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. " -- for a contrary view, see https://www.av8n.com/how/htm/aoastab.html#sec-pitch-equilibrium — quiet flyer, Jul 08 '19 at 19:20
@JohnK -- in more detail-- "I took the same Skyhawk and put a small pilot in the front seat, a moderately large mad scientist in the back seat, and 120 pounds of luggage in the rear cargo area. That put the center of mass right at the rear of the envelope, so the tail had to produce considerable positive lift in order to maintain equilibrium. The airplane still had plenty of stability. (As far as the pilot could tell, it was just as stable as it ever was.)" — quiet flyer, Jul 08 '19 at 19:21
@JohnK -- continuing-- "The easiest way to determine whether the tail lift is positive or negative is to observe the direction of motion of the tip vortices, as discussed in section 3.14. To observe the vortices, I attached a streamer of yarn, about half a yard long, to each tip of the horizontal tail, at the trailing edge. The streamer gets caught in the vortex, so its unattached end flops around in a circle. When the tail is producing positive lift, the circular motion is in the direction shown by the green “circulation” arrows in figure 3.29, i.e. downward on the inboard side." — quiet flyer, Jul 08 '19 at 19:24
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? — Fred, Jul 08 '19 at 20:16
@quietflyer I'm not sure I buy his thesis. If the tail was making positive lift, the load on the elevator itself, which wants to trail when hands free were it not for servo force of the trim tab, would be up, and the trim tab's servo force would have go the opposite way to counteract it, applying a down force at the elevator TE, not an up force. I've never seen a trim tab work backwards like that. Certainly doesn't on my airplane. — John K, Jul 08 '19 at 21:42
@JohnK so you are saying that we can observe that a trim tab always applies an upforce to the trailing edge of an elevator, and this can be taken as evidence that the horizontal tail as a whole (including elevator) is creating a down force. Hmm. There could be a question for Aviation Stack Exchange here--. — quiet flyer, Jul 09 '19 at 00:18
As an interesting thought experiment, think through the action of the anti-servo tab on an airplane with a stabilator tail, like a Cardinal or Cherokee, where all of the static stability comes through the anti-servo's action to drive the surface to a given incidence to hold a trim speed, and think through how that tab could function if the tail was lifting up instead of down. It's mechanically geared to only work with a down lifting surface. I think this issue has been beaten to death many times on here and there is unlikely to be a resolution that satisfies everybody for some time to come ;) — John K, Jul 09 '19 at 00:45
Let's beat it some more. Take a tandem with 2 identical wings, mount the fore wing at 5 degrees greater incidence, where do we put the CG? It must be a little closer to the fore wing, which is at a higher AOA on the Coefficient of Lift Curve (it will also stall first). Theoretically all well and good. Someone named Langley tried this around 120 years ago. Issue with tandems (and biplanes) is wing interference. Even if the rear "wing is a much smaller flat plate with downforce, the design remains a "tandem" and interference remains an important design issue. — Robert DiGiovanni, Jul 10 '19 at 01:37
Would there still be interference if the gap and stagger is 2 chord? I heard at that gap and stagger, the 2 wings act independantly. Is that true? — Fred, Jul 11 '19 at 00:34
@JohnK Centre of all lift must be behind CG. Stabiliser down force still has good reasons — Koyovis, Aug 07 '19 at 21:59
@Koyovis yes CG must be forward of the neutral point, and as a result the tail always lifting down, except in transient states. That's what I've been arguing. — John K, Aug 07 '19 at 22:06

score 2 · Accepted Answer · answered Jul 06 '19 at 22:00

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.

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

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