The MiG-29's tail axes are located fairly close to the leading edge, and rotate along an axis oblique to the chord. What are the possible consequences to this arrangement (e.g. actuator torque, flutter potential)?
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$\begingroup$ The horizontal surfaces can be shifted further aft, increasing their effectiveness and allowing to make them smaller. $\endgroup$– Peter KämpfCommented Feb 8, 2021 at 6:48
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1$\begingroup$ But through the several evolutions it's gone through, right up to the MiG-35, the tail design hasn't changed, and the Russians are known for keeping things simple wherever possible, so there has to be an advantage or constraint that keeps it that way? $\endgroup$– bud bertCommented Feb 9, 2021 at 12:11
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3 Answers
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There is nothing too special about the MiG-29 stabilator.
The "slanted" (swept) axis of rotation is fairly common: as far as I remember, all MiGs that had a stabilator had this arrangement.
This has primarily structural advantages: it is easier to integrate the shaft (or axle) along the structural elements of the stabiliser, making it lighter. Also, the hinge moments are more consistent along the shaft. (For the straight shaft on a swept stabiliser, there will invariably be twisting forces that must be absorbed by the structure. Say, the root section may experience positive moment with respect to the shaft while the end section may have negative moment).
Also, as Charles already noted, a swept axis may allow to move the stabilator a bit further back (while keeping the same overall hinge moment), increasing its lever arm and thus efficiency. This is because the attachment point at the root can be closer to the leading edge. But this is only a gain when the actuator cannot be moved further back for some design reasons (like on MiG-29).
On the other hand, the leading edge may interfere with the fuselage when deflected, necessitating a "stump" at the root (which you can also see on most jet MiGs). At the same time, you need larger deflection in order to achieve the same aerodynamic effect (see also this answer).
The perception that the shaft is too close to the leading edge is a bit of an illusion. In fact, it is just a bit forward of the 1/4-chord line: exactly where it should be, as can be seen in these diagrams.
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The further aft the stabilator is, the longer the moment arm, which means they can be smaller and still have the required pitch moment. But if the majority of the stabilator surface is behind the aircraft fuselage, (as in this case), then, even when the rotation axis is swept back, as it is on many Mig aircraft, the bulk of the aerodynamic surface can be behind the rotation axis, and if so the rotation axis will be forward of the fore-aft aerodynamically neutral point of the surface, (called the aerodynamic center (AC) - about 25% of the way back from the leading edge), which means the actuator that moves the stabilator must be larger (more powerful), to hold the stabilator in position, since the airloads when the stabilator is displaced from neutral will be higher.
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1$\begingroup$ The hinge line sweep allows the hinge line to stay near 25% of chord, so even with the rearwards shift the actuators need not be larger. $\endgroup$ Commented Mar 10, 2021 at 15:33
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$\begingroup$ Peter, Are you saying that the Stabilator pivot axis is not normal to the fuselage reference line, that it is swept back at an angle? Wouldn't that cause the forward inboard edges of the stabilator to interfere with he side of the fuselage when it was deflected? And as close as the pivot is to the leading edge of the stabilator, ir would have to be swept back quite a bit to intersect with the 25% chord. Do you know what the axis sweep angle is? $\endgroup$ Commented Mar 11, 2021 at 2:29
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$\begingroup$ No, I don't and only learned it from this question (and rotate along an axis oblique to the chord). You are correct about fuselage interference; this is why the moveable part sits on a stump and does not directly abut against the fuselage (see here for example). $\endgroup$ Commented Mar 11, 2021 at 6:36
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$\begingroup$ Ahhh, I see that the inboard leading edge is aligned with the horizontally tapered fuselage extension (which I would assume holds the actuator and linkages), so that when the stabilator is deflected the forward tip, although it must move inboard, does not interfere with anything. Interesting design. Just looking at how far forward the pivot point is, I would think the swept back axis is still forward of the AC though. $\endgroup$ Commented Mar 12, 2021 at 14:29
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$\begingroup$ I am editing my answer to reflect this. $\endgroup$ Commented Mar 12, 2021 at 14:30
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This arrangement is for a better fulcrum (not pun intended here) in pitching. Advantage is that they hide the plum of exhaust heat at some angle of view form the enemie aircraft
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2$\begingroup$ The horizontal tails being thin and oriented more or less radially from the exhaust plume, I don't see much shielding of the exhaust & plume IR signatures except from a front aspect, which is pretty much the same for most all fighters. $\endgroup$– bud bertCommented Feb 9, 2021 at 12:18