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What's the difference between an airplane that stalls nose down or just mushes in a stall?

I understand most GA planes go nose down after a stall, but some GA planes don't drop their nose at all, they mush (they just drop in altitude). How are these planes different? Is their CM curve near horizontal, instead of negative, so that they fly almost neutral with little static margin? Or do they have a high horizontal tail volume and so with stick back, they just stay almost horizontal, or other?

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You are absolutely right that it has to do with the pitching moment (Cm) characteristics of the aircraft. Take a look at the sketch below illustrating three different pitching moment characteristics:

enter image description here

In all the cases, you can assume that the CLmax occurs around 12deg AOA (not presented here). Cm1 is the aircraft that would present a distinct nose-down upon reaching CLmax. This is also known as a pitch bucket, because of its shape, and is good for stall identification and recovery as the aircraft tends to un-stall itself.

Cm2 is the mushy case. With increasing aft column pressure, the AOA will continue to increase, while the airspeed may stagnate. No stall identification is naturally provided to the pilot.

Cm3 is anything from stick lightening to outright pitch up, depending on the aggressiveness of the instability. This is a common characteristic for high speed aircraft that have large LE sweep and the stall does not originate from the root of the wing.

As mentioned by Carlo, the degree of nose-down or nose-up would also be CG dependent. Aft CG would always have smaller pitch bucket and worse pitch up (if exist).

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  • $\begingroup$ This is a good analysis of how static stability will affect stall characteristics. CG placement matters! Definition of "mush" can be discussed. It can be a semistalled aircraft nosing down in a shuddering mush (172), or, you describe it, a low aspect wing (or tailplane) with a flatter near stall lift curve and nuetral static stability entering the max lift region with little fanfare and effect on performance. This is why, 100 years ago, low aspect "squares" and deltas were popular tail designs. $\endgroup$ – Robert DiGiovanni Jul 3 '19 at 9:19
  • $\begingroup$ @RobertDiGiovanni I'm not sure I would characterize aspect ratio with stall identification, since the aircraft level pitching moment is dependent on airfoil stall (straight tapered wing), spanwise stall (swept wing), downwash, and whether there are slats/flaps. $\endgroup$ – JZYL Jul 3 '19 at 20:18
  • $\begingroup$ the difference in stall angle between high and low aspect wings will make your eyes pop. Rather interesting, had I written "150 years ago" it would have been "popular sail designs". $\endgroup$ – Robert DiGiovanni Jul 3 '19 at 20:48
  • $\begingroup$ @RobertDiGiovanni Understood. Vortex roll off is very different between the two cases, and more prominent in delta wings. This makes the lifting surface stall at higher AOA. But the OP was asking about stall characteristics and identification of an airplane. $\endgroup$ – JZYL Jul 3 '19 at 21:04
  • $\begingroup$ I agree that planform and CG have a lot to do with it. Many factors here. I would add sharpness of leading edge and airfoil thickness. Many trainers have rounded leading edges and thick airfoils, which make for a softer stall. More on slats later (I want to help the Max 8). $\endgroup$ – Robert DiGiovanni Jul 3 '19 at 21:41
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It's different planes and different flying conditions. Picture a plane trying to do a quarter loop. Without sufficient energy it will not make it to 90 degrees (pointed straight up).

Now, considering most GA planes have a thrust to weight ratio of around 0.20 to 0.25, what affects how far into the "loop" we go before stalling? Do we "mush" or pitch down sharply?

Energy is entry speed, so if we dive and then pull up at full power (zoom) we will go higher into the loop, maybe even completing it. But if you don't make it to 90 degrees, the plane loses airspeed, stalls (with continued elevator deflection), sinks, and pitches sharply down.

Two factors contribute to pitch down. At full stall center of lift moves back to 50% chord and tail volume (as plane sinks) will "flip" the nose down.

Now picture entering the loop with no power or zoom, your plane does not nearly go as high to vertical, it simply loses airspeed and does a much gentler "mush".

Other factors include elevator authority. The B-52 bomber has a comparatively "weak" elevator compared with an Extra 300. You don't want to stall one of those.

And light weight, which enables a much sharper loop entry.

Also placement of the CG. Many trainer aircraft, including the Cessna 172, as set up for students with a forward CG (within limits) to "weaken" elevator pitch up authority and encourage "mush".

But even with the 172, if CG is aft, stall characteristics can change dramaticly.

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That behavior is going to largely depend upon where the CG is located. A fwd CG should offer a sharp break in the stall with the nose abruptly pitching down when the stall occurs. An aft CG loads airplane will have a more demur, mushing and sinking stall and, in extremes cases where the CG is aft of the approved range, may be unrecoverable simply because both wing and tailplane have stalled and aerodynamic forces no longer permit the pilot to lower the nose to break the stall.

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  • $\begingroup$ The behavior will vary from aircraft to aircraft, but a rearward set CG (great for fighters) will make it much easier to increase AOA (in straight and level (the kind of flight passenger carriers want to be in) flight), and pull the nose up into a full blown stall. Forward set CG helps mitigate this effect by getting the nose to drop ballisticly before the wing fully stalls. This is the "mush" that I enjoyed so much with the 172. It unstalls it self (just let go of the yoke). Rear set CG may require down elevator to recover. Warning buffet does help. $\endgroup$ – Robert DiGiovanni Jul 2 '19 at 22:04
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The simple answer is that it depends on the degree to which the airplane is stalled. When an airplane is in the stalled regime the lift on the wings does not go to zero but usually gradually decreases (see graphic). The horizontal stabilizer cannot support the weight of the nose on its own and needs a contribution from the wings to keep the nose from dropping. If the wing is not producing enough lift to support the weight of the plane but producing enough lift to keep the nose in the air then you have a mushing stall situation. If you stall more deeply, then the wing cannot support the weight (or more specifically the moment of force due to gravity) at the nose.

enter image description here * Graphic taken from See How it Flies Website

In the Piper Cherokee that I fly the plane will break when I stall it aggressively (lots of back pressure on the yoke) but will mush if I stall it more gradually.

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