Apparently, the C42 nose pitches down shortly after stall, like in this video. I know part of the video might be pilot input to recover from the stall, but I've read on various flying forums that this will happen naturally without input anyway. Why is this?
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3$\begingroup$ It's built in to the design of most aircraft as an inherent safety feature - one of the many reasons why flying is remarkably safe. $\endgroup$– BenCommented Oct 25, 2018 at 20:30
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1$\begingroup$ I might have to take my instructor up for another few fully developed stalls some time, but that pitch-down movement looks huge, making me think that the CG is quite far forward. What's shown in the video appears similar to what I did the first time we practiced stall recovery, at which time the instructor told me "don't dive". That's in a C42B, so not exactly a C42 but still a very similar aircraft. Really, at least the C42B I've flown has only needed about neutral elevator plus some engine power to recover from a fully developed stall with two adults onboard. $\endgroup$– userCommented Oct 26, 2018 at 7:21
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4 Answers
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Most aeroplanes are designed with the Centre of Gravity being ahead of Centre of Lift, so when the aeroplane's wing cannot produce sufficient lift anymore -- due to a high Angle of Attack, the nose will drop, therefore, it will decrease the AoA and as the gravity pulls the aeroplane towards earth, the speed will increase which in combination with the lower AoA will hopefully produce enough lift to keep the aeroplane flying again.
In some cases when the CG is not forward of CoL -- e.g. mostly in transport category aeroplanes, the horizontal stabiliser is set/installed on the neutral nose-down Angle of Incidence, which makes the aeroplane fly in a more stable manner and will help the stall recovery by pushing to nose-down.
Generally flying in any type of aeroplane with CG aft of CoL is considered a no-no, for the reasons explained above: 1) Flight instability and 2) possibly unrecoverable from a stall -- which in fact is likely to happen because of the displaced CG will tend to increase the AoA which will decrease the speed, until stall happens.
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$\begingroup$ The second paragraph is incorrect. This configuration (CG is behind CoL of the wing (which is usually called Centre of Pressure, CP)) is less stable and doesn't really help stall recovery. When it becomes actually unstable (CG behind the Neutral Point, NP), the tail will have to be set at a higher AoA than the wing (generally), which means it will likely stall first, causing severe nose-up effect. In general, flight stability is related to NP, not CP. $\endgroup$– ZeusCommented Oct 26, 2018 at 4:55
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$\begingroup$ Your psychics are a bit off here. To obtain a balance with three forces either all three forces must pass through the same point or they must be parallel with the middle force in the opposite direction of the other two, and the ratio between the distances of the outer forces from the middle force must be the inverse of ratio of the magnitudes of the outer forces. $\endgroup$– XRFCommented Oct 26, 2018 at 5:01
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$\begingroup$ Also, your description of why the plane noses down in a stall is wrong. It is not about where the center of gravity and center of lift are positioned, but rather about the relative movement between the two. $\endgroup$– XRFCommented Oct 26, 2018 at 5:03
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Most (if not all) longitudinally stable aircraft will pitch nose down after a stall.
This is because the forward flying surface (regardless of conventional or canard layout) -- or forward portion of the wing, in the case of tailless designs -- must fly at a higher loading and coefficient of lift than the rear in order to maintain stability, so when lift is lost, it will be lost first at the higher-loaded and higher-coefficient surface, which will then start to drop before the lower-loaded or lower-coefficient surface.
answered Oct 25, 2018 at 11:57
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$\begingroup$ Unless I'm misunderstanding your answer it doesn't seem correct. Stall generally starts at the rear of the wing and progresses forward. $\endgroup$– TomMcWCommented Oct 25, 2018 at 18:10
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2$\begingroup$ @TomMcW, stall starts at the trailing edge and progresses forward, but the answer says the forward airfoil stalls before the aft airfoil. Tailless designs are always (at least a bit) swept, so the root is the forward airfoil and the tips are the aft airfoil and the twist ensures the proper order of stalling. Untwisted swept wing would stall tip-first, which would cause pitch up, but that is why most swept wings are twisted. $\endgroup$ Commented Oct 25, 2018 at 18:23
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1$\begingroup$ @JanHudec Non-swept tailless designs exist (Flying Plank, for instance); they depend on reflex airfoils to provide pitch stability. This keeps the aft portion lifting after the leading portion stalls, so they, too, pitch down after stalling. If they didn't, they'd be death traps. $\endgroup$ Commented Oct 25, 2018 at 18:40
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1$\begingroup$ @TomMcW, the part is included because it explains that the behaviour is universal. And forward and aft airfoil are somewhat common terms when discussing stability requirements. $\endgroup$ Commented Oct 25, 2018 at 18:51
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1$\begingroup$ @supercat As I understand it, the Wrights built the 1903 Flyer to be marginally unstable because they weren't certain they could control it if it was "too stable". After they found out how unstable it was, they built the next model with a reasonable stability margin. The Flyer wasn't a deathtrap only because it flew at 20-30 feet, at 35 mph, instead of a few hundred feet at 70 mph. $\endgroup$ Commented Oct 25, 2018 at 19:12
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The Ikarus C42 is very similar to the Cessna 152 in design and power (but around 500 lbs lighter!). This is a classic high wing design that is one of the safest aircraft one can fly. Rock stable on all three axes and very forgiving. These are a lot of fun to slip, as you will not see a lot of roll coupling with yaw (rudder) input.
As far as the video, this is a fairly normal stall recovery. As with anything, practice makes perfect. Properly weighted, this type of plane will not need a hugely aggressive push forward for stall recovery. Just hold the nose straight with the rudder.
There are really 2 lines of defense in stall recovery with an aircraft properly balanced. The first is a natural nose down with loss of speed caused by positioning CG ahead of center of lift from wing and trimming with downforce on tail (elevator up). Because the trim torque is aerodynamic, it has less force when the plane slows down and the nose drops. Planes set up this way will gently drop out of stall simply by relaxing the elevator.
The second line of defense comes from the surface area of the horizontal stabilizer (as seen from underneath the plane). If the plane starts sinking in a nose up or level position, vertical motion will help flip the nose down sharply. This is assisted with down elevator (stick forward).
The key is to hold the nose straight with the rudder. Do not use the ailerons. With the Ikarus, simply letting go of the stick should be enough to recover, but it would be highly recommended to take a lesson and learn the limitations and behavior of that plane.
answered Oct 26, 2018 at 1:42
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$\begingroup$ Not a lot of roll coupling with rudder input in a slip in a Cessna 152? I'd disagree on that one; the high wing placement creates a strong "dihedral effect". If we want to talk about WHY that is so it would be good for someone to ask that as an actual question. $\endgroup$ Commented Oct 27, 2018 at 13:46
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$\begingroup$ Have you flown one? Do not understand your need to disagree. These threads are gratis to all, let's not abuse them. The reason why high wings don't couple roll with yaw is LOWER CG. If you don't believe me, check in with the Antonovs. I know what physics books are teaching, it ain't always that way in the real world. What Cessna has done is a masterpiece of force balancing. $\endgroup$ Commented Oct 27, 2018 at 14:11
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$\begingroup$ Yes I have flown a Cessna 152 many times and I would say there is substantial coupling between slip and roll. You've got to be joking if you are saying a high wing configuration has inherently less slip-roll coupling than a low wing, for the same actual physical dihedral angle. Or that putting the CG way below the wing REDUCES slip-roll coupling. Post it as a question if you want to discuss further and hear what other folks have to say on this. Or just see av8n.com/how/htm/roll.html#sec-other-slip-roll . $\endgroup$ Commented Oct 27, 2018 at 14:17
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$\begingroup$ Haven't you ever noticed that high-wing planes are usually designed with less actual dihedral than low-wing planes, for the same general class of aircraft (e.g. general aviation trainers)? There is a reason for that. $\endgroup$ Commented Oct 27, 2018 at 14:20
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$\begingroup$ I have flown a Cessna 172 and one can slam the rudder to the stop and it won't roll worth a damn. That is what makes it so good in forward slips. $\endgroup$ Commented Oct 27, 2018 at 14:21
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As a stall is approached, the center of lift moves slowly forward. The pilot will compensate for this with either back-pressure on the stick/yoke or by trimming the aircraft. When the stall occurs the center of lift suddenly and rapidly moves towards the rear of the aircraft. Since the aircraft was balanced before, moving the force of lift backwards will cause the rear of the aircraft to rotate upwards, and the nose to drop. In a traditional aircraft configuration, the wing will stall before the horizontal stabilizer. The horizontal stabilizer normally produces lift in the downwards direction just incase it does stall, that way the nose would automatically drop. Canard configurations achieve the same effect by having the horizontal stabilizer produce positive lift and stall before the wing.
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$\begingroup$ I don't understand how the center of lift moving forward would cause a nose-down moment. $\endgroup$ Commented Oct 27, 2018 at 13:24
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$\begingroup$ It is the rapid movement of the Center of loft to the rear that causes the nose to drop. The forward movement earlier on may or may not affect trim depending on how slowly the aircraft is brought to the stall as well as different design characteristics pertaining to the wing and horizontal stabilizer. $\endgroup$– XRFCommented Oct 28, 2018 at 14:55