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I think I understand how a high wing differentiates from a mid wing or a low wing; what I am asking is how, for instance, a very high high-wing makes itself unique in flight from a very low high-wing.

So let's say you have a high-wing aircraft. Raise the wing vertically to make the high-wing even higher. What does this do to the airplane's flight characteristics?

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  • $\begingroup$ An important factor is the "pendulum effect" or "pendulum stability"-- google ASE questions or answers involving these terms-- I'll post some links when I get a chance-- $\endgroup$ Commented Feb 20, 2022 at 1:27
  • $\begingroup$ Highly related -- aviation.stackexchange.com/questions/53437/… $\endgroup$ Commented Feb 21, 2022 at 1:01
  • $\begingroup$ Re link above-- and see my answer aviation.stackexchange.com/questions/53437/… . Could be further improved by incorporating some of the specific points I've made in comments to two of the answers to the present question -- $\endgroup$ Commented Feb 21, 2022 at 1:03
  • $\begingroup$ Another highly related question (arguably a near-duplicate, except the current one is broader)-- aviation.stackexchange.com/questions/26396/… -- this is a topic that's gotten quite a lot of exposure on this site, and you can see that there's a bit of an ongoing controversy, with several contributors consistently falling into one "camp" or the other-- $\endgroup$ Commented Feb 21, 2022 at 12:40
  • $\begingroup$ Another highly-related question aviation.stackexchange.com/questions/64380/… $\endgroup$ Commented Feb 22, 2022 at 22:36

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First off, it's important to realize that a wing does not constitute a fulcrum in a pendulum. There's no magical hinge that a plane rotates about, hidden in the wing. This is known as the pendulum fallacy.

Instead, think of an airplane purely in terms of incoming airflow. If the air is coming from straight ahead, there's not much difference between a low wing and a high wing. The main difference is that interference effects at the wing root are most prominent at the suction side (top of the wing) in a low wing aircraft, which may affect performance. Mid fuselage would be ideal, like on most gliders.

A more prominent effect is when the wind is not from straight ahead but from the side, like in a gust or sideslip. This will increase the pressure at the windward side of the fuselage. For a low wing, this high pressure pushes the windward wing down, whereas a high wing will be pushed up. Ideally, you want the latter, since the induced roll will make the plane slip the other way, stabilizing towards a coordinated turn. So a high wing is superior in this regard, although dihedral can also provide this stabilization. A high wing far away from the fuselage would lose this benefit.

In reality, wing placement is governed mostly by practical issues like obstacle or water clearance, landing gear and engine placement, and not having a wing spar in the middle of the passenger cabin. Dihedral or anhedral is then used to achieve the desired stability.

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  • $\begingroup$ Consider an unswept flat wing (no dihedral), with a streamlined heavy dense weight mounted far below the wing, connected to the wing by an infinitely thin (but completely rigid) pylon. Interference effects between pylon and wing are nil. Also the weight is so compact that sideforce effects generated by the impact of sideways airflow against weight are nil. If any sideways airflow is present, won't the simple fact that the wing's drag vector now has a sideways component, acting above the CG of the whole system, create a roll torque in the downwind direction (i.e. a dihedral-like effect) $\endgroup$ Commented Feb 20, 2022 at 14:00
  • $\begingroup$ (The problem should be able to be analyzed either by treating wing and "pendulum" weight as a single rigid system, or by treating them as two separate bodies and analyzing what torque one exerts on the other -- with the same answer yielded by both approaches-- ) $\endgroup$ Commented Feb 20, 2022 at 14:01
  • $\begingroup$ (Potentially could (and may) refine this into a new question after some further thought and input -- ) $\endgroup$ Commented Feb 20, 2022 at 14:01
  • $\begingroup$ (One issue is I'm a little confused about whether (or why) we would still see the same roll torque if we analyzed the problem from the point of view of the wind frame of reference rather than the aircraft's body frame of reference. On the other hand, if the roll torque is generated by actual dihedral, or by sweep, then the answer to that is obvious.) $\endgroup$ Commented Feb 20, 2022 at 14:08
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    $\begingroup$ Considering the Catalina as a giant hang glider shifting its weight... the quandary becomes: will it roll faster than it accelerates sideways, just like balancing a broomstick. If it is "bottom heavy", the weight is dragged behind the wing, if it is top heavy, the weight (unstably) falls forward, acceleration (from the bottom) must match its rate of (angular) topple. Interestingly, aerodynamic drag helps stabilize top-heavy, and promotes bottom heavy roll, but once acceleration goes to 0 drag alone must stabilize top heavy. $\endgroup$ Commented Feb 20, 2022 at 16:14
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The most important advantage that a higher wing gives you, is the ability to land on 'airports' that have obstacles directly adjacent to the runway, such as tall grass or snow banks, or surfaces that may provoke roll, such as water. It also allows you to have the CG determine your airplanes tendency to keep speed constant, rather than only the wing and tail shape. With a very high wing, pitching up or down will change the projection of the lengthwise distance between neutral point and CG on the horizontal plane much more than on mid- or low wing aircraft. This means that at high pitch angle your static stability is larger than at low pitch angle.

A good example of such an aircraft is the Consolidated PBY Catalina (picture source).

Consolidated PBY Catalina

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  • $\begingroup$ @PeterKämpf Not really, It's something else I mean, but I don't know which word to use for it. I'll edit it out. $\endgroup$
    – user55607
    Commented Feb 19, 2022 at 8:38
  • $\begingroup$ @PeterKämpf You're right. It still doesn't sound entirely right. Please feel free to edit it accordingly. You know what I mean to say. I'm not a native speaker, which never entirely stops being an obstacle. $\endgroup$
    – user55607
    Commented Feb 19, 2022 at 8:57
  • $\begingroup$ The lower the CG sits along the vertical axis, the further it moves from where it wants to be as the aircraft rolls or pitches.. $\endgroup$
    – user55607
    Commented Feb 19, 2022 at 9:06
  • $\begingroup$ @PeterKämpf CG is probably the wrong term to use at all. When it sits right at the spot where all three axis cross, banking or pitching will not make it have influence. The further the CG is positioned along the vertical axis away from where the other two cross, , the more it will push the airplane back into straight flight. . Thnx $\endgroup$
    – user55607
    Commented Feb 19, 2022 at 9:31
  • $\begingroup$ @PeterKämpf I read some of your other posts and found them to be quite enlightning. Is what you mean the reason why it is possible to make loopings with a hang glider? $\endgroup$
    – user55607
    Commented Feb 19, 2022 at 10:10
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So let's say you have a high-wing aircraft

Allright, I choose the Dornier Do-24 ATT, a very well behaved high wing airplane with excellent seaworthiness. Below I show it in frontal and side view with lift and weight forces symbolized by arrows. I decomposed the lift into its zero-lift moment and put the lift vector at the neutral point, where all angle-of-attack-dependent lift acts, so the lengthwise distance between weight and lift vector is proportional to the static longitudinal stability and is delineated by a pair of arrows. Of course, when you shift the lift vector into the center of pressure, it will be aligned with the weight vector and the zero-lift moment will disappear.

Do-24 ATT sketch 1

This is my baseline high wing design, and next I raise wing and tail by a grotesque amount. No less will do to make the differences obvious.

Raise the wing vertically to make the high-wing even higher. What does this do to the airplane's flight characteristics?

Do-24 ATT sketch 2

If we neglect the additional vertical surface at the back, what has changed are inertias in all axes, which have grown by the increased distance between the horizontal surfaces and the fuselage. Also, the center of gravity has moved up a bit.

What are the consequences for the flight characteristics?

  1. Maneuvering will be more sluggish and will require larger control surface deflections.
  2. Rolling will cause a sideways shift at the pilot's station which might be a bit confusing, but one can get used to this.
  3. Stability hasn't changed. Static stability is identical to the baseline and since the tail lever arm is still the same, pitch damping is also the same. The increased fore-back motion of the wing during a pitch oscillation will mostly make itself felt in inertial changes, aerodynamically this is of minor consequence.
  4. The higher engine placement means higher pitching moment contributions from the engines. More downforce on the horizontal tail is needed to compensate the pitch-down moment from the high engines, and throttle changes will require high pitch trim changes.

Now what about lateral stability? And what about pitch changes away from level flight at cruise speed? For this we need another sketch. The baseline first:

Do-24 ATT sketch 3

On the left, the airplane flies a coordinated turn with a bank angle of 45° and on the right it is in level flight close to stall at 15° angle of attack. Turning adds a centrifugal force which acts at the center of gravity and needs a bank angle and lift increase so lift is sufficient to balance the resulting mass forces (denoted as R here). Since both forces act along the centerline, no imbalance or instability comes with the high wing arrangement. However, the side sketch now reveals a larger distance between the mass and lift forces, which means that the airplane becomes longitudinally more stable at low speed. The zero-lift moment has to become larger to trim this high pitch angle by incasing the negative elevator deflection. Note that the green circle has grown in size and weight to reflect this. The increased elevator travel helps to keep stick forces at low dynamic pressure up and requires more elevator travel for stalling than in a low wing configuration.

And now we do the same for the high wing version:

Do-24 ATT sketch 4

Again, not much has changed, only that the stabilizing effect of a high pitch attitude is now even more pronounced and the zero-lift moment needs to become even larger than before. This will require a bigger tail, or only a small range of angles of attack can be trimmed. Turning feels the same, apart from the need to overcome the higher inertia with more forceful commands, and again no instability can be seen.

But a low wing ... a low wing will still show the same results. Angle of attack changes mean less change in static longitudinal stability and lateral stability is unaffected. Only the effect of the fuselage on the yaw-induced rolling moment will require a low wing to have more dihedral which makes it ever so slightly less efficient. But mayhem and carnage fail to manifest themselves.

And what does an uncoordinated turn do? Now the airplane will sideslip and the weight vector will not be aligned with the centerline in the frontal view. But the lift vector, still orthogonal to the wing, will, so no rolling moment from weight or wing lift develops. Only the side force of the vertical tail and the fuselage might cause a very small roll contribution which can easily be balanced with a bit of aileron. Again, mayhem and carnage fail to manifest themselves.

The central fallacy here is the comparison with the broom, balanced on a fingertip. Airplanes (and drones or rockets, for that measure) are different. Conservation of momentum dictates that all rotations take place around the center of gravity, and lift, being the result of surface pressures, is always orthogonal to the surface of the wing. In consequence, regardless of wing position, a bank angle will not cause a destabilizing rolling moment.

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  • $\begingroup$ +1 Ir's a good answer. Would that still require all the comments on other people's answers? $\endgroup$
    – Koyovis
    Commented Feb 20, 2022 at 23:44
  • $\begingroup$ @Koyovis Tried to nudge them to the truth but the force is strong with the pendulum fallacy ... $\endgroup$ Commented Feb 20, 2022 at 23:47
  • $\begingroup$ See my recent comments under Sanchises's answer about roll torque generated by wing's drag vector during a sideslip, acting well above the CG -- -- also the rolling effect of any sideforce generated by the wing due to wingtip endplates, etc, or even due to side area exposed by dihedral, gull-wing geometry (dihedral inboard, anhedral or flat outboard), or even anhedral geometry-- the paraglider case is particularly instructive because during sideslip it manages to generate a net dihedral-like roll torque with an exteme anhedral wing geometry-- all due to wing being so very high above CG-- $\endgroup$ Commented Feb 20, 2022 at 23:53
  • $\begingroup$ -- all due to wing being so very high above the CG of the whole system with so very much side area exposed to the sideways wind component during sideslip -- $\endgroup$ Commented Feb 21, 2022 at 0:01
  • $\begingroup$ Will work on merging those (and these) comments into a new answer and then deleting them -- !! – $\endgroup$ Commented Feb 21, 2022 at 0:01
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You can imagine that any change to the outside shape of an aircraft or anything that affects its centre of gravity or angular inertia will affect its flight characteristics. On the face of it, a high wing will impart more stability but it may compromise other things such as placement of engines and undercarriage, airflow over the empennage and control surfaces, characteristics while in ground effect and so on. In aircraft design there are very many factors to take into account and it’s difficult to consider one in isolation from the others. Trivially, high wing has more inherent stability than low wing but there are many ther factors.

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  • $\begingroup$ Please explain why a high wing has more stability, and why this should be trivial. You don't mean the pendulum effect fallacy, do you? $\endgroup$ Commented Feb 19, 2022 at 9:23
  • $\begingroup$ @Peter Kämpf forgive me I’m primarily a paraglider pilot and rely heavily on this phenomenon as I have no empennage. $\endgroup$
    – Frog
    Commented Feb 19, 2022 at 19:46
  • $\begingroup$ So while the pendulum effect may be regarded as a fallacy, a claim supported by the fact that most rockets have the engines at the bottom, surely it’s intuitive that an aircraft with a large weight positioned high above it on a tower would have rather poor flight characteristics, no? $\endgroup$
    – Frog
    Commented Feb 21, 2022 at 5:48
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Weight distribution is critical in determining the effects of making a high wing even higher.

While trial and error with balsa models will keep one busy, some educational background is very helpful in eliminating the more embarrassing moments.

As a designer, try to plot any changes in center of gravity and center of drag.

Generally with aircraft, raising the wing moves the center of drag higher. This can make the plane more difficult to control in a crosswind. As seen with the PBY 3 Catalina, the designers raised the center of gravity by placing the engines on the wings to reduce this effect$^1$.

Having the Cdrag above the Cg means that ailerons must be held in a turn or the plane will tend to roll away. This is the "self righting tendency".

Again depending on the new CG, raising the wing may also give the plane a greater tendency to pitch up. As this will vary with forward airspeed and AoA, one might wonder if too much of this may actually be undesirable.

$^1$ keeping the props away from the water as well

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Placing the wing above the center of mass of the airframe means that perturbations in the three axes will generate a righting moment which tends to oppose the perturbation. The plane can then be positively stable in the hands-off condition and will to some extent "fly itself".

Placing the wing (the center of support) below the center of mass is then like balancing a pencil (with its center of mass at its midpoint) by its tip on your finger (the center of support). Once the pencil begins to rotate and fall, the rolling moment becomes stronger (as the center of mass gets offset relative to the center of support) which means those perturbations will grow instead, and the plane will be divergent (dynamically unstable, i.e. it will display "negative stability") in all axes- and if flown hands-off, it will try very hard to invert itself and fly upside-down. Mayhem then ensues, and carnage will result.

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    $\begingroup$ While a lower wing is indeed less stable in pitch, you can't be serious about the rest of your answer, can you? $\endgroup$ Commented Feb 19, 2022 at 6:33
  • $\begingroup$ @PeterKämpf Why not? I read that explanation coming from a PBY pilot. That's why I took it serious. You seem very sure. What speaks against it? I can't imagine a low CG to have no effect at all, but that's just my imagination. $\endgroup$
    – user55607
    Commented Feb 19, 2022 at 9:39
  • $\begingroup$ The misleading term is CG itself. It implicates a connection to the earths gravity, which it doesn't necessarily have. It is subject to any acceleration working on the airplane. So as the airplane banks, G-forces are working in the opposite direction to the turn along the line of acceleration ., leaving the CG in a spot where it is quite comfortable.. For the CG to stabilize the aircraft, It has to roll without turning or pitch without climbing, or climb at a constant rate.... I think.. The straight forward pendulum effect applies as the word says on pendula and an aircraft is not a pendulum. $\endgroup$
    – user55607
    Commented Feb 19, 2022 at 10:37
  • $\begingroup$ @PeterKämpf, yes I am unless I misunderstand the question- will edit. Please review it :-) $\endgroup$ Commented Feb 20, 2022 at 3:17
  • $\begingroup$ @PeterKämpf, the fuel tank atop the bomber is empty and weighs much less than the bomber and I would assume a skilled pilot or an autopilot could keep the center of lift centered under the center of mass so mayhem and carnage are avoided, yes? $\endgroup$ Commented Feb 20, 2022 at 3:31

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