High vs. low wing
Thread Starter
High vs. low wing
[SLF, again, because idiot questions are tolerated]
During the almost endless debate and reportage about the 737 MAX I came across a claim that the reason there was limited space under the wing was because the fuselage needed to be low to the ground to allow easy loading of cargo. This struck me as rubbish: undercarriage is *heavy* and no sensible engineer would make it any longer than needed, and in the era of skinny turbojets that initial clearance was plenty.
But I have got to thinking - if high-wing allows a much shorter (i.e. lighter) undercarriage why the predominance of low-wing types?
Plus: surely high-wing is inherently more stable? In hand-wavey terms, the aircraft is hanging off the wings rather than balancing on top of them. I note that civilian trainers (e.g. Cessna 152) are high-wing whilst military types tend to be low.
A brief search indicates that low-wing is more survivable in case of "uncommanded landing", but there must be more to it than that.
During the almost endless debate and reportage about the 737 MAX I came across a claim that the reason there was limited space under the wing was because the fuselage needed to be low to the ground to allow easy loading of cargo. This struck me as rubbish: undercarriage is *heavy* and no sensible engineer would make it any longer than needed, and in the era of skinny turbojets that initial clearance was plenty.
But I have got to thinking - if high-wing allows a much shorter (i.e. lighter) undercarriage why the predominance of low-wing types?
Plus: surely high-wing is inherently more stable? In hand-wavey terms, the aircraft is hanging off the wings rather than balancing on top of them. I note that civilian trainers (e.g. Cessna 152) are high-wing whilst military types tend to be low.
A brief search indicates that low-wing is more survivable in case of "uncommanded landing", but there must be more to it than that.
The aircraft rotates in roll about its centre of gravity not about the the wing and the lift force of the wing tilts as the aircraft rolls so will always be acting along a line that passes through the CofG.
You will often hear people talk about "pendulum stability" and describe it as you intuitively described it above however that would rely on the aircraft's lift always acting vertically even when rolled or alternatively the aircraft rotating at the level of the wing attachment point not the CofG) I therefore am an athiest when it comes to believing in the "pendulum effect". Happy to be proved wrong though.
I believe there may be some lateral stability differences between high and low wings due to differences in the sideslip behaviour (keel effect) of the two configurations but in most light aircraft I think you would find the wing's dihedral would have more of an impact than wing position. There are aerobatic aircraft that are high/low and mid wing (and in a Pitts eg both high and low wing placement) and typically aerobatic aircraft do not want a lot of lateral stability as it impacts on roll behaviour.
Also you will find many low wing light aircraft trainers as well as high wings (eg Tomahawks, Warriors, Skippers etc). Most differences are in visibility in cruise and in turns between high and low and differences in take-off landing behaviour due ground effect.
The wingbox that carries the load between the wings is of large size in large jets. That would come across the foreheads of passengers going down the aisle to the seats. It would also place it out of contact with a main structural element - the floor, and be separated from the main cargo/passenger mass above the holes cut for the windows.
It's less a problem below the floor.
For some planes that turn the entire fuselage over to cargo and have no side windows, such as C-130, C-17, they don't have that passenger interaction - they also benefit from mounting the engines high above a potentially contaminated landing/takeoff surface.
A goal for the 737 was operation from dirt and grass strips where luggage and cargo handling equipment was people powered, so yeah, keeping the LG short was important. However the trade is susceptibility to sucking dirt and gravel off the ground, which is one reason for longer gear on most passenger jets. Instead, the 737 with tougher stubby gear, got gravel deflector kits, though I think those were only available for the early designs.
It's less a problem below the floor.
For some planes that turn the entire fuselage over to cargo and have no side windows, such as C-130, C-17, they don't have that passenger interaction - they also benefit from mounting the engines high above a potentially contaminated landing/takeoff surface.
A goal for the 737 was operation from dirt and grass strips where luggage and cargo handling equipment was people powered, so yeah, keeping the LG short was important. However the trade is susceptibility to sucking dirt and gravel off the ground, which is one reason for longer gear on most passenger jets. Instead, the 737 with tougher stubby gear, got gravel deflector kits, though I think those were only available for the early designs.
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Son of Slot
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All visitors to the cabin are welcome to test the limits of my tolerance ... ! 
Here is a new test by Boeing: Transonic Truss-Braced Wing
Here is a new test by Boeing: Transonic Truss-Braced Wing
Of the two longstanding major manufacturers of these aircraft, Cessna and Piper, one is traditionally high wing and one low wing. Those of us who have handled both notice the differences, things like handling in turbulence in different ways. The "better ground view" that a high wing supposedly offers, in reality doesn't really happen.
There's no key upside, it's one of those things where there are 101 advantages/disadvantages for each, and it depends on the detailed objectives of the designer. For the 737, the original design used smaller diameter engines, aircraft are more designed around the capabilities of the engines than you might think, and over time the engine size has just grown. I don't think the 737 was actually designed for rough strips, there was a later adaptation (favoured mainly in Canada) that did a number of modifications to handle this, but it wasn't a mainstream market requirement. The 757, which is basically a 737 fuselage, used much bigger engines from the start, and is much higher off the ground, yet has become a key type for the cargo industry. The 737 has an issue that the basic structure frame was initially sized for the landing gear to fold up into, and as the desire for longer gear arose it no longer fits without a complete structural redesign.
There's no key upside, it's one of those things where there are 101 advantages/disadvantages for each, and it depends on the detailed objectives of the designer. For the 737, the original design used smaller diameter engines, aircraft are more designed around the capabilities of the engines than you might think, and over time the engine size has just grown. I don't think the 737 was actually designed for rough strips, there was a later adaptation (favoured mainly in Canada) that did a number of modifications to handle this, but it wasn't a mainstream market requirement. The 757, which is basically a 737 fuselage, used much bigger engines from the start, and is much higher off the ground, yet has become a key type for the cargo industry. The 737 has an issue that the basic structure frame was initially sized for the landing gear to fold up into, and as the desire for longer gear arose it no longer fits without a complete structural redesign.
Large high wing aircraft typically have wing anhedral. Large low wing aircraft typically have wing dihedral. The difference is required to give accepable roll stability for each wing position. Anhedral is destablizing and counteracts the natural stability of the high wing configuration.
As to the better ground view of high wing - depend on where the pilot sits relative to the wing. Pilot forward of the wing then little or no difference. Pilot above/below the wing makes a huge difference.
As to the better ground view of high wing - depend on where the pilot sits relative to the wing. Pilot forward of the wing then little or no difference. Pilot above/below the wing makes a huge difference.
