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Ergonomics and Seatback Angles

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Apollo

Well-Known Member
Joined
Feb 7, 2010
Messages
301
Location
Southern California, USA
Over the years, I've seen sketches of aircraft designs that placed the occupants in uncomfortable positions, yet the designer had no clue this was the case. This aspect of design is frequently misunderstood or even ignored. But the ergonomics of your completed aircraft will have a major impact on pilot comfort and the perceived "goodness" of your design.

Good ergonomics usually starts with seat design, and seat design starts with the selection of your seatback angle. The seatback angle will have important consequences on the frontal area, drag, outward visibility, instrument panel design and the size of your cabin. Seatback angles also determine the optimal angle for the seat pan. Experienced engineers will recognize these relationships and include them as part of the design process.

Seatback angles are normally specified as an angle of recline from the vertical axis. We see a typical airliner seat in Attachment-1 below. The "upright" position (reclined 16 degrees) results in an angle of 95 to100 degrees from the torso to the thighs. The spine is almost vertical and that results in higher pressure on the spine and buttocks. That position is often uncomfortable on long flights. The reclined position results in a 110 to 120 degree torso-thigh angle. This places less stress on the spine and buttocks area. Studies have shown that stresses on the spine are most evenly distributed when thighs are 135 degrees from the torso.

Note the lower legs are not far from vertical and the seated passenger takes up quite a bit of vertical space. This allows the "pitch" between rows of seats to be minimized. So if you were designing a 6 place aircraft with 3 rows of seats and were trying to minimize CG variability due to passenger loading, you might consider tall seats and upright positions. This permits a shorter cabin but requires the cabin to be taller as well.

Attachment-2 shows some typical seatback angles for older homebuilts. The seatbacks are reclined 0 to 15 degrees from vertical and the sketched pilot is shown in a position that will become uncomfortable. Angles less than 90 degrees between the torso and thighs create high stresses on the spine and buttocks, and also results in the perception of a cramped cabin area.

Attachment-3 shows conventional and modern seating examples. This drawing includes min/max values for comfortable angles between the upper and lower legs. We also see the effects of reclining the seatback 30 degrees or more from vertical:
1) The cabin height can be reduced, which lowers the frontal area.
2) The cabin length must be increased due to the stretched out body.
3) The pilot's head is lowered and visibility over the instrument panel is reduced. The panel must be lowered or reduced in height to preserve visibility.
4) The canopy becomes longer to maintain outward visibility.
Reclining the seatback to 42 or 45 degrees from vertical (as is common with many canard aircraft) results in the above effects plus the following:
5) Thigh supports are required or the extended leg muscles will get tired quickly.
6) Headrests are required to support the neck and head.
7) The instrument panel must be moved aft to remain within reach of the pilot's outstretched arm. This results in the pilot's knees being further forward than the lower edge of the panel, which may inhibit entry/exit ease when a canopy is used for access.

Some pilots and passengers have noted the "bathtub effect" with 45 degree seatbacks. They feel like they are reclined in a bathtub and just barely able to see over the edges. Clearly there are a lot of design considerations here!

Attachment-4 (contributed by Autoreply) shows a recumbent seat on a high performance sailplane. This results in very low frontal area and minimum drag. Observe how the canopy must extend well forward onto the nose. The instrument panel is reduced to an absolute minimum to allow a forward line of sight. You're not going to fit an IFR panel onto an aircraft with recumbent seating! The extreme recline results in extended neck muscles and the pilot flies with his chin laying on his chest. He may need a nice neck massage after a long day of soaring!

In summary, the optimum seatback angle will vary depending on the type of aircraft, mission profile and other design objectives. Each range of seatback angles has a significant impact on the cabin configuration, canopy design, instrument panel space, and other areas. Bad ergonomic design results in fatigued pilots and uncomfortable passengers, which is irritating at best and dangerous at worst.

This article just begins to cover the ergonomic factors that affect cockpit design. I'm sure other HBA designers can expand on this. Additional references on aircraft ergonomics are just a Google search away.

- Mike
www.apollocanard.com

Attachment-1.jpgAttachment-2.jpgAttachment-3.jpgAttachment-4.jpg
 
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