Sport Aviation – 01/85
By Tony Bingelis
WITH ALL THE new homebuilt, ultralight and lightplane (ARV) designs crowding into the homebuilt scene I sometimes wonder if there aren't more aircraft designers than there are builders. Among the current crop of favorite homebuilts are some very nice designs. So nice that I wish I could start building 3 or 4 of them at one time. But, I assure you, one homebuilt project at a time is plenty for anyone.
All of those nice designs have cockpits but who thinks of cockpit accommodations when selecting a design to build? It is the aircraft's overall appearance that we get hooked on initially. Then, predictably, our interest will switch to the aircraft's touted performance, but seldom does the matter of cockpit size and interior layout command much of our attention until sometime later. Of course, there may be an exception to this process, especially if the airplane design is exceptionally small, or if there just happens to be a prototype before us that we are invited to climb into and try on for size.
At any rate, no matter what selection process you follow in picking a project, don't get completely taken in by the beauty of a particular design without first determining other essentials, like the cockpit's ability to accommodate you comfortably, for example.
A good roomy cockpit should be rated on a par with good looks and good aerodynamic performance. If the cockpit compartment is cramped or poorly designed, you will become disenchanted with your pride and joy soon after it is completed. Avoid that dismal prospect by giving early attention to the cockpit features that are really important to you. If you are already building and have never sat in, or flown, a copy of the design, you might well be wondering how well your cockpit will fit your requirements.
The Cockpit Area In General
I guess most any builder expects a few surprises to crop up during the construction of his project. But one surprise that nobody expects or wants is to learn that his cockpit is too small for him, that it is difficult to get into and that there is insufficient legroom. Perhaps it is also too narrow for his broad shoulders.
Insufficient headroom can become a major irritant, too. But of course there is a penalty to pay for generous headroom and shoulder width accommodations in any cockpit and that is profile drag. You should, therefore, expect that cockpit space will ordinarily be at a minimum consistent with "average" comfort needs because profile drag is usually reduced with a reduction in profile area. Crowded or not, all of your cockpit housing and control functions will have to be made to fit into the confines of the compartment area set aside for the cockpit or cabin (Figure 1).
Before you undertake to install any of the miscellaneous support systems and equipment, you must decide where to install the seat or seats. The seat must be positioned and installed to assure your comfort and efficiency during the operation of your airplane. This means that you will have to determine the proper horizontal and vertical position best suited to your own physical configuration.
A cockpit designed or built especially for your own use need not have an adjustable seat. That would only involve a lot of unnecessary work and extra weight. Just make sure that you position the seat where you want it before you build it into the structure.
If you intend to allow other pilots to fly your airplane, or if you are building a two-seater, adjustable seats and/or rudder pedals could be worth installing. Any adjustable seat should be of a simple design and must be constructed and proof tested to your satisfaction before installing it in the aircraft. But remember this . . . if is far easier to make the rudder pedals adjustable than it is to engineer and build a sliding seat installation. Besides, a runaway seat on take-off could be disastrous. That sort of thing does happen, you know.
The structural design of the fuselage exerts a big influence over what you can or cannot do in locating the seat. Often it must be bolted directly to the cockpit floor (no legs) in order to obtain the minimum headroom you can get by with.
In most designs other than the low wing type, the horizontal location of the seat can be changed to a limited extent. The reason you cannot alter the seat location in many low wing aircraft is because the front spar, typically, cuts across the fuselage in the cockpit area. In small aircraft this absolutely restricts the seat location. The usual practice, in the past, has been to plunk a plywood slab down over the spars to serve as a (butt numbing) seat. I'm happy to say that most homebuilders have progressed beyond this primitive solution, or at least they aspire to install something more comfortable. Present practice is to install a dished out base for the seat bottom in the space between the two spars. As I recall, Mr. Pazmany, designer of the PL-1, PL-2 and the popular PL-4, was one of the early leaders in using this technique for installing seats in all his designs.
An immovable seat leaves you with one alternative for making legroom adjustments - adjustable rudder pedals. That is you will have to modify your rudder pedal connections so that they can be adjusted to position the pedal further forward or further back (see Figure 2). This will help obtain the proper leg angles for both short and tall pilots.
The seat back is another important part of the seating accommodation. Its angle of tilt or slope can, in effect, affect the amount of headroom and the degree of comfort or discomfort the pilot will have (see Figure 3). Unfortunately, its location, as well as its slope angle, will be limited by the structural bulkhead directly behind the seat. This bulkhead is most often positioned perpendicular to the thrust line or top longeron. In order to increase the seat back tilt angle, the bottom portion of the seat back has to be moved forward several inches. This, of course, will use up at least 6 inches of potential legroom. Some aircraft designs can easily give up that amount of space (mostly biplanes) while others simply cannot.
Entry and Egress Considerations
Now, how about getting in and out? Will your knees fit under the instrument panel? They bend only in one direction you know. If you're lucky, you probably have already had the opportunity of getting into and out of the prototype and are familiar with any shortcomings the cockpit design might have in this regard. If so, you can better anticipate the type of adjustments you may have to make, if any. For example, the bottom of the instrument panel might have to be raised slightly or the panel moved forward (away from you) to correct an entry difficulty unique to your needs.
If you have to move the instrument panel back to make room for gyro instruments between a front fuel tank and the instrument panel, be careful that you do not make it more difficult to get into and out of the cockpit.
Anyone building a two-seater should also think about his passenger's needs. Passengers are not, ordinarily, familiar with structural limitations and could damage your airplane or themselves if provision is not provided for a dignified entry and exit. A built-in handhold is a valuable aid and one that can be fitted easily to most any windshield bow. The provision for ease of entry or exit under normal and emergency conditions should not be compromised. I don't think that is asking too much, do you?
Control Stick Dilemma?
After you have resolved the seat location problem, you should turn your attention to finding the best position for the control stick. I'm sure that if you are building from plans, the control column mounting mechanism location is already established and you are "stuck" with that particular installation. Most of the time the location will be just about right. Sometimes, however, it may place the control stick in an awkward position, too far forward or aft for comfort. Of course, slight changes in the stick's neutral position can be made by lengthening or shortening the control cables (or elevator push-pull tube). However, this causes the control column to be slanted and may convey a false "neutral" sensation through your hand. A better way to make larger changes in the positioning of the control stick's neutral position is by making and installing a curved control column. A curved control stick will often eliminate or at lease minimize this kind of problem. It may also eliminate the problem of your knees or legs getting in the way and interfering when applying full aileron.
It is well to remember that a short control stick will increase control sensitivity and a long one will reduce it. However, don't make your control column so long that it just barely clears the bottom of your instrument panel because your knuckles won't.
Rudder Pedal Placement
Try locating the rudder pedals so that they permit your knees to be slightly bent (see Figure 3) so as to form an approximate 120° angle (no more than 155°) between your upper and lower legs. This will help you obtain full rudder travel and will establish a comfortable position for your legs. At the same time it will also give you the best foot angle for easy brake application.
The so-called "standard dimensions" for the spacing between rudder pedals is often impractical to achieve in most homebuilts. For example, in a single seat installation the rudder pedal location to either side of the centerline of the fuselage is determined more by where the control cables are attached than by anything else. If the rudder cables are routed along the fuselage sides, the rudder pedals cable attachment points will, naturally, be close to the fuselage sides. This may require the rudder pedals to be spaced quite far apart in a wide cockpit or quite close together in a narrow cockpit. In an installation where the cables are routed through the center of the cockpit, the pedals will probably have to be positioned closer together, either side of the centerline. Neither condition, however, should create an uncomfortable situation for you.
One feature that could create a poor cockpit condition is a wide center console. So will an extension of the instrument panel all the way to the cockpit floor. This is so because you will have to straddle the thing when you get in the airplane. Furthermore, you will be forced to keep your knees apart during the entire flight. The inability to move your legs around because of this hindrance may create a most uncomfortable condition. In addition, you may no longer be able to clamp the control column between your knees (poor boy wing leveler) while handling your navigation charts on flights into strange territory.
While on the subject of legs and knees, be sure that there are rounded smooth surfaces against which your knees may be rested. Sharp corners and unyielding structures can be torturous on a cross-country flight. Incidentally, a center console is not ordinarily a problem in a side-by-side two-seater.
Should you mount the throttle quadrant on the left side, military style, don't make the mistake of locating it too high or too far aft in the cockpit. If you do, you may find that you won't be able to pull the throttle all the way back because your elbow hits the seat back or aft bulkhead. A throttle positioned too high is tiresome if you, like many a pilot, prefer to keep your hand on it most of the time.
Visual and Visibility Factors
Before making that final installation for the instrument panel take a critical look at it. Is it too close to view comfortably? Your eyes can see more of it if the panel is not too close.
Are there sufficient areas of windshield and windows to see through? Areas free of large frames and braces that might be cluttering your field of vision? Of course, nothing can be done about moving the wings out of your line of sight, but in the early cockpit layout stages other potential hindrances can be minimized.
For your own safety, good visibility from the cockpit should never be sacrificed for the sake of esthetics. Super-sloped windshields, even if distortion-free, aren't always as practical or as aerodynamically effective as they may appear (see Figure 3).
Seat Belts and Shoulder Harnesses
Plenty has been written about seat belts and shoulder harnesses and I will only say here that they are extremely important, should be installed and must be anchored to solid structure . . . not the seat.
Believe me, there is a tremendous amount of difference in the safety, size and arrangement of cockpits among the many designs being offered to potential builders so, if you can possibly arrange it, fly it or at least try it on for size before you start to build it.
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