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Modifications... Good or Bad?
By Tony Bingelis (originally published in EAA Sport Aviation, December 1995)
All builders make them. Some are small and unimportant but others need to be carefully thought out before attempting to make the changes. It is interesting to note that builders of composite kits are less likely to attempt structural modifications than those building from plans. Perhaps those prefabricated structural components discourage the inclination to experiment with major changes.
Fortunately, most builders are satisfied with the design they select and conscientiously strive to build their airplane strictly according to whatever instructions come with the plans. If they can’t quite understand a particular detail, it bothers them until they can get the problem resolved by calling on the designer, kit manufacturer or some knowledgeable local guru.
On the other side, we have that impulsive impatient builder. He doesn’t have the patience to take the time to restudy the plans and reread the instructions or, for that matter, to resolve his problem with the designer or kit manufacturer. Instead, he thinks nothing of barging ahead, trying whatever fix first comes to mind, naively assuming that anything is O.K. if it works.
Just as problematical is the builder who can’t resist changing most anything that doesn’t look right to him. He thinks he can do it better. He seems to have forgotten what originally attracted him to that particular design and now thinks nothing of changing it into something it was never intended to be.
What compels an otherwise dedicated builder to make major alterations to the aircraft he has chosen to build . . . the one airplane that really turned him on originally? The reasons they give are many . . . some are good but just as often some they cannot adequately justify.
Some builders try to modify their airplane in the hopes of enhancing its high-end performance while almost as many want to improve the low-end performance.
But, that is not all. High on many a builder’s modification want list are other changes often contemplated at some time or other. Here are a few examples:
1. The urge to "improve" the appearance of the design (at least in the builder’s eye) by changing the shape of the tail, wing tips, etc.
2. Ways to improve the efficiency and performance of the aircraft.
3. Ideas for enhancing cockpit appearance and creature comfort.
4. Safety enhancement considerations (turn-over protection, quick release canopy, etc.).
5. Ways to simplify maintenance by providing easy improved access to most everything.
Most of these are worthy objectives and many of them can be achieved without altering the basic structural design of the aircraft.
But, even modifying the aircraft’s structure doesn’t deter some builders. To them, the challenge is the big deal . . . not the means for carrying out their desired modifications. They lock onto an idea and simply rely on "instinct and logic" to effect those changes for the "better."
Sometimes a builder will luck out and come up with a good way to carry out a particular modification, but his means for effecting the change, more often than not, will turn out to be heavier-and at times work poorly, if at all.
Structural Changes Are Risky
Sometimes a builder may unwittingly weaken the aircraft structure dangerously when in his mind he thinks he is really strengthening it.
A particular example comes to mind involving an individual who figured he would install the same thickness plywood skin all the way to the wing tip, instead of scarfing in the required thinner skin for the outboard wing section. He figured by installing the plywood skin in one piece he would eliminate all that scarfing work. This would make the wing stronger, he thought.
Unfortunately, instead of simply making the wing stronger, he only made it structurally stiffer and heavier. The increased thickness in the outer wing panel skin stiffened the wing structure, all right, but it also caused the wing flexibility to move inboard into the cockpit area where the spar was not beefed up nor designed to take the additional localized bending stress of a stiffened wing.
This modification, probably more than higher G forces encountered, later contributed to the reported catastrophic wing failure.
An uninformed builder can weaken his aircraft structure in some area other than where he is arbitrarily "beefing up" an imagined weakness.
It is also possible to inadvertently degrade the aerodynamic efficiency below that of the original tested and proven design. This can happen most any time you make structural changes involving the airfoil, span and/or incidence
Another structural change attempted by some first-time builders of wooden aircraft is to make butt joints instead of the required scarf joints. The builder rationalizes, "Making good scarf joints is too hard. Besides, a reinforced butt joint should be just as strong."
But is it?
The best wood joint is one made with a 10 to 1 (minimum) taper as it will transmit very nearly 100% of the load from one plywood panel to the other.
Anyone who is tempted to use a butt joint for ease of construction must install a suitable doubler behind it. This doubler should be at least as thick as the thickest plywood panel being joined, and 20 times that thickness in width.
This will give an ample glue surface on each side of the joint equal to 10 times the thickness of the skin. Anything less than this width would cause the stress flow being transmitted across this joint to make too great a change of direction (see Figure 1).
Everything considered, I must say that I am against the average builder making structural modifications to somebody else’s design . . . especially by a builder who lacks the engineering instinct, resources or qualifications to do so.
Other Frequently Attempted Mods
Modifications involving the engine:
These seem to be the most tempting type of modifications by far. They generally entail supercharging the engine, installing a more powerful aircraft engine, or substituting a converted auto engine for the originally recommended aircraft powerplant.
This type of change must be carefully evaluated before it is attempted. This is particularly important when the aircraft structure is not designed to handle the extra engine power and the anticipated higher airspeeds which might conceivably be achieved.
In addition, consideration must be given to any significant increase in weight which would, undoubtedly, affect the aircraft’s CG and load carrying capacity. This type of modification is especially tempting to builders of VW powered aircraft like the KR-2, the Sonerai, as well as other light aircraft originally designed to be powered by lightweight two cycle engines (Rotax, etc.).
Replacing the relatively lightweight VW and Rotax engines with a significantly heavier standard aircraft engine such as the A-65 or C-85 at the very least creates a difficult-to-solve weight and balance problem which must be thought out carefully beforehand.
Also to be considered is the possibility that a more powerful engine is likely to burn more fuel. The existing fuel capacity may not be sufficient to provide the bigger engine with a reasonable 3 hour range without somehow adding to the fuel system’s capacity. Often, this can require important structural changes in addition to fuel system changes.
Ordinarily, the larger the engine the higher the acquisition cost . . . but not always. This would depend on the supply and demand for that particular engine model and type.
The gross weight modification game:
Unfortunately, it is true many builder learn to their disappointment that their aircraft exceeds the designer’s prototype gross weight by 50 to 100 pounds . . . sometimes more. And yet, they intend to carry the same payload as advertised by the designer for his spartan-like prototype. Here’s the dilemma typically faced by many builders of two seater aircraft:
a. Certificate the aircraft listing the designer recommended gross weight on the Data Plate-and limit the payload accordingly?
b. Assume his two seater, to be practical, must be able to carry two 200 pound passengers, full fuel and a generous baggage allowance? Add this total weight to the aircraft’s empty weight and list that as the gross weight on the Data Plate?
If the airplane weighed in with an empty weight of 100 or 200 pounds more than the original design empty weight, a bad situation becomes immediately apparent.
This extra gross weight limit, of course, has a drastic influence on the takeoff and climb performance and has the effect of reducing the loads the structure can tolerate in flight . . . and especially in turbulent and gusty conditions.
It is well to remember that aircraft used for aerobatics are limited to a gross weight well under that established for normal operations. Violate this and you are jeopardizing your own safety and that of your airplane.
Unfortunately, "increasing" the aircraft’s gross weight over that established by the designer is usually a paper modification indulged in by quite a few builders. Because the builder is the aircraft’s manufacturer, he is the one who establishes the gross weight for the FAA records (usually with no challenge from the FAA).
However, an airplane is not like a dune buggy which will carry everything you can jam into it without causing the vehicle to fall to earth . . . it is already there.
The added fuel capacity mod:
This could be difficult, depending on the aircraft’s design. A metal wing with two or three rib bays originally established for an integral fuel supply could easily enough, during construction, be modified to include another rib bay or two for added fuel. Often this added fuel capacity can be provided without weakening the wing structure. It would be wise, however, to check with the designer or kit manufacturer before making the plunge. The addition of a separate fuel tank in the baggage compartment area or just behind it is a fairly common approach to providing additional fuel capacity.
However, consider this. You will be adding fuel well behind the CG. This could move your CG outside its aft limits . . . a dangerous situation especially at low airspeeds and stall conditions. Furthermore, fueling an aft fuselage tank could result in slopping fuel inside the fuselage.
Such an installation should be constructed so that any overflow fuel will be collected into a built-in scupper that accepts the overflow and allows it to safely drain overboard.
The addition of another fuel tank will definitely alter the original proven fuel system and may introduce unexpected venting problems as well as fuel flow and fuel management problems.
Don’t forget the added fuel capacity will increase your empty weight and gross weight. Think the proposed changes through carefully before making such a modification. Do you really want and need the extra fuel?
Converting a tri-gear to a taildragger or vice versa:
This can be a tricky change and should not be undertaken lightly because of the structural changes which may be required. In addition, the correct positioning of the wheels in relation to their ground contact is critical.
For example, canards such as the VariEzes and the Long-EZs could not be rotated for takeoff if the wheels were to be moved back far enough to keep the airplane from falling over backwards when parked on the ground . . . that’s why the nose down parking attitude.
Positioning the wheel location for a taildragger is equally critical. Locate the wheels too far forward and it will be difficult to impossible to raise the tail for a normal takeoff. Equally bad is having the wheel location too far aft. In this case, the slightest application of brakes can result in a sudden uncontrollable nose-over.
Wing tip modification:
Usually little benefit is obtained by modifying the wing tips. Whatever benefits wing tip plates produce is more than offset by blocking vision beyond the wing tips.
As for those sexy contoured drooped tips, it seems to be a matter of personal preference for the resultant appearance more than any anticipated increase in speed. Sometimes the low speed stall characteristics may be improved due probably to an increase in the extended wing area rather than to any "aerodynamic magic."
Drag reduction modifications:
Drag reducing fairings and covers are relatively simple to make and install. Whether or not there will be a measurable reduction in drag, however, is something else. Here again, it is a good idea to make one change at a time and test fly it to see if there really was any speed increase or an improved control response. Essentially, what you are dealing with in drag reduction is a need to close all unnecessary gaps and smooth the flow of air over the aircraft’s surfaces.
Of course, any time you can remove anything from the slipstream, drag will be correspondingly reduced. There is one thing you should not overlook. If the drag reduction efforts add considerable weight to the aircraft, the benefits will be hard to justify.
On Modifications In General
These are just a few of the modifications builders like to make, along with some of the ramifications from such efforts. The most successful modifications are those that do not involve structural changes.
On the other hand, you can expect that changes like lengthening the fuselage or increasing the wing span will place additional loads and stresses on the original portion of the structure. Always keep in mind that structural failure in flight is generally catastrophic. Whenever possible every attempt should be made to subject the modified structure to a new stress analysis.
Unfortunately, the structural concerns are not the only problems that must be addressed. Any change in the aircraft’s configuration or airfoil shapes can result in unexpected aerodynamic behavior leading to control difficulties, vibration, and/or flutter problems.
An appreciation for and an understanding of fundamental aerodynamic laws is essential if you expect to experiment with modifications in this area. This admonishment, of course, would apply to experiments involving high lift devices and STOL (short take off and landing) modifications.
If you are determined to experiment in this area, I would again suggest you attempt only one change at a time, checking it out thoroughly before going on to other changes.