In June, an article entitled “Mod Masterpiece,” appeared in Sport Aviation. It extolled many features of the absolutely gorgeous interior that Greg Hale built into his award-winning RV-10. Unfortunately (perhaps unwittingly) the article drew our attention more to the price he paid than his admittedly wonderful workmanship and customization. No, I’m not referring to the usual costs measured in dollars and building time. I’m referring to the price that airplane builders often pay in reduced utility and, more important, in impaired safety.
The article started with a pull-quote: “The RV-10 impressed us since you could load four passengers and bags and be well within the maximum gross weight and CG.” Normally, that’s true. An RV-10 usually weighs about 1600 lbs empty, so with its rated 2700 lb gross it has an 1100 lb useful load. That translates into four 170 lb people, sixty gallons of fuel, and sixty pounds of baggage. But given what article goes on to describe, this quote appears increasingly ironic.
Mr. Hale’s modifications and additions had a dramatic effect on the empty weight of his RV-10. The reported empty weight of 1848 lbs -- 248 lbs over the 1600 lbs that we’d consider “standard.” This translates into the equivalent of 1½ passengers who must be left behind or 41 gallons of fuel, which must remain on the ground if the airplane is to remain within the design gross weight limit of 2700 lbs. With full standard fuel tanks, his RV-10 effectively becomes a 2-seat airplane. Then, we noticed the spec sheet accompanying the article giving the fuel capacity as 120 gallons! If this is accurate, it means that, in addition to the cabin interior modifications, Mr. Hale apparently installed additional fuel tanks in his RV-10 and doubled the standard sixty gallons. With 120 gallons on board, his RV-10’s payload would be further reduced to a 132 lbs -- not even a single-seater anymore.
Here’s another, perhaps more appropriate, quote: “You can’t have your cake and eat it too.”
Many builders will tell you that it is not possible to meet the factory empty weight figures. In some instances this may be true – some kit suppliers have been known to optimistically quote an empty weight based on an unfinished and unequipped prototype, or weights that could never be equaled by subsequent builders. But the 1600 lb. empty weight Van’s Aircraft quotes for a 260 HP Lycoming-powered RV-10 is realistic. An example is my personal RV-10, built from a standard kit and employing no special weight saving efforts. It weighs, empty, just 1595 lbs. This includes full paint, wheel fairings, EFIS instrumentation, radio, transponder, GPS, 2-axis autopilot, ELT, an intercom system w/CD, carpeting and headliner, and landing lights. Though it may seem spartan to some, it is comfortable and totally functional for long-distance VFR flight, day or night.
From long experience we anticipate that builders will customize, and will add weight in the process. This does not mean that there are not compromises or penalties involved. At the very least, any added weight will subtract from the useful load of the airplane. This is the reason that so many 4-seat factory airplanes cannot fly with full seats and full fuel at the same time. But for homebuilt aircraft, this is a compromise any builder has the freedom to make, and many do. But adding 248 lbs of “stuff” in the example above is rather extreme. It is the equivalent of adding the weight of an entire ultra-light, engine and all. It’s almost equivalent to adding another pair of RV-10 wings.
The spec sheet also notes Mr. Hale’s airplane has a listed gross weight of 2800 lbs instead of the 2700 lbs the factory specifies. Yes, we realize that a builder of an Experimental Amateur-Built airplane can list any gross weight or flight limits he wishes. It’s just that we don’t accept that. Our factory specified gross weight is based on the best science we have available. This includes careful stress analysis calculations and extensive static load and flight limit testing. By way of contrast, we wonder what basis Mr. Hale (or any other builder who uses a higher-than-recommended gross weight) uses for establishing the 2800 lb gross weight of his airplane? If it isn’t based on the same science and testing, we simply cannot recognize it as valid, and neither should anyone else. Any “penciled in” gross weight increase is just wishful thinking. The laws of physics are not repealed by wishful thinking.
But this discussion of payload trade-offs is not the primary purpose of this paper. While we hate to see our laboriously designed 4-seat payload erode to a 2 1/2 seat limit, that is the builder’s privilege. Our primary purpose here is to point out several modifications made to primary flight control systems and safety features. We feel these are detrimental to safety, and that readers and other RV-10 builders should be aware of our concerns. Modifications undertaken for otherwise good reasons can have negative consequences.
Specifically, we see a real problem with the front seat shoulder harness attach modification. As designed the RV-10 uses a two-strap shoulder harness attached to a hard point in the structural cabin top. We used the two-strap (one over each shoulder) harness because it is the universal aircraft standard and has been demonstrated to be superior to the automotive style single cross strap. Anchoring the harness to a hard point in the cabin top provides a near ideal load path for crash restraint forces. (See illustration 1.)
The subject airplane uses a single cross shoulder harness anchored to a hard point in the fuselage under and aft of the seat. The strength of the anchor point is somewhat irrelevant in this installation, because the load path (see illustration #2) essentially applies the crash loads to the top of the seat back. The low anchor point for the shoulder harness causes the tension in the strap to bear down on the occupant’s spine, and to pull forward on the top of the seat back. The back of the Oregon Aero seat supplied in the RV-10 kit was not designed to withstand shoulder harness crash impact acceleration forces. When the seat back fails, the upper body will pitch forward because the shoulder harness essentially becomes slack. While some automotive seats do apply the shoulder harness loads to the top of the seat backs, we assume that those heavy automotive seats have been adequately designed and tested for this purpose. The RV-10 seats have been designed and tested by Oregon Aero, Inc. to withstand anticipated crash impact loads of the occupant, but not acceleration loads transmitted through shoulder harnesses.
Another safety feature of the Oregon Aero seats is the foam used to make the cushions. Its type, density, and lamination schedule have been carefully tailored and tested to absorb vertical impact loads. Any changes or replacements may not provide equivalent protection.
In addition, the modification made to the active seat belt attach points is suspect. Our design provides for each belt attached directly to anchor points in the airframe structure. Anticipated crash acceleration loads are transferred in linear tension into these hard points. In the subject airplane, the seat belts are attached to a small diameter cross shaft between the intended structural hard points. (See photo 1) Crash acceleration loads will be applied normal to this cross shaft, loading it in bending, which in turn will apply eccentric (twisting) loads to the mounting brackets in the cabin structure.
The rear seat shoulder harness modification of the subject airplane also uses a single cross-strap rather than the standard RV-10 dual-strap harness. The load path into the airframe is again an unknown – in contrast to the static load testing performed on the factory supplied harness assemblies. These transmit loads linearly to the aft fuselage structure.
Another worrying modification altered the attachment of the rudder cables to the rudder pedals. Mr. Hale used an offset stud (see photo 2) on the rudder pedal to which the rudder cable is attached. While this may provide a more attractive cabin appearance, it causes an inferior load path for the rudder control forces. Cable tension loads will apply a twisting force to the rudder pedal attach horn.
While we’re on the subject of modifications and how they might affect safety, let’s go back to that question of sixty extra gallons of fuel. There is a cute saying in aviation that “the only time you have too much fuel is when your airplane is on fire”. Unfortunately, this is not entirely accurate. The most obvious exception comes when the fuel load causes an over gross weight condition that adversely affects performance and flight safety. Even if the additional fuel weight is within gross weight and C.G. limits, the location of the added weight can adversely affect the aircraft’s polar moment of inertia. Reduced to its most understandable form, it means that the spin recovery characteristics of the aircraft will be affected. While the article never says where the extra fuel goes in this RV-10, the most likely place for additional fuel tanks would be in the outer sections of the wing—outboard of the standard wing root leading edge tanks. With regard to the polar moment of inertia, this is possibly the worst place (other than in the tail) to add weight to an airplane. Also, weight added anywhere in the wing will affect the flutter characteristics of the wing. The RV-10 wing has been subjected to Ground Vibration Testing (GVT) with standard tanks both full and empty. With significant weight of any kind, structural or otherwise, added to the wing, the flutter speed limits will change – and until the new arrangement is tested, nobody knows what the new limits will be.
Like many kitplane suppliers, we endeavor to supply very complete, thoroughly designed and tested airframe kits. It is our hope that builders will construct the airframe assemblies in compliance our proven design. Most do. Details such as instrumentation, avionics, and cabin interior appointment are often not included in kits because we know from experience that builders have very special individual preferences for these details. These are areas where builders can usually express their individuality without as much concern for safety of flight as would be the case with changes to the structure or aerodynamics. I say usually because even any seemingly insignificant part of an aircraft can affect safety of flight. With reasonable care interior appointments will remain benign.
We all know that builders of Experimental Amateur-Built aircraft have the right to make changes to their aircraft at will – whether or not their changes are based on good science. If they choose to operate the aircraft with a lesser or unknown margin of safety, that is their prerogative. However, unless the aircraft is single-seat, any passengers carried in that aircraft will be exposed to the same unknowns that the pilot has accepted for himself. We feel that this is a responsibility often overlooked by pilots. While they may be willing to accept certain risks for themselves, what should their responsibility be to their spouses, friends, children, and grandchildren?
SIDEBAR: WHO OWNS THE MARGIN?
It seems common practice among homebuilders to second-guess the factory engineers, particularly regarding gross weight increases. Because of all of the “I gotta have” added features, empty weight creep erodes the aircraft’s useful load. The simple solution for the homebuilder is to “pencil in” a new gross weight limit. It’s only 100 lbs. (3.7%) more; how much effect can that possibly have?” Imagine this example: you are on a mid-size airliner with a gross weight of 270,000 lbs. Just before leaving the gate, the captain comes on the PA system and says: “we’ve overbooked more than usual today, so we’re going to assume that the factory engineers over-designed this airplane and allowed an abundant safety margin. We’re going to take off at 280,000 lbs. instead. So move over, there are 50 more passengers coming on board.” Run the numbers; it’s the same over-weight ratio as simply pencilling in an additional 100 lbs to the gross weight of an RV-10.
Along with gross weight increases, some builders take the same liberties with horsepower increases and speed increases, betting their lives on the assumption that the airplane is designed with a huge margin of safety---it is really far stronger than in needs to be. This is not really true. Certificated aircraft, and well-designed kit aircraft, are designed to withstand limit loads at specified maximum weights. During testing, they are subjected to ultimate loads, which are higher than design limit loads by a specified margin. Yes, there is a margin between the design and ultimate strengths. But that margin belongs to the engineer. He owns the margin. It is his insurance against the things he doesn’t know or can’t plan for, and the pilot’s insurance against human error, material variations, and the ravages of time. Wise pilots respect this design safety philosophy and leave this insurance policy in effect by operating strictly within established limits. They don’t try to steal the margin from the designers.