The RV-10 is a low wing, four place, metal aircraft. It is being designed to accept engines in the 200-260 hp range. It is intended as a moderately fast touring airplane that will carry four real people, a reasonable amount of baggage and about 60 gallons of fuel. It will not be aerobatic, although we are working hard to endow it with enjoyable handling characteristics, including the typical RV characteristics of excellent low-speed handling and control. With a 260 HP engine, we are hoping for cross-country speeds approaching that of a 180 hp RV-7/8. With more economical 200-220 HP engines, cruise speeds would be about 15 mph lower, but take off and climb performance should still be good. Given the intended mission only a fixed tricycle gear is planned.

The tail will consist of a constant-chord horizontal stabilizer and elevators with riveted trailing edges, similar to the current RV-9A. The vertical stab and rudder are typically RV. Engineer Mike Schwartz has designed some innovative ribs that should make the control surfaces easier and quicker than ever to build.

The wing, at least at the moment, is a constant chord design with a 31.5’ span and single slotted flaps. The wing area calculates out at 147 sq. ft. which yields an aspect ratio of 6.75. This is greater than both the 4.76 of the RV-4, and even the 6.34 of the RV-9. The higher aspect ratio is desired (necessary?) to enhance climb and glide performance of a heavier airplane. The airfoil section we have chosen will (heh, heh,) remain a secret at this time.

The fuselage bears a passing resemblance to the rest of the side-by-side RV family, and provides forward facing seats for four reasonably sized adults. After considering sliding canopies, hinged hatches, transporter technology licensed from Starfleet, and other options, we settled (although we may still change our minds) on doors, one on each side. We’d prefer a single door, for simplicity and fuselage strength, but are afraid buyers would disagree.

The firewall forward portion is open to a variety of available engine options. We are tentatively planning on a 260 HP Lyc. IO-540 engine for the prototype. Other 6 cylinder engines, from the lower HP Cont. IO-360 and PZL Franklin 6A-350, through the larger Cont. IO-470 and IO-520, are possibilities. A lot will depend on used engine prices and availability, and on customer preference. Though there are a lot of possibilities, we’ll have to limit the number of firewall forward options which we can practically offer.

The kit? The metal portions of the kit are planned to be totally matched-hole, similar to the RV-7/9A kits we are selling today. We hope to make the standard kits even easier to build, and of course, there will eventually be RV-10 QB kits. Realistically, with an airframe at least 25% larger, twice the number of seats, doors, a baggage door, etc. will take noticeably longer to build than a comparably technology RV-9A or RV-7. Kit cost? It’s much too early to speculate with any accuracy. As with our other kits, we’ll be striving to make this one a good value.

Recently, we announced that Van’s had begun work on a 4 place kitplane, the RV-10. Since then we have received some letters and E-mail praising our efforts, a few condemning them, and several suggesting design features the writer felt would be desirable. These letters are always interesting…if nothing else they demonstrate how varied the demands of the market can be…one reason why all airplanes are flying compromises.

One writer suggested a short list of design features he thought appropriate, along with his rationalization for "doing it that way." In addition answering his letter, I felt that maybe a discussion of a couple of his suggested points might be of interest to all of you; so that you can better understand why we are following the design path we have chosen.

His suggestions:

• 1. Use a short span, low aspect ratio, "thick" wing, much as we have done on most of our previous designs. He reasons that such a wing does not impair wing drag or lift and that the RV’s good climb rate with short wings is achieved because of a low wing loading. Such a wing would also provide better spar depth and strength, the wider chord would mean less sensitive C.G. limits and it might achieve a greater angle of attack at stall.
• 2. Use a sliding canopy — at least as an option.

Let’s take a look at the reasoning in the first suggestion, point by point. It is true that we have used low aspect ratio "fat" wings very successfully on most RV designs. However, those airplanes were designed for a job the four-place will not be called upon to do. They were airplanes designed for aerobatics and sport flying. The short wing was easy to build, and the short span meant low bending moments, increased strength and rapid roll acceleration. But now the job required of the RV-10 is different. Will a wing that serves the sport RV work as well on an easy-to-fly, efficient-cruising, people-mover?

First, a few definitions:

• Wing loading is, of course, the area of the wing divided by the weight that it must lift. An 1800 lb airplane with a wing area of 100 square feet has a wing loading of 18 lbs/ft².
• Span loading is calculated by dividing the wingspan by the weight. An airplane with a weight of 1800 lbs and a span of 25’ has a span loading of 72 lbs/ft.
• Power loading is the weight of the airplane divided by the power propelling it. An airplane weighing 1800 lbs being pulled along by 200 hp has a power loading of 9 lbs/hp. (By some twisted semantic reasoning, a "high" power loading is a lower number…an airplane with a power loading of 15 lbs/hp is said to have a "higher" power loading than one with 20 lbs/hp. This was probably dreamed up by the same guy who decided that larger AWG drill sizes would have smaller numbers…).

First, consider climb rate. Basically, climb rate is linked most closely to power loading. It is determined by the engine horsepower or thrust available in excess of that required to maintain level flight. But span and span loading come into play as well. Everything else being equal, a shorter, lower aspect ratio wing has a higher induced drag, especially at low speeds, than a wing with a greater span and lower span loading. The shorter wing requires more horsepower just to maintain level flight at low speed. Since the power available is finite, this means less power remains for climb.

The RV-9A is another example of how, sometimes, power loading is more important than wing or span loading at climb speed. Although its longer wing gives it a lower wing loading and lower span loading than the RV-6A, its climb rate (for a same size engine) is not dramatically better. But, if these airplanes were flown at 50% power (or at high altitudes where power output diminishes), the benefits of the RV-9A’s lower span loading would become more noticeable. Handling qualities or control feel are also considerations. At low speeds, the RV-9A is noticeably better because of its lower span loading.

Even though the induced drag is higher at typical climb speeds with a low aspect ratio wing than it is with a high aspect ratio, most RVs have a good climb rate because they have a lot of horsepower per pound of flying weight; around 10 lb/hp or better. This more than offsets the induced drag of the short wing.

It is only when airplanes with low power, or low power loadings, are considered that the relationship of span loading and climb rate become primarily important, even critical. Consider my DG-400 self-launch sailplane. It has a span of 57 ft., a flying weight of 1000 lbs, a whopping 43 HP, and a respectable 600 fpm climb rate. Interestingly, its weight is almost the same as the RV-3. How well do you imagine the short winged RV-3 would climb with only 43 HP?

Now, let’s apply some of these considerations to a potential 4-place airplane. The table shows some rough examples of specifications a 4-place airplane might have if its wing design were driven by various goals. In the left RV-10 column, and in Fig. 1, we have a hypothetical airplane with the same aspect ratio and the same wing loading as an RV-3. Just doesn’t look right, does it? From the structural viewpoint, there’s a lot of problems here. Such a wing would require a much bigger horizontal tail, it would be difficult to attach to a sleek fuselage, etc. And on the performance end, we have calculated that, to have climb rates equal to an RV-3, such an airplane would require 321 horsepower. Bad news… the real world hasn’t provided us with a practical engine choice in this horsepower range. We are designing around engines that are readily available, betweem 200 and 250 horsepower, so we are essentially back to the "low power" scenario. Our power loading is going to be much lower than the RV-3, and if we are to achieve a respectable climb rate, we need to have a longer span/lower span loading. The induced drag the short wing assumes a proportionately larger role, and there isn’t enough power to overcome it.

The middle RV-10 column (see Fig. 2) describes an airplane with 147 square feet of wing area (our working number on the RV-10), but with the same aspect ratio as an RV-3. In the right column, we have our current RV-10 thinking.

Either of the lower wing area (147 sq. ft.) examples should yield a higher cruise speed. By choosing a higher aspect ratio wing, we can maintain the same span loading as we would have had with the large, low aspect ratio wing. Thus, despite the higher wing loading, we expect a climb rate as good as could have been achieved with greater wing area. However if we chose the low wing area along with the short span, the climb and descent rates would suffer. Since our wing loading is considerably higher, the stall speed will be higher, although we expect to limit the stall speed through the use of long, efficient flaps.

The flip side of climb performance is glide performance. With a high enough power loading, you can have a good climb rate even with a high wing loading and short span, but such an airplane won’t glide well. While we want to climb at the highest possible rate, we don’t want our power off descents to be at an excessively high rate. The same aerodynamic factors that cause an airplane to climb poorly cause it to glide poorly also. We want to avoid the "streamlined brick" syndrome, so we can end up with an airplane with descent rates and angles that minimize piloting skills and workload. Thus, a wing with a greater span and lower span loading is perhaps more important for glide performance than climb performance. For example, take the Space Shuttle. It’s all thrust and no wing on the way up, all wing (what there is of it) and no thrust on the way down. It is a dreadful glider that requires great precision to fly safely. The designers had to make awful aerodynamic choices to accomplish the mission.

How about next point: greater spar strength, wider chord for less sensitive C.G. limits and a greater angle of attack at stall?

Basically, we agree. However, while the wider chord does improve the C.G. range, measured in inches, the expanded range requires larger trim forces. This means, for a given wing area, either one needs more spacing between the wing center and the horizontal stabilizer, or a larger stabilizer or both. Either way, it means more drag. Regarding the higher angle of attack at stall: there doesn’t seem to be any particular reason that is this desirable. In fact, it may just permit the pilot to prematurely hit the tail on the ground upon landing.

Figure 1, above, shows a wing with the RV-3 wing loading and aspect ratio, scaled up to fit the proposed RV-10. Notice that the horizontal tail area must be increased as well. 

Figure 2, below, shows a wing with the proposed 147 square feet of area, but with the RV-3 aspect ratio.

Neither wing will really do the job the RV-10 requires. 



What about the assumption that a "thick" wing does not impair wing drag or lift?

Here, we must disagree, at least in principal. From the studies we have done, and from the experts we have consulted with, thicker airfoils do have higher drag, and they do have reduced lift.* The ideal thickness seems to be approximately 12%. We can see an example in sailplane racing. While sailplanes may not normally be viewed as racers, that’s exactly what they become in soaring competitions, and low drag is everything to a racer. Historically, sailplane wing spans kept getting longer and aspect ratios higher. This was the way to improve performance, which for sailplanes, meant glide angle or Lift/Drag ratios, and climb rates. Airfoil thicknesses of 18% were common in order to achieve the spar depth required to get the necessary strength with the wood and aluminum materials of the day. One high performance sailplane of the 1960's had a 20% thick wing! The benefits of the greater spans and aspect ratios offset the losses of the thick airfoils. With the advent of fiberglass construction, greater strength permitted thinner airfoils. Not much thinner, because spars needed to be made over-strength to achieve sufficient stiffness. Now that carbon fiber, which is much stiffer, is more affordably available for use in spars, airfoils have been getting much thinner. Some of the highest performance sailplanes now have airfoil thicknesses of around 13%. For a typical 15 meter ship, this means a wing root thickness of about 4 inches---only 4 inches deep and it stretches out almost 25 ft. on each side…a tough construction problem! If the drag of thicker airfoils was not a penalty, then leading sailplane manufacturers would not be working so hard to make them thinner.

However… drag rise and lift loss do not vary in direct proportion to thickness. Usually, relatively little is lost through a thickness increase of a few percent, so designers often use a thicker-than-aerodynamically-optimum airfoil. The weight savings of a deeper spar and the added internal volume (for fuel or retractable landing gear) are reasonable trade-offs for slightly impaired efficiency.

For the RV-10 airfoil we have struck a compromise between the thickness needed for spar strength and a reasonable drag coefficient. The dust settled at 16% thick. With a custom-designed airfoil section, we hope to achieve a somewhat wider range of laminar flow than with the NACA 230 airfoils we have been using. We hope this will keep the drag down even with the thicker section. We know that the lift of a 16% thick airfoil is less than that of a thinner section.

Through the use of carefully designed flaps, we should be able to neutralize this loss, too. As of now, the design calls for a flap span of 51% of the total span, after fuselage width, aileron and tip spans are subtracted. So, since the benefit of the flap affects only half of the wing, we still lose a little, overall, because of the thicker wing.

Sigh! Compromises and more compromises. Even with the span and aspect ratio we have chosen for the RV-10 wing, it will not be a fabulous glider. It's a compromise, but we feel that it is a wise one.

Now. About that sliding canopy.

We have ruled out a sliding canopy because we feel that doors will permit easier cabin (don’t say cockpit) entry for the wider variety of passengers expected to use a 4 seat airplane. A sliding canopy may actually be easier to design and manufacture, but considering the size of a canopy necessary to cover a 4 seat cabin, building it and achieving a good seal might be an obstacle for the builder. Anyway, we are progressing in the direction of gull wing doors.

For several days, our prototype builders found themselves using table saws, wood rasps and air nailers borrowed from the crating shop. The result was a cockpit mock-up, simulating the cabin of the RV-10…maybe. We needed to see if the ideas that looked good on paper actually worked in three dimensions. While computer modeling can easily define and modify dimensions, it can’t tell you that a six-foot tall human has to make a difficult to twist into the seat if the door sill is yea high, or that short people have nothing to grab when trying to pull themselves up and out of the rear seats.

The mock-up also allows us to try variations quickly and easily. A few seconds with a battery drill and a handful of drywall screws can move a seat ½", which may be all it takes to make the difference when you are trying to get a knee under the instrument panel.

One of the biggest current questions on the RV-10 fuselage is the control system. The new Lancair Columbia and Cirrus airplanes use side-stick controllers, which allow easy entrance into the airplane, lots of leg and "wiggle" room, and a clean instrument panel. And, since they are used on high-tech airplanes like the F-16 and Airbus airliners, they have a certain cachet…modern, sexy, etc.
                       
We’d like to be just as sexy as the other guys, but the control system is a big deal. There is no back-up. It must work, and it must function reliably. The simpler it is, the easier it will be to manufacture, and the better it will work. Fixing on one idea and "making it work" is not a good design technique. In the RV-10 we are tentatively planning gull-wing doors with low sills. The problem comes in running the control rods down the side of the airplane. The low sills make it easy to step down into the cabin from the wing, but don’t leave a lot of room for pushrods.

We have also played with conventional sticks between knees, and a short stick on a center console, ala Velocity or (for you Aussies/Kiwis) Victa Airtourer.

Phil Duyck has spent some time tack-welding and riveting various combinations of tubing together, building different systems to measure exactly how the rods move, what kind of room they need and where they might interfere with airframe or seating structures. No clear-cut winner has emerged yet.

This is just one of the many aspects of the RV-10 that must be worked out before we can commit to production. At this time, many details of the airplane are undefined. We don’t know the exact dimensions of the landing gear, the precise shape of the cowl, or the number of cabin cupholders. We cannot predict when the prototype might fly, or even exactly what it might look like when it does. 
                      Click on the photo for a larger image.