Dick VanGrunsven (Van) is working on a proof of concept (POC) motorglider. The project, which has been under construction for a number of years in Van's home workshop, has a very low priority. Because it is strictly a POC aircraft, there are neither plans nor any schedule for kit production of this aircraft. The information provided here is for the purpose of education and market survey. Included are copies of several articles that have been published in Van's "RVator" newsletter over the past few years. It will also include periodic updates. Its purpose is to provide a tutorial on the elements of soaring and an evaluation of the criteria considered in the design of the RV-11. In addition, this site will provide a medium for disseminating progress status information.

First, some definitions:

A glider is any unpowered heavier-than-air aircraft. For the sake of this discussion, it will be assumed that gliders are fixed wing vehicles, though they can be rotary wing or flexible wing such as parasail soaring parachutes.

A sailplane is loosely defined as a high performance glider- A more detailed definition of high performance will come later. A sailplane is a glider capable of maintaining altitude or climbing in favorable atmospheric conditions.

A self launch sailplane. As the name implies, it is first and foremost a sailplane with an engine that produces enough thrust to allow it to takeoff and climb. In its evolved state, this has come to mean a state-of-the art sailplane airframe with a retractable engine/propeller system. Early SLS had an engine atop a fixed mast, which caused too much drag to be practical for soaring once the engine was stopped. The first practical configuration was an engine/ prop atop a retractable mast. Figure 1


                       
                     Click on any image for a larger version.

Most current production SLS have engines which remain in the fuselage and drive the prop via a long belt and retractable mast Figure 2.

 
 
This permits lower drag when in powered flight, and permits a larger muffler (In the fuselage) to meet stringent European noise requirements.

A motorglider is typically an airplane with a fixed engine, usually tractor configured much like typical power planes. Actually, most of them look much like a Piper Tomahawk with long wings and fuselage Figure 3.

The term motorGLlDER implies that it is typically of lower performance, thus a GLIDER rather than a SAILPLANE. Later in this discussion the performance distinction will become more obvious.

When we refer to a sailplane being high performance, just what parameters are used as a measure of performance; Speed? Climb? In a manner of speaking, yes on both. However, there are other measures which are of more direct importance, the greatest of which is Glide ratio or Lift/drag (L/D) ratio. Also of great importance is minimum sink rate. Stall speed is also important because it will affect the speed at which minimum sink rate and maxi-mum L/D are achieved, and also will determine the turning radius for any practical bank angle.

Max. L/D is obviously important because it dictates how far forward the plane is capable of traveling for every unit of altitude lost. Sink rate is important because it tells how much time one can glide per unit of altitude lost, and thus the amount of lift necessary to maintain altitude, or the climb rate which will be available after subtracting the sink rate from the lift rate. Before we get completely lost in units and terminology of measurement, let's draw a picture and visualize what happens.

Figure 4 below shows a simple diagram of glide ratio or L/D. The numbers given are typical for a modern high performance sailplane. For any specific airplane at a given weight, the max L/D will be achieved at a specific calibrated air speed (or angle of attack). Converting the airspeed to feet per minute we divide by L/D ratio to find the sink rate. Actually, the minimum sink rate will occur at a speed lower than that for maximum L/D, but to simplify this explanation, we'll limit our comparisons to the max L/D condition.


                    Click on any image for a larger version.

In Figure 5 below I have added glide slope lines for a couple of other conditions and airplanes. First, if we take the same sailplane and fly at a higher weight, the L/D will remain the same, but because the AOA remains the same for max. L/D regardless of weight, the speed will increase and thus the sink rate will increase. (You may have heard, but wondered why, some sailplanes have water ballast tanks. It seems counter-intuitive...wouldn't more weight make it harder to stay up? It does take more energy to keep a heavier airplane up, but sailplane competitions are based on speed, not duration. The extra weight helps the glider to fly faster without sacrificing L/D, but at the expense of needing stronger lift to stay aloft or climb.)

Also in Fig. 5, I have added two more glide slopes for lower performance sailplanes. The one is for a low performance sailplane which is lighter (wing loading) and achieves its max. L/D at a lower speed. Its sink rate is also greater. Thus, while its max. L/D glide speed is 70% that of the High performance ship, its L.D. is only 50% because of the higher sink rate (drag).

Now, the third line represents a typical motor glider. These are almost always 2-seaters and are thus rather heavy and of relatively high drag because of the engine configuration and openings, landing gears, etc.

Notice that its speed for best L/D is higher, close to that of the high performance sailplane. But, because of its weight and thus, higher wing loading, its glide speed and sink rate is much higher. While this motor glider has a better L/D than the low performance sailplane, its sink rate is higher and therefore would require stronger lift to remain aloft when its engine is stopped in the soaring/gliding mode.

The more I fly unpowered airplanes (or at least airplanes without mechanical power) the more I appreciate the basic rules that all airplanes follow. We'll continue this discussion in the next issue, and delve further into the inner working of the field of soaring.

The result is that, in marginal lift, the classic motorglider cannot soar and thus must either land or start its engine and look around for stronger lift. On the other hand, the self-launch sailplane soars nearly as well as its unpowered counterpart. It has a slightly higher sink rate because of the weight of the engine it is carrying around, but its drag is essentially the same as its pure sailplane equivalent.

The above analysis of thermaling qualities of different classes of sailplanes was limited to a single thermal situation. In reality, thermals are of limited duration. The supply of "hot air" is expended and the thermal weakens, disappears, or collapses into sink, so the soaring pilot must find another. The light sailplane which climbed well now finds itself at a disadvantage to the faster, higher L/D ships which can fly fast and flat (far) while searching for a new thermal. Figure 7 below shows an overhead view of the relative areas within reach of the two different sailplanes. The high L/D example reach 400% more area than the low L/D example. Because of its speed, it can also reach visible lift (forming cumulous clouds) While the lift is best, or before it dissipates entirety. The faster ship is also less affected by winds Which can drift the lighter ships away from the safety of their home field, or seriously impede progress in a desired direction. Thus, the real world advantage is more than the simple 400% greater glide area shown. 

The motorglider example can reach a greater area than the low L/D sailplane, but it must find stronger thermals to continue its soaring flight. What level of performance do you want?

Statistics show that only 20 percent of the active glider/sailplane pilots in the US engage in cross-country flying. That means that 80% of them never venture farther than gliding distance of their home airport. There is nothing inherently wrong with that. Each pilot finds his rewards in different places. The pure joy of sustaining motorless flight is reward enough. For this majority, a sailplane of limited performance would suffice.

But, let's consider why so many never leave the security of their home airport. Perhaps it IS because they are intimidated by the prospect of an out-landing which would require either an expensive aero retrieve, a cumbersome ground retrieve, or even a damaged sailplane. With an on-board engine to provide an almost 100% assurance against out-landings, I would guess that most of that 80% majority would be willing to attempt cross country soaring flights, so it seems that reasonable X-C performance would be desirable in a typical powered sailplane.

On the assumption that our self-launch sailplane will be used for soaring X-C, we still must determine the desired performance level. Another statistic shows that only 5% of soaring pilots participate in competitions or contests, which are cross country speed tasks.

To accommodate the largest segment of the soaring pilots, it might be safe to assume that our motorized sailplane would find an acceptable market if it offered just "sport X-C" performance rather than "competition racer" cutting edge performance.

If you're thinking of designing a sailplane or motorglider, there is another pressing question...should it be single-seat or a two seater? There are many two-seat sailplanes, but their use is primarily for training, sailplane "adventure" rides, or local flying. The vast majority of sailplanes used for sport and competition soaring are single seat. The primary reason for this is that, to maintain desired performance in a 2-place sailplane, the wing area and overall size must increase dramatically. For powered aircraft, wing area need increase only about 20% to accommodate the second person. For two-seat sailplanes, the wing area must increase by 50% to 75. Since many high performance sailplanes are dismantled and stored and transported regularly in trailers, the added size and weight is a major obstacle.

Another reason favoring single seat is that serious soaring flight is often not "passenger friendly", particularly for non-pilot passengers. How many passengers do you know who would enjoy spending half of a flight doing tight turns? Sometimes for 5,10, 15 minutes at a time? You spend hours doing this, often not getting answer in particular. Gliding is fine for sightseers; but soaring is a pilot's sport. Despite these drawbacks, there is a market, and several companies sell very high performance two seat sailplanes with L/D ratios approaching 60:1. These have wingspans of around 25 meters (82'), so they require wide runways and special handling. Usually the passenger seats of these very expensive, specialized craft are occupied by other experienced pilots who are prepared for the rigors and rewards of such flying.

In the next RVator we will continue this discussion. What kind of engine? Where should the engine be? What materials should we use to build the airframe?

The wing is the heart of a sailplane—where the performance comes from. The design of the wing, its airfoil, span, planform, aspect ratio, etc. are of paramount importance.

In the Real Estate business, conventional wisdom says there are three factors which determine price; location, location, and location. For sailplane performance, its Span, Span, and Span. Or, perhaps more accurately, Aspect Ratio. The two are inter-related because of certain practical parameters such as stall speed and attendant wing loadings. Thus, for a certain purpose sailplane like single seat sport, the wing areas generally remain in a narrow range and if span increases, wing chord generally will decrease, and aspect ratio increase. (or vise/versa) While all "long" span sailplanes have outstanding performance, NO short span sailplanes have outstanding performance. You MUST have the span, plus low drag and other design optimization features.

I have included a couple of scale drawings showing the relative spans and aspect ratios of the RV-8, a typical 15-meter span sailplane, and a 26-meter super sailplane. See Figure 8 below. Now you know why some people call the RV wings "fat".



                     Click on any image for a larger version.

Aluminum sailplane wings have proven to be the favorite for outside tie-down. While composite materials are generally weather proof, their gel-coat surfaces are usually susceptible to ultra-violet degradation and cracking. Over time, this leads to lost performance and/or expensive re-surfacing repairs. So, for routine handling and maintenance ease, aluminum is favored. Despite this, most privately owned sailplanes are now Composite. Owners are willing to routinely dismantle and store their ships in a trailer than to accept the lower performance of a metal ship.

For reasons of weight and size, retractable sailplane engines are always the two-stroke variety which means they are more temperamental and thus prone to harder starting. They require pre-mix fuel and are generally less user-friendly than 4 stroke engines. However, they are sufficiently reliable for self-launch purposes.

Motor gliders have traditionally used 4 stroke engines, mounted as fixed tractor installations. The engines themselves are heavier than 2 stroke engines. However, since motor gliders are often used for general-purpose cruise flight, the 4-stroke reliability and friendly operating characteristics are important.

The parameters here are glide ratio and sink rate. Because of its inherently higher drag, the traditional fixed engine motor glider is almost certain to suffer a performance loss over a pure sailplane of comparable size, or retractable engine sailplane.

Back in the 1950s, the international soaring community chose a 15-meter (49.2 ft.) span as a practical size for competition and sport sailplanes. Thus, the majority of single seat sailplanes today are 15-meter ships, and they have become the standard of comparison. Recently, we have seen the establishment of an 18meter (59 ft) class, largely inhabited by self-launch sailplanes. The added span helps carry the weight of the retractable engine and still offer very good minimum sink rates.

Thus, let’s think in terms of a minimum 15-meter span as a basis for motor glider design concept. The best 15-meter fiberglass sailplanes achieve L/D ratios of 45 or slightly higher. The best of the 15 meter aluminum sailplanes can attain around 40:1 L/D. By rule of thumb, we might assume that the same basic wing used on a motor glider with a fixed tractor engine, a drag optimized cowl, and a feathering (or folding) prop, would achieve an L/D of 33-35. Sink rates of the better 15 meter ships come in at just under 2 ft/sec. (120 fpm), so we can hope for as little 2.5 fps (150 fpm) for the motor glider equivalent. These are numbers which translate into what should be a very acceptable sport sailplane.

In the next installment, I will explore what is available or on the horizon in the field of lower cost motorgliders and self-launch sailplanes. While it may not have been clearly evident, this has been the objective of this series of articles. Within the power pilot fraternity, I sense a latent interest in the challenge and thrill of soaring flight. However, I see an even stronger aversion to the traditional limitations of pure sailplane flight: awkward ground handling and launch requirements, regular off airport landings, trailer ground retrieves, etc. In sailplane design, like power planes, innovation and change does not come quickly. You might be interested to know just what is happening.

From the  fourth issue of the 2002 RVator

Providing an engine certainly addresses the capability issue, but it hasn’t help the affordable part. There are a number of very high-performance self-launch sailplanes being imported from European manufacturers, primarily in Germany. These have been available for the past 15 years or more, yet relatively few are in service --one statistic I read recently indicated that there were less than 1000 of all models in use worldwide. For sailplanes with such amazing performance along with the self-launch and in-flight restart capability, this is not what you’d classify as " taking the world by storm". Why so few? In a word, they are doggone expensive! The classic single-seat self-launch sailplanes is a high (or very high) performance machine, and comes with a price tag in the $100,000 to $130,000 price range. Two place ships with similar performance are even spendier: $150,000 to $225,000. For a pure sport aircraft, this sticker shock limits the market considerably.

In recent years, there have been new mid-performance sailplanes and self-launch sailplanes developed with price tags of 30-40% of the above. You may have seen magazine articles covering more affordable self-launch sailplanes such as the Italian Silent (with its unique 1-blade prop) and the Russian AC-5, both in the $35,000 to $37,000 range. Why haven’t they haven’t seen immediate popularity? There are several reasons. Very small companies with limited finances often manufacture such sailplanes and production capabilities…there just aren’t many of them available yet. Making importation arrangements and finding qualified sales representatives in the USA is a challenge. There is very little money to be made on either end, so you don’t see people rushing in with big budget marketing plans. And then, $35,000 still represents a considerable investment for a pure sport single place airplane that flies only on warm days with convective atmospheric activity.

The self-launch sailplane, whether it’s just expensive or very expensive, is primarily suited to soaring activities rather than power cruising. A motorglider, on the other hand, can be used to some degree for powered cruising.

In an early installment, I mentioned that most motorgliders, or sailplanes with fixed engines, tended to be 2 seat with fixed landing gear and feathering props. They are somewhat suited for general purpose flying but in return are also somewhat marginal sailplanes. They are better suited for training or for what might be termed a "casual sailplane pilot," someone who is content with limited actual soaring ability in return for reasonable general-purpose power flying. (My definition of an aggressive sailplane pilot is one who soars extensively and only uses his engine for launching and an occasional in-flight "save".) Statistically, motorgliders spend about half of their flight time under power.

During the 1970-2000 period, powered sailplanes seemed to fall into two categories; the low performance fixed engine configuration, and the high performance retractable engine self-launch sailplanes. There had been almost no effort put into developing a high performance fixed-engine motorglider. (One exception is the 2-seat Stemmi S-10. This unusual airplane has drawn a lot of attention because its engine is buried in the fuselage aft of the cabin with a long driveshaft driving a prop with blades that fold like jackknife blades into the fuselage nosecone. The Stemmi has a span of 72 ft. and very good true sailplane performance with a reported glide ratio of 50 to 1. Its 4-stroke Rotax engine makes it a practical X-C cruising airplane as well. These unique qualities have made it popular with those who can afford its approximate $200,000 price tag, but it is still a big, complex, expensive, high maintenance airplane). It seemed to me that there should be a niche for a moderately high performance motorglider/sailplane which would be less expensive than a retractable engine sailplane, offer better utility by virtue of having general purpose cruising capability, and still soar reasonably well. It other words, my inclination would be toward a motorglider optimized for soaring rather than general purpose powered flight.

Since I just happen to be in the kit aircraft business, the thought of filling that niche has crossed my mind….

The first thoughts surfaced about a half dozen years ago, while I was still flying my Schreder HP-18 sailplane, a homebuilt kit design that I bought second-hand. I conceived the idea of mating its wings to a motorized fuselage. Since designing and building sailplane wings is a big job, I reckoned that this would be the quick means of getting a motorglider into the air. My initial idea was to build a quick and dirty airplane to test this concept, then develop a more refined motorglider based on lessons learned and with the benefit of the "Market Survey" info gleaned from the prototype aircraft’s exposure. At that time there wasn’t anything similar in production. I did not know, from real world examples, what level of performance I might expect.

I dubbed the project, with my customary talent for coming up with unique and market-savvy names, the RV-11. See Figure 9 below. 



                       Click on any image for a larger version.

It was intended as a strictly Proof of Concept airplane. The HP-18 wings are made of metal spars and closely spaced PVC ribs bonded to aluminum skins. They are 15 meter (49.2’) span and on the HP-18, yield a reported glide ratio of about 40:1. My goal was to see what level of performance these same wings could provide on a fuselage with an engine and the added drag of the less-than-perfect shape needed to accommodate it. Soon after beginning the RV-11, I purchased my first self-launch sailplane and the HP-18 became inactive. Since the HP-18 was relatively inexpensive, keeping it around just for the use of the wings was economically justifiable. The basic metal work of making form blocks and bulkheads, longerons, and skins for the empennage and fuselage went pretty fast. But at that time, I didn’t have shop space to work in. This was an evening project and I had to keep my work out of the way of the "day job" development work being done on the RV-8 or whatever front burner project our prototype dept. had going at the time. For this reason, I designed the long fuselage to be built in two sections, front and rear, that joined together later. With smaller pieces, it was easier to keep it out of the way of our busy R&D shop mechanics.

Progress proved to be much slower than I had optimistically hoped. With a high performance self-launch sailplane to play with and maintain, I had less incentive and less time to devote to the RV-11 concept. The design-as-you-build approach and the upsetting effects of two fatal accidents we suffered during this period combined with moving into a new shop to make progress very slow.

Initially I had designed the RV-11 for a 2 cycle, 2 cylinder Rotax engine. I considered the 40 HP Rotax 447 I had on the shelf, left over from the RV-5, even though I felt it would be marginal. Eventually I settle on the 46 HP 503 model. As time dragged on and I hadn’t yet purchased an engine, the Jabiru engine appeared on the market. Rated at 80 HP, it had much more power than I needed, but its weight was "only" about 25 lbs more than the Rotax 503. Best of all, it was a 4 stroke engine which would eliminate the need for pre-mix fuel and the bulky exhaust and muffler systems needed for 2 cycle engines. It promised better cruise performance for non-soaring flight, so I designed a new firewall and modified the forward fuselage to accept the Jabiru 2200.

Sailplane retractable landing gears are quite simple mechanisms. However, the packaging considerations necessary in a forward fuselage to accommodate both an engine (prop ground clearance) and pilot caused the RV-11 landing gear position to be such that the "normal" retraction concept wouldn’t work. I figured out a way to retract the gear and wheel "half way" into the forward fuselage. The exact mechanism and gear doors would come later. When "later" arrived, brilliant ideas were nowhere to be found. I have cobbled together a Rube Goldberg affair which will hopefully be OK for this POC airplane.

I also have made more work for myself by trying to keep the fuselage profile drag to a minimum. This included such things as dreaming up the idea of reducing cowl drag by using cowl flaps for both inlet and outlet air. It may turn out to be a worthwhile idea, but has added a lot of development time to a POC airplane.

As the years passed and the RV-11 inched down its bumpy production line, a new German motorglider named the "Carat" was announced. Conceptually, it was very similar to my RV-11. Figure 10 below.

The Carat uses the 15-meter standard class (non-flapped) wing from a Schemp-Hirth Discus. The fuselage is a rather conventional low wing with a fixed tractor engine. It utilizes a unique forward folding prop to reduce drag in the power-off soaring mode. Rather than using a conventional retractable mono-wheel landing gear, it uses an innovative conventional landing gear that retracts forward and inward into the fuselage…something like a Cessna RG, but in reverse. This landing gear should provide ground handling similar to tail wheel sportplanes, yet low drag for soaring performance. They list an L/D of 35:1, a sink rate of 150 fpm, and a 75% cruise of 124 kts. (The same wing, on the Discus sailplane, achieves an L/D of 43:1) The engine is a 54 HP Volkswagen conversion. The Carat is just now entering production and importation into the USA. It will be interesting to watch and see if it finds a niche in the market at its price of about $80,000.

The performance of the Carat has answered my decades old question: Yes, a fixed engine motor-glider can achieve true soaring performance. Thus, there is hope that my RV-11 design might deliver reasonably similar performance.

That pretty well brings us up to date. The RV-11 POC airplane is perhaps 80% complete, and is progressing very slowly, particularly in the summer months where there are too many after-hours distractions. In some respects, it is rather crude since I am making it from left over parts and materials rather than refining these components as we would for a more dedicated kit development project.

This POC prototype could probably be finished in a short time if we raised its priority and brought it into our regular R&D program. However, our R&D priorities are assigned based largely on potential for return, or anticipated market. That, at least in my mind, is a large unknown. Because there has not been a comparable sailplane on the market, we don’t have a good basis for assessing the market potential. Are such sailplanes not built because there is no market for them, or is there no demonstrated market simply because there’s no product on which to base a market? I’m inclined to believe that the market for this type of sailplane would originate primarily from the power community wishing for a more practical sailplane, rather than from within the existing soaring community. The soaring community is much smaller, and pilots are typically less interested in building kit airplanes than power plane pilots.

Now that I have shared my reasoning process with you and hopefully expanded your understanding of soaring flight, I’m looking to you for input in answering the above "market potential" question. I have already received quite a number of replies from interested parties; both via E-mail and over-the-counter at Oshkosh. I welcome comments of any kind, but may not have time to respond individually to them. I will use this printed medium, as well as our website, to keep you informed regarding the progress of this prototype and general concept.

In a future installment, hopefully the next RVator, I will delve further into the RV-11 and give some details of what I think a kit motorglider evolution of it might look like, as opposed to the one-of-a kind POC presented here.