Criteria for the light sailplane
Design parameters for the light sailplane
Criteria for the light sailplane: by Mat Redsell
I define Light Sailplanes as gliders with an empty weight of less than 350 lbs.
Within this realm of Light Sailplanes we have divisions according to wing loading. There are Class A with wing loadings up to 2.5 lbs/sq. ft., Class B with wing loadings over 2.5 lbs./sq. ft. but less than 4.0 lbs./ sq. ft. and Class C with wing loadings equal or over 4 lbs. / sq. ft. to 6 lbs. / sq. foot.
Each class will have a different goal in mind, Class A will be usable in light lift, light wind conditions and concentrate on airfoils that stress slow speed and high CL's. Class B will be for more cross country, windier conditions and stronger thermals. Class C is for racing, high wind conditions and strong thermals.
Main Objectives for a Class A Sailplane.
The main objectives for my Class A sailplane is to fly slowly, launch easily (and inexpensively) and to stay aloft for as long as possible with a good margin of safety.
I'm concerned mostly with a good sink rate (I am not really concerned with the best L/D although there is some relationship there) , the simplicity of an auto or winch tow, the ease of construction, an open air cockpit, ease of maintenance, a ballistic parachute and of course it all has to be done at a reasonable cost.
More specifically, these requirements for the design of my ultra light glider is have an empty weight of 125 lbs. to 185 lbs., a wing loading in the area of 2 lbs./ sq. ft., a sink rate between 115 and 150 ft. per min and be able to fly from 20 to 60 mph, sustain a maximum 8 G loading (tested to 5 g's), and have three axis controls with spoilers. The advantage of having a Marske Wing is that one sits on the CG. which at present is stressed for pilots up to 230 lbs. I expect to spend about $8,000 in 2002 US dollars if I build it myself. Having this built for you will cost upwards of $14,500.
These design requirements listed above are well suited to the designs of Jim Marske. His Monarch, a very unusual and brilliant design, came was a result of many years of research and empirical development. It is a very stable design, easily flown by beginners, will not stall, and is highly spin resistant. Thousands of flights have resulted in proving this design concept to be a remarkably safe platform.
Despite the proven thousands of safe flights on Marske Wings, there are many frightening tales that surround flying wings in general. There are no grounds to such stories for the Marske Flying Wings. Still some experienced aerodynamicists have been skeptical: one prominent Pennsylvania college aerodynamicist even said that he would take up wood carving if the Genesis flew..... Jim presented him with a wood carving knife shortly it's first flight. It flies just like any other glider except it is much safer. Again, as with all of Jim's designs, it will not stall and it is highly spin resistant.
I find the current trend of Light Sailplanes, promoted throughout the SSA publication, emulating heavy racing gliders. These Class "C" light sailplanes look like mini- versions of the high wing-loading sailplanes---adopting their same high wing-loading!. Do their designers have any experience in the tremendous variety possible in light sailplanes and understand the beauty of slow speed in a light weight sailplane? Or are they are only catering to the public's perception of the current popular high performance sailplane?
After years of hang gliding, I looked upon the Horten Flying Wing as the most visually appealing, but it did not take me long to discover its weaknesses. The Horton wings are swept back and require great amounts of washout to resist spinning.
Through careful analysis and actual flight I ended up choosing the designs of Jim Marske for my venture into the fascinating field of soaring. One must not confuse the Marske configuration with the Horten design: the Marske wing is slightly swept forward with no washout.
Wes Whitehurst and his Carbon Monarch
The wing will not stall under normal conditions. As the wing approaches a high angle of attack the upper surface airflow separates (which is normal for all airfoils ), the elevator on the trailing edge of the wing becomes ineffective which then causes the nose of the glider to drop slightly to allow the airflow to re-attach. The wing then accelerates, to its previously set equilibrium without the use of control surface deflections. There is also no negative pitching moment to overcome with the Marske Airfoil in order to fly at any airspeed.
There is no tail boom to create drag and add unnecessary weight. With light sailplanes, weight and wetted area are the most important factors. A weight of ten pounds when the entire craft weighs 150 lbs. will make quite a difference in the sink rate; even an over-weight pilot is at a disadvantage and will increase the sinkrate. By eliminating the unnecessary tail, less wetted area and weight leads to lower drag and tighter circles.
Interference drag is much less. The intersection of the low aspect ratio horizontal and vertical stabilizer on standard tails is suspect in creating more drag than previously thought. With the Marske Wing, the Fin/rudder is offset to the rear of the wing with very little surface at right angles compared to the tailed rudder/elevator combination, thus reducing the interference drag.
The construction using a D-tube with fabric is remarkably lighter than using a fiberglass/carbon-foam sandwich skin. While one can obtain a lower drag count for an extensive laminar flow airfoil with a foam core, the added weight of these skins will defeat the apparent performance gain for a slow flying light sailplane. In the Marske D-tube construction, there has also been in discussion,the use of foam sandwich construction instead of our presently used individual ribs. In the Marske D-tube we use either two 10-oz. fiberglass layers or two carbon 5.7 oz. (not vacuum bagged to get more stiffness) which proves harder to dent or disfigure than the sandwich-constructed D-tube, which is highly susceptible to hangar rash, of easily damaged sandwiched foam since it will only have one thin covering layer. The Marske D-tube with a molded ribs structure takes longer to construct but the result is a lighter and more robust D-tube.
The Marske wing is aerodynamically stable. If the elevator becomes disconnected in a tailed glider (with the normal negative pitching moment main airfoil) because of an unchecked misalignment, you are facing a certain, critical crash but with the Marske Wing, which is normally never disconnected, it will continue to fly level and then can quite easily be landed, aided by weight shift. (This was however not true of the Genesis, a hybrid wing with a tailed elevator, which unfortunately killed a pilot when the improperly assembled elevator fell off during the takeoff roll.)
The Marske wing is very spin resistant. The only spin entry on record was a one turn rotation and then returned to normal flight without pilot input. Mush stalls are characteristic.
There is no washout in the wing, resulting in improved spanwise distribution over a wide speed range. Jim Marske has carefully designed his wings so that the wingtip will not stall even while in slow circling flight. Jim, through empirical study, has devised a taper ratio that resists the wing tip stall under normal conditions .
Design parameters for my Class A Light Sailplane:
The things I think about when choosing a design are comfort, controllability, launching technique, landing qualities, cross wind control, ultimate G-loading, empty weight, ease of setup, the open cockpit, ballistic parachute, and one's own experience.
Comfort becomes paramount, especially as we cross the 50-year age barrier. Hanging my head down and lying in an awkward position are no longer appealing. I even need a cushioned seat! The countryside is more beautiful from 1000 ft. at 25 mph than 6000 ft. alg. at 130 mph. AND I like the idea that I can land easily and safely in a short distance with little fear of damaging the aircraft.The fact that I do not fly 1000 kilometers in a day does not bother me!
Controllability is often overlooked. In a sailplane "C" with over 4 lbs. per sq.ft wing loading, you will find flight suitable for almost any turbulent weather although not necessarily totally enjoyable. So I rather enjoy the 2 lbs. per sq. ft. of the Monarch in light conditions with less than 12 mph wind with the later afternoon thermals. I also do not enjoy getting tossed around fighting for my life which can happen with very light wing loadings in the most turbulent time of the day.
Jim in his many years of building and testing has found that the combination of elevator on the trailing edge of the wing with differential ailerons near the wingtips offers the most desirable solution to control harmony. In recent years Jim has developed higher aspect ratio control surfaces which are still effective with less pilot effort. Marske Flying Wings has also pioneered the Yaw Damper activated by the ailerons to substantially reduce adverse yaw. A weight shift mechanism, that we developed years ago, is being considered for further optimization of the Marske Flying Wing performance envelope.
Launching technique is a more difficult topic. CG. hooks are a necessary requirement for preferred auto-tow launches. (Aero-towing speeds for light wing-loaded aircraft are low, usually below safe operating speeds for standard towplanes.) They are cheap, easy and work well. Without a CG. hook one is relegated to using aerotow only which is more expensive, especially if you do as many launches as I do in a week. Jim prefers the bridle located on the sides of the Monarch nose which provides for aerotow as well as well as for excellent auto-towing. Aero-towing from the CG. hooks will require more down pressure at the control stick during a tow than in most sailplanes.
Landing qualities are at the top of my list. For my flying at 1000 ft. alg. I must be well prepared to land out at all times. I want to land at speeds under 25 mph with as little energy as possible which means the gross weight must be under 350 lbs. with a 2 lbs. per sq.ft wing loading. Having the sailplane come to rest in under 50 yards is rather comforting in difficult situations. When you try a landing, take a tape measure and measure it yourself; experiment and have spot landing contests when you fly light evening lift to prepare for the short landing requirements of cross country flying.
Secondly, one must think of the safe altitude for your pattern. In the heavy wing loading ships, 800 ft. alg. is a minimum, whereas in the Monarch my pattern starts about 250 ft. where I can easily inspect the landing area. I have many times made recoveries from 250 ft. ( but never in a heavy wing loading ship)!! In the club tailed ships, I have often begun my landing pattern at the recommended 1000 ft. alg. over an unknown field to discover that what seemed like a flat field was really rather rough terrain when I got down to final!!
Cross wind control: With the very short coupled tail and main wheel aft of the CG. on the Monarch, both cross wind landings and launches pose little concern. The only concern one has is in cross wind launching is where the drifting and falling tow rope lands.
For ultimate G loading, I like a strong light spar designed for a 230 lbs. pilot to at least an 8 G-load minimum. This is an easy requirement to meet with carbon rods . One of the amusing problems with using carbon rods is that the amount required is so small that one needs to design to about 12 G's in order to get an acceptable wingtip deflection of about 12 inches per G. It is rare that one will ever see over a 5 G load in flight but I like to have the design go at least to 8 G's.. The math needed to calculate the spar is rather easy and well worth doing for those seriously considering an ultralight sailplane design. Test the wing! There is nothing more reassuring than doing a proof loading. We tested the Carbon Monarch wing to 5 G's with no evidence of any damage. In reality this 30 lb. wing supported 750 lbs. of sand....rather hard to comprehend let alone suffer through during the test! (Yes, I was nervous.)
A major consideration in ultralights is the empty weight and the Pilots weight!. For a light sailplane empty aircraft weighing 135 lbs., the difference in performance between pilot weights of 165 lbs. and 230 lbs. is much more apparent than in heavier gliders. Overweight pilots are at a performance disadvantage compared to lighter pilots in the same light sailplane; the best thing to improve sink rate is to lose weight! READ THAT AGAIN..... THE EASIEST PERFORMANCE GAIN IS IN THE PILOT LOOSING WEIGHT!
Ease of setup and breakdown are very important. My light sailplane is light enough to remain assembled on a trailer so that I just open the hangar door, hitch the trailer and head out to the runway to go flying. Most will not be so lucky and will have to assemble it and take it apart on the same day! When hang gliding, I did not consider it a hassle.... but with a rigid wing there are frustrating differences. It requires about three people for a complete assembly/disassembly. First, the parts are extremely light and threaten to blow away. Second, the controls must carefully be hooked up. (It would be ideal if the Monarch had folding wings with automatic hookups that all fit nicely in an enclosed trailer and was easily assembled by one person.)
I love the open cockpit! Most designers of light sailplanes have chosen to entirely enclose the pilot to reduce drag and the cockpit is usually small with the pilot is almost laying on his back. This is an uncomfortable position to be in for hours, and one really misses out on the very wonderful sensations available to open cockpit ultra light sailplanes. The things of note are: the smell of a coming thermal, the temperature changes readily felt on the face, the ability to hear people on the ground, and the sensation of the airspeed rather than relying on instruments. These all add to the ultimate performance of the glider! Totally open cockpits only work well for the aircraft that can fly below 30 mph on hot summer days but for colder weather or at altitudes above 3000 alg. some windscreen is preferable. I have actually settled for a half way measure of a windscreen that stretches from the nose cone to the leading edge but leaves the sides open, which is especially good in cold weather.
A ballistic parachute is expensive but the lighter and slower the sailplane, the less the parachute will cost. I have had to bail out of a sailplane and it was great to have that option of a parachute but I would have preferred a ballistic chute!. Unfortunately our club has ruled that unless you own the parachute, you can not use one in the club planes. This is more a judgment based on fear of litigation than safety. My teaching as flight instructor has been severely limited since I refuse to fly without parachutes for the student and myself.
Consider one's own experience. Those without experience will always be swayed by the fastest and best L/D gliders advertised. A more sensible policy is "fly before you buy"! ( I learned this the hard way more than once).
Get lots of experience in light sailplane flying. Without any experience your choices will be prone to expensive judgment errors. Be practical! Most of us are not racing pilots so consider short cross country flights, staying up on light days, and get the most fun for the money (autotowing) by sharing the experience with a number of friends. Analyze your flying needs carefully: do not fool yourself into thinking you are a great cross country pilot without having actually done it.... get a lot of experience and see what you really enjoy.... and, yes, take some of those calm evening flights and judge for yourself! It takes guts to stand up and say "I like slow flight in light conditions".... but you will notice that you will fly a lot more, and build more confidence than if you wait just for the booming days!
It may be startling to realize that an auto- towed flight in evening thermals (from dark vegetation) to 1000 ft. alg. can be really enjoyable. Try it and see for yourself... and if the cost is just a few cents , you will do it more often. You will also find that you want all your flying buddies to join you in this enjoyment.
A great consideration should be given to the altitude one will fly. In the present soaring circles there is a primary interest in going fast , far, and high with an aerotow as the standard method of launch. There are two possible objections to this. First, I like a sailplane to be light, and secondly, I want to auto- or winch tow for reasons already mentioned. The tension required to launch the craft by auto-tow is half the gross weight of the aircraft. Having the minimum sink speed below 25 mph will also mean a slow speed for the towing vehicle, which speed will be around 15- 20 mph, tolerable even on a farmers field. With the added wind speed (of say 5 mph), the Monarch's airspeed during climb out will be a best airspeed of 35- 40 mph.
Theory and Practice:
Airfoils are a challenge. At low Reynolds numbers (under 500,000), the drag coefficient is high and the "drag bucket" (drag "shape" occurring when plotted against Reynolds numbers) can be somewhat irregular compared to Reynolds numbers over 1,000,000.
Flying at Reynolds numbers under one million demands a special airfoil suited to that environment. Jim's theoretical and practical research over 40 yrs. has led to a modified 43012 airfoil. Upon examination it looks quite normal without any significant looking reflex until one looks to the bottom trailing edge, where the positive pitching moment is generated. This airfoil with a very high camber shifts the "drag bucket" to the high CL values and, since laminar flow can only be maintained for the first 10-15 %, only a narrow leading edge composite D-tube is needed which can then be followed by a light fabric (over composite ribs) to the trailing edge. Flying at high speeds with this airfoil does create more drag than one would desire... but the low speed high CL range is very good as has been well proven over 20 years of flying.
Most theorists note that with a regular flapped sailplane, the flaps in the 'down' position move the "drag bucket" to a higher CL value, giving less drag and, conversely, when the flaps are in an 'up' position, the drag bucket moves in favor of the low CL's for high speed. But what is also produced is a positive/negative pitching moment which has to be countered with a deflection of the elevator on the tail. That is the theory. The Marske Monarch Flying Wing has the elevator on the trailing edge of the main wing and looks like a flap to most people. We already have a high CL airfoil and in practice this apparent shift in the drag bucket cannot be felt which may be the result of the energy saved, not having to counter the pitching moment as on tailed sailplanes. In the near future we will do more studies on this.
Another interesting theory question is whether the elevator on the Marske wing is counted in the aspect ratio of the wing when analyzing the wing. After all, the tailed sailplanes do not add in their horizontal area. Again another issue for more research.
Few people realize how many ways there are to control an aircraft. Weight shift is traditional in hang gliding so I'm quite partial to the simplicity and effectiveness in an aircraft that the pilot weighs more than the empty weight of the sailplane. If your aircraft only weighs 155 lbs. and your weight is 160 lbs., your weight is a greater percent of the gross weight in that ultralight sailplane.
An obvious direction for the Monarch is to have weight shift coupled with 3 axis control. This is another area of development we are working on.
Most inexperienced homebuilders we meet prefer the moldless technique just as I did when I began building. The sailplane I built without using a mold took longer to sand to the exact shape in the long run than to have made molds, and there was no second chance if the first attempt was not successful. So I like to make molds as a way to have a finished surface without spending hours sanding the final product. Most molds can be made quite quickly: hot wiring foam blocks, covering the foam with spackling (drywall compound), sanding it smooth and then applying a layer of heat shrinkwrap. There is no waxing or PVA used. Sanding the drywall compound is certainly a lot easier than sanding epoxy and micro balloons.... and besides much lighter without the filler!
The wings must be aligned carefully. We have just finished our flight testing on the Pioneer we purchased and found it has one wing at a different angle of attack than the other .... it is fatiguing to fly with having to hold the control stick to one side!!!
The wing itself also must be carefully checked for twist between the first and last ribs in each panel.
Good skills are necessary in fiberglass and carbon construction!
Materials to be used:
Carbon is the lightest most available material we know of that is affordable. We use mainly 5.7 oz. with a twill weave bonded with Aeropoxy. It is much easier to use than fiberglass.
In some areas metal can even be replaced by carbon for a weight saving without sacrificing strength. In the aileron pushrods, carbon rods have substituted for the aluminum usually used. One difficulty in doing this is making the fittings that attach to belcranks. I devised a mechanical device that expanded in the carbon rod and then was wrapped to provide the strength to secure this bond.
In fiberglass/carbon construction one should avoid the use of any wood!
I have built one sailplane with 316-type stainless steel but encountered a lot of problems with distortion due to the heat of welding. Stainless is also softer and one must increase the thickness to compensate.
Just as weight is the killer in sink rate, its closest cousin is the drag created by the airfoil, struts, open cockpit, interference, etc. But here we have some choices to make. After years of open air flying I find it very hard to hide behind a full canopy; an open cockpit is by far superior when it comes to the fun factor. Since I also have the choice to fly with a full canopy in the Pioneer IId or in our club sailplanes, I can readily make the comparison with open cockpit. However, there is very little advantage in flying in an open cockpit if the sailplane flies consistently over 35 mph so I settle for a partial canopy on the Monarch, provided it still meets my 130 ft. to 150 ft. per min sink requirement
Now the drag of the airfoil must be considered. We can certainly get a very low drag from a specialized airfoil that has laminar flow back to 45% chord or more, but the penalty here is the weight of making a wing of this type. The high laminar flow airfoils lack in many cases the low drag at high CL values which is what is needed in an light sailplane. Now if we take a forward camber airfoil and just use fabric from the highest point (spar) to the trailing edge, weight is greatly reduced and we have an airfoil that still has high CL values. There are no major molds required for the skin after the spar though there are quite a few small rib molds.
Thoughts on the Self Launch:
I realize the holy grail is to self launch, and it appeals to those who have not yet discovered the auto-tow for a sailplane of 135 to 185 lbs., a low sink rate and a few friends to share the work and love of gliding. For those with unlimited funds, there are quite an array of self launchers available.... but they are not the holy grail! The ultimate self launch for me is the unassisted cliff launch I have done for years in my hang glider: but that too is always more enjoyable with other pilots to fly with. As a word of advice, do not consider making a self launch glider without first having flown one and making sure you are very proficient at gliding. Remember, too, that builders are not necessarily experienced flyers!
Auto-towing has been my preferred method of launch for the last three years, giving me hundreds of safe flights. I can easily have 10 -20 flights with friends in a day of enjoying the afternoon soaring or the challenging the evening thermals (or fun slide rides). There is no standing around to see if the weather improves, the cost is just a few dollars, and the satisfaction incalculable.!
Many people claim it just is not possible or safe to auto-tow on an airport today; and that may be so. But often there is field near a ridge, an old road through a field, an abandoned military field, etc.? Auto towing is simply the easiest and cheapest way to get launched, provided your sailplane is equipped for ground towing.
The real trick in auto-towing this is to have a thin, strong rope such as an inexpensive 3/16' Polypropylene rope. But we are now looking forward to experimenting with 1/8 " Spectra. We have discovered smaller diameter rope makes a very big difference in the climb rate.
Lockouts do not happen with the Marske Wing when auto-towing due to the very effective rudder!
With the Monarch there is no worry about cross wind launches: the sailplane and towrope will drift downwind unless the pilot makes the necessary corrections. Most of the correction is with the rudder supplemented by small adjustments with the ailerons. The launch can also be substantially improved by moving any weight one can as far back as possible.... and the lighter pilots will always get the greatest altitude. We are researching a seat shifting movement for CG. travel.
Future of soaring:
We have arrived at a point where most of those currently in soaring have a large financial investment in state-of-the-art sailplanes. The cost of an aerotow is starting to hurt the family budget but our SSA sailplane "experts" are all going faster if not farther. But we feel there is more to gliding for the average flyer. There is no shame in taking a bunch of "sled rides" from 1000 ft.agl or lucking into a half-hour or three hour soar in an evening thermal. In fact it is a heck of a lot of fun and very instructional!
At present, diversity is not a part of the SSA soaring world. Homebuilts have fallen by the wayside just as the understanding of sailplane aerodynamics and technology has made quantum leaps. But soaring has become a sport for the Privileged and in many ways doomed the average pilot to extinction .
Paragliders, hang gliders and the developing light sailplanes are the future for the Little Man and deserve more attention for performance and safety as well as affordability and fun to be shared by many.
The success of our soaring interest rests entirely on us grass roots guys accepting the limitations of modest budget and standing up without shame and stating, "Yes! We do enjoy light conditions and light aircraft".
We believe Marske Flying Wings offer the best inexpensive, proven sailplanes for this end.
When I showed my article to Jim Marske he came back with some interesting visual comparisons below.
Wing Loading, Stall speed, Max wind, Minimum recovery altitude and comfortable cruise altitude.
Note that the 2 lbs per sq ft (Monarch) can make a recovery at 200 ft whereas the heavy ships do not go generally below 800 ft agl for a recovery. This chart is pretty much what I have actually experienced in my flying. The Comfortable cruise altitude is where I fly most comfortably with reference to the wing loading.
|For a comparative launch height for differing gliders see the Marske table below:
This again is very much what I have found in reality. With my hang glider I certainly need a lot more altitude to find my thermal whereas in the Monarch I need a lot less altitude for the same search area, see below for comparisons.
|Horse Power minimum requirements for auto-tow and winch launching|