The Snedden M7 Ultralight
The Magic Carpet
By Pat Panzera, EAA 555743, firstname.lastname@example.org
To say that Andrew L. Snedden, of Mount Perry, Ohio, is passionate about the current state of the ultralight industry is an understatement. But unlike those who will sit on the sidelines and bemoan the “good ‘ol days” (see my May 2009 editorial), he’s doing something about it. His answer is the Snedden M7; Andrew’s first flying design (he has much more radical designs still on paper—many of which are vertical takeoff and landing derivatives).
“Safety is paramount” is what I came away with after speaking with Andrew. The pilot sitting in the middle of what appeared to be a “crashworthy” structure is no accident; neither is the very low wing-loading (read that as a very low stall speed), the high power-to-weight ratio, the steerable front wheels, and the ergonomics of the revolutionary control system including the motorcycle-like twist grip throttle.
The goal of all of this was to create an easy-to-fly ultralight that would take minimal ground training for the average person to achieve a safe and fun first flight, followed by years of the same. The overwhelming drive behind this revolutionary design was the industrywide lack of two-place ultralight trainers that aren’t actually N-numbered light-sport aircraft, operated by a sport pilot flight instructor through an authorized flight school with all the related overhead included.
In the Spotlight
The Snedden M7 made its public debut at EAA AirVenture Oshkosh 2009 where it was awarded two of the seven awards in the light-sport plane (not to be confused with light-sport aircraft) category: The Joe Diamond Award and the Ultralight Reserve Grand Champion - Silver Lindy. Of special note is the recognition of Popular Mechanics magazine, which identified the Snedden M7 as a major highlight of AirVenture 2009, although it seemed to be unable to comprehend the brilliance of the control system.
“My crew and I were overwhelmed by the overly enthusiastic people, praise, interviews, inquiries, photographs, and touching. Close-up photos and measurements were taken, some that I had never taken yet myself!” Andrew stated in a press release earlier this month. “Of great interest was the people-friendly full three-axis handlebar control system, inverted V-tail with anti-servo tabs, and four-wheel landing gear with linked ground steering.”
There is not much about this plane that one could consider “conventional”—even the seemingly innocuous Dacron covering system and the polycarbonate windscreen, traditional by ultralight standards, are secured in place by hundreds of Ty-Raps (zip ties). The zip ties are rated at 30-40 pounds each, and the wing skin is attached at the trailing edge with a little more than 100 ties per panel.
More so than its unconventional appearance, the gravity-defying performance was the real attention-getter. From any other angle than perpendicular, the craft appeared to levitate, gaining it the nickname “The Magic Carpet” from the announcers at AirVenture.
Pattern altitude by midfield resulted in a push-over that gave the appearance of a brief hover at the apex of the climb. Weighing in at 277 pounds including a 24-pound parachute allowance, and with a wingspan of 23 feet by a chord of 78 inches, the power-on stall speed is a mere 25 mph. Power-off stall approaches 27 mph thanks to the lift-inducing 65-hp Hirth two-stroke engine and the 70-inch Powerfin propeller it swings. Andrew reports that the M7 can do much more in the air and on the grass than demonstrated at AirVenture—one can only imagine seeing an airplane doing donuts and power-slides on the grass like a go-kart.
The upper wing surface is a modified NACA 23012/23013 airfoil. The bottom surface is flat and cambered toward the trailing edge spar. With only an upper rib section, the lower skin has a concave deflection during aerodynamic loading. Since the wing roots are open to the cockpit, which is in the free air, the wing profile is affected by the propwash, causing the upper and lower surfaces to be attracted toward the wing’s core. Forward speed seems to help this beneficial anomaly as the wing cross section seems to be unchanged even during high g-loads and a high angle of attack.
The cable-braced, aluminum-tube main and drag spars are separated by intermediate structural members and are bridged by the upper rib section that is riveted to each spar.
Just prior to AirVenture, Andrew opted to switch out the wingtips (shown in the photos on his website) for a set that increased span a little and gave it a different look.
Andrew Snedden preparing to wow the onlookers.
Control System—Look Ma! No Feet!
All three axes are controlled with a set of motorcycle handlebars installed to a cross member that interconnects each of the two ruddervators. The rotational axis of the handlebars is parallel with the up-down throw of the elevator portion of the ruddervators, the same way that a typical yoke works in a Cessna 172 or Piper Warrior. Crank the bars on the M7 to the right and the ailerons deflect accordingly. Push or pull the bars and the plane pitches up or down as you would expect. Twist the entire assembly and the rudders are deflected.
To get a clear understanding of how the system works, picture yourself in a C-172 or a PA-28. Now imagine that the yoke only was replaced with motorcycle handlebars, complete with a twist-grip throttle in your right hand. Now go flying. I’m sure that 100 percent of everyone reading this could instantly adapt to this new ergonomic arrangement, and there would be no noticeable difference in the precision command of the plane.
But that’s just part of the system. Now, when you want rudder input, you twist the handlebars through a vertical axis, just like you would on a motorcycle—pushing forward on one hand grip while pulling on the other. Now your C-172 is controlled just like the M7.
Andrew, an experienced USUA ultralight flight instructor, finds his new system to be vastly more natural for a beginner to learn than traditional controls. Stepping on the left pedal to yaw left seems counterintuitive and unnatural to Andrew, as it was with many of his students. And if you think about it, unless you are riding flat-track motorcycles, you don’t push on the left side of your motorcycle handlebars to go left, you pull—just like a bicycle, some riding mowers, all riding watercraft, and the list goes on and on.
Andrew’s system has also come to the attention of those concerned with making aircraft “accessible” to people with physical limits. Specifically, since this plane can be flown without the use of one’s feet, it’s certainly of interest to those with that restriction. During a chance visit with Jessica Cox (a motivational speaker and pilot born without arms), she expressed to me that she was interested in building her own plane, one with all the controls on the floor. Andrew’s system, slightly modified with stirrups perhaps, seems like it could be the perfect solution to that issue and simple to fit to just about any homebuilt aircraft. If I were an LSA-compliant experimental aircraft kit manufacturer, I’d jump all over the opportunity to work with this inspiring young woman on this project. Hint-hint.
The twist-grip throttle has a friction mechanism attached that allows the throttle position to be set and then let go, but it can easily be overridden just like the throttle lever in your plane. But unless the device is engaged, the throttle will snap closed when let go, just like on a motorcycle. I can see both advantages and disadvantages to this system when dealing with beginners.
Seemingly almost as large as the wings themselves, no one could miss the pair of inverted-V control surfaces, complete with anti-servo tabs. Arguments can be made for and against V-tails, but for Andrew’s mission—that being keeping the control surfaces out of the propwash, out of turbulence from the open-cockpit design, and out of the weeds—the arrangement works well. While the design was yet in its infancy, Andrew showed it to Mark Beierle of Earthstar Aircraft, the renowned ultralight designer, builder, and cross-country pilot. His first response was to advise installing anti-servo tabs. Andrew, having just been introduced to the concept, looked at Mark’s Earthstar (complete with anti-servo tabs), and the light bulb was illuminated; it was the final piece to the puzzle.
When rigging the plane for its maiden flight no real thought was put into the tab’s initial deflection, which made for an interesting flight to say the least. The plane wanted to pitch almost straight up according to Andrew, and the entire flight was then flown with what seemed to be 100 pounds of forward pressure. At that time, the rudder pedals (since removed and reconfigured) were tied into the pitch system, and leg pressure on both helped take some of the stick pressure from Andrew’s arms. This pressure was directly tied to throttle settings; when power was reduced, stick pressure could be reduced as well, and that made for an interesting first landing as power needed to be modulated during the descent, resulting in pilot induced oscillation (PIO) as Andrew found himself out of sync with the plane. Once sorted out, the first landing was otherwise flawless.
Andrew’s father is credited for the eyeballing of the next preset for the anti-servo tabs. His intuition was spot on, with the exception of some minor tweaking after some airframe modifications, specifically moving the empennage vertically 14 inches in two stages. At this point it has become “set and forget.”
Although the M7 has four wheels, with the front wheels marginally in front of the center of gravity (CG) and the rears well behind the CG, it still handles and otherwise behaves like a tricycle-gear aircraft. That’s due to the steerable nose wheels. If the rear wheels were to steer, then potentially it would have ground-handling characteristics more like a conventional-geared craft.
More akin to the aforementioned go-kart than an airplane, the front wheels are interconnected with a set of cables, one in front and the other behind the axle pivot. The front cable spans unbroken between the steering knuckles, whereas the rear cable is split in the middle where each end is connected to the pitman arm, driven by a tube that extends to the foot pegs. In keeping with Andrew’s concept of steering the plane like you would a bike, car, wagon, etc., he pushes on the right foot peg to turn the vehicle left. Rear drum brakes handle the stopping chores.
The four-point landing gear system and the structural members spanning between them also act as a carry-through for the trussed spars, acting in tension of course as the loads are ultimately transferred through cables to two points along each of the four spars. They also act in compression since there are x-bracing wires holding the whole landing gear system square while at the same time keeping the seat centered.
The two rear tire locations also support the triangulated box structure that supports the empennage, which according to Andrew is the “lightest, strongest possible way to make a tail.” The configuration is also conducive to the installation of pontoons, which is certainly in the near future.
At first glance, the craft appears to be controlled through weight-shift, as the seat back is definitely hung from the mast with cables. However, each of the four corners of the seat bottom are tightly strung by cables to each of the four wheels. Andrew assures me that once weight is applied to the seat, the system becomes extremely rigid.
Climbing in (maybe crawling under is a better description of how one installs oneself into the M7) is no easy chore and about the only negative aspect of this craft. Crouching and then crawling under the wing and then up into the seat is how it’s described in the Popular Mechanics article. It’s not very graceful, but then again, ingress and egress is almost always a challenge with any aircraft. Slipping into the front seat of a Pietenpol for example can be a life-alerting experience for some. Not having to negotiate the placement of a traditional stick and rudder makes up for some of the gymnastics required to get seated in the M7.
The next version of the cockpit will incorporate a kayak-like fairing, as dubbed by Grant Smith whose article on the M7 will appear in a future issue of EAA Sport Pilot and Light-Sport Aircraft magazine.
Andrew and his M7 have been received well by the people at Hirth, so much so that they invited him to display in their outdoor booth at AirVenture. As previously mentioned, he used their model 3203, 65-hp, air-cooled, two-stroke twins, naturally aspirated through a pair of Dell’Orto single-barrel side-draft motorcycle carbs.
The heavy, stock recoil starter system and fan shrouding was replaced with a very compact, lighter-weight recoil starter of Andrew’s own design. It plugs into where the optional electric starter would bolt. The design also incorporates the ability to remotely mount the pull handle from just about anywhere on the plane—certainly while seated and belted in. The cooling system is also replaced by a lightweight ram air system of his design, netting a total weight loss of 4 pounds.
Another weight loss and potential power increase is planned for by way of a new expansion chamber design that should remove 6.5 pounds from the exhaust system. Ten pounds shaved off the engine is substantial in the big picture.
Recreational Power Engineering (Rec. Power) is the factory-authorized U.S. distributor for Hirth engines and is a dealer for Powerfin propellers. Rec. Power also manufactures its own cog-belt propeller speed reduction unit (PSRU) for use with Hirth engines. It comes in two versions: the standard unit features a 7-inch offset, and a 10-1/2 inch offset is optionally available.
Rather than using a jack bolt for belt tension adjustment, the drive uses an eccentric shaft. The system also uses a single roller bearing for support on the one side of the pulley and a double-row axial bearing for carrying the thrust load on the other side. Cog belts are typically the simplest and lightest way to mate a fast-spinning engine to a slow-turning prop.
PSRU as viewed from the rear. Of note is the machined pulleys and the bearing block. Visible in this photo is also the T engine mount.
Andrew used his skills to seriously alter the “standard” unit on his plane and to further reduce the weight and hopefully increase its longevity. The pulleys were thinned down and lightened by removing excess material and boring lightening holes everywhere possible. The engine pulley has been altered to allow for a very close installation, reducing the moment arm and subsequent side load on the crankshaft bearings. About the only things left from the original PSRU are the pulleys (albeit highly modified) and the eccentric shaft. The comparatively “hefty” body of the stock unit has been replaced by a highly machined piece of 6-inch cube aluminum billet, streamlined for weight and function. Andrew’s initial iteration was made from welded pieces that began to fail, so the new design was milled just three weeks before AirVenture. Time will tell if this version will stand up to the job, as not only is it designed for improved strength, but also it weighs 1.1 pounds less than the welded piece it replaced.
The propeller shaft is hollow, not only for lightness but also for compatibility with a design that Andrew has on the table for an in-flight (manual) adjustable propeller. The shaft already has been extended in length and has extra threads for attaching the pitch-control mechanism. Several modifications are in mind for weight reduction throughout the plane that will allow for the future installation of the obviously heavier controllable prop without busting the FAR 103 maximum set for ultralights.
The engine is installed inverted as many two-strokes can be. With the propeller thrust-line above the crankshaft centerline, the offset of the PSRU is actually “down.”
The engine is hung from its mounting points, rigidly attached to an aluminum T-shaped bed mount that becomes part of the PSRU. The T is then mounted to the airframe with urethane isolators at each of the three extents of the T. The forward two points act as a fulcrum, and the single rear point can then be shimmed up or down to adjust the thrust angle.
Experimenting with the engine angle has led to a bias toward it being aimed slightly downward, placing the propwash at a higher than normal angle of attack as related to the wing. Experience has shown that this has noticeably shortened the takeoff distance, and I’m sure it helps to reduce the power-on stall speed and the perceived “hover” I witnessed at AirVenture.
Andrew and crew have tooled up for and are contemplating building 10 slightly modified versions of the M7 with a few of them designated as M10 versions. Complete with 100-plus hp and inflatable floats, the M10 will be Part 103 legal and will have lightweight cog belt reduction drives suitable for their power ratings, in-flight adjustable-pitch propeller hubs with Powerfin F blades, and Magnum ballistic recovery parachutes.
For the short term there will soon be an inward-facing wingtip video cam to show the constant 45-degree pitch angle during full-power climb, and another one simultaneously aimed at an angle of attack indicator; also shown in that same view will be the new user-friendly control system and its actual interactions with the control surfaces and vehicle attitude response. YouTube fodder for sure, which we look forward to viewing.
Snedden M7 Specifications
Weight: 277 pounds, includes 24-pound parachute allowance
Wingspan: 23 feet
Wing chord: 78 inches
Wing material: Removable Dacron, 4 ounces per 36 by 28.5 inch area
Stall speed: 26-27 power off, 25 mph power on
Top speed: 63 mph
Cruise speed: Variable, 35 mph and up
Cruise rpm: 4500 low load
Engine: Hirth 3203 with dual Dell’Orto carburetors forward slanted
Horsepower: 65 @ 6300 rpm
Reduction type: Cog belt with modified pulleys and eccentric adjuster
Reduction ratio: 2.6-to-1
Propeller: Powerfin F type, 2 blades, 70-inch diameter
Climb rate: 1,970 feet per minute with 180-pound test pilot
Takeoff ground roll time: 3 seconds with no wind
Fuel capacity: 5 U.S. gallons