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Looking at the Facetmobile From a New Angle

By Anthony J. Liberatore, EAA 99484, edriveeconomyllc@comcast.net
Photos courtesy of EAA / Barnaby Wainfan

Facetmobile

As I get older I have learned to count my blessings. One of those blessings is our abilities as EAAers to walk amongst the giants in aerospace and talk airplanes, their design, or to just share a laugh. When we meet these fellow EAAers, they are our Wayne Gretzky, and what they share with us is often akin to getting stick handling lessons from Gretzky himself. Some of these good folks are encountered by just bumping into them in the field, or you are introduced to them via mutual friends. One such introduction for me happened two years ago through Pat Panzera. I got to meet the designer and builder of the Facetmobile, Barnaby Wainfan. If you haven't met Barnaby, he has a quick wit, quite a sense of humor, and yes he loves to talk about airplanes.

To refresh your memory, the Facetmobile (N117WD )was the winner of the Dead Grass Award in 1994 at Oshkosh with its unique faceted shape and low aspect-ratio design. To many people, it defied everything they knew about aerodynamics. But many that remember it ask, “Where is it today? And in what direction is that effort going?”

Facetmobile

Well, for starters, N117WD still exists in a hangar in Compton, California. With 130 hours on the airframe, an engine-out necessitated an emergency landing that ripped off the landing gear among other damage, and the repairs have yet to be completed. Why so long to fix N117WD? Like many of us, Barnaby is a father, a husband, and both Barnaby and his wife, Lynn, have full-time careers. As many of us have experienced, life happens while you make other plans. With their busy lives and little time to do the work themselves, the Wainfans were on the verge of hiring someone to restore N117WD in 2009. However, with the economy in quite a state of flux, they decided not to go forward with that commitment.

But even though the original prototype isn’t currently in an airworthy status, that doesn't mean the concept has languished. In fact, the overall concept has advanced in a number of different directions. Interest in the Facetmobile by the radio control (R/C) aircraft community continues, and a number of homespun electric powered R/C versions have taken flight, utilizing fan-fold foam construction (1/4-inch thick underlayment foam board from home improvement megastores) as well as traditional balsa.

A number of different efforts have come from Barnaby himself; one is a follow-on design to the original Facetmobile single seater, the FMX-4. This follow on design, the FMX-5 is a two seater of which an extensive study was prepared for NASA entitled Feasibility Study of the Low Aspect Ratio All--Lifting Configuration as a Low-Cost Personal Aircraft.http://www.wainfan.com/pavreport.pdf

FMX-5 Concept
The FMX-5 Concept

While no man-carrying version of the FMX-5 with all its design enhancements over the FMX-4 exists, a small-scale version does. This scale variant of the FMX-5 is known as the “Talon Topper,” an autonomous flying scale vehicle that uses a GPS-based autopilot that guides them home after they release from the balloon.

Talon

Not only has this small payload return vehicle (PRV) version of the FMX-5 built by Near Space Corporation flown, but it has also returned successfully from altitudes greater than 98,000 feet and has broken the sound barrier on returning to lower altitudes in performing its missions. Much of these updates and more were discussed in 2009 at the Oshkosh Forum Presentation that Barnaby gave on the Facetmobile to update fans of the design.

PRVs
Multiple PRVs being prepared for missions

It was during this presentation (and my one-on-one discussions with Barnaby on Wittman Field) I confirmed my hunch that there is more clever aeronautical engineering to this faceted, low aspect ratio aircraft than meets the eye. In fact, if the concept were to go beyond one prototype and become a kit or enter serial production, it may be the “affordable” aircraft that many of us have been longing for. With that as a background, there are a series of aerodynamic features applied to the Facetmobile’s design during its inception that we will discuss in detail. These features enhance the Facetmobile's performance and add to the “sweet spot” that already exists for a properly designed, low aspect ratio aircraft.

Inverse Zimmerman
Like many of you reading this article, I have been fascinated with aircraft design, especially some of the more esoteric configurations and comparing them to one another. Looking at the planform drawings of the Facetmobile, one could not help but notice the angles that give it a tapered or almost elliptical planform shape. When I asked Barnaby if he was trying to get the Facetmobile to have an elliptical lifting curve (known to be very efficient), he smiled and said, “Why yes, why not take advantage of that?” I also asked, is it a “Zimmerman” (referring to Charles H. Zimmerman, the Chance-Vought engineer who championed the concept), a Chance Vought V-173 Flying Flapjack rotated 180 degrees? Again the answer was yes. Ironically, in the April 2005 Issue of Flight Journal, Barnaby authored an article on the Flapjack and notes how it may have been way ahead of its time.

Following our conversation, research revealed that, in the micro unmanned aerial vehicle (MUAV) arena, the configuration of choice at the moment for these small flying vehicles is in their parlance an “Inverse Zimmerman”! It seems that with the wider trailing edge there is more room for control surfaces, these control surfaces are more effective, and the vortices are shed outward to make for a better flying craft. What is also amazing about this is Barnaby was way ahead of this trend by going with the more forward-facing Delta planform, given that the Facetmobile concept was conceived many years before the MUAV arena even existed.

Flapjack
Larger view

The Flying Flapjack and the “Inverse Zimmerman” Facetmobile. Notice an overall elliptical planform on the Facetmobile (images courtesy of www.voughtaircraft.com and Barnaby Wainfan).

Airfoil?
In the most in-depth article of all time written about the Facetmobile, printed in Contact! Magazine’s issue number 71 and authored by Barnaby, he noted about the FMX-4: “What we have is a fat delta wing with funky airfoils.” What these funky airfoils on the Facetmobile have are two characteristics. One, they change in cross section across the lifting bodies span and they have sharp leading edges. As Barnaby said in his Oshkosh forum, “How sharp is my leading edge? As sharp as I can bend 20 thousandths aluminum.” In an interview with Barnaby's friend, John Dyke of Dyke Delta fame, John discussed the origins and benefits of sharp leading edges in the low aspect ratio arena. John noted: “NACA had a 0012 airfoil. Basically a symmetrical 12% airfoil, and a pretty blunt leading edge on it. NACA took it and extended that edge out and changed the lift coefficient of the blunt leading edge at only 0.6. They increased it to 1.2 by putting sharp leading edges on it. Why does this do this? That is contradictory to everything you read about aerodynamics. But smoke tests have shown that they produce vortex lift, and the angle of attack went up. It is the vortex lift that is keeping this air going - it is not turning around on you."  

The aforementioned airfoil changes across the span of the Facetmobile have not gone unnoticed by some of its admirers. These fans have taken Barnaby's publicly available data on the FMX-4 and have drawn it in CAD, and by plotting cross sections at various stations along the span, they have discovered quite a bit of wing washout. In Barnaby's December 2005 KITPLANES article dealing with washout, he noted that even with a straight or tapered wing, one could achieve the much desired span efficiency of 1 by adding washout. (Span efficiency is a unit of measure of the lift distribution of a wing with 1 being a benchmark to achieve.) Even if the twist is chosen for climb or cruise, it still can add to the overall wing efficiency. Since the Facetmobile is all wing, i.e. a lifting body, I asked Barnaby if the twist added to its efficiency. Again the answer was yes, with a smile, “Why wouldn't you do it?” In fact Barnaby admitted the N117WD had too much washout, and in cruise it responded as if he were applying a bit of up elevator, necessitating additional nose down trim at cruise to get the proper pitch attitude. Barnaby also noted the FMX-5 has less washout, and the washout is now spot-on for the Talon Toper RPV.

R/C version
Wetted with paint, Barnaby flew his R/C version to get a visual on the flow characteristics. A fatal flaw in one of his earlier designs was discovered and corrected using this method.

FMX-4
Larger view

FMX-4
Notice the different tail placements. The twin-tail is how the FMX-4 was outfitted and flown to OSH. The single tail is how the autonomous FMX-5 is currently being flown.

FMX-5
The final iteration of the FMX-5

For a little more detail on the R/C versions, see the excerpt from the Contact! Magazine article at the bottom of this feature.

In further discussions with Barnaby he noted that the airflow over the Facetmobile isn't typically linear for-and-aft. In fact with all the faceted surfaces, sometimes it is moving in other directions, sometimes sideways against the surfaces, but the flow stays attached. However, Barnaby said to me, “Anthony, if I changed the faceted angles just a bit, I could make the flow laminar.” If you think about the potential of obtaining laminar flow for a minute with these sharply faceted surfaces, it’s mind-boggling as it’s certainly counter-intuitive. We take for granted that the flat tail surfaces on Champs, Cubs, etc., work just fine, given even they have limits before flow separation, but to go laminar with flat plates instead of smooth curves again test the boundaries of what we would consider necessary for laminar flow.

Performance
While the Inverse Zimmerman planform, sharp leading edges, and the wing washout combine to enhance the Facetmobile's performance, it is its low empty-weight (via a low parts-count) that’s the icing on the cake in terms of contribution to the Facetmobile’s performance. Again these attributes are discussed in detail in the PAV report (see link above). All of these design features may have saved the day when the engine quit at 600 AGL on climb-out, with Barnaby at the controls. As Barnaby noted, looking forward, there were no good landing options. With nothing but ugly in front of him, Barnaby did what our instructors tell us never to do; he executed a 180-degree turn. What is significant about the turn is that with the Facetmobile's ability to obtain high angles of attack and fly-on vortex lift, the stall/spin scenario was avoided. Barnaby noted if he would have had a few extra feet of altitude approaching the airport fence, the damage to N117WD would have been avoided.

Barnaby was not harmed in the incident, which in addition to a low landing speed, he attributes to the aluminum tube construction and large amounts of that structure between him and the ground to absorb the energy from the impact. But nonetheless, this mishap may highlight the characteristics of this unique configuration that might have a good fit with a certain section of the aviation community, i.e. low-time pilots. With its capability to fly at high angles of attack, inherent stall/spin resistance, and the ability to land slow, low-time and infrequent pilots may just be a natural market for such a craft. I asked Barnaby, could a two-seat LSA variant of the FMX-5 be tailored to reach the vaulted Van's "4:1 ratio" of stall to top speed (for LSA, 34.5 mph and 138 mph respectively)? His response was in the affirmative. And that is not the only end of the spectrum that this configuration could be tailored to. The concept is completely scaleable, and at the Forum at Oshkosh, Barnaby noted a six-place "Largemobile" with a 260-300 hp IO-540 engine would be a six-place that would redefine camping at Oshkosh because you could certainly camp in it!

Facetmobile
A very high pitch angle can be achieved resulting is very slow flight when necessary.

With the possibilities of scaling, one might ask, when can I build one? At this point Barnaby has decided not to go the plans route due to a host of reasons, one being that while maintaining his career (and with life in general) there would be no time to offer proper support. The kit, or potentially even serial production routes possibly utilizing some very advanced construction techniques, is not out of the question, if the right offer came along. Until then, many fans of the Facetmobile could explore the R/C route and continue to follow Barnaby via the web and updates at Oshkosh for the day when the Facetmobile might become a reality for them as well.

Many thanks to the Wainfans at Oshkosh for discussing all things Faceted. 



FMX-4 SPECIFICATIONS

OVERALL

Span
Length
Height-level
Height-ground

16.0 ft
19.5 ft
6.0 ft
6.0 ft

FUSELAGE

Seats
Frontal area
Cockpit width
Cockpit height

1
43.2 sq ft
40.0 in
42.0 in

PERFORMANCE

Vne
Vc-75% power
Va
Vs
Rate of climb
Service ceiling
Land, 50 ft obs.
Endurance
Fuel capacity
Wing loading
Power loading
G load +
G load -

130 mph
90 mph
100 mph
26 mph
700 fpm
12000 ft
600 ft
3.0 hr
23 gal
3.0 psf
13.5 php
10.0 ult
10.0 ult

WEIGHTS

Gross
Payload
Baggage limit

620 lb
250 lb
——–

WING

Span
Area
Aspect ratio
Chord-root
Chord-tip
CG-fwd
CD-aft
Airfoil-root
Airfoil-tip
Dihedral
Washout
Sweep
Incidence

16.0 sq ft
205.7 sq ft
> 2.0
234.0 in
96.0 in
%
%
Wainfan
Wainfan
0.0 deg
0.0 deg
51/75 deg
N/A

ELEVONS

Span
Area
Def-up   
Def-down
Type

72.0 in
9.0 sq ft
+30 deg
-30 deg
Flat plate

FLAP, TRIM  

Span
Area
Def-up
Def-down
Type

36.0 in
4.5 sq ft
+30 deg
-30 deg
Flat plate

VERTICAL SURFACES

Span at rudder
Stab area
Rudder area
Airfoil
Deflection

46.0 in
12.0 sq ft
4.4 sq ft
Flat plate
+30,-0 deg

POWERPLANT

Make
HP
Max RPM
Max torque
Fuel quality

Rotax 503
46
6800
40.6 ft lb
premix

PROPELLER

Make
Type
Material
Diameter
Pitch

CGS
3 blade
wood
60 in
42 in

More on the Inverse Zimmerman:
http://edge.rit.edu/content/OldEDGE/public/Archives/P0206F/mav_loader.htm

Excerpt from the 14-page article printed in issue #71 of Contact! Magazine:
I was just about ready to start building the real thing, but to be on the safe side I decided to build one more RC model (FMX-3) incorporating some slight changes. It was a very good decision because there was a fundamental flaw hidden in that version. In making changes, I had gotten one of the facet break angles too steep and the flow completely separated on the back side of the fuselage. The model would barely fly. I kept trying to get it to fly, and it kept just falling out of the sky in various interesting ways. I finally figured out the problem by utilizing flow visualization. I applied some diluted water-based paint, artist tempera, on the model. The paint streaks showed what was happening to the airflow. Since the Dash 2 flew great and the Dash 3 didn't fly worth a damn, we looked at the contrast in the paint flows.

I then actually took the Dash 3 model and cut it up to introduce one more facet to soften that break. The revision to FMX-3 (now -4) was near miraculous because it flew really well.

In summary, we went through two 15% scale models and then eventually a quarter scale model to get the configuration dialed in. They were radio controlled and the original early models were 2 channel—no throttle control, but they flew fine. The final one was quarter scale at about 5 feet span, roughly. It was full house RC, with rudder, elevator, pitch trim, and throttle and flew great. I had the basic setup pretty well pegged. I had a pretty good confidence level that the airplane would fly nicely, and of course one of the things you can do with an RC model is really abuse it in a way you wouldn't abuse yourself in an airplane. We did very aggressive pushovers and then let go of the stick to make sure there was no tendency to tuck, for example. We stalled it, or tried to stall it, and discovered it wouldn't. We did extensive flying at full aft stick and maneuvering with the elevons. That model was actually built by a good friend of mine named Don Larson. Don still has it and flies it regularly. The thing is so gentle it could be used as an RC trainer. Its docile performance convinced me that we finally had something workable. In mid-1989 we started building the real thing.

 
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