Tuesday, December 29, 2009

Cheap aircraft

I have read this report before, but it still remains quite interesting, it is about the FMX-4 Facetmobile: http://www.wainfan.com/pavreport.pdf


Thad Beier said...

What makes the report so interesting is that it's not just paper -- he built and flew the machine extensively.

It's interesting that it flies in the face of so many conventions -- very low aspect ratio, little consideration to streamlining, no curved surfaces, no airfoil design, low wing loading, no high-lift devices, low L/D.

Lightness, large wing area, and high Reynolds number (due to very long chord length, not high speed) all have strong beneficial effects together.

You of course know that the plane was lost due to an engine failure and the subsequent inability to fly through a fence. I know that the Wainfans have to make a living, but it is too bad that nobody has picked up the mantle since then and built a successor to the Facetmobile.

Barnaby has built many 'model' Facetmobiles to recover instrumentation from very high-altitude balloons -- the good stability and ruggedness of the design more than make up for the low L/D.

Anonymous said...


Exo Cruiser said...

Sure, but it looks so ugly. I don't really understand the desire to have so much cruise wing. Actually you would like to have as small wing as possible to minimize the drag when cruising. For landing and take off that is great of course and if you want to have UCAS making 20 G robot turns.


Anonymous said...

Hello~happy new year............................................................

mrk said...

Hi Designer

sent an email to your gmail address but looks outdated. need to make it go through, could you please show up on karenfuxia (at) libero (dot) it
with information kilo sierra?


Unknown said...

mrk, my email is the format firstname.t.surname at gmail.com
Pay attention that my first name has two i-characters, see end of this post.

Exo: It is all about wetted area, nothing else is that important than that. Fuselage is dead wetted area where lifting fuselage encloses people plus generates lift making it possible for every component in the aircraft (almost) to contribute to lift (in other words, get some lift for almost for free, almost because there are penalties because of the unoptimal faceted shape of the craft). The facetmobil is interesting because it has relatively good performance compared to the unoptimal faceted shape it has. Wing size has nothing to do with aircraft performance, the drag comes from wetted area. The simplification on the drag coefficient is just to conceptualize it to a simpler format. In practice the air does not know what belongs to wing and what belongs to fuselage, it is all about air molecules being displaced by an object and that is thermodynamics, fluid dynamics and mechanics what then happens, how much there is force against the travel direction of the aircraft and how much there is force generated upwards and how much force on downwards (which depends obviously on the weight and G).

It is all physics, aerodynamics is not isolated art but just plain physics and all the laws of physics do apply, there is no magic in how airplanes fly and how much drag they produce.

exo: the wing is always designed around the takeoff and landing parameters. This is the trade-off that is necessary. Wing can not be only optimized to cruise only. The most important thing is to get the plane to the air (accelerate enough to take off) and the second most important thing is to have a lading speed that is safe. Drag comes later in the list, minimum drag is a good goal but the tradeoffs of safety and ability to take off overrule it always or otherwise the plane simply will not fly (the available power must be greater than required keeping the speed on runway to the plane to takeoff, there is the friction from the runway affecting this in addition to the drag of the airplane. Good example of this is Cirrus SR20, takes some time to accelerate it for takeoff speed. It is quite at the limit of wing size (=lift capability) in relation to the available power for a general aviation aircraft that needs to fit to reasonably short runways.). That is why there are high lift devices like flaps invented, to minimize the wing area (thus extra wetted area) while still maintaining enough lift for the critical takeoff and landing.

In my opinion, most interesting design would be the following:
- lifting body + little wings = combined structure. This may be quite challenging though because of the very different steepness of the Cl-curve on outer wing sections compared to the center section (some compromise has to be made possibly - the center section may be only a relatively poor wing), but anyhow I find it conceptually interesting thing.

Best Regards
Karoliina Salminen

Anonymous said...


Nathan said...

Great blog!

I was at first confused why the bottom of the craft has 3 different angles. The middle, flat section is expected; the back, angled-upwards section seems logical, to meet the top in a shape reminiscent of a wing; but the front is what really confused me, because it is angled down. Not only does it make the pilot's visibility better, like the Concorde, but the front and back are also reduced attack angle, which would make the stall more gradual. Oh, and the rear panel would stall last, making a complete stall at speed almost impossible. Clever.

When considering the angle of attack, how is it measured in this vehicle? Is it the angle of the structure (same angle as bottom middle panel), or the angle of the line drawn between the leading and trailing edge, as with a wing?

I like your refinement of this vehicle by adding little wings. You mention the different Cl curves associated with the fuselage and the wing, so I offer this possible solution: it does not have to be a fixed wing. The computer control to proportionally change the attack angle of the little wing relative to the attack angle of the fuselage would be almost trivial. The extra hardware would add weight, but not much, since it is, after all, a little wing.

I've been trying to imagine that craft in an actual airfoil shape, maybe 3:1 or 4:1 AR, a cockpit bubble pushed forward through the leading edge, and a pole behind to hold a conventional tail. In other words, the reverse of the Facetmobile concept: rather than make the fuselage fly without wings, make the wing so big that the fuselage is almost completely within it.

Of course, this would completely negate one of the main design goals of reducing expense by using standardized parts and low labor. It would, however, better combine structural and aerodynamic efficiencies.