I am going to build an ultra light room flyer type electric model.

AUW approx 1/2oz (14g) and a wing loading of around 0.4oz/sq ft.... and I was wondering what the benefit of the high mounted wing was?...

From : Don Stackhouse

The commonality of a high-mounted wing in an application like yours is in part a "tradition" item borrowed from indoor free-flight technology.

As others have mentioned, it does allow for easy adjustment of incidence on free-flight models, but at the weights typical of indoor R/C I suspect that the typical methods (which usually rely on friction fits to hold incidence settings) must be getting a little risky. Also, for an R/C model where we generally have elevator and/or throttle control, and where we operate at a variety of airspeeds and angles of attack, having an easily adjustable incidence to the wing is less desirable than on a low-powered indoor free-flight rubber model.

If you are planning to use functional wire bracing on the wing, and a stick fuselage, mounting the wing on a pylon gives a better angle for the flying wires. Without that, you would need some sort of structure sticking downwards to provide a decent anchor point, and such a structure might be a problem when landing. However, on an R/C model, if you have landing gear, that gives you a good anchor point without the extra structure that a pylon wing mounting requires. This is another example of how an idea that works well in one application is not as helpful in another, only slightly different application.

As far as stability is concerned, the effects are generally little or none, especially if the pylon is open structure, not covered. Contrary to popular belief, there is no such thing as "pendulum stability" for a high-wing arrangement. When the plane is banked, the wing's lift vector banks with it, so the vector's relationship with the C/G does not change, and therefore creates no restoring effect.

The aerodynamic interaction of a covered, slab-sided fuselage or a covered pylon with the underside of the wing can create an effect that acts like a degree or two of additional dihedral, but for a stick fuselage and.or an uncovered pylon this will not happen. We saw this when developing our Roadkill Series Curtiss-Wright Junior and Piper J-3 Cub. The Junior needed about two degrees more dihedral because its parasol-mounted wing did not get the same extra dihedral effect that the Cub received from its high fuselage.

There can be a weak influence on static pitch stability in positive-G flight. One of the biggest drag producers is the wing, and positioning it well above the C/G on a pylon or high fuselage does create a nose-up effect if the airspeed increases. Of course it has the opposite effect when flying inverted. Also, it does tend to move the major masses of the plane further from each other, increasing the moment of inertia about the pitch axis in particular, which would tend to hurt the dynamic stability.

There are some theories that placing the wing above and clear of the fuselage results in cleaner flow over the wing center section, resulting in less drag and more lift. However, I am very skeptical of whether this is actually being achieved in the real world. The interference drag between the wing and the fuselage top, the wing and the pylon, and the fuselage and the pylon, the extra structural weight of the pylon and its supporting structure, and the parasite drag and added whetted area of the pylon itself all extract their toll. Also, on a powered aircraft, having the propeller slipstream below the wing instead of in-line with it or above it, will tend to reduce lift by increasing the air velocity under the wing. Altogether, the net effects of increasing the amount of "stuff" hanging out in the airflow by adding a pylon to put the wing significantly above the fuselage most likely kills any benefit from supposedly reducing the disturbance across the wing's center section. A little work on properly interfacing a shoulder-mounted wing with the top of the fuselage should in most cases accomplish about the same benefits, without the detriments.

BTW, you can see the effect of slowing the local airflow under the wing (the opposite of what a low-positioned propeller slipstream does) in the performance specs for full scale airplanes. The published stall speed with landing gear extended is typically a little less than the stall speed with the gear retracted. The drag of the extended landing gear impedes the airflow under the wing, reducing the local airspeed, which, in accordance with Bernoulli, increases its static pressure and therefore the wing's lift.

Structurally, a pylon-mounted wing is typically not a major issue on an extensively strut or wire-braced arrangement, but a significant problem area if the wing is cantilevered from the pylon with no extra bracing. Besides all the usual forces and moments present in a conventional arrangement, the pylon introduces at least two additional bends in the load paths, and with them a number of additional torsional and bending problems.

Altogether, in most cases you would probably be better off from both a structural and an aerodynamic standpoint without the pylon.

Visually the pylon can be helpful, especially on trainer planes such as our Curtiss-Wright Junior. It increases the plane's overall "visual footprint", and also adds some more visual cues to tell the pilot on the ground what the plane's orientation is. For beginners this can be a significant benefit. Also, the greater structural flexibility that is almost inevitable in an open-framework pylon-mounted arrangement can help reduce the impact stresses on both the wing and the fuselage in case of a crash or hard landing. Typical crash damage will usually be just a strut end pulled loose. Try to anticipate that and make the strut attachments easy to repair.

Other operational considerations can mandate a pylon-mounted arrangement, such as the need to keep wings and propellers clear of the water spray on the PBY Catalina flying boats. On models such as the "Mantis" series of thermal duration R/C sailplanes it allows for a one-piece flap all the way across the center section, eliminating end gaps that can increase induced drag and reduce lift. Of course a conventional flap can be sealed against the side of the fuselage with just a little extra work and forethought, such as we did on our original "Monarch" 2-meter sailplane, without introducing all the other problems that come with a pylon mounting.

Then of course some folks think pylon-mounted wings look "cool". To each his/her own. In the case of the PBY Catalina I'd have to agree with them. However, Neil, for a plane like you're contemplating, it's probably not going to have any measurable net benefits for any objectively-based criteria.

Don Stackhouse
DJ Aerotech