About this Plan
Hercules. Free flight cargo-carrying model. Wingspan 108 in, wing area 1050 sq in, for .049 power.
Quote: "Aerodynamic Lessons from Clipper Cargo Competition. By Donald A Gurnett, 1958 Champion.
Effective last January Pan American introduced a new .02 cu in Clipper Cargo class. Cargo carrying ability will again be the goal of the event although under the new rules total weight of the model and cargo will be counted. Thus, structures may now be made strong and durable, and aerodynamic refinements may be emphasized. Other changes in the rules will limit the wing span to less than 48 in to facilitate transportation.
Considering the type of design, we will probably find most of the general principles which applied to .049 cargo will also serve in 'mini-cargo' with the exception of wing planform and structural design.
The designer will again as in .049 cargo find that a large wing area and consequent light wing loading will be desirable. However, with the wing span limited to 48 in we can no longer choose an optimum wing loading but must select the wing chord to give the most efficient relationship between aspect ratio and wing loading. A large chord would result in a large wing area but the aspect ratio may be so low that induced drag could cancel all benefits of a large wing. The opposite effect could also be noted. As effective aspect ratio is now a limiting factor, it would appear that an elliptical or correctly tapered wing would be more beneficial.
The trend in mini-cargo design will probably be towards a model of around 300 to 350 sq in wing area with an aspect ratio of approximately 8 to one.
The type of model I will fly is shown in the accompanying small drawing. The strong and simple models that should result under the new rules will be ideal for the young modeler starting out in Clipper Cargo. Certainly mini-cargo should be an interesting new event in which both ex-pert and beginner may compete with equal satisfaction.
Because my 'Hercules' was the first cargo model under the former .049 rules to lift 50 ounces, the editors thought that a discussion of its design factors might be of interest and help to other experimental modelers.
The designer of a Clipper Cargo model finds himself facing one of the greatest design challenges in modeling today, for few other events demand so much from a small engine, require such a light strong structure, straight takeoff, stability and many other problems. For the person starting the lack of information is a serious handicap.
Aerodynamically the Hercules is of a quite conventional configuration. (Although the class is finished I still have the model and so speak of it in the present tense.) The first impres-sion may be that the model is quite large. As the altitude that can be gained during the power run is very limited by the high power loading it becomes necessary to get as good a glide as possible. Once the wing load-ing gets over 5 oz per 100 sq in the glide begins to suffer even when high lift airfoils are used. Ask anyone who flies FAI power. Obviously the wing will have to be large if the wing load-ing is to be kept light enough to af-ford a good glide.
A design with a light wing loading offers two more distinct advantages besides the improved glide. A light wing loading allows the model to take-off quickly because of the lower air-speed required to become airborne. A quicker takeoff is valuable because it allows a slightly longer power climb and it gives the model an advantage in many of the small local contests where takeoff facilities are poor.
The slower gliding speed of a model with a light wing loading also means that it will he less likely to be dam-aged should it hit a car or some other obstruction. This fact plus an efficient structure help make up for the strength problems associated with the previ-ously large cargo models.
With the higher Reynolds number and better efficiency afforded by a large wing I decided that a wing load-ing of 6 to 7 oz per 100 sq in would result in a good glide plus the other factors already mentioned. As the total weight may well be 65 to 70 oz, this would mean a wing area of about
1050 sq in.
Because cargo models have a rather low moment of inertia due to most of the weight being concentrated around the center of gravity I felt sufficient stability could be obtained by using a small 25% stabilizer as well as a short tail moment arm..."
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