Control Line: Aerobatics
Ted Fancher
This month's column is the concluding portion of a discussion on designing your own Stunt model which began in the May issue.
Plan format
The format of a construction plan is fairly straightforward. A glance at any of the dozens of Stunt plans, which most of you have perused over the years, will show that the wing will take most of the left two-thirds of the upper half of the sheet, the fuselage side elevation will occupy the bottom third, and the top view of the fuselage will use the area in between, with the tail (usually only the right half) shown in its actual position on the fuse top view. Bulkheads are normally displayed in their proper relative lengthwise location above the fuse top view. Miscellaneous items such as landing gear, gear blocks, and so forth will be located wherever space allows.
Drawing the wing
Now, pick up your pencil and let’s draw that important first line. If you agree that the flap hinge line should be straight, you have just simplified the drawing immeasurably. Draw that line and use it as a reference to keep all other lines in proper angular relationship to each other. Draw it, then the root and tip chords, the leading edge, the flap root and tip chords, and then a wing tip which suits your fancy. Easy so far, huh?
The next important step is to draw in the construction details. The three most important of these are the root and tip airfoils and the spar location.
Airfoils and construction details
I like to draw the airfoils right on the wing drawing aft of the spar location. Simply draw in a centerline and draft the airfoils using the parameters from your data sheet (i.e., length, thickness, location of the high point and the leading edge radius). Draw in the flap, and then connect these plotted locations with any of your curves which will fit the plots. The actual curve shape is of relatively minor importance in comparison to the airfoil characteristics you have plotted.
Bill Netzeband, the oracle of Control Line aerodynamics, once said that an engineer's curve is about two percent better than the curve on the sole of a pair of size 12 loafers, insofar as making an efficient airfoil was concerned. I agree with him.
Spar location and CG
The spar location is one of the most critical decisions in making a drawing that will result in a serviceable and easily-trimmed airplane. It is very important to have a pretty fair idea of where the center of gravity (CG) will be and where the control leadouts will exit the inboard wing tip. I feel strongly that the bellcrank pivot should be located as closely as possible to the fore-and-aft CG, and that the spar at the root should be at that same point to support the bellcrank mount. At the wing tip, the spar should be located so as not to interfere with the leadouts when the ship is finally trimmed. If you have ever had to try to sand away a piece of wing spar to clear a leadout so that you can move the leadouts just another quarter of an inch, after the plane was finished, you'll know what I'm talking about.
Here is a rule of thumb, derived from measurements of a number of my own airplanes and in which I have great confidence: When the airplane is properly trimmed, the CG will fall in a very narrow range centered on a point which is 15% to 17% aft of the leading edge at the mean chord (actually the mean geometric chord — but for any reasonable wing shape, the two are so close to each other that they can be considered colocated). Plot this CG location on your drawing at the mean chord and then project it parallel to the hinge line directly to the wing centerline. The point thus located on the root rib will be the location of both the bellcrank pivot and the wing spars.
Although it is true that line rake is affected by a number of variables (such as airplane speed and weight, line length and diameter, air density, etc.), it is nonetheless true that the total effect will be negligible on any remotely normal Stunt ship. When your plane is properly trimmed, your leadouts will exit the wingtip at a point about three degrees aft of the center of gravity (where we have just located the bellcrank pivot), plus or minus less than one degree. This is the point at which the spar should be located on the tip rib. Allow for a small amount of fore-and-aft adjustability of this point — but I'm willing to bet you'll end up with both leadouts within 1/4 inch of the plotted location.
Now, just draw on a wing tip of your choice and as many construction details as you feel are appropriate and let's get on to the fun stuff — the fuselage.
Fuselage layout
The fuselage is fun because it is the best place to let your creativity run rampant. Aside from locating the important pieces in the right places, the body is as you like it. Spinners, rudders, cowlings, and canopies are all grist for your creative mill. Least you be carried away, however, start by plotting the following on a convenient horizontal reference line (I usually use the top of the main fuse side):
- The spinner backplate
- Wing leading edge
- Center of gravity (CG)
- Flap hinge line
- Wing (flap) trailing edge
- Stabilizer (stab) leading edge
- Elevator hinge line
- Elevator trailing edge
On a vertical reference line, plot the following locations:
- Engine thrust line
- Wing centerline
- (Another hint: Try to keep thrust line and wing centerline within 50% of the wing thickness of each other. Since the wing will be the primary source of drag, having it a large distance from the thrust line will create substantial pitch trim problems.)
- Stabilizer centerline
- Wheel/ground contact point located vertically to allow adequate ground clearance for the largest prop you plan to use, and horizontally located with respect to the center of gravity
Now, draw in the wing and stab cutouts and let yourself go. Just remember, anything you draw you're going to have to figure a way to build at a reasonable weight.
Top view, stab and final details
The last major component is the top of the fuselage and the stab/elevator. For ease of drawing and to save room, usually only half of the top view is shown. Critical measurements here are limited to the width (which should be dictated by the size of the engine crankcase). Measure that, and then let your fuselage grow naturally: crankcase plus two engine mounts plus two doublers plus two fuse sides equals fuse thickness. Firewalls and formers are simply plotted from the top and bottom views and located between the wing and fuse top view in their respective longitudinal locations. The right half only of the stab and elevator are drawn as a part of the top view and you are now down to the nitty-gritty of how detailed you want your completed drawing to be.
If the drawing is planned for publication, you will want to letter it using a lettering guide and also put in some caption material such as the model's name, the designer's name, critical dimensions, etc. Consider using Letraset dry transfer lettering for the captions, as it gives a very professional look — and besides, it gives you something to do with all the old Letraset sheets drying up in the drawer under your jockey shorts.
Include explanatory notes for design items which may not be easily illustrated by the plan, such as wing symmetry, wood types and grades, and manufacturers' names for commercial items which you recommend.
And, not least, if you have the nerve to do the original in ink: if the design is published, an inked drawing is worth as much as an additional $100 in your pocket.
I hope this has proved helpful. Perhaps another time we can spend some time discussing what those numbers on the data sheet should be and why. You really can design your way to the top with a better mousetrap, especially one tailored to your particular talents. After all, that's what makes Stunt the greatest of all modeling events. There really is more than one way to skin the cat. Fly Stunt!
Ted Fancher 158 Flying Cloud Isle Foster City, CA 94409
Transcribed from original scans by AI. Minor OCR errors may remain.
