Aerobatics
Wynn Paul
LAST MONTH we looked at some current designs of stunt planes that are available through reader services. If, however, you are just stubborn enough to want to try your own hand, the writer will take you through a crash (that's not really a good word for this hobby) course in stunt design. It appears to this writer that most stunt plane design is based on empirical reasoning rather than actual research; the only pure research this writer has seen has been Al Rabe's experiments with airfoils; the series that Bill Netzeband published (July, 1969 through January, 1971 in MAN) did not seem practical, nor was much of it substantiated. Seems we rely on what worked in the last plane and try a couple of changes for the new one; this has produced some excellent flying stunt planes, however. Hold on, here we go.
You should first decide on what basic style you want: classic, semi-scale or jet, and then pick an engine. Take a look at 25 top fliers represented by the 18 qualifiers in the 1975 Nationals plus seven others who made it into the finals of the FAI tryouts. You will see ten Supertigre 46s, seven OS Max 35s, four Fox 35s, and one each of ST 60, Max 40, K & B 40, and HP 40. It boils down to what you like, 35 or 46 power.
Next, work on the wing size. For 35s try 500 to 550 sq. in.; for 46s try 600 to 675 sq. in. Granted, we all see 35-size ships with 625 squares and 46s with 700, but more often than not the intermediate builder will come out way too heavy with a large wing. For reference, the Nobler kit has 550 sq., the Akrobat kit has 550, and the Shark kit has 650 sq. in. The next item to consider is wing aspect ratio (defined as the span squared divided by the wing area). Most recent competitive stunt ships fall into the category of 4.8 to 5.2 to 1 (examples: Gieseke Nobler — 4.83; Scorpio 46 — 4.83; Werwage's USA-1 — 5.26; Stiletto 46 — 4.97; Schaffer's Statesman 46 — 4.90). Since you know what your wing area is and what you want for an aspect ratio you can find the span.
In this short article we cannot go into airfoil design and root and tip chords. If you are attempting this you should be familiar with stunt airfoils which usually range from 18 to 21 percent thick. If you want a 35 size copy the Nobler ribs; if you want a 46 size, better get a set of Scorpio or Shark plans and use the ribs.
Moving right along to force moments or, how far do I stick the tail out behind the wing? For simplicity, stunters who run around with their yardsticks (like the author) usually measure the front moment as the distance from the back face of the prop to the leading edge of the wing at the fuselage joint. The rear moment is usually defined as the distance from the flap horn to the elevator control horn, or from "crack to crack." Some good combinations have been: Gieseke 75 FAI Nobler — 9.25-14; Shark and Pilgrim's Sundance — 10-17; Werwage's USA-1 — 10.375-17; Genesis 35 — 9.50-14.50; Klapinski's Tempest 35 — 9.125-13.75; Schaffer's 46 Statesman — 9.625-15.75; Stiletto 46 — 10-17.5. But continued on page 91.
CL Aerobatics/Paul
then, let's remember that the Sea Fury and Rabe had moments of 7.5-20.5!
Now we come to what the writer calls working areas: the stabilizer-elevator, and the flaps. The trend in the recent years has been toward some very large span stabilizers (up to 30 inches on the 46 Stiletto, and 24 inches on the Genesis 35) which obviously amounts to more area in the overall tailplane. An elevator area expressed in percentage of the total wing area (including flaps) of from 8 to 14 percent takes in the majority of competitive stunt planes in the past few years. For instance: Stunt Machine 35—12.2%; stock Nobler—8.8%; 46 Stiletto—9.55%; Scorpio 46—9.9%; Genesis 46—11.1% and Gieseke 75 FAI Nobler—9.8%. You will note that a lot of elevators are 3 inches span at the root, just the same as the sheet size of balsa. What a coincidence.
The area of the flaps expressed as a percentage of the total wing area (including flaps again) varies from 12 to 17 percent in recent competitive airplanes. For instance, look at: Stunt Machine 35—16.6%; Gieseke 75 FAI Nobler—13%; Scorpio—15.2%; Genesis 46—15%; Werwage's USA-1—12.9%; Paul's Pampawagon 46—15.8%.
The writer is purposefully leaving out the Bearcat and Mustang of Al Rabe because, as even Al warns, these semi-scale ships present special problems and solutions. Better to go straight to his articles and plans for these and other similar semi-scale airplanes.
Still with us? There are a number of small yet important items which should be noted when designing a stunt job. In brief they are: (1) bellcrank location—most plans this writer has examined place the bellcrank 3½ inches back from the leading edge. Some say it doesn't matter, this writer disagrees, but use 3½ in. to be safe. Use a Top Flite or Sig 3-in. bellcrank, and hook up the horns as on the Nobler plans. (Look at them in your local hobby shop if you don't have a framed, autographed copy in your living room like most of us do.) Be certain to bush the horns with brass tubing. (2) wing sweepback—from zero on the Orantil to 2 inches on the Nobler and 2¼ in. on Werwage's USA-1. Most competitive stunt ships have from 1½ to 2 in. (3) wing panel difference—a hot subject for some. Rabe insists on equal panels, although we notice that his rebuilt wing on the '75 Sea Fury has 1½ in. difference! However, he then has to load up on wing weight to 2.5 oz. (on a large 60 powered ship at 73 oz.). Tradition has one inch difference (Nobler, Whitley's Miss Kell, Blunt's Genesis), the long panel on the inside of the circle, just for those combat freaks who might be reading this by mistake), and a number of fliers are trying 1½ in. successfully (Stiletto 46, Pilgrim's Sundance, Genesis 46) and then Werwage uses 2 in. with no tip weight.
Lead-out location (4) has been touched briefly by Netzeband, Rabe and Charles Mackey; an initial setting of 7 degrees back from a line through the bellcrank and parallel to the trailing edge will give a good start. Mackey had us go to 10 degrees, but this was on an elliptical wing ship. With adjustable lead-outs this dimension is not too critical. (5) lead-out suspension within the adjustable fixture can go from 1/4 in. to 1/2 in., with a lot of stunt fliers using 3/8 in. and 1/2 in. These last also make it easier to build the fixture. You may want to try Klapinski's (and Bob Palmer for your older boys like McFarland and Gildin) over-and-under style lead-outs on the theory that you will get less yawing problems. For goose skates, build in an adjustable tip weight box, even though the writer has not used one on the ones in the last two Pampawagons, but what the heck, it takes another day to build and is another question to answer on the flying field. "Gee, mister, what's that thing labeled adjustable tip weight? Is that an adjustable tip weight?"
Lets see, don't forget engine offset (6). The writer wants someone to tell him how they actually measure a 2-degree engine offset. The thickness of your chalk (well,
CL Aerobatics (continued)
That's what Art Arfons used on the garage floor to design his jet drag car) should eliminate this small dimension. Another item, (7) the thrust line. That's the line running from the center line of the propeller back through the entire plane. Most stunters line up the wing center-line no more than 1 inch below the thrust line. OK, with your dihedraled MB-5 it's lower, but the writer hopes that your lead-out exit from the plane is within 1/2 inch of the thrust line or you may be in for trouble. And most stabilizer center-lines are from 3/4 to 1 1/4 in. above the thrust line. If you have solid rudder/fin better put in some offset, 1/8 to 1/4 in., or make it adjustable.
The writer strongly suggests ordering a foam wing from either Arnold Stoll, 628 West 6th St., Mankato, Minn., 56001, Tel.: 507 388 2728, or from Bob Hunt, 240 Greenbrook Rd., Greenbrook, N.J., 08812, Tel.: 201 469 1663. This will eliminate a lot of pain and possible trouble with warps, excessive weight. Both manufacturers have several very competitive wings to pick from in both 35 and 46 sizes.
There you have what normally takes about 10 pages and maybe ten years condensed into a few paragraphs, but with some prior knowledge and some other plans to use for guidelines, someone who is ready to design a plane should be able to come up with a reasonably good flier. Try to get someone experienced in stunt to look at your plans before you start looking for balsa. My address is Wynn Paul, 1640 Maywick Dr., Lex., Ky., 40504.
Let's remember Sea Fury Rabe moments 75-205. Now come what the writer calls working areas: stabilizer - elevator - flaps. The trend in recent years has been toward some very large span stabilizers — up 30 inches (46 Stiletto), 24 inches (Genesis 35) — obviously amounts to area over tailplane. An elevator area, expressed as a percentage of total wing area including flaps, 8–14 percent takes the majority of competitive stunt planes in the past few years. Instance: Stunt Machine 35 122% stock, Nobler 88%; 46 Stiletto 95.5%; Scorpio 46 99%; Genesis 46 111%; Gieseke 75 FAI, Nobler 98%. Will note lot elevators 3 inches span.
Transcribed from original scans by AI. Minor OCR errors may remain.
