Control Line: Combat
Charlie Johnson
THE COMBAT WORLD was challenged by Sherwood Buckstaff a couple months ago and in the following issue of the MACA Newsletter there was a torrent of comment. The best of the lot follows, written by Rich Brasher, Redwood City.
There is a Texan named Sherwood B. Who at the Nats will compete with me. But he'll have no chance, I'll just do a dance And kill him with simplicity.
The rest of the comments are in the March 1977 issue of the MACA N/L. In all fairness to Sherwood Buckstaff I must remind you that he has won the last two Nats Fast Combat events, and another Texan, Dick Stubblefield, won the year before.
I've seen a lot of comments about having the Nats out in California. Seems the Easterners think it is closer for us to drive 2200 miles back there than it is for them to come out here. At least it won't rain. March AFB in August should be in the 90's so bring along a good wide-brim hat, salt tablets and some shims to raise the head on your engines.
Here's a neat way to make perfectly scaled copies of any relatively flat object like a stab, airfoil section, etc. You'll need access to a photo enlarger if you want a print, or a slide projector if you don't mind tracing. Take a picture of your favorite elliptical stab or airfoil, using black-and-white film if planning on making prints or color-slide film for projecting. Almost any camera will do. Take care that the camera is parallel to the object and centered. Using either the enlarger or projector, it is possible to obtain perfect reproductions of the original in any size you desire. A Nemesis airfoil can be reduced to make a 1/2A Nemesis or made large enough to be used on a full-scale plane. You can even take a top shot of a model and project it onto blueprint paper (special variety) and get full-size prints from which you get all the dimensions. The photography bit helps, even if you get all the basic measurements from the designer, since you probably won't remember the exact leadout location and engine placement or a dozen other details. It'll be even harder to keep a design secret now. Guys like Brasher would even pick up all the broken bits of their models to keep a wing from falling into the hands of copycats. I'll bet you thought they were so ecology minded!
Something tells you that the 35-sized venturi in your Tigre X-15 looks a bit large but you don't have a machinist friend, so now a solution. You could cut a bit off the bottom of an old suction venturi but the easiest and cheapest method is to use epoxy. Wrap masking tape around the venturi after you've cleaned it well, and don't forget to cover the bottom, too, or you'll need large quantities of epoxy. The long-cure epoxies are more fuel-proof, the five-minute type OK for field work.
After the epoxy sets up, you remove the tape and drill a hole down the center.
Control Line: Combat
appropriate size (around 7mm in the case of the X-15). A Dremel works well in shaping the venturi. Don't forget to grind a passage for the fuel to get from the spray bar into the venturi. On engines like Fox you might coat the inside of the regular venturi and crank (make sure the port isn't open) with mold release or a thin coat of grease and, after the epoxy sets, you'll have a removable venturi ready for shaping. To aid in removal of the epoxy you might try molding in a piece of string to give you something to pull on in case it gets stubborn.
If you are using suction and want the maximum size venturi possible for making a lot of power but retain decent mixture control, you might try the next idea. Find some brass or copper tubing that is a little bigger than the spray bar. Drill or grind a hole in the tubing just large enough for the spray bar to fit through. This piece of tubing will be located inside the regular venturi and over the outlet jet in the spray bar. Secure with epoxy or solder. The top of the tube should be 1/4 in. below the edge of the primary venturi and extend about 1/4 in. or more below the spray bar.
What happens is that air enters the main venturi and starts to flow downward. The air enters the secondary venturi and proceeds as normal until it reaches the spray bar which really restricts flow. The only alternative the air has is to speed up in order to join its friends at the crank port, thus creating a low pressure area and a lot more suction than would have been found in a larger venturi without the restriction. The effect is that you'll be able to run a much bigger primary venturi, but still have reasonable fuel draw and easy settings. The advantages with the normal short intake stack on a combat motor aren't nearly as great as with a rear intake motor, because it takes quite a distance to establish air flow through the venturi.
The fastest way to seal foam is to use low-heat plastic like Solarfilm, Flite Coat, etc. Time-wise it will cost you only about 30 minutes per plane, but your pocketbook $3-4.00. The weight penalty is from 1-2 ounces depending on covering and size of model. One of the least expensive ways is to use wrapping paper or contact paper; the weight penalty is greater but they look cute. Possibly the lightest, cheapest, and strongest is to use the old standard, Silkspan, with white glue/water mixture as adhesive.
Cut the tissue to the approximate size of each panel and saturate with a very diluted mixture of glue and water. Do this step on an old mirror or pane of glass, because you'll want to squeegee out all the excess water/glue mixture. The foam surface can be lightly pre-glued with some mixture, but don't get carried away and fill all the pores. The damp Silkspan is now molded over the wing core. Once dry, it can be sprayed with clear epoxy with an extra dash in the center where more fuel is likely to accumulate. Riley Wootten claims 1/2 ounce finishes, but an ounce would be easily acceptable since it only costs 50¢ to do a model, and the hard shell adds to the structural strength. This method saves money but costs in time and limits the life of the plane, since fuel will eventually soak into the surface.
Wooten's new Slow Combat job needs a little work to make it competitive. As designed, it will be too nose-heavy even with an inboard tank and engine moved back to the minimum 22 in. from thrust washer to elevator. Either make your own body 6 in. longer or splice in some wood on the stock body. Slow Combat planes should be on the verge of being unstable without a streamer. Always practice with a streamer so you know what you've got and won't be disappointed at a contest when the streamer not only slows you down but affects the handling as well. The wheel and fuel tank set-up as used on Dan Rutherford's Cheater Slow is perfect for the Tezzer Too. If you decide on an outboard tank, I would suggest using a metal strap or rubberbands to secure the tank rather than epoxy as suggested in the instructions. A full tank of fuel weighs almost as much as an engine and could easily work loose after a few flights or ground pounding. Besides the safety factor you will be disqualified if any part of your plane comes off during flight (I know of a person who was disqualified when his engine spit out the crank—he also writes poems).
The Cox 15 hasn't exactly set the world on fire in FAI combat. It runs well but so does a G-15 Tigre which weighs less. When restricted enough to pull through maneuvers and hold consistent needle settings, even the Rossi 15 is not that far above a G-15 or G-20 Tigre. It probably won't make any difference at our Team Trials because most people will be flying their usual Fast Combat style with 15-sized airplanes. The emphasis won't be on consistency and good pitting but on good wild matches.
I've been using a Tigre X-15 lately in place of the Rossi and have had pretty good results with it even though it weighs 1/2 ounce more (like the Cox). The motor is not sensitive to being over-lean as much as other high performance engines; it just slows down but has never come near seizure. Fiberglass props are a necessity with the high-revving motors; the 7-4 nylon that was used on the G-15's is not up to the task. Some of the new wood Quarter Midget props are good, too. I've been using props designed for AMA Goodyears which are available from several different manufacturers. They tend to have a bit too much pitch, since top-end is more important to them than acceleration.
Try to keep the full 7-in. diameter if possible, and keep the revs up by de-pitching the tips of the prop. The wider bladed props will give a better feel to the model than the super narrow speed types. The larger diameter and wide blade will give more zip when climbing or during consecutive maneuvers, and the extra diameter helps a bit in cutting the streamer. When you think you've got the perfect setup try some "Wilcken's Wiggles" and see if the motor maintains the revs, the plane speed stays up, and check how well it stays on the lines.
MACA will be running the U.S. Team Trials over the July 4th weekend. Due to lead time of the magazine you may not read this until just before the contest, but I'm told that for an additional fee you may enter late at the site in St. Louis. Additional manpower is needed to help run the contest, so in either case you might contact MACA president Gary Frost, 22 Glynn Dr., Florissant, MO 63031.
In the last column I mentioned some neat bearings available for the new Fox. I know a few were disappointed because they didn't fit the crankshaft. The inside diameter of all the bearings is the same, but the diameter of the crank isn't. You need to polish a bit off the crank until the bearing is a nice "fingers only" push-fit on the crank. Some people tried freezing the crank, heating the bearing, and brute force. I repeat, the crank must be "fitted" to each bearing or you don't accomplish a thing. By the way, Rich Brasher has a couple cranks left from the original batch of 100. When these are gone there will be no more. Write Rich at 327 Howland, Redwood City, CA.
Charlie Johnson, 3716 Ingraham St., San Diego, CA 92109—telephone (714) 273-6530.
Transcribed from original scans by AI. Minor OCR errors may remain.
