Control Line: Aerobatics
Frank McMillan
12106 Gunter Grove San Antonio, TX 78231
Introduction
Many years ago I watched an experimental airplane I was truly thrilled with — a piped Stunt model. Actually, it was a Dynajet-powered ship. The guy flying it was a legend in the New York area (where I survived growing up) named Tommy DeVille. The flights were spectacular — so were the thunder of the jet and the large, sweeping maneuvers that were necessary to keep the jet lit.
Over Labor Day at the Dallas Southwesterns the memories were rekindled by Dale Hungerford and his new jet-powered Stunter. Dale is the type of guy who likes to take on a technological challenge. He's really got hold of one here. The plane I observed wasn't the first jet Dale has tried. He has looped both inside and outside, flown inverted and done figure eights. For those of you who haven't witnessed jet-powered Stunt, it's impressive!
Dale Hungerford's Jet Stunter
Beyond the flying, Dale has had great rewards from pushing his special field of technology. The jet isn't a standard Dynajet, but a limited-production Cosmojet, made in Houston, TX. It is somewhat larger — about 1.5 in. across the pipe versus 1.25 in. for the Dynajet. It's also designed so that settings are less critical by virtue of its inlet shapes and vibrating reed angles. In short, Dale vouches that the Cosmojet is much more reliable and easier to handle than the familiar Dynajet (or what most people envision it to be).
Dale has developed a pressurized fuel system to control the engine more reliably. He runs a lubricated bladder inside a large RC fuel tank, much like a Combat fuel system.
To vary the power he uses a series of valves to control the fuel flow. The flights I saw demonstrated that it actually works, but it still has a few bugs to iron out. Metering jets is somewhat critical. Dale uses an onboard radio-controlled servo to work the fuel valves and throttle the engine. He has a transmitter-and-belt arrangement that he straps on to assist him during flight. He operates the radio on a ham band (53 MHz), which is relatively free of interference — he got his license specifically for this purpose.
Also of interest is the fuel used — basically it's Stunt fuel without oil. This was a surprise. Dale says it is very tolerant and doesn't present the danger of fire that many of the speed formulas still do.
Construction and Control-System Solutions
As you can see from the pictures, the aircraft's construction explores new ground to host the pulsejet power plant. As a result of using a jet, high heat is generated. Dale chose an open frame to house the engine while he worked out engine runs. This presented some problems. The frame had to be strong enough to carry and support the load of the wings, which are attached permanently to the fuselage frames. The frames are laminates of plywood and carbon wrapped in fiberglass.
After the frame was conceived another major practical problem arose. The bellcrank/pushrod mechanism path was circuitous at best. Dale used a cable-in-tube approach to solve the problem, stiffening certain key areas.
Dale is to be congratulated for some innovative, technical problem solving. We hope to report more on this effort in the future.
Stopwatch and Timing
The stopwatch doesn't lie. It tells you valuable information if you listen to it. You need a stopwatch that reads to 1/10 of a second. Find a cheap one. With modern chip technology they work just as well as the really expensive ones. I know — I've had a bunch. Then throw them away when they're busted, and buy a new one. Check Pro Stunt Products. Windy has some reasonably priced ones that work.
What now? Obviously, you can time lap speeds and adjust your speed to run what the airplane likes best. That will vary from plane to plane, even with identical designs. So find out by trial and error. You also may find that even though the plane may have a broad range that it performs well in, it still is best in one narrow speed band. That is why you use the watch. It gives you the capability to make the evaluation and get back to it time and again.
Get used to using the watch on every flight. See what the engine does inverted. Does it run faster or slower? Most fliers seem to settle lap speed somewhat faster inverted — about a tenth of a second — to help particular maneuvers like the vertical eight and the square eight. If you weren't timing, it would be extremely difficult to sense the small differential, let alone make the adjustment. The airplane would know and those maneuvers wouldn't be quite as clean.
The watch can also let you know that there is a problem. Given that your setup is the way you like it, your flights should always be within a tenth of a second of each other under good weather conditions. If everything is running well and the watch detects a growing variation with no real cause, you had better start looking for something.
Troubleshooting Checklist
- Try a new plug if the current one has been in the engine for a long time. Usually that will settle things down.
- Check the fuel; a partially empty can that hasn't been used in weeks can create subtle problems.
- Monitor duration changes. If you notice a lengthening run for no reason, it can point to a failing compression seal.
- Watch for a gradual lessening of power. You might compensate by turning up the engine a few more revs, so monitor carefully and consider an engine change if the condition worsens.
The bottom line is that these techniques can be a valuable tool to help you when you aren't flying your best.
Tech tip
Doug Dahlke wrote in response to the tip I had in a previous column on control system lubrication. He suggests using 90-weight, lithium-based auto chassis lube (with or without graphite or Teflon powders). This is what I have been using. Also, I have a friend who swears that automotive wheel-bearing grease (with or without graphite or Teflon powder) is the way to go. I haven't tried either of these, but they should work. The tough part is to get the lube into the whole mechanism and keep it there for the life of the plane.
Transcribed from original scans by AI. Minor OCR errors may remain.
