Diva's Specifications:

Type: CL Aerobatics
Wingspan: 58 inches
Engine: .36-.65
Flying weight: 50-62 ounces
Construction: Balsa and plywood
Covering/finish: .2-ounce carbon mat, light-grade silkspan and modeling dope, clear automotive topcoat

Power

With more than 2,500 hours invested directly in designing, building, testing, and flying Stunt airplanes in the past six years, it is reasonable to expect consistency from the total package I put on the flightline. Here are some thoughts on engines and power.

Power is often confused with conversion of power. Often an engine is thought to have too much or not enough power, when the real problem is the conversion of power. For our purposes this translates to propeller, exhaust system, fuel, ignition and timing. Of these elements the propeller is the most important. As you read the remainder of this article you will hear this theme repeated.

Throughout the years I have come to realize the importance of power management and application. Diva has earned a reputation as a rock-solid aerodynamic package. Power dynamics are equally important. There are many ways to alter the direct power output of an engine. In precision aerobatics the most common method seems to be a combination of rework by a variety of engine specialists augmented by constant compression adjustment usually by changing head clearance with shims. There is no question that those methods can produce good results.

Having a fair amount of engine knowledge, if rework is justified, I usually do it myself, but my preference is to avoid it when possible. While shims can be effective in forcing and engine to run below its designed performance specifications, this is the least desirable method for a specific reason. Every time the head is removed and replaced, the cylinder liner, case, and head distort. This can be slight if done carefully in the shop or severe if performed at the field, which is where it most frequently occurs. This in turn creates a new set of friction points or hot spots, which produce new wear patterns. To put it simply, the engine is self-destructing.

Frequently the head-shim approach a temporary fix at best. Do you ever wonder why a good engine suddenly goes bad? I believe it is the result of distortion and shifting wear patterns. Breaking in an engine is the process of allowing the moving parts to find their own best fit at normal operating temperature and speed. This is why most engines seem to get better the more you run them. These same engines also seem to decline noticeably in the performance once the natural fit is disturbed.

Larry Scarinzi’s “Blitz” article in the July 1964 issue of Model Airplane News is must reading for understanding the fundamentals of two-stroke engines in actual practice. The article describes how to disassemble, clean, and reassemble an engine using the dry method to minimize distortion and the creation of new wear patterns. It also explains in plain language how to measure timing using simple easily obtainable tools such as a radius wheel and a flashlight. Maybe with some coaxing, Larry would publish a reprint of the article on the Internet or make hard copy reprints available.

Let’s consider an alternate approach to power management. Suppose we allow the engine to run in its designed comfort zone and focus on the conversion of power as it applies to precision Aerobatics. What a concept! Could the time and money engine designers and manufacturers invest in research and development actually deliver a very good product right out of the box with minor or no modifications? I think the answer is yes because of modern CNC-production technology quality and consistency are at all time highs.

Propellers are the key to the Stunt kingdom!

In the days of wood propellers, it was common to have several propellers of the same size, from the same manufacturer, with one clearly outperforming the others. Today we have an ever-increasing variety of composite propellers available.

While it is still no easy task to find two or more propellers that work exactly the same, it is far easier than working with wood. In the past six years a variety of carbon propellers have been tested and today the testing continues. The benefits and performance gains of propeller experimentation and understanding are definitely worth the investment of time and energy.

Did I say propellers are the key to the kingdom? Etch that statement in stone. These days, the majority of my time is spent sanding, twisting, shaping and cutting propellers. Although they are still a mystery in many regards, the biggest performance gains seem to come from small adjustments in pitch or diameter. Square tips seem to produce better-applied performance than rounded tips. Three-bladed propellers seem to convert and apply power more consistently than two blades, regardless of flight attitude or altitude. Simply put, there is a positive feel at all parts of the pattern.

Successful propellers for this model range in size between 10.25 inches and 11.25 inches in diameter. If you are serious about improving you flight scores and you don’t have a pitch gauge or access to one, in my opinion, you are at a severe disadvantage.

In looking at the published specs for the Magnum 36, it was difficult to understand how 1.2 horsepower and practical rpm range up to 16,000 would not be enough power for a 60-ounce airplane.

This engine is really not a mystery. It is just good metallurgy, good fit, and timing easily adapted to Stunt. My current competition engine has more than 100 hours of running time. It is inexpensive and easily obtainable. The only modification required is a venturi and spray-bar assembly.

Except for replacing the rear bearing the engine has never been dissembled. It is light at 8.25 ounces and extremely consistent in its delivery of usable power. It successfully powered airplanes weighing 41 to 62 ounces with wing areas from 540 to 675 square inches: a big range and no lack of power.

Here's a summary of what has worked best. I use Sig 5 or 10% nitro airplane fuel or I mix my own which is identical to Sig (20% total oil, half Klotz and half castor). The engine is a thumper and likes to run on the hard. Typical rpm is 10,600 to 12,300 at launch. Don't look for a 4-2-4 type of run. An appropriate run is a soft two-cycle. Small adjustments at the needle can produce more power when needed.

Keeping in mind all of the preceding text, here's a good starting place. Bolly 12/4 narrow two-blade propellers are available from several sources. You will need at least two of them. This is where the fun starts.

Cut them to 11.25 inches and balance them. Put in several flights with each propeller and decide which one works the best for you. Set that propeller aside and use it as the control or baseline for future experiments. Now start cutting, carving, twisting and sanding other props. I sincerely hope you can learn to enjoy this process because it will probably never end.

Most people will find a comfortable flight with lap times rather than air speed. These are most easily adjusted to your liking first with pitch and diameter of the propeller and secondly by line length. Notice the use of that “propeller” word again. That's how you get to the usable power.

The engine has a fairly flat power band and when run in that band it is very happy. My comfort zone for lap times is a bit faster than most at 5.1 to 5.2 seconds; anything slower is just not in sync with my timing.

No matter what lap times you are comfortable with, you can usually get to the usable power with the correct propeller and line-length adjustment. The most effective venturi sizes range from .250 to .270 inch. Use a  SuperTigre or Randy Smith (Nelson) needle and spray-bar assembly. You don't need muffler pressure. Lines should be cut to 62 to 70 feet in length from the center of airplane to center of handle.

Pipe

You will note from the plans that the Magnum-powered Diva uses a tuned pipe. This is the final piece of the puzzle. To my knowledge there is only one header that is mass-produced and fits this engine and it is readily available from Randy Smith's Aero Products.

Throughout the years and with a little help from metal shop owner Warren Walker, I have found ways to heat and reshape headers to fit a variety of airplanes. All you need is a common propane torch, a vise, some simple wooden tools, and of course a hammer. It is difficult to explain how to reshape these headers but it is actually easy and can be done quite quickly with practice.

There is a wealth of knowledge about how pipes work and how to adjust them. Here is a good starting place. Use the standard Smith/Werwage “40” pipe with the angled exhaust. It is available from Randy Smith’s Aero Products. The pipe length measured in a straight line from the center of the glow plug is 15.375 to 15.75 inches with 15.5 inches being a good starting place. Set the rpm at approximately 10,500 on the ground and use the needle valve to find a comfortable setting for your flying style.

The most important aspect of pipe installation is preventing leaks. The following procedures have resulted in very consistent runs and virtually no leaks. Do not us a gasket between the engine exhaust and header pipe. Use Permatex brand Ultra Copper high-temperature gasket sealant, found at most auto supply stores to seal the header to the engine. Use Loctite to secure the header mounting bolts.

On both the pipe and the header I make a flange ring from 14-gauge copper wire secured with JB weld. Finally, use two electrical ties at each end to secure the coupling hose. This will ensure a very tight seal of the exhaust system.

Find your sweet spot. That’s it! No real mystery.