Builder's Tips _ Articles

Article Links


Push-Pull, Tractor _ Contra Systems:


ABC's of Glow Fuel

Tip Links

New! Giant Servo Chart New!

Sand/Shot Bags

IC Propeller Charts

Proper way to Range Check

Builder's Tips

Thanks to Larry Hewitt, for the idea of developing this compendium of expertise from our ranks.

Until  someone comes up with a better scheme...

Any one of our members may submit a tip... Just write up your tip in an email, or as an attachment to an email, and send to ,and it will be added below.

If any one has ideas how to organize or improve the format of this section, please let me know... 
I'd like to hear some ideas on categorizing, among other things...

Don't be shy, send 'em in, 'n I'll publish 'em!... 

P.S. SMALL pictures/graphics can be included to support your text. If in doubt, send them in, I can re-scale most.


01-- Courtesy of: Jeff Whitney

Get several pounds of sand or bird shot and put it in zip-lock baggies or have your wife sew some bags. If you use baggies, it would be a good idea to "double bag" the sand or shot in case of a tear.

The sand/shot will conform readily to various shapes and help hold it in place while glue is drying or measurements are made. Be sure to make several bags, as they come in hand for sheeting wings, of blocking up a fuselage and holding it in place while wings are set on them or whatever.

02-- Courtesy of: Bob Freshwater

Standard Propeller sizes are mid range pitch props that should fly just about any plane designed for that size engine. Use these first as in most cases they will be close to being the correct size. Then use trial and error to try and fine tune the maximum performance from your ship by trying the alternate props. If none work better stick with the recommended standard prop.

Prop Chart For Two-Stroke Engines






5.25x4, 5.5x4, 6x3.5, 6x4, 7x3 







.19 - .25



.20 - .30



.35 - .36





9x8, 11x5 







.60 - .61


11x7.5, 11x7.75, 11x8,12x6 




.78 - .80






.90 - .91


















Prop Chart For Four-Stroke Engines




.20 - .21






.45 - .48



.60 - .65
























Developed by Geistware of Indiana© ., 1999.
Updated December 1, 2000

03--  Courtesy of: Bob Freshwater

Proper Way to Range Check Your Plane

Reference checking of Range:

We recommend the following procedure that George Steiner and I used successfully for four years when we ran the RF checks, inspections, etc., at the Giant Scale Air Races held in Madera, CA. The aircraft should be positioned so that the wing tip is facing you. Have a helper stand by the aircraft to observe control action. With the engine running at all speeds, the flyer should have control of all functions with out any servo jitter with him standing at 150 feet from the model with the transmitter antenna collapsed to one section and the antenna pointed directly at the aircraft's wing tip. If that criteria is not met, don't fly until you find the problem and correct the problem. Spark ignition noise can be a problem in ignition engine powered models if it is not suppressed and if you don't separate the radio components from the ignition system. The metal Bosh spark plug shield cures a lot of ignition noise problems. It is obtainable from Bill Carpenter at C.H. Electronics, Inc.. Check out his web site at He has a lot of good information about installations in his instructions.


04-- Courtesy of: Larry Hewitt

Here is a link to a website which shows a very large servo chart. Not only does this chart show the cost of different brands of servos, it also provides details regarding the performance of several types of servos.
Giant Servo Chart














ABC's of Glow Fuel

Fact (A) - It's quite likely that no other single facet of modeling generates as many myths, misconceptions, misunderstandings, errors (and more than a few lies), or as much outlandish goofiness as model fuel, one of our absolutely necessary, non-optional items for powered flight.

Fact (B) - Of all the above, the one fact that rouses the most questions - and without doubt the most wrong answers - is the ongoing nonsense about the amount of oil required in model fuel.

Myth: Model Glow Fuel must contain XX% oil to operate properly, perform well and protect the engine.

Fact: There is no such fixed number or at least not a valid one.

Why not? Think about it: In order for this to be true, all oils used in model fuel - all of them - would have to be identical in every characteristic. Does anyone honestly believe they are? I doubt it.

While lubricants compounded for full-size engines - automotive, recreational vehicle or aircraft - are rarely, if ever, suitable for use in model engines (for many reasons), nevertheless, there are a number of base lubricants that are available for our highly specialized use. However, most of these must be modified slightly or extensively by the use of a variety of additives and modifiers.

While Klotz model oils are perhaps the most well-known to the average user, and are quite good, they are by no means the only lubricants available to model fuel blenders, and there are currently a number in use. Each has its own "personality" - its own set of technical specifications and characteristics.

At this point, we should point out that we_re speaking of the so-called "synthetic oils" popularly used in modern model fuels. Castor oil, the oil of choice, and, indeed, the only suitable model engine oil for many years, is more of a common and known factor. Assuming a good grade, if a fuel uses only castor as its lubricant, then we could give you a fixed percentage, at least for the various engine groups and types.

However, few model fuels intended for R/C use today contain only castor oil as the lubricant. For the purposes of this discussion, we will only deal with fuels containing either straight synthetics, or a blend of castor and synthetics.

So, what does all that mean?

Let's draw a little picture here: Suppose at some point in your life, you become concerned about living a long and healthy life, so you decide to consult a doctor for advice as to how to accomplish this. When you come to the subject of food, you say, "Well, tell me, Doctor, if I wanna still be healthy and virile at 90, how do I eat?" The good doctor replies, "M'boy, if you will eat two pounds of food a day, you'll be fine!"

My guess is your response would be something like, "well, what kind of food, Doc? After all, no two are exactly alike, is that two pounds of lettuce or two pounds of pork chops?" If he replied, "It doesn't matter. Just as long as you eat that two pounds every day, you'll probably outlive your kids." My bet is that you'd run, not walk, out of that quack's office!

Why, then do we blindly follow someone's Word From On High when they say (in words engraved on stone tablets), Thou shalt use no fuel that does not contain XX% oil." It makes absolutely no sense to me, nor do I think it will to you, if you just stop to think about it. All foods are different; so are oils.

If that's true, why do the instructions with my engine specify a fixed percentage of oil? Simple - to protect themselves. All engine manufacturers have been burned (figuratively and literally) in recent years by "bargain priced" fuels containing either inferior oils, or insufficient amounts of oils. Every one that I've talked to will admit off the record that they know that fuels containing good oils won't need as much as their instructions say. But they also say they know they have no control over that, so they are going to print a high number, in hopes that amount of even a cheap oil will be sufficient. Frequently, it isn't.

So why not just put a lot of oil - at least 20% or more - in fuel and not worry about it? A lot of reasons, all good ones. For example:

Too much oil - any more than is necessary - makes the engine run really crappy. Think about it: methanol burns; oil doesn't - or at least it shouldn't. (Some do, but that will be dealt with in another installment.) Common sense would tell us that the less oil (nonburnable) we can safely use (to an irreducible minimum point, of course), the more methanol (burnable) we will have in our combustion chamber. More burnable ingredients = more power. One well-known magazine writer, with more than 50 years engine experience, tells me that in his experience, for every 1% oil removed from model fuel, the effect is about the same as adding 1% nitromethane. And it costs a lot less! By the same logic, the less oil we use (to the predetermined minimum, of course), the less the oil is going to be dousing the glow plug element, and we should be able to achieve a lower, smoother idle. Next to nitromethane, oil is the most expensive ingredient in model fuel. By not using an unnecessary amount of oil, especially if it's just to satisfy some Great Guru's edict, the manufacturer can keep the cost of the fuel down, which puts a smile on all modelers' faces. Remember that even an additional 25 cents in manufacturing cost translates to an additional dollar or more at the retail level. So, what is the right amount?

It all depends on, what kind of oils, in what combinations, with what additives, etc. And for what use? Sport airplanes, Racing, Helicopters, Boats, Cars, Ducted Fan? What size engines? (As engine size increases, they need progressively less oil. Why? Simple mathematics. Surface area of the combustion chamber increases at about half the rate as the displacement increases.) Most people know that the big T.O.C. and Unlimited racing engines use oil in the 4% to 5% range.

Ducted fan and helicopter engines typically need more oil, 4-strokers less. It might be surprising to most airplane flyers to know that top competition model car engines use fuel with oil contents in the single digits, even though they are turning in the 40,000 - 50,000 rpm range, and have no fan in front to cool them! As matter of fact, they will hardly run on regular airplane fuel. Before we get started on the subject heading, I_d like to offer a couple more thoughts on last month_s subject, "What’s the Oil Content?" - thoughts that have been remembered since writing the original column:

Many modelers who have been involved in the hobby for a long time, including those who have been away for years and recently returned, are very stubbornly remembering when model fuel just about had to contain something in the order of 25% oil - usually all-castor - and have a hard time dealing with the idea that virtually no one runs that much any more in modern engines.

The operative word here, of course, is "modern." The metallurgy in today's engines barely resembles that of a generation ago. The end result, as far as model engines are concerned, is that the engines today simply don't require as much lubricant - not nearly as much. I will be quick to add that those running antique engines in Old Timer events should certainly continue to use the old-time formulas - no doubt about it.

In addition to vastly improved metallurgy, we must remember than manufacturing techniques barely resemble those from years ago, in many ways. Modern CNC machinery has made it possible to routinely and cheaply make 1 or 1 million parts all exactly alike.

Those of you who have come along in later years may be shocked to know that up until the advent of this new technology, every piston was hand fitted to every liner. There was no such thing as simply machining 1,000 pistons and 1,000 sleeves, picking one from each batch and having them fit.

The belief in those days that some engines of the same size and make were markedly hotter than others was no doubt true. We've read that in those days, a .29 for example, might vary from as low as an actual .26 to a .32 - some 23% more displacement! More closely controlled tolerances have resulted in the ability to use much different fuels than a generation ago.

The second thought on the subject of total oil content came from reading the operating instructions included with a new imported 4-stroke engine - the DAMO FS 218 twin. It recommends a fuel containing 94% methanol, 5% nitro and 1% Castor Oil! Clearly, this reinforces my point that "there ain't no such thing as a fixed percentage of oil content." Now, on to this month's subject:

Before we depart the subject of oil in model fuel, let_s talk about a point that's argued vehemently all over the land - Which kind of oil is better - synthetic or castor?

Each side has its very strong proponents, and each side is right, to a point. "Old-timers" tend to still favor an all-castor fuel, or at least one containing a liberal amount of castor oil. Modelers who have come to the hobby in the last 15 or 20 years have a strong affection to synthetic oils, or at least want their fuel to have mostly synthetics. Let's take a look at both types statistically:


Strong Points/Weak Points


I'd like to insert here that there is a "Chicken Little_ The Sky Is Falling" rumor making the rounds of the Internet these days that the manufacturers of castor oil have recently changed their methods of making the product, and the castor oil we are getting now is either wholly or partially incompatible with methanol.

I have talked at some length with the "Head Techie" of one of the largest castor oil importers in the U.S., and I want to go on record as saying that, according to the best information I can find, This is total B.S. The Head Techie actually laughed out loud when I told him what was going around. He said, "You know, there isn't much we do to the stuff. We press the oil out, filter it, grade it and package it. As far as I know, nothing has changed." It apparently started with one of the fuel manufacturers. For what reason, I have no idea, unless it's to help them promote their proprietary synthetics. (Incidentally, I have read a response on the net from SIG, agreeing with the fact that it's nonsense.)

So, there you have it. "You pays your money and takes your choice." Actually, it's a little better than that, and the obvious answer is - use a combination of the two, in proportions that will come nearest to enjoying the benefits of each, while minimizing the adverse characteristics.

A few years back, the modeling community was in a "synthetic oil frenzy," and the swing was toward all-synthetic fuels. Happily - at least in this writer's opinion, we've seen a very noticeable swing back toward the center, with the majority seeming to prefer a synthetic/castor blend. We think this makes sense, and many years experience proves it.

The most frequent comment I hear from lovers of all-synthetic fuels is, "Brand XX leaves a lot less oil on my model." My response to that is, "Doesn't that bother you? If you don't see much oil on your model after flying, that tells you one of two things - or both: Either there wasn't enough oil in there in the first place, or the oil is burning off with the methanol. Neither is good. There's no way oil can burn off and properly lubricate at the same time." This is usually met with a puzzled look, then one of the light dawning, having just realized something they never thought of before.

Oil residue in model engines is a natural as barking is to a dog. We have to learn to live with it.

As an aside, not long back a friend sent me a copy of an article published in a European model magazine. In one part, the writer stated, "The Americans are the only ones rich enough and dumb enough to use synthetic oils." Perhaps overstated just a bit, but it has some validity.

There a couple of types of engines that do require an all-castor fuel, or at least one with a considerably higher castor content than most others. One would be the Fox ringed iron piston type, and the other would be the small Cox engines, because of their rather unique ball-and-socket connecting rod-to-piston design.

Pattern flyers traditionally prefer an all-synthetic fuel, for a couple of reasons, I think. One is the fact that pattern flyers practice a lot - hour after hour after hour. That much use, plus the tuned pipe setup that is almost universal with them probably, tends to cause a greater problem with varnish and carbon buildup than in sport types. (At the risk of bombarded, I also think it's largely a state of mind. "Joe Champion uses all-synthetic, so that's what I'm going to use.")

The other area where we have seen all-synthetic fuels gain in popularity in recent years has been with model helicopters, probably for the same reasons. Also, the trend toward 30% nitro fuel for serious competition has led to using a lower viscosity lubricant, and, as shown in the comparison charts above, this necessarily dictates using synthetics.

Nitromethane…..everybody knows it’s there, but few, it seems, really know much about it. Although most seem to know - at least vaguely - that’s its primary purpose is to add power, we still get an occasional call or letter asking, "Why do you use it in model fuel?" At best, there is much misinformation regarding this somewhat exotic ingredient. Let’s see what we can do to clear some of it up.

Nitromethane is just one of a family of chemicals called "nitroparaffins." Others are nitroethane and 1-nitropropane and 2-nitropropane. Nitroethane can be used successfully in small quantities. (Top fuel drag racers, which generally run on straight nitromethane, sometimes add a little in hot, humid weather to prevent detonation.) At one time, nitroethane was only about half as expensive as nitromethane, but its cost now is so nearly the same, using it to lower cost is hardly worth the trouble. Neither of the nitropropanes will work in model engine fuel. Incidentally, nitromethane is made of propane, in case you didn’t know (and I’ll bet you didn’t).

Yes, NITRO = POWER! But….there are conditions and contingencies. First of all, it doesn’t add power because it’s such a "hot" chemical. Not at all. This may come as a surprise to most readers, but the methanol (methyl alcohol) in the fuel is by far the most flammable ingredient….nearly twice as flammable as nitromethane. As a matter of fact, if nitro were only 4 degrees less flammable, it wouldn’t even have to carry the red diamond "flammable" label!

In actuality, nitromethane must be heated to 96 degrees F. before it will begin to emit enough vapors that they can be ignited by some sort of spark or flame! (I demonstrated this not long ago to a friend by repeatedly putting a flaming match out in a cap full of nitro. I might add that he insisted on standing about 20 feet away during the demonstration.)

So….how does it add power? We all know (I think) that although we think of the liquid part substance we put in fuel tanks (in our automobiles or model airplanes) as the fuel, in truth, there is another "fuel," without which the liquid part would be useless. Remember what it is? Right….just plain old air (in reality, the oxygen in the air).

Every internal combustion engine mixes air and another fuel of some sort….in our case, a liquid…glow fuel. The purpose of the carburetor is to meter those two ingredients in just the right proportions, and every individual engine has a requirement for a specific proportion of liquid fuel and air. Try to push in too much liquid without enough air, and the engine won’t run at all. That’s the purpose of the turbocharger on full-size engines….to cram in a lot more air than a simple carburetor or fuel injection system can handle.

Now…..suppose we were to find a way to run more liquid through our model engines without increasing the air supply? That would add power, wouldn’t it? Well, guess what….we can! An internal combustion engine can burn more than 2 ½ times as much nitromethane to a given volume of air than it can methanol. Voila! More Power! That’s how it works, and it ain’t all that complicated. Nor do we have to spend a lot of time thinking about it in the course of a normal day’s sport flying.

However, there are some factors we do need to consider. As a practical matter, virtually all our everyday sport flying can be done on model fuel containing from 5% to 15% nitromethane. If you’re flying something like a trainer or a Cub or similar model, there’s probably no reason why 5% won’t work perfectly well. Need a little more power? Move up to 10% or 15%. In most of our sport engines today, I really wouldn’t recommend going any higher than that. It probably won’t hurt anything, but it won’t do you much good, either.

We sell more 15% fuel than any other single blend, and for good reason. Most of the popular engines on the market today are built to run on something very near that blend. Typically, European engines will successfully run on lower nitro blends, because they are built to do so. Why? In Europe, nitro can cost between $150 to $200 a gallon! Reason enough?

Nitro does more than just add power. It also helps achieve a lower, more reliable idle. One good rule of thumb for checking to see if a particular engine needs a higher nitro blend is to start the engine, let it warm up for a few seconds, set throttle to full idle and remove the glow driver. If it drops rpm, move up to a 5% higher nitro blend. If there is no discernible drop, you should be fine right where you are.

One of the most popular misconceptions is that by adding substantial nitro, the user will immediately achieve a huge power jump. Just ain’t so. Most will be surprised to learn that in the 5% - 25% nitro range, you will probably only see an rpm increase of about 100 rpm static (sitting on the ground or on a test stand) for each 5% nitro increase. In the air, it will unload and achieve a greater increase, and it will probably idle better, too.

My pet rule is this: If you have a model that’s doing well, but just isn’t quite "there" powerwise, go up 5% in nitro. If that doesn’t do it, you need a bigger engine, not more nitro!

Most of our popular sport engines in use today aren’t set up to run on much more than 15% or 20% nitro. Increasing the nitro has the effect of increasing the compression ratio, and each specific engine has an optimum compression level. Exceed it and performance will probably suffer, not gain, and the engine will become much less "user friendly."

High performance racing engines, for example, are tuned entirely differently….compression ratio, intake and exhaust timing etc….and are usually intended to run on much higher nitro blends. One exception, of course, are racing engines used in certain international and world competition (FAI). By the rules, these engines are not allowed to use any nitro at all, and they go just as fast as those that run on 60 or 65%! The first question that comes to mind, then, is, "Why aren’t all engines designed to run on no nitro, so we can all save a lot of money?" Ask any of the world-class competitors. Those engines are a serious bitch to tune and run, and are definitely not user-friendly! In fact, they are well beyond the skill levels of most average flyers. There’s a price to everything.

Another statement we read or hear frequently is that nitromethane is acidic and causes corrosion in engines. It isn’t acidic, and the manufacturers say it doesn’t happen…..can’t happen. However, at least one noted engine expert and magazine writer insists that it does. Flip a coin. (I once asked Dave Shadel, 3-time World Pylon Champion, and a fellow who works on more high performance engines than anyone I know, how frequently he encounters rust in engines that have been using high nitro blends. His answer? "Never.")

Why does nitro cost so much? While I have no clue as to the cost of manufacturing, other than it takes a multi-million dollar investment in a large refinery to produce it, there is one pretty good reason: There is only one manufacturer of nitromethane in the Western Hemisphere. Figure it out for yourself.

Also (and this will come as a big surprise), our hobby industry only consumes about 5% of all the nitromethane produced; and full-size auto racing about another 5% or so. This means we have no "clout" whatever, and simply must pay the asking price. Where does the rest of it go? Industry. It’s used for a variety of things - a solvent for certain plastics, insecticides, explosives (yes, it was an ingredient in the Oklahoma City bombing) and I’m told it’s an ingredient in Tagamet, a well-known prescription ulcer medication (no wonder that stuff is so expensive!). Please note that while nitromethane is an ingredient in making some explosives, under normal use, it in itself, is not exploseve. (Remember….the guy used fertilizer, too.)

Hardly a month passes that someone doesn’t call to ask, "I hear more nitro will make my engine run cooler. Is that true?" Nope. The higher the nitro content, the higher the operating temperature. Fortunately, in most of our sport engines, the difference in operating temps between 5% and 10% is negligible, and there are lot of other factors (proper lubrication, etc.), that are much more important.

Finally, remember in the beginning of this, we said that nitro adds power because we can burn more of it than we can methanol, for a given volume of air? This also means that the higher the nitro content of the fuel, the less "mileage" (or flying time) we will get. In a typical .40 size engine using 15% nitro, we can usually get a minute to a minute and a half flying time for every ounce of fuel. The Formula 1 guys are lucky to get 2 minutes out of an 8 oz. tank!

What’s the practical side of this? If you go to a higher nitro blend, be sure to open your needle valve a few clicks and reset before you go flying. Otherwise, you’ll be too lean, and could hurt your engine. Conversely, if you drop to a lower nitro blend, you’ll have to crank ‘er in a little.

Well, what do you think? Is there really a difference, or is this merely a big hype by the fuel manufacturers to sell more products? Let’s see a show of hands…..ah, yes…about evenly divided. Well, let’s explore the facts.

Fact: Most 4-stroke model fuels contain less oil than comparable 2-stroke fuels.

The most common response to this is, "But 4-stroke engines have more moving parts….they should need more oil, not less!" Well, that sounds reasonable, but it doesn’t stand up under close examination. The number of moving parts has nothing to do with it. What is important? Think about it.

Fact: With rare exceptions, 4-stroke engines run at substantially slower rpms than a comparable 2-stroke engine…most in the under-10,000 rpm range vs. 12,000, 13,000 or more for a typical 2-stroke of the same size. They are engineered to deliver maximum power at slower rpms, with bigger props. What does this have to do with it? One of the main factors used in determining the proper oil content of fuel is heat. To use the well-worn term, it doesn’t take a rocket scientist to figure out that the more slowly an engine turns, the less heat it generates from friction. If you don’t believe that, rub your palms together slowly, then as fast as you can. So….lower rpms = less heat = less need for oil.

Fact: 4-stroke engines only fire every other stroke, vs. every stroke by a 2-stroke engine. Firing, or combustion, burns fuel, which creates heat. Logically, it may be deduced that if there is fire in the chamber only every other stroke, the engine has time to cool off a bit between combustion cycles. Let’s take that a little further: Using a hypothetical 4-stroke engine turning 10,000 rpm = 5,000 combustion cycles per minute, vs. a hypothetical 2-stroker turning 13,000 rpm…with the same number of combustion cycles per minute….the gap widens. The 2-stroker has 160% more combustion cycles than the 4-stroker. Even though this is partially offset by the fact that at least some 4-strokers have a higher exhaust gas temperature, the message is clear: 4-strokers remain cooler, and need less oil.

Fact: Oil doesn’t burn (or shouldn’t) - methanol does. Using a little logic, we arrive at the conclusion that a properly made 4-stroke fuel will deliver better performance than a 2-stroke fuel in the same engine.

Why? Remember…the 4-stroker is only firing every other stroke. This results in the plug element wanting to cool down between strokes, resulting in a "colder" plug. Excess or unnecessary oil, constantly dousing the element, is going to make it more difficult to achieve a slow, smooth idle. Those who contend that, "Well, using too much oil can’t hurt anything" are wrong. In addition to causing undue friction in the engine, keeping the metal parts from properly mating, etc., too much oil in 4-stroke fuel is constantly trying to cool a plug element that is already having problems. Sort of like pouring a bucket of cold water on a poor guy who is already shivering.

Again, since oil doesn’t burn, it’s doing nothing to help us develop power….it simply lubricates and goes right out the exhaust and all over everything. However, suppose we don’t put unnecessary oil in the fuel, and replace it with methanol, which does burn. Well, what do you know…greater top end power! Hey, I think we’re on to something here! Remove unnecessary oil from 4-stroke fuel, and we get a "twofer" - two benefits for the price of one….a slower, more reliable idle plus greater top end power!

Conclusion: For reasons that should be clear above, a properly blended 4-stroke fuel should deliver better all-around performance in a 4-stroke engine than a regular 2-stroke fuel in the same engine.

While it’s not going to actually harm anything to run 2-stroke fuel in a 4-stroke engine, never, ever run 4-stroke fuel in a 2-stroke engine. It’s not going to have enough oil. Now, for those of you will say that you have done it with no problems, I’ll agree…..if you have a real good ear and keep the needle valve "fat" (rich), it will probably work just fine…but the official word is DON’T! It reduces your margin of error unacceptably.

Finally: Because engine manufacturers have been burned in recent years by some fuel makers’ attempt to lower the cost of their products by using either too little oil or a cheap grade, most manufacturers today are recommending that you run a 2-stroke fuel only in their 4-stroke engines, or will specify what would seem to be an abnormally high oil content (and it probably is). Who could blame them? Since they know they have no control over the oil used in someone else’s fuel, they’re just trying to cover their fannies. So would I.

Note: I believe it’s commonly known that the manufacturers of YS engines…among the most powerful 4-stroke engines available….mandate that only fuels containing oil contents in the normal 2-stroke range be used. Their engines are unique, and the manufacturer’s recommendations should be followed, although, as with anything, there are exceptions.

Nitromethane…..everybody knows it’s there, but few, it seems, really know much about it. Although most seem to know - at least vaguely - that’s its primary purpose is to add power, we still get an occasional call or letter asking, "Why do you use it in model fuel?" At best, there is much misinformation regarding this somewhat exotic ingredient. Let’s see what we can do to clear some of it up.

During the Q_A part of countless "Dog _ Pony Shows" at hobby clubs all over the U.S., one of the frequently asked questions is, "What’s the shelf life of fuel?" The answer if both simple and easy: Properly stored, model engine fuel will last almost indefinitely. So….what constitutes "properly stored"? Let’s take a look.

Contrary to many things you might have read or heard, just about the only thing that adversely affects model fuel is the absorption of moisture from the air. Keep the air away from it, and your fuel will likely be potent longer than you are! Methanol - the major ingredient in model fuel - is hygroscopic. This means it’s virtually 100% soluble in water, and absorbs moisture from the air like a vacuum cleaner sucking up dirt.

Most modelers have no idea how rapidly this can - and does - happen, and tend to be rather skeptical about the idea. Let me paint a picture for you: Almost everyone has spilled a little fuel on the top of their fuel can in their flight box. If so, you’ve no doubt noticed that the shallow film of raw fuel takes on a cloudy, milky look. What you are seeing is the methanol sucking moisture right out of the air. Since the quantity of fuel is thin with a lot of surface area, the absorption is rapid, the water won’t mix with the oil and the fuel turns cloudy. Just remember how quickly this happens…..almost immediately….and it might give you an idea of just how quickly your fuel can be ruined if you leave the cap off, allow a vent tube to remain open, etc.

The wide surface area relative to the quantity of the fuel exposed is disproportionate, of course, to leaving the cap off the fuel jug, but I think you get the idea. In a humid condition such as exists in parts of the U.S., it doesn’t take very long at all to adversely affect your fuel. And it doesn’t take a large opening….a cross-threaded cap, a small vent line, etc. is all that’s needed to do the damage.

The solution is simple, of course….just keep it tightly sealed. And yet, sometimes that’s not enough. Most of us have seen small droplets condensed inside our fuel jugs after it’s become partially empty. This is the result of condensation of moisture as the air trapped inside the jug cools. Until about a year ago, there was little we could do about this, but there is now a method to take care of this problem. Since it’s not the purpose of this column to commercially promote our own products, those interested are invited to contact the writer at the e-mail address above, and we’ll be happy to tell you about the product that will solve the problem.

For the reasons above, it’s our opinion that it is rarely a good idea to buy model fuel in 55 gallon drums. Unless all the fuel is poured up the first time the drum is opened, a substantial volume of air is trapped inside the drum each time it’s opened. Steel containers of any kind warm and cool much more readily and rapidly than plastic, and condensation is much more evident in this type container. The result is that the last portion of the drum of fuel is quite likely to be contaminated with moisture, sometimes to the point of being unusable.

There is another downside to buying fuel in drums, especially if more than one person is using it. With no control over the type container the fuel is dispensed into….perhaps not bearing sufficient or proper warnings, etc., the liability is incredibly high if an accident of any sort should occur. Model clubs considering this type of fuel purchase for their members should be particularly aware of the potential liability….which is huge!

While it’s true that the UV in sunlight (or in fluorescent lights, for that matter) will cause pure nitromethane to deteriorate over time, it’s our experience that once the nitro is in solution and substantially diluted, the deteriorative effect is relatively minor.

To test this, some years ago we put a gallon of 10% fuel out in direct sunlight (in sunny Southern California) for a month. At the end of that time, we tested that fuel in an engine vs. fresh product and could see no difference. While it certainly won’t hurt anything to store fuel away from direct sunlight, etc., it’s our personal opinion that the adverse effect of sunlight on fuel under normal operating conditions is too little to worry about.


Push-Pull, Tractor _ Contra Systems
By Don Stackhouse

T orbjorn Molin asks:

“How much less efficient is a push pull arrangement a la Savoia-Marchetti SM-55, LeO H-24, Dornier Wal, ... than two tractors? A reasonable approximative guesstimate to get into the right ballpark is what I would be interested in. And what can you do to make it the beast of it? Counter rotating props (long live varioprop!), higher pitch/revving pusher, ...?”

Simple question, but as usual no simple answer. For the types of disk loadings we see in models, it's likely to be less efficient than two independently mounted tractors.

A propeller imparts a swirl to the air passing through it. It takes energy to accelerate the air into this rotation, and therefore represents an efficiency loss. The idea behind a contra-rotating propeller system (i.e.: two props rotating in opposite directions on the same axis; "counter-rotating" means two props rotating in opposite directions on different axes, such as on the Lockheed P-38) is to have the swirl from one prop cancel the swirl from the other, eliminating the rotation in the slipstream.

Supposedly this can provide efficiency improvements of as much as 15%. The problem with this concept is that there must be that much efficiency loss already in the basic design, that is available for recovery. This is only true in props with extremely high disc loadings (i.e.: massive amounts of horsepower being forced into a relatively small diameter prop), such as the "propfans" that NASA was experimenting with back in the 1980's. Those were trying to absorb 12,000 horsepower or more in a prop only about 10 to 12 feet in diameter, less than the diameter of the 4500 horsepower props on a C-130 Hercules! When you have that much power going into such a relatively small prop, there is lots of swirl, and therefore a lot of energy that can be recovered by the second prop. That's also why those props use so many blades; more disc loading requires more blades to absorb the power.

At the disk loadings typical of our models, there is very little swirl by comparison, and I doubt that you could expect more than a percent or so of recovery from it at best, IF you did everything exactly right (a virtual impossibility in the real world).

About the only case where swirl in models is a significant factor worth doing something about is in ducted fans. In that particular case we have a lot of power going into a very small prop, so there is lots of swirl. To combat this, we put stator vanes in the duct behind the prop to straighten out that swirl. Those stator vanes are nothing more than the special case of a contra-rotating propeller system, with the second prop in the system designed to run at an RPM of exactly zero. Even so, those stators must be designed and optimized very carefully, or the energy losses due to their own drag will be greater than the energy revered from the swirl, resulting in a net loss. This is the exact same problem most energy recovery devices (such as winglets) face, that of delivering a benefit that exceeds their cost.

The other benefit of a contra-rotating system is that it can cancel out the torque and P-factor effects of a large engine. This is one of the main reasons for its use in planes such as the later Rolls Royce "Griffon" engined versions of the Spitfire, the Bugatti racer, or the Fairey Gannet. Unfortunately, that also requires the use of a fairly complex gearbox, and gearbox-driven props of any kind have a long history of nothing but trouble. The British seem to have had the best luck with them (the gear drive that combined the two crankshafts into the single propshaft on the Napier "Sabre" 3000 to 5000 horsepower H-24 engine was particularly ingenious, and very successful), but other than those successes, the propeller gearbox has historically been the ruin of many airplanes. Gearbox problems were on of the biggest factors that kept the Northrop XB-35 flying wing bomber from being ready before the end of WW II.

In any case, torque and P-factor are generally not significant issues on models. However, the asymmetric thrust in the case of a failed engine on a twin (yes, even electrics can have those), can be a significant issue. The "centerline thrust" of a contra-rotating twin arrangement can solve this. This was one of the biggest reasons for this layout on the Cessna 337 and the Rutan "Defiant".

For a typical un-ducted contra-rotating propeller system, one of those two props is a pusher prop, and therefore you have all the problems and efficiency losses inherent in a pusher prop, which can be considerable on any size airplane. The myth of pusher efficiency assumes that by putting the prop at the back end of the airplane so the rest of the airframe in line with the prop does not feel the increased speed of its slipstream, you save on airframe drag. In actual practice this may be true, although in the vast majority of cases the savings from this are microscopic. If we convert that airframe drag savings into its equivalent in terms of propeller efficiency, we're looking at typical differences on the order of a small fraction of one percent. Recent wind tunnel studies by NASA even show that the majority of the flow behind a propeller tends to be laminar, not turbulent.

Meanwhile, putting the pusher prop at the back, so the airplane does not have to fly through that prop's slipstream, means that the prop now has to fly through all the disturbed airflow coming off the airframe. The efficiency losses from that are typically at least 2-5%, and can be well in excess of 15% in some cases, not to mention the increase in vibrational stresses and noise, the added FOD ("Foreign Object Damage") of stuff coming off the airframe, rocks kicked up by the wheels on takeoff, etc.. Keep in mind that on a propeller driven aircraft, only a very small percentage of the airframe is actually immersed in the propeller slipstream, and therefore only a small percentage of the total airframe drag is affected by the slipstream. Meanwhile, essentially all of the thrust comes from the propeller, so anything you do that hurts the propeller's ability to do its job will have big effects on thrust and efficiency.

In addition, pusher props are usually restricted in diameter because of ground clearance problems. This tends to force additional efficiency losses. Diameter is probably the single most important factor in the efficiency of most propellers, and even a small restriction on it can have big effects. This is especially true at high power and low speed, such as takeoff and climb, although less so at high speeds. This is one of the major reasons the Prescott Pusher (among others) was such a disaster. Try comparing its takeoff performance with conventional tractor aircraft in the same power and payload class and you'll see what I mean.

A pylon-mounted arrangement like you're considering doesn't have ground clearance issues, but has restrictions due to the height of the pylon. All that thrust way above the C/G and the hull tends to shove the nose down, especially on takeoff. I know of at least one amphibian with a pylon-mounted engine that has been unable to accept larger engines, because any significant power increase beyond the plane's current engine tends to make the plane want to become a submarine when you open the throttle for takeoff.

With a pylon-mounted arrangement, the forward prop sees some disturbed inflow due to the flow next to the fuselage and wing, but the aft engine also sees the disturbances from the forward engine and nacelle, as well as the pylon and any external bracing.

The net result of all of this is usually little or no measurable benefits from reducing airframe drag, but quite significant losses due to these other factors, for an overall net loss. Even the possibility of recovering swirl energy, as in the case of a contra-rotating propeller system, usually does not start to see measurable benefits until you get into the sorts of horsepowers typical of turboprops and very large piston engines. I used to be an engineer for a propeller company that happened to have more experience with pusher installations than probably anyone else in the business (Voyager was one of those). Our usual first reaction when someone approached us with a new pusher application was to try to talk them out of it.

There are a number of aircraft designers (including Rutan) who have at some time in their careers been a big proponent of pusher designs. In general, they are airplane designers, not propeller designers, and tend to overestimate the benefits to the airframe of a pusher arrangement while badly underestimating the detrimental effects on the prop. There is a tendency to think of props as these mystical devices that you just bolt to the engine to make thrust, with little thought given to the prop's own needs and idiosyncrasies. To really get a decent working relationship between a pusher prop and the airframe usually takes an incredible amount of work. Piaggio came up with one of the better pusher designs (from an aerodynamic standpoint) in their P-180 "Avante", but it took a huge amount of engineering effort including over 2000 very expensive hours in Boeing's wind tunnel to achieve it.

So, back to contra-rotating props: what do we need to do to get the most from them? First, you need enough power to make it worth the extra weight and complexity. OK, so the vast majority of models do not satisfy that requirement, but we want to have a contra-rotating system anyway for scale appearance purposes. What should we do to minimize the detriments?

The swirl dissipates through friction with the surrounding air. To recover the maximum of whatever swirl energy is available, the two props should be as close to each other as possible. However, that also worsens the vibrational effects of the blades passing each other. That arrangement also generally requires one of those complex and troublesome gearboxes I discussed above. It's a tradeoff. In the case of the Cessna 337, Dornier Do335 "Pfeil" ("Arrow"), Savoia-Marchetti S-65 racer, etc., they give up some of the possible swirl recovery, and also worsen the inflow environment and efficiency of the aft prop, to eliminate the gearbox. The mechanical simplicity may make it a worthwhile tradeoff.

The real key then to getting the most out of any pusher installation, including one with a tractor up front as well, is to get the airflow into the rear prop as clean as possible. Any fat fuselages, bracing, struts, and especially any flying surfaces or any large bodies that are to one side of the prop's axis can spell serious trouble.

For example, I know of one prominent twin-pusher that had fairly fat nacelles sitting on top of a fat wing root ahead of the props, and a fuselage to one side of the prop disk. The inflow angle over approximately one-fourth of the prop disk was fifteen degrees different than over the other three-fourths of the disk! Imagine what would happen to your glide ratio and the comfort of your passengers if during a max-performance glide you started rapidly and continuously porpoising the nose up and down over a 15 degree range. That's what was happening to the blades of those props.

The vibration problems were extremely serious, and the performance fell well short of original projections. They ended up having to go to much more powerful engines (with their attendant increase in fuel burn and other operating costs) to make up the difference.

So, the key is to keep the inflow as undisturbed as possible, and also to have it as symmetrical around the axis of the prop as possible. Anything that creates turbulence is bad, and anything that deflects the airflow to a different angle (so that the angle of attack seen by the blades varies as they sweep around the disk) is even worse. Wings, tail surfaces and deflected control surfaces can be serious problems. A thin, shoulder-mounted wing mounted well ahead of the prop on a slender, smooth fuselage (such as the case for the aft engine on the Voyager) is better than a high wing and a lop-sided fuselage right in front of the prop such as the Cessna 337.

Speaking of the Cessna, some folks like to trot out the fact that it climbs better on the aft engine alone than on the front engine alone as support for their flawed claims that pushers are generally more efficient than tractors. In truth, the aft prop of the 337 is less efficient. However, the lower aft fuselage of the 337 slopes upward at such a steep angle in front of the aft prop that, without the induced flow from the aft prop, the airflow over the aft lower fuselage separates, causing massive amounts of drag. When one powerplant is shut down and feathered, the plane climbs worse on the front engine alone because of the massive increase in fuselage drag due to the poor aft fuselage shape, in spite of the front prop's better efficiency.

OK, so we've learned that pushers are usually a detriment unless you really do your homework, contra rotation is not generally worth the trouble on models, but if we're going to do it anyway, we should try to keep the airflow into both props as clean, smooth and uniform as possible. What's that bit someone else mentioned about different diameters due to "slipstream contraction", and what about the need for different pitches and/or rpm's for the two props?

A prop makes thrust by grabbing chunks of air from in front of it, and accelerating them out behind. About half the acceleration occurs in front of the prop, and the other half behind. The reaction to the force required to accelerate the air's mass shows up as thrust. Because the air has to be accelerated to make thrust, the velocity of the air behind the prop is faster than the velocity in front of the prop.

As the velocity changes, the roughly cylindrical stream of air flowing through the prop has to obey Bernoulli's principle. If its airspeed increases, then the cross-sectional area (and therefore the diameter) of the stream has to decrease in proportion to that in order for the volume of the flow to remain constant. If this were not so, the flow through the prop would violate the law of conservation of mass and energy, which happens to be one of the most inflexible laws in all of Newtonian physics. Thus, the diameter of the inflow to the prop is actually larger than the prop at some point upstream of it, then contracts during that first half of its acceleration until it is equal in diameter to the prop when it reaches the prop disk. It continues to contract after it passes through the prop, during the second half of its acceleration. This is that "slipstream contraction" that some other posters to this thread have mentioned. This means that a second prop, aft of the first one, that is supposed to be working with the slipstream of the first prop, needs to be a little smaller in diameter in order to match the boundaries of the now-contracted slipstream.

Just how much faster (and therefore how much smaller in diameter) depends on a number of factors. For the ratio of slipstream dynamic pressure to free-stream dynamic pressure, Daniel E. Dommasch's "Airplane Aerodynamics" suggests an equation, which with a little algebraic juggling gives us:

Qt = Q + [(4 * T) / (D^2 * Pi)]

where: Qt = dynamic pressure ("ram air pressure" minus the static pressure) in the fully developed slipstream well aft of a prop.

Q is the dynamic pressure in the free stream well ahead of the prop, and outside of the propwash

T = thrust
D = prop diameter and of course "Pi" is 3.141592...

Dynamic pressure ("Q") is equal to one-half the air density, times the

velocity squared. If we plug that back into the formula and do some more algebra, we get:

Vt = SQRT [V^2 + (8T / rho * D^2 * Pi)]

where: Vt = the velocity in the fully developed freestream in feet per second "SQRT" means you take the square root of the result of the formula inside the [ ]

V^2 = the freestream velocity squared (velocity in feet per second) T = thrust in pounds

rho = air density in slugs/ft^3    (.00238 at sea level standard day conditions)
D^2 = prop diameter in feet

Other units will work as well, just make sure that you use the same system of units throughout (no fair mixing feet in one variable with inches in another, or metric units with English, etc.!).

Ok, now that half of you are getting glassy-eyed and most of the rest are running for cover in a mad panic, let's clarify that terrifying blast of algebra with a practical example: Suppose we have a twin-engined model that weighs 1 pound, and we're planning to modify it into a twin contra-rotating arrangement. Let's also assume that the L/D (essentially the same as the glide ratio) at our expected cruise speed of about 25 mph ( multiply by 22 and divide by 15 to get 36.67 fps) is about 4:1 (I know that sounds low, but remember, typical cruise speeds are higher than best gliding speed, and besides, this airplane has a bunch of extra stuff hanging out in the breeze). This means our drag is equal to the weight divided by the L/D, or 0.25 pounds. In level flight, that is also equal to the total thrust.

Let's also assume the front prop is doing about 55% of the work (0.138 pounds of thrust) to allow for the lower efficiency of the aft prop. We'll define the prop as having a 6" diameter (0.5 feet). Plugging all of that data into our formula: Vt = SQRT [36.67^2  + (8 * 0.138 / .00238 * 0.5^2 * 3.1416)] which is equal to 43.99 feet per second, or 30 mph. That's a velocity ratio of 1.2, or 20% more than the free stream velocity.

This means that if the aft prop is far back enough to sit in the fully developed slipstream from the forward prop, it will need either 20% more pitch (the preferred solution) or 20% more rpm (which opens several other cans of worms). In addition, the slipstream contraction will be SQRT (1/1.2), or 0.913 . That means the aft prop should be 91.3% of the diameter of the forward prop, or just a little less than 5.5" diameter. See, that wasn't so hard, was it?

If you plan to do this a lot, I suggest coding these formulas into your favorite spreadsheet program, such as Excel.

I helped advise a guy recently who scratch-built a VERY giant-scale electric model of the Voyager. As I recall, his original setup used the same size props on both ends. It flew much better when we put a prop with more pitch on the aft motor.

So, that's all there is to it! Just correct for slipstream effects on the rear prop, and keep the inflow into it as clean and undisturbed as possible. You will probably not have as much prop efficiency as a pair of tractor props with nice clean inflow, but it shouldn't be too bad.

 Don Stackhouse @ DJ Aerotech