If It Flies ...
Dean Pappas | [email protected]
Our aeromodeling skills serve us in areas besides aircraft
Also included in this column:
- The difference between wet and electric power
IF IT FLIES, other people are interested in it too! It tickles me when I see all sorts of fliers in the hobby/sport who, like me, seem unable to stay between the lines. They dabble, at the very least, in more than one segment of aeromodeling. That failure to "stay between the lines" is something we share, whether we recognize it or not.
If we really stayed between the proverbial lines, we probably wouldn't be building or flying model airplanes at all. It's not something people you meet every day do. What percentage of the general population do you figure has anything to do with model airplanes? Maybe one in 1,000? I'll bet it isn't even that many. We're a bit different—"plane" crazy, you could say! We stand out from the general population not just because we fly model airplanes, but because we build them—and rebuild them! We're different because some of us design them and because many of us teach others to fly them. This "wrapping our arms around" the many skills and bits of expertise that make up aeromodeling is uncommon these days. They include many useful things, and the more different corners of aeromodeling you look into, the more of these "outside-the-lines" skills you will accumulate.
The other day my neighbor was edging his lawn. As I wrestled with my autumn-time foe, the swimming-pool cover, I could hear the edger in the next yard bogging down—I could tell from the sound. That sound normally bugs me because it means a spoiled flight and an early landing. I can identify it from 500 feet away, and you can too if you are an experienced wet-power flier. Right? This time that sound meant I had an excuse to grab a screwdriver, walk next door, motion to my ear-protector-clad neighbor as he stopped to greet me, and deftly twist the high-speed mixture screw out one-quarter turn. I watched and listened only long enough to verify that the job was done, and then I returned to the back yard smug, satisfied, and ready for a rematch with my big, green fabric nemesis. That is a skill any reasonably successful wet flier takes for granted! Normal people would have taken that power tool to a small-engine and mower-repair shop and spent a meaningful amount of money to have it "fixed." Forget that! That's right; we aeromodelers are different that way, but for some reason we also stand out as being different from each other.
"Hey Dean, you fly wet and electric both, right?" my clubmate, Dave T., asked a couple months ago. "Maybe you should write something about the differences between wet and electric flight." Dave's suggestion was helpful, and I'll get to it shortly, but as I started to contemplate that subject, a shoe wedged itself between the gears in my noggin. I did not think being heavily involved with both wet and electric power was such a distinction, but, in fact, I had gone through quite a learning curve in the last couple years! At least in the clubs I belonged to, until just a few years ago electric fliers were a distinct group; but that isn't the case anymore. Now it seems like almost every wet flier has a flat foamie or park flyer in the hangar, although relatively few of those fliers mess with larger, high-performance electrics. Meanwhile, there are fliers who have spent years in the sport, becoming experts, without ever owning a glow or gas engine. Could you have imagined that just 10 or so years ago? I didn't.
The pedagogical problem
There must be people looking to make that transition from wet power to high-performance electric, and, even more interestingly, there are accomplished fliers who have only a vague idea of how to properly set up a glow engine. I'll describe that shoe in the mental gears I mentioned. It's the problem you run into when you try to explain something basic to someone who is already accomplished at something else.
It's easy to insult someone without meaning to. When we step out of our personal comfort zone, or area of expertise, we become beginners. It can either be a pain in the neck or an opportunity for discovery and fun.
So I'll take Dave's suggestion and look at the differences between setup and technique for electric and glow flight. If it gets basic, please think of it as a reminder of all the many useful things you've mastered without thinking much about it.
I'm going to write a bit about the big picture. I'll start with a discussion about how wet and electric power have different characteristics, which has a great deal to do with how their torque vs. rpm characteristics are shaped. Later we can get bogged down in all sorts of specific stuff, making comparisons between electric and wet power as they apply to many styles of flying. There's a lot of it, so I'll probably chop the subject up into little bits from month to month.
Wet versus electric torque and power curves
I'll start by describing a glow engine's torque characteristics. Within their preferred operating rpm range, glow engines make more horsepower as they turn faster. When you put slightly less propeller on them and let them rev up some, you get more performance.
That's true only as long as the propeller is fairly well matched to the airplane. If you overdo this propeller reduction, you get noise with no added performance.
The funny thing is that more noise fools many pilots into thinking they have more performance. This touches base with the human-factors part of the discussion months ago about making your airplanes quiet. That loose end is still dangling.
A glow engine typically has a horsepower peak at roughly 15,000 rpm. Depending on the engine and muffler design, it will have a torque peak at a much lower rpm—maybe roughly two-thirds of the horsepower-peak rpm. Quieter mufflers often bring the peak power rpm down a bunch, while affecting the torque-peak rpm somewhat less.
The "trick" for getting a friendly engine setup is to use a propeller with enough pitch to fly at the desired speed when the rpm is in the middle of the engine's rpm sweet spot, and enough diameter to load the engine down to just above the torque peak at a standstill. Loaded that way, takeoff performance will be strong and the engine will unload into that sweet spot between maximum torque and maximum horsepower while flying around.
An accompanying diagram shows this relationship, but it is just a picture until we explore what it means in comparison to the same picture for an electric motor.
Electrics have an entirely different torque vs. rpm curve. Once you have chosen how many cells in series you will use, if you want more power from an electric you must lower the rpm by putting a bigger propeller load on it. If you are familiar with electric power, this comes as no surprise. But think of the huge change in thinking this represents for many of us.
Revving up has always meant more power; it's almost programmed into our DNA. But with electrics you have to grunt to make more power. When you look at the electric torque vs. rpm curve, you see that it's not a curve at all, but an almost perfect straight line. Torque and current draw fall linearly with increased rpm.
Horsepower is not a straight line, though. The horsepower curve is a nice, neat parabola, with a peak value that is measured in watts. There is no magic to that; there are 746 watts per horsepower, so watts and horsepower are really the same thing.
(When you see "Ps" in the specifications for a model engine in an advertisement, that's the symbol for a metric horsepower, or one Pferdestärke. It's approximately 735 watts, so the difference between a U.S. horsepower and a metric horsepower is only a bit less than 1.5%.)
The rpm where the horsepower peak would occur is at half the no-load rpm. That's true with a motor, but operating there may be impractical. The no-load rpm, at the right side of the diagram, is the same as the Kv of the motor multiplied by the battery voltage.
The motor constant Kv is expressed in rpm per volt—not kilovolts! I don't know how many times I see that boo-boo in advertisements and catalogs.
Operating within the happy zone
Every engine design has an rpm range within which it will run happily. Yes, "happily." I realize that isn't a precise term, but it will have to do for now. There are engines intended for normal sport use and engines designed for high-rpm use, such as racing.
With all the necessary emphasis on flying-site retention these days, there are more engines optimized to make gobs of torque at low and moderate rpm, for the purpose of turning larger propellers quietly. That's a good thing, and the side benefit is that many fliers will learn that larger propellers fly better. There are notable exceptions to this general rule, and that only means we have yet another loose end to tend to some other time.
Look at the torque curve presented for a muffled engine. It is mostly flat or gently sloping downward with higher rpm for a healthy portion of the engine's usable rpm range. That's the part of the range between the torque peak and the horsepower peak. This is the rpm range where the engine is going to be most forgiving. Stable torque readings mean the engine is breathing properly.
Because any propeller changes load characteristics as the airplane's speed changes, glow (and gas) engines will see a substantial rpm change between running on the ground, climbing, or in a high-speed pass down the runway. The rpm can change as much as 20% and sometimes more. That's because the load the propeller presents to the engine drops as the airplane goes faster. (Yes, there are exceptions to the last statement!)
How do you keep the engine running in the sweet spot? By changing the propeller's pitch and diameter. If the propeller has too little pitch for the airspeed at which your airplane is intended to fly, the load will drop off dramatically during a high-speed flyby. As a result, the rpm will rise substantially—maybe even past the peak-horsepower rpm.
That's not terribly useful and it makes a bunch of noise. This is actually how many competition fun-fly and 3-D airplanes are set up intentionally, but they are never flown at high speed because they tend to flutter and explode!
When selecting a propeller for one of these 3-D setups, a very low pitch is chosen and then the diameter is adjusted to load the engine to near the torque peak. Why? Because torque provides the grunt needed for hovering.
For flying, as opposed to hovering, start by picking a pitch that will fly at the hoped-for airspeed when the rpm is maybe 10% higher than the ground rpm. Most glow engines intended for sport have torque peaks of roughly 10,000–11,000 rpm, so we tend to load them to 11,000 or 12,000 rpm. That's approximately 200 revolutions per second.
Let's say you want to fly at roughly 85 mph, which works out to 110 feet per second. The pitch needs to be nearly 6 or 7 inches. The arithmetic is (110 feet per second divided by 200 revs per second) multiplied by (12 inches per foot). I got 6.6 inches of pitch. I'd start with a 7-inch pitch and then add as much diameter as I can before the engine "lugs," or bogs, at full throttle on the ground.
With an electric setup the propeller selection is done similarly, but first you have to figure out what your motor's running rpm will be. Estimate the voltage of your batteries. Let's say you are using a 6S, or six Li-Poly cells in series. That works out to 22.2 volts.
Reduce that figure by approximately 10–20% to account for the voltage lost across the winding resistance of the motor. We could calculate it more accurately, but that is for another day. Using a 20% reduction as an example, we have about 18 volts left.
Multiply that by the Kv of the motor, and that will be the running rpm, give or take a few percent. Now pick the pitch the same as in the preceding. But here comes the difference.
Assuming you selected the correct pitch for the desired flying speed and your motor's realistic full-throttle rpm, you must pick the diameter to get the desired current draw. If the diameter is too big, the current draw goes way up. If the diameter is too small, the current draw (or horsepower, or watts, or Pferdestärke, or whatever!) will be too low to fly the airplane properly. How do you choose how much current draw you want?
From a practical point of view, you want only as much as is needed to get satisfactory performance. You would probably start slightly small on diameter and work your way up until the climb performance is acceptable.
There are good online electric-flight calculators to help you make a good first guess at propeller size. Many motor manufacturers have them on their web sites, and there are computer programs you can buy. There are other considerations when choosing how high of a current to run with electric power.
The diagram for the motor has an extra curve on it, labeled efficiency. We typically choose full-throttle rpm so the current and efficiency are within reasonable bounds.
In this case you might choose a maximum current based on how long you want to be able to fly without using more than roughly three‑quarters of the battery, or you might choose the maximum current based on your batteries' "C rating." You also need to look at your motor's maximum current rating. Exceeding any of these will shorten the life of your equipment.
If you are interested in building an electric model to break a duration record, you will aim for the current and rpm that give the best efficiency. If you look at the electric diagram, you will see that this happens at relatively low current.
If you are building an airplane for CL electric Speed, where less than a half minute at full throttle is needed, you will probably want to crank the current all the way up to the C rating of your battery or the short-term maximum current rating of your motor.
Overload characteristics
What happens if you overload the system with too much propeller?
If the glow engine is overloaded, it will turn at much less than the torque-peak rpm. This is on the left side of the diagram. The drop in torque happens because the engine can't breathe properly at such a low speed. That's a long subject, and I will probably try to draft an expert to write about it someday.
Because the engine is not breathing properly, the fuel-to-air mixture becomes inconsistent. The symptom is that the needle valve becomes difficult to set, and the setting often changes dramatically as the engine unloads in the air. If you do get the right mixture, takeoff and climb performance will suffer. Throttle response usually suffers in this case, especially when you punch the throttle for a go-around on landing.
Overloaded electrics are easy to spot—by the smell. It's the smell of melting motor windings, blown ESCs, and overheated batteries. There's no need to discuss this further!
In the next column I will describe how the running characteristics of wet and electrics affect their use in a variety of applications from CL Stunt, to RC sport, even to FF. There may not be an inherent advantage to one or the other, but they have their strong points and vulnerabilities, and different applications highlight them well.
Until next time, go fly an airplane—any kind of airplane! MA
Sources:
- Michael Ramel
- www.f3a-e-factor.de/eng/
Transcribed from original scans by AI. Minor OCR errors may remain.
