Electric and glow conversions (both ways), Part 2
Dean Pappas | [email protected]
Compare and contrast
The process of converting existing airframes and familiar designs to electric power highlights something a few of us have observed, not just in RC but in CL and maybe FF as well. Although there are many little details to tend to that are particular to this kind of power, the basics are the same. Torque turns the propeller; when torque and rpm are present, power results; and power must be harnessed to the airplane by a propeller in the appropriate size range.
If there is a difference to be found in this basic relationship, it is that the rpm characteristic of the electric motor’s torque curve means that the range of “useful” propellers is a bit more restricted.
Domino theory of weight
In the last installment of this column, I promised to discuss the “domino theory of weight” as it applies to airplanes. Once it is described, your response will probably be much like mine the first time I heard it described in any detail: a slap to the forehead, followed by an almost Homer Simpson–like “D’oh!” This is a normal response when you are told something you already knew in some form but had never grasped the big picture.
I knew this bit of wisdom as a youngster, but former CL Precision Aerobatics columnist Windy Urtnowski came up with the ideal name for it slightly more than 20 years ago. The domino theory has nothing to do with geopolitics and everything to do with cause and effect. I’ll get to that shortly.
Recap: what “well powered” means in a propeller-driven airplane
For the sake of continuity, I’ll recap a few key aspects of what “well powered” means in a propeller-driven airplane.
- Electric power and gas have fundamentally different “grunt vs. go” behaviors.
- A gas/glow engine typically gains horsepower with rpm over an extremely wide rpm range, but the torque curve is almost flat over an exceptionally wide range and falls off at both high and low rpm. The size of propeller that allows the engine to reach its sweet spot in flight is very much dependent upon the airframe’s weight and drag.
- The motor makes both more torque and more power when you load it down with a bigger or higher-pitch propeller, as long as the voltage or throttle is unchanged. Assuming that the voltage does not change, the torque drops with rising rpm, unlike with the engine. This means that the electric power plant inherently draws more power in an attempt to try to maintain airspeed when the load is increased during a climb.
- The electric-like characteristic of torque and power increasing with added load can be replicated in gas/glow by using an intentionally mistuned exhaust system. Although useful, this produces less than optimal horsepower. This was done to create the CL Precision Aerobatics tuned-pipe setup.
- The motor displays much less flexibility in its personality, but that basic personality is adaptable to almost everyone, provided we do our homework.
- The process of picking the correct propeller for both types of power plants is similar, but still different enough to give the user fits, depending on the type of airplane.
The three example power setups (recap)
What if the dominoes aren’t lined up? In the April column we powered a hypothetical 5-pound airplane with three different packages, each with more power than the one before.
- The first was a trainer with something like 300–400 watts. Think of it as roughly the equivalent of a strong .25-size glow engine. A reasonable guess is that a motor sized for that load will weigh 5 or 6 ounces, and a battery that is capable of delivering that much power for takeoff and a few maneuvers during an eight- to ten-minute flight would weigh maybe 8 ounces (4S 2200–2500 mAh). The total is roughly 14 ounces including a speed controller.
- Next we bumped up to almost double that power in a .40-size example, looking for about 700 watts. That much power delivery will almost certainly require an 8–10 ounce motor, depending on the type chosen. The battery would weigh either 12 or 16 ounces, depending on whether or not you would be happy with short flights. Throw in an ESC and you have a power plant that weighs about 1 pound 10 ounces.
- The last combination was the equivalent .60 setup. We figured that such an engine in muffled sport trim was equivalent to roughly 1,000 watts. This will typically require a 13-ounce motor and approximately 20 ounces worth of battery (5S 5000 mAh or 6S 4000 mAh), yielding a power plant that weighs 34–35 ounces.
Lining up the dominoes
It's all in how you line up the dominoes. For the three aircraft to weigh approximately the same (about 5 pounds), the airframes plus radios would have to have been lighter as you go up in engine size.
A muffled .60 weighs roughly 14 ounces more than a muffled .25 engine, so the .60-powered hot rod would have to have been built for performance, while the Kadet LT-40–like trainer would have had a bit more beef in it for the inevitable student landing. It would take a bit of effort to build or assemble a 600–700 square-inch design and remove 14 ounces, but it is doable.
Now try the same with the electrified versions of the same airplanes. The difference in power-plant weights is somewhat more than with “wet” power, although the difference doesn't seem dramatic at first glance. Instead of roughly three-quarters of a pound difference between the small and large power plants, we end up with something closer to 1.25 pounds. We would have to take yet another half pound out of...
Transcribed from original scans by AI. Minor OCR errors may remain.
