Chart for what motor to which size plane. (2024)

scirocco

Registered User

Quote:

Originally Posted by TedD60

When I started building and flying model a/c back in 1962 there was a system to help determine what plane needed which engine. Engines were classed: 1/4A, 1/2A, B, C , D.

For instance, a 1/2A engine equaled .049 and my Wen/Mac .049 would get a c/l plane having a wingspan from 18 to 24 inches around the circle pretty well. Those engine size groups worked with f/f and r/c as well.

We had charts that even included prop size and pitch for guys who designed and built their own a/c.

Kit manufactures, magazines and hobby shops all worked from those sizes. This system was OK from about the 1930s until electric motors came along ... and every thing went crazy.

Now, as a builder, every time I ask which motor, prop, speed "thing", batteries (i.e., MPSB) do I need for the plane I am drawing up, I'm told that can only be determined by experience and trial and error, a completely unsatisfactory answer.

Electric motors for model have been around over 30 years. It's frustrating beyond words that the MPSB problem hasn't been reduced to a formula or a simple chart and absolutely ridiculous to think that the manufactures and hobby shops haven't come up with something. Not everybody is an electrical engineer of has access to electrical motor specs required to make even a WAG.

This is the sort of thing that people leave this hobby over.

I appreciate your frustration - almost the same question comes up regularly.
I hope I can offer you a way forward, but working with electric power requires a somewhat different mindset. There isn't and I don't think will ever be the simple chart you are looking for, but instead there are well established methodologies and tools that mean you don't need to be an electrical engineer to succeed, nor does it need to be done by WAG or physical trial and error - anything but that! And as others have said, just as with glow engines there is the experience of others to draw from, and IMO this forum is a good place to ask. You might get a range of options but in my experience that's at the margins and not fundamental, but you're also getting suggestions that aren't for example limited to the narrow range of motors that say Horizon Hobby sells or the even smaller range you might find in an LHS.

But why can't it just be a 'simple' chart? As others have said, IC and electric operate so differently that the range of variables for electric solutions compared to the variables in picking a glow engine totally defies straightforward if a then b classification. To illustrate, a given IC engine needs to get to a high and narrow rpm range to develop useful power and its load needs to be controlled so that a) it can get to that range and b) doesn't destructively overspeed. That means a small number of feasible propeller sizes. A given electric motor can be useful over a much much wider load and rpm range. At the correct voltage it simply cannot overspeed by being underpropped, and torque doesn't depend on rpm . The implications are that feasible prop diameter may vary by 50% or more and battery voltage can vary by a factor of two. All with the same motor, but for very different applications.

It might seem easy to pick an engine for a glow model, but a lot of that is because the available energy in the glow fuel is so abundant there is practically no need to optimise. Objectively viewed from a whole system perspective, most glow power setups are woefully badly matched to the airframe they are powering but succeed through sheer brute force, with minimal effort put into system efficiency. Pretty much the opposite of how full scale aviation operates. When we decide to pursue electric power we gain cleanliness, reliability, instant power, quietness, no minimum idle rpm, etc but we give up that massive energy density of liquid fuel so to get a workable electric system we simple have to optimise more how it works. An inefficient brute force approach is possible, but the consequence is unacceptably short flight times.

Electric motors have another key advantage over IC - they can be sufficiently accurately modeled or simulated in software, as can their propellers and batteries. Also there are well established rules of thumb for what it takes to fly a model at varying performance levels. That is the watts per pound chart that redshift 2 posted, which in may ways is the electric alternative to the motor classes you mentioned - but rather than separate classes for both model size and engine size it is a continuous range that allows power goal setting for any model the way its owner wants it to fly. Together, being able to set a realistic power target and virtually estimate how a set of components might meet that target IMO actually makes a one size fits lowest common denominator selection chart unnecessary and counterproductive.

So 3 simple steps and one not quite so simple step can get you there with a tailor made system. and there are dozens of people on here who can help you with the less simple step (which is the simulation phase) if needed.

Step 1. Got to have a goal - how much power is required? The glow situation - you walk into an LHS or go to a website and there on the box or at the top of the webpage is "Whizzbang 46". Somebody has decided for you that a 46 size engine is appropriate power for the model.
For an electric system, let's say it's a low wing aerobatic model and you have a fairly adventurous flying style so the watts/lb rule of thumb says you should go for about 150W/lb, or perhaps you just want basic aerobatics so 100W/lb will be plenty. The box says it will weigh 7lb, so depending on how you want to fly it, you might want a max power target of 1050W or be happy with 700W. You can do this for pretty much any model regardless of the label on the box (how many people put 60s in "46 size" models?) and know that you will get a solution that flys the model how YOU want, not the way someone else thought you might fly it.
The max power target also gives you right up front an idea of what minimum weight motor you will need. Electric motors are pretty simple and the maximum power they can handle depends on how well they can get rid of the waste heat generated in operation, and that is strongly related to the weight of the motor. For long term continuous operation, which is actually rare for RC models, it's about 3W per gram. For more typical sports flying operation where full power is used rarely and intermittently 5W/g is feasible. Thus with a target of say 1000W only motors over about 200g are viable candidates. We still don't know what other characteristics our motor must have, but it's a start that will help immensely in the final step.

Step 2. What size prop is appropriate? With glow this decision is made for you by what the glow engine can handle, and is often much smaller than makes sense for the airframe. For electric systems, the best bet is to pick a prop that suits the model, then build the rest of the system around that. A decent answer for prop sizing for electric is unless you explicitly want very high speed is to use as big a prop as will fit with enough ground clearance considering your runway and the gear geometry, up to about 30% wingspan. Always measure - don't take the range of prop sizes for the suggested engine/s as the 'right' prop sizes. For short geared models you might end up with a glow sized prop and that's fine, but otherwise every inch is worth getting!

Step 3. How to provide the power determined in step 1? Start by looking at how any existing batteries you have might do the job. For sports flying, a good goal is to keep max current under 60A and have enough capacity so that max current divided by pack capacity in Ah is no more than 20 and preferably less than 15, ie load is less than 20C or 15C. Space, weight, budget and specific performance goals might drive battery choice outside those guidelines but some basis for voltage selection is better than none. A caveat though is that even with the most brilliant battery plan, if we find in the final step that there isn't a motor available or available in budget that gets to the power target with the desired prop size, cell count may have to be revisited.

Step 4. What actual motor/s will use the power determined in step 1 with the prop diameter from step 2 using the voltage from step 3? This is the less simple part but there is an exceptionally good online motor calculator (more accurately an electric power system simulator) that is ideal for the task. It's Ecalc that 2michaely mentioned above. The full version costs about the same as one wrong prop choice per year, or if using online tools is not your cup of tea, ask a question in here with the steps 1-3 information clearly set out, and you'll have at least one answer from a member here in a very short time.

We already know from step 1 what our minimum motor weight should be and should have a reasonable idea of the max weight (mocking up motor and battery weights or using a balance calculator is a good idea before ever going shopping). What we're trying to do with ECalc is to figure out what the other key motor characteristic, Kv (the motor velocity constant or how fast it wants to turn per volt applied with no load) needs to be. That's all. Ecalc is working through all the detailed electrical engineer voodoo - we're just trying to get a match on weight and Kv but why this step is less simple is it involves applying a software program to some virtual but guided trial and error, but you might be pleasantly surprised by how quickly the results of the first few erroneous trials can converge to a viable solution.
While there are other calculators at least as good at modelling the physics of the battery/motor/prop combination, one of the key reasons I suggest Ecalc is that it has a powerful motor database filtering tool that we can use to narrow the thousands of motors in the database down to just those in our feasible weight range - and then automatically plug their details into the motor part of Ecalc, which is what makes the virtual trial and error process relatively quick and easy.
The process starts by telling ECalc what battery you plan to use (step 3), ESC rating (based on max current from step 3, plus at least 10% headroom), skip the motor for the moment and tell it prop size (step 2, note that one of the generic choices is fine and pitch should be to start at least half diameter).
Come back to the motor area and click the search button (only available with an account subscription). In the motor search window, set the desired motor weight range and set the results to sort by Kv. Select a motor in the middle of the range, or even at random, return to the main ECalc window and click on Calculate. No need to worry too much about brand and motors labels at this stage - we're just trying to find a workable weight and Kv combination.
Odds are it will be a bad choice and there will be lots of red warnings, but the results to focus on initially are in the Motor @ Maximum column. If the electric power result is over the target from step 1, that means that the motor selected had too high a Kv. Go back to the search results and pick a motor with lower Kv and recalculate. If power is still too high, go yet lower with Kv. If power is now too low, pick a motor with Kv inbetween the first and second. Repeat until in the right ballpark for power and now pay attention to the warnings and advisories, if any.
There may be a couple of candidates with results a bit either side of the power target. If estimated power is more than you need but within limits for the motor and the ESC that may just a matter of using throttle management. If power is a little low, consider bumping pitch up - electric systems manage higher pitched props very well.

The Ecalc database is enormous but cannot be comprehensive so not every motor is in there, and it is the nature of the beast that the database contains the better documented which sometimes = more expensive branded motors. But armed with knowing motor weight and Kv to get the results you want, you can shop motors of any brand, as the key influences on performance are weight and Kv, not brand and not motor nomenclature, which is not standardised anyway.

That's my methodology. 2 rules of thumb; the first based on model weight and flying style to get a target max power, and the other based on max power required to get a minimum motor weight; plus some guidance on prop size and battery selection; followed by some targeted virtual trial and error in a highly effective power system simulator to get to a specific motor Kv. Hope it helps

Chart for what motor to which size plane. (2024)

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