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Apr 21, 2013, 07:48 AM
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Guide 1: Designing and optimising a motor and prop system for for your plane


I have a habit of going into too much detail when answering questions - which can be bad - but I think not sharing information at all can be worse. So I have come up with the solution! Turning those long, winding posts into blogs where they are not in the way of people who don't want to read them, but are available to those who do. I will start by copy and pasting a few large posts over to this blog. So, without further ado:

Designing and optimising a motor and prop system for your plane

I Haven't really done it enough times to have a set in stone procedure, in reality I suggest a more organic approach (loop around to previous steps, look for missed opportunities, etc) but if I had to make a step by step procedure for picking a motor and prop system, it would be as follows. Note that this is from the perspective of someone who considers 20 minute flight time a bit too short, 30 minutes average, and an hour good, so your mileage may vary.

Step by step:

1) Start by finding the largest diameter prop you can on there, which may be limited by flight dynamics with truly silly diameter, or a prop crash with the ground or fuse in a lot of cases, or even aesthetics or prop weight. We will play with pitch and Kv to suit this diameter, and modify it later if we must.

2)Generally use the cell count you already use on all your other stuff, it's more convenient to be able to share batteries amongst planes

3)Pick a Kv that will get you enough thrust [1] on a prop of the aforementioned diameter, that has an average P/D ratio for your application (I consider 0.6-0.7 to be an average P/D ratio for most applications, go lower if you are flying 3D, or higher if you have managed to fit a large prop for the plane, or need to go really fast). Take into account the drop in Kv*V when the motor is loaded. If in doubt, assume for now that that drop will be 75% for an average setup wanting a fair amount of thrust for the weight, or increase to 80-85% if your setup will be using a very efficient motor or be running an oversized motor for increased efficiency at full throttle.

[note 1]: Obviously that's easier said than done - if you do a lot of static tests and know what you want in terms of static thrust, you can 'simulate' a static test with E-calc, fudging a lot of data like motor resistance because we don't know yet - it doesn't matter that much at this stage. Otherwise you can use prop data from the various databases to simulate an in air test.

4) Pick a pitch that, at the RPM you figured out in step 3, gets you a pitch speed around 10% higher than your intended top speed. At least for the planes I fly, that tends to be fairly realistic. If this ends up having a P/D ratio of more than 1 and you were hoping for easy flying (not a pylon racer), you probably won't get it: your thrust target may be unrealistically low, or you just managed to fit a massive prop on the plane somehow - your flight dynamics might be bad with such a large prop. Drop diameter and increase RPM to deal with this. If you have P/D ratio less than about .4 or .5, that hurts cruise efficiency a bit - your prop might be too small or your thrust target very high (if you are flying 3D that's normal). Note also that P/D ratios at or above about 0.8 might get noticeably stall-y at low speeds, if you think that's a problem in your application, drop it a bit lower.

5) Go back to e-calc and figure out the approximate static power draw of the system. Fudge whatever numbers need to be fudged to get an approximate figure. Use this power number to pick a motor size. 3 static watts per gram if you are using an average motor and planning on leaving the damn thing on all flight, you can go a bit higher for higher quality motors or intermittent full throttle. I think 4w/g static is often pretty decent in the air for those not flying at WOT all day. This gives you a minimum motor weight.

6) Go to your vendor or vendors of choice and try to get a motor with your desired Kv, in at least the weight you chose in step 5. Get as close as you can on the Kv (consider varying cell count if you want). If you can't get close enough, you may need to change the pitch or diameter a bit to add or remove thrust and top speed, or if it's a faster but bigger motor, it might just work anyway but have your plane going a bit faster than you planned at WOT. Stick to motors who are ok on the voltage you chose, according to the manufacturer, unless you do the later optimization steps. This is because those ranges seem to be the best tradeoff of inductive vs resistive losses.

This is probably a ballpark power system, but now we can optimize prop geometry and motor choice (for cruise, because I am an FPV flier so that's all I've really thought about) with some added maths. You may want to skip this step as it starts to run into a serious "I don't care enough to bother" issues with most people. It also has significant garbage in=garbage out issues as we start to deal with a lot of measurement errors. You can tune out here if you like and go to more traditional optimization methods ("How does it feel in the air").


First we deal with prop geometry, because the motors operating point is going to depend on how much the prop unloads in flight, which depends on prop geometry.

The great thing about prop geometry is that it scales really nicely if you only optimize it for a single speed. The troublesome part is getting the data for that single speed, which in turn makes this the mathiest and most wishy-washy part of the whole process in this post.

First of all, find some prop data for a given speed (as close to a cruise speed as you can find data for is nice if possible). I have recently been using the APC prop database, which has incredibly comprehensive data, but I am just now discussing with someone the possibility that it may be pretty innacurate. The traditional method I've seen here has been this website: http://www.ae.illinois.edu/m-selig/props/propDB.html. Data is less comprehensive/more difficult to use and it misses some props, but it is arguably more accurate.

Now that we have data, the goal of the operation is to use the coefficient of thrust to get the plane operating at the peak of its efficiency curve in cruise at our chosen speed, and we vary diameter (leave pitch constant if possible, or you have to go back and change other things again) to do so.

This unfortunately requires you to know how much drag your plane has (and therefore how much thrust is needed). Drag varies with square of speed more or less, so you only need to find this figure for a speed that is in the ballpark of your prop data speed, and then modify it.... getting the drag at any speed for any plane however, is again easier said than done. Since most of us don't have wind tunnels, the most viable way I have found to get a drag point is to get it from a similar proportioned plane with telemetry. If you know the RPM and air speed of a plane at steady state cruise, then you can reverse engineer out the thrust that prop is generating using these prop databases, which gives you your drag point, which you can scale to the speed of your requirement. If you have a similar plane with more or less weight or somesuch, you can also give flying it at a negative or positive AOA a shot to nullify out the weight. Requires AOA indicator. Good luck with that one, I haven't attempted it personally. If you don't have RPM sensor, but you have accurate voltage and current data (a lot of FPV flyers have voltage/current data), you can use motor data to reverse engineer out the motor RPM from that.

Now that you have your drag point, and thus thrust requirements, and prop data at a given speed, check if your prop is running at higher or lower advance ratio than its peak efficiency. If its lower, check if the next size up prop would be more efficient. You can do the opposite too (go to lower diameter if you are over the max efficiency point), but note that you will be decreasing your static thrust, which could be problematic. Rest assured, increasing prop size to a more efficient one will increase your static thrust and get your plane flying a little faster in the air, but it shouldn't burn out your motor since it will unload more. That means you should actually see less current draw at any given steady state speed in the air, which means your motor should actually run a bit cooler in flight, unless you keep the throttle pegged (because you would be flying slightly faster thus using more current as usual) or do prop hangs or something.

Thank god that's over, now we can just go optimize the motor choice. You can optimize it at several points (get it 'approximately right' everywhere), or just a single cruise point. I'll only talk about a single point, it's up to you where this is. I suggest it to be either cruise, or around halfway between cruise and full throttle, depending on how you want to fly. Cruise may be where your plane spends most of its time, but if it's a low speed cruise (like 60% throttle), full throttle sucks juice down so much faster that it can actually be a more significant power drain. That's why halfway between them can be a better idea, which would leave them both "kind of OK".

The goal here is, assuming you have multiple motors to choose from with similar Kv but differing no load current and resistance figures, to pick the one that runs our prop most efficiently. First, get your loading case for the motor. Go back to the prop database (oh god not again D: ) and use the torque, RPM, and coefficient of power figure for your prop at your desired advance ratio, to find the torque and RPM we want to optimize for. Find this point on all the motors in question using either something like Ecalc, or your own motor curves (not too hard to make but beyond the scope of this post), then pretty much pick the one with highest efficiency. To be a little more complete, my personal preference is to get one that is operating past its peak efficiency point at full throttle, and below it at cruise, which leaves that whole zone between cruise and full throttle pretty decent. This probably means your motor ends up a bit heavier than it needs to be according to the watts/gram rule, so check how much weight you are adding, and if it is significant, consider the benefits of using a lighter motor and using that added weight to add battery instead.
Last edited by Nereth; Apr 21, 2013 at 07:59 AM.
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