Prop Selection and Care for FPV Quads

Unless you have been living under a rock, it is pretty hard not to notice that FPV Quad Racing  has become the biggest thing to hit RC this decade!  These tiny little craft, often weighing less than a pound, sport thrust to weight ratios in excess of 8 to 1, and can accelerate from 0 to 100 MPH faster than any exotic sports car, and turn faster than a cheetah trying to catch its next meal!

With props spinning in excess of 40,000 RPM on some of these craft, propeller selection, as well as proper maintenance of the props and power system, becomes more important than ever.  Unfortunately, many pilots have no idea what is happening in their power systems, and the demands that are being put on them often cause in-flight failures that can be easily avoided.

The typical power system for the smaller 200mm FPV quads consists of a set of four 2204 or 2205 motors, four matching ESC’s, four 5 inch props and a 4-cell Li-Po battery with a capacity of around 1300 to 1500 mah.  At full throttle, each motor can pull 20 amps of current, so a set of four motors is trying to pull 80 amps of current from a little 1500 mah battery pack.  This represents a discharge rate of 53C, and really pushes the poor little batteries to their breaking point.

The biggest problem that I see is that pilots slap props on their quads, without ever looking up how much current the prop actually pulls from the motor.  In most cases, the prop pulls more than the recommended full throttle current from the motor, which will cause the motor to fail eventually.  It may take 10 flights, 20 flights or even more, depending on how hard the motor is being pushed, and this gives the pilot a false sense of security.  Because the motor does not fail withing 20 seconds of the first flight, the pilot thinks that there is nothing wrong, and then is surprised when the “Magic smoke” comes pouring out of the motor 20 flights later.

There are so many different variables with props these days, that the size of a prop alone really tells you nothing about it’s performance.  The photo below shows three different 4 inch props from HQ, the 4×4.5, the 4×4 and the 4×4.5 Bull-Nose.

HP-Props-Comp

Just going by size alone, a 4×4.5 prop should pull more current than a 4×4 prop would, and if you have two different props that are both 4×4.5, then they should pull the same current, right?  Unfortunately this is not the case, and this is where people get into trouble when they put props on their quad.  If someone recommends a 4×4.5 prop, you also need to know the brand of prop and the blade style to get the correct one.

All three of the props in the photo above were run on a Cobra CM-2204-2850 motor powered by a 4-cell Li-Po battery (14.8 volts).  The numbers below the props show how much current the prop pulls in this situation.  The 4×4.5 prop pulls the least current, even though it has more pitch than the 4×4 prop does.  This is because the blades are shaped differently, and the blade area on the 4×4.5 prop is only about half of what it is on the 4×4 prop.  Because of this, the 4×4 prop pulls 32% MORE current than the 4×4.5 prop.

The HQ 4×4.5 prop actually has the same blade shape as the HQ 6×4.5 prop, with 1 inch removed from each tip as seen in the next photo.  Because the blades are much wider, and the pitch at the tip of the blade is at a higher angle.  This dramatically increases the thrust of the prop, 871 grams for the 4×4.5-BN compared to 575 for the normal 4×4.5, but it also dramatically increases the current draw as well.

HQ-4045-BN-Cut-L2

When you compare the current draw of the 4×4.5-Bull-Nose to the normal 4×4.5 prop it nearly doubles, going from 15.5 amps to 28.4 amps!  This is where people get into trouble.  As I said earlier, these numbers come from the Cobra CM-2204-2850 motor running on 4 cells.  This motor has a maximum recommend current rating of 24 amps, so either the 4×4.5 or 4×4 prop are well within this current rating, but the 4×4.5-BN prop exceeds the recommended current rating for the motor by almost 20%!

In the short term, the motor will run with this prop, and appear to be fine, but every time the motor goes to full throttle the motor will be stressed, and over time, this can lead to failure.  The failure can come in a number of ways, but the two most common are having the magnets work loose from the motor, and having the stator winding develop a short inside the motor.

I like to use the analogy of a paper clip to explain this “accumulated stress” phenomenon.  Start by taking a paper clip and straighten it out.  Then if you bend it in the middle about 30 degrees, nothing happens, it simply bends and stays in the new shape.  You can then straighten it back out, and once again, nothing really happens, the paper clip once again holds the new shape.  If you continue repeating this process, you will probably be able to bend and straighten out the paper clip 40 or 50 times, and each time, nothing seems to happen.  Then suddenly on the 51st bend, the paper clip simply breaks in half with virtually no resistance.  Why would this happen??

This simple demonstration shows how accumulated stress can build up in a part, after a repeated series of events that did not cause a failure, and suddenly cause the part to fail.  There are many parts of a motor that act the same way.  The glue that holds the magnets in place will fail over time, if they are repeatedly stressed beyond a certain safe level.  The insulation coating on the stator wires will eventually wear through after multiple heating and cooling cycles that push the temperature of the motor core too high.

Time and time again I hear from a pilot that had a setup that was, “Working perfectly for 30 flights, and then failed out of the blue for no reason.”  The truth of the matter is that the motor was being over-stressed on every flight, and it took 30 flights for the damage to accumulate enough to cause a complete failure.  Then the person wants to call out the motor manufacturer and demand that the product be replaced under warranty, because it flew fine for 30 flights.  The truth of the matter is that is the motors are pushed beyond their design limits, even by a little, they WILL eventually fail, it is simply a matter of when.

The other big issue that has been popping up lately is the use of the so-called “unbreakable” props.  There are videos all over YouTube showing people running these types of props on their quad while they throw sticks and stones into the props, or use the quad to trim the shrubs in front of their house to prove how durable the props are.  These props instill a false sense of security into the modelers because they think that since the blade did not break off, that everything is fine.  Unfortunately, this is far from the truth!

When you consider that the props on some of the smaller quads spin in excess of 40,000 RPM, having the prop perfectly balanced is of the utmost importance!  Compounding this problem is the fact that a large number of pilots NEVER balance their props before putting them on the quad.  They simply bolt them on and go fly. Even the smallest amount of imbalance at these speeds can wreak havoc on motor bearings, magnet glue and motor windings, not to mention the MEMS gyros and accelerometers on the flight controller board.   Any time a prop gets the smallest nick in it, it is most likely no longer in balance, and will begin to damage the Quad.

Quite often, in the heat of competition, a quad will bounce off a gate, hit a tree branch or have a mid-air collision with another quad, but keep right on flying.  Cases like these can be the most damaging because the quad will continue to be flown for the rest of the race with a prop that is potentially destroying the quad as it flies, because it is so out of balance.  It is a regular occurrence to see a quad hit something on the course and then cartwheel across the ground or a parking lot, only to have a turn marshal flip it over and have it take right back off again to finish the heat to get the points.  Then 3 flights later, the motor fails in flight and the pilot blames the manufacturer for making “a piece of crap motor” and demands that it be replaced under warranty, when the truth of the matter is that they broke it through their own negligence.

Another big problem occurs when pilots upgrade their quad to higher Kv motors, such as going from the Cobra CM-2204-2300 to the CM-2204-2850 or upgrading from the CM-2206-2100 to the CM-2206-2400 motor, but keep using the same props.  This will almost always create a situation where the maximum current rating of the motor is exceeded, and causes damage to them.

Mathematically, whenever you increase the Kv of the motor, and keep the voltage and prop the same, the current draw will increase as the cube of the percentage of Kv increase.  For example, lets say that you have a set of CM-2204-2300 motors on your quad, running on a 4-cell battery and spinning HQ 5x4x3 props.  The current draw for these is 16.6 amps and the motor is rated for 17 amps, so it is being pushed right to the max motor current.  If you increase to the 2850Kv version of this motor, the Kv value goes up by 24% or a factor of 1.24 times greater.  Sine the current increases as the cube of the increase you would take 1.24 x 1.24 x 1.24 and get 1.907 for the current multiplier.  Now if you take the original 16.6 amps and multiply that by 1.907, the current increases to 31.6 amps!  OUCH!

The new CM-2204-2850 motor has a maximum current rating of 24 amps, so running the 5x4x3 prop at over 31 amps WILL cause the motor to fail.  Not right away, because you do not run at full throttle constantly, but after a few flights the motor will fail.  Even the smaller 4x4x3 prop is too big for this motor on 4 cells, because it pulls almost 26 amps at full throttle. to be completely safe, you would have to drop back to a 4×4 2-blade prop to keep the full throttle current under the maximum rating of the motor!

Just like with full size airplanes, the Pilot In Command is ultimately responsible for EVERYTHING!  Take care of your motors, use props that are the right size and balanced, and you will get hundreds of trouble free flights from your quad racers.  If you need more power, put on a bigger motor!  Do not allow yourself to fall into the trap that simply putting on a bigger prop will get you the power you are looking for.  Electric motors will try their hardest to put out the power demanded of them, but they will eventually drop dead of exhaustion if you push them too hard!

We spend a LOT of time here at Innov8tive Designs performing extensive prop testing on our motors.  All of this data is put into charts that are available on each motor page on our website.  An example of this type of chart can be found at this link  Cobra 2204-2850 Prop Data Chart .

The data collected for each prop on the chart is shown with a corresponding color code, which is explained at the bottom of the chart.  In short, Blue highlight means that the prop is really too small for the motor, pulling less than 50% of its rated current. Green highlight means that the prop is good for any condition, and pulls between 50% and 80% of the max rated current.  Yellow highlight means that the prop should be used with caution, because it pulls between 80% and 100% of the max rated motor current.  For Multirotor use, props in the yellow region are usually a safe bet, because you seldom fly at full throttle for more than a couple seconds before you slow down for a turn.  Red highlight (actually pink) means that the prop pulls more than the maximum current for the motor.  These props should not be used under any circumstances because they will over-stress the motor, and over time, will damage it.

Some people have asked us why certain props are not included in the charts.  The simple answer is that as a user, a bit of common sense must be applied to the chart.  If a 6×4 prop pulls too much current, we do not test a 7×4 prop because it will be even worse.  If a motor is rated for 20 amps, and a 5×4 prop pulls 19.7 amps on 3 cells, it makes no sense to show it on 4 cells, because it will pull way too much current.

When you look at the charts on some of the motors, there are numbers for 3, 4, 5 and 6 cell batteries.  As the cell count goes up, the prop size goes down accordingly, because the larger size props will pull too much current when the voltage goes up.  Here are a few quick rough rules of thumb for switching from one size batter to the next.

  • Going from a 2-cell to a 3-cell battery on a motor, keeping the same prop, will give you approximately a 100% increase in current.
  • Going from a 3-cell to a 4-cell battery on a motor, keeping the same prop, will give you approximately a 60% increase in current.
  • Going from a 4-cell to a 5-cell battery on a motor, keeping the same prop, will give you approximately a 40% increase in current.
  • Going from a 5-cell to a 6-cell battery on a motor, keeping the same prop, will give you approximately a 30% increase in current.

If you see a prop on the 3 cell section of a prop chart, and multiply that value by 1.6, you will get the approximate current draw for a 4-cell battery.

Hopefully this helps explain some of the issues associated with selecting the right props for an FPV Quad, and gives some insight on the proper care and feeding of your electric power systems!

Best Regards!

Lucien

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