T-Rex 500 5S motor test

Written by Ashley Davis Wednesday, 05 August 2009 08:52

Motor Test

Introduction

There are a few motor solutions for the T-Rex 500 but three of the most popular for 5S operation are the stock Align 500L, the Scorpion HK3026-1900kv and the Medusa 36-50-1600 V2. All three are capable of delivering 3D performance but how do they compare?

The following test looks at amp draw, RPM hold under full load and RPM during pitch pumping as per the normal TrexTuning bench tests. So let's get on with it and separate the good from the mediocre.

Equipment

Before we move into looking at the test data I would first like to introduce the various motors that have taken part in this test. I would also like to credit the various suppliers whose motors are participating in the test. Additionally, I need to detail exactly what pack has been used in these tests and what speed controller. Following this I will explain how the test was conducted and give some explanation to the charts that have been produced.

Motor	kv	Supplier	Pinion	Shaft size
Align 500L	1600	Align UK	16T	3.17mm
Medusa 36-50-1600 V2	1600	Medusa Products	16T	3.17mm
Scorpion H3026-1900	1900	Scorpion	14T	3.17mm

The lithium polymer pack used throughout this test was a Revolution 5S 5300mAh 22C. This pack was used with a FMA Cellpro 10S charger. This pack was specifically selected to remove any pack voltage variances from the motor testing. Of course you couldn't possibly fly with this pack but it produced a good consistent voltage for the testing of the motors, which is what we are testing (not packs). The speed controller used in this test was an Align stock T-Rex 500 60A ESC that comes as standard with the T-Rex 500 kit.

The test equipment used in this test was an Eagle Tree data logger fitted with a brushless RPM sensor.

The test machine was a T-Rex 500 with 430mm blades and belt drive.

My thanks to Aurorra for the supply of the Revolution Pack and Cellpro 10S Charger and thanks to Eagle Tree Systems for the data logger.

The Test

The motor test consists of four phases. The first phase is an initial spool up to operating RPM. I set all of the motors to run at around 2800-3000 rpm unloaded using pinions only. The initial spool up is 30 seconds operating at 0 pitch on the blades. The second phase of the test is the application of full positive pitch for the duration of 10 seconds. This is the main loading part of the test and is designed to ascertain what RPM the motor can hold at full pitch at the end of 10 seconds. The third phase of the test the motor is given a 20 second rest at 0 pitch. The fourth phase of the test consists of 10 seconds pitch pumping from full positive to full negative. At the end of this phase the test is complete and the motor is checked to make sure it has not hit an excessive temperature (over 80° C.). The following table summarises the test.

I should point out at this stage that none of the motors tested went over the control temperature of 80° C. Therefore I will not mention this further in the test results. However, I will come back to motor temperatures in the conclusion of the test as there is relevant data to be shared.

The Charts

The following is an explanation of the charts used to differentiate the performance of the various motors. I will use the data generated by the 500L motor to explain how to interpret the charts.

Below is the combined chart for the Align 500L.

Green line = RPM
Blue line = Amps
Red line = Volts

The left vertical axis shows volts, amps. The right vertical axis shows head speed.

The chart shows spool up and 0 degrees pitch until 0.37 minutes when full pitch was applied. Then follows a peak in amps and a dip in RPM for 10 seconds to 0.55 minutes. Following this the pitch is reduced to 0 degrees pitch for 20 seconds and a recovery can be seen in RPM and the amps required fall until 0.87 minutes. Lastly pitch pumping with lots of fluctuation in amps and RPM from 0.87 minutes for 10 seconds.

To ensure fair play pack supply voltage was checked and seen to be consistent across all three tests.

Amp Draw & RPM

This part of the test will focus on the amp draw of the various motors as they are put through the standard test. The amp draw consumed by the motors in isolation does not give an indication of the performance of the motor. The set of charts that follow give an indication of what sort of flight time one might expect to get on each motor. The data also shows how hard the pack will be pushed to deliver the performance. The amp draw needs to be compared to the RPM performance in order to get an indication of efficiency. We will also focus on RPM in relation to amp draw for each graph. I'll give approximate figures in the graph descriptions, the tabular data at the end contains the full details.

What we are concerned with here is the average amp draw across the entire test. In this we see the 500L with the lowest figure of 30.59A, midfield is the Medusa with 32.19A and last the Scorpion with 35.24A. So overall the 500L would provide the longest flight time amongst these motors and the Scorpion the shortest time.

Secondly lets look at average RPM to give an indication of overall performance for 3D. Here we have the Medusa in first place with 2854 RPM, midfield is the Scorpion with 2845 RPM and last the 500L with 2705 RPM. It is important to note that the 500L starts with 100 RPM less than the other two motors, I did try a 17T pinion but it over geared the motor and caused worse results rather than better as the motor got too hot.

OK, so let's look closely at full pitch.

So at full pitch we have a slightly different placing, the Medusa is first with 50.66A, midfield is the 500L with 50.79A and last the Scorpion with 58.77A.

Looking at RPM we have a slightly different result than the overall test figures, first we have the Scorpion with a minimum RPM of 2543 RPM, midfield the Medusa with a minimum RPM of 2500 RPM and last the 500L with a minimum 2369 RPM. Clearly the 500L has a lot less torque than the other two motors and wouldn't hold RPM over a prolonged period.

OK, so what about the pitch pumping section. Here we once again use the average draw figure to give an indication of the most efficient motor.

Once again we have the same running order, 500L first with 40.24A, midfield the Medusa with 42.05A and last the scorpion with 46.88A.

Looking at RPM we have the Scorpion in first place with 2687 RPM average, midfield is the Medusa with 2640 RPM and last we have the 500L with 2511 RPM.

Here's the tabular data for the previous three sections:

Motor	Overall Amps	Full pitch Amps	Pitch pumping avg Amps
Align 500L	30.59	50.79	40.24
Medusa 36-50-1600 V2	32.19	50.66	42.05
Scorpion HK3026-1900	35.24	58.77	46.88

Motor	Overall avg RPM	Full pitch min RPM	Pitch pumping avg RPM
Align 500L	2705	2369	2511
Medusa 36-50-1600 V2	2854	2500	2640
Scorpion HK3026-1900	2845	2543	2687

Position (Amps)	Motor
1st	Align 500L
2nd	Medusa 36-50-1600 V2
3rd	Scorpion HK3026-1900

Position (RPM)	Motor
1st	Scorpion HK3026-1900
2nd	Medusa 36-50-1600 V2
3rd	Align 500L

Additional Factors for RPM

When going back over the data we can see that the Scorpion has more torque than the Medusa but over the entire test it seems to have a lower average RPM, how can this be?

Well, if you look at the 20 second rest period following the full pitch test you can see that the Medusa returns to the same RPM prior to the full pitch test where as the Scorpion does not. This would indicate that heat generated in the Scorpion is causing some lack of RPM recovery i.e. the motor has gone a bit stiff to rotate. All three motors actually suffer with this problem, the 500L being the worst and the Medusa fairing best. At the end of a flight all three motors have generated heat and some stiffness can be felt in rotating the motor by hand. The reason the Medusa fares better in this short test is that unloaded it recovers a little better than the Scorpion and in that 20 seconds just managed to get a higher overall average RPM. In a continuous 3D flight the Scorpion would actually win out as it has better torque handling than the Medusa but a quirk of this particular test is that the 20 second rest has worked to the Medusa's advantage.

Efficiency Analysis

I could at this point just select the highest performing motor and ignore the amp draw characteristics. However, whilst this may be the right thing to do and fit some people's requirements it doesn't take into account all of the characteristics that a potential purchaser may be interested in. Not everybody just wants ballistic power at the expense of reduced flight time.

Personally I would want to make sure that I'm getting great performance but equally maximising my flight time as well as taking care of my pack. This is where it is worth taking a couple of moments to talk about the amp draw from these motors.

The test is conducted with the helicopter strapped down. This means that during the test the blades have to move static air. The motor is having to work significantly harder than if the model was moving through the air. This effectively pushes up the amp draw requirements during the test. In flight all of these motors will pull less amps, suffer from less head speed drop and generally perform better. We need to keep this in mind when looking at the figures and assessing whether the motor in question is suitable for our requirements. This needs to be especially considered in relation to the flight time figures in the table below as in free flight the flight time would be longer.

The table below gives an average flight time based on the test average amp draw figures utilising a 3300mah pack. This may seem unrealistic to a real flight based on the way the test is conducted but it helps put into perspective what difference the amp draw makes. The flight time is calculated using only 80% of the packs available capacity (2640mah):

Motor	Average Amps	Flight time
Align 500L	30.59	5.18 minutes
Medusa 36-50-1600 V2	32.19	4.92 minutes
Scorpion HK3026-1900kv	35.24	4.49 minutes

Position (RPM)	Motor
1st	Align 500L
2nd	Medusa 36-50-1600 V2
3rd	Scorpion HK3026-1900kv

Weight & Price

The last consideration with regard to performance is the weight of the motor. In this section I will also include an indicative price for each of the motors. Although price has little to do with performance it is a selection criteria when choosing a new motor. The following table summarises the weight and price of the various motors on this test.

Motor	Weight (g)	Price ($)
Align 500L	200	90.99
Medusa 36-50-1600 V2	230	104.99
Scorpion HK3026-1900kv

The ranking on price is as follows:

Position	Motor
1st	Scorpion HK3026-1900kv
2nd	Align 500L
3rd	Medusa 36-50-1600 V2

The ranking on weight is as follows:

Position	Motor
1st	Scorpion HK3026-1900kv
2nd	Align 500L
3rd	Medusa 36-50-1600 V2

'In flight' thoughts

All three of these motors have been extensively flown on 5S. My impressions of them 'in flight' are that the Scorpion produces the strongest performance due to it's exceptional torque, it really does hold it's RPM and it's the lightest motor on test. Following this is the Medusa which produces a good head speed but ultimately suffers in prolonged torque moves like four point tic tocs or big bens, it's also the heaviest motor in the test by a full 30g. Lastly the 500L which produces reasonable 3D let down by a lack of torque. It also gets scorching hot leading me to believe that it is not best suited to 5S use as it does not get as hot when used in it's normal 6S configuration. This is from the perspective of hard 3D flying rather than sport flying or mild 3D.

While all three will do 3D there is a marked difference between each motor in performance that any pilot of an average skill level would notice. The jump from 500L to Scorpion is pretty big in performance terms.

Other Considerations

Having owned and run these motors for a while there are some other considerations that I'll share in this section.

First the 500L. This motor gets very hot when run 5S on a 16T pinion and although I've not yet killed a motor myself there are numerous reports of 500L motors dying when run in 5S configuration. This doesn't surprise me given how very very hot it gets during a full 5 minute 3D flight. It gets hot enough to be able to burn a finger, not good.

The Medusa suffers with three problems, one is that the mounting screw holes are very soft and it is extremely easy to strip the threads when tightening up the motor in the motor mount. Caution is needed and just don't go crazy tightening down those motor mount screws. Secondly it requires the motor mount to be modified slightly in order to get the motor to sit properly in the motor mount. Lastly, the motor shaft is extremely short and doesn't fully support the pinion, although so far this has not been an issue and I haven't had any pinions break.

The Scorpion wears out it's bearings in 50-100 flights, fortunately they are very easy to replace and there are numerous threads and some videos on how to complete the procedure. I see this as just necessary maintenance to keep my motor in tip top condition. The bearings are not expensive and you can alleviate this task almost entirely by fitting high quality ceramic bearings. Personally I just replace them periodically, you can hear when they are going.

Conclusion

As with all tests it eventually becomes necessary to pick the winner. Listed below are how each motor performed in each individual test:As always I pick an overall performance winner and an all round winner.

Motor	Amp Draw Overall	RPM Overall	Efficiency	Weight	Price
Align 500L	1st	3rd	1st	2nd	2nd
Medusa 36-50-1600 V2	2nd	2nd	2nd	3rd	3rd
Scorpion HK3026-1900	3rd	1st	3rd	1st	1st

Picking the performance winner

The performance winner is the Scorpion HK3026-1900kv which although it didn't return the best overall RPM figure when put to the real test in flight the combination of lightest weight & best torque provides for a stunning performance on 5S.

Picking the overall winner

The overall winner is the Scorpion HK3026-1900kv. Whilst not being the most efficient it is the most fun to fly and the overall penalty on amp draw is not that huge. It's also the cheapest and lightest in the test.

As a foot note it's all to easy to get overly drawn into these test facts/figures but I would like to point out that any of these motors can produce good 3D performance and none of them were disappointing to fly. The data is provided to allow an informed choice. In flight data on these motors can be found in the trextuning blog where I on occasion put captured flight data and compare both packs and motors.

Lastly, my thanks to all the suppliers who made this test possible by providing not only motors but lithium packs, test equipment and their expertise in helping me get the best out of their motors.