For the FPV and multicopter industry to accelerate it's annual leaps in technology, it is my belief that it needs standardized test procedures so that:
- a manufacturer/inventor knows
... before product launch how their product compares (saving money and helping focus research)
... which product is best so they can tear it down and improve up it
... receive royalties when protected products are copied, helping pay for further research
- the consumer can
... properly reward companies that produce the best by buying their products
... more easily build/buy optimal ships
To my knowledge there are four common technical criteria for a multicopter rotor (as opposed to an airplane that because of wing lift has slightly different test criteria):
1) Maximum Net-Lift--This is how much more the rotor can lift after it lifts itself. Until it lifts itself, it is doing nothing except vibrating the ship. Max Net-Lift is critical because in design one wants a minimum lift margin of 2X or 3X hover. Max Net-Lift thereby becomes a decision gate. One only needs to consider rotors that exceed design needs.
2) Net-Lift Efficiency--How efficiently a rotor lifts the ship and payload that in the end determines how long the ship will stay in the air. For an FPV racer, this criteria is not very important as the ship only needs to stay in the air less than 2 minutes. But a camera, inspection, or sensor multicopter needs to stay airborne for 15 minutes to 3 hours to be useful. This is a criteria with degrees of benefit.
3) Response--How quickly the rotors can achieve a commanded change in thrust. This is a criteria with degrees of benefit. This greatly impacts:
... Acceleration (critical for off-the line starts, changes in direction, etc. while FPV racing)
... Stability in apparent wind (critical for any kind of flight)
... Control of off-center payloads (critical for camera, inspection, and sensor ships)
... Smoothness of camera/sensor platform
4) Maintenance Schedule--If the maintenance schedule is defined and valid. How often bearings and other components need to be changed or checked. This is critical on ships carrying expensive payloads. This is a decision gate criteria.
There is, of course, one other non-technical criteria, cost, that is impacted by the maintenance schedule (durability of the ship). And there are other technical criteria, but not always common between applications (e.g., noise, vibration).
These links are to video of draft standardized test procedures. I'd like to collect input and collaborate with other test labs on defining industry-wide standardized tests useful to our industry.
Efficiency - https://www.youtube.com/watch?v=6EGwVhoqEz0
Response - https://youtu.be/aw3cCJwSn38
If buyers use the data from the test labs and manufacturers know it, then and only then will the industry become more serious in narrowing choices to "good" choices for consumers and integrators that build ships using off-the-shelf rotor components. For this to work, lab data needs to be:
- known by the consumer (which quickly informs manufacturers)
- easy to access (e.g., Consumers Reports, Consumers Research, JDPower, etc.) through multicopter forums.
Data results might take the following forms.
Net-Lift Efficiency - Maybe a chart showing the full range of net-lift efficiencies comparing different rotors along with a bar chart that summarizes to a single average of the middle 40% of the thrust curve.
Response - Maybe a table showing how much of targeted thrust is achieved at various Hz rates or just a single number denoting the Hz rate where 80% of the commanded target thrust delta is met.
Maximum Net-Lift - Maybe a Bar Chart showing side by side comparisons.
Scope: These tests would only apply to rotors: ESC, Motor, Prop combinations.
Any way ... your thoughts please. Is this needed? Would people use it? What tests are needed for component selection? Are these tests valid?
Replies
Forrest, did you try a response test using a staircase shape input thrust change curve instead of a up/down square wave form? I wonder if it would change the maximum Hz response.
great idea, and your are absolutely correct. it would change the statistics greatly. but no i haven't.
here are the realities of testing response.
1a) the load cell responds at a specified rate with a DC signal that fluctuates around a mean
1b) the DAQ reads at a specified rate a DC signal, so the longer it takes to read, the longer it's average
1c) thus the faster i read, the more samples i need to get an adequate average (right now i'm up to 299 samples to get a number that is repeatable to 2%)
2a) ESCs, when responding to a commanded thrust change, do so in a nice curve that looks like the following.
2b) so unless the commanded thrust change is high, the response will start to flatten out as the ESC tries to lock into the target value without overshooting too much.
2c) the thrust change has to occur over a long enough period to measure with existing test equipment
2d) thus larger changes are more accurate to measure
That brings up the importance of a standard. Right now i'm using about a 380 gram target and measuring the first 200 grams of net-change. But i'm not even sure how to measure net-change. All i can do is pick a method and test everything the same and hope that it tells me a lot about rotor response.
Unlike measuring efficiency on a net-lift basis, which is mathematically and physics sound (it's a simple subtract function), measuring net-response if quite different. a = F / m. so the impact of mass is a ratio. but a ratio of what mass? just the item mass to total ship mass being tested? or the total mass of the rotor system to the mass of the ship being tested?
To be honest, at this point i'm a bit confused as to how to proceed.
So what i've found is that if i command about a 400 grams thrust change
That's a very good question : what should net-reponse measure ?
It would be nice to be able to produce, by measurements, a net-response that is independant of the ship components and masses, because masses vary by brands/models of these components. So maybe the net-response should measure a "time delta" to maintain a constant net thrust when a standardized external disturbance occurs (for example a predefined instant weight increase, simulating wind forces or something in that vein) ?
you're right. this is a statistic that needs to be unfiltered by mass. it will be up to the engineer to use that statistic correctly to determine which ESC or motor or prop delivers the best response for their particular ship attributes. a heavy lift ship will show little impact from a 10 gram difference in ESCs for example, but a 300 gram FPV racer would see a much larger delta in acceleration. The engineer needs to use the stastitic to consider both: - ship coordinate acceleration (delta change to ship mass as a proportion to total mass) - ship rotational acceleration, which is greatly impacted by how far the mass is from ship center. So that answers the question. Thanks. I'll just publish the raw data without adjustment.
Great idea Forrest! Being in a world governed by IEEE for material standards, I find the RC industry lacking depth of details and validation. What's printed on the box doesn't mean that is what you are getting...from my short experience with MR parts. And you are correct, it would motivate manufactures to step up their game to compete.
Over 10 grams net lift per watt. This has got to be the best post ever! ;-)
Hey Forrest,
Really like the way you are approaching this. Such testing procedures, and good resulting data, is much needed indeed.
One thing I'd suggest is to add power testing procedures, in other words and simply (to be further brainstormed) how much electrical power can a given motor take? Data for this is all over the place, from non-existing to wildly exaggerated. In my experience it's all a matter of temperature, and how high and for how long one is willing to go and be "comfortable", reliability wise. And while worrying about power requirements is generally not an issue when building for maximum efficiency (given doing so will typically result in operating conditions well below maximum power and hot motors), I think it is an important one when designing copters with utmost reliability in mind.
In my experience I've found that typical rules of thumb are ok as a first approximation (e.g. warm to hand but not hot, or max power in watts 4-6times weight in grams, but really crude. Actually measuring temperature after long flights is better, but still not a good indicator of how "hard" the motor(s) were pushed and how, if any, temperature and power impacted the motors' reliability.
The best related data I've seen so far is from T-motor, who does give "operating temperature" in propeller tests (10 minutes, full throttle) yet even this is still a bit vague in terms of judging reliability and long term performance. And definitely not "standardized" ...
One beginning of an idea for testing, one that I've sort of used: run the motor at various throttle levels for some pre-determined time depending on its intended use, and test for temperature convergence/stabilization. If temperature stabilizes early at a reasonable level (what exactly is reasonable, though ... ) we now have an upper bound and a good data point. If temperature keeps rising yet never reaches a maximum (in the time allowed) deemed safe, we have a good data point too. Etc ... maybe some time/power/temperature plots?
Also like your use of net lift. Could maybe be expanded to net lift with propeller, i.e. Lift minus weight of motor minus weight of used propeller? After all one can't talk about or measure net lift without a propeller anyways, and weight of propeller can definitely make a difference when testing different ones so useful for comparisons and measuring efficiency.
Intrigued by your thrust measurement apparatus, and you real time computer display setup looks top notch. Can you talk more about your dynamometer? Appears very precise yielding (crucial) good repeatability ...
Just a few thoughts ... Great work!
Excellent puts.
Motor Limits - Higher watts is key making a sluggish rotor respond better. But that also increases heat. So we really do need to communicate that data too. I like your idea on convergence. If heat "runs away", then we know it's too hot. So maybe a chart showing how watts impact temp. I've been using an axle temp of 52C as convergence limit. So maybe a watt rating based on axle temp converging on 52C?
Net-Lift - Glad you like that. Have to admit, it does make sense. Right now, net lift includes the mass of the entire rotor system so one can evaluate, apples to apples, the impact of prop size:
- ESC (naked, wires not included)
- Motor (and bolts to the motor mount)
- Prop (including it's attachment items)
- Structure (the larger the prop the longer/heavier the structure; 2 grams per inch over 12")
Test Device - Ask away. The load cell is an Omega LC703.
This test stand looks like it would make the data collection process easy and scriptable
https://www.rcbenchmark.com/product/rcbenchmark_dynamometer/
having low-cost test devices would certainly help that would be able to measure:
- amps
- volts
- thrust (primary with optionally rpm in addition)
- temperature
It's great to see devices like this emerging, which are great for smaller props and lower thrusts.
A commercial device generating information to the public needs to include thrust in a manner that is accurate. The load sensor housing, because of prop wash, needs to be:
- away from the prop where there is little force from prop wash
- have a small or aerodynamic profile to not be impacted by prop wash
- should be pointed to within 15 degrees of vertical if not vertical
- 1.5 prop diameters away from test bench edges that can provide ground effect.
- heavy, if especially testing large props, to rid the device of harmonics between the prop and load cell
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