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Jun 15, 2014, 10:56 AM
Jack
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Multis - Static Thrust to Flying Weight Ratio Testing - 1.2:1 is the answer!


Static Thrust to Flying Weight Ratio Testing - 1.2:1 is the answer!

As a fairly new to multicopters person, I was curious as to what kind of power it takes to make a multi fly and hover. So I recently ran some hover checks to compare the thrust readings I get on a motor/prop combo on the test bench to compare that to what it takes to get a quad into stable hover.

The details on my test bench are here:

https://www.rcgroups.com/forums/show...&postcount=151

And the quad used in the testing is an iFlight RC iQ450 "flamewheel clone" kit quad.

http://www.altitudehobbies.com/multi...r-flight-combo

The quad has been used and maybe even abused a little, it now has home made G10 arms and some other minor mods. The fishing bobber chin bar is to help a newbie pilot with orientation in flight. And it give my old eyes some help when it gets up around 100-150 feet or so.

But it is still a very nice flyer! I just recently upgraded to the home made G10 arms and I'll never break another arm on this guy!

The quad has iPower iBM2212Q-1000 Kv motors and was running iCF 10 x 4.5 CF props for the tests and the quad was flying at an AUW of 954g.

I flew the eLogger V4 data logger on the quad and did a number 10-15 foot or so stable hovers. I looked at the data from the eLogger and zoomed in on the periods when I was in a stable hover with little or no throttle inputs.

The third image is of a hover of about 39 seconds and you can see the averages of the data for that period of time. The key value for me in the info there was to find the average motor RPM in a stable hover. Most of the power and RPM variations during that time were from the CC3D flight controller. I made only occasional very small throttle adjustments and control inputs.

I did a number of hover checks and found that the iQ450, on the average, hovers at the following numbers:

Stable hover power averages:

4582 RPM
11.17V
11.28A (2.82A per motor)
126W (31.5W per motor)
Battery capacity consumed = 184 mAHper minute (46 mAH/minute per motor)

It is virtually impossible to measure propeller thrust in flight. But the thrust readings taken in static testing are useful for estimating the input power needed and the thrust that can be obtained from a given prop at a given RPM.

Static testing thrust will always be a little different from what happens in flight because the prop will unload a little, the current will drop a little, the motor will have better cooling. And the numbers will typically drop a little more in forward flight because of the transitional lift or wing effect of the props when in forward motion.

What I wanted to learn was a factor for static thrust to flying weight that could be applied to estimate how much thrust in readings from a static tests would be enough to get a quad of a given weight into a stable hover.

I set the same prop and motor up in the test bench and ran it at 4,500 RPM. I read the thrust from the test bench and the rest of the data is from the eLogger. I use a servo tester for a throttle on the test bench and could tell the throttle percentage from the displayed PWM values on that. You can see the data from a typical static test in the fourth image. In the fifth image I am zoomed in on the data for a period of 32 seconds at a steady throttle setting of about 4500 RPM. There averages for that 32 second period were:

4482 RPM
12.35V
2.74A
34W
Thrust 288g

And now we can get our answers!

Comparing the known weight of the 952g quad in a in a stable hover at a known RPM to the static thrust obtained at the same RPM gives me the factor I was looking for.

Static thrust @ 4500 RPM = 288g of thrust

Four motors would produce a 255g x 4 or = 1152g of thrust.

4500 RPM also gives a stable hover on a 952g quad

1152g static thrust / 952g AUW = 1.21 to 1 thrust to weight ratio factor from comparing static thrust to a stable hover.

So now I can look at prop testing thrust data for other props, as long as it is from honest sources like flybrushless.com, and use the thrust figures to plan out power system requirements and choose components.

It was also interesting to learn the following:

- The hover was taking place at almost exactly 50% throttle (plus or minus a percent or two)

- The motors were each using about 35W of input power

- The total input power was about 140W

- That it takes about 150W per Kilogram (actually 147W/kg in this case) to fly a quad

- That I could expect to use about 184 mAH/minute of battery capacity

Experience has verified that the flight duration I am getting now, about 10 minutes when the cells are taken down to 3.6V, is very consistent with 184 mAH/minute number. And the 80% rule of thumb for useable battery capacity puts the 2200 pack at 1760 mAH and it is working out that way too.

When I cut the flights off at 3.5V or 3.6V per cell (with the help of the easily audible alarms from the 2S-8S battery monitor/alarm) the recharged capacity has been in the 1400-1600 mAH range for the 3S 2200 mAH Hobby People 30C pack I am using. And if I want to use larger or smaller 3S packs I can make accurate estimates of the flight duration.

I started out setting the low voltage alarm at 3.7V, I have since back it down to 3.5V because the battery is coming down cool and it is not getting warm in recharging. That tells me that the battery has broken in nicely and that I can expect to get a long service life from it.

If you are a curious person and enjoy playing with numbers, this kind of testing invites all kinds of comparisons to be made with various props, various types of flying, and other factors. I am an old guy and new to quads, I don't do any "high energy" flying yet. I just enjoy getting it back into the house and in still flyable condition.

As a final bit of eye candy and to help explain my addiction to the eLogger, the last image is of my highest flight to date (I fly only Line of Sight).

Jack
Last edited by jackerbes; Oct 29, 2015 at 07:56 AM.
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Jun 16, 2014, 03:36 AM
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Great study, thank you for patience to gather all of these for us

One notice though: the 150W per Kilogram figure is for your specific setup and shouldn't be taken as valid for all platforms.
In fact this is the number that gives us the efficiency of your platform, and sorry to tell you, is not great in your case
As far as I know nobody issued a general recognized and accepted ranking for multicopters efficiency, but on my scale average is somewhere at 120-130 W/kg. The high efficiency performers are under 100 W/Kg, and setups that need improvement are over 150 W / kg.
From quad setups I flown so far I remember quickly now:
- rctimer 2830 850kv Gemfan 10x4.5 SF 1200 grams 11 Amps 3s = 120W/kg
- ax2810q 750 kv apc 12x3.8 1100 grams 11 Amps 3s = 110W/kg
- park300 1080 kv phantom prop 9443 550 grams 6A 3s = 121W/kg
Jun 16, 2014, 05:58 AM
Jack
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Thread OP
It is not stated as having a high degree of accuracy, only that it would give you a good starting point. And if it is a little on the high side, that is better than being on the low side, right?

I think almost any multi that is given a thrust to weight ratio of 1.2:1 at about half throttle will turn out to be a decent flyer. And if you want to fly high energy profiles or in higher winds or things like that, you simply need to start with a little more thrust to weight. If you look at the thrust and find the thrust value you need is up around 75% throttle or higher, you will have a platform that is underpowered. And that, as you face the realities of the pack voltage falling in use, will not get you as much duration too.

The most subtle aspect of any power system is how closely the battery voltage and Kv come to matching up to give you that magical but unknown and hard to find RPM where the prop is most efficient.

A figure that is little known or used, and that would be better used in advertising than that much used but almost meaningless "Efficiency g/W" (grams of thrust per Watt of input power) would be if they advertised the "RPM as % of Kv*V" (loaded RPM as a percentage of the Kv x the no load voltage) for a motor and prop combo.

To me, that is the best measure of efficiency because as long as the prop attains a given RPM it will have approximately the same thrust regardless of the motor turning it. When the RPM as % of Kv*V is up into the 80% range and higher the motor is giving you it's best efficiency.

Jack
Last edited by jackerbes; Jun 16, 2014 at 06:08 AM.


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