|Jun 19, 2011, 03:09 AM|
|Jun 19, 2011, 10:46 AM|
It is also quite close when I compare my measurement of APC 8x4E to the Taipan 8x4 in Propcalc, the two props seem similar in shape (??).
Propcalc is calculating Ct, Cp and efficiency from the prop's shape and airfoil, with good results I think. Unfortunately, I don't have any prop to compare with it.
As Jack says, the problem is that using Ct and Cp is uneasy. And I'm sure that anybody can do very well without.
On the other hand it's possible to make an excel file that do the same thing as the many calcs available, but at different speeds...
Flybrushless.com gives a huge database from where it is possible to modelize numbers of motors in an easy way.
A "light" flight modele can be set (weight,wing area...)
Batteries can be easy to modelize too.
I think that with nothing else but being able to measure amps, volts and rpm, there can be a tool that can be very useful for someone looking for high performance in a particular domain:
-max efficiency in climb near stall speed,
-max flight time,
I've got a "small" worksheet that can do good job, but since there's some motors data from Doctor Kiwi at flybrushless i don't know if i can make it public.
|Jun 19, 2011, 11:28 AM|
That's true i should do that, it would give "theoretical" strength to my data.
I've got to look closer at this.
That's a little complicated because propcalc starts from airfoil to optain prop's coeffs, what i directly measure; and need to know the prop's shape which is uneasy.
Drivecalc don't use propcalc's data (does it?), that's "dommage"!
|Jun 19, 2011, 01:40 PM|
I can give some info about accuracy:
|Jul 02, 2011, 09:00 AM|
If you would like, and send me the excel file I may be able to work you test data into the site somehow, but I'm not sure of where it would go. It seems like it would be part of the props data, however since you are measuring amp draw that confuses the issue some since that would be dependent on the motor. Please let me know if that's something your are interested in.
In the mean time it looks like I have a lot of reading to do
|Jul 02, 2011, 08:11 PM|
I'll ask the good doctor when he'll be back!
I've made all this things with the wind tunnel because at first I was wishing to build an electric 1/12 scale Yak, just like a combat warbird, even if it's nearly unknown in france, so I had maybe wrong idea about it.
Anyway, I've tried to built a tool able to compare such things as max speed, flight time, max thrust at stall speed, and of course the efficiency of the prop...
it works quite well for my well known motors... might do the same with Dr.KIWI's motor data.
Greg, thank you very much, I love you!!!
This is exactely what I was looking for!
This is from the pdf on this page (John B. Brandt and Michael S. Selig):
"proper propeller selection for UAVs can have a dramatic effect on aircraft performance"
I had doubts about my airspeed recording accuracy, seems to be justify...
I can just compare, for the moment, the APC 9x4.5 thin elec figures.
The static tests from University of Illinois where runned at 4000 up to 6900rpm, i've made it at a bit more than 9150 rpm.
The pictures below show the comparaison between University of Illinois results and mine, poor man...
Static test looks OK but I'm making a little mistake on airspeed recording so that my advance ratio and my efficiency are a bit higher.
(I've bought an e-logger with pitot tube 2 days ago, so i shoud be able to compare speed at different position in the test room, waiting for it...)
Anyway, those kind of mistake will be a problem only the day we'll all know the airspeed of our aircraft and the exact real capacity of the batterie...
A tool using those kind of results should be better used in order to compare from a known setting to another.
Sorry I'm talking, talking, talking... and it's getting late...
|Jul 03, 2011, 09:39 AM|
This morning I compared my results with those from Greg's link to the University of Illinois, for APC 9x4.5E at different rpm.
With a correction factor of 0.78 on airspeed (that's a lot!) what I obtained is shown on the three graphs below. The little red dots shows what I measured.
I don't doubt that the guys from UIUC did a very good job in estimating airflow speed. But in a way I found their results very pessimistic. My 78 cm wingspan Yak has a recorded top speed around 115 km/h, with 6x4 running at 19,000 rpm. With an efficiency so low as what I found with airspeed correction, this means the drag is very low, almost the same aerodynamic coeffs than the original Yak-3, I can't believe this...
The pitot tube should give answer about this...
Thanks again Greg ! now I know I'm not totally fool!!!
|Jul 03, 2011, 04:16 PM|
Je suggere que tu mettes les unités sur l'axe couple
I suggest torque unity indication on vertical axis
Louis (92160 Antony France)
|Jul 03, 2011, 05:11 PM|
Sur les graphes c'est le coeff de puissance pour P = Cp*rho*n^3*D^5, sans unité.
On the graphs it is power coeff Cp for P = Cp*rho*n^3*D^5, dimensionless.
Torque = P / omega
So APC 9x4.5E at 9100 rpm:
Cp = 0.04
rho = 1.225 at sea level (1.1 at home, 900 m high)
n = 9100 rpm / 60 = 152 rps
D = 9 inch * 0.0254 = 0.2286 m
P = 0.04*1.225*152^3*0.2286^5 = 107 Watts
omega = 9100 rpm * pi / 30 = 953 rad/s
Torque = 107/953 = 0.112 N.m
Thrust = Ct *rho*n^2*D^4 so if Ct = 0.11
Thrust = 8.5 N = 865 grams
More or less of course !
|Jul 24, 2011, 05:33 PM|
I couldn't find an easy and simple way to present the props data, and it needs a study of wind correction and maybe better torque measurements.
Lot of things to do!
So i've just tried to translate my "little" excel sheet in approximative english
It's a bit complicated so i wrote something about it on a .doc file.
Everything can be download here:
It's really not perfect and easy to use, i'm afraind, but it might give an idea of what i'm looking for.
BTW, this is not really a "find my setup" software...
|Jul 25, 2011, 05:20 PM|
Rapid look on Tetacalc
Your "loose motor quality coef" (300 to 700) is not easy.
Better to speak "efficiency" or something like that :
bad motor 0.6
medium motor 0.7
quality motor 0.8
HQ motor 0.9
What is your ref N2826-1350 51g ?
|Jul 25, 2011, 07:18 PM|
Thank you for looking at this!
Yes you're right, this coeff is not easy, i can't really explain it . !!!
It's just : (coeff / kv˛) / weight(kg)˛ = motor armature resistance Ri
A 80% efficiency motor should have the coeff around 300, 75% efficiency ---> 550/600.
Tetaclac is not really serious because it's just my tool, so it has some maybe "extravagant" things !
The N2826-1350 is an EMP motor, so equivalent Turnigy, 0.3 mm laminations.
the "calc" is a simple math model very similar to Drivecalc I think. The basic formulation is something like that:
rpm = kv * (U - ( Ri+a.rpm.I ) ) and Pout = ( rpm/kv ) * ( I - Io[rpm] )
The "dyno" one is another model, very accurate, where:
rpm = kv[I/U] * ( U-Ri.I-L[rpm] ) and Pout = ( I*rpm/kv ) - Po[rpm]
Well, it works well at home and seems to be accurate enough.
I don't know what are exactly the math behind the other calculators, so i made mine in order to be able to use props charts like that. I think it can be interesting to have real props performance data, even if wind tunnel data are just wind tunnel data. The calc is just a tool to read the data and compare props, any motor math model should be fine.
Anyway, i love the challenge !
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