Two square propellers: Graupner 7x7 Super Nylon and APC 7x7 Sport
This is a translation of my original article, please leave comments for any correction that can be made! my english isn't perfect at all !
Yesterday and today I tested two propellers in a wind tunnel, according to "standard procedure"! If you will, I recently developed ...
Each test lasts between 6 and 7 minutes, according to the following:
- Propeller set, wind-tunnel door open, voltage of the power supply pre-set, transmitter ON,
- Camera ready to capture the thermometer ( "motor air gap" temperature...), scale, and weather station (time, temperature, pressure and relative humidity),
- Power supply ON -> 2 continuous shots at controller beeps (the emission of the beeps is detectable on logged data so that both shots and data can be synchronized)
- scale ON via servo, shot,
- First run for static conditions, shot, shot, shot ...
- Back to zero, shot (to estimate the scale tare before and after the run, if the difference is very strong there is some trouble),
- Closing the door of the wind-tunnel and start the "dynamic" run, full throttle, wind-tunnel off for now, shot, shot, shot ...
- Wind-tunnel speed is increased step by step, shot, shot, shot every step.
During the first of the 3 runs, the prop was spinning a bit too fast, exceeding the wind-tunnel capacity (high pitch needs a slower rotational speed compared to the airspeed).
At the end of the run, the pics and data are uploaded in the PC.
The logger data are sent to excel, pics data are added (30 to 40 lines with time, weather, scale and motor temperature).
Some excel formulas eat all this (corrections, synchronization,..) and the performance coefficients of the prop are obtained, in two forms: raw data with measured points and arranged data with an averaged curve that can be used into my homemade software Tetacalc...
Testing a prop can last less than 1/2 hour ! Of course the accuracy of the data is relative, it's hard to make scientific research with it but... good enough to be useful.
On the graphs below J=V/nD represents the ratio between airspeed and rotational speed and prop diameter:
J = advance ratio,
V = airspeed (m/s),
n = rotational speed (tr/s),
D = diameter of the propeller (m).
Every props in this report have a 7 inch diameter, so 0.1778 m. In order to use more current units in RC world (rpm, km/h, inch...), you can use this formula:
J = 656.17 * (km/h) / (rpm * inch)
Example: flying at 60 km/h and 12,000 rpm, with a 7 inch prop, the advance ratio is:
J = 656.17 * 60 / (12000*7) = 0.47
For explenations on coefficients, you can visit this site (in french... sorry!):
This is a simmers site... reminds me things...
It would be a good advice to be sure to understand well all the things about advance ratio, thrust and power coefficients and efficiency, before having a look at the following graphs.
The keys of the graphs indicates the date of the test (yyyymmdd), the prop's brand, the dimensions and sort (electric, slow fly...), the average rotational speed during the test, and the motor used during the test (a same motor means a better combarability more than better accuracy).
Note: I had some difficulties to properly centered the graupner, which had a higher noise level than the APC when spinning, so it might had a lower efficiency than what it could have been.
The comparison is made between 4 differents props, including the two 7x7, so square props, and previous data for the 7x4 and 7x6 APC thin electric. The 7x6 being the one that's mounted on my Yak actually, with the same motor as during the wind-tunnel tests.
Extrapolations of thrut and power:
Below, charts for finding thrust, power needed and efficiency of the propeller according to flight speed and rpm.
Those curves are extrapolated from a single test, at the rotational speed indicated (ex: source test: Graupner 7x7SN - 6473 rpm). In the case of homemade test, the rotational speed isn't constant, it's the voltage that remains constant, the rotational speed increases as the prop "unload", in a similar way as what can be seen in flight.
The props are quite relatively sensitive at variations of their rotational speed, see the UIUC site. The following curves don't care about this!
The red curve "Réf." represents the test conditions.
The dotted red curve "Calc." represent a practical case, here:
- Motor: EMP N2826-1350, Kv 1350 - 51g,
- Voltage: 11,1V,
calculated by the homemade software.
Those two curve then show an increase of the rpm with speed.
The values are given for standard atmosphere, 0m height.
NOTE: "poussée" means "thrust", "puissance" means "power" and "tr/min" means "rpm" !
Graupner 7x7 Super Nylon:
APC 7x7 Sport: