Propeller measures
We plan to do a set of measures in a wind tunnel. Preliminary tests have shown that for the speed we are concerned for very slow flyers, the change is less than 10% at 1.6 m/s. Propeller plotsThe important parameters are the trust and the power. We need also the torque and the rotation speed (RPM) in order to understand the matching of the propeller and the motor. E.g, for propeller #5 (Wes Technik 16/12, 160mm, 1.35g), our measure gives three interesting plots. The thrust vs RPM is in theory a straight line (on the log plot) of slope
2/1 (two decades vertically, one decade horizontally) since the thrust
is proportional to the square of the rotation speed. At low speed, the
curve must be below the theoretical line, due to lower Reynolds number.
At high speed, deformation of the blades due to a lack of mechanical strength
degrades also frequently the curve. Propeller's measures (selection 090804)
|
#
|
Make/material
|
Diameter
[mm] |
Pitch
[mm] |
Mass
[g] |
Thrust range
|
Qp
|
Qpm
|
|
17
|
260
|
180
|
1.32
|
5-10
|
240
|
180
|
||
18
|
230
|
150
|
0.48
|
5-25
|
220
|
240
|
||
19
|
200
|
125
|
0.55
|
5-25
|
210
|
230
|
||
20
|
122
|
100?
|
0.59
|
2-12
|
120
|
210
|
||
21
|
167
|
140
|
0.5
|
5-15
|
140
|
280
|
||
22
|
160
|
140
|
0.52
|
5-15
|
175
|
330
|
||
23
|
160
|
140
|
0.44
|
5-15
|
145
|
330
|
||
24
|
160
|
140
|
0.58
|
2-12
|
115
|
200
|
It is difficult to conclude. The long rod seems to be good for the aerodynamics (19 better than 21-24), but it is a catastrophy for the rigidity, that is the ability to absorb higher torques. A wide area next to the shaft does not seem to be good (22 better than 23and 24), except for rigidity.
Changing the size of a propeller influence its parameters. In theory,
if L is the diameter, and for a constant thrust
Rotation speed is proportional to 1/L^2
Torque is proportional to L
Power is proportional to 1/L
Reynold number is the same (no aerodynamic changes)
There is hence a theoretical advantage to increase the size of the propeller, at the expense of a larger gear train and an increased propeller fragility and rigidity, plus a look incompatible with scale models.
Good luck for doing your own experiments with propellers.
CH-1092 Belmont/Lausanne Switzerland Tel +41 21 728-6156 Fax -6157 |