There are many factors which govern propeller selection. The most difficult propeller to select, is the first one - especially when trying an all new set up. Once you have a 'working' propeller it is much easier to predict the outcome of changing propellers.

The recommended method for prop selection is :

a) Find the correct prop size in glass,

b) then try a carbon fibre propeller of the same size,

c) to improve this, thin down the carbon fibre propeller. The extra strength of carbon fibre makes this acceptable.

Models will influence propeller selection. A large, high drag, slow flying model will need a larger diameter (or area) propeller with lower pitch than would a smaller, lighter faster flying model. For example a 120FS powered Piper Club would be best using 16 x 6 where as a 120FS Pattern model is best using between a 14 x 13.5 to a 15 x 11 or even a 16.5 x 12 for the latest engines.

A good analogy is to compare motor vehicle gearbox ratios, high gearing for racing vehicles, low gearing for load carrying vehicles.

Please consider these causes and effects.

HIGHER PITCH More load on the motor - lower RPM.

HIGHER PITCH (same RPM) Higher air speed - slower acceleration, more noise

LOWER PITCH Less motor load - higher RPM, etc.

LOWER PITCH (same RPM) Lower air speed - better acceleration.

HIGHER DIAMETER Greater motor load - lower RPM.

HIGHER DIAMETER (same RPM) More torque (pull) - vertical pull, more noise.

LOWER DIAMETER Less motor load, higher RPM.

LOWER DIAMETER (same RPM) Less torque - more power.

Blade Area: Changes in blade area will have a similar effect to variations in diameter. However an increase in propeller diameter (hence area) is far more effective than simply increasing blade area. Most wide blade propellers are inefficient compared to higher aspect ratio blades.

MULTI BLADE PROPS

There is no exact formula for selecting a 3 or 4 blade prop in comparison to a 2 blade prop, pitch remains much the same but the diameter must decrease. A 4 blade 11 x 7 is not double the load of a 2 blade 11 x 7.

Selecting the correct 3 or 4 blade prop is very much dependent on the brand and type of prop.

Most nylon type 3 blade props are very inefficient to the point where (for a given engine size), the diameter reduction makes for a drastic reduction of performance. For example, a .60 size engine using a 2 blade 12 x 6 will need to go to a 3 blade 10 x 6 for similar engine rpm.... clearly not acceptable, especially where vertical performance (= diameter) is a criteria.

A well designed (narrow bladed) CRE or GRE prop can be used at a diameter between 90 to 95% of the equivalent 2 blade prop. For example, in the Bolly range the 11.25 x 6.5, 3 blader would be the equivalent of a 12 x 6, 2 blade prop.

 

AIRSPEED Vs RPM AND PITCH DATA CHART

This chart is a good guide to the airspeed of the model. As a general guide, a good propeller will be about 90% efficient and the engine will unload by at least 10% in the air, therefore ground rpm x pitch should closely represent in air performance.... assuming a clean airframe and correct propeller choice.

PITCH RPM x 1000

Inch

5

6

7

8

9

10

11

12

13

14

15

16

17

2.5

 12 14 17 19 21 24 26 28 31 33 36 38 40

3

 14 17 20 23 26 28 31 34 37 40 43 45 48

3.5

 17 20 23 27 30 33 36 40 43 46 50 53 56

4

 19 23 27 30 34 38 42 45 49 53 57 61 64

4.5

 21 26 30 34 38 43 47 51 55 60 64 68 72

5

 24 28 33 38 43 47 52 57 62 66 71 76 80

5.5

 26 31 36 42 47 52 57 63 68 73 78 83 89

6

 28 34 40 45 51 57 63 68 74 80 85 91 97

6.5

 31 37 43 49 55 62 68 74 80 86 92 98 105

7

 33 40 46 53 60 66 73 80 86 93 99 106 113

7.5

 36 43 50 57 64 71 78 85 92 99 107 114 121

8

 38 45 53 61 68 76 83 91 98 106 114 121 129

8.5

 40 48 56 64 72 80 89 97 105 113 121 129 137

9

 43 51 60 68 77 85 94 102 111 119 128 136 145

9.5

 45 54 63 72 81 90 99 108 117 126 135 144 153

10

 47 57 66 76 85 95 104 114 123 133 142 152 161

10.5

 50 60 70 80 89 99 109 119 129 139 149 159 169

11

 52 63 73 83 94 104 115 125 135 146 156 167 177

11.5

 54 65 76 87 98 109 120 131 142 152 163 174 185

12

 57 68 80 91 102 114 125 136 148 159 170 182 193

12.5

 59 71 83 95 107 118 130 142 154 166 178 189 201

13

 62 74 86 98 111 123 135 148 160 172 185 197 209

13.5

 64 77 89 102 115 128 141 153 166 179 192 205 217

14

 66 80 93 106 119 133 146 159 172 186 199 212 225

14.5

 69 82 96 110 124 137 151 165 179 192 206 220 233

15

 71 85 99 114 128 142 156 170 185 199 213 227 241
 
  18 19 20 21 22 23 24 25 26 27 28 29 30

2.5

 43 45 47 50 52 54 57 59 62 64 66 69 71

3

 51 54 57 60 63 65 68 71 74 77 80 82 85

3.5

 60 63 66 70 73 76 80 83 86 89 93 96 99

4

 68 72 76 80 83 87 91 95 98 102 106 110 114

4.5

 77 81 85 89 94 98 102 107 111 115 119 124 128

5

 85 90 95 99 104 109 114 118 123 128 133 137 142

5.5

 94 99 104 109 115 120 125 130 135 141 146 151 156

6

 102 108 114 119 125 131 136 142 148 153 159 165 170

6.5

 111 117 123 129 135 142 148 154 160 166 172 179 185

7

 119 126 133 139 146 152 159 166 172 179 186 192 199

7.5

 128 135 142 149 156 163 170 178 185 192 199 206 213

8

 136 144 152 159 167 174 182 189 197 205 212 220 227

8.5

 145 153 161 169 177 185 193 201 209 217 225 233 241

9

 153 162 170 179 188 196 205 213 222 230 239 247 256

9.5

 162 171 180 189 198 207 216 225 234 243 252 261 270

10

 170 180 189 199 208 218 227 237 246 256 265 275 284

10.5

 179 189 199 209 219 229 239 249 259 268 278 288 298

11

 188 198 208 219 229 240 250 260 271 281 292 302 313

11.5

 196 207 218 229 240 250 261 272 283 294 305 316 327

12

 205 216 227 239 250 261 273 284 295 307 318 330 341

12.5

 213 225 237 249 260 272 284 296 308 320 331 343 355

13

 222 234 246 259 271 283 295 308 320 332 345 357 369

13.5

 230 243 256 268 281 294 307 320 332 345 358 371 384

14

 239 252 265 278 292 305 318 331 345 358 371 384 398

14.5

 247 261 275 288 302 316 330 343 357 371 384 398 412

15

 256 270 284 298 313 327 341 355 369 384 398 412 426