BASICS

The following is a basic outline of prop theory and conclusions on prop design. A more comprehensive outline would require more mathematics and references than is practical to include here.

For most model aircraft, the propeller is the only means of converting the power of the engine into flight performance. This makes the choice of propeller vital for the performance of a model.

Treat a propeller as a wing, and the theory will be easier to understand. Blade shape, pitch and airfoil selection all become understandable. The only difference is that a propeller is a rotating wing, with the 'Reynolds' number (via different blade width and air velocity) changing along the blade.

One of the biggest problems with propeller design (for all props including ‘full’ size), is that it is impossible to know what a prop is doing in a ‘dynamic’ state. Often static testing does not relate to dynamic performance. It is very easy to develop a prop which gives excellent ‘pull’ on a static test, however a prop designed to these criteria often don’t work that well in the air.

The choice of prop construction material will have a marked effect on prop design and performance.

BLADE SHAPE

As the average model propeller operates at a Reynolds number (scale effect) similar to a R/C Glider. A high aspect ratio, elliptical based shape generally works best, whether it be for glider wings or a propeller. If you take the average Bolly Prop and give it a well rounded or swept back tip, this overall shape will be as practically close to an ellipse as possible. This is especially so with the 3 and 4 blade Bolly Props. Practical considerations sometimes prevent the 2 bladed propeller from being the ideal shape, ie, undercarriage lengths and ground clearance.

The choice of blade shape is also determined by the propellers end use. For slow flying high drag models or models which need small diameters, a lower aspect ratio, wider chord prop is more suitable.

1, 3 AND 4 BLADE PROPELLERS

The rules vary when dealing with single blade and racing propellers. A small 2 blade 5.8 x 5.8 with 12mm chord is equivalent to a single blade of 6.2 x 5.7 with 17mm chord. The single blader is more efficient due to the extra diameter and chord (much higher Reynolds number). Large diameter single blade propellers are not practical due to the high weight of the counterbalance required.

For some racing applications a high aspect ratio blade is not possible due to the extreme loads imposed at high RPM, especially if the tip is expected to exceed mach 0.7.

Contrary to popular belief, multiple-blade propellers do not operate in severely disturbed air from the previous blade (when in forward flight). The reason multi blade propellers often appear inefficient is the need to use considerable lower diameter propellers (in comparison to 2 blades), for the same horsepower available. Diameter for diameter a well designed 4 blade prop will in some circumstances perform better than the equivalent 2 blade propeller.

It is rare to find an efficient 3 or 4 blade propeller manufactured from nylon based materials. The reason for this is the most efficient shape (thin, narrow blades) for these propellers is difficult to produce in anything but a carbon or glass composite construction. For this reason Bolly have one of the best and most efficient ranges of 3 and 4 bladed propellers in the world.

AIRFOILS

As with a glider wing we want the maximum lift over drag performances. Practical strength and aerodynamics means an optimum airfoil thickness of around 15 to 18% near the root, progressively thinning to 10% at the tip. About 12% at the 3/4 span in optimum.

The blade airfoil will vary slightly with use, with the Clark Y shape as a standard. The root which does less of the work and the tip which has ‘tip speed’ problems should have less camber (semi-symmetrical). For some applications the centre portion of the blade may need a lower or higher cambered airfoil.

REYNOLDS' NUMBER

The Reynolds' number (Re) is a theoretical number used to describe the 'Scale Effect', ie - an exact 1/8 scale replica of a Boeing 747 wing will not behave identically to the full size version. The higher the Re, the greater the efficiency. The equation is - Re = 68500 x Velocity x Length (chord or wing or prop)

PITCH

Without doubt this is the least understood factor of propeller design. Pitch is the theoretical distance the propeller will advance along the axis of rotation in one complete revolution.

The fact that a propeller of constant pitch will have a twisted blade is also not often comprehended, ie - at 5 inch radius the propeller will travel a circle of 31.4 inches, at 10 inch radius it will travel 62.8 inches (double the distance) it will need half the pitch angle to travel the same distance.

 

 

 

 

 

 

 

BLADE ANGLE (degrees) AT GIVEN PITCH & DIAMETER DATA CHART

Pitch (inches)

Radius (mm) 2 3 4 5 6 7 8 9 10 11 12 13 14 15
20 22.0 31.2 38.9 45.3 50.4 54.7 58.2 61.2 63.6 65.7 67.5 69.1 70.5 71.7
30 15.0 22.0 28.3 33.9 38.9 43.3 47.1 50.4 53.4 55.9 58.2 60.2 62.0 63.6
40 11.4 16.8 22.0 26.8 31.2 35.2 38.9 42.2 45.3 48.0 50.4 52.7 54.7 56.5
50 9.18 13.6 17.9 22.0 25.8 29.5 32.8 36.0 38.9 41.6 44.1 46.4 48.5 50.4
60 7.67 11.4 15.0 18.6 22.0 25.2 28.3 31.2 33.9 36.5 38.9 41.2 43.3 45.3
70 6.58 9.82 13.0 16.1 19.1 22.0 24.7 27.4 30.0 32.4 34.7 36.9 38.9 40.9
80 5.77 8.62 11.4 14.1 16.8 19.4 22.0 24.4 26.8 29.0 31.2 33.3 35.2 37.1
90 5.13 7.67 10.1 12.6 15.0 17.4 19.7 22.0 24.1 26.2 28.3 30.2 32.1 33.9
100 4.62 6.91 9.18 11.4 13.6 15.8 17.9 19.9 22.0 23.9 25.8 27.7 29.5 31.2
110 4.20 6.29 8.36 10.4 12.4 14.4 16.3 18.3 20.1 22.0 23.7 25.5 27.2 28.8
120 3.85 5.77 7.67 9.56 11.4 13.2 15.0 16.8 18.6 20.3 22.0 23.6 25.2 26.8
130 3.55 5.33 7.09 8.83 10.5 12.2 13.9 15.6 17.2 18.8 20.4 22.0 23.5 25.0
140 3.3 4.95 6.58 8.21 9.82 11.4 13.0 14.5 16.1 17.6 19.1 20.5 22.0 23.4
150 3.08 4.62 6.15 7.67 9.18 10.6 12.1 13.6 15.0 16.5 17.9 19.3 20.6 22.0
160 2.89 4.33 5.77 7.20 8.62 10.0 11.4 12.8 14.1 15.5 16.8 18.1 19.4 20.7
170 2.72 4.08 5.43 6.78 8.12 9.45 10.7 12.0 13.3 14.6 15.9 17.1 18.4 19.6
180 2.57 3.85 5.13 6.40 7.67 8.93 10.1 11.4 12.6 13.8 15.0 16.2 17.4 18.6
190 2.43 3.65 4.86 6.07 7.27 8.47 9.66 10.8 12.0 13.1 14.3 15.4 16.5 17.7
200 2.31 3.47 4.62 5.77 6.91 8.05 9.18 10.3 11.4 12.5 13.6 14.7 15.8 16.8
210 2.20 3.30 4.40 5.49 6.58 7.67 8.75 9.82 10.8 11.9 13.0 14.0 15.0 16.1
220 2.10 3.15 4.20 5.24 6.29 7.33 8.36 9.39 10.4 11.4 12.4 13.4 14.4 15.4
230 2.01 3.01 4.02 5.02 6.02 7.01 8.00 8.98 9.96 10.9 11.9 12.8 13.8 14.7
240 1.92 2.89 3.85 4.81 5.77 6.72 7.67 8.62 9.56 10.4 11.4 12.3 13.2 14.1
250 1.85 2.77 3.70 4.62 5.54 6.45 7.37 8.28 9.18 10.0 10.9 11.8 12.7 13.6
260 1.78 2.67 3.55 4.44 5.33 6.21 7.09 7.96 8.83 9.70 10.5 11.4 12.2 13.1
270 1.71 2.57 3.42 4.28 5.13 5.98 6.83 7.67 8.51 9.35 10.1 11.0 11.8 12.6
280 1.65 2.48 3.30 4.12 4.95 5.77 6.58 7.40 8.21 9.02 9.82 10.6 11.4 12.2
290 1.59 2.39 3.19 3.98 4.78 5.57 6.36 7.15 7.93 8.71 9.49 10.2 11.0 11.8
300 1.54 2.31 3.08 3.85 4.62 5.38 6.15 6.91 7.67 8.43 9.18 9.93 10.6 11.4
310 1.49 2.24 2.98 3.73 4.47 5.21 5.95 6.69 7.43 8.16 8.89 9.62 10.3 11.0
320 1.44 2.17 2.89 3.61 4.33 5.05 5.77 6.48 7.20 7.91 8.62 9.32 10.0 10.7

Pitch should always climb from the root to the tip, with the rate of increase being less at the tip - even constant over the last 20% of the diameter. The quoted pitch should always be the 'peak' measurement, although to be more reliable and consistent quoting the pitch at 80% of diameter is the usual practice.

The pitch distribution described above is often described as 'progressive pitch', ie - the pitch progressively increases along the blade from root to tip. Some prop manufacturers quote pitch as say 6 - 10. This refers to a 6" pitch at the root, and 10" of pitch at the tip. This should simply be regarded as a 10" pitch propeller.

The often quoted alternative to progressive pitch is 'constant or helical' pitch, ie - the identical pitch measurement from root to tip. This type of prop tends to be extremely efficient .... but at only one rpm range.

The mathematical explanations to why progressive is best are very involved. Basically it involves the angle of attack that the airfoil is operating at, coupled with the need to slightly washout the root and tip to reduce drag.

The diagram below shows how most Bolly Props are pitched...... The diagram has 4 lines.

    1. Theoretical pitch distribution line
    1. Practical pitch distribution line ... this is what works better in practice (usually)
    1. Low noise pitch distribution line ... for props to generate less noise, the tips need to be ‘washed out’.
    1. Pylon line, for the best high speed performance a ‘pitched up’ tip is often best.

The lines are drawn separated for added clarity. They should all be identical at about 70% of radius.

SLIP

Is a word often used to describe the propellers lack of efficiency. To say a propeller has 15% slip is in fact saying a propeller is only 85% efficient at converting the pitch to forward motion.

It is also often described as the difference between the angle of the blade and the angle of the relative airflow (which is less than the angle of the blade).

EFFIcIENCY

Most Bolly Props are in the 90 to 95% efficiency at converting pitch to airspeed, ie - very efficient. Some props we have tested struggled to have 60% efficiency at converting pitch to airspeed.

NOTE... this assumes a correctly matched prop to airframe.

NOTE... the above is different to overall efficiency, which is the measurement of converting the engine energy into kinetic energy via the propeller. This measurement is considerable less than above.

PROPELLER NOISE

Over the last few years noise has become an important issue to many model clubs. The exhaust noises are fairly easy to reduce, leaving the propeller as the main source of noise. The fact remains, to convert engine power into propulsion must by its very nature be noisy, FACT = the more powerful the engine, the more noise it will produce.

There is a big difference between true and accurate noise levels as measured and the perceived noise of the average person. An outstanding example of this is the noise level produced by a model with a high rpm small engine, (.049) compared to a large scale model (60cc). To the ear both may sound equally noisy, but a noise meter will tell a different story .... the large model will be far more noisy, and if the noise is measured from a long distance away, the small model will hardly register at all.

The overall design and even the materials from which the propeller is constructed, also have a significant effect on noise. The softer materials such as nylon produce a ‘softer’ noise, but as they are more flexible, the flexing will also create extra noise. To make such a prop rigid will require a thicker blade (heavier and more expensive to produce), which in turn operate at lower rpm (due to absorbing more power), and reduce the propellers overall efficiency. As can be seen, designing for low noise as opposed to high efficiency is a compromise.

The most obvious factor in prop noise is tip speed and shape. Reducing tip speed and using a good tip shape is the most productive method of reducing noise. This will generally mean using a lower diameter higher pitch prop than before.

Tip shape

Propeller noise is predominantly a product of tip noise. The propeller tip is travelling much faster than any other part of the blade, speed = noise.. A high drag tip shape will always create more noise than a low drag shape, the tip shape will have an effect on the props overall efficiency

A well rounded or raked tip should always be used, never leave the tip square. The very end of the propeller should be rounded on all sides, not only the leading and trailing corners, as would a model wing tip - never leave it cut off square.

Although not necessarily the best shape a well rounded tip is easily reproducible and less prone to damage than some more exotic shapes.

By all means, use other shapes. A raked tip of between 20 and 35 degrees is often used successfully.

Important .... not only should a tip be rounded etc in plan view (as shown), it should also be rounded in side view, as if it were a well rounded wing tip i.e. the tip should be a fairly sharp edge as per the leading or trailing edge.

 

TIP SPEED Vs RPM & DIAMETER DATA CHART

The speed of sound is approximately 760mph.

A propeller should NEVER be used over 550mph without paying attention to tip shape, due to compressibility and shock waves as mach 1 is approached.

An optimum maximum tip speed for achieving a low noise is 400mph. There appears to be a marked increase in noise above this speed. The best example of this is R/C Aerobatics where low noise is an advantage. These models have developed a basic set of rpm vs. diameter equations, which are reasonably accurate to the below chart ... As can be seen, F3A models avoid going above 375mph tip speed.

15" diameter = 8,500 rpm

14" diameter = 9,000 rpm

13" diameter = 9,500 rpm

12" diameter = 10,000 rpm

RPM x 1000

Dia 5 6 7 8 9 10 11 12 13 14 15 16 17
6 89 107 125 143 161 178 196 214 232 250 268 286 303
7 104 125 146 167 187 208 229 250 271 292 312 333 354
8 119 143 167 190 214 238 262 286 309 333 357 381 405
9 134 161 187 214 241 268 295 321 348 375 402 428 455
10 149 178 208 238 268 297 327 357 387 416 446 476 506
11 164 196 229 262 295 327 360 393 425 458 491 524 556
12 178 214 250 286 321 357 393 428 464 500 535 571 607
13 193 232 271 309 348 387 425 464 503 541 580 619 657
14 208 250 292 333 375 416 458 500 541 583 625 666 708
15 223 268 312 357 402 446 491 535 580 625 669 714 759
16 238 286 333 381 428 476 524 571 619 666 714 762 809
17 253 303 354 405 455 506 556 607 657 708 759 809 860
18 268 321 375 428 482 535 589 643 696 750 803 857 910
19 283 339 396 452 509 565 622 678 735 791 848 904 961
20 297 357 416 476 535 595 654 714 773 833 892 952 1011
21 312 375 437 500 562 625 687 750 812 875 937 1000 1062
22 327 393 458 524 589 654 720 785 851 916 982 1047 1113
23 342 411 479 547 616 684 752 821 890 958 1026 1095 1163
24 357 428 500 571 643 714 785 857 928 1000 1071 1142 1214
25 372 446 521 595 669 744 818 892 967 1041 1116 1190 1264
26 387 464 541 619 696 773 851 928 1006 1083 1160 1238 1315
RPM

 
  18 19 20 21 22 23 24 25 26 27 28 29 30
6 321 339 357 375 393 411 428 446 464 482 500 518 535
7 375 396 416 437 458 479 5000 521 541 562 583 604 625
8 428 452 476 500 524 547 571 595 619 643 666 690 714
9 482 509 535 562 589 616 643 669 696 723 750 776 803
10 535 565 595 625 654 684 714 744 773 803 833 863 892
11 589 622 654 687 720 753 785 818 851 884 916 949 982
12 643 378 714 750 785 821 857 892 928 964 1000 1035 1071
13 696 735 773 812 851 890 928 967 1006 1044 1083 1122 1160
14 750 791 833 875 916 958 1000 1041 1083 1125 1166 1208 1249
15 803 848 892 937 982 1026 1071 1116 1160 1205 1249 1294 1339
16 857 904 952 1000 1047 1095 1142 1190 1238 1285 1333 1380 1428
17 910 961 1011 1062 1113 1163 1214 1264 1315 1366 1416 1467 1517
18 964 1017 1071 1125 1178 132 1285 1339 1392 1446 1499 1553 1606
19 1017 1074 1130 1187 1244 1300 1357 1413 1470 1526 1583 1639 1696
20 1071 1130 1190 1249 1309 1368 1428 1487 1547 1606 1666 1725 1785
21 1125 1187 1249 1312 1374 1437 1499 1562 1624 1687 1749 1812 1874
22 1178 1244 1309 1374 1440 1505 1571 1636 1702 1767 1833 1898 1963
23 1232 1300 1368 1437 1505 1574 1642 1711 1779 1847 1916 1984 2053
24 1285 1357 1428 1499 1571 1642 1714 1785 1856 1928 1999 2071 2142
25 1339 1413 1487 1562 1636 1711 1785 1859 1934 2008 2082 2157 2231
26 1392 1470 1547 1624 1702 1779 1856 1934 2011 2088 2166 2243 2320

SPINNERs


As can be seen from the diagrams, a large diameter spinner or fuselage doesn't have as large an effect on propeller efficiency as expected. Using an exposed propeller nut will reduce propeller performance, instead a short blunt spinner flaring into the fuselage should be used.

Many large scale models have a problem with large radial cowls. It is important to pay attention to the shape. Where possible, allowing air to flow through the cowl will help.

When designing a model pay close attention to the shape of the ‘front end’. The shape and drag of this area is very important for performance.