Propeller Selection Guide
Please see Chart A and Chart B when referring to the following specifications.
Propeller selection assistance is available from your local sales representative or by contacting us.
General Information
Normally, the selection of the optimum propeller for your ultralight - and predicting propeller performance - would be a difficult and sometimes exasperating experience. The Ultra-Prop makes this task easy.
The data presented on the two charts were obtained on a dynamometer with props operating under very similar conditions to those you experience. That is, the ambient temperatures were 80-90 degrees F and the dynamometer presented a forebody similar to most ultralights.
The horsepower (HP) levels on the charts are Propeller Shaft HP. Not Rated Engine HP. Temperature, engine condition, reduction unit, intake and exhaust system differences can make your installed HP more or less than the engine manufacturers' rated HP. Don't be surprised if your static thrust is different from predicted.
Note that we use the term "pitch blocks" or "pitch angle" as opposed to the conventional pitch or advance in inches. This series of Ultra-Props has no twist in the blade, so our designation is appropriate.
Ultra-Props are carefully engineered and tested for good performance and high structural integrity. Your particular ultralight, however, may have different vibration characteristics than those we have experienced - that is, your engine, reduction unit, extension shafts and inflow may present different vibration inputs to the propeller. When you install your Ultra-Prop, assure yourself that the installation is at least as smooth as that you have had with a balanced wooden prop. In most cases the Ultra-Prop will be significantly smoother. If not, carefully check your power system installation. If nothing is amiss, contact us for engineering assistance.
If a loud "buzzing" noise is noted at high RPM with high-pitch blocks, it is because the tips are stalling. Do not operate the propeller in this range. Either a lower pitch or smaller dia. prop must be used.
Tests have shown that at and below 52-in. dia., no blade tip stalling will occur with 18-degree pitch blocks below the rated 3100 RPM.
Between 52" dia. and the full 59" dia., the approximate tip stalling RPM or onset of buzzing is shown in the table below for various pitches.
| Diameter | 18 deg pitch | 16 deg pitch | 14 deg pitch |
|---|---|---|---|
| 59" | 1900 RPM | 2400 RPM | 2700 RPM |
| 58" | 2050 | 2450 | 2700 |
| 57" | 2100 | 2700 | 3100 |
| 56" | 2250 | 3100 | |
| 55" | 2400 | ||
| 54" | 2550 | ||
| 53" | 2800 | ||
| 52" | 3100 |
Sample Calculation Using Charts
Use these charts to find the number of Ultra-Prop blades and their angle of pitch to match your specific ultralight.
To begin, you must know your propeller RPM, your engine HP and the diameter of your present prop or diameter that will match your airframe structural limit.
The following typical data is used in our calculation example:
Engine: Rotax 277, 25 H.P. @ 6200 RPM
Reduction Unit: Integral Rotax gear box 2.58:1
Prop RPM: 6200/2.58 = 2400 RPM
Prop dia.: 56 in. (assumed airframe structural limit)
First, we solve for a 59" dia. Ultra-Prop (See Chart A). We find that the 2400 RPM line intersects with a 25 HP curve in the 2-blade, 3-blade, and 4-blade envelopes. Reading the locations of these 3 points (circled) on the pitch degree lines identifies three 59" dia. Ultra-Prop options as 14 degree 2-blade, 11 degree 3-blade, or 9 degree 4-blade. To solve for a 52" dia. Ultra-Prop (See Chart B) we find that the 2400 RPM and 25 HP intersections identify only two different options: 18 degree 3-blade, or 13 degree 4-blade.
Since our example requires solving for a 56" dia., we must interpolate between the 52" and the 59 " dia. data we read from the two charts. A table such as the following is helpful:
| 2-Blade | 3-Blade | 4-Blade | |
|---|---|---|---|
| Prop Dia. (") | 52 56 59 | 52 56 59 | 52 56 59 |
| Pitch (degrees) | 14 | 18 (14) 11 | 13 (11) 9 |
| Thrust (lb.) | 158 | 163 (179) 182 | 170 (180) 188 |
To complete the table for 56" dia. (that is, to insert the numbers in parentheses), we note that this diameter is 56-52/59-52 = 4/7 = .57 of the way up from 52" dia. The pitch angle for 56" dia. can be approximated as follows:
For the 3-blade prop:
Incremental pitch angle = .57 (18 deg. - 11 deg.)
= .57 x 7 deg. =4 deg.
Pitch at 52" = 18 deg. minus incremental pitch decrease of 4 degrees equals 14 degrees.
We must subtract the 4 degrees from 18 degrees since we are going up 4/7 to 59". The 4-blade prop pitch for 56" dia. is computed the same way.
Completing the table for a 2-blade prop is not practical, since all required information is not available from the charts. We know that the pitch required for a 56" dia., 2-blade prop would be higher than the 14 deg. we saw for a 59" dia. 2-blade prop. Looking at the table for tip stalling, we see that at 16 deg., 59" dia. and 2400 RPM we would already be at critical stall angle.
To complete the table for estimated static thrust, the 3-blade calculations are as follows:
Incremental thrust
Increase = .57 x (182 lb. - 163 lb.)
= .57 x 29 lb.
= 16 lb.
Thrust at 52" = 163 lb. minus the Increm. Increase = +16 lb.
Thrust at 56" = 179 lb.
Estimated thrust for the 4-blade prop is calculated the same way.
Which prop should we choose? The 3-blade prop should be used since it is the lightest. Also, its odd number of blades makes it the smoothest running. If the 4-blade is selected, the thrust difference in this case is insignificant.