Model Turbines

[Turbine GuyParticipant @turbineguy]

I thought that by reducing the height and thickness of the blades and the blade spacing I could improve the performance of Radial Turbine 2. The following drawing Radial Turbine 2 R3 shows the dimensions and placement of the new rotor I call Radial Rotor 2D. The second drawing is Radial Turbine 2 R2 that I show for comparison. The changes I made to revision 2 were as follows. The blade height was reduced from 0.199” to 0.107”. The blade thickness was reduced from 0.039” to 0.012”. The blade spacing ras reduced from 0.197 to 0.124.

I thought that the large blade height of most radial rotors was the primary reason for their high windage losses and cutting the blade height almost in half would give a significant reduction in the total rotational loss. The actual reduction in total rotational loss was 0.08 watts. I thought that reducing the blade thickness and blade spacing would give a significant increase in the rotor velocity coefficient. The rotor velocity coefficient actually decreased from 0.39 to 0.37. I noticed that the dental ball bearings were making a rubbing noise they did not have before, so I ordered some new dental ball bearings. I will retest Radial Turbine 2 R3 when the new ball bearings arrive and see if they were the reason for the disappointing results.

[Turbine GuyParticipant @turbineguy]

I received the new dental ball bearings I said in the last post that I was going to try. This was to see if the old bearings were partly responsible for the high rotational losses. The new dental ball bearings did not reduce the rotational losses so I thought increasing the close clearance between the housing and the rotor OD discussed in the 10 December 2025 post might help. I 3D printed the rotor I call Radial Rotor 2E and used it in Radial Turbine 2 R4 shown in the following drawing. This rotor had a gap/rotor radius ratio of 0.02 that gave the best performance for the rotors tested in the report discussed in the 10 December 2025 post. This turbine was identical to Radial Turbine 2 R3 shown in the last post except for the new rotor with a larger gap on the OD. This change actually increased the total rotational loss by 0.4 watts. Apparently a small gap reduces the gas circulation in the blades enough to offset the increase in friction. I thought that increasing the gap on the rotor OD would give a significant decrease in the rotor velocity coefficient due to leakage. The rotor velocity coefficient actually decreased from 0.37 to 0.09. This test confirmed what I have found for all the other types of rotors I have tested that the gap between the nozzle and the blades needs to be kept as small as possible.

[Turbine GuyParticipant @turbineguy]

I thought the large rotational losses of the radial turbines might be partially due to the supersonic velocities resulting from the high inlet pressures on Radial Turbines 1 & 2 and from imbalance of the rotors. I 3D printed the rotor I call Radial Rotor 3 and used it in Radial Turbine 3 shown in the following drawing. This rotor was balanced and required an inlet pressure low enough to keep the flow sonic or below. This did increase the efficiency as shown in the 5/18/2026 test in the following update of my turbine performance spreadsheet.

[Turbine GuyParticipant @turbineguy]

I decided to run Axial Turbine 5 R1 (AT5 R1) with a gearbox. I didn’t see an easy way to attach to the speed reducer that came with Radial Turbine 1 (RT1) so I thought I would try using one I purchased about 25 years ago and never used. This speed reducer is an Electrify GD-600 Gear Drive shown in the following photo attached to AT5 R1 using a 3D printed adapter . I wasn’t sure how good the ball bearings would be after sitting this long, but I thought I would go ahead and try using it. I ran the gearbox with the ball bearings and 18 tooth pinion that it came with that gave a 2.5:1 ratio. I also ran it with a 12 tooth pinion that gave a 3.75:1 ratio. The results I got were not very good so I ordered a new set of ball bearings and a 9 tooth pinion that gives a 5:1 ratio used in the following photo. The following spreadsheet shows the performance with the new ball bearings and the 5:1 ratio. These results were a lot better but not as good as expected. The second spreadsheet is the test result of Radial Turbine 1 with the 5.5:1 speed reducer discussed in the 23 January 2023 post on page 18 of this thread. According to what I have found for model boats in the size range of these model turbines, the maximum power should be at a speed of around 2,000 rpm. My testing of model steam engines on air and with my smallest boiler got the best efficiency at speeds of 2,300 rpm for air and 2,400 rpm for steam at a pressure of 25 psig. The output powers for these speeds were 1.5 watts and 1.7 watts respectively.  The output power for RT1 at 2,200 rpm was 1.4 watts and the output power of AT5 R1 at 2,300 was 1.7 watts.  Even though the turbine efficiency of AT5 R1 is considerably higher than turbine RT1 the outputs at these speeds was about the same and matched what the best of my model steam engines.

[Turbine GuyParticipant @turbineguy]

I will try to show why I expected better gearbox efficiency in the tests for the ElectriFly gearbox shown in the last post. In the 6/23/2026 tests the power output for 10.5 psig was 2.8 watts with the gears and 4.2 watts without the gears so the gearbox efficiency was approximately 67%. In the 1/22/2023 tests for Radial Turbine the power output for 25.0 psig was 1.7 watts with the APC 6×4 EP propeller and 2.2 watts without the gears so the gearbox efficiency was approximately 77%. The 77% efficiency of the gears that came with Radial Turbine 1 that used metal bushings instead of ball bearings seemed reasonable. I expected a measurable increase in efficiency with the use of new ball bearings and low viscosity oil in the ElectriFly gear drive instead of the approximately 10% decrease in efficiency. I tried everything I could think of to find the problem with this drop in efficiency but the performance in the spreadsheet of the last post was the best I was able to get. Another difference between the gearboxes is the plastic main gear used by ElectriFly and metal main gear used by the gearbox that came with Radial Turbine 1. Some sources say the metal gears are more efficient due to the flexing of the plastic gear teeth. Most sources say the plastic gears are fine for low power but don’t define how low the contact force has to be.

Pololu tests their gearmotors and publishes the results like the following one for a 15:1 ratio gearbox. This example shows a maximum overall efficiency of 31%, a power output of 1.5 watts, a torque of 1.5 kg-mm (2.08 in- oz), an output speed of 990 rpm, 6 volts, and 0.83 amps. The input power is approximately 5.0 watts. Motors like used for this gearmotor have efficiencies below 70% so assuming this efficiency is probably a little high, but should give a fair approximation of 3.5 watts power going into the gearbox. The overall efficiency of the gearbox is approximately 43% and it has 3 stages so the stage efficiency would be approximately 75% similar to what I found for Radial Turbine 1’s gearbox. The Pololu gearbox has metal gears with the gear shafts running without bushings in the brass support plates like shown in the following photo. This is typical of many of the gearboxes used in the micro gearmotors and also is using 3 or more stages.

[Turbine GuyParticipant @turbineguy]

I thought I would see if I could connect Axial Turbine 5 R1 (AT5 R1) to the gearbox of the Pololu 15:1 ratio gearmotor discussed in the last post. I did not realize how tiny this gearmotor is as shown in the following photo next to AT5 R1 mounted on the ElectriFly speed reducer. It is also next to the 12 tooth pinion used in the ElectriFly speed reducer to illustrate how small it actually is. The propeller is the APC 8×6 EP used in some of the tests shown in the next to last post and is the size used to test my best small model steam engine that was also mentioned in the next to last post. If the test results given by Pololu shown in the last post are correct this tiny gearmotor would have enough torque to turn this big of a propeller over 2,000 rpm to the speeds mentioned in the next to last post but would not be able to have the 8” diameter propeller mounted directly on its shaft. The small bushing that supports the output shaft of the Pololu speed reducer would not be able to keep the propeller from wobbling. It is remarkable how much torque this tiny gearmotor can produce and how tiny the gears in it are. Apparently this is typical of the micro gearmotors that are readily available at very low cost.

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