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.

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