During a hot vibration discussion, one respondent posted about overlapping props. If I remember right, for one particular ship, he felt that the optimal overlap was about 30% for an octa H.
Previously I had only tested up to a 5% overlap, assuming that more would be bad. So much for assumptions. The data is shown below. The data is the result of taking:
- over 400 data points
- using a T test stand where four motors/props were mounted on top spaced vertically at 0, +16mm, 0, and +16mm
- horizontal overlap amount was variable for each series of tests from 0% to 55% relative to prop radius.
- props tested were 11x4.7 fiber reinforced plastic slow fly.
- prop rate was 0% (mid throttle) generating about 960 grams of lift.
- motors were 750 KV
- the vibration sensor was located at the top of the neck of the T oriented to measure x/y
- the dB meter was located 1 m distance from motors
Test equipment was:
- APM 2.5 controller
- gram scale
- T test stand
- Mastech HY1530EX DC Power Supply
- Radio Shack Sound Level Meter
- Extech SDL800 Vibration Meter
showing the two left motors and scales in % overlap
showing the four motors on top of the T test stand
The results were:
Note the following for this particular setup:
- Interestingly, thrust (the blue line) might increase for low overlaps. Need more samples to confirm or reject this possibility.
- Frame vibration (the red line) might not increase much at all for low overlaps, but does increase dramatically after a point (about 12% ish overlap)
- dB (noise level) increases dramatically right away.
- Not shown on the data was that I had planned to test at even greater overlaps. But, I started to hear what might have been occasional sonic pops by turbulence breaking the sound barrier (or some other phenomenon). This caused me to also test for vibration and dB.
Limitations of Test: I'm going to try alternative custom frames to the V using low KV motors. When the new test props arrive, I'll test the lot and select the optimal one for the most efficient low KV motor. Then I'll retest overlap using that setup with multiple vertical options also. Until we see at least one more setup, caution is advised on accepting the results here as applying to more than this particular setup. Only shown are relative changes, not the actual numbers. The actual numbers (grams thrust, dB, and frame g forces) are impacted by this setup (1" bar, mass of the T-frame, etc.). In layman terms, the slope directions can be generalized within this context, but not their magnitudes.
Of Interest to Vibration: With overlapping props, optimal is based on the criteria of the pilot. So in that mix, consider that sound pressure on an exposed APM will go up 30% ish at the frequency of rotation, about 200 Hz, when props are greatly overlapped.
Sound pressure is more likely to impact APMs that are in free space (mounted on moon gel or double sided foam). On the other hand, the forces from frame vibrations also go up with overlap. If frame forces are too high, then some decoupling of the APM from the motors/frame might be required if free access to more mass isn't available. This might explain the alt-hold fly-away experienced by the pilot that posted a reference to that event. This can also explain why with stiff frames, the least vibration occurs when the APM is hard mounted to the frame with the addition of moon gel or double sided tape worsening the condition.
For an APM adequatley shielded by stiffness and mass, to unnecessarily expose that APM to 60 to 90 dB sound waves by putting the APM on pliable material could be as harmful as not supporting the APM with pliable material when the frame has high vibrations that are not mitigated by stiffness and mass. It is up to the pilot (or commercial enterprise that made the copter) to make that trade for the safety of the ship, pilot, bystanders, and property.
Conclusion: Some gains in reduced frame size and efficiency might be achievable by a stiff frame with small prop overlap when the APM is hard mounted to frame/mass (so as not to be affected by the increased forces from air pressure). But more sampling and testing will be required before this hypothesis can be accepted with reasonable certainty.
More testing will follow to answer some of those questions and others.