sonar sensors affected by propeller background noise

Hi all,
I have a small RC airplane (fixed-wing) that is capable of hovering (i.e. the fuselage is vertical and the propeller balances the weight of
the aircraft). I mounted an ultrasonic sensor on the tail (pointing down towards the ground when hovering) and used it to successfully achieve altitude hold during a hover. See the link below if help is needed visualizing:
http://prism2.mem.drexel.edu/~billgreen/Videos/3D_flying/fullAutonomousHover/fullAutonomousHover.wmv
Now I would also like to maintain a constant distance from a wall on the left side of the aircraft (still while hovering). When I mount another sonar sensor about half way down the fuselage pointing outward in the direction of the wing, it is affected by the noise of the propeller. Moving it further down the fuselage towards the tail doesn't seem to improve it at all. Just wondering if the orientation of the sensor has anything to do with its vulnerability to the background noise created by the propeller? That is, the axis of the sonar sensor on the tail is parallel with the longitudinal axis of the plane while the axis of the sonar sensor on the fuselage is perpendicular to the longitudinal axis of the plane (parallel to the wing).
I can't change the orientation of this sensor, so I was wondering if anyone had any suggestions? Some way to muffle the background noise created by the propeller?
Thanks, -weg
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snipped-for-privacy@drexel.edu wrote:

http://prism2.mem.drexel.edu/~billgreen/Videos/3D_flying/fullAutonomousHover/fullAutonomousHover.wmv
Looks like a fun project.

I'd guess the airflow from the propeller is bending the sonar beam as its emitted; this doesn't affect the tail sensor since its beam is parallel to the flow.
Could you mount the new sensor out near a wingtip? Any attempt to shield the fuselage-mounted sensor will result in drag...
- Daniel
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snipped-for-privacy@drexel.edu wrote:

Very cool. How does it regulate spinning about the vertical when in the hover? Does it use accelerometers?

Well, just a thought. Maybe it's not just the background noise in general, but a direct reflection off the wall from the prop to the sensor? You could try blocking that direct path by either putting a colling around the prop to make it a ducted fan or just adding a surface to the wing that extended out in front of the wing to the side of the prop. To test the idea, I guess you could hold a sheet of something to the side of the prob (above the wing) as it hovered to block the path.
I would assume the noise was creating problems because one of the harmonics was in the frequency range of the sonar. Maybe a different prop will allow it to run at a different speed which might move the harmonics out of the range of the sonar. A larger prop that spins slower I would think would help to reduce the harmonics at the high frequency used by the sonar (40 KH?)
Or maybe the noise is coming more from the motor than the prop??? Or maybe the vibration of the body itself is producing the noise and the fact that you seem to have a body which is just a vertical surface causes it to radiate out maximal noise to the sides? A different engine mount might help to isolate the motor vibrations from the body and stop the body form acting like a speaker cone (if that is what is happening?). Or some shielding to keep the noise radiating from the motor might help?
I also wonder if there might be vibrations traveling through the body of the plane that are being picked up by the sensor. Better vibration isolation mounting of the sensor might solve that if that is the case.
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Curt Welch http://CurtWelch.Com /
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I mounted two miniature DC motors on each wingtip, facing in opposite directions. Thus, when both motors are activated, it creates a rotational force which counters the torque roll effect. I measure the roll angle of the aircraft and implement PD control to generate the control signal to the motors.
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snipped-for-privacy@drexel.edu wrote:

Ah, I didn't spot them in the video. I assumed you must have been using either special close in ailerons to create the torque (since normal placement of ailerons on the tips would do no good in the hover) or special independent controlled elevators.

What type of sensor do you use to measure the roll angle?
BTW, I like the answer from the other poster better than the answers I thought up for your question - that the air rushing past the sensor is the cause of the problem.
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Hi Curt,
I'm using a 6 DOF IMU from Microstrain.
-weg
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