Consolidated PB2Y Coronado - Page 5 - RC Groups
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Mar 22, 2016, 10:57 PM
Park Jet Guru
jetset44's Avatar

Reducing prop resonance?


Does anyone have any experience or ideas about reducing prop resonance on models like this? You probably noticed in the video the wa-wa-wa-wa sound this model frequently makes, and it drives me crazy. It only does it at around 1/3 throttle, but unfortunately that's where this model typically likes to cruise. Low throttle and full throttle sound fine with no resonance, it's only at 1/3 throttle where this occurs.

I already tried re-balancing the props and strengthening the firewalls to make the structure stiffer, plus I tried reversing the rotation of the props. None of those really worked. When the resonance starts, you can actually see the prop tips vibrating back and forth about 1/4", so it's quite strong. Any ideas? I wonder if quad copters ever have to deal with this? I would expect some resonance in a multi-motor model, just not this much...
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Mar 22, 2016, 11:00 PM
Registered User
localfiend's Avatar
Are the props scale sized? I wonder if it would be an issue if you could go to a smaller size prop, which would leave more distance between them.

5x5 3 blade props would probably work for your motors, and they should come in both rotations.
Mar 22, 2016, 11:12 PM
Park Jet Guru
jetset44's Avatar
Yes, the current 6x4 props are scale diameter. I certainly could try smaller diameter 3-bladed props, though. Good idea!
Mar 22, 2016, 11:13 PM
Vacuum Tubes Forever BCT2/BCR3
Could your tip float be having the Coanda effect?

http://theoriesandideas.com/wp-conte...poonCoanda.gif

JohnB
Mar 23, 2016, 02:51 AM
Foam is where the heart is
brett.c's Avatar
Have you tried calibrating each ESC individually?
This can help a bit. I even set individual trims on my radio so that I could fine tune the motors. Didn't help much but did help with yaw issues.

I've built several twins and they all warble at lower throttle settings.
I often thought it was synchronisation problem.

Reminds me of when my grandfather told me he could always tell if they were German bombers overhead because they used unsynchronised engines which gave an uneven drone.

Now, if someone were to develop a synchronised multiple output ESC
Mar 23, 2016, 06:26 AM
Ken's CAD Models
dz1sfb's Avatar
Quote:
Originally Posted by jetset44
Does anyone have any experience or ideas about reducing prop resonance on models like this? You probably noticed in the video the wa-wa-wa-wa sound this model frequently makes, and it drives me crazy. It only does it at around 1/3 throttle, but unfortunately that's where this model typically likes to cruise. Low throttle and full throttle sound fine with no resonance, it's only at 1/3 throttle where this occurs.

I already tried re-balancing the props and strengthening the firewalls to make the structure stiffer, plus I tried reversing the rotation of the props. None of those really worked. When the resonance starts, you can actually see the prop tips vibrating back and forth about 1/4", so it's quite strong. Any ideas? I wonder if quad copters ever have to deal with this? I would expect some resonance in a multi-motor model, just not this much...
Steve,
I've had this issue with quadcopters as well. The solution has been more robust propellers. Some of the lighter hubbed props would vibrate terribly whenever they had to deal with gyroscopic precession. Some of the newer propellers (2 and 3 blade) run so smooth and quiet that I am totally happy with the experience and performance.

These are fabulous http://www.banggood.com/2-Pairs-Gemf...p-1022940.html

and these are junk http://www.banggood.com/2-Pairs-Gemf...p-1031383.html

Not sure what props you are running , but I've been very happy with APC brand as well. Very stiff props.

Hopefully it's as simple as that,
Ken
Latest blog entry: Mini H Quad Project
Mar 23, 2016, 06:29 AM
Ken's CAD Models
dz1sfb's Avatar
Quote:
Originally Posted by jiberk
Could your tip float be having the Coanda effect?

http://theoriesandideas.com/wp-conte...poonCoanda.gif

JohnB
I like that thought.

Ken
Latest blog entry: Mini H Quad Project
Mar 23, 2016, 08:28 AM
5,200 Led Lighted Bike-Bob P.
**neons**'s Avatar
Very nice Steve. It is a very light weight plane. Has many similarities to my Martin Mars model. Flies very responsive and looks nice taxiing. Happy flights.
**Neons** Bob
Mar 23, 2016, 10:32 AM
Short bursts, Don't waste ammo
RCAV8R1964's Avatar
Are the rpm's matched throughout the throttle curve? If not try picking one motor for the master and match the other motors to that one.
Last edited by RCAV8R1964; Mar 23, 2016 at 10:44 AM.
Mar 23, 2016, 10:15 PM
Park Jet Guru
jetset44's Avatar
Great comments!

jiberk - I don't normally think of Coanda effect on something like this, but I suppose it's possible. Something is sure causing the floats to sink in.

dz1fsb - I'm using APC 6x4s, which are pretty stiff propellers. What you're saying makes good sense, though, so trying an even beefier prop is on my list of ideas. As is trying 3 blade props (perhaps on just the inboards).

neons - GREAT looking model! I've never seen the Martin Mars in the 1943 baby blue Navy scheme before, I only recall the red and white water bombers.

brett c/RC8VR - Great thoughts. I'll check the RPMs and ESC settings to see how they much and can be adjusted.
Mar 24, 2016, 07:46 AM
Registered User
Mark 2's Avatar
Great design, I can't wait for the plans, as a flying boat lover this will be next on my list for sure.
For what it's worth, I have a couple of Parkzone Mosquitos, which have really flexible contra rotating props. One does this exact same thing at about 60% throttle, and it drives me nuts, the other, identical setup doesn't do it at all. I have tried recalibrating the ESCs with no effect, I think I will change one of the motors next to see if that's what is doing it.
Mar 24, 2016, 08:03 AM
Ken's CAD Models
dz1sfb's Avatar
Quote:
Originally Posted by jetset44
Great comments!

dz1fsb - I'm using APC 6x4s, which are pretty stiff propellers. What you're saying makes good sense, though, so trying an even beefier prop is on my list of ideas. As is trying 3 blade props (perhaps on just the inboards).
Knowing that you are running APCs already leads me to believe that the harmonic resonance you are getting is related more to the synchronization of the motors. One trick you might try is to isolate the source by unplugging say the outboard motors first and then vis-versa. You seem to have plenty of power to fly on two.

Ken
Latest blog entry: Mini H Quad Project
Mar 24, 2016, 11:07 AM
Short bursts, Don't waste ammo
RCAV8R1964's Avatar
Hobby King makes a Kv tester. I bet this would be a valuable tool in solving problems like this. I think I'm going to get one myself.
Mar 24, 2016, 02:15 PM
Foam is where the heart is
brett.c's Avatar
Amazing how much the full scale ones flex

Propeller resonance (5 min 14 sec)


Perhaps there is a harmonic interplay between motor pairs on each wing which induces propeller resonance.
Mar 25, 2016, 07:44 AM
Ken's CAD Models
dz1sfb's Avatar
Very interesting brett.c! It certainly underscores the need to inspect and maintain our propellers.

Ken
Latest blog entry: Mini H Quad Project


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