I have just purchased 4 new Graupner 10x5 Propellers to replace the existing APC 10x4.5 slow fly props on my TBS Style Quad as I was not happy with the vibration levels.
Thought I would share the results.

Vibration levels for APC Props

Vibration levels for Graupner Props

I had spent a lot of time trying to balance the APC props but could never get much improvement. I just balanced Graupner props very quickly and got the above result

David Ardis

Views: 2073

Comment by Gary McCray on November 28, 2013 at 4:53pm

Hi David,

It does look like you have a small improvement, but since the 2 charts are shown at different vertical resolutions they are a bit misleading,

You need to show the second chart such that the numbers on the left exactly correspond to the number on the top chart.

Taking that into account it does not appear that the Graupners are very much better, although both charts look like you are in the range of + and - 1 or 1/10G which normally is a very acceptable vibration range.

Best Regards,

Gary

Comment by Rob_Lefebvre on November 28, 2013 at 7:03pm

Interesting.  We should do more testing like this.  I would believe that the SF props cause a lot of vibration.  They are quite flexible.  I've been doing some testing which involved strapping my quad to the bench and running it with props on, and it's really surprising how much you can see the prop disk wandering around, even at less than half throttle.

I've purchased some of the new APC MR props, and they are quite a bit stiffer, I bet they create less vibration.

However, that's not the full story. In fast forward flight, stiff propeller will probably cause MORE vibration. This is because of disymmetry of lift across the prop disk, due to the parallel component of the oblique flow across the disk.  The advancing blade has a higher effective Angle of Attack than the retreating blade, which causes a torque.  The torque comes and goes as the prop aligns with the incoming airflow.  This results in a torsional vibration.  

Helicopters have teetering or hinged rotor heads to alleviate this, because they allow the blades to "flap".  However, rigid blades used on multirotors do not allow this, thus the vibration.  The more flexible SF props may in fact result in less vibration in fast forward flight, because they can flex to allow a bit of flapping.

Comment by shanthosh ravikumar on November 28, 2013 at 9:15pm

may i know what setup did you use to measure the vibration ??

Comment by David Ardis on November 29, 2013 at 4:02am

Gary. You are are right but I was not expecting that much change between two sets of balanced props.

Thomas. Unfortunately I cant get APC Multi rotor props the postage is more than the props.

Shanthosh. I was measuring the vibration in flight using the APM 2.6 all test where done indoors all that changed was the props.

May do a few full power runs with each set of props and post the results tomorrow.


Developer
Comment by John Arne Birkeland on November 29, 2013 at 4:34am

Slow fly propellers where never meant for this. They are made for low rpm's on light weight planes, not high rpm lifting like we are doing.

Comment by Rob_Lefebvre on November 29, 2013 at 4:46am

Yeah, it's a miracle these the APC SF props work as well as they do. And indication that APC is a high quality product that is overdesigned.  I've been using them on a medium-lift octo with a fair amount of power.  But I'm switching over to APC MR's.

Comment by Oliver on November 29, 2013 at 7:44am

Robert: That's an interesting point about vibration in fast forward flight, and is of course a well-known factor in trad-helis where a common solution is to hinge the individual blades so they can "lead and lag" in the plane of the rotor disk (my understanding is that the failure to discover that was a big reason why helicopter development stalled for decades in first half of the 20th century). But this "flapping" is not up and down, as a soft blade might do but rather fore-and-aft (as it were) in which direction even soft blades are pretty stiff. It would be interesting to see comparison vibe logs....

I would think that soft prop blades would potentially introduce vibrations just from bending back and forth with load changes while maneuvering, especially on a heavy lift machine. People seem to forget that the rotor blades literally support the entire weight of the aircraft, including big inertial loads induced by asymmetrical commands.

A lot of props used for multis were not designed for loads like that, nor are they optimized for producing static thrust. Gary M. & I have been grumbling about that for a long time, but it seems that nobody in the hobby is set up to really investigate what's what with props, let alone build any, and the available information all seems pretty much anecdotal. I guess there's not a whole lot of incentive (yet) for a manufacturer to get into this, it's probably cost intensive with not much potential return.

For the record I'm flying the biggest really stiff carbon-filled balanced Graupners that will fit on my distinctly pudgy hex and they seem vibration-free (but my idea of fast forward flight with this truck is maybe10 mph, LOL).

 


MR60
Comment by Hugues on November 29, 2013 at 9:01am
Having tested different brands of props at different sizes&pitch, the conclusion was you can't draw a general conclusion on a brand better than another. In the same brand some prop size are really good while other sizes are really bad (in terms of vibrations).
Personally the best i have experienced in 14-15" size are the wood XOAR props because they are very light, rigid and produce two times less vibrations than the APC of the same size.
Comment by Rob_Lefebvre on November 29, 2013 at 9:04am

Oliver, lead/lag is a different thing.  Blade flapping really is up and down, and is taken care of by either the teetering hinge at the center of the head, or the flapping hinges at the root of the blade.  Flapping and Lead/Lag do get intermingled in some weird ways that I haven't actually understood yet.  It makes a difference if the lead/lag hinge is inboard or outboard of the flapping hinge.  

In forward flight, the entire rotor disk actually tilts over, this is due to the flapping to relieve disymmetry of lift.

Comment by Oliver on November 29, 2013 at 9:38am

Robert: Right, I see what you're saying, this flexing is allowed on most R/C helis by the feathering shaft being  held in o-ring-like mounts, the stiffness of which have a lot to do with the handling character of the aircaft. Some effectively similar but simpler mount could be contrived for fixed pitch multicopter blades but I wonder if it would be worth any decreased vibration and/or increased efficiency. The idea of accomplishing it with the flex being in the blades themselves is pretty spooky unless the prop is designed for that, and then there would maybe be harmonic vibration issues and control mushiness... sort of like automotive suspension via chassis-flex, if you know what I mean.

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