My prop is hitting the sound barrier... OR... - RC Groups
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Jul 10, 2011, 06:00 PM
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My prop is hitting the sound barrier... OR...

I recently installed my new 1500kv (Turnigy 2730-1500) motor on my Swift II. I'm using a 7x3.5 prop, GWS nylon, with the standard prop saver.

Anyway at half throttle it's fine. Maybe at 75% throttle there's a bit of noise (above expected noise.) But at 80%+ it's extremely loud!

The 1500kv motor with a 3S battery could be going 18,900 rpm. One rotation in 0.19 seconds. 3.5" radius, so it travels 2 * pi * 3.5 = ~22 inches every 0.19 seconds = 115 inches/second, or a little under 3 metres per second, which is way lower than the speed of sound ~330m/s.

So what's causing the massive increase in sound at high speeds?
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Jul 10, 2011, 09:04 PM
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Bruce Abbott's Avatar
Your calculations are wrong (0.19 seconds per revolution eqauls 316rpm) but you are correct that it's not breaking the sound barrier. The reason for the noise is that the prop is fluttering at high rpm.

You need to find a way to reduce rpm (run 2S Lipo or lower Kv motor) or use a stronger prop (eg. APC LP07030).
Jul 10, 2011, 09:57 PM
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Are you sure it's the prop and not a motor bearing?
Jul 11, 2011, 01:36 AM
Registered User
Originally Posted by Bruce Abbott
Your calculations are wrong (0.19 seconds per revolution eqauls 316rpm) but you are correct that it's not breaking the sound barrier. The reason for the noise is that the prop is fluttering at high rpm.

You need to find a way to reduce rpm (run 2S Lipo or lower Kv motor) or use a stronger prop (eg. APC LP07030).
Oops, never do math when tired.

That's 0.0019 per second or almost 300m/s which is close to sound... however I find it difficult to believe that the prop could reach that speed without destroying itself.

The simplest option for me is just to not use full power, I usually run at half power anyway.
Jul 11, 2011, 06:40 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Smaller diameter 3-blade prop? More ground clearance too

Vriendelijke groeten Ron
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Jul 11, 2011, 10:53 AM
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Doug Bartley's Avatar
WARNING!!! Very dangerous, even at half throttle your likely spinning these thin flexible props way beyond their designed capacities! Please look up the max prop speeds for GWS. These props where never designed for these rpms. Serious injuries can occur!! The noise is from the blades flexing, then they explode!
Use Apc, Aeronaut Cam Carbon, Graupner speed etc designed for these rpms.Even with these, very carefully check the balance. FWIW Doug B
Jul 11, 2011, 07:15 PM
jackerbes's Avatar
There are no "nylon GWS props" that I am aware of. I think they are all some unspecified plastic. This page seems to list most or all of them, I'm not sure it is put up by GWS or just someone selling their props:

And GWS does not publish any RPM limits for the propellers. Based on extensive testing it is generally accepted that the Direct Drive and Hyper Drive props be limited to 90,000 / diameter in inches, the RS (Reduction Series) or slow fly prop should be limited to 45,000 / diameter in inches.

You won't find that published or endorsed anywhere but some well respected prop and motor testers agree with the numbers.

Jul 28, 2011, 03:54 AM
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AirBornOne's Avatar
3.5" radius, so it travels 2 * pi * 3.5
I think it's more like 3.5 * 2 * pi or more directly D * pi.
I'll leave the rest to you.
And do watch out for the little pieces your prop turns into at those speeds.
The piece that blinds you can be to small to see with the naked eye.
Jul 28, 2011, 04:11 PM
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Bruce Abbott's Avatar
Here is a list of GWS props that I have tested. The EP6030 started to flutter at 16k, so I'm guessing the EP7035 should be OK up to 15k.

	     OK    flutter   flutter
   Prop      rpm     rpm     rpm*dia
  ======    =====   ======   =======
  EP2508    19650
  EP2510    35070
  EP3020    22130
  EP4025    34838                     disintegrated at 37000rpm!  
  EP4530    19930   21490     96705
  EP5030    19290
  EP5043    12660   16891     84455
  EP6030    13440   16060     96360   disintegrated at 17000rpm!
  EP6050     5190
  EP7035    12577
  EP7060     6678
  EP8040    13880
  HD8040x3  13280
  EP8043     7967
  EP8060     6986
  HD8060    11250
  EP9047     6607
  EP9050    11290
  HD9050x3  11387 
  EP9070     5681    6005     54045
  EP9070X3   6461
  EP1047     4405
  EP1060     9937
  EP1080     4602
  HD1080     8552
  EP1080X4   5843
  EP1147     4426
  EP1170     8618
  HD1260     9493
Jul 28, 2011, 06:46 PM
jackerbes's Avatar
Thanks for post that, Bruce. That is some great info to have, especially in view of the fact that GWS does not publish any recommendations on RPM limits.

It is a little intimidating to think about intentionally taking props up to the point of disintegration. I can handle the props coming apart, it is the horrible things that would take place with the motor being that badly out of balance that really intimidate me.

Great job! Thanks again!

Aug 12, 2011, 10:02 AM
Registered User
I use the same prop and have only had noise once.It was caused by the bell pulling away from the stator.

There was too much space between my e-clip and the spacer HK uses behind rear bearing.

I used a soldering iron and heated just the very tip of shaft,with prop-saver removed and take out the two allen screws that help lock in the shaft.

Heat will melt whatever HK uses as loc-tite on the shaft and you can easily move shaft and get rid of play.
Aug 13, 2011, 12:54 PM
Registered User
Did a search for the Swift II and nothing shows for it. Is the motor on a pylon or front with a spinner? Can you post a picture?
In eaither case, noise is not good.
I have modified my EDF jets with the BVM type air diverter. This gives a substantial increase in rpm and thrust. On the 4th flight of the modified Venom F-18 Hornet, I heard a raspy POP and then silence. Made a good glide and landing. This picture is looking into the inlet at the diverter (white on the right side) and the impeller. The impeller (hub and blades) disinigrated and blew out the exhaust. I exceeded the burst strength of the plastic at that higher rpm.

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