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blade harmonics

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tc1917(at)bellsouth.net
Guest

Posted: Wed Apr 08, 2009 2:55 am Post subject: blade harmonics

I still have the problem of harmonics with my blades on my slingshot.  I 

have narrowed it back down to the blades.  even though they have been sent 

back twice and super balanced by myself and lazer set, it is still there, 

buzz buzz buzz.  drives you nuts.  put a good anr set in my helmet and that 

makes it tolerable.  friend has a new set of wd blades down in florida. 

gotta get down there and put them on my plane.  if it makes it go away I am 

going after wd for new blades big time.  this noise is awsome and sounds 

like I am flying a five o'clock charlie in korea.  good luck.  on two of my 

blades you can see the cloth and one has a smaller tip.  told wd this and 

they didnt do anything but grind them down further.  gotta prove it first. 

will think twice about my next set of blades.  ted cowan, alabama, slingshot 

912.  p.s. if it werent for this harmonic problem I think I have the perfect 

flying machine, zoom zoom.  at least for me.

jlsk1(at)frontiernet.net
Guest

Posted: Wed Apr 08, 2009 4:04 am Post subject: blade harmonics

Hey Ted, I just started flying my MK-3C with a 912 with the Titan exhaust,(& 

WD Prop) & have a similar harmonics. Now don`t get me wrong, mine isn`t as 

pronounced as yours, & I`ve only flown it 2.2 hours with the 912, but here`s 

something I`m gonna try, unless you already have & proven it doesn`t work.

When I look at the exhaust/prop view from the side, my exhaust is hitting 

the blades a few inches up the blade in the flatter area of the prop. My 

exhaust goes straight up now, but if I rotate it 90 degrees either way, it 

looks like it will hit the prop more in the "hub -to -blade transition area, 

in the round part of the blade just as it exits the hub. It will be another 

week or 2 before I get to try this, So if you already have, or do soon, let 

me know what happened. Also, mine isn`t so bad that a tweak of the throttle 

lessens it & If it doesnt` go away, I`ll probably live with it.

None on my friends have mentioned it, & I have`nt either, on purpose, to see 

if anyone else notices.

Thanks,

Jim Kmet

Cookeville, TN

MK-3C 912

MK-3C 582



---

jbhart(at)onlyinternet.ne
Guest

Posted: Wed Apr 08, 2009 6:44 am Post subject: blade harmonics

At 05:53 AM 4/8/09 -0500, you wrote:

 	  Quote: 	
		  




	


Ted,



Have you considered trying a lower inertia propeller?



Jack B. Hart FF004

Winchester, IN

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Wed Apr 08, 2009 7:22 am Post subject: Re: blade harmonics

 	  tc1917(at)bellsouth.net wrote: 	
		  I still have the problem of harmonics with my blades on my slingshot.  I 

have narrowed it back down to the blades.  even though they have been sent 

back twice and super balanced by myself and lazer set, it is still there, 

buzz buzz buzz.  drives you nuts.  put a good anr set in my helmet and that 

makes it tolerable.  friend has a new set of wd blades down in florida. 

gotta get down there and put them on my plane.  if it makes it go away I am 

going after wd for new blades big time.  this noise is awsome and sounds 

like I am flying a five o'clock charlie in korea.  good luck.  on two of my 

blades you can see the cloth and one has a smaller tip.  told wd this and 

they didnt do anything but grind them down further.  gotta prove it first. 

will think twice about my next set of blades.  ted cowan, alabama, slingshot 

912.  p.s. if it werent for this harmonic problem I think I have the perfect 

flying machine, zoom zoom.  at least for me. 	




Well I think I've found a possible culprit on my setup. There's a resonance between the engine/mount and the frequency at which the blades vibrate fore and aft. I discovered this yesterday by accident when I happened to tap one of the blades, the entire top of the plane went "thoom" like a drum in concert with the blade. And it sounds like that frequency is the frequency I'm hearing in the cab....



Also yesterday I went over the whole butt end of the plane to make sure nothing was loose or any problems with the mounts. No problemo.



In my case it's clearly not a problem with the prop itself - it's just an interaction between it and this particular plane.



The only option with the WD looks like changing the diameter and I've BTDT on a couple other planes (that means a few different sets of blades that all still do the same thing). Going down in diameter is not acceptable as I need the climb performance. And I can only go up another inch before I run out of safe clearance, doubtful an inch is going to do it.



So it's time to get a little busier on this - next idea is a different prop type. That's the only way I can see right now to change the blade resonance enough, get the whole fundamental changed back there.



LS

_________________
LS
Titan II SS

aslsa.rng(at)gmail.com
Guest

Posted: Wed Apr 08, 2009 7:23 am Post subject: blade harmonics

Ted, Have you considered getting a prop spacer and gaining a little space between the exhaust pipe opening and the prop blades?



Rick



On Wed, Apr 8, 2009 at 5:53 AM, Ted Cowan <tc1917(at)bellsouth.net (tc1917(at)bellsouth.net)> wrote:

 [quote]--> Kolb-List message posted by: "Ted Cowan" <tc1917(at)bellsouth.net (tc1917(at)bellsouth.net)>

  

 I still have the problem of harmonics with my blades on my slingshot.  I have narrowed it back down to the blades.  even though they have been sent back twice and super balanced by myself and lazer set, it is still there, buzz buzz buzz.  drives you nuts.  put a good anr set in my helmet and that makes it tolerable.  friend has a new set of wd blades down in florida. gotta get down there and put them on my plane.  if it makes it go away I am going after wd for new blades big time.  this noise is awsome and sounds like I am flying a five o'clock charlie in korea.  good luck.  on two of my blades you can see the cloth and one has a smaller tip.  told wd this and they didnt do anything but grind them down further.  gotta prove it first. will think twice about my next set of blades.  ted cowan, alabama, slingshot 912.  p.s. if it werent for this harmonic problem I think I have the perfect flying machine, zoom zoom.  at least for me. 

  

 

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Dana

Joined: 13 Dec 2007
Posts: 1047
Location: Connecticut, USA

Posted: Wed Apr 08, 2009 8:40 am Post subject: blade harmonics

At 11:22 AM 4/8/2009, lucien wrote:



 	  Quote: 	
		  Well I think I've found a possible culprit on my setup. There's a 

resonance between the engine/mount and the frequency at which the blades 

vibrate fore and aft....it's clearly not a problem with the prop itself - 

it's just an interaction between it and this particular plane.


	


If that's the case, going to a slightly stiffer or softer engine mount 

rubber isolator might help, change the resonant frequency.



If it's a pulsating mmmmMMMmmmmMMMmmmmMMMmmmm, though, an exhaust/prop 

interaction seems the more likely culprit.  Comparing the frequency of the 

cycle with the engine and prop rpm's may help to narrow it down.



-Dana



--

  Welcome to the Federal Bureau for Reducing Bureaucracy!

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Wed Apr 08, 2009 9:27 am Post subject: Re: blade harmonics

 	  Dana wrote: 	
		  

If that's the case, going to a slightly stiffer or softer engine mount 

rubber isolator might help, change the resonant frequency.



If it's a pulsating mmmmMMMmmmmMMMmmmmMMMmmmm, though, an exhaust/prop 

interaction seems the more likely culprit.  Comparing the frequency of the 

cycle with the engine and prop rpm's may help to narrow it down.



-Dana



--

  Welcome to the Federal Bureau for Reducing Bureaucracy! 	




Yeah it's a pulsating thing like you describe, it could very well be aggravated by exhaust pulses. I do have a 3" extension installed tho and the blades are a good ways away from the exhaust outlet.



The lower frequency doesn't seem to correspond to anything that I can tell, though a wild guess is it coincides with the alignment of the crank and prop as it rotates around the 2.43 gear ratio. this _does_ change in frequency with rpm so that could be it.



The higher frequency hum seems to be a constant frequency and, I think I repeat think..., it's the same frequency as the "thoom" I get when tapping the blade on the ground. That's probably the resonance of the blades/engine/mount etc.



Next I go back up (if we ever get flyable weather again) I'm going to listen to it some more. 



No idea if different compliance mounts are available on my mount.... I'll try to research that and see.....



LS

_________________
LS
Titan II SS

djwatson(at)olg.com
Guest

Posted: Wed Apr 08, 2009 9:49 am Post subject: blade harmonics

LS,



Ran into something like this years ago when I still had a Kolb, try to 

direct your exhaust up down or to the side if you haven't already done so, 

also, unbolt the prop hub and rotating it by one bolt hole and see if it 

makes a difference



V/R,

Dennis from MD.

---

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Wed Apr 08, 2009 10:08 am Post subject: Re: blade harmonics

 	  djwatson(at)olg.com wrote: 	
		  LS,



Ran into something like this years ago when I still had a Kolb, try to 

direct your exhaust up down or to the side if you haven't already done so, 

also, unbolt the prop hub and rotating it by one bolt hole and see if it 

makes a difference



V/R,

Dennis from MD.

--- 	




Oh yeah, this may be possible with the exhaust now that I thinka bout it.



Ironically, my FS II had this exact same prop (WD 68" taper).... completely different airframe and motor but it was totally silky smooth with no vibration whatsoever..... just that healthy 503 howl.... 



LS

_________________
LS
Titan II SS

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Wed Apr 08, 2009 11:42 am Post subject: blade harmonics

> Oh yeah, this may be possible with the exhaust now that I thinka bout it.

 	  Quote: 	
		  

 Ironically, my FS II had this exact same prop (WD 68" taper).... 

 completely different airframe and motor but it was totally silky smooth 

 with no vibration whatsoever..... just that healthy 503 howl.... 



 LS


	




Never heard of a Titan Exhaust/912 prop noise/vibration/harmonic problem.  I 

experimented with the outlets on my Titan left, right, and straight up.  No 

problems.



I know of three Kolbs that flew with Rick Thomasson's exhaust designed for 

the Pulsar.  All three suffered from a terrible rattling/noisy exhaust beat 

caused by the retreating blade hitting the downward aimed exhaust pipe on 

the right exhaust system.  I knew I was going to have that problem so I cut 

the outlets perpendicular to the ground and eliminated the noise.  There was 

no vibration or harmonic associated with this system.



I know of three airplanes that have had this similar problem:  a Titan with 

912, a SS with 912, and a SS with 582.  The SS with 582, with 3 blade warp 

drive, was caused by loose fabric on the inboard rear corner of the right 

wing.  I don't know what Ted Cowan's problem is, probably disturbed air flow 

and the way it is hitting the prop blade, or blades.  Items mounted in front 

of the prop all contribute to the way air flows to and through it.  Ever 

play with an electric fan?  Set one up, turn it on high, then start messing 

with the air flow to the blades.  You'll get all kinds of very unusal 

complaints coming from those blades.  Why wouldn't a pusher respond 

similarly?



john h

mkIII

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

Dana

Joined: 13 Dec 2007
Posts: 1047
Location: Connecticut, USA

Posted: Wed Apr 08, 2009 1:35 pm Post subject: blade harmonics

FWIW, I get a similar pulsing hum at cruise rpm on my UltraStar.



-Dana

--

  I hadda I LOVE WINDOWS  sticker on my car...but it crashed.

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