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tc1917(at)bellsouth.net
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 Posted: Sun May 31, 2009 2:34 am Post subject: harmonics |
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Okay, boys and girls, it is time for a good Kolb story. I am sure all of
you remember me and my little slingshot 912 with the rum rum syndrome. A
good friend of mine down in Florida loaned me his just reconditioned WD prop
to check the noise issue. It seemed to drop the noise in half. I contacted
WD (remember I stated that I needed proof to verify the prop noise) and
after a long conversation, they decided that I had the wrong blades. (duh).
They sent me three brand new 68" taper tips with nickel edges. Some of the
best looking and balanced blades I have ever seen come out of there.
Anyway, I purchased a sound meter (db) as per suggestion of someone on this
kolb list. I started taking soundings and ended up putting some serious
noise reduction materials under my engine and everywhere I could behind me.
This really helped. I started out with the same dbs inside as there were
outside under the wing. Mike Bigelow sent me his Kiev 70" three blade on
the agreement if I liked it fine, but if I did not, I could send it back.
What a deal. He is certainly a great guy to do this for a fellow kolber.
Anyway, after a lot of testing and readjusting both blade setups, I finally
concluded that overall, the WD blade out performed the Kiev. The Kiev was
definitely quieter on the ground or at low RPMs but did not have the climb
and/or cruise of the taper tip WD. It WAS a lot lighter but with a four
inch extension, it set my blade thrust back too far (I found this was the
case with the WD blade also). So I was back to my WD 68" taper tip. I also
added a perforated stainless shroud around my exhaust pipes and that was the
cats meeyow. When I turned it straight up, it really made a difference. My
numbers were nice. Behind me I was 104 db (started out 123 and under the
wing was off the scale of 126) and was now 116 db under the wing cruising at
4800 rpm and over 80 mph indicated. That was a staggering 20 db difference
from when I started!!! Mike happily agreed for me to send the Kiev set up
back to him so it is now available again. If I were flying a MkIII or
something, this would be the prop I would want. By the way, I still have
the harmonics but it is now subdued and since I repitched my prop to add
about two hundred rpms, I found that the engine is actually quieter on the
wom wom at 5000 rpms than it is at 4800. All in all I now have a tolerable
situation and still have my zoom zoom. If anyone wants the name of the type
of insulation I used and pics of the new exhaust extension, let me know. If
I could stop the wom wom, I would not even hear my engine at cruise. Cool.
Oh yes, by the way, the man at Kiev told me has a rans 12 with same engine
and it does the same thing and he knows of several others and no one can do
anything about it. Seems Rotax doesnt want to us to know they know about
this problem which means it IS in the engine. Shame on them. Sorry for the
length. I appreciate what all have done and suggested. If I can help
anyone with this stuff, let me know. I had a $50 reward for anyone who
could solve my problem but I think between Jim Holbrook lending his blades
and Mike Bigelow effectively doing the same, I will give each $25. Does
that seem fair to everyone? Ted Cowan, Alabama, Slingshot, 912UL zoom zoom.
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russ(at)rkiphoto.com
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| Posted: Sun May 31, 2009 3:35 am Post subject: harmonics |
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Ted
Delighted you've solved, or at least, lessened your noise problem.
But I guess you realize you've guaranteed yourself a
new nickname -- from, now on, it's ''ZOOM ZOOM" Cowan
And thanks for sharing -- l'm sure this is useful info for many,
do not forget. er, archive
On May 31, 2009, at 6:33 AM, Ted Cowan wrote:
| Quote: |
Okay, boys and girls, it is time for a good Kolb story. I am sure
all of you remember me and my little slingshot 912 with the rum rum
syndrome. A good friend of mine down in Florida loaned me his just
reconditioned WD prop to check the noise issue. It seemed to drop
the noise in half. I contacted WD (remember I stated that I needed
proof to verify the prop noise) and after a long conversation, they
decided that I had the wrong blades. (duh). They sent me three
brand new 68" taper tips with nickel edges. Some of the best
looking and balanced blades I have ever seen come out of there.
Anyway, I purchased a sound meter (db) as per suggestion of someone
on this kolb list. I started taking soundings and ended up putting
some serious noise reduction materials under my engine and
everywhere I could behind me. This really helped. I started out
with the same dbs inside as there were outside under the wing.
Mike Bigelow sent me his Kiev 70" three blade on the agreement if I
liked it fine, but if I did not, I could send it back. What a
deal. He is certainly a great guy to do this for a fellow kolber.
Anyway, after a lot of testing and readjusting both blade setups, I
finally concluded that overall, the WD blade out performed the
Kiev. The Kiev was definitely quieter on the ground or at low RPMs
but did not have the climb and/or cruise of the taper tip WD. It
WAS a lot lighter but with a four inch extension, it set my blade
thrust back too far (I found this was the case with the WD blade
also). So I was back to my WD 68" taper tip. I also added a
perforated stainless shroud around my exhaust pipes and that was
the cats meeyow. When I turned it straight up, it really made a
difference. My numbers were nice. Behind me I was 104 db (started
out 123 and under the wing was off the scale of 126) and was now
116 db under the wing cruising at 4800 rpm and over 80 mph
indicated. That was a staggering 20 db difference from when I
started!!! Mike happily agreed for me to send the Kiev set up back
to him so it is now available again. If I were flying a MkIII or
something, this would be the prop I would want. By the way, I
still have the harmonics but it is now subdued and since I
repitched my prop to add about two hundred rpms, I found that the
engine is actually quieter on the wom wom at 5000 rpms than it is
at 4800. All in all I now have a tolerable situation and still
have my zoom zoom. If anyone wants the name of the type of
insulation I used and pics of the new exhaust extension, let me
know. If I could stop the wom wom, I would not even hear my engine
at cruise. Cool. Oh yes, by the way, the man at Kiev told me has a
rans 12 with same engine and it does the same thing and he knows of
several others and no one can do anything about it. Seems Rotax
doesnt want to us to know they know about this problem which means
it IS in the engine. Shame on them. Sorry for the length. I
appreciate what all have done and suggested. If I can help anyone
with this stuff, let me know. I had a $50 reward for anyone who
could solve my problem but I think between Jim Holbrook lending his
blades and Mike Bigelow effectively doing the same, I will give
each $25. Does that seem fair to everyone? Ted Cowan, Alabama,
Slingshot, 912UL zoom zoom.
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Sun May 31, 2009 6:09 am Post subject: Re: harmonics |
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If you haven't done this then I would do a carb balance to eliminate it from the possibilities.
Have you balanced your carbs lately? That wom wom or vibration that sounds like that will show up at lower rpms and go away just as you stated if rpms are increased if they are out. Carbs need balancing every 100 hrs. or at least each annual if you don't put on 100 hrs a year. They will be out and it's too easy to do for you not to do it. The cross over tube can only help so much. Think of your 912 as a left engine and a right. You don't want the left trying to run at 5000 and the right trying to run at 5050 or worse 5100. Do the mechanical and pneumatic sync. Just doing the mechanical without doing the pneumatic is a waist of time. How do I know because I get to sync at least 30 or more sets of carbs a year and they are all out of balance by 100 hrs. or the annual, just some more than others.
P.S.
Before you get into a mess trying other expensive or labor intensive fixes try the cheapest and easiest first like the carb balance. You have nothing to loose. When you put these props on are you at least doing a static balance on them? You need to rule out other issues before you do something to just mask the problem.
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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056
Last edited by Roger Lee on Sun May 31, 2009 1:44 pm; edited 1 time in total |
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captainron1(at)cox.net
Guest
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| Posted: Sun May 31, 2009 9:04 am Post subject: harmonics |
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Hi Ted
For 50 bucks I'll help you.
I am guessing that you have the regular factory kolb single tube motor mount.
Obviously you get an airframe engine resonance problem. Its easy to cure, all we got to do is remove the resonating frequency above the operating rpm range of your motor. I am guessing that you already tried different rubber mounts, that's the quickest and easiest solution and should work. A harder mount will move up the resonance frequency and a softer rubber mount will lower it. If you tried that and it didn't work then the only other thing is change the tube mount resonance. The only way I can thing for you to do that is weld two parallel braces to the kolb tube which are also welded to the tray mount. That will also have the effect of increasing the frequency range above what you have right now. The stiffer the engine mount the higher the resonating frequency, Like I mentioned earlier the trick is to move the onset frequency above the operating range of the motor. You may just have been unlucky in terms of the mount tube thickness or some other minor difference from most of the others who do not report that problem. I certainly do not think its the motor itself, those are balanced and tested very carefully in their assigned rpm operating range.
===========================================================================================================================================================================
---- Ted Cowan <tc1917(at)bellsouth.net> wrote:
=============
Okay, boys and girls, it is time for a good Kolb story. I am sure all of
you remember me and my little slingshot 912 with the rum rum syndrome. A
good friend of mine down in Florida loaned me his just reconditioned WD prop
to check the noise issue. It seemed to drop the noise in half. I contacted
WD (remember I stated that I needed proof to verify the prop noise) and
after a long conversation, they decided that I had the wrong blades. (duh).
They sent me three brand new 68" taper tips with nickel edges. Some of the
best looking and balanced blades I have ever seen come out of there.
Anyway, I purchased a sound meter (db) as per suggestion of someone on this
kolb list. I started taking soundings and ended up putting some serious
noise reduction materials under my engine and everywhere I could behind me.
This really helped. I started out with the same dbs inside as there were
outside under the wing. Mike Bigelow sent me his Kiev 70" three blade on
the agreement if I liked it fine, but if I did not, I could send it back.
What a deal. He is certainly a great guy to do this for a fellow kolber.
Anyway, after a lot of testing and readjusting both blade setups, I finally
concluded that overall, the WD blade out performed the Kiev. The Kiev was
definitely quieter on the ground or at low RPMs but did not have the climb
and/or cruise of the taper tip WD. It WAS a lot lighter but with a four
inch extension, it set my blade thrust back too far (I found this was the
case with the WD blade also). So I was back to my WD 68" taper tip. I also
added a perforated stainless shroud around my exhaust pipes and that was the
cats meeyow. When I turned it straight up, it really made a difference. My
numbers were nice. Behind me I was 104 db (started out 123 and under the
wing was off the scale of 126) and was now 116 db under the wing cruising at
4800 rpm and over 80 mph indicated. That was a staggering 20 db difference
from when I started!!! Mike happily agreed for me to send the Kiev set up
back to him so it is now available again. If I were flying a MkIII or
something, this would be the prop I would want. By the way, I still have
the harmonics but it is now subdued and since I repitched my prop to add
about two hundred rpms, I found that the engine is actually quieter on the
wom wom at 5000 rpms than it is at 4800. All in all I now have a tolerable
situation and still have my zoom zoom. If anyone wants the name of the type
of insulation I used and pics of the new exhaust extension, let me know. If
I could stop the wom wom, I would not even hear my engine at cruise. Cool.
Oh yes, by the way, the man at Kiev told me has a rans 12 with same engine
and it does the same thing and he knows of several others and no one can do
anything about it. Seems Rotax doesnt want to us to know they know about
this problem which means it IS in the engine. Shame on them. Sorry for the
length. I appreciate what all have done and suggested. If I can help
anyone with this stuff, let me know. I had a $50 reward for anyone who
could solve my problem but I think between Jim Holbrook lending his blades
and Mike Bigelow effectively doing the same, I will give each $25. Does
that seem fair to everyone? Ted Cowan, Alabama, Slingshot, 912UL zoom zoom.
--
kugelair.com
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http://www.matronics.com/Navigator?Kolb-List |
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