May 2005 Additions to the rec.aviation "Rogues Gallery"...

[Jay Honeck]

May was an active month for the Gallery!

Click on http://www.alexisparkinn.com/rec_aviation.htm to see:

*- From Sacramento, CA, more pix of Jack Allison's beautiful Piper
Arrow...

*- From the United Kingdom, pix of Chris Blythe's French-built Wassmer
52...

*- From Manassas, VA, more pix of Bryan Chaisone's way cool Robinson
R22 helicopter...

*- From Chicago, IL, pix of Paul Hekman's fabulour Piper Arrow...

*- From St. Charles, MO, pix of Brian Larkin's awesome Piper Archer...

*- From Chetek, WY, pix of Scott Lifkin's slick Corben Junior Ace...

*- From Sandpoint, ID, pix of the new panel in Mike Rapoport's Helio
Courier...

What is this Rogues Gallery, you ask? It's a photo webpage created
exclusively for users of these rec.aviation newsgroups, designed and
maintained so that we can all have a better idea of who (and what) the
regular users of this group look like.

About to respond to a guy's post, and wondering what he flies? Take a
peek at the Gallery -- he's probably listed in there!

Want pix of YOUR pride & joy out there, for all of us to drool on?
Simply send me a few (4 - 8) pix of your bird, along with some
narrative about yourself (where you're based, how long you've been
flying, how long on the newsgroups, etc.), and I'll get 'em up ASAP!
Send 'em to

Blue skies!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[john smith]

Jack Allison: what happens when the rpm runs in the yellow band of the
tach? [you know, you can get rid of that pesky yellow segment by
installing a three-bladed prop :-)) ]

Mike R: How about a lesson on Courier fuel management?

[Jack Allison]

john smith wrote:

> Jack Allison: what happens when the rpm runs in the yellow band of the
> tach? [you know, you can get rid of that pesky yellow segment by
> installing a three-bladed prop :-)) ]

Um...nothing because the prop doesn't get run there. Three basic power
settings that I was taught: 1) Takeoff: Everything full forward 2)
1000 AGL: 25 squared for the climb 3) Cruise: 21-24 inches, 2400 RPM.

How does the 3-bladed prop get rid of the yellow arc?

--
Jack Allison
PP-ASEL-IA Student
Arrow N2104T

"When once you have tasted flight, you will forever walk the Earth
with your eyes turned skyward, for there you have been, and there
you will always long to return"
- Leonardo Da Vinci

(Remove the obvious from address to reply via e-mail)

[Jay Honeck]

> Um...nothing because the prop doesn't get run there. Three basic power
> settings that I was taught: 1) Takeoff: Everything full forward 2) 1000
> AGL: 25 squared for the climb 3) Cruise: 21-24 inches, 2400 RPM.

Now that you've had some time to diddle around, what kind of speed are you
seeing at, say, 23 squared, Jack?

Also, are you and Jim Weir gonna "convoy" to Iowa City en route to OSH?

And you really need to get on Jav (Henderson) and get him to fly to OSH with
you guys this year, too! He appears to be coming up with excuses like
"family" and "work" that simply can't be allowed to interfere with essential
things like Oshkosh in July!

:-)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[john smith]

Jack Allison wrote:
> How does the 3-bladed prop get rid of the yellow arc?

The yellow arc in the middle of the green arcs is an area of
"resonance". That is, it can cause vibration in the airframe. With the
two blade prop operating in that rpm range, a frequency exists where the
whole airplane is in synch. Ever see the video of the Tacoma Narrows
bridge oscillating in the wind? The structure becomes "fluid-like" and
distorts in proportion to the amplitude/strength of the wave.
The three bladed prop has a resonant frequency outside that of the
airframe/engine/prop system.
This is the best answer I can give. I understand it, I may not be
explaining well.
I believe Jim Weir could probably give you a better expaination. He's a
teacher and very good at reducing technical things to simple english.

[Jack Allison]

Jay Honeck wrote:

> Now that you've had some time to diddle around, what kind of speed are you
> seeing at, say, 23 squared, Jack?
Need more diddle time for this stuff. It's on my list though. For
flying instrument stuff like holds and approaches, I can tell you that
18 inches/2400 RPM works out to approx. 100 mph and gives my brain more
of a chance to keep up with the plane :-)

>
> Also, are you and Jim Weir gonna "convoy" to Iowa City en route to OSH?
That's the plan. Jim and I have pinged each other a few times and I
need to actually fly the 20 minute hop to Grass Valley and meet
him...another "on the list" thing :-)

> And you really need to get on Jav (Henderson) and get him to fly to OSH with
> you guys this year, too! He appears to be coming up with excuses like
> "family" and "work" that simply can't be allowed to interfere with essential
> things like Oshkosh in July!
Ok, I'll take the challenge. Hey Jav H, be a man, come to Oshkosh this
year or Jay says he'll release several virus attacks against your
servers, he'll load up so many pictures in the Rogue's gallery that you
won't have any storage capacity, and we'll all create so much traffic
that your servers will explode in huge fireballs. Come on man, an Arrow
sandwiched between two straight leg Skylanes in a flight of three...the
poor newbie Piper owner will have girly man high wings all around him!

There, how'd I do Jay? :-) If Jav doesn't read this thread, perhaps we
can elevate the abuse to it's own thread.


--
Jack Allison
PP-ASEL-IA Student
Arrow N2104T

"When once you have tasted flight, you will forever walk the Earth
with your eyes turned skyward, for there you have been, and there
you will always long to return"
- Leonardo Da Vinci

(Remove the obvious from address to reply via e-mail)

[nrp]

It isn't an airframe issue, it is an engine-prop combination issue. It
is a high frequency (about 220 Hz or a little below middle C on a
piano) torsional resonance involving the crankshaft and the propellor.


The mode of damaging vibration is such that a rotating observer sitting
on the also-rotating spinner would see the back of the crankshaft
rotationally deflecting to-and-fro while also seeing the propeller tips
going fro-and-two. The spinning observer gets a smooth ride.

Props are highl;y stressed. A spinning propeller (wood or metal) is
almost 1/8 of an inch larger in diameter than when standing still. The
prop blade leading and trailing edges accumulate the most fatigue
damage from this 220 Hz bending mode.

OK - the mode shape definition is a little honky but that is the best
way I know to describe it.

[Dave Butler]

nrp wrote:
> It isn't an airframe issue, it is an engine-prop combination issue. It
> is a high frequency (about 220 Hz or a little below middle C on a
> piano) torsional resonance involving the crankshaft and the propellor.
>
>
> The mode of damaging vibration is such that a rotating observer sitting
> on the also-rotating spinner would see the back of the crankshaft
> rotationally deflecting to-and-fro while also seeing the propeller tips
> going fro-and-two. The spinning observer gets a smooth ride.
>
> Props are highl;y stressed. A spinning propeller (wood or metal) is
> almost 1/8 of an inch larger in diameter than when standing still. The
> prop blade leading and trailing edges accumulate the most fatigue
> damage from this 220 Hz bending mode.
>
> OK - the mode shape definition is a little honky but that is the best
> way I know to describe it.

nrp, your posts on this topic are always illuminating.

I've heard the yellow-arc issue described as "detuning the crankshaft
counterweights" or some such.

If the counterweights are involved, and the the torsional vibration issue is a
propeller/cranskshaft phenomenon, do you change anything in the crankshaft
counterweights when you change the propeller?

Dave

[Morgans]

"Dave Butler" > wrote

> If the counterweights are involved, and the the torsional vibration issue
is a
> propeller/cranskshaft phenomenon, do you change anything in the crankshaft
> counterweights when you change the propeller?

Nope, you still don't quite have it.

Did you know that if you have two tuning forks than vibrate (resonate) the
same note, and you hit one to start it resonating, and you put the second
one up to it, the second one will start vibrating? Well, they do.

Same idea with the crank and the prop. The crank vibrates at one frequency,
and at a certain RPM, that is the frequency that it wants to vibrate at. If
the crank has no other things vibrating at the same frequency touching it,
it is stiff enough to not be a problem.

Now you add a prop that *does* vibrate at the same frequency as the crank,
and run it at that critical RPM, the prop starts its vibration, and the
crank is doing the same thing. Think back to the tuning forks, and now the
one excites the other, and it keeps on exciting each other, getting louder
(think more movement) and louder, until something breaks.

So if you put a different prop on, (3 blade) that does not vibrate like a
tuning fork at the same critical frequency as the crank, the crank still
vibrates at its' frequency, but the prop does not, so it does not help the
crank vibrate bigger. (louder) No problem. The restriction for the
combination is removed.
--
Jim in NC

[Dave Butler]

Morgans wrote:
> "Dave Butler" > wrote
>
>
>>If the counterweights are involved, and the the torsional vibration issue
>
> is a
>
>>propeller/cranskshaft phenomenon, do you change anything in the crankshaft
>>counterweights when you change the propeller?
>
>
> Nope, you still don't quite have it.

I guess I didn't make my question very clear, since you don't seem to have
understood it, or didn't answer it.

>
> Did you know that if you have two tuning forks than vibrate (resonate) the
> same note, and you hit one to start it resonating, and you put the second
> one up to it, the second one will start vibrating? Well, they do.

Yes, I know that.

>
> Same idea with the crank and the prop. The crank vibrates at one frequency,
> and at a certain RPM, that is the frequency that it wants to vibrate at. If
> the crank has no other things vibrating at the same frequency touching it,
> it is stiff enough to not be a problem.

I question this explanation. I think it's rather that the whole assembly, prop
plus crankshaft, has a resonant torsional vibration, not that the the prop and
crankshaft vibrate independently and reinforce each other. I stand ready to be
corrected, though.

>
> Now you add a prop that *does* vibrate at the same frequency as the crank,
> and run it at that critical RPM, the prop starts its vibration, and the
> crank is doing the same thing. Think back to the tuning forks, and now the
> one excites the other, and it keeps on exciting each other, getting louder
> (think more movement) and louder, until something breaks.

See above.

>
> So if you put a different prop on, (3 blade) that does not vibrate like a
> tuning fork at the same critical frequency as the crank, the crank still
> vibrates at its' frequency, but the prop does not, so it does not help the
> crank vibrate bigger. (louder) No problem. The restriction for the
> combination is removed.

I concur that whether your tuning fork analogy is right or not, changing the
prop changes the vibration characteristics.

Now, back to my original question (as I intended, at least): since the torsional
resonance that the crankshaft counterweight vibration dampers were designed to
damp is no longer present, do you remove or otherwise modify the crankshaft
counterweight vibration dampers?

Dave

[john smith]

Dave Butler wrote:
> Now, back to my original question (as I intended, at least): since the
> torsional resonance that the crankshaft counterweight vibration dampers
> were designed to damp is no longer present, do you remove or otherwise
> modify the crankshaft counterweight vibration dampers?

If you remove the dampers, aren't you further changing the vibration
characteristics and frequency?

[Dave Butler]

john smith wrote:
> Dave Butler wrote:
>
>> Now, back to my original question (as I intended, at least): since the
>> torsional resonance that the crankshaft counterweight vibration
>> dampers were designed to damp is no longer present, do you remove or
>> otherwise modify the crankshaft counterweight vibration dampers?
>
>
> If you remove the dampers, aren't you further changing the vibration
> characteristics and frequency?

Yes, I suppose so. Is that a good thing or a bad thing?

Dave

[Morgans]

"Dave Butler" > wrote

> Now, back to my original question (as I intended, at least): since the
torsional
> resonance that the crankshaft counterweight vibration dampers were
designed to
> damp is no longer present, do you remove or otherwise modify the
crankshaft
> counterweight vibration dampers?
>
> Dave

Hum, that would be a substantial job, since if you mess with the
counterweights, you are changing the balance relationship between the
piston/rod assembly, and the crank. I think you would be adding another set
of vibrations into the engine. Not a job for the homebuilder, I think.

It seems to me that if you added mass into the engine/prop combination, by
using a heavier flywheel, or harmonic balancer, you would have a chance at
fixing the problem.
--
Jim in NC

[Dave Butler]

Morgans wrote:
> "Dave Butler" > wrote
>
>
>>Now, back to my original question (as I intended, at least): since the
>
> torsional
>
>>resonance that the crankshaft counterweight vibration dampers were
>
> designed to
>
>>damp is no longer present, do you remove or otherwise modify the
>
> crankshaft
>
>>counterweight vibration dampers?
>>
>>Dave
>
>
> Hum, that would be a substantial job, since if you mess with the
> counterweights, you are changing the balance relationship between the
> piston/rod assembly, and the crank. I think you would be adding another set
> of vibrations into the engine. Not a job for the homebuilder, I think.

Sorry, I didn't realize this was crossposted to r.a.homebuilt.

My point was
- when you change propellers, you are already messing with the vibration
characteristics.
- there are specific engine parts (those dynamic crankshaft counterweights) that
were presumably designed to dampen vibration with the original propeller.
- someone asserted that the yellow-arc on the tachometer could be removed if one
installed a 3-bladed prop.
- someone else (nrp) noted that the yellow-arc was there to avoid a torsional
vibration mode that occurs with the specific propeller/crankshaft comibnation.
- the question was intended to address certified airplanes, and I wondered
whether the STC for the propeller change addressed any other changes besides
just bolting on a new propeller.
- nrp seems to know a lot about torsional vibration, and I was hoping he would
respond w.r.t. the effect of changing propellers and not "messing with" the
vibration dampers that were installed to ameliorate the vibration with the
original prop.
- it was just a curiosity question. Never mind. Thanks.

>
> It seems to me that if you added mass into the engine/prop combination, by
> using a heavier flywheel, or harmonic balancer, you would have a chance at
> fixing the problem.