Conference vmszoo::rc

The 8/92 Model builder I picked up last night has a table of prop tip speeds 
for 11-15 inch props. The article recommends keeping tip speed below 400mph
in order to meet AMA noise guidelines. They also recommend a series of speed
and diameters for best noise reduction (a portion shown below).

Prop	Engine		Recommended	Cutoff range
Dia.	Speed		Tip speed	400mph tip speed threshhold

11"	11000rpm	359.91mph	12k  < 400mph < 12.5k
12"	10000rpm	356.93mph	11k  < 400mph < 11.5k
13"	9000rpm		348.01mph	10k  < 400mph < 10.5k
14"	8500rpm		353.96mph	9.5k < 400mph < 10k
15"	8000rpm		356.93mph	8.5k < 400mph < 9k

From Propellers: Myths & Facts by Rick Allison.
Table credit given to E. Hawkinson, 1992, National Society of RC Aerobatics

    
     Jim,
    
        Looks like interesting data. I was aware of trying to keep prop
    speed sub-sonic, but not aware of the 400 mph sound threshold.
        It looks like 12,800 for a 10", and 13,500 for a 9" would be 
    somwehere near the 400 mph limit, but it doesn't look like they would
    exceed it.
        Easy enough to figure out, what's the circumfrence of a 9" and 10" 
    circle? :) 
    
                                                        Dan      

    
    Your 10" diameter shows a tip speed of 381mph at 12,800rpm, and the 9"
    is traveling around 361mph at 13,500rpm.
    
    The tip speed on a 14" diameter at 9000rpm is 375!

Hey, I just copied the table.

Re .2 The circumference of a 10" prop is...

Round!

Sorry, long lunch...

    
      Sorry officer, looks like I am well within the 400 mph speed limit!
    :) HMMMMMM, maybe it IS the engine making the noise! NNAAAHHHHHH,
    couldn't be! :)

    
    Does anyone out there has any information on tuned pipes for 4-stroke
    engines ? Formulas - experiences - other ?  I'd like to setup a YS120
    with a muffeled tuned pipe. could I also use 2-stroke tuned pipes ? If
    so, what size should it be for above mentioned engine ? (don't like the
    idea of spending $130.- for the regular setup)
    I know, somehow the length could be calculated, but miss the details.
    
    
    	Bernd

>                     <<< Note 1431.6 by RZSCSI::KNOERLE >>>
>                         -< tuned pipe on 4-stroke ? >-
>
>    
>    Does anyone out there has any information on tuned pipes for 4-stroke
>    engines ? Formulas - experiences - other ?  I'd like to setup a YS120

Bernd, in general the concept of a tuned pipe does not work with 4-stroke
engines.  Now in the model press there have been some claims to the contrary
but my best recollection from Clarence Lee's column is that they don't 
work.  I have seen some for sale that are "designed" for 4 strokes but
regardless of the side you take on this controversial issue all parties
agree on one thing.  They don't add nearly as much to the horse power
as do tuned pipes on 2-stroke engines.  So the bottom line is they are
not cost effective and unless they are going into a plane that needs weight
they probably don't add sufficiently to justify the weight increase.

Perhaps our HTA gang has seen some on pattern planes?
I have never seen one on scale or sport planes.  In pattern they
frequently hide the exhaust in the fuselage so it isn't obvious
what they are running.

Bye          --+--
Kay R. Fisher  |
---------------O---------------
################################################################################

    Well KAY???
    
    
    Technally speaking a tuned pipe only workd on a two stroker.  With
    multi- cylindered four strokers tuned headers (headers of exactly
    identical length) that are joinde together can help purge the exhaust
    volume of one cylinder with the expanding gasses in the header from 
    another cylinder.
    
    Four our four strokers,  the addition of a stock Hatori 650 pipe
    (common for .60 2/stroke engines) is used to muffle the exhaust note
    of the engines.  Then and additional muffler is added to further
    reduce the noise.  Depending on if these is any overlap between the
    exhaust valve being open and the intake valve opening their is some
    merit to the use of a tuned exhaust. If the duration of the overlap is
    sufficient, when the overlap occurs the engines exhaust system works
    like a 2/stroke exhaust system.  The engine will be exhausting gasses
    and intaking fuel at the same time. At this time some fuel can be drawn 
    out of the cylinder with the gasses.  I an energy wave is present in
    the pipe, and if it gets to the exhaust port at the right timing it
    will help force some of this fuel mixture back into the cylinder.
    Added fuel = more power.
    
    
    Tom

    
    I was not expecting a power boost from a tuned pipe on a 4-stroke, but
    less loss of power when adding a good muffler system. My immagination
    tells me, as Tom said, if the expanding wave of the exhaust gases will
    not be reflected (pressure wave) before the exhaust valve will be
    closed it should help at least getting the cylinder better empty
    (slight negative pressure) what again should help getting a better
    (cleaner) filling of the cylinder.
    Assume we are spinning 10000 rpm. and assume the exhaust valve would be
    open exactly 1/2 rev every second rev. The pressure wave should not be
    reflected before the valve would be closed or opened again.
    The delay would be 3 ms or 9ms (for the next cycle). That means the
    length of the pipe should take the exhaust gases more than 3ms and less
    than 9 ms to be reflected. 
    
    exhaust open-->
                   |Input open->
                                |compression->
                                              |explosion--->
                                                            |exhaust open
    |--------------|------------|-------------|-------------|-----------|
    0              3            6             9            12ms
    
    first wave
    
    |---------------|                                       |-------------
     no pressure wanted                                   no pressure want
   
    
    If the pipe would be too short, the exhaust gases would be pressed back
    into the cylinder - less power & more heat. If the pipe would be way to
    long (I doubt that this could be achived) the preassure wave could
    affect the second cycle.
    Now if there would be (I know there is) a way to calculate the speed of
    hot gases (what I know are different than the speed of cold gasses) one
    could calculate the length of the exhaust system for getting best
    performance.
    
    Any Physisists out there ?   

    
    Mind clears slowly. Now what I recall, the speed of sound is approx.
    330 meters/second. The opening duration in previously mentioned example
    was 3 ms. Now let see what distance would that relate to ?  
    330 ms x 3ms = 99cm . Now divide that by 2 (gas need to travel forward
    and back) that would give me approx. 50cm pipe length. 
    I did calculate this with the setup on my 2-stroke and I'm comming
    pretty close to the actual setup.
    What does that mean, 50cm ?  If I want no gases to come back into the
    cylinder while the valve is open @ 10000rpm my pipelength (dist. Valve to
    plate or konus (sp?) inside the pipe) needs to be at least 50 cm 
    (20inch). 
    I take this as a starting point. I know that the speed of hot gas is
    different than cold gas, the opening of the exhaust valve could be
    different (more or less than 180 degree) and the rotational speed 
    during the exhaust cycle could be less than the average rpm would tell.
    Even the oil and particle content within the gas could make a different
    
    Am I far off track, nuts, missing the point, anything else ?
    
    
    Bernd 
                                                 

The speed of sound is a function of temperature so that part makes sense.

I guess my question would be where you measure from. The reflection would 
begin at the widest part since after that the restriction would cause the 
reflection.

    Probably the best bet is to do a bench run with the engine propped to
    turn about the right RPM.  Vary the pipe length with a sliding pipe and
    tune for max RPM.  If RPM gets too high, put on a bigger prop and
    continue.  You want Minimum backpressure with a 4-stroke to help
    extract exhaust where you want backpressure with a 2-stroke to ram the
    raw fuel mixture back into the cylinder.  A 4 stroke gets virtually no
    mixing between intake and exhaust unless there is a bunch of valve
    overlap.  Ramming anything back in the exhaust will only hurt
    performance.  A large, nonrestricting pipe would be best for a 4-stroke
    to quiet the exhaust without greatly increasing the backpressure.
    
    Charlie
    

    Charlie,
    
    The best way of developing an initial pipe length I've ever seen and
    used is;
    
    1,  Pick a target length propr an pitch.
        Shorten the prop bby 1/2' from each blade.  With a std. two blade
        prop this ends up cutting 1" off of the diameter.
    
    2,  Install the prop on the engine with the pipe installed.  Run up the
        engine, and adjust form max rpm with high speed needle. 
    
    3,  Reduce length of pipe.  With most pipe designs this no longer
        requires cutting the header.  Rather the pipe diameter is designed
        to have the header slip inside of the pipe.
    
    4,  Rerun the engine adjusting needle for max rpm.
    
    5,  Continue this process shortening the pipe app. 1/4" at a time until
        the rpms become flat.  Ie, there is no gain.  Then extend the pipe
        to the setting that first produced the max rpm's.  
    
    For the newer long strokes running 10-15% nitro with 11/14,11/12,
    12/11, 13/10 props the pipe starting length usually is app. 17"
    measured from the glow plug to the high point of the pipe.   The most
    common .60 sixed pipe used today is the Hatori 650 or a carbon fiber
    copy.  I run my YS with a Shadel piston/liner with a hatori 650, apc
    12/11 prop at 17" on 12.5% nitro fuel with 20% oil, 2% of which is
    castor.  Pulls app. 11.5- 12000 rpm on the ground.  Rubber mounted
    with an air cleaned on the venture it test out to 94db on pavement.
    More vertical than I need. Plane weighted 8.25 lbs.
    
    
    Tom

    
    Tom, your rpm on a 12/11 prop is impressive. If I recall the tip speed
    noise problem, that seems to exceed the recommended tip speeds for 12"
    props though.
    
    The subject we currently discuss is the pipe setup for 4-stroke
    engines. If your 2-stroke setup description will be applicable on
    4-strokes as well I'll try it. One point though: You measured the
    distance to the beginning of the pipe, but I know that the important
    measure (on 2-strokes) is the distance from exhaust port to the plate
    where the wave gets reflected. 
    Now what Jim said, since you don't want the pressure wave beeing
    reflected to the exhaust valve you might want to consider the part of
    the pipe that gets wider ?  Hmmmmmm.... 
    
    
	Bernd

    
    I measure from the glow plug to the High (Wide) point of the pipe.
    
    Not because measuring from the Plug is better. For me it's more
    consistant.
    
    Tom

    I know nothing about motors and pipes, being an electric and
    glider flyer.  However, I have made the following observation.
    The fastest ship(next to my electric glider) at the club I belong to
    is a full blown pattern ship with retracts, a Rossi rear exhaust
    .61 and tuned pipe.  It turns an APC 11x12 prop and it is also
    the quietest two stroke at the field.
    
    My question is why don't all power flyers use tuned pipes?  They 
    really seem to help the noise and apparently do not adversely
    affect power.  The ship I mentioned has unlimited vertical and
    great speed.  The pipe is tucked under a shroud whichmust help
    reduce drag immensely.
    
    It seems so funny to me to hear a loud, screaming engine when
    I see what can be done.
    
                                                    Regards,
    
                                                    Jim

It's all a matter of money. It's an expensive solution and most people try to 
get away as cheaply as possible. This usually means that they use the 
manufacturer's muffler and the manufacturer is also trying to get away as 
cheaply as possible.

Again, ease and simplicity. They want easy access to the engine so they don't
cowl it. They want cheap solutions so they settle for what the manufacturer's 
provide and they don't (as novices) feel comfortable making the plane more
aerodynamic since they're just novices. Drag isn't necessarily bad on a trainer 
type plane. It just makes it fly slower. For a beginner, that isn't a problem.

Slab fuselages are easier to kit than nice round crosssection ones. It takes too
much sanding to actually convert the slab to round (a shame since the tree 
started out round 8^)

I agree with all your points with one small exception. The new kids on the 
circuit are using the latest stuff.  Look at the jets and such. The oldtimers 
you're discussing are proving the old adage "you can't teach an old modeller, 
new techniques" 8^)

    
    
    talking about tuned pipes...
    
    I think there are a number of reasons why they are or are not used by
    the general power RC public.  The validity of the reasons I will not
    debate but they are why most modelers do not use pipes.
    
    
    	1,  Cost as associated with the standard purchased RC engine.
    
    	Most all manufacturers equip their engines with cast exhaust
    expansion chambers.   The average modeler (unless field conditions
    warrant differently) will install what comes on the engine and thats
    that.
    
    	2,  Reputation, old bones in the closet...
    
    	When pipes were first introduced and in the following years they
    gained a reputation for their finiky running, noise and their limited
    need in relation to the average modeler.  These feelings seem to be
    passed on from one generation of modelers to the next.  I've seen many
    times modelers with a great deal of experience spout off this or that
    dribble about pipes without knowing the first thing about the latest
    generation of quiet,reliable pipes.
    
    3,  Going with the crowd.
    
    	You will note that few people are really experimentors.  They much
    prefer to follow than take a risk and lead.
    
    
    
    Lastly, I think there is a difference between glider pilots flying and
    the average sport pilot.  The biggest difference is that the majority
    of the RC glider pilots are competition oriented.  Even if they don't
    attend a contest their mind set is one of competition.
    
    The average sport pilot is one for putting around the sky or burning
    holes in the sky.  they are not competition oriented. Rather just the
    opposite.  It's this mind set that deters most from trying new
    technoligies until they are necessary to meet their needs.
    
    On the other hand if you look at the ranks of the competition RC power
    pilot you'll see that they are riding the technology curve almost as
    closely as a glider pilot.  It's the drive of competition trying to
    stay on the leading edge to be competitive that is the motivating
    factor.  Even if the technology is not required to be competitive the
    mind set says that it is required.  Case in point is pattern...
    
    To fly any of the first three classes in pattern all that is needed is
    a proven design that is built straight, fairly light and reliable.
    Being straight and reliable being the two most prominent concerns.
    What does the average sportmans or advanced pilot build?  In most cases
    whatever ship this or that FAI pilot is flying.  Why?  Competition and
    the percieved need for the technology to equal the playing field.
    Funny thing is that the best way to equal out the playing field is to 
    FLY,FLY,FLY.............................................
    
    Even funnier is the fact that they actually judge you on the flying,
    not on what technology you used.
    
    
    Ironic, isn't it:-)
    
    
    But then, who am I, it's just my opinion. Oh, sorry, thatthe other
    file...
    
    
    
    Tom
    

Now, Now, Tom 8^)

I think you've probably hit the nail on the head in a couple of ways. The 
people pushing the technology are the ones competing. They want to have an 
equal footing when they step up to the flightline.

The other side of that coin is that the average club member isn't a contest
flier but he does train the younger fliers.

There's a good group in the Acton fliers in that we're all trying new stuff 
and we're all trying to push the envelope. It feeds on itself and we're each 
pushing the others to try different things. After flying at lunch with these
guys I've really evolved in my building and flying. I go out to practice now
rather than bore holes in the sky.

    There's a paraphrased quote from George Bernard Shaw in the latest UK
    edition of Digital Today that says:
    
    "Reasonable people adapt themselves to the world, so all progress
    depends on unreasonable people."
    
    So if anyone out on the flying field tells you you're being
    unreasonable, you can thank them profusely.
    
    Malcolm 

    In the past, tuned pipes meant MORE NOISE, very high RPM's, high fuel
    consumption, short glowplug life, etc.  They were used to boost
    performance.  You also lost all of your midrange and throttle response.
    Anyone who has heard a short stroke engine turning 20,000 RPM with an
    unmuffled pipe knows what I mean!
    	Now, the engines are designed for lower RPM, props have much more
    pitch, and the pipes are muffled and usually enclosed in the fuse.  All
    of this has greatly reduced the noise and added to the acceptance of
    tuned pipes.  You can't just throw a pipe on a standard sport 40 and
    expect it to work, be quiet, etc.  Everything must be designed as a
    system.  Port timing, pipe length, prop diameter and pitch, engine
    stroke, all have to be matched to get optimum performance at a low
    enough RPM to be quiet.  In competition, evolution works!  The best
    setups become popular and the ones that don't work die out. 
    Competition is the best environment to drive innovation because
    everyone wants to have the best setup - even if they don't need it to
    win.  It may be a psychological thing but you want to give yourself
    every edge.  That's why you see top of the line pattern ships in the
    Sportsman class even if you can fly the Sportsman pattern with a
    trainer. :-)
    
    Charlie
    

    Re: -1
    
    I LIKE THAT QUOTE!  I am glad that the "obsessive" non-conformist nuts
    who are constantly trying to make a better mousetrap are around.  It
    makes life much easier for us lazier types, who reap the benefits
    of their experimentation and hard work.
    
                                                     Regards,
    
                                                     Jim

re .20

	I think the guy you talked to that spurred your discussion 
on latest vers old way of doing things gave you the reason power
people are a little behind. He said to you " That's no fun " and
he's right. After all we are all doing this because it is "fun "
to him "fun" is doing it the old way. Your Idea of fun is sitting
on the leading edge.

			to each his own

				Larry 

Re .27

Speaking as a newcomer. Couldn't agree more!

It seems to me that this hobby is a very wide church, and should remain so. I 
know from other activities I have been involved in that if discussion becomes 
in-fighting then everyone is harmed.

"Relax - it's only a toy !"

Nigel

>    In fact after I ripped the wings off my ARCUS during a dive last
>    week one of the club members commented - "Now you can build a
>    real airplane".  A "real" airplane in this case is a plane with
>    a glow engine motor. 


Hang in there Jim. As my club's resident and only copter flyer I get this a lot.
Even when I show up with a Gremlin or some other fixed wing ship I get this. 
Course, in our club, a "real" airplane is defined as one having at least a
G-38 hanging off of the front.

Have uoy considered building an electric with a washing machine motor up front?
That would get thie attention! 8^)

    I'll plead guilty to being hung up on motors.  I always have been and
    always will be.  I also fly electrics but not very seriously.  I like
    electric motors but I HATE nicads.  I see no advantage whatsoever in an
    electric power plant other than reduced dB output.  Since I fly at a
    power field, that's no advantage to me.  Good battery packs are
    expensive and have a finite life.  Good electric motors cost far more
    than an equivalent power glow engine.  Like the man said, to each his
    own - but when it comes down to performance vs cost, you can't beat a
    glow engine with an electric.
    
    Charlie
    

    Re: -1
    
    Charlie,
    
           I totally agree with everything you have said.  The power
    of electrics is well below that of a good IC engine.  I also
    do not seek to get people to switch from IC to electric, but
    rather to dispell some myths and open up some new avenues.
    
    That being said I will mention a few comparisons between the
    two.  Electrics are less powerful and heavier than IC engines.
    They also cost more and are quieter.  Electrics have a shorter
    flying time but are much more reliable than glow engines.
    
    The amount of time tinkering with the IC motors at every field I have
    visited is nothing short of incredible.  Electrics offer a new
    challenge.  It's just about all been done with IC powered planes.
    I have seen enough glow engine CUBS to last 2 lifetimes!  I think
    electrics make more sense for multiengine applications where the
    inherent reliability and synchronicity of the engines makes many
    successful flights much more likely.  The number of crashed twins
    built and flown by the best in the world is tremendous.  I have
    seen more aborted twin IC flights because the pilot could not
    get the two motors to run simultaneously than successful flights.
    Usually loss of an engine which happens frequently, results in a crash.
    
    Two cycle glow planes do not sound or fly in a scale fashion at all.
    The four strokes are much more convincing for scale applications.
    If a person takes a plane designed for wet power and simply substitutes
    an electric motor, the combination is often disappointing.  Different
    design and building techniques are needed for electric power.
    
    I really like scale glow powered planes, particularly with 4 stroke
    engines.  I would say that 90% of the clubs I have visited are
    comprised of mostly constant wing chord trainers with loud, high revving
    two stroke engines.  The obvious aerodynamic efficiency of these
    designs and homely looks will never excite me.  Seeing any of the
    common designs come by rather slowly with the motor screaming at
    10,000 RPM only serves to underline these inefficiencies and lack
    of flying grace.  I realize you must start at the beginning and
    learn to fly one of these types before progressing to a more
    interesting/efficient design.  It just seems most get forever
    stuck with these screaming, draggy, slow designs.  Electric power
    would at least make the plane fly more like it looks-slow and
    draggy.
    
                                                 Regards,
    
                                                 Jim

    Jim,
    	I'm in general agreement with -.1.  However, electric is not
    realistic sounding for scale either.  Also, the problem with glow
    powered scale is the pilots not the engine.  You don't have to fly at
    full throttle but most of them do.   I agree that most cubs I've seen
    fly too fast, take off in three feet, etc.  I've also seen good scale
    pilots use less than full throttle to keep the scale realism and have
    it quieter as a bonus.
    	On reliability, I agree that multi engines is a natural for
    electric.  It's easier to get matched thrust output and it's unlikely
    that one engine will quit while the other is pulling full blast - the
    cause of most twin crashes.
    	I don't recall spending much time making my glow engines run.  (far
    less than the time spent waiting for packs to cool and recharge) 
    Reliability is a function of the experience of the guy running the
    engine.  You need to do things right whether it's an electric or a glow
    job.  Battery care requires just as much experience and knowledge as is
    required to run a glow engine reliably.  Again, I'm not knocking
    electric flying but I also don't think that it requires less knowledge
    to do successfully.  On the contrary, it requires a much more optimized
    setup to work well due to the much reduced energy available per pound
    of propulsion equipment.  There is about a 100 to 1 difference between
    energy per pound in glow fuel vs a battery.  (actually this is a
    comparison I did between gasoline and lead/acid so it's not that accurate
    for glow fuel vs nicads)  You need the right prop/engine/battery
    combination and a light airframe to have acceptable electric flight. 
    Wiring must be low loss as well.
    	The main advantage I see with my electric (sailplane) is that I can
    climb out, shut down the motor, glide around, turn on the motor
    again....  Try that with a glow engine.  My Eclipse will climb out to
    winch height at least twice on a charge and I can fly it at a field
    that doesn't have a good place to put out a highstart or winch.
    
    Charlie
    

    This is way off the topic of tuned pipes.  How come the new
    moderator doesn't move this stuff?  (Oh wait a minute - that new
    moderator is ME... :-)

    Anyway, I have to jump in here regarding power vs. weight.  The
    disclaimer is that I haven't done scientific experiments here, but
    some R/C electric power "experts" say that a cobalt electric motor
    weighs approx. the same as an equivalent power glow engine. 
    HOWEVER, the nicad weighs approx. 6 times more than the equivalent
    energy amount of glow fuel.  So, it's not the motors that are that
    much different in power, just the batteries are different in energy
    density.

    As an electric flier, you can choose a battery that weighs approx.
    the same as the (full) glow fuel tank and have 1/6 the flight time
    or choose a battery that weighs 6 times more, but will fly just as
    long.  (Or anything in between...)

                       _____
                      |     \
                      |      \                          Silent POWER!
      _        ___________    _________   |            Happy Landings!
     | \      |           |  |         |  |
     |--------|-  SANYO  + ]-|  ASTRO  |--|              - Dan Miner
     |_/      |___________|  |_________|  |
                      |       /           |     " The Earth needs more OZONE,
                      |      /                       not Castor Oil!! "    
                      |_____/

Re .-1

Look guys, please remember us poor 'uns living in the UK. If I could buy my gear 
for those kind of prices I'd be a happy man indeed!

Had a holiday in California earlier this year, and my wife swears that two 
thirds of our baggage allowance on the way home was taken up with assorted hobby 
bits'n'pieces.

Which gives rise to an interesting question: Why is R/C so much more expensive 
over here?

Cheers

Nigel

    RE: -.1
    
    Nigel,
    
    must have to do with the size of the market. I used to complain just as
    you about the prices in Germany. But this year, I find that a lot of
    stuff is available in Germany at competetive prices. I wouldn't buy any
    of the items of Jim's list in the US since I'd get them at home for
    comparable prices. The motor is even cheaper since it's from Graupner
    (DM 10 in Germany which translates to about $ 7). Of course, if you
    really worry about your batteries and plan on going to bigger electrics
    one day, you might want to spend A LOT more for a decent charger. But
    that's not the issue here. To start with electrics has become very
    affordable in the US as well as in Germany, and I suspect, it will be
    cheaper in the UK as soon as there is considerable interest and the
    market develops.
    
    BTW, I found even computer radios are competitively priced in Germany
    now. The biggest advantage when buying here seems to be wooden kits.
    
    Sooo - do a good promotion for our hobby, get new people involved and
    more and more interested in electrics, support a good competition of
    hobby shops and you will see prices dropping - I assume...
    
    Hartmut_without_financial_business_education