A question only a newbie would ask

On Wed, 11 Aug 2004 02:31:22 GMT, Roger Halstead
wrote:

Now that number concerns me. A piece of steel tube the length of a
normal muffler weighs almost that much. IF they claim no performance
degradation the muffler must be straight through and would hardly be
very effective, or at least hardly more effective than the stock units
which weight a *lot* more than 2#. Are you sure that wasn't Kg?

Straight through mufflers can be deceptively effective. They aren't
literally just a straight through pipe, the pipe is actually drilled
with many holes which allows the exhaust pulse to bleed off into an
outer chamber. On street cars, this outer chamber is often packed
with fiberglass, producing "Glass Packs", famed for their suppressed
rumble.

The "Swiss Muffler" is a variation on that theme. It's a long drilled
tube, or a tube rolled from stainless steel mesh, surrounded by a
solid outer tube. The space between the two is packed with stainless
steel wool. So the exhaust pulse can go straight through, but it's
energy is bled off through the holes in the inner tube.

This is very effective, but the ones I've seen are routed outside the
body. Since they parallel the wind stream, they don't greatly effect
overall drag, but they definately are there to see.

http://www.piteraq.dk/flight/muffler.html

My opinion is that anti noise regulations for GA aircraft are likely
in the future here in the USA. We know it isn't impossible to do
because the entire European continent flies with such laws.

Corky Scott

On Wed, 11 Aug 2004 04:15:37 GMT, "G.R. Patterson III"
wrote:

It's going to depend entirely on whether or not the aircraft in question is
"overpowered". If the engine and prop combination is such that there's power to
spare, adding a blade can effectively use that power. You'd actually do better with a
larger 2-blade prop, but there will be excellent reasons why this can't be done with
that aircraft (if it could, the manufacturer would have installed one).

If the aircraft *doesn't* have any power to spare (mine doesn't, for example), adding
another blade (or a larger prop, for that matter) will only decrease performance.

Maybe, maybe not. I'm no prop design expert but I was intrigued by a
prop planform design written up in the latest Contact! magazine. It's
for experimental airplanes only, but it offered very good performance
and low noise at maximum rpm.

It had a sort of triangular planform, with the outer portion of the
blade narrowing way down outside the cowl. This seemed counter
intuitive to me because I'd always heard that the fuselage cross
section basically blanked out much of the thrust produced by the prop
close to the hub, and because the prop didn't turn very fast at that
point anyway, there just wasn't much thrust to be had.

But this designer found that wasn't true.

He narrowed the tips and cut them off abruptly because they operate at
Mach .85 or so and wide tips at those speeds create lots of drag and
noise.

So the prop looks really berzerk. It has an extreme pitch angle at
the hub, then the planform widens dramatically to the outer edge of
the cowling where it abruptly and also dramatically begins narrowing
down. It sweeps inward then begins straightening out to the point
where by the time the tip is reached, the planform is essentially
straight. The pitch angle flattens out as it gets further away from
the hub.

The airfoil is does not have a flat back either, it's airfoil shaped
on both sides, and I gather the airfoil is different at different
locations from the hub.

This is a relatively high speed prop for a high speed airplane, a
Lancair 360.

The designer was listening to a prop "expert" lecture (I think this
was at Oshkosh but don't recall when). The guy repeated the old wives
tale about multi bladed props being less efficient than two bladed
props, and that a single bladed prop was the most efficient propeller
of all.

The designer pointed out that at cruise, the rate of advance for a
standard rpm prop was in the neighborhood of nearly 15 inches PER
BLADE, on a three bladed prop. That meant that each blade bit into
air unsullied by the previous blade. So when exactly did a three
bladed prop become inefficient?

The expert paused for a moment, then said that the point of greatest
inefficiency occured while the airplane was completely stopped.

That's a "so what". Know one cares how efficient a prop is while
standing still, we only care how efficient it is while pulling the
airplane through the air. And while at cruise, the rate of advance
guarranteed that the prop blades, even three or four bladed props,
always saw clean, undisturbed air.

Corky Scott

On Wed, 11 Aug 2004 at 11:26:14 in message
, Todd Pattist
wrote:
So we have a blade hitting air that is 15" away from air
that was hit by another blade. The pressure disturbance
from the first blade propagated at the speed of sound over
that 15" and has arrived at the "new air" 15" ahead long
before the airplane and the second blade arrive , so there's
some effect there. I'm also not a prop designer, but you
need more to convince me than just saying they are 15"
apart.

Let us assume a cruise rpm of 120knots == 202 ft sec

Then assume cruise rpm is 2000rpm

@ 2000 rpm is 33 revs per second

advance per revolution = 202/33 or 6 feet.

So three blades would arrive at the same spot every 2 feet. I am not
sure that means much because we know the flow is rotated by the prop as
well.
--
David CL Francis

Roger Halstead wrote:

Ideally a muffler could combine pulses into a steady string and
average them out. The higher the RPM and the more cylinders the
higher the frequency. The larger the pulse volume the more difficult
to smooth, The higher the frequency the easier to smooth.

Agreed.

Playing with a bit of math and an over simplification where I always
make at least one mistake:

I'm not particularly "challenged" in any way, but frankly, I don't
understand the point of your calculations. But the principle is simple
and I agree: The bigger the volume of one (!) pulse, the bigger the
volume of the muffler. Everything beyond this pronciple is beyond me,
too, and left to the specialized engineers. All I know is that I can
hear the difference between a good and a bad muffler.

Stefan

Newps wrote:

Any muffler, no matter how well engineered, puts back pressure on the
system.

No. As the exhaust system is an oscillating system, a well tuned one
even *reduces* back pressure to the engine!

If it didn't it wouldn't muffle anything.

Back pressure to the engine relates to one side of the system, noise is
created on the other side.


Rip off the exhaust system from a modern car. You'll be surprised how
much power you loose! (Don't ask me why I know.)

That's because the computer is now befuddled

In my particular case there wasn't any computer in the car. Ok, I should
have avoided the "modern" part.

Stefan

Roger Halstead wrote:

Why less RPM? Speed maybe, but why more fuel consumtion?

To get less noise with a particular prop you have to reduce the RPM.

Yes, but we talked about changing the prop.

To go with more blades to reduce the noise is reduced efficiency,

This needn't to be so.

The noise from the 2 blade prop is on take off.

The noise which causes most problems is the take off noise. En route
noise can be reduced by flying high and avoiding populated areas. Of
course a quiet plane is a nice thing en route, too.


To me, working on the muffler(s) is like trying to keep
the mouse from squeaking while the lion is roaring.

Nobody prevents one to work on both.

Stefan

Roger Halstead wrote:

One thing between Europe and the US, in general they are not flying
the same kind of planes, or the ones they do are in the lower
performance area with only a couple exceptions. Flying over there is
unbelievably expensive compared to the US.

That's certainly so. There are several reasons for this: Generally more
dense population (I'm not talking Manhattan, of course), hence more
real noise problems. Smaller distances, hence private planes are
considered luxury. And generally more environmentally minded, hence the
high taxes on fuel and more strict noise regulations. Oh, and no
colonies for an ensured oil supply! (duck and run)

Stefan

On 11 Aug 2004 11:26:14 -0500, Todd Pattist
wrote:

So we have a blade hitting air that is 15" away from air
that was hit by another blade. The pressure disturbance
from the first blade propagated at the speed of sound over
that 15" and has arrived at the "new air" 15" ahead long
before the airplane and the second blade arrive , so there's
some effect there. I'm also not a prop designer, but you
need more to convince me than just saying they are 15"
apart.

Not sure I need to convince you Todd. I'm just stating what the guy
who IS a prop designer is saying. If you want to debate, I'll look
the guy's name up and you can contact him.

But from what I can discern, the rate of advance is such that each
prop blade is biting into clean, undisturbed air. You should see this
prop, it's really unusual looking.

Corky Scott

On 12 Aug 2004 09:21:10 -0500, Todd Pattist
wrote:

Corky Scott wrote:

Not sure I need to convince you Todd. I'm just stating what the guy
who IS a prop designer is saying. If you want to debate, I'll look
the guy's name up and you can contact him.

You don't have to convince me, I'd just like to know the
answer. Monoplanes are more efficient than biplanes, so it
makes sense to me that single blades are better than
multiblades. How much "better" is the question. I would
expect anything within about a prop "span" to have some
effect.

The designer of the prop I mentioned wrote in the article about the
fallacy of single bladed prop. Regardless the dubius advantage of
biting into clean air, the problems associated with the unbalanced
thrust produced by the single blade spinning around, despite it being
counter balanced weightwise, are for all practical purposes
insurmountable.

The prop tries to rip the engine out of it's mounts all the way
around. The imbalance is pretty much impossible to dampen out and
results in monumental vibration.

Corky Scott

On 13 Aug 2004 08:20:13 -0500, Todd Pattist
wrote:

I was looking at the issue from the theoreticalaerodynamic
sense and whether 15" of separation is enough. I agree
there are lots of practical difficulties with single blade
thrust, but that does not mean they are insurmountable in
all cases. It's not unusual to see them on indoor model
aircraft where efficiency is particularly critical and the
"unbalanced thrust" is low and less of a problem.

Not absolutely positive, but I think that the difference in Reynolds
numbers between one of those 2 oz indoor mylar covered rubber band
powered airplanes and a Lancair 360 would skew the information so much
as to be non comparible.

Practically speaking, there appears to be a point where a two bladed
propeller simply cannot deal with the available horsepower and another
blade is necessary. That is why during WWII you can see the
progression from two bladed props, to three bladed, four bladed and
eventually, five blades.

Corky Scott