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Date: Wed, 15 Jul 2009 09:01:00 -0700
Message-ID: <1ab24f410907150901t3453f7aepe384f88ae4a49b21@mail.gmail.com>
Subject: Intake velocity, stack lengths.
From: Lynn Hanover <lehanover@gmail.com>
To: flyrotary@lancaironline.net
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 Fellows,
>
>  I have been thinking up this crazy idea of trying to make intake
> runner tubes out of carbon fiber. My question is regarding length of
> the velocity stack. Does the length make a difference? Could a
> velocity stack be more or less the entire length of the tube?
> Meaning, if I have a 12" intake tube, could the entire length of the
> tube be a gradual taper to the diameter of the block opening? Would
> that mess with the speed the air is traveling in the tube?
> Aerodynamics is not something I have a very good handle on, and am
> hoping someone out there in Fly Rotary land can shed some light.
>
> Thoughts???
>
> BTW,  Any reasons why carbon fiber should not be used for intake
> tubes? May make the velocity tubes a mute point.
>
> Thanks for any thoughts.
>
> Ben Schneider


It would take the rest of your life to read all there is to read about
intake (tuned) lengths. So here is my view on it.

The pipe organ idea is in play, but in my opinion is gifted too much
interest. In actual operation just about anything works prety close to just
fine. I just reviewed some formula Super Vee stuff and note that a number of
intakes were tried. Super Vee was a class where a 1600 CC VW (Rabbit)
engines were used in race cars. The stock parts had to be used but could be
modified by machining, The intake runners could be up to 32MM, and the
mechanical fuel injection (probably from the diesel I think) could be used.

HP well above 180 was the result, from VW parts. A straight 32MM tube with a
bellmouth. Not very exotic.

The tuned length of any system has many harmonic peaks. They look like the
primary frequency but are removed from it by a factor of 4. So you have
harmonic peaks at multipals of 4. So, 4,8,16 and so on. So there are
secondarys that show up to either side of the primary, and some of those are
reenforced by combinations of the secondaries, so the 16th harmonic can be
quite clear and easy to find on a scope.

The outcome is that it would be foolish to build an intake with one primary
frequency in mind. Say you want to cruise at 6,000 RPM, and built tubes to
peak at that frequency. If you cannot keep the engine at that RPM exactly,
the effect is lost. Then you find the engine in a null between peaks and
performance is now less than that of the thrown together mess on that plane
in the next hanger.

On takoff you would want to have say 6,500 or 7,000 available. Where would
you tune for that?

So I drop back to what works in most situations. In general long columns of
air operate at lower frequencies. Short columns at higher frequencies. Large
diameters at lower and small diameters at higher, and so on.

The ideal shape for intake runners is tapered. Yep, thats it. not often seen
due to complexity of construction, but if you want to talk
ideal.................

The horn shape at the end of the tube is to prevent a vena contracta that
reduces the effective diameter of the tube. A short rounded edge that turns
back 180 degrees is the very best and works well in confined spaces like a
plenum. For carbs, not so good. At harmonics of, and just off peak
frequencies, you will get a ball of fuel standing in space just outside of
the horn. In those cases, a longer tapered horn works better to cover up a
number of frequencies and make the ball less obvious.

The tuned length thing works less well in bent systems, and since the nearly
best length will involve runners over the top of the engine, there is at
least a 180 turn involved, and for side port engines, two turns involved.
So, to start off we are in tuning trouble with a bent system that tends to
null our best math picture of ideal.

Round runners are ideal for efficiency in moving air through the smallest
cross section. However, in a turn
the "D" shape takes over from the round, in that the mass of the airflow
tends to move along the outer wall, and at some speeds may be seen moving
along the inside wall in the wrong direction, and also may
slow and upset the flow along the outer wall. In most cases the "D" shape
may have slightly less cross section than the original round runner. Also
there may be improvements should a slightly course or rough surface be left
along the flat of the "D" so as to keep flow attached and reduce the higher
velocity along the outside. Uniform velocity is better than a number of
velocities or even reversed flow.

The taper............

Large tube diameters have lower velocity and small higher for any fixed
depression. If you imagine the rotor as a blade and the intake flow as a
sausage being pushed through the blade, then you are one sick
puppy.............

However, it works for me.

So would high velocity at the port face, (where the rotor cuts off each
chunk) be better than low velocity?

The higher velocity for any unit of time means more sausage (or fuel air
mixture) entering the chamber.

Yes high velocity is better. But drag in the runner develops at the square
of velocity, so if the runner is the same diameter for all of its length,
drag will be high, the boundary layer will be thick and easy to upset.
Suppose then that the taper was only in the last several inches of the
runner. Or if you want the ideal, the taper starts at the horn and runs the
length of the runner tightening to the port size at the manifold face. Less
drag overal. Thinner more stable boundary layer, and the highest possible
velocity at the port face.

Another thought..........

The primary and secondary ports feed just one rotor each set. The primary
ports are those in the center iron, and are smaller than the secondary ports
in the end irons. The engine operates for most of its life on the primaries,
as little HP is called for in normal driving. The primaries are small, so as
to maintain high velocity, and crisp throttle response.

I would join the two intake ports and run a single runner for each rotor
housing. Less volume consumed.
Lighter intake system. Smaller plenum. Probably no loss of performance. The
molded part then could be tapered, have the "D" shape, the rough inside
turn, a length of straight tube to mate to a slip tube to "tune" runner
lengths.

Lynn E. Hanover

--0022152d6389593f50046ec0a934
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Content-Transfer-Encoding: quoted-printable

<div>=A0Fellows,<br>&gt;<br>&gt;=A0 I have been thinking up this crazy idea=
 of trying to make intake=A0 <br>&gt; runner tubes out of carbon fiber. My =
question is regarding length of=A0 <br>&gt; the velocity stack. Does the le=
ngth make a difference? Could a=A0 <br>
&gt; velocity stack be more or less the entire length of the tube?=A0 <br>&=
gt; Meaning, if I have a 12&quot; intake tube, could the entire length of t=
he=A0 <br>&gt; tube be a gradual taper to the diameter of the block opening=
? Would=A0 <br>
&gt; that mess with the speed the air is traveling in the tube?=A0 <br>&gt;=
 Aerodynamics is not something I have a very good handle on, and am=A0 <br>=
&gt; hoping someone out there in Fly Rotary land can shed some light.<br>&g=
t;<br>
&gt; Thoughts???<br>&gt;<br>&gt; BTW,=A0 Any reasons why carbon fiber shoul=
d not be used for intake=A0 <br>&gt; tubes? May make the velocity tubes a m=
ute point.<br>&gt;<br>&gt; Thanks for any thoughts.<br>&gt;<br>&gt; Ben Sch=
neider</div>

<div>=A0</div>
<div><br>It would take the rest of your life to read all there is to read a=
bout intake (tuned) lengths. So here is my view on it. </div>
<div>=A0</div>
<div>The pipe organ idea is in play, but in my opinion is gifted too much i=
nterest. In actual operation just about anything works prety close to just =
fine. I just reviewed some formula Super Vee stuff and note that a number o=
f intakes were tried. Super Vee was a class where a 1600 CC VW (Rabbit) eng=
ines were used in race cars. The stock parts had to be used but could be mo=
dified by machining, The intake runners could be up to 32MM, and the mechan=
ical fuel injection (probably from the diesel I think) could be used. </div=
>

<div>=A0</div>
<div>HP well above 180 was the result, from VW parts. A straight 32MM tube =
with a bellmouth. Not very exotic.</div>
<div>=A0</div>
<div>The tuned length of any system has many harmonic peaks. They look like=
 the primary frequency=A0but are removed=A0from it by a factor of 4. So you=
 have harmonic peaks at multipals of 4. So, 4,8,16 and so on. So there are =
secondarys that show up to either side of the primary, and some of those ar=
e reenforced by combinations of the secondaries, so the 16th harmonic can b=
e quite clear and easy to find on a scope.</div>

<div>=A0</div>
<div>The outcome is that it would be foolish to build an intake with one pr=
imary frequency in mind. Say you want to cruise at 6,000 RPM, and built tub=
es to peak at that frequency. If you cannot keep the engine at that RPM exa=
ctly, the effect is lost. Then you find the engine in a null between peaks =
and performance is now less than that of the thrown together mess on that p=
lane in the next hanger.=A0</div>

<div>=A0</div>
<div>On takoff you would want to have say 6,500 or 7,000 available. Where w=
ould you tune for that? </div>
<div>=A0</div>
<div>So I drop back to what works in most situations. In general long colum=
ns of air operate at lower frequencies. Short columns at higher frequencies=
. Large diameters at lower and small diameters at higher, and so on. </div>

<div>=A0</div>
<div>The ideal shape for intake runners is tapered. Yep, thats it. not ofte=
n seen due to complexity of construction, but if you want to talk ideal....=
.............</div>
<div>=A0</div>
<div>The horn shape at the end of the tube is to prevent a vena contracta t=
hat reduces the effective diameter of the tube. A short rounded edge that t=
urns back 180 degrees is the very best and works well in confined spaces li=
ke a plenum. For carbs, not so good. At harmonics of, and just off peak fre=
quencies, you will get a ball of fuel standing in space just outside of the=
 horn. In those cases, a longer tapered horn works better to cover up a num=
ber of frequencies and make the ball less obvious. </div>

<div>=A0</div>
<div>The tuned length thing works less well in bent systems, and since the =
nearly best length will involve runners over the top of the engine, there i=
s at least a 180 turn involved, and for side port engines, two turns involv=
ed. So, to start off we are in tuning trouble with a bent system that tends=
 to null our best math picture of ideal. </div>

<div>=A0</div>
<div>Round runners are ideal for efficiency in moving air through the small=
est cross section. However, in a turn</div>
<div>the &quot;D&quot; shape takes over from the round, in that the mass of=
 the airflow tends to move along the outer wall, and at some speeds may be =
seen moving along the inside wall in the wrong direction, and also may</div=
>

<div>slow and upset the flow along the outer wall. In most cases the &quot;=
D&quot; shape may have slightly less cross section than the original round =
runner. Also there may be improvements should a slightly course or rough su=
rface be left along the flat of the &quot;D&quot; so as to keep flow attach=
ed and reduce the higher velocity along the outside. Uniform velocity is be=
tter than a number of velocities or even reversed flow.</div>

<div>=A0</div>
<div>The taper............</div>
<div>=A0</div>
<div>Large tube diameters have lower velocity and small higher for any fixe=
d depression. If you imagine the rotor as a blade and the intake flow as a =
sausage being pushed through the blade, then you are one sick puppy........=
.....</div>

<div>=A0</div>
<div>However, it works for me. </div>
<div>=A0</div>
<div>So would high velocity at the port face, (where the rotor cuts off eac=
h chunk)=A0be better than low velocity?</div>
<div>=A0</div>
<div>The higher velocity for any unit of time means more sausage (or fuel a=
ir mixture) entering the chamber. </div>
<div>=A0</div>
<div>Yes high velocity is better. But drag in the runner develops at the sq=
uare of velocity, so if the runner is the same diameter for all of its leng=
th, drag will be high, the boundary layer will be thick and easy to upset. =
Suppose then that the taper was only in the last several inches of the runn=
er. Or if you want the ideal, the taper starts at the horn and runs the len=
gth of the runner tightening to the port size at the manifold face. Less dr=
ag overal. Thinner=A0more stable boundary layer, and the highest possible v=
elocity at the port face.</div>

<div>=A0</div>
<div>Another thought..........</div>
<div>=A0</div>
<div>The primary and secondary ports feed just one rotor each set. The prim=
ary ports are those in the center iron, and are smaller than the secondary =
ports in the end irons. The engine operates for most of its life on the pri=
maries, as little HP is called for in normal driving. The primaries are sma=
ll, so as to maintain high velocity, and crisp throttle response.=A0</div>

<div>=A0</div>
<div>I would join the two intake ports and run a single runner for each rot=
or housing. Less volume consumed.</div>
<div>Lighter intake system. Smaller plenum. Probably no loss of performance=
. The molded part then could be tapered, have the &quot;D&quot; shape, the =
rough inside turn, a length of straight tube to mate to a slip tube to &quo=
t;tune&quot; runner lengths.=A0</div>

<div>=A0</div>
<div>Lynn E. Hanover=A0=A0</div>
<div>=A0</div>
<div>=A0</div>
<div>=A0</div>
<div>=A0</div>

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