Jerry Hey wrote:

Paul wrote

Scoops like that don't work Jerry. If they did there would be no
need of boundary layer air bleeds like are found on all airplanes
built since World War two. What happened to the tunnel idea?

The pictures of air bleeds that I have seen seem to have been very
shallow perhaps only a quater of an inch or so.  How thick is the
boundary layer under the belly ?  Seems like a scoop hanging down
below this level  would be in clean air, and it would generate
considerable pressure to enter the inlet.  Like I was very careful to
say, these are just aluminum sheet metal that can be discarded and
new versions created fairly easily.  Perhaps in the end, I will have
just a scoop with an airbleed.  It would be the easiest of all
configurations to build.  How deep should the air bleed be?.   Jerry

I don't know. It is hard for me to say. That is why I wanted you
to do the velocity rake trick. Here is the old message.

"You are in the boundary layer.
Make a velocity rake which is a series of six simple pitot tubes
spaced 3/4 of an inch apart and measure the pressures below the fuselage
with this plastic tube water manometer and tell me what they are.
Use the cockpit for a static reference pressure. The lowest pitot
tube in the velocity  rake should be about four inches below the skin.
The numbers will be the difference in the height of the two water
columns in inches. Fill the U tubes about half way up. Drip irrigation
tees and angle connectors are available in the garden shop to make
a water manometer. PL"

Paul Lamar

I have been lurking here for some time and find this whole topic very
interesting.  I have been wondering if you could place some vortex
generators ahead of your inlet to eliminate the boundary layer problem?  -
Amos

Might work.

Vortex generator generate low pressure in the vortex core behind
them and they work best when the boundary layer is thin.
This local low pressure keeps the flow above and behind the vortex
attached to the skin for awhile.  It is like a magnet for air flow.
It delays complete separation of the airflow. The short
explanation is; they energize the boundary layer.
This additional local airflow increases the low pressure over
the curved surface behind the vortex generators.

The boundary layer has a lot of distance to build up on the
bottom of the fuselage so it is probably pretty thick back
there under the cabin.  If they work the pressure
would be lower which would not be  good for a NACA duct.
The local velocity close to the skin  should be higher
which is good for the NACA duct. Net change????

The velocity rake and pressure taps should tell the tale
but it might take days of experimentation to find the optimum
position of the vortex generators. Sounds like fun to me if I had
nothing else to do.

Don't forget you still have the problem of the rad air resistance slowing
the velocity of the air going into the NACA duct disabling it.

A fried rotary engine can ruin your whole day.

Another reason  NACA ducts work on canard pushers, besides the prop
sucking the air in, is the air resistance of the fins on an air cooled
aircraft engine is probably less than the air resistance of a radiator
with equivalent cooling capacity.

Paul Lamar

"Bass, Jerry" wrote:

I guess I don't understand boundary layer bleed.  I do understand the need
for smooth expanding inlet tunnels for air to coolers, and such.

I think I have a lazy injector now.  One lobe is always hot and I tried
Tracy's method of richening up one lobe, and nothing changed.  I had
previously thought that one injector was leaking causing a rich lobe.

I will trouble shoot the two injectors of the hot lobe this week and report.

With intakes very near the front of the airplane like yours boundary layer
thickness is less of an issue.
      
Paul Lamar
 
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