tuft testing

Corky Scott wrote:
On 29 Oct 2004 05:17:03 -0700, (sidk) wrote:


No, there are no other cooling air inlets. The cooling air flows down
thru the pressure cowling, down thru the cylinders, and exits out the
back around the prop spinner.
This photo shows a rear, in-flight view. The small, dark object on
the bottom cowling is the left exhaust

http://img96.exs.cx/img96/5820/Velocityinflight.jpg

This Velocity was one of the earlier versions (early '90s). The
builder/previous owner told me that this was the third attempt...
The first (IRRC) was pitot type scoops on the bottom cowling. Then
pitot type scoops on the bottom of the strakes a few inches outboard
of the bottom cowling with internal ductwork (about 2 1/2 id) piping
the air into the engine compartment. Finally, apparently the Velocity
people came up with this current design and the plane was retrofitted
shortly before I bought it.

Sid


Sid, I guess I'm stating the obvious then, the cooling system as
designed does not appear to flow enough air through the ductwork to
cool the engine properly.

You can probably improve the inflow by attaching scoops on top of the
Naca inlets. I'd recommend the scoops be standoffs, like the P-51
belly scoop so that you do not have turbulent boundary air flowing in
and disrupting the flow. You can even shape them to emulate the shape
of the Naca inlet where they attach to the opening. This should be an
easy thing to fabricate and attach for a test. Just tape them over
the inlets and fly. If you see the cooling improve, you're on the
right track.

Corky Scott



Sid, does the root airfoil of your propeller compare well with what the
factory has?

Air will flow with any pressure gradient. The factory design looks to
me like it is trying to accelerate the boundary layer along the back
half of the fuselage by having it 'sucked' through by the propeller. Of
course this will only work if the root of your propeller actually sucks.
Some blades are designed to no do anything at the root (Ivo is one, I
believe), and won't give you the pressure gradient you need.

--
http://www.ernest.isa-geek.org/
"This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Ernest Christley wrote

Sid, does the root airfoil of your propeller compare well with what the
factory has?

Don't think there is a "factory standard" (?) ... mine is an MT
(electric) constant-speed, "made for pushers".


Air will flow with any pressure gradient. The factory design looks to
me like it is trying to accelerate the boundary layer along the back
half of the fuselage by having it 'sucked' through by the propeller. Of
course this will only work if the root of your propeller actually sucks.
Some blades are designed to no do anything at the root (Ivo is one, I
believe), and won't give you the pressure gradient you need.

Yes, I have wondered about the possibility that maybe near the hub the
blades might be churning the air so much that there is some slight
back-pressure trying to push the cooling exit air back (forward) into
the cowling exit holes. I had taped some tufts just inside the exit
hole in the left side but none of the photos were able to see this
region.

At this point, I think a test by temporarily fastening some scoops to
the tops of the existing NACA ducts (as Corky posted above) is
relatively easy to do and may disclose a lot.

Sid

"Sid Knox" wrote:
At this point, I think a test by temporarily fastening some scoops to
the tops of the existing NACA ducts (as Corky posted above) is
relatively easy to do and may disclose a lot.

Be certain that the temporary scoops are well attached. Losing a scoop in
flight is almost certain to cause it to pass through the prop disc, turning
an interesting experiment into a way-too-exciting flight.

Russell Kent

Russell Kent wrote:

"Sid Knox" wrote:
At this point, I think a test by temporarily fastening some scoops to
the tops of the existing NACA ducts (as Corky posted above) is
relatively easy to do and may disclose a lot.

Be certain that the temporary scoops are well attached. Losing a scoop in
flight is almost certain to cause it to pass through the prop disc,
turning an interesting experiment into a way-too-exciting flight.

Russell Kent


Yeah, especially 200' up at the end of the runway!
John