tuft testing

Corky Scott wrote in message
Sid, I hope that there are some other scoops somewhere besides the two
I see on top of the airplane. From what I've read over the years if
the two scoops I see are it for cooling, there are two really glaring
problems. 1. Naca scoops are really marginal for engine cooling
intakes, this is something that has been known for many years. 2.
Naca scoops do not work when placed in a low pressure region of the
fuselage. Your two scoops are on the downside of the rear of the
fuselage and for sure are in a low pressure area.

What works? Pitot type scoops placed in an area of positive pressure,
or out in the free air stream ala P-51 Mustang.

But if you have pitot type scoops on the belly of your airplane,
nevermind.

Corky Scott

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

On 29 Oct 2004 04:48:39 -0700, (sidk) wrote:

Downdraft cooling is prone to problems at low airflow velocities
because of the tendency of heated air to rise. Low airflow velocities tend
to coincide, unfortunately, with situations that need the best possible
cooling, namely climbs (high power, low airspeed).

Yes, and the increased AOA during these conditions probably make the
situation even worse.

Sid

This is where exhaust augmentation would improve things, if properly
done. Unfortunately, with a rear mounted engine, exhaust augmentation
is nearly impossible, simply not enough room.

Normally, exhaust augmentation has it's greatest effect exactly when
the engine needs it most: during takeoff and climb when airspeed is
low and power is high.

Corky Scott

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

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)."

Generally the tufts are placed aft of the leading edge where there is no
laminar flow. Since the tape is in the turbulent boundry layer it has no
significant effect, any more than protruding rivet heads or skin laps do aft
of the laminar region. I don't think tufts on the leading edge are a good
idea. Putting them across a wing is mainly to observe the degree and
progression of flow separation at stall. Say if you wanted to install a set
of vortex generators on your leading edge and needed to find the most
beneficial location, you would tuft the wing behind them and film the tuft
behaviour at stalling AOA with the VGS at various chordwise locations to
find the optimum spot to place the VGs.

I'm an airline support techie for a regional aircraft maker and last summer
I put tufts on the flap of a regional jet that was mysteriously rolling with
flaps fully down and with seemingly symmetrical flight spoiler deployment.
The tufts revealed that when the flight spoilers were retracted to about 8
degrees one side had attained smooth flow in the slot between the spoiler
and the flap, but the other side was still transitioning from turbulent to
smooth because it was a tiny bit higher then the other (but within service
limits). This angle happened to correspond with the first detent on the
deployment lever (it's fly by wire) so the lever could be selected to this
position and all of a sudden the aircraft would start to roll. Move the
lever a tiny bit higher or lower and the roll would stop as symmetrical
spoiled or unspoiled lift was regained. I stuck them on pretty much by
eyeball using speed tape cut into little triangles.

John



"keepitrunning" wrote in message
om...
I was reading and saw a picture of a wing being tuft tested. I was
wondering, since I do not have a clue, how much difference the tape holding
the tufts would cause. In the picture it appears that 1/2 inch blue
masking tape was used and it was torn rather than cut. The tufts were
spaced a couple of inches apart and there were seven rows and eight
columns. His results were that the wing was not showing any separation.
Just wondering if there are accepted methods of doing this type of testing?
How would you determine where to test? Any discussion and comments would
be appreciated.

Corky Scott wrote in message

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

Yes, that's a good plan except in my case I cannot "...Just tape them
over
the inlets and fly." because if one gets free it will go thru the
prop! However (as seen in the photo), there is a series of top
cowling screws that I can use to secure a temporary scoop.

Sid

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

WD-40 removes duct tape goo like magic...
-Bob Korves

"sidk" wrote in message
om...
Here is a link to a photo from last month's tuft-testing on my
Velocity with hopes that some clue would appear as to why my engine
cooling is worse than marginal. Tape was duct tape and some was on
for several months (waiting for weather, photographer, digital camera,
photoplane, etc...) and was a pain-in-the-butt to remove.
For the test, the left (pilot) side NACA has two VGs, the right side
none.

http://img35.exs.cx/img35/5182/HPIM0430.jpg

Sid Knox

Velocity N199RS
Starduster N666SK
KR2 N24TC
W7QJQ

http://img35.exs.cx/img35/5182/HPIM0430.jpg

"keepitrunning" wrote in message
. com...
I was reading and saw a picture of a wing being tuft tested. I was
wondering, since I do not have a clue, how much difference the tape
holding
the tufts would cause. In the picture it appears that 1/2 inch blue
masking
tape was used and it was torn rather than cut. The tufts were spaced a
couple of inches apart and there were seven rows and eight columns. His
results were that the wing was not showing any separation. Just
wondering
if there are accepted methods of doing this type of testing? How would
you
determine where to test? Any discussion and comments would be
appreciated.

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

"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