Figure (as pdf file)
Flight 3, May 3:
We cut down the thickness of the porous diffuser to reduce the power
needed to pump suction air through it. We also re-plumbed the system to
enable us to achieve higher flows, by eliminating constrictions and in some cases replacing needle valves with ball valves. The window plate was
rebuilt to align the LTI with (what we believed was) the local flow angle at the LTI tip. We believed that the uncorrected sideslip at that location
might be contributing to our inability to achieve laminar flow at flow rates near isokinetic.
Some turbulence control was achieved, and we were able to pull all the
flow we needed to exceed isokinetic flow. However, as we approached
isokinetic speeds, the flow became turbulent. The upper panel in the figure below shows the turbulence intensity vs time for the entire Flight 3. The
periods of high turbulence are all when the tip velocity exceeded 70-80 m/s (vs the airspeed of 110 to 140 m/s). We were only briefly able to
stay laminar while isokinetic.
Flight 4, May 4:
The turbulence at higher flows suggested that the LTI tip's sharp leading
edge might be the problem. When sampling sub-isokinetically, the flow is
diverging near the tip, and separation due to misalignment is inhibited. But misalignment should cause greater problems when the flow is closer to
isokinetic. To test this idea, we glued on a curved leading edge for Flight 4. The rounded leading edge had a diameter (the junction of the inner
4.5:1 ellipse and the outer 3:1 ellipse, the forward-most point) of 0.496 inches, compared to the inlet throat diameter of 0.44 inches. Thus, if the
stagnation point of the flow were at the junction, the effective cross-sectional area of the inlet would be increased by 27%.
The lower panel of the figure above shows the dramatic improvement which
this curved leading-edge caused. The flow was laminar under virtually
all conditions, short of turning the suction pump off. Clearly the impact of misalignment is eliminated by this curved leading edge. Since normal
motions of the aircraft and changes in fuel load during a flight can change the attitude of the entire plane by several degrees, our inlet has to be
insensitive to misalignments on the order of 5 degrees or less.
This curved leading edge, however, implies that 1) the total flow should
increase by 27% and 2) at least 27% of the flow will have curved around
the inner ellipse, potentially causing problems for particles due to bending streamlines. Reducing the diameter of the forward-most point would be
Originally I had imagined that we would shroud the LTI to ensure proper
alignment. But the more Russ Seebaugh has shown me about the ways
in which shrouds accelerate air and bend streamlines, the more I think it would be a thesis project in itself to design an optimum shroud and then
demonstrate that a super-micron particle distribution was not significantly changed by it. It seems to me that the impact of this small curved leading
edge might be easier to define and it may potentially cause less modification to the size distribution than a shroud. If we can minimize the area
increase due to the curved leading edge, that will make it even more attractive.
Flight 5, May 4:
It seems that the misalignment during earlier flights must have been
greater than we thought, even though we oriented the LTI according to data
from a Rosemount 858 probe mounted 6" below the LTI. To examine the possibility that a bow-wake from the LTI was changing the flow angle
at the 858, we removed the LTI and flew a short flight with just the 858 probe on the plate. Our fears were confirmed: removal of the LTI caused
a change of between 2 and 3 degrees in the flow angle at the 858. Thus, we did not have the LTI properly aligned into the flow on flights 3 and 4.
However, the curved leading edge took care of that misalignment during flight 4.
Flight 6, May 31:
We flew the LTI (without the 858 probe) using a smaller curved piece
on the inlet tip. This one scaled the ellipses back so that the diameter
forward-most edge is 0.471 inches, an area increase of 14%over the sharp edge (throat) area. It performed beautifully, even when sampling
super-isokinetically. (The flight 4 figure is very much like that for Flight 6.) The only time it became turbulent was during the slowest speed of the
speed runs, when the aircraft has to pitch upward by about 5 degrees. Even then it was possible to find laminar conditions, though. This suggests
that we are able to accommodate deviations of 5 degrees or more with this smaller curved edge. At 14% area increase, I have fewer concerns
about the fraction of the flow that may have suffered curved streamlines. Furthermore, since the flow is laminar, any particles bouncing off the
curved edge are likely to be sucked into the wall with the 80% of the flow that is being removed.
Flight 7, June 1:
We tried further reducing the curved piece, to a size that only increases
the area by 7%. (This edge looks to the naked eye to be like the sharp
edge, but you can feel a slight difference with your finger.) Unfortunately, this was not enough curvature: the inlet behaved as it did in Flight 3,
becoming turbulent as we got close to isokinetic flow. Thus, the 0.471 inch diameter curved piece we flew in Flight 6 provides the optimum
C-130 Flight, May 30:
To align the LTI properly on the C-130 for the PELTI flights in July,
we need to know the local flow angles a the LTI mounting point on the
upper-right fuselage of the C-130. Since we do not have models of airflow at this exact point, we used the opportunity of the C-130's trip to DIA
for its post-TOPSE washing to get flow data from the 858 probe at that location. A series of maneuvers was flown at two altitudes to see how
the flow responds to changes in attitude and airspeed. This data will be invaluable in orienting the LTI for PELTI.
The present version of the LTI only expands the sample flow to 3/4"
ID pipe. However, the sample tube needs to be 1 3/8" ID, to slow the flow
before it makes a 90 degree turn to enter the fuselage. This extra expansion will change the geometry of the LTI considerably. Bernie Lefleur has
done lab work on this issue and has found that he can maintain laminar flow with three sections: the first is the same that we have flown (0.44 to
0.75" ID), but will be fabricated of more porous stainless (60 um pores) that is to be delivered in a few days. The second piece (0.75 to about
1.0") will have considerably higher flow resistance (lower porosity and thicker material), since it has less need for suction at lower flow velocities.
The last piece (1.0 to 1.375") will be solid stainless. The trick in combining these is to keep the second piece from taking most of the suction flow,
which is needed most at the forward tip, where the velocity is highest. This new LTI will have a cone length of about 10".
5 June - Drawings of PELTI LTI completed
9 June - 60 um porosity front diffuser ready for lab testing
12 June - All parts of final PELTI LTI machined and ready for testing.
15 June - Electra test flight of PELTI LTI
22 June - First PELTI shakedown test flight of entire C-130 package
29 June - Final PELTI test flight
5 July - Depart Jeffco for St. Croix. Overnight in Miami 6 July.
7 July - First research day in St. Croix
25 July - Last possible research day in St. Croix
Incidentally, our test aerosol has arrived in force. The dust season has begun in St. Croix. A local report forwarded by Joe Prospero said:
"The dust moved into the Virgin Islands on the night of 29 May and continues
to be heavy this morning, 31 May. Red dust is settling on surfaces and
residents are complaining of sinus problems and headaches (beginning 30
May). Š This is one of the heavier events I have witnessed in 17 years here."
While I'm sorry for the residents of the Virgin Islands, this bodes well for our finding good dust layers to fly in to test the LTI.