LoPresti Cowl and Cooling Seminar
This info was provided by John Cox with the following note:
"A key point is the LoPresti Poor Boy Tufting process to read cowl pressure corrections without the time and expense of Tufting. They take a quart of Turbine oil and mix it with a quart of engine oil and add black toner cartridge carbon to make a spray able solution. It looks like shit but can be sprayed with a cheap
garden sprayer and washed off with degreaser. It allows the project
coordinator to read the lines of backflow, high and low pressure more
Now, on to the info...
What Makes an Inlet Good?
Yes, we really will
total pressure behind a propeller blade does not change much with useful
regions close to the spinner propeller blade sections can have bad profile
due to stiffness requirements.
inches away from the spinner are low in energy and can even have reverse
that have their inside edge in contact with spinner contour so the inlet
will always ingest this chopped up flow.
This chopped up flow contributes to bad inlet performance.
inlets are set off from the spinner sufficiently so they only see high
Control and direct
throats can be reduced in size thru the use of inlet diffusers.
diffusers are a specially contoured expanding duct which is an extension
of the inlet throat.
expansion reduces the flow velocity beyond what exists in the throat and
recovers additional pressure recovery.
small high speed inlet can recover more additional pressure then a large
slow inlet with equivalent duct length because there is more velocity to
additional pressure recovered is proportional to the square of the
velocity ratio (again by Bernoulli’s equation).
path lengths for diffusers can be rare in general aviation engine
installations. Typically there is
only about 4 inches or so for diffusers.
a closed duct is impractical because the path may be cluttered with engine
items like prop governor, alternator or oil cooler so full diffuser
performance cannot be achieved.
Determining factor in
gathering maximum pressure
design requires placing inlets in regions of high static pressure and
exits in regions of low static pressure.
- A region
of high static pressure would have a low velocity but constant total
inlet wants total pressure, it does not care if some of the energy is
portioned into static pressure or not.
don’t want regions of high static pressure, you want high total pressure. There are positions very close to the
prop where the energy level exceeds ideal slipstream levels by 25% or
LoPresti inlets are positioned in this energy rich environment. Also, if
you put inlets in high static pressure areas there is a good chance that
velocities are so low as to be full of low energy turbulence that would
slide into the throat.
that is the case, total pressure in the throat can be significantly below
ideal slipstream energies and could be considered another type of inlet loss.
- LoPresti inlets sit proud of basic body
contour high enough so low energy flow is diverted away from the inlet
entrance and only ingest air at maximum energy.
(Why you WANT them)
- You can force an exit to cause low static
pressure with a step in contour, an open cowl flap or a fixed suction bulge.
- These areas o suction are a major contributor to
pressure drag and should be avoided if proper design practices are followed.
- What you want is areas of natural suction in the
slipstream away form the body surface.
- The normal exit location on the cowl bottom
centerline near the firewall is a great location which is far enough from the
prop for free suction in cruise and minimum positive pressure in climb. This cruise suction does not contribute to
What you see is NOT
what you get
performance in climb can be difficult to predict accurately.
inlets are feed different levels of drive energy, are running at different
throat velocities, can have different diffusers attached that scrub different
amounts of extra pressure and make un-even contributions to plenum
can we make sense of all this confusion?
It gets easy when you are armed with a few facts about inlet
if you have a chosen design flow rates, you know the average of throat
velocity by volume divided by area.
you need to solve for a reference velocity for each inlet form its
supplied total pressure. Each inlet
has its own reference velocity to calculate its own velocity ratio. The absolute velocity in each inlet that
is governed by the flowing rule: The ration of absolute velocities of an
inlet pair is equal to the square root of the ration of throat total
- This means” If you don’t like math..
Don’t design your own cowl.
- LoPresti cowls are optimized for cruise
cowls would not cool at climb without the introduction of a potent cowl
- If no cowl
flap were added inlet/exit geometries would have to be compromised and the
drag would increase dramatically.
- The importance
of having a cowl flap increase with cruise speed.
flaps in climb can supply more than 1/3 of the work load that would
normally be handled by the inlet alone allowing a generous reduction in
- How do
cowl flaps work?
- The suction
make can be predicted by Bernoulli’s equation.
- The extended
flap contour causes an additional acceleration to the local flow followed
by a sudden expansion.
- It can
be shown that the generated suction is equivalent to the tangent of the
deflection angle squared.
- These predictions
are accurate beyond 30 degrees of deflection.
drag can be reduced by closing off the exit to choke the flow but the
speed comes at a price of higher temperatures.
- When we
are in the final throws of new cowl testing we always investigate
different exit sizes and cowl flap length. We call that “tuning” the
losses are reduced due to increased exit speeds. What is happening is you are taking drag
out of the momentum pocket and putting tit back in pressure drag pocket.
the exit chokes the flow.
- The back
pressure increases and the pressure drop goes down.
the flow rate is reduced the inlet slows down.
slower inlet can recover more pressure so pressure drag is increased.
- The pressure
drag increase is exactly equal to the reduction in momentum losses so
airspeed remains unchanged.
- This is another example that
illustrates that cooling drag is both momentum losses and pressure drag.