Gary,et al
I thought I would add a few comments with respect to some 320/360 unique issues.
"Most Lancair inlets are oversize and the velocity going in the inlet is probably half the free stream velocity."
Every Lancair model after the 320/360 saw a greatly increased inlet size in terms of in^2 per hp. Even visually the 320/360 inlets "look" small. When I measured the actual mass flow in the inlet, it was very near free stream velocity. That means there is no pressure recovery ahead of the inlet at
all. This goes a long way to explaining why many of the little two-seaters run on the warmer side.
If the inlet size is to remain this small, the design approach needs to emphasize a different set of parameters. All of a sudden internal flow control becomes super critical. This means proper internal inlet radii and diffusers to control flow expansion. The upper limit of what is possible in terms of pressure recovery is much reduced over what can be achieved with external pressure recovery. This less than optimum pressure recovery is traded against cleaner external flow. The tiny inlet with a 1.0 inlet velocity ratio will merely carve out a section of the oncoming air mass and leave the rest undisturbed.
All of the points Fred made regarding flow down stream of the inlets apply here just the
same. The negative consequence are perhaps just amplified. Leaks, for example, increase the total flow volume. Since the inlet velocity in these tiny inlets is already extremely high, leaks serve only to drive the inlet velocity to unmanageable levels - and losses.
Unfortunately many of the old studies on GA engine cooling do not provide much useful information for really small inlets as they require many features behind the inlet to be controlled as well. Studies that merely change inlet size will easily conclude that inlet velocity ratios above 0.7 are suicidal. At least one researcher added notes in the text stating that "better results could perhaps be obtained with the use of diffusers..."
Thus the success of small inlets remains very
much tied to the details of each implementation.
Chris Zavatson
N91CZ
360 std 1,400 hrs (lock-up free)
From: Gary Casey <casey.gary@yahoo.com>
To: lml@lancaironline.net
Sent: Thursday, February 14, 2013 5:36 AM
Subject: [LML] Re: IV (not IVP) Intake pictures
Paul,
Needless to say, this is a complex and difficult subject. You are right in that if you could get a high velocity airflow directed at the cylinder it would likely be more efficient than slowing the air and then speeding it back up again to go over the fins. But, the likelihood of getting all the right jets of air to blow in all the right places is almost zero. So the alternative is to slow the air down to a very low velocity and then allow it to "bath" the cylinders with air, the velocity picking up where the area between the fins is least. But that's not to say that the bigger the plenum the better. Inside large plenums you can bet there are high velocity jets and whirlwinds everywhere. So it's a matter of degree. Most
Lancair inlets are oversize and the velocity
going in the inlet is probably half the freestream velocity. That means that fully 3/4 of the kinetic energy is gone before the air gets into the plenum, hopefully converted to pressure energy. Then some more gets lost swirling around in the plenum, probably not converted to pressure. Most engine-mounted plenums I have seen are probably half to 3/4 of the volume of the cowl plenum, so I think there isn't much to be gained by using the cowl as the plenum. But the cowl is free and weighs nothing (it's already there), so most people use the cowl. You could design a minimum-volume plenum, but that gets you back to my first comment.
As you would guess, more data is better, and more confusing than less, so
sure, measure the lower cowl area as well.
And several places in the upper with a pitot tube. The static pressure measurement in the upper mostly tells you whether or not the inlets are efficient and not a lot about leakage. After all, the engine itself is likely the biggest "leak" and other leaks are small by comparison. So they won't affect the plenum pressure by enough to detect.
All just my opinions, of course!
Gary
Fred: the whole explanation is contrary to what I believed. It seems the best solution is to provide a large reservoir for the stream to enter, settle, then be fed down through the cylinders. I would have thought feeding the high speed flow
onto the cylinder might be more efficient but I can imagine how that would leave hot spots or low pressure areas inside the cowl.
I'm still a little confused as to why the larger reservoir is needed unless it is simply to assist slowing the flow and providing more even coverage of the cylinders. A plenum is a smaller reservoir and I'm guessing the lower losses of a plenum are dramatic compared to losses in a normal cowl?
So, given your summary, if a plenum is not used, the best method for measuring the seal of a typical cowl would be differential pressure measurements from pitot at inlet to static inside the cowl? If I measure static pressure within the cowl at a certain airspeed, can I make a stab at the losses in the cowl from the seals by measuring airspeed or do I need to measure static at the bottom cowl area as
well?
Paul
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