I have constructed a full plenum on my Lancair IV around my IO-550
engine with good results. A key to getting the best performance is not only installing
the plenum, but also making a strong effort to seal all leaks from the high pressure
area under the plenum to the lower pressure area under the engine. I think
that attention to sealing of leaks, particularly big leaks such as those under
the crankshaft at the nose of the engine, is probably more beneficial to
cooling and drag reduction than the plenum itself. But that is conjecture as I
have no independent data.
As to the questions Ralf raises:
Question: can I use the standard baffling? – I
understand that the idea of the plenum is to reduce leaks and losses. Therefore
the pressure within the plenum should be higher than before. I was planning to
bolt the plenum to the standard baffling (which is pretty flimsy). The force that
is created by the pressure is now taken away from the upper half of the cowl
and has to be held down by the baffling. How
much pressure do I get in a plenum? With the area of the engine I could get an
idea of how much force I can expect on the baffling. Is this an area of concern
or am I over-thinking everything again?
I used the standard Lancair baffling, and then cut it a lot
and threw much away (particularly up front) fabricating new parts of
fiberglass. See photos in the archives under Builders Tips. Look at the
cooling drag reduction report I wrote which is in five PDF files with lots of
photos including many of the plenum I built and the steps taken to minimize
leakage. Expect to spend many hours on this project. It will pay off.
To specifics:
Not all the ram pressure is carried by the plenum. Maximum
inlet ram pressure available at the inlets at 200 KIAS is about 23 inches of
water. You get about 18 inches in the plenum (about 90 pounds per square
foot). But the loss across the engine is typically 7 inches at lower
altitudes, but may be up to 15 or 20 at the high altitudes in the turbo
airplanes. Then there is additional loss as the flow exits the cowl. If you
estimate that the plenum is perhaps 10 square feet, the load separating the
cowl or plenum from the top of the engine is perhaps 900 pounds at cruise
(turbocharged Lancair IV, 280 knots true, 24,000 feet) and could go to double that at Vne (274
knots) in a descent. So the attach points need to be numerous and strong. All
those reports of bulging cowls indicate the scope of the problem. I
stiffened my top cowl with ribs, but made them excessively deep in my opinion
which made the fabrication af fitting of the plenum more problematical than
need be. My cowl does not bulge. At all. Zilch. I made it too rigid and
created other problems.
Be particularly careful to keep the plenum at least ¼ inch
or more away from the top and side of the cowl, or you will get a rub due to
vibration or while flying through rough air.
Fred Moreno