R

RV8R999

Well Known Member
I'd like to fab better inlet baffling. The idea is to form a completely enclosed inlet which fits tightly within the cowl inlet section. This area in my view is the most leak prone section of the standard baffling. My idea is more fully enclose this area as part of the baffle structure and not rely so heavily upon the upper cowl itself.

What I need is a flexible matrix in which I can use glass or carbon fiber to provide strucure but retains it's shape while remaining flexible.

Any ideas?

Thanks

Ken
 
Had the upper cowl off today to swap some injector sizes. Nope, the urethane compound isn't shrinking.
 
Interesting work Dan. Are you seeing a wrinkling in the structure? I'd expect the glass would would force distortion in the shape if the matrix begins to shrink.
 
Are you using the shore 40?
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Shore 70A, with a single ply of 9 oz plain weave glass. The idea is working, but this particular design/application needs refinement. It really needs a reinforcement fabric with less modulus.
 
RV8R999 said:
Is it not rigid enough?
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Less modulus....a "stretchy" fabric.

Here's the problem. Prop rotates clockwise (viewed from the rear) so in reaction the engine rotates CCW on the mounts, around the crankshaft axis. Viewed from the left side, cylinders 2&4 move downward in relation to the cowl inlet.



Despite being encapsulated in a very flexible matrix (urethane rubber), the fiberglass fabric continues to do what you would expect; it is stiff in tension and compression. It does not allow the "tube" to deform into the desired smooth S-curve, and the result is more like thinwall tube buckling; wrinkled duct walls. Right now the objection is mostly cosmetic (it doesn't seem bother the low velocity flow of the large inlets), but this is an experiment. How can it be improved?

The primary force we wish to resist is ballooning due to dynamic pressure, 136 lbs per sq ft at 200 knots and sea level. One possibility would be to change the fiber orientation; all the fibers wrapped around the tube rather than 50/50. Instead of the current bidirectional fabric I could try a uni fabric or perhaps a wrap with 1" glass tape:



Another approach would be to simply thicken the rubber matrix cross section. The compressive properties of a fiber/matrix composite are heavily influenced by the matrix. This may be the reason standard fabric reinforced rubber automotive hoses resist buckling until bent severely.

A third possibility would be to mimic a standard hump hose:



Any other ideas?
 
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136 psi?

Dan, with this,
"The primary force we wish to resist is ballooning due to dynamic pressure, 136 lbs per sq ft at 200 knots and sea level."

are you saying that the pressure on the cold side of the engine baffling is that high? I must be understanding that wrong. Can't imagine the cowling and relatively flimsy sheet metal baffling withstanding that without blowing the cowl off. Course I've been wrong before.

Keep up the great work though. We all benefit from it and are grateful for it.

Thanks
 
That's what I missed!

Now I want to bury my head in the sand for asking a dumb question.:eek:

Had psi in mind, not psf.

Nevermind.
 
With lees modulus would you worry about it retaining shape? Or is it supported in tension through the ring fed and attachment to the plenum at the aft end?

How about a unidirectional layer radially as you depict and another at a 45 deg angle from aft outboard toward fwd inboard?

I take it the deformation remains after shutdown? Wonder if the heat plays a roll in it taking a set?

It can't be much as the angular displacement of the engine is small. Prob greatest during start prior to any significant heating affects.
 
RV8R999 said:
136 lbs/ft^2 equates to .94 psi
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Yep, dynamic pressure, q=1/2pV^2. Assuming 200 knots VNE, it ranges from 136 psf at sea level to 100 psf at 10,000 ft. The Rocket and Lancair guys deal with more; it's 305 psf at 300 knots and sea level.

0.94 psi doesn't sound like much, exactly why I use the per square foot term; the force on your top cowl or plenum lid is indeed high. For example, assume 4 sq feet for a plenum lid; that's 544 lbs. If you don't have a plenum lid the force is against the top cowl, which is why most of you can see the center of the trailing edge of the top cowl raise up in flight

The intake duct is about 8" long and 6" diameter at the small end, which would be 1.57 sg ft area if a straight cylinder. It's actually more than 2 sq ft as it is not a cylinder. The reinforcement fabric needs to withstand 40-50 lbs per linear inch. Not bad, but something to keep in mind when forming the bonded overlap seams.
 
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RV8R999 said:
With less modulus would you worry about it retaining shape?
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That is a good question.

How about a unidirectional layer radially as you depict and another at a 45 deg angle from aft outboard toward fwd inboard?
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Maybe like this, a radial uni ply plus a ply or two of BID on top and bottom?



<<I take it the deformation remains after shutdown? Wonder if the heat plays a roll in it taking a set?>>

Not really, certainly not badly. I was surprised how well the ducts straightened themselves when I removed the cowl, which now leads me to discount heat. Some of the deformation may be the result of the engine settling into a final position in the Lord mounts.

It can't be much as the angular displacement of the engine is small. Prob greatest during start prior to any significant heating affects.
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I'm estimating 1/2" vertical displacement or more at the valve cover end during start or misfire.....enough to require some engineered flex in these ducts.

So what did you have in mind for your inlets Ken?
 
I think your proposed layup would work. Will be interesting to see.

My plan is to fab similar inlets but for standard cowl. I'm in no hurry to do so as I'm enjoying flying too much. Just thinking ahead.
 
You might consider a stiffener in the top to hold the desired shape, like small diameter rods or wire. Then fasten the rubber diffuser across the bottom and part way up the sides, with a slip joint at the top?
 
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