[Mark Wright]

Used PMC 780 and a 3D print as a plug after scanning plenum and inlet rings to fabricate the inlets.

Positives - fit is great. And it is fun playing with new materials such as the liquid polyurethane!

Negatives - I tried to lay up some fiberglass on top of the polyurethane and wet it out without putting it between plastic film and forcing the PMC into the fiber. Doesn't work. The inlet position is not ideal, but I'll experiment with moving the rings vertically to provide better airflow once flying. Would like a baseline before making changes.

Results - an enjoyable project and fairly simple to do. Remake with saturated fiberglass, with a small reinforcement of a single layer of fiberglass at the sharp corners. Might try using carbon fiber on the next attempt.

 

[D-Dubya]

Interesting concept! Does it have enough flexibility to "give" without tearing during the engine shaking associated with a normal start-up and shutdown? Or is that still to be determined?

 

[Mark Wright]

Interesting concept! Does it have enough flexibility to "give" without tearing during the engine shaking associated with a normal start-up and shutdown? Or is that still to be determined?

I am standing on the shoulders of giants with this concept of using polyurethane. DanH pioneered the idea. I'm just imitating to the best of my ability.

It does have good flexibility and is an alternative to using the neoprene sleeve to connect rings to plenum. Dan indicates that his have held up well for over 1000 hours

 

[scsmith]

Do I remember correctly that the Urethane does not have good durability when exposed to a lot of water?
I think for many of us it would be minor concern, but it might matter to some folks.
Or am I confusing a different material?

 

[DanH]

Do I remember correctly that the Urethane does not have good durability when exposed to a lot of water?
I think for many of us it would be minor concern, but it might matter to some folks.
Or am I confusing a different material?

Polyurethane comes in two general types, polyester and polyether. Polyester has good resistance to oil, but not water. Polyether is water resistant, but not so good for oil.

No significant oil concern at the cowl inlets, so a polyether compound is the better choice.

I knew none of this when I built flexible inlets 15 years ago. First time I parked the airplane in the rain, I was surprised to see white patches form in the bottom of the inlet where water formed a puddle. When the water dried, the rubber turned black again. Doesn't seem to hurt anything, and it has seen many wet-dry cycles since. Yep, must be polyester.

Original thread, circa 2009: https://www.vansairforce.net/threads/tip-rubber-ducts.43391/#post-344338

Photos below are relatively current. The grey patches aft of the inlets are another experiment, plenum lid soft spots made from one ply of 9oz glass filled with Loctite 598/Permatex UltraBlack silicone sealant. RV-8 builders with angle valve engines are well aware of how little clearance is available above the front cylinders and on the left side. The soft spots allow the plenum lid to bang the inside of the upper cowl without concern. Fabrication is just like the cylinder baffle wraps; squeeze the sealant into the glass fabric between two sheets of HD plastic, 4 mil or more. These too are holding up fine, so there is potential for more complex fabrications using UltraBlack, and offering better resistance to both oil and water.

 

[Webb]

Do you have a picture of how the plenum is supported?

 

[Mark Wright]

Do you have a picture of how the plenum is supported?

I'm pretty sure you are referencing the supports for Dan's plenum, but here is how I did mine. Aluminum angle attached to the top of the baffling with #6 screws. Used stretcher/shrinker to obtain correct contour of the angle aluminum.

 

[wawrzynskivp]

Used PMC 780 and a 3D print as a plug after scanning plenum and inlet rings to fabricate the inlets.

Positives - fit is great. And it is fun playing with new materials such as the liquid polyurethane!

Negatives - I tried to lay up some fiberglass on top of the polyurethane and wet it out without putting it between plastic film and forcing the PMC into the fiber. Doesn't work. The inlet position is not ideal, but I'll experiment with moving the rings vertically to provide better airflow once flying. Would like a baseline before making changes.

Results - an enjoyable project and fairly simple to do. Remake with saturated fiberglass, with a small reinforcement of a single layer of fiberglass at the sharp corners. Might try using carbon fiber on the next attempt. View attachment 63467

If you are going to redo consider Kevlar as your fiber material. Only downside there is the need to modify your shears when cutting your patterns.