Pusher cowling ideas

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Vigilant1

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Remember the basics of recovering lost cooling air momentum here- you must have less outlet area than inlet area to do this. In order to cool in the climb and accomplish lower drag in cruise, you'll need a variable area exit door, just like all WW2 liquid cooled aircraft did.
In every airplane, every bit of drag reduction helps improve performance. At some point, however, some of the lesser steps to reduce drag on this particular airplane are like fitting sleek racing mirrors to a dump truck to improve fuel economy (NOT that this plane is a dump truck).
I think Eugene is focused primarily on improving handling, but he's also shown an interest in drag reduction. I'm not sure where a variable exit door for this cowling would fit in the hassle/drag reduction ratio heirarchy of projects.
 

Eugene

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As I said at the start of this thread, it will be very difficult to achieve a noticeable amount of drag reduction without a dedicated radiator duct (not passing cooling air past the engine as well).
Not much I can do for outlet in the back, but air intake can be done correctly. Wondering if 3/4 inch lip would be good enough? Approximately 3/8 radius.

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Eugene

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I have another question that needs to be answered. 14 inch round plywood represents back plate of the spinner and it will be moving with engine under power forward. Cowling is naturally stationary peace. What would be reasonable space between spinner and cowling. 3/8 -1/2"? What you see on my pictures below is only about 1/8". And this will need to be increased I think.

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Vigilant1

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I don't have a precise gap to suggest, but it needs to accommodate a lot of things (engine/gearbox vibration, any sag of rubber isolation rubber as it ages, any slight variations in cowling location due to mounting hardware tolerances, etc). And, it is a big spinner, so any slight tip or yaw will be exaggerated at the edges of that large plate. So, I'd go with a generous gap (3/4"?) and include provisions for a sacrificial wear strip attached inside the cowling that you can tailor to width to bridge that gap. Just some polypropylene, vinyl, or similar material that is just rigid enough to hold its shape, but will easily give/wear away if you have a minor misalignment or major event. That'll let you close the gap aerodynamically without taking a big risk, and it makes the inevitable fine adjustments easy.

PS Your work looks really good.
 
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Eugene

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I don't have a precise gap to suggest, but it needs to accommodate a lot of things (engine/gearbox vibration, any sag of rubber isolation rubber as it ages, any slight variations in cowling location due to mounting hardware tolerances, etc). And, it is a big spinner, so any slight tip or yaw will be exaggerated at the edges of that large plate. So, I'd go with a generous gap (3/4"?) and include provisions for a sacrificial wear strip attached inside the cowling that you can tailor to width to bridge that gap. Just some polypropylene, vinyl, or similar material that is just rigid enough to hold its shape, but will easily give/wear away if you have a minor misalignment or major event. That'll let you close the gap aerodynamically without taking a big risk, and it makes the inevitable fine adjustments easy.

PS Your work looks really good.
Thank you, good idea about flexible weatherstripping! I talk to my Russian friend and looks like they have a rule to do 5 mm gap for tractors and 10 mm for pushers
 

Vigilant1

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At the risk of stating the (very) obvious or repeating something that has already been said:

You've got to make da*mn sure this cowling, in whole and in parts, stays put.

If it (or a significant bit of it) comes off in flight, the best you can hope for is a forced landing and a need to replace your cowling, prop, and engine (overspeed). The worst is a lot worse given what the kinetic energy of an unbalanced prop/engine torn loose from its mount can do and the location of that engine, prop, and cowling relative to the folks on board and especially the fragile tail booms and tail surfaces.
Again, apologies for the obvious point.
 

Pops

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I don't have a precise gap to suggest, but it needs to accommodate a lot of things (engine/gearbox vibration, any sag of rubber isolation rubber as it ages, any slight variations in cowling location due to mounting hardware tolerances, etc). And, it is a big spinner, so any slight tip or yaw will be exaggerated at the edges of that large plate. So, I'd go with a generous gap (3/4"?) and include provisions for a sacrificial wear strip attached inside the cowling that you can tailor to width to bridge that gap. Just some polypropylene, vinyl, or similar material that is just rigid enough to hold its shape, but will easily give/wear away if you have a minor misalignment or major event. That'll let you close the gap aerodynamically without taking a big risk, and it makes the inevitable fine adjustments easy.

PS Your work looks really good.
Yes, your work looks good. What you are doing is not easy and is labor intensive . Plus you have to enjoy sanding. Did I say, I hate sanding. When you are finished and take the first test flights, it will be all worth it.
 

Eugene

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Did I say, I hate sanding. When you are finished and take the first test flights, it will be all worth it.
Sanding drywall compound with 80 grit paper is relatively easy and quick way of shaping future cowling. Relatively interesting and creative process to me.

And yes I was told before that in comparison to normal people I have unusual amount of patience for everything I do. So, it's not about destination, it's about journey. But destination will be rewarding I'm sure couple years from now.
 

Eugene

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OK, cooling air outlet is going to be much larger than I need. Wondering if I can make it smaller by simply cut opening in flat section just like I outlined with black marker.

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