Prop spacers + exhaust

[Wragie]

This is sort of a weird question and I'm asking to see if real world is what
my theory tells me is right.

What I would like to know is basically what if any changes were noticed
when you changed from or to a prop hub extension. Anything is helpful
here so if all you noticed was more exhaust on the blades or quieter or
faster and so on thats what I'd like to know.

I am curious if there is a optimal separation distance from the trailing edge
of the cowling to the prop. If it doesn't seem to matter then I'm fine,
if it needs an extension then I need to replace my crank as I have
the thin flanged lycosaur crank which is unfriendly towards these.

And now for something completely different. Almost.

I'm also wondering if anyone has tried to modify the exhaust  so that it
exits outside of the prop arc. I know that would take a bit of doing but
I'd be willing to bet someone somewhere has done it.  Again wondering
if the improvements were that measurable.

I now return you to your regularly scheduled reality....

Dave

[Waiter]

I'm not sure about the first question, as I always had the 6 inch extension.  However, I did notice a significant increase in cooling by increasing the prop to cowl clearance.  My cowl was originally withing an inch of the prop, I cut the cowl so I now have almost three inches, and my CHTs came down, oil temp came down, etc.

As far as exhaust hitting the prop, if you have a two bladed prop, time it at 7/2 o'clock position , and the exhaust will never hit it.

Waiter

[Drew]

Waiter,
I wonder if the temps dropped because of the increased clearance or if they dropped due to the increased outlet area (assuming that your cowl was getting narrower towards the prop).

It would seem to me (however, with no data), that the noise level would drop by getting the prop away from the cowl.  It seems like there is going to be a lot of noise generated by the prop beating within an inch of the cowl.


BTW, how much did your temps drop?  If the outlet area did change, any way for you to estimate the initial and final area?

[Waiter]

I don't recall the exact amounts (10 years ago) but I do remember being very satisified with the results. i.e. 35 - 50 deg cooler.

I don't think the outlet area increased significantly, maybe 2 or 3 percent.

I do remember looking at oil flows and I could actually see air flow was coming in the back.

My theory is that the bow wave from the prop would actually push air back into the rear opening. Maybe someone with more insight could add to this!

Waiter

[Wragie]

Actually that is what Lopresti found when he went over the Mooney's
cowling. Remember when they went from the big pie plate opening next
to the spinner and then they were two tiny 4" dia hole as far away from
the spinner and center as possible. And after that everyone had that style
f they were going fast or just pretending to.

The prop does drag air with it as it rotates and does stuff it back into the
cowl. Lopresti found the cooling improved as well. If I recall right he thought
it was due to how much of the time the holes were covered by the blades
and stuffing the air in and dragging it back out as the blade passes. The
closer to the prop hub the worse it does this.


Thats actually kind of interesting Walter, I wouldn't have thought you
would see that on a trailing edge cowl. Guess it shows just how strong
that wave really is.

Dave

My theory is that the bow wave from the prop would actually push air back into the rear opening. Maybe someone with more insight could add to this!

Waiter

[Jack]

Actually that is what Lopresti found when he went over the Mooney's
cowling. Remember when they went from the big pie plate opening next
to the spinner and then they were two tiny 4" dia hole as far away from
the spinner and center as possible.]

When I was ground testing a Q-200 we did some tuff testing on the inlet cowls.  On the pilot's side, no air was coming in through the inlet. After trying numerous configs, we finally settled on adding a chin and we had air flowing nicely. We left the other inlet  side alone and it flowed fine.

I use the rule of thumb of double your outlet area from the inlet for cooling.

Interesting on the cowl removal at the prop for increased cooling.  One of my future goals, install some of those small computer cameras in the cowl and record airflow while in flight.

Cheers;

Jack