In watching the video on the new Luscombe company I recently posted here, I noticed the Catto prop on the 0-200 was extended out quite a ways from the cowling. Some mention was made of that, it was somewhat glossed over, but it gave me the impression it was done as a method to achieve better cooling.

Right now my Prince prop is bolted right up to the Rotax reduction flange, with just a 1/4" spacer. This of course saves weight and money over using a longer prop extension and is why I did it. The back of my prop is just 1/4" from the cowl. But after a recent experimental period of seeing if a full sized spinner would show noticeably less drag than my small skull cap one (it doesn't, a big sexy spinner look may look fast, but at the speeds I fly at, nothing changed a whit, other than I spent money and added weight) I ended up with a 1" spacer, that is no longer needed now that I am back to the skull cap. As it cost me a bit over $300.00, (and no takers yet, after trying to sell it for $200.00) and after getting a hint from the Luscombe video that better cooling may result with the prop out a ways, I am now considering putting it back on. To date, my interaction with hub spacers has always been just as a way to clear the cowl, anyone out there have definitive experience with extending it further than needed, to get better cooling? I'll try it no matter what, I already have the longer bolts, but would like to hear of anyone else's experiences with moving the prop out a ways and any effect on cooling.

I don't have any experience to share, but I'm subscribed for your results.

Generally speaking, you need to approach that as a whole cooling system, not simply moving the prop. If you don't have inlets and ramps that work in conjunction with the prop distance, it's not going to matter much. On air cooled aircraft engines, you are generally trying to increase the distance between the front cylinder and the prop blast so you can have more room to build expansion areas that slow the incoming air and in turn increase the overall pressure. More pressure means a bigger differential between the cold side and the hot side, which translate directly into better cooling.

That said, how that Luscombe was done, I don't really see how they used that distance effectively at all, so I'm wondering if the net gain was very much.

We ran a 6"+ extension when we were racing IF1 engines so we could minimize the inlet size and use that distance to effectively reduce the velocity and increase the pressure. The max divergence duct that doesn't crease turbulence is somewhere around 6-7 degrees from what my reading has told me.

Yes, I can see any results highly dependent on the type of prop, the aircraft's speed (not much) and the type of cowl inlet, a real "who the hell knows?!" Since I have the 1" spacer, I'll throw it on there and call it good, and see what it does if anything. I don't really have a cooling problem, but like most, come summer and a long hot climbout, I wouldn't turn down a bit more cooling. I'll get that spacer on in the next week or so and post the results, can't hurt, and since I haven't sold it yet it will at least make me feel like I'm getting my money's worth out of it.

When I was playing with hovercraft we used part of the prop wash to fill the skirt. This was done by using a partition wall and the closer the prop was to the wall the tighter the skirt got. To me this indicated more flow and pressure. I see on the lopresti mods they seem to move the inlets further out on the prop diameter and closer to the prop. In doing wool tuft tests on our Bearhawk and my 172 we found a huge amount of recycling going on. The prop was shoving the air into the cowl but an amazing amount was coming back out around the prop shaft area just behind the spinner. We tightened that area up by various means and saw a noticeable increase in cooling.

Hi all,

I can speak to the prop extension on the Model 8 LSA. It was purely out of desperation that we selected the prop & extension combination. We urgently needed to get the plane ferried and learned that our Sensenich ground adjustable prop was not airworthy. Sensenich offered a new prop in 6 weeks, so I called Catto and they offered us an O-200 prop overnight. Catto props have a wood core and the LSA has an O-200D with a "daisy petal" prop hub. The two don't mate well to each other so an adapter is required. Saber happened to have one in stock, and yes it was a bit longer than I would have preferred.

While not necessary for an LSA, we treated this project as we would any other aeronautical engineering modification. The installation was subjected to a design review, ground test, and flight test under the supervision of a DER who specializes in FWF modifications. We discovered during flight test that the plane performs very well in climb and cruise, with lower EGTs.

The prop we have mounted now is optimized for a float plane installation and we consider it temporary. Catto is making us a new prop and we intend to test it on the Model 8 LSA (with O-200D), starting with a .75 inch extension. We'll monitor speeds, rates of climb, RPMs, EGTs and CHTs and see what happens.

Yes, I agree that there are many, many variables that go into optimizing prop/engine/cowl combinations for drag reduction and improved cooling. And there are a some very talented engineers in the world who specialize in this discipline. They are a rare breed, and none are on our staff.