A friend is looking at doing the conversion from a 2-blade Mac 72" to the Top Prop Hartzell Scimitar 3-blade. His plane is the Cessna XP II Hawk.
They advertise increased pulling power and faster cruise speeds, but it's an expensive 1-3kts cruise to me...

What say you experts? A good STC for this aircraft, or not?
The Cessna is mostly doing long cross country work, lots of mountain flying, with occasional short field take-off.

Hartzell will eat him alive with hub ADs. Plan on a hub every 5 to 7 years....and they're not cheap......but they reduce Hartzells liability

Tell him to look at an MT if they're approved.

MTV

And they are [approved]....

Forgot about the horrendous Hartzell hub. I have been told of the AD horror before.

What's wrong with the bulletproof Mac, which is fine for thousands of hours, condition inspection, etc?

Any 3 blade will save maintenance dollars on all the stuff the 2 blade Mac shakes apart. I would also avoid Hartzell. I've found them to be a chickenshit outfit.

I love mine on the continental 470 in my skylane. Everyone comments about how smooth it is. I like it because it's easy to keep clear of the dirt compared to a 2 blade. I pay a guy $300/year to keep it balanced, sent it to the prop shop once because it blew a seal. I was disappointed that it won't work on a pponk. I hope I can sell it for a decent price when I put a new engine in.

If you want to get semi scientific about prop impacts on performance, and if you can get your hands on the new prop to measure the blade chord along the span, you might look into John Lowry's Bootstrap method. Google avweb bootstrap to find a series of three articles from the late 1990s. I'm just in the process of finishing an article on this topic for the Bearhawk newsletter.