Art Mattson STC Propeller Modification - 02/02

BEFORE	AFTER

History

Last summer, I changed the prop on the plane to something more in line with the STC restrictions. This both significantly improved the takeoff performance and similarly reduced the cruise speeds. Static RPM with that prop was 2040. On 10/28/01, I ran a series of GPS speed tests with the following results:

With the same prop, fuel flow measurements were taken on the fly during cross country trips, so that the systems have plenty of time to stabilize. Here are some data with that prop. (Note, at the time I took these data, my airspeed indicator was off, so I have not included speeds here.) This engine/prop combination definitely seemed to have a "sweet spot" near 2400 rpm which gave maximum bang for the buck.

Prop Change

In February, 2002, I sent the prop to Art Mattson for his STC'ed tip modification. More information about the mod is available at the AMR&D web site. At the same time, I had him re-pitch the prop to a "strong" 66. As part of the modification (and later rework), the diameter was reduced to about 73¼″. The hope was that the combination of the slightly reduced diameter and the tip mod would leave me with decent static RPMs. This definitely worked! Static rpm with the modified prop was 2025. On 3/11/02, I ran as similar a series of GPS speed tests as I could with the following results:

I don't have comparison cruise data yet, but I will post it as soon as I do. I expect to see higher fuel flows associated with the higher speeds, hopefully mitigated slightly by the more efficient prop tip shape.

Summary Comments

We can see that with the prop mod, I was able to get a significant speed improvement of 5 or 6 knots without losing static RPMs. Seems like Mr. Mattson is telling the truth! It also seems that the efficiency vs. altitude curve may have changed, with the new prop giving 6-7 knots higher speed at the lower altitude, but only 4-5 knots at the higher altitude. However, these differences are probably within the error band of the test. Regardless, I think this was definitely a worthwhile change.

Shipping Notes

One problem with getting the tip modification done by Mr. Mattson is that you need to send the prop to him. This is not a trivial matter, as the prop is quite heavy and difficult to package. On Art's suggestion, I made up a wooden shipping container just big enough for the prop. In the box, I carefully wrapped the prop so that it could not shift around in transit. I sent it FedEx Ground, which cost about $45, and it thankfully arrived OK. I was not so lucky on the more important return trip, however. Art sent it back via USPS and the box arrived with one end broken out. Inspection showed that one of the modified tips had been destroyed, necessitating a complete re-do of the entire mod, and the loss of just under another ¼″ off each tip. Major bummer. Here are some pictures of the damaged and reconstructed tip. The line on the repaired tip is a construction reference only. Click on any photo to see it bigger.

Damaged - Bottom Side	Damaged - Top Side
Repaired - Bottom Side	Repaired - Top Side

Andy Elliott
10 March 2002