Aeromatic Prop & TC A-799 Specs

[Bob Carter]

Kent Tarvers recently received approval for his Aeromatic Prop. In researching the application of the Aeromatic Prop on 170's, the TC # A-799 specifies that a C145-2H with a dampened crank is the only engine eligible for the installation. According to Tarver: "The Aeromatic prop needs no governor, cockpit control nor a hollow crankshaft... It is entirely controlled by dynamic forces, centrifugal forces, air loads...etc." My engine is a C145-2 with a dampened crank and I assume that most C-170's do not have the "H" engine designation. Can someone explain the requirements for the “H” engine designation????

[GAHorn]

While the Aeromatic may not require any hydraulic support, if the type certificate/approval stipulates the "H" engine....we are stuck with that requirement unless Mr. Tarver can convince the FAA to revise it. He's the person best in position to do that.

The type certificate of the engine is the best source of specific details on the engine differences and can be found at:

http://www.airweb.faa.gov/ in the Regulatory and Guidance Library.

Specal attention to Note 4.

"NOTE 4. Model designation suffix "H" on model C145-2 denotes the incorporation of crankcase and crankshaft
provisions for use of a hydraulically controllable propeller from the engine oil pressure. Model designation
suffix "P" denotes engine eligible for pusher installation as permitted by a special crankcase and front main
bearing."

The problem with the Aeromatic versus the McCauley or Sensenich all metal props is that the Aeromatic is a wooden blade design, and as such suffers from many of the same deficiencies of other wooden props (plus a few more potential problems.)
1. The wood blades, in order to be sufficiently strong, are thicker than metal blades, therefore less efficient. Much of the advantage of changeable pitch is lost due to that loss of efficiency.
2. The blade attachment to the hub is a complicating maintenance factor, and has in the past been subject to AD. It is certainly less strong than any one-piece, all-metal design.
3. Inspection procedures are more complex than all-metal designs, as is erosion, crack, or other repair schemes due to the plastic overlay/covering of the wooden blades.

In short, while it'd be nice to have a simple, adjustable-pitch prop for the 170....the fact is that simple, all-metal, fixed-pitch props are a durable, reliable, relatively inexpensive compromise. (I'd love to find an airworthy original McCauley ground-adjustable all metal prop, but...if someone called me with one for sale...I'd have the same dilemma probably anybody else would ....be reluctant to part with the necessary funds to adequately pay for it and just continue to operate my fixed pitch McCauley, unless .....my prop needed replacement anyway.)

[Bob Carter]

The TC A-799 states: "With this propeller installation the airplane performance has been demonstrated to equal or exceed that presented in the Airplane Flight Manual with a fixed pitch wood propeller over the altitude and temperature range shown" (emphasis added).

The reference to a fixed pitch wood propeller seems confusing when the performance information in the Flight Manual is for fixed pitch metal props. Is there any data to support the Aeromatic/Cessna claim??

[GAHorn]

The aircraft type certificate was awarded with a wood prop, and any subsequent installation must either meet-or-exceed that....or the performance data must be documented/re-certified.
The McCauley metal prop (MDM/DM series) was documented in the CAA (FAA) Approved Flight Manual (Not the Owner's Manual) which is required to be onboard the aircraft per the type certificate. That AFM documents the change in performance from the original wood prop.

(While the Owner's Manual re-states/duplicates the AFM performance data,..the Owner's Manual is NOT an Approved document, and cannot be used to replace the required AFM to be onboard.)

If you want documentation that the Aeromatic meets/exceeds original wood prop performance...then you should contact the person making that claim and require that of them. That the statement is made in the aircraft type certificate is generally accepted that it is now FAA approved data.

NOTE that the type certificate places other requirements on the installation of the Aeromatic prop.
"Installation of this item must be in accordance with Cessna dwg. No. 0550103 and adjustment and operation must be in accordance with Koppers "Adjustment Instructions and Operation Limitations No. 45." Item 108 with seaplane lip, Item 402(c), and C145-2H engine with dampered crankshaft are required when this propeller is installed. With this propeller installation the airplane performance has been demonstrated to equal or exceed that presented in the Airplane Flight Manual with a fixed pitch wood propeller over the altitude and temperature range shown. This item can only be used on C145-2H engine."

So, if you want to have the Aeromatic, you'll also have to have the seaplane lip and the C145-2H with a dampered crankshaft.
There are reasons why this prop fell on hard times other than the old AD note about lag-screws in rotten wood blades, which appears to be the current TC-holder's main focus. The prop just wasn't that good in performance because of it's wood blades, it was objectionable to many operators because of it's "gear-shift" sound/feel in climb when it suddenly and sometimes seemingly without warning changed pitch, it had a limited number of engines it could be installed upon, it was more maintenance than metal props, it was more expensive, it was more difficult to keep balanced (wood absorbs water/humidity) and it required other airframe modifications to be legal.

[Jr.CubBuilder]

It might be the cats meow with composite props, but I suppose that's another can of worms.

I wish MT would make a prop for the C145/O300 engined 170 and 172 aircraft.

[cessna170bdriver]

When I first met ron74887 he was running an Aeromatic prop. Get him to tell you the hassles of keeping the thing adjusted. I don't think he minded the "gear shift" type action as much as not knowing when it was coming . To hear him tell it, that prop could be a challenging hobby in and of itself. There are plenty of times where we could use a lower gear, but, I guess we should all be happy with our existing direct-drive single-speed pneumatic torque-to-thrust converter.

Miles

[ron74887]

Bob, if memory serves me right, A-799 already approved the Aeromatic prop on all engines with the dampened crankshaft. I had one on an )-300-A and enjoy it the only problem was adjusting it for diofferent altitudes. If you fly the same area all the time and don't change much altitude on takeoffs it was great. It will scare you to climb at 2700rpm and maintain 60 or so and not see over the cowling-- just not normal in a 170. with engine screaming and maitaining speed and then leveling off the prop shifts- smoe times you had to let off on power to make it shift. performance was no less than the fixed pitch. I flew to DC with 5 other planes and had no trouble keeping up. Took it off before the San Deigo convention because of altitude changes and being at gross or above did not want it shifting on takeoff roll. I got the prop from Doggie and he was flying from 2500 at one airport to 7500 and was having trouble with it shifting. The setting was the hardest thing to get right and like I said if you fly at the same altitude its is great. you can easily turn 2700 on take of and sometimes 3000 if you don't set the weights correct ( don't ask) The prop I had is in Michigan now and as far as I know is still up there Ron