Aeromatic Propeller discussion

[bagarre]

Bruce and Ron, Congratulations and thanks! I'm one who has benefited from your tireless behind the scenes work as I bought a project with an O-300B.

As for the virtues of the Aeromatic propellers.... they dropped out of the market because they were unprofitable due to lack of popularity. While on paper they seemed to be a cure-all.... in fact they were not durable, required excessive maintenance compared to ordinary wood and metal propellers, and offered minimal performance advantage only in limited operations. I've flown only one airplane with one and it was disturbing when it made sudden pitch changes and was unreliable. When it failed annual inspection due to wood-rot at the common lag-bolts that held the blades onto the hub, the owner hung it on the hangar wall too.

This might be worth splitting off to a new topic.

They were supposed to be smooth operating and not a two speed prop so it's possible the one you flew behind wasn't the best example.
The concept and design was a sound idea. Modern materials could cure the issues they had. Carbon fiber blades for example.

When I did my research on the prop, the gains weren't significant enough to justify the cost with thick wood blades and 1930's castings.
It would be interesting to see a carbon bladed and billet machined setup (maybe even basic electric pitch over-ride) developed in the Experimental world. A prop like that would be very effective on a Vans RV.

[flyboy122]

My Dad has one and played with them extensively in the 60's and 70's. According to the hanger talk between him and his buddies setting them up was voodoo magic, but if you managed to figure it out they worked great. The way they talked they were essentially 2 position (high and low). They worked really well on Fairchilds, Airmasters, etc... that had a reasonable speed range. (We had it on our Meyers OTW biplane and NOTHING makes that thing any faster, so it didn't really do anything....hence it hung on the wall.) Dad has an actual manual for setting it up somewhere.

I thought I heard a few years ago that MT was making replacement blades for them. Not sure what ever came of that.

DEM

[GAHorn]

...They were supposed to be smooth operating and not a two speed prop so it's possible the one you flew behind wasn't the best example.
The concept and design was a sound idea. Modern materials could cure the issues they had. Carbon fiber blades for example.

When I did my research on the prop, the gains weren't significant enough to justify the cost with thick wood blades and 1930's castings.
It would be interesting to see a carbon bladed and billet machined setup (maybe even basic electric pitch over-ride) developed in the Experimental world. A prop like that would be very effective on a Vans RV.

They were two-speed. Forward speed would place relative wind onto the face of the blade and force it to coarse pitch. When slowing down, the blades were supposed to remain coarse, unless high power were applied, in which case the blades would shift back to fine-pitch.

Wood bladed props have advantages... metal blades have other advantages... CF still others... But they are all compromises and have disadvantages as well. The actual blade material wasn't the failure of the Aeromatic, I don't believe. But the availability and reducing costs of constant speed props which DO have variable pitch-control made the Aeromatic design obsolete.
A perfect example of how blade material change would not likely make the Aeromatic profitable again is the McCauley Two-Position (metal) prop. That prop is also very rare due to lack of significant improvement in performance and unnecessary complexity for so small advantage. I believe only two of those are still in existence and McCauley will not support them anymore.

[bagarre]

You're right.

[GAHorn]

I wish it weren't so.

Dave Bengston has one of the two-speed McCauley props on his B-model and he kindly allowed Bruce F. and myself the chance to fly it and make a subjective judgement on it's performance. I believe both of us felt it had slight improvement in takeoff/climb performance to what we were used to. I don't recall Bruce's opinion on cruise, but I don't recall it being any better than what my B-model does with an EM7655 on it. (The TO/Climb would likely be my opinion even with a standard prop however, since I am accustomed to my cruise prop. My airplane is a bit sluggish when it comes to takeoff distance or climb performance.)

BTW, comparing the two prop designs which fit the 170 to each other (the two position compared with the EM or DM/MDM blades)
it appears the max travel of the 2-position blades result in pitch angles equivalent to 43" fine pitch, to 59" coarse pitch. This would definitely lean towards improved takeoff/climb performance when compared to the standard fixed pitch props, and indicate not-so-much improvement in cruise (due to the drag coefficient of increasing speed.)
This entirely subjective guess is based upon the data in the McCauley propeller overhaul manual, and did not take into consideration any blade profile differences.

[n2582d]

They were two-speed. Forward speed would place relative wind onto the face of the blade and force it to coarse pitch. When slowing down, the blades were supposed to remain coarse, unless high power were applied, in which case the blades would shift back to fine-pitch.

It's not a constant speed nor a two-speed. Here's what Tarver has to say in this regard.

The Aeromatic propeller is a fully variable pitch propeller that is virtually equivalent to a constant speed propeller. But it is not quite the same thing. ... Typically when you apply full throttle for takeoff you engine will rev up to about 50 rpm less than red line. After you have reached flying and or climb speed your engine will be turning red line rpm. After you reach cruise altitude and level off and gain speed, the propeller will increase pitch as you gain airspeed. Consequently you are now in cruise mode with more pitch much like a constant speed prop. This is not a two speed prop, it modulates itself based on the speed of the airplane and other dynamic forces.

One problem with the design is that it loses around 20 rpm per 1000'. This is what the hi-cruise model was designed to compensate for.

[GAHorn]

Tarver's description is misleading as far as the original, air-deflection/driven model goes (the only type I've flown.) The prop has a low pitch stop and a high pitch stop and those are the two positions it operates at except for short periods when transitioning between those two stops. If the pilot attempts to fly at a speed that keeps it off the stops then the prop never stabilizes and wanders...hunts... according to each and every airborne gust...an uneasy experience and difficult to maintain if one should so desire. But, even if it were possible to stabilize the prop at some point between the two stops, why would one so desire? It would require a cruise power setting much lower than optimum. What advantage the prop design offered would be discarded by such......?

The hi-cruse prop was a different design which used oil chambers and engine oil pressure to regulate blade position as I understand it, but it also did not compete successfully with the constant speeds for reasons that I think are obvious. Like the McCauley it was usually operated at low or high pitch, I believe. BluEldr might be able to shed some light on this prop.