What is the story on the 3-blade McCauley propeller for the Sierra, which is placarded against ‘continuous operation above 26″ Manifold Pressure’? I do this on every takeoff. What does McCauley call ‘continuous operation?
Technical Editor:
Here is a response from McCauley, courtesy of Paul Werbin:
From: “Bell, Chris”
To: [email protected]
Sent: 12/7/07 5:42 PM
Subject: RE: Another question on prop Beech Sierra
The placard is there because above 26in MAP the blade stresses induced by the engine are above our allowables. This was found during testing for STC approval. Basically, operating in the placarded range will cause fatigue damage to the blades and hub.
Also, continuous operation is longer than 5 minutes.
Best Regards,
Chris Bell
Senior Field Service Engineer
McCauley Propeller Systems
Tel: 316-831-4021
Tel: 800-621-7767
Fax: 316-206-9899
Email: [email protected]
www.mccauley.textron.com
________________________________________
I had the 3-blade put on my own 1977 C24R on 11/7/2007. Long story on why I had it done, but mostly just being lazy on the way home from Gaston’s. I only have a few modest flights on it thus far, so my mental jury is still out. It is not all sunshine and roses. Here are my initial observations.
– There is somewhat less harsh vibration during start-up and shut-down.
– There is less prop noise as heard from outside the plane. We have ANR headsets so I can’t tell any difference inside the plane.
– There is no apparent difference in cruise vibration; and yes, I had it dynamically balanced. I suspect that those who report reduced vibration either had unbalanced 2-blades, or are confusing the different (higher-frequency) sound with reduced vibration (a common scenario).
– There is a new vibration mode not present before. I failed to make notes, so I can’t tell you the parameters that lead to it. It is more of a fine buzz than a vibration; and it does not occur during normal cruise settings. It’s more of a reduced-power phenomenon.
– The 2100-2350 red arc goes away. However, two new limitations are imposed. No continuous operation at 26” MAP or higher, and no continuous operation below 15” MAP when above 100 MPH. I don’t have the book with me here, but I think there is also a constraint against exceeding 2600 RPM until after passing 7,500 feet DA. These new limitations imply, to me, that the McCauley cannot withstand continuous power exceeding 75% of rated power, and therefore is not as inherently strong as the Hartzell. But I could be wrong; I am certainly not an engineer.
– The MAP/speed constraint could be a definite impediment if the owner operates in congested airspace, and must regularly deal with ‘slam-dunk’ approaches. Unlike the Hartzell, it won’t be possible to use the McCauley with flaps and extended gear, to come down at 2,000-3,000 FPM when needed. I won’t need to do that up where I now live; but it was a commonplace need when I was flying into Jacksonville, Tampa, Daytona, Orlando, Miami, etc. I find the 26” MAP limitation particularly interesting, since it is necessarily violated on every low-altitude takeoff. One of these days I will have a conversation with McCauley about it
– One aspect is both good and bad. There is greatly reduced clearance between the cowling nose and the aft surface of the spinner assembly. The good news is less lost cooling air, and less drag. The bad news is that your cowling nose will tend to get scratched up during R&R; and if your engine cushions are sagging at all, you will probably have cowling interference.
Another small down-side is that a conventional crows-foot wrench cannot be used to torque the McCauley. Unlike the Hartzell, it does not have adequate clearance for the head of the torque wrench. The technician must have a torque adapter, such as the extended ¾” open-end wrench style made for use on the Hartzell. An up-side is that the .032” safety wire used on the McCauley makes installation and service a bit easier. The Hartzell requires .041” wire, and can be a pain to safety. I can also say that, when I am working on a 3-blade Sierra, I hit my head on the prop five times more often. Hopefully time will fix that….
While I had disbelieved the stories from others, I have to admit that I believe the 3-blade is indeed giving slightly better takeoff and climb performance. I cannot pretend to understand the physics of what can be leading to that. There is no improvement in cruise. I am not yet ready to say that there is no loss of cruise performance; but if there is a loss, it is in the one to two knot range. I and Paul Werbin both suspect a slight loss, but it is too soon to say for sure.
What I would really like is a proper 2-blade CSP from McCauley. What I do not want is another 2-blade prop from Hartzell; I have become fed up with them and their product (when it comes to the compact-hub props).