I found the following Usenet posting under rec.aviation.owning. Please note that some specifics apply to McCauley props, and Cirrus uses Hartzell props, but I thought it was interesting enough to post.
For what it’s worth, I push my SR22 by the prop, being careful to push at the root of the prop, in the center of the blade, so that it doesn’t apply any twisting force on the blade.
From: John Roncallo
Subject: Pushing on the Prop, Reply from McCauley (Was Pushing on the Prop)
Date: 2001-03-13 18:18:05 PST
After seeing the McCauley Web Site with the discussion about pushing on theprop, I decided to e-mail them for a little more data. Below is a copy of the email I sent to McCauley and the email I got back. Mr Mayhill was kind enough to respond with an attachment that I converted to text format for all to read. I thought it was very good. Unfortunately the graphic he refers to in the text did not reproduce even in the RTF document he sent me. I don’t think it has convinced me not to push or pull on the prop but I will keep in mind the concept of pulling from the trailing edge or pushing from the leading edge when handling a CS prop.
Thank you for your question; I believe I can shed some light on this issue for you. Below is a page on this question I wrote up for you. It is in Word .rtf format. Please feel free to pass it on to anyone else you know that may have this same question.
Let us know if you have any further questions.
Michael R. Mayhill
Director of Product Support
McCauley Propeller Systems
From: John Roncallo
Subject: Pushing on the prop
I read your article on the web titled “Preventing Damage”. The last sentence reads as follows:
“Pulling or pushing with the propeller could severely damage actuating components inside the hub.”
As an aircraft test engineer, I find this very hard to believe that I might be able to damage a propeller’s actuating components by pushing or
pulling on a prop by hand. Do you have any real data that would support this. Although I use a tow bar to move my airplane, I find the tow bars
to be inadequate at supporting the load in that they have a tendency to slip off. Typically I use the tow bar to steer, and the prop to push.
Also since you are talking about actuating components, can I assume that your no push policy is recommended only for constant speed props. Is it OK to push or pull on fixed pitch propellers.
Why SHOULDN’T I Tow My Airplane By The Propeller??
A person absolutely, positively can cause serious damage to the actuation parts inside their McCauley propeller if towing the airplane by pulling on a propeller blade. This may at first seem very odd since the aircraft is “pulled” by the propeller whenever it flies; what’s the difference? This is a very common misconception.
There are two things many people overlook - the absence of both centrifugal load, and natural Centrifugal Twisting Moment (CTM) on the blades. On typical single engine application, the blades see approximately 15-20,000 lbs. of force trying to “throw” them from hub while in flight. This force also strengthens the blades by “straightening” them. Thirdly, the blades are
also trying VERY hard to go “flat” or to low pitch when the propeller is spinning due to the “CTM” within the blades. Keeping this in mind, the blade actuation system includes a composite phenolic link that is designed to be used solely in compression; tension will snap it. This link translates linear piston motion into an arc motion thru an actuating pin installed on the outer perimeter of the blade base. The blade rides on a ball bearing assembly and rotates when “pushed” by this link. This is how the blades change pitch in flight.
During normal flight, the blades are actuated thru this link under compression ONLY. The link is compressed because the piston (moving under oil pressure from the governor) is pushing it from one direction, while the blade CTM (see above) is pushing it from the opposite direction. The link compression at cruise of a typical propeller application such as a new Cessna 182 is approximately 1000 psi. A rough representation of the link can be seen in the graphic below joining the piston and the blade actuating pin (sticking up from the blade). The actual link is much shorter - about 2" long. While the graphic is very
crude, it does correctly illustrate the relatively tiny amount of phenolic material on the ends of the link vs. the large amount of material normally under compression in the middle. This end material is there simply to keep the link attached to its neighboring parts; it is NOT designed to be pulled upon.
[Graphic was supposed to be here but it didn’t come out]
If an aircraft is regularly towed by pulling on the blades, it is very possible that eventually the blade will be twisted the “wrong” way by
hand, putting the link under tension, snapping off one of these ends.
This is not hypothetical stuff; it’s proven fact and we’ve witnessed MANY cases of it over the years at McCauley.
If you must pull a single engine aircraft by its McCauley propeller, hold the blade as close to the base as possible, and the blade by its TRAIL EDGE. This will put the link under compression, much reducing the chance of links being snapped.
Regarding pulling on a fixed pitch propeller: Obviously the risk of damaging actuation parts does not exist, so most of the above does not
apply. However, let’s not forget the lack of centrifugal load again even f/p props are designed to pull the airplane when this load is present in normal operation. Centrifugal load GREATLY improves the blades’ resistance to bending; they are naturally held stiff and straight while spinning. As this strength is missing when stationary, there is a good possibility of bending a f/p propeller blade if it’s pulled too hard by hand on the ground.
If you must do this (and most of us have), hold the f/p propeller as close to the base of the blade as possible. The reasons for this: 1)
The blade is fatter and much less likely to bend, 2) the “bending” force you are applying to the blade is much less at the base than the tip since the “moment arm” is shorter.