It's very common to see pilots moving a light aircraft by pushing or pulling on the prop, either because there's no tow bar available or simply out of preference or convenience. Is this a reasonable thing to do or can it damage the prop or engine in some way?
A couple of my CFIs have asked me to help them park the airplane (in absence of a tow bar) when I was pushing the airplane by putting one hand on the hub and the other on the propeller near the hub, and them (the CFIs) pushing the tail down and nose wheel elevated. But according to AOPA, it is not a good thing:
... never push or pull on the prop blades. Forget what anyone has told you about pulling near the hub or the strength of the propeller. Aluminum blades can bend, and it doesn’t take much to put the blades out of track with one another.
This PDF says the same thing:
Avoid pulling the airplane around by the prop. Yes, this seems the perfect solution to a vexing problem of how to change the airplane’s position without having to walk around and get the tow bar, but it’s worthwhile to make the extra effort. Neither the engine nor the prop particularly benefit from the loads imposed by horsing the whole airplane around.
Classic example of one corner case leading to a blanket belief.
Grabbing a propeller by the tip, especially a wooden prop, can cause damage. In use, the tips generate most of the thrust but are also held straight by the rotational forces.
Pulling on a prop right beside the hub will cause no harm - that's where the propellor thrust goes anyway. If you grab the prop on both sides the airframe won't know that the engine isn't running.
So, here's a Rule of Thumb: if you cannot touch the prop hub with your thumb while pulling on the prop, you are holding it too far out.
Well, grabbing the propeller at the blade root and pulling from there cannot hurt it. Why? Because the thrust imparted by the propeller to the aircraft is applied at exactly the same place. In other words, the propeller pulls the airplane through the air at the hub. You are so close to the hub that you're not going to create a significant bending moment along the blade.
On the other hand grabbing the propeller at the tips would be questionable at best, and ill-advised!
I asked around at my gliding club about this, the consensus seems to be that it won't harm the plane as long as I didn't use excessive force.
The reason for doing it is to not have to walk from wing to wing to move the plane. The main counterpoint is that you shouldn't move a plane by yourself unless you're very sure you won't hit anything (another plane, the hangar walls, etc).
By deviating from the standard procedure (using the tow bar) you incur into the following risks:
Mechanical damage: you risk damaging propeller itself if you don't pull from as close as possible to the hub. Furthermore, being more complex, variable pitch blades obviously imply more risk.
Arguably, the forces in play during taxiing are similar to those exerted manually through pulling. Also, in my flight school, during pre-flight inspection we actually perform some push/pulls to assess that the propeller, hub, and drive are ok -- suggesting that there is no mechanical stress derived from doing so.
Safety risks: although arguably rare, you may accidentally start the engine through hand-swinging. I was instructed in my flight school to shut down the engine through the mixture control mainly to prevent this from happening during the pre-flight inspection previously described. This is also why we perform magneto ground checks during shutdown.
How do you taxi your plane? I use my engine to spin my propeller, which pulls on the hub and pulls my plane hard enough to make it roll on the ground. On takeoff, I do this so hard that the 2000+ lb plane rolls 60 knots or so before it leaves the ground.
Now, with that in mind, do you think pulling on the prop, close to the hub, enough to move the plane 10 feet is doing any damage?
Regarding "it must be ok to pull the plane by the prop because the prop pulls the plane through the air"... Yes....through the air. The air is much less sticky than the ground. Also, the propeller doesn't pull the plane through the air, it pushes air behind the plane creating a lower pressure area in front of the plane and a high pressure area behind it. This "pulls" the plane into the low pressure area.