The propeller had to be taken off and reattached easily and securely, thousands of times so that the car could operate and the plane could fly, and also in order to satisfy the FAA. I asked my father to write up how the propeller was changed and here is his response (April 20, 2009) -

The Franklin 150 hp aircraft engine has an 8" diameter flange on the front end of the crankshaft. That flange has eight 3/4" holes which normally are used to bolt the propeller onto the flanged end of the crankshaft. The bolts are normally already inserted through the propeller hub, and are aligned and matched through the crankshaft flange. Locknuts are applied on the backside of the crankshaft flange that lock the propeller hub in place.

The AIRPHIBIAN required the prop to be removed when used as an automobile requiring the design of a quick, safe, foolproof system for removal or the propeller.

Crankshaft/pullerbolt about to be inserted into crankshaft.

My Father said that the machinist that they used, Fred Underhill, put a lot of his own intellect into many of the innovations that were incorporated into the Airphibian and one of them was the Propeller Hub Mounting.

Interestingly enough, if you look at Fig 2 of the Patent, you'll see that the drawing is of a 2 bladed propeller, as are all the configurations in the patent itself. But all the photos show 3 bladed propellers.

A patent was granted for Hub Mounting for Propellers on Roadable Airplanes No. 2,509,096 on May 23, 1950. It was filed by R E Fulton Jr and Octavio J Alverez on June 23, 1945. It actually is a very well designed piece of machinery.

In this patent drawing the propeller is about to be attached to the back plate. The bolts that hold the prop to the front plate are the long bolts with the snub nosed buttons that engage in the holes in the back plate. The crank handle is ready to be turned to start the 1 1/2" Puller bolt to thread into the machined female thread in the engine crankshaft. The 2 pointed parts at the end of the crank handle engage in two holes at the end of the hub when the crank handle is pushed into its home inside the hollowed out engine crankshaft.

Figures 15, 16, 17 and 18 show the overtorque prevention device built into the crankhandle. The crankhandle is actually made of layers of metal leaves that flex slightly as the handle is turned to tighten the puller bolt. There is a clip, #105, connected to one of the leaves attached to the handle of the crankhandle only. When the crankhandle is tightened enough the small returns on the 4th side of the clip will spread and move to the next leave, indicating that the bolt has been torqued enough. It is easily reset by pushing it back in place.

In Fig 3 the propeller is ready to be used. Everything is attached, the crankhandle is in place and the spinner hub is locked on. The key to the spinner hub is also the ignition key