When a buddy upgraded the motor in his plane, the "perfect" prop was too big and
struck the ground easily on the takeoff roll. Dropping down a size left the thrust
lacking and pitch speed too high. I realized that this was the perfect opportunity to
try my new multi-blade idea, so I built this hub to mount three shorter blades in such a
way that the pitch was higher. (slower-turning props are often more efficient) With
three blades, the thrust is now back up where it should be.
One day, I ran into a gentleman online who was designing a model of a turboprop commuter plane, the
Bombardier/DeHavilland 400 "Dash 8". Wanting to be scale in appearance, he was asking if anyone had any
ideas on how to make a 6-blade prop.
I had played with this idea in the past, so I joined in the discussion, offering up my design for a hub to mount
multiple folding propeller blades. "Scale guys" really don't like having floppy props mangling their expertly-crafted
babies, so the idea got a slightly cool reception.
There were several options discussed which involved modifying existing propellers in what I deemed unsafe
ways, but one "near miss" inspired me to modify existing props in such a way as to retain their hub strength.
What you see here is the result of that brainstorm...a prop that looks reasonably "correct" on the plane, and can
also be flown.
There has long been the argument that multi-blade props are less efficient than two-blade props. Internal
combustion engines often lack the low-RPM torque to "spin up" a multi-blade to the RPM where the engine runs
most efficiently...thus giving the impression that the multi-blade is inefficient. The thing with electric power is that
an electric motor has the torque at low RPM to pull the extra load of more blades up to operating RPM. This
makes the marriage of a multi-blade prop to an electric drive a perfect union!
These two drawings represent 8 inch diameter
6-blade props. The left one with a 2 inch spinner
and the right with a 2 1/2 inch. Going to larger
diameters necessitates the use of 2 1/2 inch
spinners due to larger prop hubs.
The below photo shows a "mobile testbed" that the uninitiated would call an automobile. Rest easy, I did not
drive around town like this!...My test area was an access road that was only used during working hours on
weekdays. I was playing with these multi-blade folding props in the summer of 2001 and had a 13 X 11
making over five and a half pounds of thrust at 5400RPM. The blades were blindingly expensive, the hubs
time-consuming to make, and I moved on to smaller planes...so those experiments ended.
Here is a set of 10 1/2 inch diameter five-blades with 9 inch pitch. I am
thinking that these would go nicely on a P-38 version-that-wasn't. Note
that one is right-hand pitch, the other left-hand.
A close-up showing the center cap screw that holds the light Dave
Brown aluminum spinner in place.
A close-up showing the prop slot that I custom-notch on my milling
And then I got frustrated looking for decent props for my small
transports and bombers...
Please excuse the dirty fingers...this pic was taken right out
of the shop.