Saturday, November 25, 2006

AV - Propellers

Before you become too deeply involved in discussions of propellers I suggest you make a few. Traditional designs using Clark or RAF airfoils (which have a flat bottom) are quite easy to carve and with only a modest amount of care (and an accurately made set of templates) the results of your work will closely match or even exceed the performance of most of the props being offered as suitable for VW engines. (Hint: Lay-out your templates with a DeltaCAD. Glue the layouts to the metal then simply cut & file to the line.)

Such props are far from ideal, no matter who makes them, but they WILL fly the plane and do so economically, the major investment being your time.

Once you've carved a few props -- and flown behind them -- not only will the work of Paul Lipps make more sense but you'll be able to actually apply some of his advice to a real propeller.

But don't expect miracles. Most homebuilts have a lot of drag and drag increases as the square of velocity. Even the most efficient prop in the world can't change the laws of aerodynamics.

What you should expect to see is improved efficiency -- making better use of whatever amount of horsepower you have available, which in the case of converted VW engines is never very much. An optimized prop will often allow you to maintain your accustomed cruise speed on less power, meaning your fuel consumption goes down, your effective range is increased and the number of required fuel-stops is reduced.


As someone pointed out, citing only pitch and diameter when selecting a prop is little more than a joke despite the fact that's how most homebuilders do it -- and the fact it WILL fly the plane. But if you've never carved a prop you've got to start somewhere and the truth is, the carving is the easy part. The tricky bits are selecting the wood, gluing-up the blank, doing the lay-out, making your templates and learning how to chunk your way down to NEAR the final shape without ruining it.

Break the task down into a logical sequence of chores and you'll find that making a prop is lot easier than you think. And besides, you only have to carve ONE blade. (Then make the other match :-)


1 comment:

flybynightkarmarepair said...

some of my experience attempting to follow bob's advice:

The prop/clock went to my dad for his birthday a few years back. Mom says it made him cry ;-)