Homebuilt Propellers Page

The following are pictures showing the construction of Propeller for a Velocity Aircraft.  I learned about building a prop from Nigel Field.  Nigel flies a Subaru powered Vari-eze with his own homebuilt prop.
	The prop is made using a soft pine wood core.  Each blade is carved separately and then all three are butt jointed at the hub.  The real structure within the prop is the fiberglass skins and spars (shown below).  The core is simply used to provide and hold the shape.  The shape of the prop was determined using Bates Engineering's Prop Optimizer PC program.  Visit: Bates Engineering PO Box 3894 Stateline, NV 89449-3894 775-586-9038 www.AeroOptimizer.com The prop shown is a 68 inch diameter prop.  Five 1 inch boards are laminated together using Resorcinol wood adhesive to make up one blade.  The blade outline is first cut with a bandsaw to remove as much wood as possible.  Then a hacksaw, a chisel and a hammer are used to remove even more wood.
	The software provides coordinates for ten stations along the blade.  They are drawn on cardboard and used to check contour.  A spoke shave is used to arrive at a near net shape.  Final shaping is done with a power sander.
	The blades are joined at the hub with epoxy.  Fine tuning of the blade profiles is done once they are joined.  Angle of attack, and blade thickness is matched at equivalent radial distances for each blade.
	The prop extension/flange is shown on the left.  Note the centering boss on the mating face.  To accomodate this boss, a carbon fiber spool is made on a wooden plug.  This spool slips snugly over the boss.  Its height corresponds to the prop hub thickness.  It is used to allow fiberglass spars to be wrapped around.  The spool has a combination of carbon UNI wrapped circumferentially and BID running at 45° to its longitudinal axis.  The wood is opened up approximately 0.125" bigger than the spool diameter so that it may be floxed in position.  Jigging of this spool was not used on prop#1.  We noticed that the spool end further from the boss ended up slightly off center.  That didn't hurt anything other than resulting in some head scratching when balancing came along.  The next props will be jigged.  Alternatively, a solid aluminum bar could also be used where the prop extension has no centering boss.
	The spars are UNI glass layed-up in grooves carved into the wood using a hammer and a chisel.  Careful planning is required to avoid hitting the bolt holes.  When laying in the spars, one must think of how they will be sanded flush.  Because they taper along the blade length, the longest fibers MUST be run along the bottom.  Keep them straight and place the shorter ones in last.  Avoid packing it down in such a way that the long fibers migrate to the top.  The spars must wrap around the spool.  Ideally they should go around the spool and back out onto the same blade.  If it gets tight around the spool, then a single bend onto the adjacent blade is OK ONLY if it goes around the spool by 120° (Go the long way around the spool, not the short way)..
	The bolt holes are drilled into solid formed-in-place flox bushings.  The flox pads go into the hub approximately 1.5" - 2".  The crush plate is used to position the holes accurately.  The holes will only be opened up after all the glass layups are complete.  I recommend outlining the flox pads with a felt tip pen to facilitate drilling dead on center.  You'd hate to drill through a spar cap right at the end.
	The tips get carbon fiber reinforcement as shown below.  I stepped the wood core to have solid carbon on the tip (approx. 4" long), then 2" with 4 plies, then 2 more with only 2 plies.  This makes the transition to the glass skin more gradual, to reduce stress concentrations.  The carbon used is UNI running along the blade axis.
	The UNI glass layers are then layed up.  The first plies cover from the blade tip to over the hub.  I made them go around and down the edge of the hub opposite the blade.  This adds to the crush strength of the hub.  To accomodate the glass going over the hub edge, the hub was slightly rounded off (by sanding).
	Once both sides are glassed with the UNI, 2 plies of BID are applied over each face of the hub and extending onto the blades by approximately 4 inch. The hub will be made square with small lay ups of BID glass along the hub edges, sanded smooth..  For props destined to be used on bigger engines, we found that we had to add 2 plies of UNI at 45° to the blade axis.  This was found to be necessary after running the prop and finding some blade flutter at 2100 RPM.  The cross plies add to the torsional stiffness.
	Finishing requires some micro balloons and resin slurry.  I recommend peel plying the whole prop when laying up the glass.  It makes it real smooth. . The bolt holes are coated with epoxy sloshed in the holes.  The epoxy can be diluted with alcohol or heated up to allow it to penetrate the exposed wood. Sanding and filling can be used to balance the prop
	Primer and top coat can then be applied.  All this is done by keeping an eye on the balancing.  A balancing rig can be built from small bearings and a shaft.  The prop can be final painted right on the balance rig.  You can get it so close that a paper tissue on one tip will cause an imbalance.
Return to Homepage
Go to Velocity Page