Acknowledgement
I wish to acknowledge Nigel Field of Embrun, Ontario, for his pioneering
spirit that has resulted in a number of propellers being built using this
technique.
Caveat Emptor
This description describes the techniques and processes used to build a three
bladed wood/glass composite propeller for a Cozy MK IV canard aircraft using a
Subaru EG33 power plant modified for experimental aircraft use. This propeller
is untested and in no way proven at this juncture. Persons choosing to adopt the
ideas presented herein do so at their own risk.
Why Build A Propeller?
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Discussion with propeller
manufacturers indicated that they were suited only to building propellers
for known engine configurations.
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Typically propeller
manufacturers are craftsmen not aerodynamicists.
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Only a few propeller
manufacturers actually know what makes a propeller work.
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Recommended propeller is $1,700 (US) plus shipping
and Taxes at boarder. (~$3,000 Canadian).
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Airframe is, to some extent, an unknown quantity
since it has retracts and other drag reducing concepts.
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Auto conversion torque curves are given for
automobile configuration and may not be accurate in converted form.
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In all probability a second propeller will have to
be found once the flight characteristics are known. $$$$$$$$.
Two or Three Blade ?
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Increasing the number of blades decreases
efficiency is a common myth resulting from the practices of propeller
manufacturers for production aircraft.
Similar
efficiencies may be expected from two, three, and four bladed devices
provided the diameter and overall blade area is maintained in all cases
Double
the number of blades and halve the chord will give very similar performance.
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Increasing the number of blades increases the
complexity (cost or building time).
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Increasing the number of blades increases hub
weight.
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In pusher configurations, dirty air from the
trailing edge of the wings interacts with the blades a results in vibration.
Odd blade numbers increases the vibration frequency and reduces the
vibration amplitude.
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Total airflow speed = 250 mph
Max speed estimated at 245 mph
Induced velocity > 5 mph
At 10,000 ft 0.85 mach = 625 mph
Maximum rotational tip speed, before aerodynamic compressibility issues
occur, yields a maximum propeller diameter of 66 inches.
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Efficiency Vs Diameter
- Bigger Propellers are more efficient than small
ones:
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At cruise there is little
difference in efficiency irrespective of diameter (230 hp).
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Induced velocity is similar
for all propeller diameters (230 hp)
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Cozy max prop diameter is
70. Reduction to 66 is insignificant to performance.
Blade Templates
Blade Elevation & Planform
Shaping The Planform
Select pine may be found at Home Depot.
Bonding The Laminates
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Laminates are bonded using
Resorcinol.
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Dowel used to keep
alignment when bonding the laminates.
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Nail used to keep alignment
at blade tip.
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Shaping The Backside
| Saw cuts are applied at about two inch intervals. These saw
cuts are made very carefully between the trailing edge line and the planform
marker on the leading edge. It becomes very easy to remove the excess wood
without gouging into the core. Be careful to look how the grain is lying so
that the wood splinters off away from the back face of the blade. Use sharp
chisels. Finally sand the backside using a sanding block. |
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Gluing the three blades in place
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Precise measuring of the
blade tips ensures uniform angular spacing.
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Three blades glued together
using Flox
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Wooden spacers are used to
fill voids.
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Blade tips are retained to
ensure the blade does nod twist during manufacture
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Adding The Centralising Boss
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Central boss is a machined
part over which the spar material will be stretched.
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A rebate is routed into the
wooden face to accept the central boss.
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The locations for the six
propeller bolts is incorrectly indexed in this photograph and will
subsequently be indexed by 30 degrees.
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Inserting The Hard Points
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Hard points are added to the six propeller
bolt locations.
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These are drilled to a depth of about 1/2
inches and are one inch in diameter.
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This provides a good bonding surface for the
subsequent lay-ups of glass fibre.
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Cutting the rebate for the spar
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0.25 (w) x 0.5(d) inch
grooves are routed into the face of the blade to a radius of 13 inches.
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The depth of the groove is
reduced to zero at a radius of 18 inches.
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The central axis of the
groove lies down the central axis of the blade.
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This is repeated for the
back side of the blade
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Spar Lay-Up
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E-Glass roving is layed,
starting from the end of one blade over the hub and then down/along the
next blade. The prop is then rotated through 120 degrees and the process
repeated for each blade until the spar troughs are filled.
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E-Glass
roving provides initial strength over central hub.
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Final strength achieved
from E-glass skin.
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Post Lay-up
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Before Clean-up |
After Clean-up |
Installing the Carbon Tip
Skin Lay-ups
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UNI plies run in the
direction of the radial lines.
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All blades are skinned
sequentially so that the hub has interlocking lay-ups.
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Lay-ups extend over the hub
centre and down the back face of the hub.
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Lay-Up Schedule
Back of Blade
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1 Ply of BID full length of
blade and over hub centre.
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2 Plies of UNI full length
of blade.
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1 Ply of UNI to 24 radius
and over hub centre.
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1 Ply of UNI to 20 radius
and over hub centre.
Flox in corner of trailing edge.
Front of Blade
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1 Ply of BID full length of
blade and over hub centre.
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2 Plies of UNI full length
of blade.
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1 Ply of UNI to 24 radius
and over hub centre.
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1 Ply of UNI to 20 radius
and over hub centre.
1 Ply BID from blade start to tip but covering from
trailing edge to trailing edge.
2 Ply UNI from hub centre (back) to Hub centre
(front)
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Strength Calculations
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Construction is from
wood/E-Glass composite.
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Assume glass takes all the
stress.
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Spar structure provides
sufficient strength to support the blade under all load conditions.
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Calculations based on Engine
rev limiter operating at 6500 rpm, prop speed of 3500 rpm. (Max power is at
5400 rpm)
Blade Strength
Stress Limits
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Weakest part of blade is at 20
radius.
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Blade has a safety factor of
5.7 @ Engine speed of 6500 rpm (Prop speed of 3510 rpm)
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Blade has a safety factor of
8.3 @ Engine speed of 5400 rpm (Prop speed of 2920 rpm)
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Blade is at breaking point at
an engine speed of 15,560 rpm (Prop 8400 rpm)
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Blade stations 18, 20, & 24 are
at similar breaking conditions at the same propeller speed.
Balancing
Finishing
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Apply a number of coats of
micro spheres mixed with epoxy and sand to shape.
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Use 36 grit for initial shaping
then 80 grit for final work.
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Spray with Dupont 131 fill &
sand primer until all blemishes are covered.
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Sand with 200 grit then 400
grit.
The Final Product
Last Updated: Thursday August 31, 2006 |
