Rutan Derivative Canard Forum Slide #1July 19, 2007

A Method of Desensitizing Wooden Propeller Hub Mounting Bolt Security To Environmental Effects and

Sub-Optimal Maintenance TechniquesOr

How To Keep Your Wood Prop On Your Plane With Less Work

Marc J. ZeitlinJuly 26th, 2007

2:30 PM – 3:45 PMForum Tent 06 – Sporty’s Pavilion

July 19, 2007 Rutan Derivative Canard Forum Slide #2

What Will I Talk About?• Characteristics of Wood Props• Need for Maintenance• Proposed Solution – Representation / Theory• Specific Solution• Prior Art / Proposals• Pre-Flight Testing• Flight Testing• Current Configuration• Increased Robustness / Decreased Sensitivity• Cost / Weight Penalty• Questions / Answers / Availability

July 19, 2007 Rutan Derivative Canard Forum Slide #3

Characteristics of Wood Props

• Relatively soft• Low crush strength• Low stiffness• Susceptible to

moisture / humidity / shrinkage / expansion

July 19, 2007 Rutan Derivative Canard Forum Slide #4

Need For MaintenanceExamples of Wooden Propellers:• Sensenich – 50 hrs or annually• Hertzler – every 25 hours, anytime going

between different climates• Jabiru – every 25 hours• Miles Gemini – every 10 hours

Technique (not usually specified):• Remove safety wire• Loosen bolt first to break static torque• Ensure movement – measure torque while turning

July 19, 2007 Rutan Derivative Canard Forum Slide #5

Proposed Solution –Representation / Theory - 1• Prop Bolts very stiff – Prop Hub size determines force

Prop Hub(Somewhat Squishy)

Very Stiff Spring

(Prop Bolt)

Hub shrinks, compression force falls, bolts cannot

compensate

Before Compression After Compression After Shrinkage

July 19, 2007 Rutan Derivative Canard Forum Slide #6

Proposed Solution –Representation / Theory - 2• Same motion of hub• Small change in spring force – SOFTER SPRING DETERMINES FORCE CHANGE

Soft Spring

Before Compression After Compression After Shrinkage

July 19, 2007 Rutan Derivative Canard Forum Slide #7

Specific Solution

• Belleville Washers• Flexible arrangements

– use serial stack• Force/deflection

curves adjustable –can obtain any force/deflection required

July 19, 2007 Rutan Derivative Canard Forum Slide #8

Prior Art / Proposals• Jabiru - recently switched to belleville stacks on certain

engine prop combinations

• Miles M65 Gemini w/Cirrus Minor II Engines, from 1947 – used belleville stacks

• Paul Lipps / Vance Jaqua – belleville stacks installed on Lancair 235 w/O-320

• Burt Rutan proposal 25 years ago – never tested

• Independently proposed to me by 3 sources – c-a mailing list; web forum; co-worker

July 19, 2007 Rutan Derivative Canard Forum Slide #9

COZY MKIV Installation

• Took proposals, did calculations, picked hardware

• Determined 6-stack would provide more than adequate expansion / shrinkage capability

July 19, 2007 Rutan Derivative Canard Forum Slide #10

Pre-Flight Testing - 1• Find

Hardware

• Two bolts have stacks of 6 bellevilles captured between two wide area washers – 4 standard bolts

• Can see good linearity

Bolt Torque / Washer Compression vs. Turns

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5 3 3.5 4

Bolt Turns

Bol

t Tor

que

(ft-lb

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Was

her C

ompr

essi

on -

(in)

Torque - LTorque - RCompression - LCompression - R

July 19, 2007 Rutan Derivative Canard Forum Slide #11

Pre-Flight Testing - 2

• Remove “turns” from the equation – just show washer compression vs. applied torque

• Still very linear

Washer Compression vs. Bolt Torque

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 5 10 15 20 25 30 35

Bolt Torque (ft-lb)

Tota

l Was

her C

ompr

essi

on (i

n)

Compression - LCompression - R

July 19, 2007 Rutan Derivative Canard Forum Slide #12

Pre-Flight Testing - 3• Switch from six

std. bolts to six bolts – all with Belleville washer 6-stacks

• Change in slope as “slop” is taken out of system (both types of bolting systems) – then more torque for same compression needed

Hub Compression vs. Bolt Torque

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 5 10 15 20 25 30

Bolt Torque (ft-lb)

Hub

+ F

lang

e th

ickn

ess

(in)

Compression - StdCompression - Belleville

July 19, 2007 Rutan Derivative Canard Forum Slide #13

Pre-Flight Testing - 4

• Six bolts with belleville washer 6-stacks

• Very linear torque and compression with bolt turns

Washer / Hub Compression / Bolt Torque vs. Bolt Turns

0

5

10

15

20

25

30

35

0 1 2 3 4 5

Bolt Turns

Torq

ue (f

t-lb)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

Com

pres

sion

(in)

Torque

Hub Compression

Individual WasherCompression

July 19, 2007 Rutan Derivative Canard Forum Slide #14

Pre-Flight Testing - 5• Same slight

change in slope as “slop”is taken out

• Still predictable and relatively linear with Bolt Torque

• Might be easier to measure turns rather than torque, however

Hub and Washer Compression vs. Bolt Torque

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0 5 10 15 20 25 30

Bolt Torque (ft-lb)

Com

pres

sion

(in)

Hub CompressionIndividual Washer Compression

July 19, 2007 Rutan Derivative Canard Forum Slide #15

Flight Testing - 1• First few flights – two bolts with bellevilles – four standard bolts• Calcs show prop will stay even if two bolts don’t• See washers NOT completely compressed – allows for shrinkage AND expansion of

Prop Hub

• No difference from standard installation perceived - ~3 flight hours accumulated in this configuration at various speeds, RPM, flight attitudes – near 1 million cycles for fatigue

July 19, 2007 Rutan Derivative Canard Forum Slide #16

Flight Testing - 2• Next few flights – four sets of belleville washers, two standard bolts• Have confidence based on previous flight hours• Run another ~3 hours – build ~2 million cycles on first set, ~1 million on

second set• Again, fly various RPMs, flight attitudes, speeds, G-loading

July 19, 2007 Rutan Derivative Canard Forum Slide #17

Flight Testing - 3• Next set of flights – all six bolts with belleville washer stacks• Built more confidence based on previous flights• Run another ~3 hours – build ~3 million cycles on first set, ~2 million on

second set, ~1 million on third set• Again, fly various RPMs, flight attitudes, speeds, G-loading

July 19, 2007 Rutan Derivative Canard Forum Slide #18

Flight Testing - 4• Remove two bellevilles from each stack – now have 4/bolt• Run another ~40 hours – build ~13 -15 million cycles on all belleville

washers• Again, fly various RPMs, flight attitudes, speeds, G-loading

July 19, 2007 Rutan Derivative Canard Forum Slide #19

Current Configuration

• Using 7” bolts – could reduce to 6.5” and remove hardened washer to save ~0.1 lb.

• Added AN3 bolts to FRONT of crush plate

• Will be able to add spinner backplate / mounting structure WITHOUT having to remove/touch prop bolts

• Don’t have to have backplate BETWEEN prop and prop extension

July 19, 2007 Rutan Derivative Canard Forum Slide #20

Increased Robustness / Decreased Sensitivity

• Maintenance simplification (measure spacing, # turns)

• Maintenance frequency reduction (Jabiru – from 25 hrs. to 1/yr. example)

• Safety enhanced

Hub Compression Force Loss vs. Hub Shrinkage

0%

10%

20%

30%

40%

50%

60%

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Hub Shrinkage (in)

Hub

Com

pres

sion

For

ce L

oss

(%)

Standard

Bellevilles (6-stack)

Bellevilles (4-stack)

Bellevilles (2-stack)

July 19, 2007 Rutan Derivative Canard Forum Slide #21

Cost / Weight Penalty

• ~3.5 lb for ½” Bolt SAE-2 Flange with 7” Crush Plate and 6 washers/stack (~2.5 lb in current configuration – 4 washers/stack)

• Approximately $120 gets replacement bolts, bellevilles, flat washers

July 19, 2007 Rutan Derivative Canard Forum Slide #22

Questions / Answers / Availability• My Email: marc_zeitlin@alum.mit.edu

• Website: http://www.cozybuilders.org/

• Currently ONLY configured for ½” bolts – 3/8”bolt configurations may be tested in the future.

• I can supply Bellevilles at cost+; Bolts available from Saber MFG, washers from ACS, Wicks, etc.