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REFLECTOR: Propellers and prop extensions

Following are two notes on the subject of propellers that I think you will find
interesting.  Simon's note contains a message from Nat Puffer to one of his
builders, the second note is from from Steve Boser at the Sensenich Wooden prop
Company in response to my forwarding Simon's note and requesting their input on the
subject.  I think you will find it interesting reading.


"Simon Aegerter [KPIE]" wrote:

> Jim,
> here's an interesting exchange of notes on the canard mail list:
> ---------------
> From: "Nat Puffer" <cozy@extremezone.com>
> To: "Eric Westland" <ewestland@altavista.net>
> <canard-aviators@canard.com>,
>         "Cozy Builders" <cozy_builders@canard.com>
> Subject: [canard-aviators] Re: COZY: Prop Question?
> Sender: owner-canard-aviators@betaweb.com
> Precedence: bulk
> [The Canard Aviators's Mailing list]
> Dear Eric,
> The 180 and 200 hp Lycomings are at the extreme limit of the amount of
> horsepower which can be transmitted from a 7" diameter flange to a
> wooden propeller by friction (I have been told by more than one source).
> All the wooden propellers I have seen for a 7" diameter flange have at
> least a 7-1/2" hub. When you apply the amount of torque on the bolts
> required to provide the friction necessary, you are on the verge of
> crushing the wood, even when the prop hub is 7-1/2" in diameter.
> I don't know whose propeller you purchased (and you don't have to tell
> me), but I don't think much of the designer/manufacturer if he said it's
> okay for a 200 hp 4-cylinder engine. He has used up a significant amount
> of the safety factor you are entitled to. I suggest you tell him the
> designer of your airframe states that his propeller isn't satisfactory
> and you should ask for you money back. I have an extra propeller I could
> loan you if this causes you a bind.  Clark Lydick is in the same
> building as Judy Sabor. Why don't you get his opinion as well? I don't
> like to see anyone putting one of my builders at risk!
> Best regards,
> Nat
> > From: Eric Westland <ewestland@altavista.net>
> >
> > Well, after waiting much longer than I should have, my 2-blade prop
> > showed up for my Lycoming 200 hp engine.  It looks just fine, but when I
> > went to mount it on my 7" extension from Judy Saber, I realized that the
> > prop's hub was slightly undersize, 6.65" in diameter to be precise.  So
> > I called the maker and he assured me this was OK.  I'm not so sure if
> > it's OK or within the range of acceptability, what do you folks think?
> > I guess I just figured it would be as large (or slightly larger) as the
> > flange and crush plate surfaces.
> >
> > Eric Westland
> -------------------
> Nat Puffer is the designer of the Cosy, as you know of course. Both the facts
> (7" is the lower limit) and the attitude (...someone putting my builders at
> risk) are remarkable from out perspective, I think.
> Seeya
> Simon

*** The following message is from Steve Boser at the Sensenich Wooden prop Company
*** in response to my forwarded

   Subject:      prop flange size versus horsepower
   Date:          Fri, 22 Jan 1999 16:32:19 -0500
   From:          "Steve Boser" <steveb@sensenichprop.com>
     To:          "Carl Denk (E-mail)" <cdenk@ix.netcom.com>, <Jim_Agnew@IBM.NET>,
                     "Nat Puffer (E-mail)" <cozy@extremezone.com>

I've paraphrased some earlier sensenich documents regarding this subject,
I've paraphrased some earlier sensenich documents regarding this subject,
and have added some specific notes regarding the Lycoming O-320, O-360, and
spool spacers.

Drive-Torque Capacity of Crankshaft Flanges, 1999

Experience has shown that it is important to ensure that the drive-torque
capacity of a crankshaft flange / propeller hub is adequate.  Since only one
of the four strokes accomplished by a four stroke engine makes a positive
contribution to rated engine torque, the torque contribution by the engine
during the other three strokes is negative. It follows that the
instantaneous peak torque must be greater than rated engine torque.  The
ratio of instantaneous peak torque to rated torque will vary with the number
and arrangement of engine cylinders and the type of prop extension, among
other things.

Most common flanges designed to drive wood propellers can be idealized into
two distinct torque-transmission systems.  The flat hub face can be thought
of as driven by static friction or the propeller can be considered as driven
by the drive-bushings incorporated in the flange.  It is not possible to add
the drive-capacity of one system to that of the other.  If the propeller is
considered to be driven by static friction, the drive bushings will not feel
an imposed load and when the drive bushings experience a fluctuating torque
load, some movement of the hub against the flange must occur so that the
static-friction mechanism can not apply and scorching of the hub boss will

It is well known that the maximum friction force parallel to a contact face
is defined by the compression force perpendicular to that face multiplied by
a coefficient of friction dependent on the two materials in contact.  This
fact can be used to calculate the maximum resisting torque due to
compression of the wood propeller hub against the engine flange if the
compression stress in the hub and applicable friction coefficient are known.
Sensenich has derived an equation calculating maximum allowable peak torque
by static friction, with engine flange diameter being the single most
important factor.

If it is assumed that the drive bushings in the flange must bear the torque
load, then the maximum allowable peak torque equals the product of allowable
bearing stress for the drive bushings against the side of the holes provided
for them, the total drive bushing bearing area, and the drive bushing radius
from the crankshaft axis (equals bolt circle radius).

Calculations for both torque transmission systems reveal drive bushing
torque capacities at best only a quarter of static friction torque
capacities.  This is why maintaining proper bolt torque compression on the
wooden propeller hub is critical, since the drive bushings can only provide
a short term back-up.

Installations with satisfactory service histories indicate that the drive
torque capacity of that crankshaft flange is adequate.  It follows that the
instantaneous peak engine torque is less than the static-friction
drive-torque capacity of the flange.

One new factor in the last 20 years is the increased use of extensions or
spacers to move the propeller forward (or rearward) for more streamlining.
Any type of extension/spacer which moves the propeller off the crankshaft
flange will reduce the torsional rigidity of the crankshaft system and lower
the RPM at which torsional resonance can be excited.  Spool extensions in
particular** are torsionally "softer", which can cause the propeller to lag
behind and then race ahead of nominal engine RPM.  This lag/lead may appear
as increased peak torque at the propeller / engine interface.
Thus, a propeller mounted on a spool extension requires a larger drive
torque capacity than a direct installation on the same engine.

**Torsional stiffness of a shaft is directly proportional to the fourth
power of its diameter and inversely proportional to its length.

I will get together some tables with all the values when time becomes
Below is a general rating of torque capacities:



Lycoming O-320 engine / 160 HP

stock engine flange / none / 6" diameter / satisfactory, no heavy aerobatics

spacer (solid extension) / 2-4" / 6" diameter / satisfactory, no heavy

spool / up to 4" / 6" diameter / satisfactory

spool / 4-6" / 6" diameter / marginal, especially for props with medium-high
mass moments of inertia

spool / 4-6" / 6.5"+ diameter / satisfactory

spool / +6" / 6"+ diameter /  unknown, test data not available

Lycoming O-360 engine / 200 HP

stock engine flange / none / 6" diameter / unsatisfactory

spacer (solid extension) / 2-4" / 6" diameter / unsatisfactory

spacer (solid extension) / .5-4" / 7" diameter / satisfactory, with drive
bushings for backup

spool / 4-6" / 6" diameter / unsatisfactory

spool / 4-6" / 6.5" diameter / marginal, no aerobatics or high mass moment
of inertia props

spool / 4-6" / 7.0"+ diameter / satisfactory

// James F. Agnew
// Tampa, FL
// Velocity 173 FG Elite ( http://www.VelocityAircraft.com/ ) under construction