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Propeller mass moment of rotational inertia printer friendly version
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Bob Grimstead
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Location: Perth, Western Australia or West Sussex, England
Registered: Dec 2006
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Posts: 2027

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Posted Thursday, February 23, 2012 @ 05:11 AM  

Hi Guys,

We all know (or should know) that a rotating propeller acts like a gyroscope. The faster it rotates, and the heavier its blades are, the more the stresses on our crankshaft. So propellers on our 3,600 rpm Rectimos need to have blades that are as light as possible.

I seem to remember that there was once a post on this part of the FForum with a link to a VW propeller specialist's views on rotational inertia, and the maximum acceptable weight for a VW prop, but I cannot now find that.

The best I can find is this.

http://sportplanedesign.com/article5-Propeller-Inertia-and-Why-It-Matters

Can anybody direct me to that original link?

Thanks.

Yours, Bob

--------------------
Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

Bob Grimstead
Captain

Gender: Male
Location: Perth, Western Australia or West Sussex, England
Registered: Dec 2006
Status: Offline
Posts: 2027

Click here to see the profile for Bob Grimstead Visit http://www.redhawksduo.co.uk Send email to Bob Grimstead Send private message to Bob Grimstead Find more posts by Bob Grimstead Edit or delete this message Reply w/Quote
Posted Thursday, March 1, 2012 @ 03:34 AM  

Well, I've now found the facts anyway, as previously transcribed into my Fournier documentation archive:
'Great Lakes say use a wooden prop, and keep the weight below six pounds, to minimise crankshaft stresses. (torque bolts to 11 - 14 lb ft.) Great Lakes also say never use a prop with aluminum hub and carbon fibre blades.'

The original standard Hoffmann HO-11 133s, 70l prop weighed 4.85 pounds (2.2 kg).

Our Heliptera propellers weigh 2.6kg (5.7lb)

Our new Hercules props weigh 2.23kg (4.9lb)

So, as well as a significant gain in climb performance and a small improvement in max and cruise speeds, they confer a great improvement in crankshaft stresses during aerobatics.

Yours, Bob

[Edit by Bob Grimstead on Thursday, March 1, 2012 @ 03:48 AM]

--------------------
Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

jb92563
Second Lieutenant

Gender: Male
Location: Lake Elsinore, CA, USA
Registered: Mar 2007
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Posted Thursday, March 1, 2012 @ 11:54 AM  

Also, the less mass out towards the prop tips the better as the mass near the hub is less of an issue since it contributes less to the props inertia.

I got to thinking about this and the shape of eLippse props that are thick inboard and very tapered and thin outboard.

Looks like a good combination for the other characteristics on top of the performance gains.

Unfortunately Paul Lipps died last year and I can only hope that his son puts Pauls prop design notes and program on the
open domain or arranges for someone to continue development and research on this prop technology.

One day I hope to try such a prop on the RF4D.

Also on the topic of props, my new Culver which has only seen about an hour of ground running has its plastic prop tape rather chewed up already.

Anticipating that problem I bought some sheet nickel and stainless steel 0.003" thick to apply on the prop leading edge to protect it.

Its about as thin as Aluminum foil but much tougher and will prevent errosion of the out 1/3rd LE of the prop blade.

Of course it will require some carefull balancing after it is applied but I've done this before and have a very sensitive prop balancer.

[Edit by jb92563 on Thursday, March 1, 2012 @ 11:55 AM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

Jorgen
Captain

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Location: Lund, Sweden
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Posted Thursday, March 1, 2012 @ 02:12 PM  

Thanks Guys,
Bob, I see you appear satisfied with the Hercules prop:
http://www.youtube.com/watch?v=IvP_l-A73Lg
I guess you will report about it- or did you already and I missed it?

May the 4's be with you/ Jörgen

Bob Grimstead
Captain

Gender: Male
Location: Perth, Western Australia or West Sussex, England
Registered: Dec 2006
Status: Offline
Posts: 2027

Click here to see the profile for Bob Grimstead Visit http://www.redhawksduo.co.uk Send email to Bob Grimstead Send private message to Bob Grimstead Find more posts by Bob Grimstead Edit or delete this message Reply w/Quote
Posted Thursday, March 1, 2012 @ 09:14 PM  

Hi Jorgen,

Yes, I've written the article, which appeared in Pilot about a month ago. Now that it's off the bookshelves, I've e-mailed a pdf to Collin and Steve, for them to make it available to all Fournicators.

The story detail:

The new Hercules prop increased WGN's sea-level climb rate from 620fpm to 770fpm, in identical conditions less than one hour later, which I make a 24 per cent improvement in climb rate. Both those numbers were taken in the first minute of climb from 300 feet amsl.

Climbing at full throttle for five minutes, the numbers were 580 to 712 fpm , which is a 22 per cent improvement. I believe that the lower figure for the longer climb was because the end altitude was significantly higher, and so the engine ws becoming much richer and therefore producing less power
at the higher altitude.

I specifically asked Rupert to maximise climb rate rather than improving speed, but it also increased top speed by one knot/mph and cruise speeds by two knots/mph.

They cost around 750 GBP (pounds Sterling) but they confer a significant performance improvement (for us, anyway).

See: www.hercprops.com for more info, but don't be impatient. Like the guy who built a better mousetrap, Rupert is currently swamped with orders!

Yours, Bob

--------------------
Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

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