Prop Tip Question

[aviationluver]

I was reviewing some interview questions and one question mentioned that C-130 prop tips are square and CP-140 prop tips are rounded. Why is that?

[megan]

Page 116 may answer your question, at least in part.

http://www.dtic.mil/dtic/tr/fulltext/u2/776998.pdf

A better question to have asked is why the CP-140 are rounded and the L-188 Electra, ostensibly the same airframe, had square.

[dixi188]

megan,
Not all Electras had the Aero Products square tipped props, some had Hamilton Standard with rounded tips like the C130 and P3.
I believe the HS props made the Electra about 10 Kts faster in the cruise.

As to why the difference, the reason is probably lost in the mists of time.

[pattern_is_full]

According to:

https://www.airspacemag.com/need-to-know/are-more-propeller-blades-better-108981404/

...square tips are more effective per radius - up to the point (airspeed + tip speed) where they develop "compressibility burble" sooner than rounded/elliptical tips.

C-130 is slower, and needs the extra prop "push" on takeoffs, from the square planform. CP-140 has a higher cruise speed (405 kt vs. 330 kt) where the burble is more of a problem.

As to the Electra - I see a (very) few pictures of them with rounded prop tips. Probably just one of those "Oh, why didn't we think of that" changes made over time, and as the airframe was updated (e.g. CP-140) over decades.

http://imgproc.airliners.net/photos/airliners/4/3/8/0384834.jpg?v=v40

[megan]

Not all Electras had the Aero Products square tipped props, some had Hamilton Standard with rounded tips like the C130 and P3The only airline to use the Hamilton was KLM dixi, unless an operator changed following production.

[tonytales]

I believe American Flyers also used the Ham Standards with the rounded tips.

[thetimesreader84]

Flew the Aeroproducts Electra for a short while, and jumpseated on both frequently.

By and large, the HS props worked better at lower levels, whereas Square props generally worked better above FL170 or so. But we are talking about 10-15kts at best.

Apparently the AP prop could get a more coarse blade angle, and a more “cruise” profile, but that’s just anecdotal.

[Will Co]

Considering that the blade is effectively a wing that rotates, the rounded tip might induce less drag than the square blade, at hight relative angle of incidence that is. On the other hand the square tip gives more blade area for the given span. It would all depend on the nominal angle of attack the blade, I suppose.

[megan]

I believe American Flyers also used the Ham Standards with the rounded tipstony, you prove the old saying, "Don't believe anything you read and only half of what you see". The KLM info came out of a Lockheed document. Seems American Flyer had a mixed fleet going by photos.

[Bergerie1]

BOAC's Bristol Britannia 102 aircraft had square tipped propellers whereas those on the 312 had round tips. Why the difference between the two types when the only main difference was the size and range of the two types? Was it something to do with the engines?

[tonytales]

The early Britannia's had hollow steel square tipped DH props related to the Ham Standards square tipped hollow steel props on the B377 Stratocruiser. Like the Boeing, the Brits were superseded by round tipped solid aluminum blades due to many problems with the hollow steel blades.

[megan]

problems with the hollow steel bladesGiven the prior poor performance maintenance wise with hollow props, on the L-188 the Aero was a hollow steel forging and the Hamilton solid dural.

[Bergerie1]

tonytales,

Thank you, I knew someone would have the answer!

[tonytales]

As a small thread drift, if Ham Standard hollow steel props had problems, you should have seen the Ham Standard hollow dural blades on the Lockheed L-1649. Heard a crowd outside our maintenance office at the JFK IAB. An Air France L-1649 had parked outside our door. L-1649 had slower turning props than the L-1049's and so had wider chord prop blades which, to save weight, were new type hollow aluminum.
The Nbr. 3 prop had one blade that had split open down the leading edge. Some internal foam stuffing was sticking out. Pretty quickly the hollow dural blades were replaced by solid dural blades.
Hollow steel blades were a specialty of Curtiss Electric props and Aeroproducts as pointed out. From a line maintenance point of view the blades caused us little trouble.