Prop shapes

[saudipc-9]

I tried this one a while ago and didn't really get a good answer.

Why are the prop tips on a P-3 rounded and on a C-130 squared off?

Full marks for any9one who can answer this for me

[411A]

Propellors used on the Allison 501/T56 series engines are from two manufacturers, Aero Products (Lockheed Electra, G201 series) and Hamilton-Standard (P3, C130).

Round or square-tipped blades can be found on some models of the C130...and indeed the same is true for the Lockheed Electra.

[saudipc-9]

Thanks for that but I was hoping that there was an aerodynamic reason for the different shapes

[Oktas8]

Squared off tips are less efficient, but allow higher solidity and therefore more power can be delivered to the airflow by a propeller of given diameter.

C130 - short field takeoff is a primary performance requirement, so high power output is necessary.

P-3 - oceanic endurance is a primary performance requirement, so efficiency becomes the overriding concern.

All the above is just off the top of my head, so I await correction.

O8

[Mr Proachpoint]

That is a bloody good question. Hopefully I can shed some light on it. If you go right back to wing planforms and all that shiit, you'll remember that an elliptical wingtip will provide the least amount of induced drag for a given wing. The same goes for a propellor. If the tip tapers, induced drag is reduced and hopefully the efficiency will balance or better the loss of thrust. A military aircraft such as the C-130 with its squared off tips is probably owned by a government who accepts loss of efficiency against dollars per hour. A C-130 produces lots of required thrust with an accepted loss of efficiency. A P-3 Orion however, has a different type of prop for two reasons. The P-3 was developed from a civil aircraft where efficiency was important, hence round tips. AND a P-3 requires efficiency to patrol the oceans for hours and hours on end.

My argument goes to pieces when you see a Convair 580 however.............................

[411A]

That's because the CV580 used Aero Products propellors, ie: the same as most Lockheed Electras'.

[stella arrival]

I think I can help with this one. The Electras have both Hamilton Standard props and Aeroproducts props. The Ham Standard blades are cast from solid pieces of metal (not sure what exactly). The Aeroproducts prop blades are hollow and constructed from two skins covering longitudinal ribs. From a manufacturing point of view, it is very difficult to produce eliptical pattern blades from the hollow construction method, hence they leave the blades squared off, but in the case of the solid blades, the manufacturer has chosen to round them off. They may have decided to do this for neatness or personal choice on the part of the designer, but it makes little difference. Flight tests were carried out using a Convair 580 which had one of each type of propellor and it was discovered that there was no performance difference whatsoever between the two.
At a day to day level, the Ham Standard props feel to be more responsive to power lever movements, and there are certain differences in the ways in which the props need to be handled.
The propellors are not interchangeable with one another - if a Ham Standard prop is removed from an aeroplane, it can only be replaced with another Ham Standard, not an Aeroproducts prop.
Hope this helps to some degree, although it doesn't relate specifically to the C130.

[Old Smokey]

The problem with fast, fairly high flying turbo-props with large propeller radii is that the propeller tips will be the first part of the aircraft to reach the speed of sound, with resultant significant loss of thrust for the power delivered to the propeller.

Compare the L188 Electra which was developed into the P3. The L188 was optimised to fly fast (about TAS 330 Knots) and reasonably high (F/L 250). The L188 had square propeller tips, with a slightly broader blade than the P3, i.e. the same propeller area to absorb the power. Some loss of aerodynamic efficiency was acceptable to avert the much greater loss of efficiency in having the propeller tips reach Mach 1.0 at fast and high cruise.

The P3 on the other hand was optimised to spend most of it's mission time at low level, where sonic propeller tips are not a problem, and the higher aspect ratio rounded tip propellers provided the optimum efficiency. It still left the P3 with a 'respectable' high speed / high altitude transit capability, but not in the same league as it's 'parent' L188. (This loss of high speed capability may have been compensated for in the increased power (by about 500 HP per engine) on the Allison variant fitted to the P3).

One of the prime reasons why many modern fast turboprops have 5 bladed propellers is to reduce the propeller radius, thus reducing tip speed and avoiding sonic speeds.

[Flight Detent]

Hi Old Smokey,

Sounds reasonable to me,

the P3 B/C models, with the uprated T56-A-14 engines, developing something like 4,600 SHP/engine, cruises around 360 TAS, at a max operating altitude around FL280.

Cheers,

FD

[Old Smokey]

Thanks Flight Detent,

I was a bit unsure of just how much more power the P3 versions of the Allison produced. The L188 was 3750 HP per engine, the 4600 HP per engine you mention is additional 850 HP per engine / 3400 HP per aircraft. I would think that this would more than compensate for efficiency losses at high altitude, whilst optimising performance at the 'mission optimised' low level.

Regards,

Smokey