Question

[Falcon500]

What was the reason for the transition from rounded propellor (or however it is speeled  ::)) tips to squared tips as seen on late model C-130's and C-119's.

Im not a real pilot but i figured the people in these parts (this section of simv) would know.

[C]

Mmm, tricky question that I might have to get back to you on...

[Mobius]

A larger prop will give you better acceleration, but will also cause more drag at higher speeds, so maybe the acceleration benefits of the larger propeller where outweighed by the speed benefits of clipping the tips off.  I would be more willing to say it has to do with controlling the vorticies off the tips over anything else.  I don't know exactly how it would affect it though.

[ozzy72]

It could be something to do with the tips approaching the speed of sound as this causes a lot of trouble for props....

[C]

It could be something to do with the tips approaching the speed of sound as this causes a lot of trouble for props....

This was my initial thought - generally you see square tipped props on large (final generation radial) pistons and high powered turboprops. So, as you say, square tips will slow the tips down. The other secondary effect of this is that it may stop the props overspeeding with large power changes, which maybe in this day and age is becoming more managable using electronic control systems.

[Mobius]

Or, could it be like a dog-toothed wing?  Would a larger vortex allow smoother airflow over the prop, which would give the prop more bite?  If the prop were overspeeding, why wouldn't they keep the same shape and just make the prop a different size, or lower the engine RPM?  I could just be talking out of my rear here though, so I don't know.

[C]

[quote]If the prop were overspeeding, why wouldn't they keep the same shape and just make the prop a different size, or lower the engine RPM?

[Mobius]

Yeah, there's just too many variables to make a good guess, I guess.

[Mobius]

The square tipped blades on a Herc prop are for high thrust at low speed, useful for dragging the aircraft out of short unimproved strips. Problems of transonic flow at the tip are avoided primarily because the shape and therefore the drag of a Herc prohibits a really high speed cruise. Hercs cruise at around 300KTAS.

The Orion however, only operates out of longer hard surfaced runways and spends the majority of its time in high speed cruise and so has a rounded tipped props.

If you compare the two aircraft you will notice that the Orion has a much thinner wing and smaller diameter fuselage than the Herc, also the Orions MAUW at 135,000lbs is 20,000lbs lighter than the Herc.

In reference to square tip vrs round tips. Yes, they produce more drag, BUT, if you are trying to put the maximum amount of power into a propeller, for a given rpm and pitch, and assuming a constant number of blades, you can only then increase the area of blade. If you have already reached the limiting diameter (i.e. trans-sonic tips), that only leaves increasing the blade's chord, or it's camber. Square tips are an improvement in the former. Square tips = "wider" chord = more area = more thrust (just like a helicopter blade). For a given blade diameter, a round tip prop cannot produce the same thrust as a square tip, unless it has a wider chord elsewhere. It is however, like everything, a compromise.
The peculiarities of the engine it is mated to is a big factor in chosing the design, as is it's anticipated operating environment. A given aircraft might perform better using square tips instead of round tips, because of the power curve characteristics of the engines and the flight envelope and role of the aircraft itself. As Checkboard points out, a big avantage of a square tip is it's resilience to stone chips (perhaps SFTOL operations), and ease/economy of manufacture.
It's clearly a case of horses for courses.

Couple of interesting quotes I found after I googled the question, so they may not be legitimate, but they provide a possible answer.

[Falcon500]

 :D I got it.... they do it because they can!



that more area deal is a good enough expanation for me  ;D I googled it and ended up finding that i could make molds of wooden props to turn into composite casts for auto-gyros

[Brett_Henderson]

I've always wondered why conventional props have their pitch decreasing as you move further from the hub/spinner... where these big, "paddle" props look like they keep the same pitch, all the way out to the prop tip.

On conventional props.. I'd guess that since the further out you go, the faster that part of the blade moves.. so it must be to keep thrust/air-flow more stable ?

[SilverFox441]

Decreasing pitch as you move further from the hub keeps thrust levels equal across the prop disc.

The relative speed of the blade tip is higher than near the root, therefore lower pitch at the tip produces an equal amount of thrust to the higher pitch closer in.