Propellers?

[Ex-Bay]

Further to the help given about Jet Engines, etc., can someone tell me about the modern propeller ? Those 6-bladed gizmo's with the weird shape (they remind me of a submarine's prop), as can be seen on the Herky Bird J), Antonovs and similar.

So what's the gen ? What makes them better ?

 

[Weebl]

So what's the gen ? What makes them better ?

Their ability to move air.

What bay did you used to be ex?

 

[John Lloyd]

Here ya go Ex, being of the Plumber persuasion I would equate a swept prop with a swept wing, going round n round n round n round n round, now I'm getting dizzy so I fell back on Wikipedia to explain

http://en.wikipedia.org/wiki/Scimitar_propeller

 

[Billy Whizz]

Try and get a trip round Dowty at Gloucester - you get to see them being made etc. Just avoid the 2 hr history lesson!

They are still hand made - if ever a factory was in need of Lean!

Beats the hell out of a day in the classroom on a Q course and is even quite interesting for a Rigger!

 

[Ex-Bay]

Their ability to move air.

What bay did you used to be ex?

The short answer is Radio.
I checked Wikki and got some info (including some frightening Maths).
Thanks for the pointers.

DT_Xtremez_30:

 

[mild mannered janitor]

i could be wrong but i think its to provide more surface area and therfor generate more thrust without making he props too long.
if the props are too long the tips would aproach the speed of sound and catch up with the prop before it equalling massive damage

 

[rest have risen above me]

i could be wrong but i think its to provide more surface area and therfor generate more thrust without making he props too long.
if the props are too long the tips would aproach the speed of sound and catch up with the prop before it equalling massive damage

How's that work? As the props are solid and don't bend. Assuming this principle then the tips of a Conway engine would be in a right pickle as the tips of the blades travel at approx 2820kmh.

 

[propersplitbrainme]

If the props are too long the tips would aproach the speed of sound and catch up with the prop before it equalling massive damage

Ummm, not quite. If the blades are too long the tips would go supersonic leading to inefficiency, but they don't (can't) physically catch up with the blade ahead of it!

Modern prop blades are of a wide-chord design, similar to the difference between an old 'snubbered' thin fan blades and modern wide chord designs. As stated above, the result is they shift more air than conventional designs.

 

[Billy Whizz]

Am sure I remember the guy from Dowty saying they look like they do now cos they're more astetically pleasing! DT_Xtremez_28:

 

[OneTimeSooty]

i could be wrong but i think its to provide more surface area and therfor generate more thrust without making he props too long.
if the props are too long the tips would aproach the speed of sound and catch up with the prop before it equalling massive damage

If I remember correctly the term is "Disc Solidity" - hence why J Hercs have 6 blades whereas K's have 4, and A400M will have 8.... And look at the Shackleton's contra rotating props (2 props mouted one behind the other, each turning seperate directions) which is really efficent, but the added gearing out-weighs the benefits. And remember there's the all important weight issue as well - the engine's got to get that thing spinning in the first place.

As for the J prop looking like a submarines impeller - they're not quite that bad surely? They do have blade twist - makes them more efficent throughout the flight range (J props are constant speed, variable pitch) but nothing quite as pronounced as anything I've seen in water!

Just been told that "lamina flow" of molecules helps with the efficency of props - the amount of air that will "stick" to the prop, so that the airflow flows over a boundary layer of air, not the prop itself.

Hope that helps..

 

[fat lazy techie]

Further to the help given about Jet Engines, etc., can someone tell me about the modern propeller ? Those 6-bladed gizmo's with the weird shape (they remind me of a submarine's prop), as can be seen on the Herky Bird J), Antonovs and similar.

So what's the gen ? What makes them better ?

Having had the pleasure of working on both subs and Js (albeit briefly on the J) i have to say the one doesn't remind of the other. Things may have moved on since my sub time, but i'm pretty sure the props were shrouded and had many more blades. i may be wrong though.

 

[shoutingwind]

So what's the gen ? What makes them better ?

the awesome sumpies that work on them!!! okay so i'm biased DT_Xtremez_30:

6 blades also means its easier to turn the props to check them- not so far to reach up! LOL

 

[grumpyoldb]

Is that to wind the rubber bands up before flight?DT_Xtremez_22:

 

[MAINJAFAD]

Having had the pleasure of working on both subs and Js (albeit briefly on the J) i have to say the one doesn't remind of the other. Things may have moved on since my sub time, but i'm pretty sure the props were shrouded and had many more blades. i may be wrong though.

British subs since the introduction of the Trafalgar class boats have indeed had a ducted fan type main propeller, it cuts down the noise radiated into the water. Submarine drivers hate noise, as it gives the position of thier boat away.

 

[Ex-Bay]

Having had the pleasure of working on both subs and Js (albeit briefly on the J) i have to say the one doesn't remind of the other. Things may have moved on since my sub time, but i'm pretty sure the props were shrouded and had many more blades. i may be wrong though.

Perhaps I didn't make myself clear. It's the general 'scimitar' shape of the blades that reminded me. I had an idea about the sub's prop, but I was having trouble with putting something like it on a 'plane.

I think I'm getting the hang of it now. (pause for prayer).

 

[shoutingwind]

Is that to wind the rubber bands up before flight?DT_Xtremez_22:

nope- thats on the old ones, the computer does it itself on the J.

Are the dynamics of a prop the same in water and in air?

 

[uber pikey]

'J' props are very prone to impact damage due to the composite materials used and the internal heating mat getting an open circuit from stone chips and some times delamination, the 'K' alloy prop takes a lot more impact damage to make it U/S, so in some respects they out weigh each other however the 'J' prop is far more advanced when it come to pushing air aroundDT_Xtremez_19:

 

[MAINJAFAD]

Wiki expains the working of both here, though in simple terms, they both work in the same way, the limits on performance due to the medium in which they operate are different. In the case of the aircraft, its the Mach number on the prop, as the blade tips reach Mach 0.9, shock waves form and the prop loses its effectiveness. In the case of a prop in water, the Mach limit is not a problem (seeing that the speed of sound in water is much higher and water doesn't compress), however the low pressure face of the prop will suffer from cavitiation caused by the water on the face of the blades boiling off above a critical speed, this causes the prop to lose its effectiveness (plus make shedloads of noise and vibration, as well as damge to the blades).

 

[I Look Like Kevin Costner]

The "J" propeller is a "High Speed" design that as the outer section of the blade is swept back to reduce (by the resultant airflow due to the sweep) the formation of shockwaves by the resultant velocities of airflow and rotation. The term High Speed refers to the fact that an aircraft fitted with such a prop can fly faster that an aircraft fitted with a conventional prop of the same size / rotation velocity.

Straight from my Module 17 text book.....

 

[planesailing]

Propellers?

More power, more blades, the Shack had six per power plant 2150 HP give or take.