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View Full Version : Props: 3, 4 or 5 bladers...



tomtheyak
06-09-2006, 08:08 AM
I realise that what determines the best number of blades on a prop is related to engine revs, gearing and power but what specifically makes designers choose one over another? Are there other factors (drag, weight, aerodynamic charecteristics)?

Look at the Luftwaffe a/c - even very powerful types like the 190D9 (1776HP), Ta-152 (1750HP) or 109K4 still have 3-bladed props, whereas say a MkXIV Spit, has 6 blades on an engine of 37L & 2035HP. Is this just cos of revs?

tomtheyak
06-09-2006, 08:08 AM
I realise that what determines the best number of blades on a prop is related to engine revs, gearing and power but what specifically makes designers choose one over another? Are there other factors (drag, weight, aerodynamic charecteristics)?

Look at the Luftwaffe a/c - even very powerful types like the 190D9 (1776HP), Ta-152 (1750HP) or 109K4 still have 3-bladed props, whereas say a MkXIV Spit, has 6 blades on an engine of 37L & 2035HP. Is this just cos of revs?

joeap
06-09-2006, 08:41 AM
Excellent question.

WOLFMondo
06-09-2006, 08:47 AM
I think its just about absorbing the power. I know the Spit XIV got a 5 bladed prop because its ground clearance wouldn't allow a larger 4 bladed prop. I don't know about the German props and why they were mostly 3 fat blades.

Theres also do to with the speed the tip is travelling at, ground clearance etc to be taken into consideration with the length of the blade.

I'm sure Viper will come and along and explain it all.

Kurfurst__
06-09-2006, 08:51 AM
It's really a matter of choice.. prop blades are like wings, the more area you get, the more air it can grab. There are several ways of increasing prop area, like :

- increasing blade lenght
- increaseing blade width
- increasing the number of blades

Basically lenght is limited by the ground clearance, so it's good for large machines. Also there's the problem that the prop tips should not exceed the Mach number, so it needs to be geared down.

Increasing the number of blades to increase total blade area is the simplest, but least efficient way, since each new added blade adds turbulance and decreasing the effiency of the next blade; there's also some added weight. The gains and losses must be considered, ie. a net gain is neccesary.

The least perceivable difference is the design, airfoil of the blade itself.. they can be optimized for altitude, high/low speed etc..

The third way, on which the LW went was to increasing blade with considerably to increase blade area, look at late planes, like the D-9, the blades are extremely broad, and some last war year docs I have show that there were comparable gains from redesigning three blades for broader, nd/or thinner blades as if an extra blade would be added. Even swept blade designs were considered (good for near-Mach numbers).

Alltogether you can't say which one is really better, because there are so many variables - I think there's a Spit IXHF test around that showed really no advantage from going from 4 blads to a specific 5 blade design, but again, another 5-blade design could've been better.

Blutarski2004
06-09-2006, 11:39 AM
The principal limiting factor in blade length is tip velocity, then ground clearance. As tip velocity approaches Mach, blade efficiency deteriorates rapidly.

ROTC4612
06-09-2006, 01:04 PM
I know that the FW-190 Dora had the big "paddle blades" because it gave better performance at high altitudes.

Taylortony
06-09-2006, 01:21 PM
Was the reason the Corsair has a cranked wing, for the prop to fitted on without it would have meant a hugely long undercarraige, so the crank allowed the prop and shorter legs http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

It is to do with tip velocity, as said you can't run a blade at supersonic speeds, therefore you use a wider shorter blade to absorb the engine power and create maximum thrust, addding blades is also good but again that is limited by what you can get in the prop hub and weight.. you also you start to get efficency problems on a prop after about 6 blades, contra-rotating is another way to get round that too, but the rear prop has to be a smaller diameter than the front ( about an inch ) to prevent tip vortices on the front prop destroying the airflow over the rear prop tips.....causes allsorts of problems..

You will see modern blades have all sorts of dogs danglies in the shape dept...... this is to put back the onset of supersonic tip speeds........... the Lynx Helicopter came out with burp rotors to alleviate the problem of high speed flight being wider at the tip than the rest of the blade..........

you can even get Q tips on props where the ends are bent over 90 degrees similar to the winglets you now see on aircraft wings http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

tigertalon
06-09-2006, 01:41 PM
All luftwaffe single engined fighters also carried weapons inside prop arc, so they had to be syncrhonised. Sometning to consider when you decide about blade-count and frequency at which they will block the firing line.

(edit: spelling)

erco415
06-09-2006, 03:52 PM
Consider too that each blade you add decreases top speed, though climb will be better and (usually) vibration characteristics will be better. There have been one blade props, they didn't work so great...

Haigotron
06-09-2006, 04:03 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">As tip velocity approaches Mach, blade efficiency deteriorates rapidly. </div></BLOCKQUOTE>

but why though? i cant seem to grasp this concept...

LStarosta
06-09-2006, 04:45 PM
Because the rel. airspeed of the blade is axial speed + rotational speed, the tip of the blade will go supersonic before the rest of the airplane. When the tip does go supersonic, there is an enormous increase in drag and shockwaves are produced, causing the propellers ability to produce lift (thrust) to decline rapidly.

Jets completely forgo this by decelerating the air to subsonic speeds before it hits the compressor face, which is why a jet engine is capable of sustained operation at supersonic speeds.

BfHeFwMe
06-09-2006, 05:46 PM
Sometimes they simply preform better for the mission assigned. AC-130A's were originally fitted with three blade props, than conversions began along with the rest of the regular fleet. The crews didn't like it, they switched back due to better accelleration and manouver at the lower levels they had to operate. These gunships continued for decades as the only three blade 130's until scraped in the 90's.

Haigotron
06-09-2006, 05:50 PM
LStarosta

I thank you http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

The-Pizza-Man
06-09-2006, 05:55 PM
The new C-130Js have 6 bladed props, apparently they are a lot more efficient than the old props.
http://www.bourget.free.fr/bourget2001/002.JPG

diomedes33
06-09-2006, 05:58 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Haigotron:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">As tip velocity approaches Mach, blade efficiency deteriorates rapidly. </div></BLOCKQUOTE>

but why though? i cant seem to grasp this concept... </div></BLOCKQUOTE>

What LStarosta said and here's an article (http://www.aerodyn.org/Drag/speed-drag.html#tdrag) that shows how the drag changes as it approaches supersonic speeds. Drag means more resistance, which means more power needed to drive the prop.

BfHeFwMe
06-09-2006, 06:50 PM
Don't forget the different engine series encased behind those differently shaped nacelles too. http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

Spinnetti
06-09-2006, 09:52 PM
All good points..

As said - Tip speed and ground clearance are the biggest factors. more blades are less efficient, but when you run out of clearance you don't have much choice.. fortunately jets came along as the options were running out. Another consideration is that diameter is where most of your thrust (accelleration) comes from, and pitch is where your speed comes from. Its always a balance of finding the right combo for the operational characteristics of the aircraft.

Rooks-
06-10-2006, 02:01 AM
Prop configurations lend themselves to different characteristics as I understand. My brother who worked on a lot of props said that 3-blades tended to be more efficent for cruising.

La7_brook
06-10-2006, 02:37 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by tomtheyak:
I realise that what determines the best number of blades on a prop is related to engine revs, gearing and power but what specifically makes designers choose one over another? Are there other factors (drag, weight, aerodynamic charecteristics)?

Look at the Luftwaffe a/c - even very powerful types like the 190D9 (1776HP), Ta-152 (1750HP) or 109K4 still have 3-bladed props, whereas say a MkXIV Spit, has 6 blades on an engine of 37L & 2035HP. Is this just cos of revs? </div></BLOCKQUOTE> the 109 g6as propeller had the new vdm9-12159 featuring slightly wider blades fited over the g6 ones

GH_Klingstroem
06-10-2006, 02:42 AM
Exellent thread!!
Note the elliptical leading edge and the swept back shape of the propeller blades on that c-130J!
All to increase the lift at high speed of the blade! good pic!

Vike
06-10-2006, 03:58 AM
Many thanks all for these infos!! http://forums.ubi.com/images/smilies/25.gif

@+

luftluuver
06-10-2006, 05:57 AM
Anyone know why the Fw190 (DB engined V31/U1) which was tried with a 4 blade but the Dora ended up using a 3 blade?

How did the Tu-95 Bear achieve 900kph?

The-Pizza-Man
06-10-2006, 07:03 AM
by having four 15,000 hp engines
http://flymig.com/maks_pictures/images/TU-95.2.jpg

tigertalon
06-10-2006, 08:00 AM
Yep, Bear is indeed one of most impressive ac ever built. Can you imagine building a WW2 fighter with one of this gigantic engines?? http://forums.ubi.com/groupee_common/emoticons/icon_eek.gif IMO it would just slightly fall behind spit25lb (as we have it now) in climbing compartment.

p1ngu666
06-10-2006, 10:54 AM
i remmber hearing the usn could track bears because they made such a racket there submarine tracking devices would pick them up http://forums.ubi.com/images/smilies/16x16_smiley-surprised.gif

but i love the bear http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif