RAM's Greatest Questions, chapter 1

[RAM]

Ok My question is that...

IN late WWII most Allied ETO fighters had a 4 bladed propeller, Spitfires had even 5 blades, Mustangs, P47s...4 blades

then why Me109 Had 3 blades until the end? and Fw190? and TA152??? what are the differences between propellers and why germans didnt put more blades on their fighter props?

thks


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Ram, out

JG2 "Richthofen"

[JoeMud]

Cause they switched to jets?  

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"JoeMud and the Volcano"

Now showing at your local AH drive in!

[Tern]

 

Originally posted by JoeMud:
Cause they switched to jets?  

Production problems, would be a safe guess.  Like, the allies kept bombing it?  
Great question RAM.  I'd love to hear the authoritative answer.

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Tern
"Live to Fly!  Fly to Fight!  Fight to Live!"
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"There I was, inverted at 50 feet and 120 kts. and the only thing running was the radio."

[Pongo]

To handle the extra horse power of the late war engines the Germans chose a broad bladed type of propeller. Some of the bomber designs used a 4 bladed thiner blade propeller. Why? the broad blade looks cooler.

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Pongo
The Wrecking Crew

[Kieren]

(Foolishly realizing he should leave this to the engineers)

You may be surprised to learn that the theoretical most efficient propellor has only one blade!

There are a host of reasons why this isn't done in practice, but the best way for me to explain would be to have you think of the blade as a miniature wing.

Just like in aircraft, the airfoil for this "wing" is selected with its desired performance in mind (high lift, high efficiency, etc). Examine the blades and you will also notice differences in aspect ratio (the comparison of the length to chord).

Engine power and gearing are important. How fast will the blade turn? One of the biggest reasons more blades are added is because it is the only possible alternative. To make a 2-bladed prop develop more power, you might have to lengthen the blades. There is an obvious and not-so obvious limitation here. The obvious is the ground that gets in your way (this is why the Corsair's wings are bent). The not-so-obvious is the fact that the tips of the blades approach the speed of sound at full rpm. Example: Huey helicopters "thump" because those long blade tips are right at the speed of sound.

The point to all the rambling? The Germans and Allies took different approaches to the same problem. From my uneducated eye it appears the Germans went with high-lift, low aspect ratio blades, and the Allies went for efficient, high aspect ratio blades.

Once again, engineers shoot holes in this.  

[Duckwing6]

One thing with Props is that the more blades you have the less efficient they get ..

more  blades are usually used to decrease prop diameter and to reduce tip speed, and decrease the ground clearence problems.

Also you can add prop blades to absorbe more horsepoer (Torque) when you can't increase blade length and or chord length any further.

-> The multibladed props where usually used on the "growth" versions of these planes which had up to 1500Hp more than the original design.

[Kieren]

A thought to add to this discussion, and aeronautics guys correct me:

Isn't most of the "lift" of the propellor generated in the outer 20% of the blade?

[-lynx-]

Precisely Keiren (read it somewhere too  )!

The problem was that with the engines becoming more and more powerful a compromise had to be reached between power transferred from the engine to a bigger prop, prop diameter (to avoid tips going supersonic = drop in thrust) and, of course, ground clearance. Brits even tried counter-rotating 3 bladed probs - effectively a 6 bladed prop...

Griffon-powered Spit with bubble canopy and NO torque effects *drool*  .

Pongo - have to disagree: 5/6 bladed beasts look way more menacing  .


p.s. More blades - less efficiency? Can anyone explain why? Each blade is hitting totally "independent" and undisturbed chunk of air. They simply do not affect one another at cruising speed. Latest experimental turboprop powered civilian aircraft feature 6+ low aspect blades and show miracles in efficiency/airspeed achieved for a prop-driven plane...

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-lynx-
13 Sqn RAF

[This message has been edited by -lynx- (edited 04-05-2000).]

[Ghosth]

Drag vortices, that is the blade in front does disturb the air for the one following.

While at high speed this is somewhat reduced
it is still a factor. The more blades, the shorter time (and distance) for these to disappear before the next blade comes up.

(Which is why Kieren said a single blade is more effiecent)

Not that I'm an engineer or anything. But if you've ever stood in prop wash its NOT smooth  


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Maj Ghosth
XO 332nd Flying Mongrels

[AKDejaVu]

The area behind the blade would be subject to the wash.  But each blade should be cutting into clean undisturbed air.  I immagine the major factor would be the dispersal of the wash... does it flow in a manner that enables it to move forward enough to disturb the next blade in line.  I immagine it does at some point... thus the lower aspect blades.

AKDejaVu

[This message has been edited by AKDejaVu (edited 04-05-2000).]

[Hangtime]

(hang smiles)

No flies on u boys. Pretty sharp. Your answers are correct.  

Hang

[funked]

I'm an engineer but I know nothing about propeller blades.  I know how the pitch control system works though.  

From what I've read and seen, the Allies went with more blades to absorb the increased power in late-war engines.  

The Germans stuck with three blades but went with very very broad blades.  I've had the opportunity to inspect a Ta 152H propeller and the blades are amazingly wide - maybe 30 inches at the widest point.

[-lynx-]

Well - Ta152 was a high alt plane, it had to have wider blades to grab onto whatever thin air there was up there .

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-lynx-
13 Sqn RAF

[spinny]

Hitler's astrologer told him the numbers 4 and 5 were not good numbers for him...come to think of it, he was right.