[BlackBerry]

Crummp, you are the expert on propeller aerodynamics. With your help, I've finally got the whole story.

In world war ONE, UK, Germany, USA developed RAF-6, Gottingen, and ClarkY airfoils for propellers respectively. These airfoils are "high drag high lift" conventianal airfoils. At the time, 2-blade fix pitch airscrew were used.

Before WWII, people found it's nessesary to add the 3rd blade to absorb growing horsepower of engine. eg. Bf109B/D->Bf109E. When you add more blade, there are two contrary effects:

1)good thing: better power loading ability
2)bad thing: more drag

At late 1930s, UK/USA/Germany engineers found it's almost no benifit from the 4th blade because the improvement on power loading is completely counteracted by drag increase added by the 4th blade. Allied tested RAF-6/ClarkY with 3-blade and 4-blade configration, drew that conclusion, German Mtt and Focke Wulf also tested , with same result.

http://aerade.cranfield.ac.uk/ara/19...report-640.pdf

You can also bet that all the engineers during WWII did their homework. I know Mtt and Focke Wulf both tested 4 bladed designs on their aircraft. It was found that what one design made up in efficiency, it lost in power loading and vice versa. As such Focke Wulf concluded that was no appreciable difference other than weight savings on the 3 bladed propeller.

During late period of WWII, every country faced same difficulty: how to improve prop efficiency when more powerful engine equipped with aircrafts?

German engineers found a clever method: use broad chord in 3-blade prop thus they could improve power loading while maintain lower drag than 4-blade. Result was quite good:

I have all the data on VDM propeller series. The wide chord wooden props for the Luftwaffe dropped top level speed by about 4 percent but increased turn and climb rate by about 15 percent. The Luftwaffe conducted several indepth studies. I am sure the USAAF did the same. Blade width does help efficiency to a point.

http://forum2.totalsims.com/viewtopic.php?t=2475

The German propeller designer took the approach of widening the blade chord to increase power loading and using a better material. The allies added more blades and accepted the weight increase. Both are perfectly acceptable approaches to increasing performance with very little to choose from.

Allied finally accept 4-blade prop, eg P51A-P51B. in your opinion, both methods are perfect, and with little to choose.


Question: Since german 3-broad-blade obviuosly outperformed their old 3-blade design , so were allied new 4-blade prop. I've posted the proof of efficiency advantagde of P47's 4-blade hamilton over 3-blade. However, in late 1930s, allied reports on RAF-6/ClarkY already said there is little difference between 4 and 3 blade. What's the problem?


The answer is lamimar airfoil developed during WWII, NACA-16 series. I agree with you with the difficulty maintaining of laminar effect in actual combat envirenments. OK, let's regard NAVA-16 as conventianal airfoil, that is, NACA-16 is "fake" laminar flow airfoil. The next question is: Is there enough difference between two kinds of conventianal airfoils? Of course. In an aerodynamics textbook says:"RAF-6 is suitable for taking off while ClarkY is better in criusing and high speed flight." Notice that there is only slightly section shape difference between RAF-6 and ClarkY. Therefore, being a vast different shape, NACA-16 behavior should be "special". But in some allied test, 3-blade NAVA-16 is even slightly worse than 3-blade ClarkY especially during taking off. Notice that the test speed is probably within 400MPH.

To our dismay and disappointment, the 16-series propeller showed no advantage at high speeds; in fact the Clark Y appeared slightly
better.

Page 124 tells the story...

http://www.scribd.com/doc/46042585/T...rams-1920-1950

at high speed......how high? 0.7 Mach TAS?

Is the NACA-16 the "new age ClarkY" just like Clark/RAF-6 comparation? that is to say, "new clarkY"--NACA16 is worse than old clark in taking off and better in REALLY high speed when propeller tip approching critical mach number? This is the key of mysterious diving performance difference.

After WWII, as piston engine's power increased to 2400-3000HP, people impelmented 5-6 blade low drag NACA-16 airfoil to absorb it, and this configaration worked perfectly at high mach subsonic flight. This fact reminds us that whether the 4-blade NACA16 propeller outperforms 3-blade high drag/high lift wide-chord airfoils at high diving speed(=0.7mach or so)? There is small clue as Crummp said in 2005:

I have all the data on VDM propeller series. The wide chord wooden props for the Luftwaffe dropped top level speed by about 4 percent but increased turn and climb rate by about 15 percent.The Luftwaffe conducted several indepth studies. I am sure the USAAF did the same. Blade width does help efficiency to a point.

Why decreased in max. level flght? Dose that suggest us the 3-broad blade prop was slightly outperformed by old 3-blade with narrow chord around 0.5 Mach? ? Because of the more drag of 3- broad blade especially when tip mach number approching 0.85 ?This is only level flight, just 0.5Mach/650km/h or so, how about 0.7Mach/850km/h? Could broad design shortcoming--more drag-- become more abvious?

I think you will find that 80 percent is actually very generous. Very few early war props are even close to this figure. The wide chord Schware Wooden Props found on late war LW fighters were just a tad over 80 percent.

while 4-blade NACA16 achived 90% in P47....

http://digital.library.unt.edu/ark:/...dc63942/m1/40/

In my opinon, there is the possibility of 4-blade NACA16 greatly outperformed 3-broad blade at high diving speed(0.7 Mach). To prove this ,we need more data while crummp tons of resource will play the key role.

I have all the data on VDM propeller series.
----Crummp