With 4-blade prop. please.:)

lol. yes, vector quantities.:grin:

http://en.wikipedia.org/wiki/Propeller_(aircraft)

A further consideration is the number and the shape of the blades used. Increasing the aspect ratio of the blades reduces drag but the amount of thrust produced depends on blade area, so using high-aspect blades can result in an excessive propeller diameter. A further balance is that using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform, limiting the local Mach number - a significant performance limit on propellers.

A propeller's performance suffers as the blade speed nears the transonic. As the relative air speed at any section of a propeller is a vector sum of the aircraft speed and the tangential speed due to rotation, a propeller blade tip will reach transonic speed well before the aircraft does. When the airflow over the tip of the blade reaches its critical speed, drag and torque resistance increase rapidly and shock waves form creating a sharp increase in noise. Aircraft with conventional propellers, therefore, do not usually fly faster than Mach 0.6. There have been propeller aircraft which attained up to the Mach 0.8 range, but the low propeller efficiency at this speed makes such applications rare.

There have been efforts to develop propellers for aircraft at high subsonic speeds.[4] The 'fix' is similar to that of transonic wing design. The maximum relative velocity is kept as low as possible by careful control of pitch to allow the blades to have large helix angles; thin blade sections are used and the blades are swept back in a scimitar shape (Scimitar propeller); a large number of blades are used to reduce work per blade and so circulation strength; contra-rotation is used. The propellers designed are more efficient than turbo-fans and their cruising speed (Mach 0.7–0.85) is suitable for airliners, but the noise generated is tremendous (see the Antonov An-70 and Tupolev Tu-95 for examples of such a design).


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We will find the proof of 4-blade vs 3-blade, sooner or later.;) When a/c diving, it often creats a sharp increase in noise, which means tip of the blade reaches its critical speed.

http://digital.library.unt.edu/ark:/...dc62616/m1/19/
P47D tested with a Hamilton Standard 6507A-2 3-blade airfoil(NACA-16)

Cp=P/(r*n^3*D^5)

r=air density

We can see when dive to low altitude @0.7 Mach, prop efficiency will be as low as 63%. Fw190A8's 3.3m propeller advance ratio is quite bigger than P47's, so its efficiency should be quite less than 63% if the VDM prop. shares the same airfoil with 6507A-2.

But don't forget Hamilton Standard 6507A-2 has a NACA-16 airfoil. see here

http://digital.library.unt.edu/ark:/...adc63942/m1/3/


Allied also tested Hamilton Standard 6507A-2 with both 3-blade and 4-blade configuration at 0.4 Mach, also here

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

http://digital.library.unt.edu/ark:/...dc63942/m1/43/


Fig 16 and Fig 18

Appearently, 4-blade NACA-16 airfoil shows around 5%-10%+ efficiency advantage over 3-blade cousin even at medium speed. That's one of the reasons p47D picked up 4-blade airscrew.

It's probably that when dive to high-speed fw190a8's 3-blade Gottingen prop. efficiency is much inferior than P47's 3-blade NACA-16 airfoil. But I know Crumpp will argue that Gottingen(WWI standard) outperforms NACA16 at high speed. I'll remind you that NACA-16 airfoil was widely used after WWII until 1970s when computer calculating method helped people designed better airfoils. To sum up, in high speed diving:

1)P47 has less advance ratio than fw190a8.
2)P47 has NACA-16 lower drag airfoil than fw190a8.
3)P47 has 4-blade prop rather than fw190a8's 3-blade.

In future, if someone finds the proof or calculates out that fw190A8 Gottingen 3-blade prop only has 30% efficiency in high speed diving while P47 has 75% with 4-blade NACA-16 airfoil, don't be surprised because that will perfectly explain why Fw190G was badly outdived by P47D at 65 degree in Italy 1943 summer.


How about 0.7 Mach comparation of 3-blade vs 4-blade efficiency? That's more interesting.:-P But I guess the truth will change il2's diving FM.

That is a poor assumption to make.

This is were your power loading effects the design. A two bladed propeller will not absorb 2000 hp very well.

A 3 Bladed propeller can absorb 2000 hp very well.

Yes, 3-blade is usually enough for 2000hp, but in some special envirenments, I doubt about it.

Crumpp, you have tons of information on a/c, try to find sth. interesting to make il2 diving FM perfect. I know you have all the data of VDM propellers. :)

Wow...interesting thread guys (but I did only read the actual page).

But I think you forgot something BckBr: the large bladed prop will fly easier in the airstream during the dive and will then have a tendency to raise the rpm much higher than a 4 bladed one.

More rpm -> pilot will have to reduce throttle during the dive in order to keep eng safe
More rpm -> more tip blade speed hence more drag

Transonic drag being far higher than low subsonic drag, low rpm is better either for your eng (max pow dive) and for your total drag coef.

But if you are comparing the Jug with the Fw, it 's far better to keep in mlind their difference in weight and the weight/power ratio.

With the latter, you'll understand easily that gravity did play a huge part during WWII in term of improvement of aircraft perf. Hence, a nose down Jug had far better "propulsive" power than a FW190 in the same configuration.

EDIT: oh... and let's not forget that the Jug had a metal prop when the 190 used ones made out of woods. The technology is quite different ( the latter being somewhat newer). Large blades might hve been something difficult to achieve with casted aluminium

TomcatViP, when fw190a8 flying at max. level speed, it uses 2700 rpm engine. Therefore if fw190A8 dive to a speed much more than its level speed, the engine rotation is definitly 2700rpm. Such as 2500 rpm in a high speed diving is stupid:loss efficiency a lot because of increase of advance ratio.

Let your CSP governer to maintain blade angle and rpm, don't bother thinking about it. And fw190a8's small prop's tip speed is inevitably around critical mach number in a dive, you can't avoid it.

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


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:

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

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.

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:

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?

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. :)