Why still no dive acceleration difference?

[Crumpp]

those supersonic propellers max tip Mach is more than 1.3 while WWII p47 max tip Mach is below 1.15. quite different story.

Come on man....

You understand the basic's of rotational mechanic's right?

On any radius of the circle, the point closest to the origin travels at a slower velocity than a point distal to the origin.

That is why as a generality, a smaller disc is better for Vmax performance.

[Crumpp]

I can't compare 2 propeller efficiency by using a single propeller curve.

I am not exactly sure what you are looking at.

CSP's are not compariable at different advance ratio.

[BlackBerry]

Originally Posted by Crumpp

Come on man....

You understand the basic's of rotational mechanic's right?

On any radius of the circle, the point closest to the origin travels at a slower velocity than a point distal to the origin.

That is why as a generality, a smaller disc is better for Vmax performance.

1.jpg
There are two shock wave areas in propeller.

One is near tip, the other is around root.

Ma----compound speed, =squareroot of (rotating speed^2+ TAS^2), Mach

Mak----critical shock wave stall speed for a certian airfoil, Mach

The propeller portion near root is usually thick and not very streamline, so Mak is quite low which means easily render shock wave. The tip portion, on the other hand, has a very high(near sonic)Ma, so shock wave inevitable although this portion is quite thin and streamline.

Afterall, there is a trade off upon propeller's diameter above Vmax, if you use a bigger one, the shock wave area near tip is quite big, bad thing. But you get a smaller advance ratio, that's a good thing.The art is to find a optimum point where whole propeller reaches maximum efficiency at a certain speed above Vmax.

Speed is an important concept in combat, just like altitude. Pilots know what's the best altitude for their aircraft, eg, P47D, are willing to fight fw190/bf109 above 6000m altitude. If fly a La7, the lower altitude, the better. Why altitude is so important? one reason is "engine output".So is speed.

If your opponent will lose 500HP at a certian speed between Vmax amd Vne due to lower propeller efficiency, you also wanna drag him to such high speed and beat him in an energy fight style.


The samller aircarft, the lower drag coefficent and smaller weight, thus easier reach high speed and better output/weight ratio. One couldn't have it both ways.Shouldn't those tiny soviet/German aircrafts pay the price during high speed dive?

[Crumpp]

There are two shock wave areas in propeller.

Right, as a function of angle of attack and not diameter....

You are getting into the weeds without keeping an eye on the big picture. Propeller designers are aware of of this and design accordingly. It is too easy to spot a bad propeller design very early on.

It is a general principle that smaller diameter is better for Vmax performance.

Keep the discussion to diameter effects.....

Take a lesson from Professor Von KlipTip.....

6. What is the importance
of propeller diameter?

Ideally, the propeller diameter should be
greater for efficient low airspeed operation
and smaller for high airspeeds.

http://www.google.com/url?sa=t&rct=j...KCOmcUBIgbeK8Q

[Crumpp]

Speed is an important concept in combat, just like altitude. Pilots know what's the best altitude for their aircraft, eg, P47D, are willing to fight fw190/bf109 above 6000m altitude. If fly a La7, the lower altitude, the better. Why altitude is so important? one reason is "engine output".So is speed.

All of this comes out in standard performance calculations. In this case, subsonic incompressible flow theory works very well at predicting subsonic propeller design behaviors.

For example, you don't have to add anything when crunching the numbers for a FW-190 regardless of the propeller.

If you plug in the data for a metal propeller, your drag is less which means less lift and your sustainable turn performance envelope is reduced.

If you plug the data for a wide chord wooden propeller, your drag increases resulting in more lift and your sustainable turn performance envelope increases!

It is all in the math!

But one fact is very clear: prop efficiency may lead to hundreds of horsepower difference above Vmax, so a simulation game must pay enough attention to detailed efficiency curve. otherwise, a big difference from history is inevitable.

What is the basis you will manipulate the curve? Propeller design is extremely complex and once again, it is too easy to spot a bad design very early on. Like first flight early, LOL.

This appears to be a license to manipulate aircraft behaviors based on intuiation and supposition.

Propeller design is just too complicated and easy to spot a bad design. There is a reason why a generic curve is acceptable!

[BlackBerry]

This is NACA16 airfoil 3-blade vs 4-blade efficiency compare at 0.4 Mach.

Obviously, 4-blade NACA16 outperforms 3-blade NACA16 WITHIN Vmax. There is 8% difference. What's the meaning of 8% efficieny for a 2000HP engine? 160HP!

What's the 18lbs spitfireXIV and 21lbs spitfire XIV Griffon 65 engine difference? （2220HP-2050）*85%=145HP!
What RAF did in order to achieve 21 lbs boost with spitfire XiV? Gear midification, 150 octane fuel, and so on.

4vs3.jpg

But there is only 2% difference between 3-blade and 4-blade RAF6/ClarkY.
1.jpg
2.jpg
3.jpg

So NACA16 shows its outstanding/distinct character WITHIN envelope/Vmax.

RAF6, ClarkY and Gottingen airfoils are all conventional and of WWI peroid when biplanes dominated the sky. NACA16 was developed after 1939, new airfoil. And NACA16's advantage is NOT directly outperforms conventional airfoil in 3 blade configuration, its benefit only available when you add the fourth blade. There are two benifit:

1)Within Vmax. With the 4th blade, naca16 get 8% more efficiency while RAF6/ClarkY/Gotingen get 0% even negative.

2)above Vmax, with the 4th blade, naca16 could maintain stable efficiency（drops slightly） when advance ratio reaches 3.0. Those conventional airfoils usually in 3-blade configuration, and a 3-blade propeller efficiency drops sharply when advance ratio=3.

[BlackBerry]

What are those picture telling us?

4-blade hamilton Standard 3155-6 outperforms 3-blade 6507A-2 above envelope.

CSP's are not compariable at different advance ratio.

OK, let's compare efficiency at SAME advance ratio. 4-blade Hamilton Standard 3155-6 could maintain 82% efficiency (in free stream)when advance ratio=3, that's a splendid achievement. How about 3-blade Hamilton Standard 6507A-2 at 3 advance ratio(J)?

V=J*(n*d)=3*23*4=276m/s=994km/h=0.81Mach

From the picture I posted, we can estimate 3-blade Hamilton get around 40% at 0.81 Mach.

We know fw190 dive limit is 466mph=750km/h IAS=900km/h at 3000 m altitude. Let's exam 3-blade vs 4-blade configuration at 800km/h TAS, only 666km/h IAS @3000m altitude, it's very safe for a fw190, quite smaller than Vne, isn't it?

800km/h = 0.66 Mach

For 4-blade Hamilton Standard 3155-6, no worries, efficiency around 85%, well done.
For 3-blade Hamilton Standard 6507-A2, 70-72%, not bad.
For fw190 3.3m diameter 3-blade propeller, advance ratio=2.78, let's assume it performance just like Hamilton Standard 6507-A2 at 2.78 advance ratio(0.75 Mach), we get 52% efficiency!

There are 30% efficiency difference between allied 4-blade propeller and German/soviet 3-blade, 30%, wow, that's 500-600HP engine output bleeding, serious problem if allied aircrafts drag them to 666km/h IAS=800km/h TAS@3000 m/10000ft altitude. We know even La7 could withstand 666km/h IAS, isn't it?

Don't forget German wide chord airfoil even worse than narrow old airfoil at Vmax.


Crummp, I think I've expressed my opinion clearly with my proof/data, my suggestion is to take away 500-600HP from German/soviet aircrafts above Vmax and within Vne. If you could provide the evidence that 3-blade propeller achieve 80% efficiency at 2.8 advance ratio, you'll win. Now it's your turn.

[Crumpp]

What are those picture telling us?

You need to learn to read those plots. The 4 bladed propeller cannot reach an advanced ratio of 2.78 for much of the Cp range.

The blades stall and it produces no thrust!!!

Read the report!!!