Why still no dive acceleration difference?

[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!!!

[Crumpp]

my proof/data

Is not correct.

Let's looks at the report. The first thing that stands out as a glaring anomaly in the chart you posted is the fact a 4 bladed propellers appears to be more efficient than a 3 bladed propeller.

This violates a basic principle, sort of like all those people who want to claim their higher wing loaded aircraft can outturn a lower wing loaded airplane. Sounds nice but is not going to happen.

That principle is the fewer blades, the higher the efficiency.

The NACA is not claiming a 4 bladed propeller is more efficient. In fact, they quite notably point out several times in the report that none of the data is corrected for wind tunnel installation.

In English, it is not good for specific comparison and they plainly state that in the conclusions. They just hung the propellers and went with it to get an idea of the general trends.

The NACA conclusion are the ONLY thing that can drawn from this report.

You calculated for an advance ratio of 2.78. The 4 bladed propeller produces NO THRUST for most of the power loading conditions at J = 2.78.

When the polar line ends, the blade is stalled!!!

Your theory is not based on facts. It would be a fundamental error to toss aside convention of n = ~.85 for it.

[Crumpp]

This is not the same airfoil....

[BlackBerry]

Originally Posted by Crumpp

Is not correct.

Let's looks at the report. The first thing that stands out as a glaring anomaly in the chart you posted is the fact a 4 bladed propellers appears to be more efficient than a 3 bladed propeller.

This violates a basic principle, sort of like all those people who want to claim their higher wing loaded aircraft can outturn a lower wing loaded airplane. Sounds nice but is not going to happen.

That principle is the fewer blades, the higher the efficiency.

Will a 4-blade propeller outperforms 3-blade one?

It depends on many factors such as diameter, airfoil, revolution, chord width, blade thickness, TAS, an so on. You know propeller is very complicated.

But for Hamilton standard 6507A-2(~4meters, Naca16 airfoil), 4-blade configuration is better than 3-blade, this is a fact you should accept.

In fact, in late WWII, Rotel, the name is a contraction of "ROlls-Royce" and "BrisTOL", had introduced the first five-bladed propeller to see widespread use



http://en.wikipedia.org/wiki/Rotol

21lbs boost Griffon 65 engine of spitfireXIV is around 2200HP, with a five-blade , wood propeller.

The fastest Mustang----XP51G, with a 2200HP engine, with rotel five blade wood propeller.

The XP-51G was a development aircraft that combined the light weight airframe developed for the XP-51F with an experimental Rolls Royce RM-14SM engine, capable of producing 2,000hp at 20,000 feet. The new aircraft achieved a top speed of 495 mph, and a climb rate of 5,000 feet per minute, well over 1,000 feet per minute faster than the P-51D. However, the new Rolls Royce engine was too complex and did not always produce its best power.

1945 early, the 13lbs boost TempestMKV, 2700HP sabreiib engine, with rotel five blade wood propeller.

After WWII, people developed 6 and even 8 blade propeller.

The NACA is not claiming a 4 bladed propeller is more efficient. In fact, they quite notably point out several times in the report that none of the data is corrected for wind tunnel installation.

In English, it is not good for specific comparison and they plainly state that in the conclusions. They just hung the propellers and went with it to get an idea of the general trends.

The NACA conclusion are the ONLY thing that can drawn from this report.

IMG_0106.JPG
IMG_0107.JPG

3-blade vs 4-blade compare when developing YP47M. Do you mean these are just to get an idea of general trends?

You calculated for an advance ratio of 2.78. The 4 bladed propeller produces NO THRUST for most of the power loading conditions at J = 2.78.

When the polar line ends, the blade is stalled!!!

1350 rpm=23rps, 4 meter Hamilton standard 6507A-2, when advance ratio is 2.78, the TAS=2.78*23*4=256m/s=920km/h=571mph.

When P47 dive to such speed, no propeller thrust? How does il2 FM calculate propeller in this situation? Still 85% efficiency?

[BlackBerry]

Originally Posted by Crumpp

Your theory is not based on facts. It would be a fundamental error to toss aside convention of n = ~.85 for it.

The test air speed is below 0.4Mach or 0.2 mach . Of course, 0.4 Mach is just a cruise speed, at medium-low speed, 3-blade CSP could get constant propeller efficiency within envelop.

4-blade propeller efficiency drops from 90% to 85% while 3-blade remaining constant 81%.....


If you test propeller with slow air speed and small propeller rpm, a 4-blade propeller could maintain constant efficiency up to 3.6 advance ratio.

It is the high Mach number decrease propeller efficiency. No air compressibility, no significant effociency drop.

Btw, in late WWII, almost 100% allied aircrafts equipped with 4-blade propeller.

[BlackBerry]

Note that ClarkY 4,6 blade test was carried in a low speed wind tunnel, only 110mph, only 550rpm. So there is no air compressibility in the test, 4-blade propeller could maintain 80% efficiency up to 3.6 advance ratio.


I have no direct proof of 4-blade NACA16 efficiency at high Mach number. I just suspect that 3-blade 3.3m propeller lose efficiency much more than a 4-blade Naca16 around 0.65-0.75 Mach----above level speed envelope, within dive limit.

[BlackBerry]

The fastest Mustang----XP51G, with a 2200HP engine, with rotel five blade wood propeller.

The XP-51G was a development aircraft that combined the light weight airframe developed for the XP-51F with an experimental Rolls Royce RM-14SM engine, capable of producing 2,000hp at 20,000 feet. The new aircraft achieved a top speed of 495 mph, and a climb rate of 5,000 feet per minute, well over 1,000 feet per minute faster than the P-51D. However, the new Rolls Royce engine was too complex and did not always produce its best power.

Flight Tests on the North American
P-51D Airplane, AAF No. 44-15342

Summary
Clean Configuration (with bomb racks).

1. Maximum speed at critical altitudes

High Blower

War Emergency power (3000 RPM and 67") 26000' 442 MPH
Military power (3000 RPM and 61") 28000' 439 MPH
Normal Rated power (2700 RPM and 46") 29400' 420 MPH

XP-51G max level speed=495 mph, P-51D=442 mph.

If everything being equal, XP-51G need (495/442)^2= 125% engine thrust of P51D. Obviously, 2200HP Rolls Royce RM-14SM is around 125% output of 3000 RPM and 67" stanard Merlin(1760HP at altitude).

Roughly, we can say Britain Rotel 5-balde wood propeller is as effective as those 3-4 blade CSP at speed envelop. Namely, around 80% efficiency at 495mph=800km/h TAS.

It seems that allied believed in wwii that 4 or 5 blade propellers are better than 3-blade when speed is high(>700km/h?).

[Crumpp]

Do you mean these are just to get an idea of general trends?

Yes

It seems that allied believed in wwii that 4 or 5 blade propellers are better than 3-blade when speed is high(>700km/h?).

Power absorbtion is what they were going for....

3000 hp engines where the next step on the horizon.

Large amounts of effort for little gain in a 3000 hp piston engine aircraft.

Jets eclipsed any further piston engined development.