FM Question/New F4U's climb slower??

[hitech]

It is the prop that is making the difference. At minimal power settings the prop drag is providing so much drag, that any extrapolation about drag co's of the airframe is totally meaningless.

HiTech

[Widewing]

Originally posted by hitech
It is the prop that is making the difference. At minimal power settings the prop drag is providing so much drag, that any extrapolation about drag co's of the airframe is totally meaningless.

HiTech

Which prop is making the difference, that of the P-51 or the La-7?

I can't find much info on the prop used on the La-7, but looking at photos it certainly does not have what Hamilton Standard called "high activity" blades. The prop appears to be about 10.5 feet in diameter. I don't see any visual difference between the the prop on La-7s and La-5FNs. They look identical.

That said, the AH2 La-5FN bleeds speed considerably faster than the La-7 does. Granted, the La-7 incorporated some aerodynamic improvements to increase speed. But, the La-5FN was a tad heavier. As it is, the AH2 La-5FN bleeds off speed 7% faster than the La-7 does. Whereas the P-51D bleeds speed 3.7% faster than the La-7.

Note that the C.205 is a bit heavier than the La-7, and swings a prop of similar diameter and blade chord. Yet, it bleeds speed at a much greater rate than the La-7.

This is where I'm confused. If the P-51 has a much higher drag propeller (apparently high enough to more than offset a 33% difference in mass), that causes it to bleed speed faster than the La-7, then why is there a significant speed bleed difference between the La-7 and La-5FN? After all, they have nearly identical weight and the same propeller. Something isn't adding up for me.

Is there something that I'm missing?

My regards,

Widewing

[hitech]

Widewing: What is your point.

[Tilt]

Originally posted by Widewing
then why is there a significant speed bleed difference between the La-7 and La-5FN? After all, they have nearly identical weight and the same propeller.

They did not have the same propeller. The la7 had a new propeller. I will dig out details when I get home.

The key (and unusual) aspect of the La7 development was that there was no engine upgrade or additional thrust component (other than any efficiencies gained from a new propeller).

Apart from any benefits aquired from the new prop the La7 was purely an exercise in pure  drag reduction plus some saving thru weight  (induced drag) reduction.

Some theorise that the La7 had a greater applicable period of WEP 10 mins over the La5FN's 2 minutes. This (La5FN @ 2 mins) is not a categorical record however.

Edit

Having said all that I would have thought that in comparison with eg a Yak9U the La7 should lose speed more quickly in a high speed glide. I will check what  prop data I have for both.

[Squire]

What Tilt said: They took the LA-5FN and reduced its drag to create the LA-7. Same engine, but with aerodynamic refinements. They should not be the same E retention at the same weight.

Tilt is the resident LA expert here, no question.

[Saxman]

I think WW's point is that with all the other factors, how can THAT much difference (a full 6 seconds with the La-5 at full RPM, and the La-7 operating at REDUCED power!) between the La-5 and La-7 be solely a difference in the prop and some aerodynamic cleanup, if E retention is as much a product of mass (as in the F4U, which btw Viking you GREATLY underestimated. She weighs in at roughly 12,000lbs under combat loads, not quite but close to twice that of the P-51).

[hitech]

Saxman:

p51d 2,500 RPM: 45.22 seconds
La-7:2,400 RPM   39.41 seconds

And we are not talking about reduced power, all test are done at min throttle. What we are talking about is more prop drag do to higher RPM.

[Viking]

Originally posted by Saxman
… if E retention is as much a product of mass (as in the F4U, which btw Viking you GREATLY underestimated. She weighs in at roughly 12,000lbs under combat loads, not quite but close to twice that of the P-51).

While I may have somewhat underestimated the F4U's mass, it is nowhere near twice that of the P-51D which had a normal loaded weight of 9,500 lbs and a max. load of 12,000+ lbs.

[Tilt]

They key difference between the Vish 105 prop (la5fn) and the Vish 105-v4 La7

The V4 was a so called "constant speed" prop assembly with additional "anti flap" properties

Both were 3.1m in dia.

Both weighed 141 kg.

Whilst I realise the debate re prop drag has arisen to show that simple glide tests do or do not indicate the drag characturistics accurately...................

Once you have established there is an influence of prop drag then  I would have thought that rpm etc becomes meaningless as the angle, blade area and back geared engine resistance would vary from ac to ac. at any given rpm.

Take a 41.2 litre 14 cylinder engine with a compression ratio of 7:1 (back geared 1:1.45)and compare turning  it to a 30 litre V 12 with a compression ratio of 6:1 (back geared  1:2.1). We can all argue over which would be easier for the prop to turn but I doubt they would be the same.

I have noted that since the aerodynamics were universally modified that some ac do dive away from the La7 far more effectively than before. The P51 being one of them. It is a matter of record that a 109 G4 would eventually obtain a higher rate of acceleration in dive than an La7.


BTW WEP power as we know it on the Ash82FN was 2500 rpm (1200mm mg) being 100 rpm above the 2400 mil power setting (1020mm mg). The engine had a rating of upto 2600rpm but this was only ever used so fleetingly limited to a duration of 30 seconds presumably at lower altitudes.

[Widewing]

Originally posted by hitech
Widewing: What is your point.

I'm not trying to make a point, I was trying to understand why there are odd looking variations in E retention.

Here's some test data, in descending order in terms of slowest bleed to fastest. 350 mph to 150 mph, power at idle. Measured in seconds/Max Takeoff Weight

P-47N: 49.91/16,300
P-47D-40: 49.84/14,500
P-47D-11: 48.34/13,582
F4U-1A: 45.91/12,050
190A-8: 45.65/9,682
F6F-5: 43.97/12,300
F4U-4: 43.65/12,400
190A-5: 42.94/8,583
Ta 152: 42.00/11,500
La-7: 39.41/7,300
Ki-61: 38.73/7,650
P-51D: 37.96/12,100
P-51B: 36.72/11,200
190D-9: 36.46/9,840
La-5FN: 36.75/7,379
109K-4: 35.72/7,440
SpitXIV: 35.69/8,500
C.205: 35.61/7,498
N1K2-J: 35.53/9,040
P-38L: 33.41/17,500
109F-4: 32.06/6,393
FM-2: 31.31/7,431
SpitVIII: 30.91/7,875
Ki-84: 30.69/7,965
Tempest: 30.28/11,400
Typhoon: 30.21/11,400
SpitXVI: 29.82/8,500
P-40E: 29.48/8,400
Yak-9U: 27.78/7,050
SpitV: 27.15/6,785
A6M5: 24.46/5,920

My regards,

Widewing

[Widewing]

Originally posted by hitech
Saxman:

p51d 2,500 RPM: 45.22 seconds
La-7:2,400 RPM   39.41 seconds

And we are not talking about reduced power, all test are done at min throttle. What we are talking about is more prop drag do to higher RPM.

By the way Saxman, propeller RPM for these two would be:

P-51D: 1,293 RPM
La-7: 1,142 RPM

Add to this that the P-51D swings a 11'2" prop, while the La-7 swings a 10'2" prop. I believe that the P-51's prop generates more drag than that of the La-7 when windmilling at higher RPM. The unknown seems to be; is that drag so much that it overcomes the 3,000 pound weight differential? It does seem that in this particular case, it does (at least as its modeled) as testing at similar prop RPM gives the P-51 a very large advantage in E bleed.

My regards,

Widewing

[hitech]

I'm not trying to make a point, I was trying to understand why there are odd looking variations in E retention.

As I said in the beginning. It is do to prop drag, also understand that prop drag in AH is a close approximation, and isn't really a detail that we spend a lot of time on with each plane.

HiTech

[Viking]

Originally posted by hitech
... also understand that prop drag in AH is a close approximation, and isn't really a detail that we spend a lot of time on with each plane.

HiTech