1950: Lavochkin La-9 v.s Grumman F8F-1B Bearcat

[Shortround6]

Vladimir Kotelnikov's book " Russian Piston Aero Engines" makes no mention of a a two stage ASh-82 of any kind except a turbo charged model called the M-82-NV(ASH-82NV) which was test flown but not put into production.

He is just a single source and may be in error but since he lists 11 different models of the M-82/ASh-82 engine including Czechoslovakian and Chinese versions I would say that the chances of any model ASh-82 having a two stage supercharger are exceedingly small.

 

[drgondog]

I am still wondering why a plane with the power-to-weight of the La-9 doesn't climb at least 1,000 feet per minute better than it does.

I don't Know but the primary factors for ROC is excess Power = Power available versus Power Required divided by Weight. where Thrust available times Velocity - Drag times Velocity = Excess Power.

In climb, with a tangible angle of attack, one of the Drag components "Profile Drag" is significantly higher - in the order of 25-35% over zero lift Profile Drag. Also max ROC in a steady climb (versus zoom) is at a low speed in the range of 160 +/- mph or square in the middle of highest Induced Drag.

The Wing and Wing Body, including esoteric considerations of mid wing versus high/low wing are considered but the La 9 and F8F are both low wing. The tapers seem similar so Induced drag due to planform are nearly the same - I don't know about wing twist but would suspect that the la 9 didn't have near the low speed roll authority requirements as the F8F so would tend to give the bod to the La 9 given equal Aspect Ratio and Oswald efficiency.

Net - given the W and the power of La 9 engine, there is something about the Wing high angle of attack profile drag that is suspiciously high if the stated ROC is so relatively low. The La 9 as presented is about the same ROC of a fully loaded (internally) P-51D at 67", which for that gross weight says that either the Laminar flow wing is awful in higher AoA Profile drag(doubtful), the actual power delivered is much less than spec'd or the wing area is grossly overstated (and subsequent W/L).

I agree with you Greg that it is a head scratcher. The Sovs did not like fighters with high wing loading/low rate of turn attributes for the designed mission strike zone

 

[Shortround6]

I have always been a bit curious as to the effect the fuselage may (or may not) have on the overall lift of the wing. This is in regards to the size (area of wing) that the fuselage takes out (occupies). Many formulas or explanations disregard the size of the fuselage or regard it as a constant (or something.) Some say the fuselage acts a bit like an endplate or something (sorry, it's been a few years since I looked at any of this and I may not be using the right terms).

As an example of what i am trying to get at you have the P-39 with 213.2 sq ft of gross wing area and because of it's skinny fuselage it has a net wing area of 197.7 sq ft. or 92.7%
The P-40 had 236 sq ft gross wing area and 217.6 net wing area for 92.2% so if you are comparing those two planes it doesn't make any difference if you are comparing gross or net.
However something like the Brewster Buffalo had 208.9 sq ft gross and 178.1 sq ft net for a difference of 85.2 %. The F4F had 260sq ft gross and 222 sq ft net (?) for 85%.
A P-47 has 300Sq ft gross and about 259.5 Sq ft net ( Wing root cord X max fuselage width taken out) for 86.5%.

Granted the LA-9 started as a V-12 powered fuselage and may be skinnier than most radial engine planes but it also starts out with a wing that is only 189sq ft.

It seems most fighters are going to be between 85% and 92-93% unless their wing area or fuselage size falls way out on one end of the spectrum so maybe it doesn't make much difference.

Perhaps it does help explain why certain fighters had trouble as weight increased compared to others?

 

[GregP]

I have seen one viseo clip of the La-9, probably Ray Hanna but I'm not sure, doing an airshow and it looked pretty much like an F8F Bearcat show that I have seen recently as many as 8 or 10 times. Even the loops were big and round emphasizing the vertical climb.

I'm starting to wonder if the stated climb rate is anywhere close to being accurate or if the data came from Soviet disinformation.

However, I don't have any resources from the former Soviet Union except for a couple of books on jets ... no La-9 anywhere. The rest of my resources are authored in the west and the writeups on the La-9 pretty much parrot one another.

I think that I'll have to bow out of further speculation based on no data to the contrary.

I am under the strong impression that the taper on the La-9 is much greater than on the F8f but, other than that, the La-9 is somewhat sparsely covered in the part of my library that I have unpacked. A lot of it is still in boxes including a few aerodynamic texts, but I cannot recall any picked up books that detail the La-9 any better than what I have.

So I'll defer to the people who have some data here.

 

[tomo pauk]

A quick look at power curves will further reinforce the information that ASh-82FN was sharing the single stage supercharger with earlier engines of the same family - above ~5,5 km, the power was in the ballpark with M-82 and M-82F (or ASh-82 and ASh-82F, so called from April 1st 1944 on). The -82FN received direct fuel injection, and greater (but still, when compared with Western engines, moderate) over-boosting was allowed under the rated heights.
At 5.4 km, all the engines of the family were doing around 950 mm Hg of boost, or 37.4 in Hg, or 1.28 ata, the -82FN was pulling 1000 mm Hg (~39.4 mm Hg, ~1.36 ata) at 4.65 km (~15260 ft) - or about the same manifold pressure like the V-1710 with 8.8:1 S/C gearing. The ancient Merlin III (1-stage S/C) was making +6.25 psig at 16250 ft, or 42.4 in Hg, or ~1078 mm Hg, a notable advantage in supercharging over M-82 family of engines.

 

[GregP]

Hi Tomo,

I'm wondering if the moderate overboosting of the ASh-82 was mainly due to fuel quality in the field rather than any inherent engine characteristics. I am given to understand that Soviet fuel was a bit low on performance number rating and could mostly be covered by octane ratings, but that may not be the case from 1943 onwards. Again, somewhat a dearth of hard data. Some of what I think comes from Soviet-written, English language-translated combat reports ... but it is entirely possible they had decent fuel from sometime in 1943 onward.

 

[tomo pauk]

The 'new' Soviet engines (AM 35A, AM 38, M-105, M-82 etc) were specified to use fuel of 'no less than 95 oct number' - while a bit better than 87 oct, that's barely better than Japanese 92 oct or what USA used on many engines (91 oct fuel, not for front line combat aircraft of course). So we won't see boost levels of what the fuel with 100 oct was offering (without the use of ADI), let alone he fuel with rich response at 130 grade.
The compression ratio of M-82 family of engines, 7:1, also does not allow for that great a boost. The Merlin was at 6:1, for example; plus the radials were always a bit lacking wit allowable boost. When Mikulin's team ddesigned the AM 42, they decreases the compression ratio down to 5.5:1, that allowed for manifold pressure of 1565 mm Hg, or: 61.6 in Hg, circa 2.2 ata, +16 psig - while still on 95 oct fuel, no ADI, no intercoler. They didn't made an own goal with increasing compression ratios for new versions of engines when 96-100 oct fuel was to be used, as DB and BMW did.

 

[grampi]

We must remember that acceleration is not only a function of power-to-weight, this only true at zero airspeed and max power, rather it is a function of excess power-to-weight. If an aircraft is using all of its power to maintain a given airspeed, it will not out accelerate a lesser powered aircraft that is not using all of its power to maintain that airspeed. This is one of the advantages of the P-51 had since it almost always used less power to maintain a given airspeed than other WW2 prop fighters. Now according to chart of spitfire performance the F8F-2 (the charts for the F8F-1B seem unbelievable showing combat power at 2750 hp at SL) generating 2500 hp at SL with a max speed of 387 mph, whereas the Russian site (believable ?) shows the La-9 generating 1850 hp at SL with a max speed of 397 mph. If these numbers are true, the La-9 is much cleaner than the F8F and therefore will need less power to operate at any given airspeed. At SL and 387 mph, the La-9 will out accelerate the F8F because it has the excess power to do so. At other envelop points, calculations would have to be run to see which aircraft would out accelerate the other, but it is possible for the La-9 to also out accelerate the F8F at other envelop points.

One thing that makes me question the notion that the La9 will outaccelerate the F8F is the difference in climb rate...the F8F climbs at more than 1000 fpm faster than the La9...granted, lift plays into this, and the F8F has a higher lift wing than the La9, but does that wing account for all of the difference in climb rates? I would be shocked if that were the case, which leads me to believe the F8F simply makes more efficient use of it's power than does the La9, which also leads me to believe that F8F would accelerate better than the La9...you made a good point with the P-51, which has a similar laminar flow wing to that of La9, and the "D" model has a climb rate that's almost identical to that of the La9, and I highly doubt a P-51 will outaccelerate a Bearcat...

 

[Shortround6]

I think that is a major problem for the Russian engines. The temperature rise in the supercharger is much more than many people realize. For instance there is a chart in one old book that gives the temperature rise for an auxiliary-stage supercharger (think P-38 or P-47) for air compressed from standard altitude condition to 29.92 in. Hg. abs. (sea level pressure delivered to main supercharger or carb) and assuming the a supercharger temperature coefficient of .65.

Going from the chart and using degrees Fahrenheit.

5000=just under 50 degrees.
10,000ft= about 80 degrees
15,000ft=about 130 degrees
20,000ft=about 175 degrees
25,000ft=about 220 degrees
30,000ft= about 260-270degrees.

Another chart shows the HP required by a hypothetical infinitely variable supercharger providing sea level air pressure to a main supercharger and shows both the out side air temperature and the temperature of the air delivered to the main supercharger. At 35,000ft or so the out side air has stopped getting colder (this is a wartime book) and is stabilizing at minus 67 Degrees F. temperature at the inlet of the main supercharger is around 240 degrees. Supercharger needs a bit over 200hp.

At 20,000ft the temp is about minus 12 degrees F, the inlet temp is over 150 degrees and power needed is just over 100hp.

These temperatures are for a 'standard' 59 degree F day at sea level. adding 30 degrees at sea level just adds adds 30 degrees to all temperatures until you get very high.

If you are trying for higher than sea level pressure, say an ATA of 1.3 no matter how you measure it, the power required and the temperature will both be quite a bit higher. The higher temperature may be the reason the Russians didn't allow over boosting in high gear. The detonation limit was a combination of the pressure, the cylinder compression ratio, and the intake air temperature (or more properly, the temperature of air or mixture in the intake manifold/s) with a few other things thrown in.

A plane at 15,000ft is trying to compress the outside air about 2.3 times in order to get 1.3 times the standard 29.92 in HG pressure. It is this extra work of compression that can drive the temperature of the intake charge up so much. Please remember that the superchargers were only about 65-75% efficient so 25-35% of the power going to the supercharger went directly to heating the air and not compressing it. The cooler air at moderate altitudes (say 8-20,000ft) could in no way compensate for the higher temps caused by the superchargers.

 

[Shortround6]

Something on the acceleration and climb we may be missing is the propeller. Bearcat uses a 12' 7" diameter propeller. It has just over 122 sq ft of prop disc area and is moving a lot of air.

I have no figures for a LA-9 But a LA-5 or LA-7 prop was 3.1 meters in diameter (10.23 ft?) according to one website (open to correction) and that gives an area of about 82 sq ft. or roughly 2/3s that of the Bearcat.

The Mustang had 97.7 sq ft of disc area. We might argue over how much the different engines/cowls masked the center of the prop disc but it seems that when it comes to moving a lot of air at low speed the big prop on the Bearcat may have an advantage.
The big prop may have a disadvantage or two at other speeds or altitudes (and is probably heavier (much) than the prop used on the LA series of fighters.

 

[GregP]

You know, although I look at a Bearcat every time I go to the museum and notice the gear legs that break at the top to fold VERY strangely for ground clearance, I totally space-cased the prop disc area.

Thanks, Shortround! It goes a long way toward some explanation that at least makes sense.

 

[grampi]

You know, although I look at a Bearcat every time I go to the museum and notice the gear legs that break at the top to fold VERY strangely for ground clearance, I totally space-cased the prop disc area.

Thanks, Shortround! It goes a long way toward some explanation that at least makes sense.

I think the top of the Bearcat's gear is called a trunion (spelling?) It was a very ingenious design to keep the gear length as short as possible, while providing enough ground clearance for that huge prop...

 

[drgondog]

One thing that makes me question the notion that the La9 will outaccelerate the F8F is the difference in climb rate...the F8F climbs at more than 1000 fpm faster than the La9...granted, lift plays into this, and the F8F has a higher lift wing than the La9, but does that wing account for all of the difference in climb rates? I would be shocked if that were the case, which leads me to believe the F8F simply makes more efficient use of it's power than does the La9, which also leads me to believe that F8F would accelerate better than the La9...you made a good point with the P-51, which has a similar laminar flow wing to that of La9, and the "D" model has a climb rate that's almost identical to that of the La9, and I highly doubt a P-51 will outaccelerate a Bearcat...

a P-51H with fully operating 1650-9 at WEP/WI 90" probably will out accelerate an F8, at least at certain initial high airspeed envelopes - maybe even a B/D at say 380mph and beyond. The F8F has a really fat wing at 18% which brings higher profile drag than the 15% NAA/NACA 45-100.

The F8F also has a higher CLmax but that is irrelevant to the climb discussion because max climg angle of attack is nowhere near max CL. Excess Power divided by Weight as noted above is the equation for ROCmax.

I just looked at Green's data for the LA-9 which I find hugely unbelievable regarding top speeds, particularly "428mph at SL and 405 at 11480 feet".

Green states that the climb to 16,400 feet is 4.2 minutes and max ROC = 3,840 fpm which I do find believable.

Looking at the plan views for the La-7 and -9, the -9 has a greater span, less forward sweep of the trailing edge, greater sweep of the leading edge and what appears to be a wing taper ratio of ~ .25 to clipped wing tip. All high performance fighters tended to be designed to a .25 to .6 ratio to compromise tip stall characteristics (small taper ----> lower section CL at same AR for the tip region but results in less weight of the wing due to lower bending moments - all else being equal, larger taper ---- higher section CL and better tip control at high AoA but drives to a heavier wing).

The greater taper ratio moves the lift distribution inboard while the small tip ratio the max CL trends near the tip and also results in more induced drag.

P-51 Taper Ratio = .46
F4U TR = .75
F6F TR = .52

The Mustang and F6F are right at minimum planform Delta Induced drag for non Elliptical wings.

I haven't looked at F8F dimensions but look by inspection like 51/F6f

 

[davparlr]

One thing that makes me question the notion that the La9 will outaccelerate the F8F is the difference in climb rate...the F8F climbs at more than 1000 fpm faster than the La9...granted, lift plays into this, and the F8F has a higher lift wing than the La9, but does that wing account for all of the difference in climb rates? I would be shocked if that were the case, which leads me to believe the F8F simply makes more efficient use of it's power than does the La9, which also leads me to believe that F8F would accelerate better than the La9...you made a good point with the P-51, which has a similar laminar flow wing to that of La9, and the "D" model has a climb rate that's almost identical to that of the La9, and I highly doubt a P-51 will outaccelerate a Bearcat...

Generally speaking, speed and hp required to maintain speed at SL is a good point of reference for comparison (also, these values are often available) of the overall aerodynamic efficiency of an airframe. However, this may not provide a reference for other performance criteria such as climb or airspeed at altitude. For example, as mentioned by SR, the propellers of the F8F and the La-9 are quite different. The F8F has a huge four bladed prop whereas the La-9 is a smaller three bladed and more slim. I don't know much about props but the F8F propellers may be great for generating the power for climbing but may not be best for top speed. The La-9 propeller may be just oposite.

 

[drgondog]

Bigger disk area at same RPM will move greater mass but isn't the R2800 running at 2800rpm and 2:1 for 1400rpm - what is the actual prop rotation for the La 9 engine?

 

[tomo pauk]

Engine max rpm was 2500 (ASh-82FN, only for take off), the reduction ratios were 11:16, 9:16 and 0.56. That gives prop RPM of 1718.75, 1406.25 and 1400, respectively. The fighter (smaller prop) should use higher rotation than a bomber (bigger prop).

 

[Shortround6]

Props are usually a compromise of some sort. Many books on powerplants published during the 30s, 40s and 50s often had one or two chapters on propellers but also noted that just covered the basics. A real study of propeller design needed a book (or more than one) instead of a few chapters and pretty thick book at that. And even at that Prop design was almost as much art as it was science. A small 3 blade prop could have less drag (need less power to turn) than a larger 4 blade prop and at low altitude and high speed actually "perform" better. The low altitude air is dense and the engine/prop doesn't have to move as much of it (volume) at sea level as it does at 20,000ft. At high speed the velocity of the prop stream is a better match to the aircraft speed for turning thrust into power/speed.
An old text book has a rather simplified chart for fixed pitch props showing how a lot of factors inter-relate. The Chart has power across the bottom. Diameter of 2 blade prop going up the side and five curves for different propeller speeds. The other constants were speed of 200mph and an altitude of sea level.

Of interest are the list of corrections below the chart.
The right sized propeller for 100mph is 33% bigger in diameter than a prop for 200mph using the same power and revolutions. A prop for 300mph can be 13% smaller in diameter and on for 400mph can be 19% smaller than than for 200mph.
The right sized prop for 10,000ft is 7% bigger in diameter than one for sea level. For 20,000ft it needs to be 15% bigger and for 30,000ft 22% bigger.
Going to 3 blades instead of 2 allows the prop to be 8% smaller in diameter and using 4 bales allows the prop to be 14% smaller than a 2 blade prop.

If you goal is high speed at low altitude you can get away with a fairly small propeller. What that does to your climb rate I am not sure. But since climb speeds were usually around 200mph or less the small prop is at a disadvantage for climbing.

I will note again this is a rather primitive chart and not only will using variable pitch or constant speed props change things (or smooth some things out) but it also assumes a constant airfoil and blade shape for it's "recommendations". Comparing real props with different airfoils and different shapes can really introduce variables (P-47s with paddle blade props? same diameter, same number of blades, not much difference in speed, big difference in climb).

 

[Zipper730]

These are not linear units of equivalence, they are national standard units of manifold atmospheric pressure.

And these are the exact conversions the nations used at the time (I downloaded the attachment)? For example 14.696 was used for the measurement and not 14.69595 PSI, and so on...

I'm compiling a bit of a chart of my own and it's important to have accurate information.

I can post an "extended" chart if anyone is interested, that covers idle to Reno racing levels.

Sure...

The Russians typically used mm H2O, not mm HG

And this was gauge pressure?

 

[Admiral Beez]

I suppose it had rubbish range, but the Lavochkin La-9 would have looked great on an aircraft carrier.

 

[Clayton Magnet]

One poster mentioned the Griffon Spitfire v the P-51. I believe the appropriate comparison would be between the P-51H and the Spitfire and I suspect the P51 might come out on top.

Why would the P-51H be a better comparison? Griffon Spitfires were flying operationally in late 1942, about a year after the first ALLISON Mustangs showed up. The Mk.XIV arrived about the same time as the B/C model Mustangs, late 1943, early 1944. A more appropriate comparison for the P-51H would be the Spiteful, or post war Spitfires like the Mk.22/24