Endurance of the La-7 and its engine
In my preceding post I used the data presented by people smarter than I (and better librarians too) to present hard numbers on the endurance of the US heavy iron navy fighters. This was an easy exercise because of all the information made available.
Now I turn to the La-7 and its Ash-82FN engine, where we have a lot of information, but not all we need. We also saw some inconsistent information. I hope to convince you that the technique I used for the plentiful American data can tells us something about the Russian data.
The bottom line is that the La-7’s engine seems about as efficient as its American counterparts, possibly exceptional at very high output settings. That said, no matter what power settings you consider, An F6f or an F4u-1 running on internal fuel only should always fly for twice as much time, or more, as an La-7. It turns out that, possibly entirely by coincidence, most of the complicated issues we argued about in this thread simply can’t change the general conclusion. I wonder if that would be true if we looked at other light planes with small fuel tanks…
Some background on the La-7 and its engine
The 1944 edition of the LA-7 weighs in at 7164 lbs (3250 kg) gross. It has a maximum speed of 408 statute MPH (658 KMH), Operating range is 413 statute miles 665 km). See Gordon and Khazanov Soviet Combat Aircraft of the Second World War.
The Ash-82FN is a 2514 CU (41.2 liter) radial engine, just a little smaller than Wright’s 14-cylinder version of the cyclone. Its cylinders are derived from the cyclone, but with a shorter stroke. The engine could generate 1850 HP when run flat out on 100 octane fuel. Its rated power was about 1460 HP. Note that front line units typically had fuel of 90-95 octane fuel.
By the time the La-7 is being produced the engine benefits from direct injection and over-boosting (i.e. more than 1 atmosphere of pressure in the cylinders) at high and low super charger speeds.
Tilt’s November 4 post includes a trove of flight test information and other data for the La-7 as of the last 9 months of the war. It is from a translation of a Russian report on tests of several production models.
From that post we know for certain the fuel tanks is 460 liters or 122 US gallons. There’s a great deal of information on the combat maneuverability of the plane, which we usually never see. I hope someone has verified that the AH flight model tracks this pretty well.
Endurance for the La-7 compared to USN Heavy Iron
Tilt’s report spells it out so no calculations, other than converting to American measurements is necessary (see the attached table). Running the plane at what HTC calls military power (2400 RPM at 40 inches HG) provides a brief 32 minute ride at 16,500 feet, 35 minutes at 3,300 feet. Plug whatever fuel multiplier you want into this and its clear the La-7 has very short legs.
Compare the F6f with full internal fuel load running at a comparable engine setting (2550 RPM, 44 inches HG, 7k alt) and you have an estimated endurance of 68 minutes (see my previous post). Just shy of running these planes to the firewall, the F6f flies about twice as long as the La-7. The La-7 is burning 164-6 gallons an hour while the F6f is burning 34 percent more (220 gallons an hour).
Let’s compare the planes at a cruise setting. On full internal fuel, the La-7’s maximum range is 413 miles and maximum flying time is 1 hour 45 minutes at 3,300 feet. At the most economical settings tested, it burns 52-5 gallons and hour.
My analysis of the SAC data (see my previous post) shows the F6f can fly 3 hours and twenty minutes, burning about 75 gallons an hour, reaching a peak altitude of 15k. So the F6f can cruise for twice as long while burning about 42 percent more fuel an hour.
In other words, power settings do not affect the relative endurance of the F6f and the La-7. In either case, the F6f on internal fuel only should last twice as long as the La-7.
Note that my result is very conservative. I could have used the minimum fuel consumption numbers from F4U1DOA’s charts. In that case, the F6f is burning essentially the same amount of fuel per hour as the La-7 and it fly’s 5 hours or more on internal fuel. I don’t use those numbers because I don’t think there are very many pilots in Aces that would be comfortable holding these planes in the air with only 500 HP and a steady hand.
Specific Fuel Consumption of the Ash-82FN engine
One of the problems we had in the thread is that we did not have good information on the fuel economy of this engine. Thanks to Tilt’s treasure of data we now know all we need about the plane’s endurance, and a lot more about its performance as a fighter. Before we saw that data, we were trying to match up data on horsepower, fuel capacity, and endurance from disparate sources.
This is what AH tells us about the Shvetsov M-82FN:
Emergency Power- 48" @ 2400 RPM (5 minutes) [probably should be 2500 RPM]
Military Power- 41" @ 2400 RPM
Normal Power- 36" @ 2200 RPM
Max Cruise- 30" @ 2000 RPM
We can use these numbers to infer power settings in some of our other data. For example, its likely that near sea level the first setting is consistent with the 1850 HP number we’ve seen and the second setting is probably consistent with the engine’s rated horsepower of 1460, although Tilt has some numbers suggesting rated horsepower was 1650.
For example, at one point we had a calculation that at the engine’s rated horsepower (1460 HP at 2400 RPM and 40 inches HG) fuel consumption was 310-55 liters and hour. Could this be right? What is the implied Specific Fuel Consumption? 355 liters is 94 gallons and weighs 563 lbs (I am rounding). That implies and SFC of 0.39.
That is not an impossible number for a really good radial engine running on a lean fuel mixture. But there is no way this engine will generate its rated power in auto lean if no American radial can. For the very good P&W R2800, SFC is 0.8 at normal power and 0.9 to 1.0 at the engines rated power. Both settings require an auto rich fuel mixture.
Suppose the 310-55 number was kilograms per hour rather than liters. In that case the resulting SFC is 0.55 lbs/hp/hr, certainly reasonable for a radial engine on auto lean, but still too low for an auto rich setting, which this almost certainly has to be.
I wonder if this range of numbers is really a reference to range in kilometers.
Let’s use Tilt’s flight test data to compute a more plausible SFC for the Ash-82FN. At an altitude of 3,300 feet, 2400 RPM and 40 inches HG, the La-7 is burning 164 gallons an hour. These engine settings and the low altitude suggest we should be generating the engines rated power of 1460 HP. In that case, we have SFC = 0.67, which is remarkably good. If the engine was about as efficient as the P&W R2800, we should expect it to develop only 1,090 HP.
What is the Ash-82FN’s specific fuel consumption at a cruise setting? Using Tilt’s flight test data we see the engine settings are something like 1500 RPM at about 23 inches HG. We know the fuel consumption, but we don’t know the resulting horsepower. Suppose the engine is as efficient as a contemporary Double Wasp can possibly be (0.44). Then the resulting horsepower would be
HP = (Lbs/hr)*(1/SFC) = 331/0.44 = 752.
Suppose we use the trick from my analysis of the cruise settings of the F6f. At the cruise, the engine is generating about 60 percent of normal power, which worked out to be 50 percent of the engine’s rated power. Fifty percent of the Ash-82FN’s rated power is 730 HP. That would make the engine’s specific fuel consumption at a cruise setting something like 331/730 = 0.45, a very respectable but still plausible number.
Other modeling issues
An important question all game designers have to answer is whether they want to model aircraft based on data generated by prototypes or production models. For the US, UK, and Germany there would not be much difference. That is emphatically not the case for Russia, where production aircraft were always 20-40 MPH slower than prototypes and had significant problems with reliability.
Russian engine designers were able to get significant performance gains throughout the war, but for many engines, they did not get service lives up to anything like the American standard. The Ash-82FN engine was not even mass produced until the fall of 1943. At the time their service lives were about 100-150 hours. By the war, the American engines could go about 500 hours and typically longer before an overhaul. Even when the engines were fine, the propellers had flaws and the finish of production aircraft was typically poor.
In the case of the La-7, the same engine in La-5 broke down more often, in part because poor design of the air intake resulted in more dirt in the engine. Engine failures in the La-7 were very common occurrences.
None of these things are captured in games, and that is why many late war Japanese and Russian planes perform better than they typically did in the actual war. What's more, these planes never accounted for a significant share of the front line inventories of those countries.
Is it wrong for a game to ignore such things? Depends on the philosophy of the game. There’s lots of things we might not want a game to model.
-Blogs
