Spitfire MK.XIV and La-7

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Constant speed props are only constant speed at a given engine RPM. (As I'm sure you know) The idea is you increase the pitch of the prop to achieve more thrust, and increase the air/fuel flow to the engine to maintain the same rpm. My point was you could not reduce rpm on the merlin/griffon at high altitudes and still get strong performance from the engine/prop on a large paddle type prop, because the torque curve falls off more sharply below peak power.

Increasing the reduction gear ratio would indeed slow the prop relative to the rpm, but doing so would sacrifice low alititude performance in favor of high altitude performance. Since it was desirable on the Spit XIV to have good performance at all altitudes, it was desireable to have one prop RPM that would work well at both - thus the smaller radius prop with more blades.

I never said it the 4 bladed prop was the absolute best, just that it was the more generally used solution. On R-2800 powered planes, they could back off the rpm a little bit without much loss of torque. The Germans chose to use a 3 bladed prop with very fat paddles for their Ta-151H.

The truth is there were many tradoffs to be considered, and very little was known about high altitude performance. The 5 bladed prop was a good solution for the Spitfire XIV, give all the considerations - especially the lack of ground clearance for a larger radius prop. It is very clear the British were not entirely satisfied with this solution, they kept experimenting with different Spitfire prop setups till well after the end of the war.

My only point in this thread is that 5 blades are not necessarily better than 4, or even 3 blades.

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Lunatic
 
I simply don´t buy your theory of torque! I don´t know about Griffon, but max boost was allowable with the Merlin down to 2850 rpm (normal max rpm 3000). I am aware that in e.g. the P-61 pilot manual it is mentioned that speed will be better at very high altitude if the rpm is reduced from 2700 to 2600 (B-series engine), but this simply due to tip speed problems. Since most Griffon aircraft had smaller diameter props running at speed not higher than those of R-2800 powered aircraft, I don´t it was any problem with them. At least it is not reported in books like Spitfire the History that has huge amount of such data.

As for 5-bladers, the Sea Fury has one. In my opinion the best answer would have been contra-rotating props. Too many sources tend to claim that the performance increase was not worth extra complication. In my opinion, that is not the point. Consider the F4U, numerous aircraft and pilots were lost because of the torque roll at slow speed. With contra-props, this would be history. Also directional trim changes would a thing of the past. E.g. the British report on the Spitfire fitted with a contra prop mentions that it was possible to to fly from take-off the landing without touching the rudder bar at all. A most valuable feature, imho.
 

But that's exactly the point. The smaller radius 5 bladed prop of the Spitfire did not need to reduce rpm. My point is that a 15% reduction in rpm on the Griffon results in a larger loss of torque than on the R-2800, so it is desireable not to have to reduce the RPM. On the R-2800, you can reduce the RPM and increase the prop pitch and sustain higher thrust without bogging down the engine resulting in dropping RPM. Remember at peak power, horsepower and torque are synonomous. Below peak power however, they are not. More cubes means a better off-peak power torque/hp curve. That's why the constant speed props were so valuable in the first place - because the engine could be held at its peak power where the torque and hp curves meet.


On this we are in total agreement. Contra-rotating props would have been far superior to adding more blades or larger paddle blades. They were however somewhat complicated and the kinks still had to be worked out of how to design the props so the front prop would not interfere with the back prop. These issues just didn't get resolved in time for WWII.

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Lunatic
 
Are you sure? Cubic capacity has no effect here as such. E.g. an engine producing 2000 hp from 36 litres at 2700 rpm produces exactly the same torque as one producing 2000 hp from 46 litres at 2700 rpm. Remember, torque is directly proportional to power but inversely proportional to rpm assuming constant MAP operation. I have examined dozens of aero engine handbooks and in none of them a torque curve has been presented.
 

Aero engines handbooks almost never go into any kind of torque curve info, since the power is always considered at peak power RPM.

I agree the 46 liter engine and the 36 liter 2000 HP 2700 rpm engines bouth produce the same torque at 2700 rpm. The torque curve issue comes in to play when you back off from the peak. The 46 liter engine will produce more torque at 2000 rpm than the 36 liter engine.

My experiance with this comes mostly from cars. Believe me, a chevy 327 can be made to put out the same 400-425 HP that the 396 or 427 could (though they could be pushed even futher) fairly easily. But the torque curves were entirely different. The 396 might make 400 HP at 6000 rpm, the 327 at 7500 rpm. But the 396 would make a lot more torque at 3000 rpm than the 327 did at 3750 rpm. The smaller displacement engines are more sensitive to the torque/hp curves.

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Lunatic
 
RG, your logic is utterly flawed here. You are comparing unspercharged car engines to supercharged engines which is way off the base. First, torque and power curves for can engines are done with full throttle, i.e. at constant MAP. In aircraft engines this is strictly no-no. They won´t tolerate full boost for much below max rpm. Therefore, with a supercharged aero engine the torque curve allways starts reducing with rpm as the MAP and therefore IMEP is lowered accordingly. I.e. the curve is completely different is shape in comparison with that of a unsupercharged car engine.

And it is also a fact that bigger engine has no greater torque rise if all the other design parameters are the same. And to add to the insult, a two litre turbocharged engine giving 200 hp has greater torque and torque rise than a 4 litre unsupercharged engine giving 200 hp.

Thse are the facts.
 

The curve is the same for supercharged engines. As long as you are comparing like to like there is no difference. Also, I'd love to see your source for a turbocharged 2L engine of giving greater torque than a normally aspirated 4L engine given the same peak HP.

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Lunatic
 
It is easy. Just go to some car maker site that has models with and without supercharging. Then calculate power/torque figures for both. You will see that the smaller turbocharged engines develop more max torque per hp (max hp).
 
pasoleati said:
It is easy. Just go to some car maker site that has models with and without supercharging. Then calculate power/torque figures for both. You will see that the smaller turbocharged engines develop more max torque per hp (max hp).
Click to expand...

That is not the torque curve. The torque curve show torque vs. rpm at different rpm. And in this case we are really concerned with "unused torque", which represents the ability of the engine to rev up to higher rpm quickly, or to sustain rpm under a higher load.

Anyway, what would that matter, both aircraft engines under consideration are supercharged.

=S=

Lunatic
 
Well, get a copy of Torque Meter issue 1, volume 3 and check page 26 (TM is the house journal of the "Aircraft Engine Historical Society"). On that page you can find a torque curve for the Allison V-1710-81A. The data is from "the demo runs Bud Wheeler did at Oshkosh and are probably the first set of performance curves on an Allison since the 1940s" according to Daniel Whitney, the leading authority on the Allison. This torque vs. rpm curve clearly shows a steadily DECREASING torque with decreasing RPM. The shape is completely different from your beloved V-8s. I don´t think there can be a better rebuttal of those silly torque claims of yours!
 
Why is that?

Normally the torque curve climbs early in the rpm band and then starts down, the HP curve climbs more slowly and continues to climb with rpm until it crosses the torque curve (and usually it does not turn down within engine rpm limits).

I never said torque would increase with RPM, only that more cubes equals more HP. Used torque always increases with RPM, but available torque (the ability to accelerate rpm) does not.

=S=

Lunatic
 
The thing though is.. why decrease RPM at all? If you want to use the paddle blader (and have the ground clearance) at a lower RPM, just use a lower gear ratio on the reduction gear. The torque below operating RPM doesn't matter, since you don't need to run below operating RPM anyway.
 
RG i dont understand how you can say that the La-7 is more aerodynamic, as it is a Radial engined fighter !

Sure the intakes under the Spit's wings induce drag, but even more so does the big flat front nose of the La-7's radial engine. On the other hand the Spit Mk.XIV's nose section is very areodynamic !

The Spitfire Mk.XIV is actually a very areodinamicly clean aircraft:


About the five bladed propellar, well it was a big advantage at High alt, and it was also less noisy.

The Spit Mk.XIV is superior to the La-7, no doubt, even at low alt. The Spit XIV is faster, climbs better at all altitudes, it most likely turns better, and it has equal roll rate.

At sea-level the Spit XIV could climb with 4,800-5,200 ft/min depending on the type, something the La-7 couldnt hope to match. At best the La-7 would climb with 3,608 ft/min.

The Spit XIV was also noticably faster with 721 km/h vs the La-7's 680 km/h.

I know wich plane i would want to be sitting in, in a clash between these two.... The Spit Mk.XIV without doubt !
 
Soren,

Yes the Spit 14 running on 150 octane fuel was about 10-12 mph faster than the La7-FN running on 95 octane fuel at about 6500 feet. It climbed a little better too.

We have few figures for the -FN(V) version, but know it could sustain WEP power for 10 minutes, as opposed to 2 minutes for the -FN version. Reliable speed data is unavailable, but we know the standard (normal power) sustained speed was 660 kph (410 mph) at 6000 meters, 40 kph higher than that of the -FN versions which was 620 kph (385 mph) at that altitude. The maximum speeds you are using are for the La7-FN version, which was 655 kph (407 mph) at 6100 meters. Obviously the -FN(V) version would be faster, probably about 430 mph at 6100 meters. The La7-FN(V) was probably as fast as the Spitfire below 18000 feet, and faster below 12,000 feet.

As for climbs, the La7-FN(V) climbed from wheels up to 5250 feet (1600 meters) in 1 minute, an RoC of.... 5250 fpm. It climbed to 9843 feet (3000 meters) in 2.2 minutes, an RoC of 4474 fpm. It climbed to 13123 feet (4000) meters in 3.2 minutes, an RoC of 4100 fpm. It climbed to 16404 feet (5000 meters) in 4.3 minutes, a ROC of 3815 fps. So, at least below about about 16000 feet, the La7-FN(V) climbed every bit as well as the Spitfire - and it could sustain this for 10 minutes, as opposed to the 5 minutes of the Spitfire. Also, it was allowed 30 second boosts of 2600 rpm performance, giving even better climb and speed.

As for manuver, the La7 was a very manuverable plane. It was considered much more manuverable than both the 190 and the 109. Roll rate was very good at all speeds up to about 400 mph. Turn rates were also excellent and energy retention in a turn was quite good. The La7 could exceute a 225 mph 360 degree turn at 1000 meters in under 20 seconds in either direction with no loss of speed.

The La-7 had one of the most streamline cowls of any radial engined fighter of WWII. It was more streamlined than any of the FW/TA designs, and comperable to the P-47J and Tempest II. It certainly had a little more nose drag than the Spitfire, but it had no wingscoop drag.

The La-7 also had leading edge slats, I'd think you'd be in love with it Soren!

=S=

Lunatic
 

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Yeah, a lot of people are fond of talking about radiator drag, but one has to remember that the inlet for cooling air for the radial engine produces at least as much drag, maybe even more. The Spitfire used the Meredith effect to recover some of the energy lost to the drag. and I don't see any evidence that the Lavochkin does so.

Note that for radial engined racers, they cut down on the cooling inlet to almost nothing. 2 inches in the case of Furias. See:

http://www.supercoolprops.com/ARTICLES/gwhite.htm
 

What are you babbling about "used" and "available" torque? Do you really understand what you write? Face this: if an engine is running at x rpm at full throttle, there is no "available" torque to accelerate anything unless the load is reduced!!! You are completely mixing part throttle and full throttle operation and inventing garbage like "available" torque.

Now, in ordinary car engines the power decreases relatively steadily as the rpm is decreased from the full power rpm down. On the other hand, torque curve starts rising with decreasing rpm (from that full power rpm) until it reaches the peak at a certain point that is highly variable with each engine. All this at FULL THROTTLE, i.e. CONSTANT MANIFOLD PRESSURE.
In supercharged aero engines the curve is of different shape as it CANNOT BE OPERATED AT CONSTANT FULL POWER MAP EXCEPT AT HIGH RPM. Get it???
 

First off, the Spitfire did not use any significant "Meredith Effect". Just because it's a liquid cooled engine radiator does not mean it exploits the effect. To achieve any significant Meredith effect, the system must include an expansion chamber prior to the radiator, a compression chamber after the radiator, and a thrust regulating exhaust nozzel. And finally, the exhaust flow must be directed into the vacuum wake of the fuselage to cancel out some or all of the parasitic drag. The Spitfire (and 109) lacked any of these features. Look in detail at the cooling system of the P-51 (as outlined in other threads on this forum) or the Mosquito to see examples of cooling systems which do significantly exploit this effect.

Secondly, radial engines can exploit the effect, though I'm not sure exactly what the mechanism is. I know the Zero gained about 5-10 mph from the effect, and this part of the desing was stolen and transfered to the F4U Corsair for a gain of 10-15 mph. It may also have been exploited by the Soviets on the LA7 design. Also, the oil cooler looks like a very small P-51 radiator, some meredith effect may have been generated there (note this is a significant difference between the La5 and the La7). The Soviet's had the opportunity to examine the P-51 in 1943 and 1944, and both the Yak and La designs seem to have borrowed the snorkle scoop from it, maybe some meredth effect technology was also borrowed.

Thirdly, the La7 cowl cooling inlet is in fact very narrow just like that of the Tempest II and P-47J. It is exactly what you are describing when you say "cut down on the cooling inlet". The combination of a large bullet spinner feeding into a small cowl inlet was definitely a part of the La7 cooling system design. Take a look at the attached image - just how much more do you think the cooling inlet can be cut down?

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Lunatic
 

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