Petlyakov Pe-2 and Tupolev Tu-2 with Merlins

[Wuzak]

The Pe-2 was fitted with Klimov M105s with about 1200hp, and reached a maximum perofrmance of 360mph.

The first prototype of the Tu-2 was fitted with Mikulin AM-37s of abou 1400hp, development of which was abandoned, and achieved 395mph in level flight. Subsequently the Tu-2 was fitted with the Shvetsov ASh-82 radial of 1850hp and lost performance.

I wonder if RR Merlins, which made about 1400hp running on PN100 fuel at the time, had been available for use in these Russian planes how more useful they could have been.

Would they have become more attractive to the western Allies as fast bombers akin to the Mosquito? Both would have had similar performance to the Mossie, and with a larger load carrying capacity.

There were over 2,000 Tu-2s made, and 11,000 Pe-2s. Is that enough for use by the other Allies, or did the USSR need them all? I have always been under the impression that the Il-2 was of far more importance for the close support role that.

[GregP]

The Tu-2 and Pe-2 were EXCELLENT aircraft. I have always wondered if fitting them with western engines would have made them better aircraft.

The Russian engines, particularly the Mikulins, were never very well developed. This does not mean they could not have been developed or that they did not make power ... it means they were never very reliable and did not hold a tune for very long.

The ALlison and the merlin were quite reliable, but I am not sure they would be as reliable nor as robust in the Russian winters. The Russians put gasoline into the oil to thin it in the mornings for startup, and generally had less than wonderful quality fuel, sometimes with wood chips floating in it.

Could the Merlins and ALlisons have performed in the Russian winter?

The Allison-powered Bell P-39's did. The Russian also flew a few Spitfires, but I do not know the results of the service.

In my opinion, both aircraft would have performed as well or better with the western engines, but that is just an opinion. I suspect better since the Merlina and Allisons were more highly developed than ANY of the Hispano Suiza derivatives such as the Mikulin.

How much better is hard to say. When the Curtis P-40 was changed from an Allison to a Merlin, it gained 3 mph and the altitude performance was essentially the same. But the Merlin used was a single-stage, single-speed supercharger just like the Allison used.

If it had been fitted with a two-stage, two-speed unit, who knows what the performance would have been?

Put ina late model Merlin with the latest developments nad the Tu-2 could have been a 430 mph aircraft ... I think ...

[Wuzak]

I believe the Russians also operated a few P-51s.

In Russian service it may have been a problem for the engines. But these aircraft would have fitted easily into Western Europe operated by the RAF or USAAF if so desired.

[Kutscha]

I believe the Russians also operated a few P-51s.

In Russian service it may have been a problem for the engines. But these aircraft would have fitted easily into Western Europe operated by the RAF or USAAF if so desired.

The Soviets received 10 early model P-51s from the British for testing. They also could have salvaged some from Poltava.

The Finns claimed to have shot down some P-51s but this is highly disputed.

The Soviets also replaced the Merlin engines in their Hurricanes, iirc. Not robust enough in Soviet use.

[Romantic Technofreak]

Also in my opinion, "Peshka" and "Tushka" were excellent aircraft. Especially in the hands of Western Allied pilots, they would have performed very well in their intended role as tactical bombers.

About engine variation, I had a dispute with Montana and others if my formula is correct. Of course, if you look for "air drag" you find it varies on a square base. But in my eyes, in reverse using a square root base for variation of the engine performance gives "too good" results. Long time ago, I read that if you want to double the performance of an aircraft, you need to increase the engine power by 8 times - this is exactly the cubic base. Later I calibrated my formula by the values for the Ar 240 given in my oldest WW2 aircraft book - it matched exactly.

So, I think there are other influences too that slow down an aircraft alongside with the air drag, and their altogether influence can be assumed by taking the cubic root of the engine power.

Of course I do calculations for you if you request and give me data.

Regards, RT

[Lightning]

Hi RT,


So, I think there are other influences too that slow down an aircraft alongside with the air drag, and their altogether influence can be assumed by taking the cubic root of the engine power.
Regards, RT

As power and speed are increased, propeller performance changes. That would be one factor. When a significant increase in power is provided, the propeller must be able to efficiently transfer that power to the air. A propeller that was adequate at the lower power may be completely inadequate at the increased power. A change in the propeller's design is thereby called for.

Regards,
Lightning

[Red Admiral]

Its more or less true that drag varies with v^2. Well, its a good enough assumption for this speed range.

However the thrust produced by the engine reduces with velocity. Thrust = Power / v

At maximum speed, Thrust = Drag

Hence, Power / v = kv^2

So power = kv^3

[Wuzak]

It is interesting that the Tu-2 went quite a bit faster with the less powerful in-lines than the radials used in production (1400hp vs 1850hp). I think this was because the in-lines were in a very clean installation. Even so, the radials were very tightly cowled with a large spinner and small annulus for cooling air.

In the case of the Merlins vs the Mikulin AM-37s the power would have been similar for the single stage two speed Merlins on 100 octane fuel.

[Lightning]

Hi Red Admiral,

At maximum speed, Thrust = Drag

Thrust=Drag at all constant straight-and-level speeds from slow flight to maximum speed.

As the propeller "screws" its way through the air, it gets less and less "bite" as the airplane's speed increases. This is because the relative speed between the back side of the propeller blade and the receding air mass (the surrounding atmosphere) is decreasing.

Depending on its pitch, the blade will advance a certain distance through the air in one revolution. As rpm increases, the blade will advance that distance in less time i.e. the airplane will be flying faster. If at maximum rpm the blade is capable of advancing through the air at 400 mph, that will be the airplane's limiting speed. Any higher speed would cause a reaction force between the propeller's front side and the air, thereby opposing any speed increase.

An example of this situation would be a person holding his arm out the window of a moving car. If the car is moving through the air at 30 mph, and the person whips his out-stretched palm backwards at 30 mph with respect to the car, the relative speed between his hand and the air will be 0 mph, and he will feel no force of air against his palm. His palm, therefore, has no "bite" into the air mass.

The above is a somewhat idealized explanation in that it assumes a propeller efficiency of 100% which is, of course, impossible. There is always some slippage, and a figure of around 85% is more realistic. Also, the propeller, in addition to being an "airscrew" (our British friends actually use this term), is also an "airfoil." It provides a certain amount of "lift" along the direction of flight--sort of like a small wing. These factors combine to complicate matters a bit, but the basic principle given above is valid.

Regards,
Lightning

[Red Admiral]

If at maximum rpm the blade is capable of advancing through the air at 400 mph, that will be the airplane's limiting speed.

Whilst this is theoretically true, there is a speed for which a propeller will no longer produce thrust, I can't see it being a common case. There is still drag on the airframe. It is this which acts against the thrust from the propeller and the aeroplane's weight if its in a dive. Propellers tend to be optimised to give maximum efficiency at the airframe's maximum speed.

Just about all aeronautical propellers use aerofoil section blades. Flat plates don't work very well. 85% efficiency is very optimistic unless there is a very low disc loading. 70-75% is more reasonable.

[Lightning]

Hi Red,

Whilst this is theoretically true, there is a speed for which a propeller will no longer produce thrust, I can't see it being a common case. There is still drag on the airframe. It is this which acts against the thrust from the propeller and the aeroplane's weight if its in a dive. Propellers tend to be optimised to give maximum efficiency at the airframe's maximum speed.

I agree. What I should have said is, "...that will be the propeller's limiting speed." This would be the case if there were no airplane attached to the propeller--a somewhat impractical situation. The airframe's drag holds the speed below the propeller's "zero-thrust" speed, so thrust is being developed. This thrust equals the drag in straight-and-and-level, unaccelerated flight.

Just about all aeronautical propellers use aerofoil section blades. Flat plates don't work very well. 85% efficiency is very optimistic unless there is a very low disc loading. 70-75% is more reasonable.

I mentioned the propeller's being an airfoil in my posting. Also, I would say that 85% is a an optimistic--but achievable--number, but I would also consider 70% to a bit on the ordinary side. Let's agree on 75-80%

Regards,
Lightning