Radial vs in-line engines

[Sternjaeger II]

Originally Posted by Crumpp

Good summary, Sternjager II and all.

In general, yes but as long as the designer pays attention to the installation of an aircooled motor, the Cd0 is not necessarily higher. Look at the Zeke, Corsair, F6F Hellcat, or Focke Wulf series. All have average to below average Cd0 for the period. In fact very few inline installations approached the Cd0 of the Zeke.

Consequently there are inline installations that have much higher drag than radials.

yeah, but in the end of the day you need a higher number of HP to compensate for the drag, and even if you implement ram fans like on the FW190, you still need air to go through the cyl heads and out from the sides in a very turbulent fashion. No matter how "polished" your radial design is, it's still a radial

Only when comparing dry weights. Add in the radiators and coolant and their is little to choose in terms of weight.

I suppose it depends on the specific plane really. A Thunderbolt would have a chunky turbo supercharger installed in the fuselage and oil coolers in the engine cowl, that adds a lot of weight as well. Let's not forget that the Merlin has a higher power to weight ratio other than a lighter dry weight.

In terms of TBO, liquid cooling wins out. Temperatures are much more stable across the engine in comparison to air cooled engines. Air cooled cylinders can vary over 100 degrees in normal operation just from fuel metering alone.

http://www.liquidcooledairpower.com/lc-longertbo.shtml

mmmmh, again, it really depends on engines. It's not a good idea to run a Merlin, even with transport heads, beyond 600 hours, while a well maintained radial has more than double that TBO. In fact, apart for certain non certified ones, I can't think of any radial with a TBO below 500 hours, while there are many many many inlines that are below that (the DB family being the extreme example).

Temperature is not the only factor, and whilst a coolant system failure can be catastrophic, a radial can survive prohibitive temperatures, thermal shock and component failure, and still do its job.

[Igo kyu]

The Sabre http://en.wikipedia.org/wiki/Napier_Sabre was a 2,000 hp inline.

[Sternjaeger II]

Originally Posted by Igo kyu

The Sabre http://en.wikipedia.org/wiki/Napier_Sabre was a 2,000 hp inline.

Sleeve valves? No thank you

The Sabre was a bit of a trouble child, and considering the sheer weight and size of the thing, you could probably compare it to a radial more than an inline.

[madrebel]

in terms of power per cylinder the v12s easily beat the 14 and 18 cylinder radials.

[Crumpp]

you still need air to go through the cyl heads and out from the sides in a very turbulent fashion.

And a radiator does what?

It's not a good idea to run a Merlin, even with transport heads, beyond 600 hours,

You can't compare a Merlin or TBO's from the 1940's with today's engines. Most Merlin's are modified so that they will be reliable and last some time.

Mike Nixon can give you a quote...

Improved Engine Life
Engines restored today actually have enhanced service lives due to the application of improved materials and technology.

http://www.vintagev12s.com/services.htm

[Crumpp]

a radial can survive prohibitive temperatures, thermal shock and component failure

A radial cannot survive prohibitive temperatures or component failure any more than an in-line. In-lines are however immune to thermal shock from descents. Assuming of course, thermal shock exists.

[Sternjaeger II]

Originally Posted by Crumpp

And a radiator does what?

It generates thrust in some cases.. as a Mustang driver I'm sure you heard of the Meredith Effect

You can't compare a Merlin or TBO's from the 1940's with today's engines. Most Merlin's are modified so that they will be reliable and last some time.

I agree, different oils, fuels and servicing make for revised TBOs, but not by much, the real difference is made by engine management. We take off with 75% throttle on the Mustang, and are surprised to hear that several operators still firewall their throttles for takeoff..

Mike Nixon can give you a quote...

http://www.vintagev12s.com/services.htm

we buy material from the US for our overhaulings: critical components like bearings, pistons, piston rings, valves all contribute to the life expectancy of these engines, but constant monitoring is necessary nonetheless.

[Crumpp]

It generates thrust in some cases

Great theory that does not work out in most attempts. None of them produced any thrust AFAIK but a few meredith designs did have the effect of reducing drag somewhat. It requires very specific parameters which were not present in most installations. In fact, the radiator ducting on the P-51 was headache and plagued by duct rumble. That is flow separation in the duct and a source of high drag.

the real difference is made by engine management.

You are correct in that the throttle makes the largest difference in any engine on making it to TBO but that applies to any engine.

The real difference in the Merlin is made by valve springs, better coolant connectors, oil feed improvements to the cam, better bearings/races and some good machine work on the heads, just to name a few.

In the case of the Merlin, there are specific upgrades to improve reliability that are highly recommended if you want the motor to last. Those upgrades overcome the shortcomings of the design.

[Crumpp]

Meredith effect on the Mustang.



Only the RAE bought into the thrust production theory. Both the NACA and the RLM disagreed.

Understand that of the three, it was the RAE that was trailing in aerodynamics. The British engines were good, probably the best of all the combatants but their aerodynamic sciences was behind the other major combatants. That is why you have RAE claims for things like Mach .98 dives out of the Spitfire that later get retracted as they discovered the static port placement was completely wrong for any degree of accurate speed measurement in the transonic realm.

[*Buzzsaw*]

Originally Posted by Sternjaeger II

Sleeve valves? No thank you

The Sabre was a bit of a trouble child, and considering the sheer weight and size of the thing, you could probably compare it to a radial more than an inline.

Salute

By 1944, the Sabre had overcome its teething problems, and was a reliable performer while producing 2600 hp from 36.5 liters, and that was without a fairly primitive supercharger. With an updated supercharger, it produced unheard of amounts of horsepower in later models, 3500 in the initial Sabre VII, out just after the war, and up to 5500 hp in the final generation Sabre VII engine which did not go into production.

It got its performance from higher rpms allowed by smaller piston and shorter stroke, sleeve valves, (which breathe better) and better volumetric efficiency from the H block design.

If piston engines driving props had remained the cutting edge of aircraft propulsion, then the H block engine would have been in the forefront, but because Jet turbines were obviously superior, the Sabre was discarded, and the simpler but more reliable Radials were kept in production as propulsion for 2nd line aircraft.