Radial vs in-line engines
 

I was thinking about WWII planes and I realized that most radial engines were much more powerful than in-line engines.
Let's take the Corsair or the Thunderbolt for example, with their 2000+ HP P&W, whereas Spits and 109s were a little above 1000. Is there a technical explanation for that?

because your comparing 1940 engines with later engines,

Is that so? OK, I was wondering about that too.

Then, what decided between an in-line and a radial engine when they designed these machines? I guess radials were easier to cool but they produced more drag, but I might be wrong.

Wait, I'm not sure those late war Griffons or DB-s ever reached the 2200 HP of a P&W ? So there might be something after all?

The way I've always looked at it is that a big radial may produce a lot of power and be very torquey, but they're is enivitably a quite larger amount of drag introduced to the nose of the plane. A 109 on the other hand for example has a more "bullet shaped" nose with the inverted v-12, thus being more streamlined and needing less power to pull it through the air.

seafire 47 had 2300 hp but was post war most fighters were radials post war,so i guess radials were the way to go,im pretty sure they took more of a beating,im sure the real experts on here will tell us why

MK 14 spit was 2035hp

the debate Radial VS Inline is as old as the planes they were installed onto.

They both come with pros and cons, here are the most common ones:

Radial PROs
Very high TBO/extremely dependable
Resistent to damage/gives protection to pilot
air cooled, no need for cooling ducts/radiators etc
huge displacement/HP

Radial CONs
lotsa drag/bad forward visibility
oil thirsty
strong gyroscopic torque (the rotating crankshaft counterweight and big prop blades can cause the plane to torque itself out of delicate situations like pre-stall if full throttle is applied, still present on inline engines, but not as strong)
avgas thirsty
needs big supercharger
heavy

Inline PROs
streamlined engine
lighter than radial
more fuel efficient
lighter superchager
better fwd visibility

Inline CONs
needs liquid cooling/radiators
more susceptible to damage (can't fly with one or more pistons damaged)
shorter TBO


I suppose it's down to pilot's preferences and for the tasks assigned.

One of the bigger question marks in the history of warfare for me was the use of Mustangs in Korea, when P-47s would have probably done a better job.

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.

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

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 ;-)


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.

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.

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.

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

And a radiator does what?

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...

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

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.

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

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..

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.

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.

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.

Meredith effect on the Mustang.

http://img337.imageshack.us/img337/8...lingthrust.jpg

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.

I'm not discussing the quality of British aerodynamic research, I'm just saying that the Meredith effect is not a made up thing, and the Mustang is probably the design that benefits most from it.
As you know, they took a great deal of care in the design of the radiator system on the P-51: the radiator intake is detached from the fuselage to avoid turbulent airflow from the fuselage, and the radiator exhaust port could be opened/shut automatically so that it wouldn't bother the pilot. It surely was an efficient and revolutionary system, which allowed for a better performance with a very low drag coefficient (if compared to others) because of its design.

Its clever aerodynamics, light weight and reliability made for a superb system compared to the conventional turbo supercharged radials.

uh, I forgot to ask Crumpp, can you please point me to the source of that page? Sounds like an interesting read.

It is from The American Institute of Aeronautics and Astronautics (AIAA) library database and is from a presentation at an engineering conference. It is from the only modern design analysis on the P-51 Mustang and was done with an eye on improvements for one of the Reno racers. That being said, I got my copy directly from the author and can give you one if you like.

yes please! :)

Certainly. Send me a PM with your email and I will get you a copy.

You do realize it contradicts almost everything you posted in your last post about the P51.

Particularly:

Supersonic aerodynamics and compressibility were still pretty new and not well understood at the time the P51's radiator was designed. Therefore, they did not correctly slope the intake for normal shock formation. The slope was too steep and separation occurred.

That means high drag. This is confirmed in both later NACA wind tunnel testing and RAE flight testing. It is highly unlikely the P-51 series achieved any of its designers goals of laminar flow or Meredith effect. Interesting enough, the B-24 with the Davis wing in a complete accident of fate, did achieve laminar flow!

Hang on, why you're taking supersonic aerodynamics and compressibility into the equation? No plane of the era was designed to operate at such speeds.

My point was that if compared to other radiators of the era, the Mustang one was by far the more aerodynamically efficient, and surely superior to radial engines.

So you're now telling me that the Mustang wing is not a laminar design? :confused:

You had a chance to read through the report?

Well, that is what the report is talking about, Sternjager. Let me know when you have read through it.

Understand too, just because the flow is supersonic does not mean the aircraft is supersonic.....

On the whole, the Mustang radiator is not so aerodynamically efficient. The duct design is poor at best.

No, I said the Mustang did not achieve laminar flow. That is not the same thing as "designed for laminar flow."

It was designed for laminar flow just as it was designed to achieve the Meredith effect, neither of which occurred.

Ok, I gave you the report and a few days to digest it. Now lets explain it so you can get a grip on what is going on with the P51 radiator system.

First let's talk a second about supersonic aerodynamics. Just because the airplane is going subsonic does not mean the LOCAL mach number is not supersonic.

How does that happen? Well basic physics explains it very well. I am sure you are familiar with a venturi. If we take a given diameter pipe filled with gas at a constant mass flow and suddenly decrease the diameter, what happens to the velocity of the gas traveling thru the pipe?

Answer is the velocity of the gas increases!! It goes faster. Look at the design of the P51 radiator system and you will see this in the flow interaction with the oil cooler intake.

That is what happens in the P-51 ducting. The air enters the intake and very quickly encounters the oil cooler intake just before the radiator element expansion chamber. The volume is smaller because of the oil cooler intake so the velocity of the air flow increases.

At some point, it increase enough to go supersonic. Whenever we have supersonic flow at the local mach number a normal shock will form. A normal shock has specific characteristics. At the point of the shock, a "wall" of air will form. In front of the normal shock is supersonic flow and behind this "wall of air" is a flow reversal followed by subsonic flow. This flow reversal is essential a vacuum at the boundary layer which is why it is called suction.

This suction dynamically increase the amount of pressure drag. The effect is our aircraft slows down as the drag dramatically increases. The airplane slows down....

Once it slows down to subsonic flow, the shock will disappear. Our thrust available has not changed so the aircraft will immediately accelerate as the pressure drag has dramatically decreased. Once it accelerates enough to create a local supersonic flow our normal shock will reform and the cycle starts all over again.

This cycle happens rather quickly and the pilot will perceive it as a "rumbling" noise in the ducts of the intake as the airplane accelerates/decelerates rapidly in a very short time period.

How did this happen? How could the designers at NAA make such a mistake?

Well we just did not understand normal shock formation at the time.

Today we know it is all about the angle of the shock. The relationship is fixed to velocity at the sine of the normal shock is equal to the reciprocal of the local mach number. We also know that in any corner, oblique shocks are formed further dissipating our available energy.

They did not know that then however and were just beginning to understand compressibility and normal shock formation.

Got it now?

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.

5500HP in a 36.5L eng in 1944 (=150HP/L) ?!!!

I think your typing outran your thoughts Buzz :rolleyes:

Some info

Thx Alpha. Always a nice idea to put back on the table the very basis.

That is the question of range and the Jug was not as maneuverable as the fiftyone but there are other reasons.
This would make an intreresting read.
http://www.airliners.net/aviation-fo...ad.main/31094/

Niels