Engine Survivability

[trackend]

I have a question for you guys
Which design of engine tended to be more durable against combat damage. Was it the inline ,the radial with its separate cylinders or were both of these superceded by the Jet with its internal compressor blades.

I can see merits and drawbacks in all but whats your opinion?

 

[Keke]

I my opinion there is too many aspects for only one correct answers that question.

Aspect for radial:

++ air cooled, so no leaking coolant
+ could produce power some time even a some missing cylinders
-- big frontal area

for inline:

+ medium frontal area
+ one bullet can't cause instant power loss
- big radiators and lot of coolant piping cause a large vulnerable area

for jet:

++ low frontal area
+ speed advance at previous
-- one hit in vulnerable area can cause total engine failure and even total loss of the plane (in case of the engine explosion)

So i my opinion at WW2 era every engine type has a own strengths and own weakness. In my opinion every one can appraise some point more important than other, so I can't name some engine type better than another.

I have although have a honor to discuss with Ilmari Juutilainen (WWII ace from Finland) and he say, that he was trying to selected always a most vulnerable area of plane to the aiming point.
So we have a one aspect more - the pilot knowledge of enemy planes.

I have also seen pictures of Merlin powered Mustang racer, which has a connecting rod failure. There is a big holes in the both side of block, but in story claimed, that pilot could make a powered safe landing.

 

[HoHun]

Hi Trackend,

>Which design of engine tended to be more durable against combat damage. Was it the inline ,the radial with its separate cylinders or were both of these superceded by the Jet with its internal compressor blades.

From what I gathered from various Avsig posts by a Vietnam war pilot/backseater (the latter after the introduction of the F-4), the naval aviators' opinion back ten was that jet engines were far more durable than piston engines due to their simplicity. Fewer systems (no ignition to necessary to keep it going, for example), fewer critically vulnerable points, far fewer moving parts.

With regard to radials versus V-engines, radials certainly have a better reputation, but I'm not sure that reputation was ever confirmed by actual combat results (beyond the anecdotal, that is).

I wonder if there is a way we could check it - maybe if we could get hold of loss statistics for RAF night bombers ... if they show the loss ratios for different sub-types with radial and V-engines, that might give us the chance to verify "common wisdom" on engine durability.

Regards,

Henning (HoHun)

 

[timshatz]

Good posts. Would've gone with radials myself but after reading the above posts, could see where jet engines would get the edge.

 

[red admiral]

Theres not a great deal of difference in reality between the robustness of air cooled radials or water cooled inlines when hit. One problem is that WWII gives a huge data sample, there are bound to be outlying examples such as R-2800s still running missing a cylinder, a P-51 returning to base with its radiator shot away, a B-17 carrying on flying after a direct hit from an 88mm shell...

 

[kool kitty89]

Air cooled inline/V engines are a whole different category as well, though only a few Millitary a/c (other than trainers/comunication/liaison, and some prototype combat a/c) actually used them. Most notable would probably be the Fw 189.

Still something to think about though.

 

[KrazyKraut]

Theres not a great deal of difference in reality between the robustness of air cooled radials or water cooled inlines when hit. One problem is that WWII gives a huge data sample, there are bound to be outlying examples such as R-2800s still running missing a cylinder, a P-51 returning to base with its radiator shot away, a B-17 carrying on flying after a direct hit from an 88mm shell...

Actually, from a statistical point of view, the bigger the data sample, the more conclusive the results should be. I doubt anyone ever bothered to really dig into that subject though.

 

[red admiral]

The USN did some studies during 1944 of around 500 aircraft in the Pacific. 38% rtb with an engine hit, 20% with a hit to the fuel system. Only 89% rtb with hit to structure. Not really sure whether it can be called conclusive.

My point was that the data sample is so large that there are a number of highly improbable events that do not accurately reflect reality.

 

[<simon>]

I think would be the worst just cause of the amount of fuel pumping around in there.
Is this solely for air-to-air combat?
I remember reading somewhere that only the bravest of jet pilots would make any kind of low ground attack, because any shrapnal, rocks, bit of plant etc that managed to get airbourne would more likely get sucked right through the engine and cause a complete failure.

 

[PWR4360-59B]

This one is easy.
You will find many accounts of the Radial engines getting all shot up and still
getting its plane back home. With the other types a hit to a critical area will cause a rapid failure. Jets are the worse, because if it takes a hit in almost any place its going to grenade, if its in the compressor or turbine section the imbalance will explode it. If it is in the combustion section same deal all the hot gases spew out and fry what ever is outside it.
Liquid cooled engines will live a short life if hit, sooner or later they will overheat and sieze up. The radials where known to run just fine with a cylinder blown off, now if that was a master rod cylinder maybe not so good.

 

[Keke]

This one is easy.
You will find many accounts of the Radial engines getting all shot up and still
getting its plane back home. With the other types a hit to a critical area will cause a rapid failure. Jets are the worse, because if it takes a hit in almost any place its going to grenade, if its in the compressor or turbine section the imbalance will explode it. If it is in the combustion section same deal all the hot gases spew out and fry what ever is outside it.
Liquid cooled engines will live a short life if hit, sooner or later they will overheat and sieze up. The radials where known to run just fine with a cylinder blown off, now if that was a master rod cylinder maybe not so good.

IMHO, it's not so easy. If radial engine get hit and can still run, it breaks lack of oil sooner or later (probably sooner). Also radials has bigger frontal area to hit.
As you write, jets has a disadvantage for it's vulnerability, but it has a low frontal area, so more difficult to hit direct on engine. Jet planes has also speed advance against liquid or radial engines, so jets are more difficult to hit.

 

[Glider]

An interesting comment from a Japanese Pilot Sgt Yoshito Yasuda on the Hurricane IIB

The hurricane was a unique plane with twelve 7.7mm mg's which caused deadly damage if we were shot from behind. Its diving speed was much faster than the Ki43. Therefore, when we fought with Hurricanes we attempted to counter its firepower with the better manoeuvrability of the Ki43 and tried to hit its radiator, bringing the engine to a stop. Even with the poor firepower of the Ki43, Hurricanes could be shot down merely by one hole in the radiator.

 

[red admiral]

You often hear comments like that, but one hole in the pilot or fuel tank will also bring down a plane.

 

[Glider]

True, but the pilot normally has armour plate behind him and and engine in front, the fuel tank normally self seals and even if it doesn't, it can take a fair amount of time to empty plus of course there is often more than one fuel tank.
The radiator is simply hanging out in the open saying, hit me and I go down. It worked for the Japanese against the Hurricane (and no doubt others) and the Germans against the IL2, who am I to argue?

 

[steven_wh]

With jets you have to consider that axial engines with their multiple compressor stages were more vulnerable than the single stage centrifugal type.

Also there were two types of centrifugal in use in WWII,the single-sided with direct air inlets, and the double-sided. The double-sided were less affected by debris or hits to the compressor, because they were mounted deep inside a nacelle.

Centrifugal jets, e.g. Klimov, Nene, or J-48 were less susceptible to damage than axial in actual combat in Korea.

Jet engines use kerosene which is less much volatile than aviation petrol and this improves their safety somewhat.

So, in order of vulnerability:
- axial flow jets
- single-sided centrifugal jets
- double-sided centrifugal jets

Steven

 

[HoHun]

Hi Glider,

>Even with the poor firepower of the Ki43, Hurricanes could be shot down merely by one hole in the radiator.

Here is an interesting article on Ki-43 armament:

Nakajima Type 1 Model 1 Army Fighter (Ki 43-I) Armament

With regard to the "one hole in the radiator", the question is not so much one of the number of holes, but rather one of the number of rounds fired to achieve that single hole.

As we know, the 7.7 mm machine gun - technically entirely sufficient to punch holes in radiators - was recognized as badly inadequate as an air-to-air combat weapon in WW2.

This means that as a reality check, we can conservatively assume that - for example - four 20 mm cannon were better than 12 7.7 mm machine guns. (This is the "exchange rate" when the Typhoon received increased armament.)

The total probability of a kill is:

Pk = n * Ph * (Ac * Pkc + Au * Pku)

Pk = total probability of a kill
n = number of rounds fired
Ph = hit probability
Ac = percentage of critically vulnerable target area as projected in the plane perpendicularly to the line for fire
Au = percentage of uncritically vulnerable target area in the same plane
Pkc = average kill probability for a single round hitting a critical area
Pku = average kill probability for a single round hitting an uncritical area

If we consider a burst of a single second for each of the Typhoon's battery variants, this comes down to:

Pk7.7 = 240 * Ph * (Ac * Pkc7.7 + Au * Pku7.7)

versus

Pk20 = 40 * Ph * (Ac * Pkc20 + Au * Pku20)

Assuming hit probability to be equal for both weapons, it follows that

Pk7.7 / Pk20 = 6 * (Ac * Pkc7.7 + Au * Pku7.7) / (Ac * Pk 20 + Au * Pku20)

Due to the cannon armament being historically considered superior, we also know that

Pk7.7 / Pk20 < 1

Now we can make assumptions on the target area presented by the radiators in relation to the overall target area. Let's say for example that

Ac = 5%, Au = 95%.

By definition, 7.7 mm hits against uncritical areas have

Pku = 0

(Note: This ignores cumulative damage. This makes our estimate even more conservative in favour the capability of 7.7 mm guns against critically vulnerable areas.)

So we get:

6 * (0.05 * Pkc7.7) / (0.05 * Pkc20 + 0.95 * Pku20) < 1

Now there is a German statement regarding fighter vulnerability claiming that it took an average of six random 20 mm hits to bring down a fighter. That seems conservative since I've never heard of a single-engined fighter coming back with more than eight 20 mm hits, but we mean to be conservative for now.

Accordingly, Pku20 can be considered roughly 1/6. Additionally, a hit by a 20 mm projectile to a vulnerable area apt to be critically damaged by a 7.7 mm round is likely to be close to 100% critical, so let's say

Pkc20 = 90%

So we get:

6 * (0.05 * Pkc7.7) / (0.05 * 0.9 + 0.95 * 1/6) < 1

or

0.3 * Pkc7.7 / (0.045 + 0.0158) < 1

or

4.93 * Pkc7.7 < 1

Thus we arrive at

Pkc7.7 < 20.3%

In short, historical experience (conservatively analyzed) shows that in reality, an average of at least five hits by 7.7 mm machine gun round were necessary to destroy a part like the radiator that "could be pierced by a single machine gun bullet".

Now it's possible to make different quantitative assumptions than those I made above and arrive at different figures, but the real lesson here is that one should not confuse the optimum effect on the target with the normal effect on that target.

Of course the notion of the "single-bullet kill" is an attractive thought model, but as Priller noted in his "JG 26" history, during the Battle of Britain some Messerschmitts came back to France safely with as many as 80 bullet holes. Others might well have gone down to single bullets - but the effectiveness of air-to-air armament has to be measured by the average results, not by the extremes.

Regards,

Henning (HoHun)

 

[mad_max]

Ho Hun posted:

That seems conservative since I've never heard of a single-engined fighter coming back with more than eight 20 mm hits, but we mean to be conservative for now.

We all know of this one. 21 holes from them, but it's just one.


P-47 Thunderbolt: Aviation Darwinism - The Cradle of Aviation Museum - The Cradle of Aviation Museum

 

[HoHun]

Hi Madmax,

>We all know of this one. 21 holes from them, but it's just one.

Wow, thanks a lot Of course I knew of this incident, but I hadn't been aware that he collected a total of 21 cannon hits!

As a reality check for my above reality check, the likelihood of surviving 21 cannon hits under the above assumptions is:

Psurvival = (1 - (0.05 * 0.9 + 0.95 * 1/6)) ^ 21 = 0.84%

These are not odds anyone would like to face! However, with the great number of fighters exposed to cannon fire in WW2, I think we should have heard of similar cases if roughly one in hundred fighters exposed to such a barrage would have survived like Johnson's. So I'd say this case by its exceptional nature confirms that my above assumptions were conservative.

Unless of course other cases were not as widely published as Johnson's ... I'm not sure how likely that is, though.

Regards,

Henning (HoHun)

 

[mad_max]

Of the few that returned I've read of seemed most were under 5-6 hits. Let's see if I can find them on the old HD in the corner.

 

[slaterat]

Here's one from Bloody Shambles vol 2 p 320

RAF pilot Sandeman Allen

" The zeros came in underneath and we had a simple target for the first few vital moments, after which we were so heavily out numbered that we got into serious trouble. I was credited with 2 zeros destroyed ,one probable and one damaged, but crawled back to the aerodrome with 28 cannon shell and 43 bullet holes in the machine..."

The hurricane airframe was amongst the toughest single engine fighters of the war.

Slaterat