Small Allison vs Merlin Mustang tech question

Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules

BarnOwlLover

Staff Sergeant
934
337
Nov 3, 2022
Mansfield, Ohio, USA
I've been looking at photos of the Merlin and Allison Mustangs, and I do know that their cowlings are very different (mostly due to some dimensional differences and mainly the Allison having a downdraft supercharger/carb while the Merlin had an updraft supercharger/carb). But I'm curious about prop spinner cover dimensions, and how similar/different they are, which is hard to tell given the different cowling profiles. Is there much difference as far as spinner cover dimensions?
 
I've been looking at photos of the Merlin and Allison Mustangs, and I do know that their cowlings are very different (mostly due to some dimensional differences and mainly the Allison having a downdraft supercharger/carb while the Merlin had an updraft supercharger/carb). But I'm curious about prop spinner cover dimensions, and how similar/different they are, which is hard to tell given the different cowling profiles. Is there much difference as far as spinner cover dimensions?
I think the received wisdom of what a spinner should be shaped like changed in this era. The Spitfire Mk II had a spinner that looks like half a sphere while some Mk Vs have what looks like a cone. If you look at late war post war planes like the Hornet and Sea Fury it is clear that the spinner was designed as part of the planes aerodynamics or cooling set up.
 
Hi, Spinner size and aerodynamics is very complicated. Just look how radial engine cowlings developed from fairly simple Townend type rings to complex integrated cowlings with intricate baffling, internal streamlining and complicated exit cowl flaps. The Spinners grew from little caps on the hub to become large aerodynamic forms that matched the cooling flow. Some even used additional cooling fans, not to mention the hollow spinners.
For the Liquid cooled engines there is the obvious streamlining of the prop hub but then, the aerodynamics of the engine installation and the rest of the airframe come into the picture.
The single engined fighter has a particular issue with the top lines of the cowling WRT the pilot's forward view and sightline. Generally, there is a conflict in aerodynamics where the best pilot view requires a good high position to sight over the nose, against the high thrustline of many upright inline V-12's. IMO, the spinner size on many Merlin powered fighters was limited by its high thrustline and the need to have a sightline over the nose. In the Hornet for example, it appears that the larger spinner was worth using aerodynamically because the cowl could be higher over the engine to give a larger spinner backplate diameter.
The relatively lower thrustline of the inverted V-12's is interesting. Early Bf 109 versions used a small spinner with a very close fitting cowl that maybe had a small cross section area but seems quite angular. The sightline over the nose is so close to the top of the engine that the engine actually cuts through the top of the cowl at the front of the crankcase.
With the Bf 109 F onwards, Messerschmitt was able to greatly increase the spinner backplate diameter and use a large aerodynamic spinner, without raising the top cowling line more than a few millimeters (it still almost touches the front of the crankcase).
Similar observations could be made of the Fw 190 D and Ta 152, where they were able to actually use annular radiators because of the low thrustline, while still having a fair sightline. Of course, the spinners for those aircraft are not as large but, the annular radiators are quite advanced.

Cheers

Eng
 
I also wonder why the Merlin cowling on the Mustang looked smoother than the Allison cowling did.

And I wonder if the Merlin Mustang and Hornet's spinners are similar in size, since both engine cowlings seem to have similar frontal area (?).
 
Last edited:
I've been looking at photos of the Merlin and Allison Mustangs, and I do know that their cowlings are very different (mostly due to some dimensional differences and mainly the Allison having a downdraft supercharger/carb while the Merlin had an updraft supercharger/carb). But I'm curious about prop spinner cover dimensions, and how similar/different they are, which is hard to tell given the different cowling profiles. Is there much difference as far as spinner cover dimensions?
haven't looked at drawaing package yet but JL McClelland's Design Analysis Number 7 on the P-51, states 29" for B/D Spinner diameter. Speculatively, I'm pretty sure that NA-73 through NA-99 had a smaller diameter spinner.

Loweing the wing 3" for B/D imposed different lower cowl line which appears to have forced a change in prop spinner diameter to increase.
 
I think the received wisdom of what a spinner should be shaped like changed in this era.
I always thought it odd that the Spit I had a dome shaped spinner while the Hurricane always seemed to have a pointed one. Then the later Spits got pointed spinners.

And it is interesting to see how during WWII the importance of spinners for radial engines seemed to diminish. Many airplanes sported big spinners for their radial engines in the early 40's but those mostly seemed to either go away or get much smaller.
 
I always thought it odd that the Spit I had a dome shaped spinner while the Hurricane always seemed to have a pointed one. Then the later Spits got pointed spinners.

And it is interesting to see how during WWII the importance of spinners for radial engines seemed to diminish. Many airplanes sported big spinners for their radial engines in the early 40's but those mostly seemed to either go away or get much smaller.
I think the dome shaped ones were de Havilland types fitted to the Mk II. At the same time P-40s were cones almost to a sharp point. but they have the air intake behind the prop above the engine. Ive no idea how much actual science is behind it or how much was "suck it and see" or "we have always done it like this". The early Mustang had its intake moved forward to just behind the prop as airflow into it was reduced in turns.
 
Here is my favorite shot showing the difference in the spinners.

Of course the Hurri also has that ring behind the spinner to catch oil from the prop. Took me decades to figure out why Monogram molded that in their 1/48 kit.

SpitsandHurri-1.jpg
 
Anyone have a guess or estimate how the Allison and Merlin P-51 spinners compare in diameter and overall size to the DH Hornet's since the Hornet was mentioned as far as prop spinner design?
 
I'd have to go looking for the numbers, and perhaps still not find them, but I think I recall that when I asked this question the answer was that the Allison one does have a bit smaller diameter. I'll report back if I do find the data.
 

Users who are viewing this thread

Back