German vs. British propeller design

[Doc72]

Hello everybody,

just another question that came across my mind recently: what are the reasons behind the different developments of propellers on German and British fighters in WWII?

Both the Spitfire and the Bf-109 started their career with a two-bladed fixed-pitch wooden airscrew. On both aircraft, these were quickly replaced by two-bladed, then three-bladed variable-pitch propellers. With ever increasing engine power the Spitfire soon used four propeller blades (Mk. IX onwards), five blades (Mk. XIV) and eventually even a contra-rotating prop with 2x3 blades (some Mk. 21 and the last Seafires).

By contrast, even the last Bf-109 K in 1945 used only three blades. The German airscrews changed in terms of increased chord and area but always remained three-bladed. This is even true for later and larger fighters, the Fw-190, Ta-152 and Do-335.

I suppose that three-blade airscrews are less labor-intense and cheaper to produce than those with four or five (let alone contra-rotating designs), but somehow the propeller has to match the increasing engine output. With the rpm limited (the propeller tips shouldn’t get supersonic) and the diameter limited (problem of striking the ground), the chord/area and the number of blades seem to be only adjustable parameters. Therefore, it’s funny that the two countries followed different paths.

Any ideas?

[Graham Boak]

The engineering choice made is very much dependent upon those made earlier: on the data base that you have built up and can use as a guide to moving forward. It could simply be chance that one nation followed one route and the other another. Even on the three-blade designs, later war Allied propellers were wider than their predecessors, showing the British and American designers were not unaware of the design possibilities.

However, it isn't quite as simple as that. If you look at the German bombers, they went to multiple blade designs rather than broader blades. And the final German fighter designs (Bf 109L?) were also moving to 4-blade designs, perhaps because they had reached the limit of broad-blade designs. I do wonder - I don't know for sure - whether this was also a function of the expected operating environment. Both RAF fighters and German bombers were reaching for higher altitudes, whereas the German fighters were designed around lower operating altitudes.

Another possibly relevant point is that 3-blade designs were more prone to vibration than four blade - witness the Typhoon story.

Edited January 13, 2016 by Graham Boak

[Bedders]

Might it have been that the German aircraft still used interrupter gear for the guns firing through the propeller arc, and that if they went beyond three blades things became...complicated?

Justin

[Darby]

3 bladed ones are easier to paint

[Giorgio N]

Both 3 and 4-bladed designs have pros and cons as broader blades tend to be less efficient but at the same time propellers with more blades can suffer from the effects of aerodynamic interference between blades. Even with the most powerful engines used by Luftwaffe fighters at the end of the war, their 3-bladed props seem to have worked. With highher powers things may have been different and the same Germans used 4-blade propellers for a number of engines.

[John_W]

Just thinking about other aircraft, and the Spitfire seems unique in it's increasing blade count. There was no four bladed Hurricane or five bladed Mustang.

(prepares to be corrected)

[Max Headroom]

There was a proposed Hurricane V with a Spitfire IX type prop but for whatever reason it wasn't pursued

http://www.airwar.ru/image/idop/fww2/hurr4/hurr4-6.jpg

Can't recall a proper Mustang with more than four props unless you count ex Shackleton Griffon engined Reno racers as legit

Isn't there something about more props/more prop area that invokes the rule about gyroscopic effect? As I understand things the Avia 199 with its wide blades (and narrow undercarriage chord) was very prone to this.

Trevor

[Stonar]

I have a suspicion that the 'it made synchronising of the cowl guns easier' argument is relatively modern, but it must also have an element of truth to it.

I believe it was simply different nations/manufacturers arriving at different compromises, aerodynamic, production etc.

Cheers

Steve

Edited January 14, 2016 by Stonar

[Graham Boak]

I don't buy the synchronising guns argument - didn't the RE 8 in the first World War have a forward-firing gun and a four-blade prop? Also at least some SE 5s? It must reduce the firing rate more, but is not intrinsically more difficult.

The Hurricane Mk.V ground-attack aircraft with a Merlin 45 was not proceeded with because it didn't give a significant improvement in performance. A different argument, but it's my belief that the RAF in India would have benefited significantly from the Merlin 45 in 1942/43, but can understand that the Spitfire had priority. Plus something had to be done with all the Merlin XXs Ford was building: once true mass production was in flow it couldn't be turned on and off like a tap..

[Giorgio N]

Just thinking about other aircraft, and the Spitfire seems unique in it's increasing blade count. There was no four bladed Hurricane or five bladed Mustang.

(prepares to be corrected)

The Spitfire and the Sea Fury are the only aircrafts that come to my mind that used 5 blade propellers. The Seafire 45 even went to 6 as used a contraprop unit...

At the same time other aircrafts did fine using 4 blade units...

Part of the reason is power: no wartime P-51 ever had the kind of power that the 2-stage Griffon developed, hence they could easily handle the power with 4-blade props. The same can be said of other aircrafts that while may have been powerful were not as powerful as the latest Spitfires. The Hurricane never even had the power of the Spitfire IX, a 4 blade propeller would have been of little use.

Other aircrafts had similar power installed but also had larger diameter propellers and this larger diameter allowed them to make proper use of the power available.

The Sea Fury was another type with a 5 blade propeller and it's interesting to note how the Tempest Mk.II with the same engine managed to use a 4.blade unit.. but check the different diameter!

I have vague memories of a lesson on propellers at college and with conventional propellers 4 was considered an ideal number of blades. Very powerful engines were considered to better benefit from a contraprop arrangement rather than by increasing the number of blades on a single disc as this introduces complicated interferences between blades. Of course contraprop units have their own interferences to consider... Different story for modern multiblade propellers that are designed to avoid such interferences

[Greenshirt]

Been 35+ years since my schooling on propeller disc theory but I too remember those same relationships. ISTR the larger blade count for the Spitfire meant an ability to harness the power without increasing blade diameter, limited on the short legs of the Spitfire. Solved on other aircraft by either bending the wing (Corsair), or articulating the landing gear as it retracted (Corsair, Hellcat, Bearcat).

It would be nice to read an article or two, written by the designers, explaining their thinking of the time...

Tim

[JasonC]

The Sea Fury was another type with a 5 blade propeller and it's interesting to note how the Tempest Mk.II with the same engine managed to use a 4.blade unit.. but check the different diameter!

Interesting point. Perhaps the diameter on the Sea Fury was limited to avoid the prop tips 'pecking' the deck as the aircraft pitched forward after catching the wire. Wikipedia gives the Tempest II a prop diameter of 12'9" (c.f. 14'0" for the Tempest V). Unfortunately I can't find a corresponding diameter for the Sea Fury.

J.

[Don McIntyre]

Actually a prop with an odd number of blades is inherently more stable than a prop with an even number of blades (IIRC what they taught me in ASW classes...).

[Graham Boak]

Perhaps you could define "more stable" in this context? Certainly the Typhoon went to a 4-blade prop to reduce vibration, suggesting that the prop, in itself, was less "stable", although the 4-blade did also give a small increase in power. However thinking overall aircraft stability, having more blades (assuming much the same width of each blade) does give a larger "disc solidity" which destabilises the aircraft. It can be said to be equivalent to fitting a fixed fin forward of the cg. This is why the Typhoon had to have the larger tailplane, and why this had to precede the 4-blade on the production line.

Edited January 14, 2016 by Graham Boak

[Bedders]

It's interesting too that of the racing Sea Furies, a number of them have been re-engined and fitted with 4 blade props rather than sticking with the 5-blades of the Centaurus. That may just be becuase four-bladers are easier to maintain/replace these days.

[Hotel Papa]

For the carrier planes handling properties come into play. That was the reason the Seafire went to the Contra-Prop. (Take off and landing characteristics of the first prototypes were criticised) Might also have been a reason for the five blades on the Sea Fury.

[Parabat]

I find this discussion of great interest as I was thinking about this while building my Fw190D-9, and again in discussion on another thread concerning the D-11 and D-13. These later Doras had broader blades, as did the Ta152H, which was designed as a high altitude fighter. So maybe the theory on high/low altitude does not really come into play on this.

[Stonar]

For the carrier planes handling properties come into play. That was the reason the Seafire went to the Contra-Prop. (Take off and landing characteristics of the first prototypes were criticised)

You might be referring to the tendency for the Griffon Seafires to spin to starboard (usually crashing in front of the carrier) on take off. This wasn't a result of the increased torque of the Griffon but a result of asymmetric airflow over the two wings. The static torque problem, trying to turn the aircraft in the opposite direction to the propeller, was mitigated by 'up floating' the port aileron slightly. The asymmetric airflow was the different slip stream incidence caused by the propeller of the Griffon powered versions twisting the slip stream far more than the Merlin versions. The propeller, whose diameter was the same, had to absorb nearly twice the power at the same rpm. The asymmetric airflow caused the inboard section of the starboard wing to stall. This was the first step in a series of events that could lead to disaster as the stall spread outwards, the wing lost lift, increased incidence (already high when 'wrenching off' from a carrier) and finally fully stalled.

The problem was solved, as you say, with the adoption of contra rotation propellers.

My father was a FAA pilot who trained in the early '50s, hence avoiding the Seafire. He trained in the US, converting to the Sea Fury when he returned to the UK. He had a few second hand 'horror' stories to tell of the Seafire! The US types he trained on at Kingsville and Pensacola were designed for carrier operations, the Seafire was adapted to them. It is an important distinction.

Cheers

Steve

[Graham Boak]

The story is, as I understand it, slightly more complicated still. It is told in David Brown's classic "Seafire" and also more recently in Peter Caygill's Ulitimate Spitfires. The port and starboard wing roots are built at different incidences at allow for this propeller flow. However, this has had grown to be an unavoidable basic mismatch in that if incidence difference is set sufficiently large to cope with the low speed high power case, at high speeds the induced rolling moment becomes too large to be trimmed out.

it would be interesting to understand how the Sea Fury and Bearcat avoided this: having larger fins comes to mind and possiply the increase in the F8F-2 may be relevant.

[Stonar]

.

it would be interesting to understand how the Sea Fury and Bearcat avoided this: having larger fins comes to mind and possiply the increase in the F8F-2 may be relevant.

I know that the Skua (admittedly not the most refined aircraft ever built) had a lift spoiler fitted to the leading edge of the wing on the down going propeller side to equalise the stall characteristics of the two wings, power on. I believe similar devices were fitted to other aircraft but don't know about the two you mention. I've never noticed such a thing on a Sea Fury.

Cheers

Steve

[Graham Boak]

Strips as stall spoilers are fairly common, usually to ensure consistent behaviour with the nose dropping rather than one wing. The Corsair being perhaps the best WW2 example, but I think that you'll find others if you look closely. They could be seen more recently on the Grumman Tiger - the light aircraft, not the fighter.. Having been staring at Arado Ar96s recently, they seem to have some kind of trip at about 10% chord on both wings. Not the usual placing, but it is difficult to see just what else they could be.

The Seafire problem was rather less amenable.

[Work In Progress]

Just thinking about other aircraft, and the Spitfire seems unique in it's increasing blade count. There was no four bladed Hurricane or five bladed Mustang.

(prepares to be corrected)

Not a variant that went into service but for completeness I'll leave the XP-51G here. Still Merlin powered, but with Griffon levels of power at 1,910 hp from its 145M.

[John_W]

Not a variant that went into service but for completeness I'll leave the XP-51G here. Still Merlin powered, but with Griffon levels of power at 1,910 hp from its 145M.

Quite pretty actualy.

The contra prop thing has got me thinking about the Shackleton and the Bear now, but this is the wrong part of the forum.

Also been thinking about blade counts on helicopters as well...