Propellers In German Wwii Aircraft

[sunday]

Gents,

 

A while ago, reading about propeller technoloy, came to me the idea that, while Western allies used extensively four (and more in the case of the Brits) blade propellers, Luftwaffe fighters seems to made a scarce use of them. They used mainly three-bladed ones, with quite big chords.

 

Does anybody knows why?

 

Best regards,

 

Jose

[DKTanker]

Gents,

 

A while ago, reading about propeller technoloy, came to me the idea that, while Western allies used extensively four (and more in the case of the Brits) blade propellers, Luftwaffe fighters seems to made a scarce use of them. They used mainly three-bladed ones, with quite big chords.

 

Does anybody knows why?

 

Best regards,

 

Jose

156462[/snapback]

 

The fewer the blades the more efficient the propeller. OTOH, the greater the number of blades the smaller the propellar arc. Usually more powerful motors have more blades to decrease the amount of space required.

[lastdingo]

Me109 and Fw190 used synchronized guns. One more blade would have ruined the ROF and therefore the firepower especially of the Fw190A series.

 

Further, 4-blade props are best at high altitude, but 109 and 190 were not very good at high altitude due to lack of multi-stage superchargers or turbochargers anyway (just very very few late variants had that - the others had the heavy GM-1 oxidator injection system at best).

 

DKTanker, it was about tip speed reaching sonic barrier, not so much about span itself.

[sunday]

The fewer the blades the more efficient the propeller.  OTOH, the greater the number of blades the smaller the propellar arc.  Usually more powerful motors have more blades to decrease the amount of space required.

156469[/snapback]

 

Thanks, it's reasonable.

 

But the P-51D with 1450HP had four blades, while the Fw-190D9, with its slighty more powerful Jumo 213A (and with MW50, a 30% more powerful) had only three.

 

I was thinking more in terms of propeller technology: Did Germans use hydraulic propellers? Was there any difficulties with high blade count propellers? Was there some design considerations of the technical culture type that differed between Western and German designers? Maybe a standarization effort by part of the Americans?

 

Best regards,

 

Jose

[Pachy]

Was there any difficulties with high blade count propellers? Was there some design considerations of the technical culture type that differed between Western and German designers?

156481[/snapback]

As Lastdingo said, gun synchronization was an issue. No late-war US or British fighter had weapons firing through the propeller arc, but of course every variant of the Bf 109 and FW 190 did (Do 335 too, for the anecdote).

[sunday]

As Lastdingo said, gun synchronization was an issue. No late-war US or British fighter had weapons firing through the propeller arc, but of course every variant of the Bf 109 and FW 190 did (Do 335 too, for the anecdote).

156487[/snapback]

 

Yes, and this helps to explain the change from the P-51A (and A-36) to the P-51B. About the Do-335, maybe they could have deleted the MG-151 ...

 

Me109 and Fw190 used synchronized guns. One more blade would have ruined the ROF and therefore the firepower especially of the Fw190A series.

 

Further, 4-blade props are best at high altitude, but 109 and 190 were not very good at high altitude due to lack of multi-stage superchargers or turbochargers anyway (just very very few late variants had that - the others had the heavy GM-1 oxidator injection system at best).

 

DKTanker, it was about tip speed reaching sonic barrier, not so much about span itself.

156479[/snapback]

 

Well it also is very reasonable and pretty much convinced me about the single-engine fighters, especially accounting for the dismal performance at altitude of most of them. More so remembering the strange things Germans attempted with one engine inside the fuselage driving a supercharger to feed the engines in the wings ...

 

What about the He-219? It had pressurized crew compartment (so it looks like a high altitude fighter) but three-bladed screws again.

 

I googled the Ju-86R recon plane and it had four bladed ones, but was a very special airplane, with only 2000HP total.

 

Notwithstanding the Uhu, I think the question is settled. Many thanks,

 

Jose

[Mikel2]

In those planes with guns firing through the propeller hub, how did that work? Was the propeller hub geared and offset from the engine, allowing for room for a gun?

[Colin Williams]

I understand Hitler wanted 4-blade propellers of a particular shape, but Ernst Udet told Adolf that the spinning swastika just didn't work, getting bumped off later for being cheeky.

 

[5150]

Propeller diameter shouldn't be so easily dismissed. There's a balance to be found in blade length/speed of the tips/landing gear length. Chord of the blade comes into play as well. All things being equal, fewer blades are preferable to more blades.

[DB]

In those planes with guns firing through the propeller hub, how did that work? Was the propeller hub geared and offset from the engine, allowing for room for a gun?

156502[/snapback]

 

Prop driven aircraft used reduction gearboxes.

 

For example, see this trial:

 

http://www.fourthfightergroup.com/eagles/bs310.html

 

for comparison of a different reduction gear and prop combination.

 

David

[sunday]

Prop driven aircraft used reduction gearboxes.

 

For example, see this trial:

 

http://www.fourthfightergroup.com/eagles/bs310.html

 

for comparison of a different reduction gear and prop combination.

 

David

156507[/snapback]

 

Yes, gearboxes of the epicycloidal ilk, with the input shaft and the output shaft in the same axis. Radial air cooled engines can not use guns firing through the hub because of the master rod getting in the way. But I think inline engines should use a first stage with spur (and surely helical too) gears to offset the axis of the prop from the axis of the crankshaft.

 

There is a nice .pdf explaning part of the development of the Pratt&Whitney R-2800 here.

 

Also, this site is wonderful.

 

[edit: more of the same ... sigh]

Edited March 22, 2005 by sunday

[DKTanker]

Propeller diameter shouldn't be so easily dismissed. There's a balance to be found in blade length/speed of the tips/landing gear length. Chord of the blade comes into play as well. All things being equal, fewer blades are preferable to more blades.

156505[/snapback]

 

The P47 and F4U being excellent examples. Even though they both used 4 blade props they still required some engineering to gain ground clearance. Not telling you anything but for those that weren't aware; the P47 landing gear would extend 8 or 9 extra inches while being lowered to gain ground clearance and the wing of the F4U is cranked as it is to gain ground clearance for the prop. Interesting really, both aircraft used the same engine, I believe, with two entirely different solutions for ground clearance.

[5150]

Yup. The later Spitfires are examples of another approach to the problem, where the airframe couldn't be modified to harness all of the power so many more blades had to be added. It wasn't because more blades and contra-rotating props were desirable in any way--it was just because there was no other way to make use of the power installed.

[GregShaw]

Yes, gearboxes of the epicycloidal ilk, with the input shaft and the output shaft in the same axis. Radial air cooled engines can not use guns firing through the hub because of the master rod getting in the way. But I think inline engines should use a first stage with spur (and surely helical too) gears to offset the axis of the prop from the axis of the crankshaft.

Ummm, most of them did use a simple spur gear. Take a look at an Allison or a Merlin, V-60, crankshaft at the bottom, propellor shaft at the top. That is not an inline, epicyclic gear, it is a simple spur gear. In fact the P-38K was rejected for production because the larger diameter spur gear necessary for the higher reduction ratio of its engine raised the thrust line several inches. Which would have required a completely new spinner/cowling than that in use already, so the WPB rejected its production, unfortunately.

 

Greg Shaw

[Jeff]

I understand Hitler wanted 4-blade propellers of a particular shape, but Ernst Udet told Adolf that the spinning swastika just didn't work, getting bumped off later for being cheeky.

 

156503[/snapback]

 

This guy got close and managed to stay airborne. Though I bet it took a proctologist to get the seat cushion out of his butt.

 

[lastdingo]

Yup. The later Spitfires are examples of another approach to the problem, where the airframe couldn't be modified to harness all of the power so many more blades had to be added. It wasn't because more blades and contra-rotating props were desirable in any way--it was just because there was no other way to make use of the power installed.

156677[/snapback]

 

The counter-rotating propellers could negate most of the torque, making starting much easier. The landing gear of the late Spitfires was still as narrow as ever, but the engine power about twice as much as originally.

Torque was not only a problem during starts, but also in a dogfight when you want to roll against it. Most prominent example was the Sopwith Camel (for a different reason) - and the Spitfire was already very inferior to the 190 concerning roll speed and control.

 

Overall, I do think that counter-rotating propellers had significant advantages ("desirable in any way"). And since they were used mostly in pre-war Russian designs (Tu-95 Bear for example, or the recent An-70), I think they are at least not hopelessly inferior in aerodynamic efficiency.

[Ol Paint]

Actually, counter-rotating propellers are usually MORE efficient from an aerodynamic perspective, thanks to swirl recovery, lack of a requirement to use drag-inducing surfaces to counter torque, etc. However, they are mechanically complex and difficult to get to work reliably/maintain. Well done, they work great, but you have to be willing to pay the mechanical price.

 

Douglas

[Geoff Winnington-Ball]

This guy got close and managed to stay airborne. Though I bet it took a proctologist to get the seat cushion out of his butt.

156733[/snapback]

 

LMAO!!!!!!!!!

 

I'll also bet that engine was junk once he got it down...

[Doug Kibbey]

The P47 and F4U being excellent examples.  Even though they both used 4 blade props they still required some engineering to gain ground clearance.  Not telling you anything but for those that weren't aware; the P47 landing gear would extend 8 or 9 extra inches while being lowered to gain ground clearance and the wing of the F4U is cranked as it is to gain ground clearance for the prop.  Interesting really, both aircraft used the same engine, I believe, with two entirely different solutions for ground clearance.

156525[/snapback]

 

Yep....the reason for the Corsair approach (bent wing ) was that for projected carrier landings, longer gear=more spindly and fragile gear, which was not an option as it was for ground-based aircraft like the P-47.

[Pachy]

In those planes with guns firing through the propeller hub, how did that work? Was the propeller hub geared and offset from the engine, allowing for room for a gun?

156502[/snapback]

Other have replied on the reductor/offset issue, but you also need the engine to be more or less specifically buit. On the Hispano 12Y and its Klimov derivatives as well as the DB601 the cannon is between the cylinder banks. The Merlin has its carburators between the cylinder banks, so there was no way to fit a cannon here.

[Argus]

Patchy, the Carbies were back before the supercharger, the only thing between the Merlin's banks was the induction manifold. You're thinking of the unsupercharged Kestrals.

 

The Merlin and Griffon were just not designed to take a coaxial gun, the priority was on drag reduction and the like.

 

shane

[sunday]

Ummm, most of them did use a simple spur gear. Take a look at an Allison or a Merlin, V-60, crankshaft at the bottom, propellor shaft at the top. That is not an inline, epicyclic gear, it is a simple spur gear. In fact the P-38K was rejected for production because the larger diameter spur gear necessary for the higher reduction ratio of its engine raised the thrust line several inches. Which would have required a completely new spinner/cowling than that in use already, so the WPB rejected its production, unfortunately.

 

Greg Shaw

156722[/snapback]

 

Yes, it make sense, more so with the low reduction ratios used.

 

So I suppose the epicycloidal ones in radials were intended to not get the gearbox in the path of cooling air, and to maintain the thrust line coaxial with crankshafts' axis.

[GregShaw]

Yes, it make sense, more so with the low reduction ratios used.

 

So I suppose the epicycloidal ones in radials were intended to not get the gearbox in the path of cooling air, and to maintain the thrust line coaxial with crankshafts' axis.

157116[/snapback]

I doubt that cooling would have been an issue, the gears where oil cooled, not air cooled IIRC. I suspect that keeping the thrust line in line with the crankshaft was a legacy of the older direct drive radials. They would be usable for the same applications as the direct drive models without having to worry about different thrust lines.

 

I'm sure there were other advantages as well. Probably had some effect on deflecting cooling air up into the cylinders, making cooling more effective. Besides, a big lightly loaded planetary gear isn't going to increase the frontal area of a radial like it would an inline. And you can probably get away with lower quality materials than those necessary on a highly loaded spur gear of an inline.

 

Greg Shaw

[Tony Williams]

The Merlin and Griffon were just not designed to take a coaxial gun, the priority was on drag reduction and the like.

 

A pity, that - it was the perfect location for a cannon. A Spitfire with three 20mm cannon would have the same armament weight as the usual 2x20mm and 4x.303, but with considerably improved effectiveness.

 

Tony Williams: Military gun and ammunition website and discussion

forum

[DougRichards]

Gents,

 

A while ago, reading about propeller technoloy, came to me the idea that, while Western allies used extensively four (and more in the case of the Brits) blade propellers, Luftwaffe fighters seems to made a scarce use of them. They used mainly three-bladed ones, with quite big chords.

 

Does anybody knows why?

 

Best regards,

 

Jose

156462[/snapback]

 

Some German WW2 aircraft did use 4 bladed props, for instance the He177, the early Dornier Do3, Dornier Do317, Junkers Ju388. All were large aircraft, that were not meant for particularly high speed.