Why no paddle prop on Spitfires?

[vinnye]

I have seen a variety of propellors used on Spitfires - from the early 2 blades, to three then four blades.
Even later still contra rotating set ups.
But, I can not remember ever seeing a paddle type propellor being used - similar to the P47, P51 and even some Mosquitos and Lancasters?
If it worked on these aircraft, then why not on the Spitfire?

 

[Shortround6]

Please define "paddle" type prop?

top to bottom.

Spitfire prop.
P-47N (paddle) prop
P-47C (toothpick) prop

 

[swampyankee]

Overall, if you've got a fixed diameter, and a fixed total blade area, the induced losses are less for a propeller with more blades. Also, there are manufacturing issues involved: the paddle blades are (most likely) heavier, so they'll need a different hub, and different bearings, and the forces needed to change blade pitch go up, iirc, by the cube of the blade chord, so the entire pitch change mechanism has to be different. Going to five blades lets you get the area without needing to manufacture a completely new set of blades and bearings. The pitch change mechanism would also be lighter with five blades vs four of the same total area.

So, I can ask, why did the US manufacturers go to paddle blades when increasing the number of blades would probably be an easier, and possibly better solution? I suspect it's manufacturing again, as a four-bladed propeller would have a lower parts count and the paddle props would be somewhat less susceptible to FOD. There may also have been vibration problems, at least with the radial-engined aircraft, that meant the 5/rev from a five-bladed prop excited some frequency or another in the R-2800.

 

[wuzak]

I'd also ask what is the main reason for changing over in those other aircraft, and does that apply to the Spitfire?

For the Mosquito, the paddle blade props were for high altitude performance. For the P-47 I'm guessing it was for climb performance. Possibly the same for the P-51.

Note, for the Sea Mosquito they changed back to needle blade props, but with 4 blades instead of 3. This was to give better take-off acceleration.

 

[swampyankee]

Aircraft propellers are almost always operating in stall during part of the takeoff run, so increasing blade area would probably increase thrust during the takeoff run. The increased area would also reduce efficiency during cruise and high speed flight, except, possibly at extreme altitude.

Optimum blade area (total blade area) is determined by the diameter (usually fixed by the landing gear), rpm (usually fixed by the engine), horsepower, and operating altitude. When you uprate the engine you've got three choices, then: increase propeller diameter, which is usually not possible, increase propeller rpm, which may be possible, but normal practice is to keep tip speed under about 850 fps, so the possible increase in rpm is small, or increase total blade area. Increasing chord -- the preferred method in the US -- is one way; the other is to increase the number of blades, which seemed to be the preferred method in Britain. Which is better? From strictly aerodynamic grounds, increasing the number of blades is better as long as the blades are not so small as to have problems due to low Reynolds' Number effects.

 

[vinnye]

I found some images of Spitfire propellors ;
2 bladed ;
http://www.ipmsusa3.org/reviews/Details/Aircraft/ultracast_48/ultracast_48_spitfire_prop1_t.jpg
3 bladed ;
http://farm1.static.flickr.com/145/393079521_8af6a4dfae.jpg
4 bladed ;
http://i.istockimg.com/file_thumbview_approve/1195831/2/stock-photo-1195831-spitfire-propellor.jpg
5 bladed ;
http://farm1.staticflickr.com/65/219049320_18df494efa_o.jpg
All of the above I would not describe as paddles.

 

[vinnye]

I also found this thread discussing props on the P47.
http://www.ww2aircraft.net/forum/aviation/p-47d-paddle-blade-propeler-she-dogfighter-986-2.html

 

[OldSkeptic]

In very simple terms, the P-47 was a hot ship at high altitude right from the start but climbed like a brick, well a brick with another brick added.

Therefore the key issue was to improve its climb (I originally mistyped limp instead of climb , but that was accurate) performance even if it meant trading off some straight line speed (which, at high altitude it could easily afford).
The Spit was one of the climbing kings (along with 109 and they traded places throughout the war for first place, with that 'intruder' the 190 having a very short place in the sun). Therefore more speed was the key issue

Thus the drive for performance improvement was focused differently.

 

[Shortround6]

I found some images of Spitfire propellors ;


All of the above I would not describe as paddles.

What constitutes a "paddle blade" prop then?

This is a P-47 "paddle blade". What cord or length to cord marks the difference or is it tip shape?

Or is it marketing or a short cut in describing propellers fitted to the same airplane?

 

[wuzak]

Any blade looks skinny on a P-47....

 

[Aozora]

I have seen a variety of propellors used on Spitfires - from the early 2 blades, to three then four blades.
Even later still contra rotating set ups.
But, I can not remember ever seeing a paddle type propellor being used - similar to the P47, P51 and even some Mosquitos and Lancasters?
If it worked on these aircraft, then why not on the Spitfire?

What is your definition of paddle bladed? Unless you are stating that paddle bladed props of necessity had to look like those on German aircraft, such as the Ta 152H, there were plenty of Spitfires with paddle bladed propellers; eg the Mk III, VBs and VCs with Rotol propellers

Spitfire XIV

Spitfire F. 21

and the Spiteful XVI

If these are not "paddle bladed' propellers then your definition of paddle bladed is far too narrow...

 

[vinnye]

I guess my "picture" of a paddle prop is one of the P47 types.
They definitely appear to be more rounded at the tip - whereas some of the Spitfire types have a more pointed tip.

 

[spicmart]

After the war the german type of paddle blade props all but
vanished (I think) with the allied types prevailing apparently.
I once read that the german props were better for climbing
performance whereas the allied ones gave better possible max speed.

May it that in peace time max speed was deemed more preferable
than climb rate, as it was not important any more to gain altitude/energy
advantage against a foe because there wasn't any?

What do you guys think?

 

[Shortround6]

There are books that devote hundreds of pages to propeller design.

ALL propellers are a compromise so there is no one BEST design.

A propeller that works great at sea level (or even at Reno) may NOT be a good propeller at 25-30,000ft. The air up there is 1/2 to 1/3 as dense so you need more diameter and more blade area for the same amount of power.

A really good prop for 30,000ft is not a good prop at sea level. The extra size of the larger blades just create more drag.

A lot of prop design (or selection) depends on which compromises you wish to make and where on the scale or balance you want to be.

Please remember that both propeller design AND construction were continuing to evolve. Even if somebody had the idea/theory for some of the prop blade shapes used in the late 40s or 50s they might not have been able to fabricate them.

 

[swampyankee]

There are books that devote hundreds of pages to propeller design.

ALL propellers are a compromise so there is no one BEST design.

A propeller that works great at sea level (or even at Reno) may NOT be a good propeller at 25-30,000ft. The air up there is 1/2 to 1/3 as dense so you need more diameter and more blade area for the same amount of power.

A really good prop for 30,000ft is not a good prop at sea level. The extra size of the larger blades just create more drag.

A lot of prop design (or selection) depends on which compromises you wish to make and where on the scale or balance you want to be.

Please remember that both propeller design AND construction were continuing to evolve. Even if somebody had the idea/theory for some of the prop blade shapes used in the late 40s or 50s they might not have been able to fabricate them.

Having been an aerodynamicist for the major US propeller manufacturer, there are numerous books, papers, and monographs devoted to propeller design, many of which sit in corporate libraries and don't circulate, never to be seen by outsiders

There are a lot of design issues with propellers, which I won't even start to go into here, but I'm just amazed at how reliable and robust propellers are in service.

After the war the german type of paddle blade props all but vanished (I think) with the allied types prevailing apparently. I once read that the german props were better for climbing performance whereas the allied ones gave better possible max speed.

May it that in peace time max speed was deemed more preferable than climb rate, as it was not important any more to gain altitude/energy advantage against a foe because there wasn't any?

What do you guys think?

Trying to dig into my prop aero memories, here...

You can, by playing with twist, airfoils, chord distribution, etc, shift the efficiency peak of a propeller to a different condition, but the difference is likely to be small. Overall, I think the reason that Allied types prevailed post-war is that the US and UK did more engineering development of propellers than did the Germans. In addition, I suspect that Allied quality during the war was much better than that of the Germans: while automated machinery (programmed by cams) did exist during ww2 (google Brown Sharpe Screw Machine, four-slide machine, gang milling, ....), it was usually not able to accurately and smoothly reproduce airfoil surfaces, so quite a bit of hand work would be required (guys with grinders and templates.)

So, what do I think? German-style propellers had fairly little commercial success prior to WW2, and their near-complete concentration on military aircraft meant that they had no skill marketing except to the Luftwaffe. By the way, I believe Junkers propellers were based on Hamilton Standard licenses; VDM, as far as I can tell, were developed without any explicit outside help (as I've said before: aero guys talked to each other, a lot, and there was relatively little effort made to keep the basic technology of constant-speed propellers secret. Proprietary, yes, but not secret.).

 

[tftoc]

Having been an aerodynamicist for the major US propeller manufacturer, there are numerous books, papers, and monographs devoted to propeller design....

Which ones would you recommend from an engineering perspective?


IIRC the wide chord props on the mosquitos and lancasters etc were Hamilton Standard props license built by DH. They are recognisable by the lack of taper in the outer portion of the blade.

 

[swampyankee]

Which ones would you recommend from an engineering perspective?


IIRC the wide chord props on the mosquitos and lancasters etc were Hamilton Standard props license built by DH. They are recognisable by the lack of taper in the outer portion of the blade.

iirc, DeHavilland, Junkers, and FIAT all held licenses from Hamilton Standard before WW2. VDM and Rotol were not HSD licensees; I don't know about Ratier.

For books and papers, the easiest place to search is NASA's Technical Report Server: Home Page - NASA Scientific and Technical Information (STI) ProgramNASA Scientific and Technical Information (STI) Program | Collect, organize, provide access, and preserve NASA (this is the search I tried: NASA Technical Reports Server (NTRS) )
Other places are Cranfield, DLR, and ONERA. Largely, the same aerodynamics is involved in helicopter rotor design and in wind turbine design. Fred Weick's book is classic, albeit outdated.

MIT has some OpenCourseWare: 11.7 Performance of Propellers
Hydrofoils and Propellers | Mechanical Engineering | MIT OpenCourseWare (yes, it's marine propellers. There are differences in the fluid mechanics, in that cavitation is only applicable to marine propellers and compressibility is only is applicable to aircraft propellers. Lifting surface theory was applied first to marine propellers, as marine propellers tend to have much lower aspect ratios, so lifting line theory is a worse approximation than it is for aircraft propellers).

 

[Shortround6]

I could be wrong but the main part of the license is for the "hub". The pitch change/control mechanism.

Some "hubs" could accept a variety of blade shapes and lengths as long as the hub or the attachment sleeves were not over loaded.

Blade shape could depart from the original Hamilton Standard or other patent holder considerably and yet the patent on the hub mechanism would still be valid.

Trying to patent a blade shape or air foil is like trying to patent a wing shape or air foil.

You could patent blade "construction" if it was new, like a folded and welded hollow blade vs a wood blade or flat plate solid blade( and even wood blades differed from WW I or private plane wood blades).