Thoughts on Propellers

[GregP]

The P-51 Mustang had a few different propellers on it. One was a Hamilton-Standard and another was from Aeroproducts. As I understand it, the Hamilton-Standard gave superior performance to any other prop on the Mustang. If I have that backwards, please correct me ... but I think not.

Anyway, I happened to live in Arizona, U.S.A. for 23 years and was mear the Champlin Air Museum while it was based there and Doug Champlin still owned it. Among his aircraft (WWI, WWII, Korea, nad Vietnam era) was a beautiful Focke-Wulf Fw 190D-9.

There are some interesting stories surrounding that particular FW 190 D-9 (now owned by the Museum of Flight in Seattle, Washington, U.S.A.

As an interesting aside, it was found that Doug Champlin's Fw 190 D-9 actually had the wings from the Smithsonian's Fw 190 D-9 and vice versa. This had now been corrected and the Doug's D-9 is now up for sale ... $2M US I think.

Anyway ... to the point. The Fw 190D-9 had a Junker Jumo engine in it. The propeller was made of wood. Nothing wrong with wood, but I wonder what the performance might have been if a metal prop of correct airfoil were fitted. Who knows, maybe the Ta-152 could have broken 500 mph with a metal prop, but Germany was in dire straights at the time and I'm sure fabricating a metal propeller for an experimental fighter in 1945 was probably not in the cards.

I might be dreaming, but I believe Dowty-Rotol also used wood blades on many of their 5-blade units for late-model Spitfires and wood blades on Tempests and early Fury and Sea Furt aircraft.

So ... does anyone out there know of any comparative tests that were run on WWII fighters with different propellers, possibly wood versus metal, along with some results?

As an example, a modern homebuilt designed and kitted in Oregon, U.S.A. is the Vans RV line of small, high-performance kitplanes (RV-3, -4, -6, -6A, -8, -9, -11, etc.). I had a friend who made one and he tried different propellers. He settled on a diameter and installed both wood and metal props on his RV-4. His top speed with the wood prop was 194 mph or so. A metal unit of the same diameter and pitch gave him 214 mph.

That is significant and I am wondering about WWII fighters since they were and are the peak of development of piston-engined aircraft. Any people with knowledge of this subject? Please quote sources if you know them. I am looking for facts here, not opinions.

[Romantic Technofreak]

Sorry, Greg, but I think you are the only real aircraft engineer here. I remember your hint with the P-51´s prop. We like to discuss with you, but I fear in so special things nobody else is competitive!

[ygb]

Aerodynamically, a wooden prop should have the edge. Structurally as well.
I'm pretty sure NACA used wood models for their earlier airfoil wind tunnel tests. If metal would have been easier to manufacture to the desired surface finish, why didn't they use metal instead?
Until the 50s or 60s, the only realistically usable composite material was wood.

In the example with the RV-4, there must have been differences in the prop's airfoil sections, spanwise twist distribution and such. In a full-throttle situation like that, the prop is turning at a constant rpm, so there is no inertia effect (due to different masses) playing a part. That would be the only noticeable difference between a wooden prop and a metal one, provided they are identical in all other respects.

However, a prop made of wood would (very likely) be thicker than an equivalent metal one, because of the different properties of wood and metals.
A thinner airfoil section can be made less draggy than a thicker one (for normal airfoils), so the metal prop would have the edge there.

[GregP]

I have nothing against wood props, particularly the WWII constant speed units. For aerobatics, I believe wood is superior due to the way it handles aerobatic stress.

I brought up the subject just to see if anyone had heard of comparative tests between wood and metal props on planes of the 1500 - 3000 HP variety.

Apparently not and I am satisfied with that. This is NOT an attempt to state tahht one or the other is better ... just fishing here.

[tenmmike]

not realy metal vs wood answere but a lot of ppl forget that the germans used 3 bladed props for a good reason 3 blades interrupt the cowel mounted guns less the a 4 blade will..=higher rof

[Corsarius]

Sorry Greg, I'm with RT on this one. My only thoughts on propellers is that they go round and round.

[Romantic Technofreak]

Incidentially there is a discussion about the same item on 12 o´clock high:

http://disc.server.com/discussion.cgi?disc=37919

Obviously it is impossible to branch directly into the article, so please scroll a little [u]to 3 versus 4 blade propellors — Dunash, Sat Jan 24 03:25</u>

[simon]

The only thing I'm aware of was the quite famous exploits of Geofrey De Havilland in the immediate build up to the Battle of Britain. On little more than a promise from Beaverbrooke that he would get paid he travelled round all the Hurricane and Spitfire squadrons in Britain supplying variable pitch propellors and training for their fitting and maintenance to the units. Apparently the change from two bladed fixed wooden props to three bladed variables added about 25mph to the speed of the fighters.

This is the only sort of example of this kind of thing that I'm aware of, and I'm afraid overall I agree with Corsarius and Romantic Technofreak, Propellors go round, they make planes move, that's about the limit of my expertise... ;)

[Lightning]

I believe durablity was also an important attribute of metal propellers on combat aircraft. They handled foreign-object dammage (FOD) better than wood. This could be a problem taking off at full power from an unimproved combat strip in the middle of a war zone.

Wood would also not tolerate the cold and damp conditions of Europe and the Alleutions nor the hot,humid weather of the tropics as well as metal (not to mention being sand-blasted in the desert).

There were numerous cases of aircraft returning to base with bullet holes through their propeller blades. A wooden blade would never have survived.

During aerial combat, there is a lot of debris in the air. Many times a fighter had to fly through the pieces of the plane it had just shot down.

Bombers had to wade through shrapnnel from A.A. fire.

Wooden propellers just could never take this kind of punishment.

[GregP]

I looked at the 3-blade versus 4-blade thread in 12 O'Clock High. The explanation was weak, but close.

A specific engine power will turn a specific propeller area at a specific RPM and pitch. The diameter is chosen for ground clearance as stated in the other forum, but the coarse pitch of the prop system is determined by the maximum dive speed expected, the fine pitch is determined by the takeoff thrust neded for the required takeoff run. Once this had been determined, the area can be calculated.

Once you know the area that can be turned at the required RPM, you can figure the blades. The aerodynamicists among us will regognize that aspect ration (span to chord ratio) affects efficiency. A long, narrow blade is more efficient than a short wide one. So ... some designers choose five narrow blades, some chose 4 wider ones, and some choose 3-fat blades for a given WWII fighter engine.

In the case of the Germans, the above post is right. Most German aircraft had nose-mounted machine guns, so rate-of-fire coupled with synchronization was important for weight of fire considerations. Hence, the Bf 109 had 3-blades for most of its service life. Ditto the Fw 190. To use the available power, their blades were wide in the middle part of the prop, but narrowed toward the tips for efficiency.

The formulas are interesting to a techo-geek, but probably boring for the rest of us.

[Lightning]

Another consideration: Tip speed is greater for a large-diameter propeller than for a smaller diameter at the same rpm. It ls imperative that the tips remain below the speed of sound for obvious reasons. Four blades provide the same thrust with a smaller diameter than three blades at a given rpm, and thefore at a lower tip speed.

[Wuzak]

quote:Originally posted by Lightning

Another consideration: Tip speed is greater for a large-diameter propeller than for a smaller diameter at the same rpm. It ls imperative that the tips remain below the speed of sound for obvious reasons. Four blades provide the same thrust with a smaller diameter than three blades at a given rpm, and thefore at a lower tip speed.


Interesting that the Spitfire MkXIV had propellor tip speeds just supersonic...

quote:5.2 Propeller tip mach number. The propeller tip mach number reached a value of 0.993 at the FS full throttle height, and 1.015 at the highest altitude tested.

http://www.spitfireperformance.com/jf319.html

[montanamotor]

On props:

Metal prop-blades are WAAAAAY simpler and, cheaper to produce than wooden ones: Hang a lump of steel in a profile-copy-grinder, swittch to "on" and off you go with the metal-blade.

Manufacturing wooden prop-blades since 1940 in Germany were a specialty of SCHWARZ company, situated close to Potsdam/Berlin. These blades weren't just made out of laminated wood, but were STEAM-PRESSED during this process down to 1/3rd of their former thickness. Hence, the material carried 3 times the tensile strength of "naturally" laminated wood.

The added leading edge of said "wooden composite blades" (that's their real name) then was usually made from aluminium or sometimes, steel.

The structural and, aerodynamical qualities of wooden blades were in many publications considered SUPERIOR to metal-blades. Wood-composite-blades have far better damping of wibration in any direction - radial, longitudinal - than steel, They tend to be substantially lighter, and they can be ground to any profile conceivable. Steel (and, Aluminium) on the other hand, are cheaper to produce, show better resistance against humidity, against weather-imposed wear and - ground contact: If bent, a metall blade can often be reshaped and cleared for further use. But - you never bend a wooden prop...

About piercing props with bullets: No problem with wood. as long as it is not a HE-grenade that hits the blade. A hole remains a hole. That's it. But this goes for metal-blades, too.

For more info's on wooden blades: Check the websites of german companies Hoffmann-propeller and, MT-propeller, who both offer high-end props made from wood-composite (MT being the legal successor and, patent-holder of said compressed composite wood manufacturing-process). They are definitely in the know.

It is true that, in modern top-class acrobatic airgraft, like Extra 300, metal props prevail, because they withstand the multiple change of extremely high load factors better than wood does. But this concerns the blade-ROOTS, not the blade itself. Where the blade is attached to the propeller hub, it's a metal-on-metal-affair for metal-probs - but a wood-on-metal-affair for wooden composite-blades. Wood is softer than metal. In an environment, where extremely high and, quickly alternating loads occur, a metal-hub step-by-step wears out the wooden prop-blade, up to the point where, a save fixing to the hub is no longer guaranteed. Here, the greater flexibility of the wooden blade, compared to metal, works against it.

Anyway: To this extreme extend it never happened in WWII-fighters. A little number for comparison-reasons: A Spitfire rolls at a 120-degree/second rate maximum. An Extra 300 rolls at a 450 degree/second rate maximum. The relation is similar in magnitude for the other twoo axles, also. Fighters simply CANNOT fly such violent maneuvers as modern acrobatic aircraft do.

About the de Havilland-prop-exchange: This was more a "constant speed-vs.-fixed prop" thing than anything else. And - why the british two-bladed wooden props were so chunky, when being compared to the german ones: Well - the brtish wooden props just weren't made from COMPRESSED laminated wood, but were made of of plain laminated wood. They HAD TO BE so thick and wide in root and profile, simply to withstand the G-forces imposed on them. Ya; know: Compressing the blades in a steam-heated form was a GERMAN patent... Sorry.

CHeers!

Montanamotor[8D]

[Red Admiral]

You want a wooden prop like this. http://www.enricopezzi.it/fam_pezzi/mario_pezzi/images/MP_1937%20C161_02w.jpg

Typically larger diameter props have fewer blades as this is more efficient. As you go to faster speeds the prop blades get shorter but you can have more of them for higher efficiency at higher speeds. Generally, 1-blade on big wind turbines, 2-blade early aircraft, 3-blade modern smaller windturbines and aircraft, 4-blade for increased power + speed available e.g. P51, 5-blade late spitfires, 6-blade contraprops late spitfires again, propfans with masses of blades now for 450-500mph cruising speed.

[Corsarius]

In the two years since I last posted on this thread I have learned that the propeller is actually just a big fan in front of the aircraft.

It's main reason for existing appears to be that it keeps the pilot cool. Just stop it in mid-flight and watch him start to sweat.

[Wuzak]

quote:Originally posted by Red Admiral

You want a wooden prop like this. http://www.enricopezzi.it/fam_pezzi/mario_pezzi/images/MP_1937%20C161_02w.jpg

Typically larger diameter props have fewer blades as this is more efficient. As you go to faster speeds the prop blades get shorter but you can have more of them for higher efficiency at higher speeds. Generally, 1-blade on big wind turbines, 2-blade early aircraft, 3-blade modern smaller windturbines and aircraft, 4-blade for increased power + speed available

I'm not sure that you would ever want a one blade prop - it would lead to other problems due to balance issues.

As for wind turbines, the ones we have here in Tassie have, I'm quite sure, 3 blades. The largest of the wind turbines we have is around 2MW generating capacity - so not small!

[GregP]

Actually, 1-bladed props work BEST, with proper counter-weight in the prop hub. I have used them on one one or two occasions on RC aircraft to great benefit, but there ARE ground clearance issues and if you nick it, there's all that custom work to do again to make another one.

Eventually, I stuck with 2-blades for RC ... and have never regretted it.

[Red Admiral]

The one-blade props have a blade and a counterweight on the other side. They look a bit strange.

Wind turbines typically have 3-blades, I think this might be to cope with varying windspeeds so they are more efficient. At higher windspeeds you're going to be generating more power, so you want a prop that is more efficient at higher speeds - 3 blades.

Corsarius, what about a pusher prop though :)