cherry blossom
Senior Airman
- 513
- Apr 23, 2007
Is the German preference for three bladed propellers related to the use of a synchronised armament?
Propeller Design
- Thread starter TheBadger
- Start date
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FLYBOYJ
"THE GREAT GAZOO"
And your reference for that?
If you're talking a fixed pitch prop, the pitch can be adjusted for better climb or cruise. A constant speed prop negates this.
Shortround6
Major General
Some constant speed props can be adjusted for initial pitch( using the wrong word here). The props had a certain pitch range they would operator over (like 20 degrees or 30 or 35) and the prop blades could be mounted so that the blade at a certain point would have a certain pitch. However this might (repeat might) only be an advantage in certain conditions.
It depends on the speed range. A 20 degree pitch change might be all that is needed for a 250mph airplane that takes off at 80mph while a 400 mph airplane that takes off at 100mph might very well want the 30 degree or higher pitch change. Depending on what they are trying to do they may find that shifting the pitch range a few degrees might give better performance. A 20 degree pitch hub is only going to change 20 degrees, however the 30 degree hub is going to try to move 30 degrees as the governor dictates. The pilot and mechanics cannot limit the hub to 28 or 27 degrees for example.
Please note that the actual pitch of the prop is going change between max speed at sea level and max speed at 20,000ft. So where you set the pitch for max speed may depend on a few other things?
A P-47 was doing about 330-335mph at sea level at max rpm. It did around 425-430mph above 25,000ft at the same engine/prop rpm.
I don't know if the prop was at max pitch at sea level or not. It may not have been in order to keep the engine from over revving.
This may have been another factor. The props may have been set to keep the rpm at a certain point while diving at max pitch and not hit the pitch limit in level flight?
Just thinking as I write this so I may have jumbled things up.
Multi engine aircraft often had greater movement of the hub in order to allow the prop to go to full feathering.
FLYBOYJ
"THE GREAT GAZOO"
I know we've discussed this before...
In an aircraft with a constant speed prop (at least the ones I've flown), you have a take off setting where you get smaller "bites" from the prop so you can get yourself accelerating down the runway (like low gear in your car. Without going into great detail, once you're in the climb you have a setting that involves RPM and manifold pressure. At cruise you adjust for optimum prop/ manifold/ RPM setting for best range.
View: https://www.youtube.com/watch?v=hCeJxCbHy3w
Shortround6
Major General
I may well have gone off on a tangent.
It is possible to design propellers for different uses even using a constant speed hub. As was mentioned earlier the Germans seemed to have used broader blades.
Prop design is close to magic/witchcraft so unless you have the designers notes we are just guessing.
However a speed prop is best if it is small in diameter and has (at low altitudes) thin or narrow blades.
At high altitude you need more diameter to grab more air.
For climbing you want to use a bigger diameter prop to grab more air, a big reason bomber and transport propellers were bigger in diameter. They weren't going to go as fast and the bigger diameter could move more air before tip speed became a problem.
Props are spinning airfoils and are subject to a lot of things that wings are subject to. The prop blades can have different airfoils, different thicknesses, different cords Which affects the thickness ratio. Bigger blades (either longer=diameter) or wider (cord) have more surface area and more drag. and different thickness (we talk a lot about thickness percentage differences). Most props top out at about 80% efficiency but a prop designer may have biased which direction he lost some of that 20% in.
It is possible to design propellers for different uses even using a constant speed hub. As was mentioned earlier the Germans seemed to have used broader blades.
Prop design is close to magic/witchcraft so unless you have the designers notes we are just guessing.
However a speed prop is best if it is small in diameter and has (at low altitudes) thin or narrow blades.
At high altitude you need more diameter to grab more air.
For climbing you want to use a bigger diameter prop to grab more air, a big reason bomber and transport propellers were bigger in diameter. They weren't going to go as fast and the bigger diameter could move more air before tip speed became a problem.
Props are spinning airfoils and are subject to a lot of things that wings are subject to. The prop blades can have different airfoils, different thicknesses, different cords Which affects the thickness ratio. Bigger blades (either longer=diameter) or wider (cord) have more surface area and more drag. and different thickness (we talk a lot about thickness percentage differences). Most props top out at about 80% efficiency but a prop designer may have biased which direction he lost some of that 20% in.
I actually do not remember anymore where I read this. But it must have been on one of the forums I frequent.
I remember it was about the shape of the blades.
MiTasol
1st Lieutenant
Only if it is a ground adjustable prop like a Ham Standard.
All fixed pitch wood props are totally locked on what they were manufactured at and the fixed pitch one piece metal props can only be re- pitched (twisted) in a dedicated prop shop.
FLYBOYJ
"THE GREAT GAZOO"
You can have wide blades made so a bigger "bite" is taken to get the most efficiency, the most common example is the P-47 paddle blade prop. I do know there were some radical prop blade configurations made on post-war Bf109s, but I don't think there was an intent to get just one performance parameter (climb) out of the aircraft.
FLYBOYJ
"THE GREAT GAZOO"
Or should have mentioned a hydromatic propeller
Apparently post-war the round blades found on German aircraft seemed to have vanished pretty quickly. The squarer blades, like those on Allied aircraft, seemed to have prevailed. Nowadays you see angular blades everywhere. In peacetime speed is more important than climbing ability. Maybe that's the reason?
FLYBOYJ
"THE GREAT GAZOO"
Speed makes good headlines and sells airplanes
Deleted member 68059
Staff Sergeant
- 1,058
- Dec 28, 2015
The hubs in constant speed units are unbelievably expensive and complex. If you have an aircraft with a 3-blade hub and need to get more power
absorbed, you`d need to be REALLY keen to develop a whole new 4-blade hub, over just getting some new wooden blades made up with
a wider blade chord (also insanely expensive, but at least just about feasible).
Lower blade count and narrower blades are both better for efficiency (at least in subsonic applications, for trans-sonic or supersonic
I dont know enough to say). The main goal seems to be to minimise wash disturbance for the trailing blade to pass through. A
wider blade isnt in itself necessarily more inefficient, but it (so I read) makes the wash bigger which makes the efficiency of the trailing blade
worse, and since they`re ALL the trailing blade...haha... you see how it goes.
FLYBOYJ
"THE GREAT GAZOO"
Propeller science 101 - there is always a compromise!
Shortround6
Major General
There may have been a problem in that engines available were ex (or never installed) bomber engines with a different gear ratio. The 109 didn't have the ground clearance to fit the larger diameter bomber props. The props used were an attempt to use a smaller diameter prop to harness the power of the engine while turning slower than a fighter propeller would.
There were some other problems with the installations
MiTasol
1st Lieutenant
Ground adjustable is only able to be adjusted on the ground by slacking the clamps and moving the blade to a different index mark.
Hydromatic is fully automatic constant speed. Bracket (counterweight) props can be two speed or constant speed
A film from ww2 about the propellers.
GregP
Major
One other item.
The Germans liked the guns in the fuselage. That means they get more bullets through the prop with fewer blades. So, they mostly stuck with 3 blades and went to broader chord to absorb more power.
The U.S.A. and the UK generally, not always, put guns in the wings. That means the designer didn't have to maximize bullets through the propeller arc and was free to put on whatever propeller he wanted. As power increased, that meant broader prop chord, more blades, or both. By the time they were getting 2,000+ hp, that meant REALLY broad chord for a 3-blade or fat 4+-bladed props.
Look at the chord on a model of a Ta 152:
Note the inboard guns go through the prop arc.
Look at the prop chord on a late-model P-47:
That's one of the so-called "paddle-bladed" props from Curtiss-Electric. The Hamilton prop was more symmetric in shape.
Look at one of the prototype Tempests. Nothing special about the prop chord, but it GOT that way later:
Then there's this monster to absorb the power from a Griffon:
Can't help but notice those radiators!
The Germans liked the guns in the fuselage. That means they get more bullets through the prop with fewer blades. So, they mostly stuck with 3 blades and went to broader chord to absorb more power.
The U.S.A. and the UK generally, not always, put guns in the wings. That means the designer didn't have to maximize bullets through the propeller arc and was free to put on whatever propeller he wanted. As power increased, that meant broader prop chord, more blades, or both. By the time they were getting 2,000+ hp, that meant REALLY broad chord for a 3-blade or fat 4+-bladed props.
Look at the chord on a model of a Ta 152:
Note the inboard guns go through the prop arc.
Look at the prop chord on a late-model P-47:
That's one of the so-called "paddle-bladed" props from Curtiss-Electric. The Hamilton prop was more symmetric in shape.
Look at one of the prototype Tempests. Nothing special about the prop chord, but it GOT that way later:
Then there's this monster to absorb the power from a Griffon:
Can't help but notice those radiators!
