4 bladed props

[richardc7]

Many WW2 birds were sporting 4 bladed props at the end of the war. Anyone know why?

[FLYBOYJ]

Many WW2 birds were sporting 4 bladed props at the end of the war. Anyone know why?

Based on higher HP engines, engineers determined that on some aircraft hey were able to get the maximum thrust efficency from a 4 blade propeller.

[SoD Stitch]

The late Marks of Spitfire (those with the larger Griffon engines, in particular) even had five-bladers by the end of the War.

[ccheese]

I've got a pic of a bf-109 [actually a HA-1009] with a RR Merlin in it
sporting a four bladed prop !!

Charles

[krieghund]

a three blade prop has a higher efficency but the diameter of the prop to accept the greater HP would exceed the length of the landing gear so the only way around was to add more prop blades. However, there are some sources that claim that a four blade is more efficent at high altitude.

Hope that helped

[Burmese Bandit]

Tip speed plus landing gear length is the short answer. If an engine has a high output you need either more prop area or a higher prop rpm to use that power efficiently. Higher prop rpms will make the prop tips exceed the speed of sound, causing tip vortex problems. Longer props with more area are more efficient, and they solve the problem of lower rpms, but you need a longer landing gear to get ground clearance for the props (hence the highly problematic and expensive telescopic landing gear of the P-47). Another solution is to add more blades to the props. This solves the problem, with the exception of propeller torque - but that could be solved with contrarotating props on the same axis, which is dicussed in another thread on this forum.

[Airframes]

Charles mentioned a 'Buchon' with a four-bladed prop, which was not uncommon after the BoB movie. I've got some pics (slides, unfortunately) of one of the BBMF's Hurricanes, in the early '80's, with a four-bladed prop. Didn't look at all right, but apparently, according to the BBMF pilots at the time, it made a heck of a difference to overall performance. BTW, this difference is allowing for the fact that the Merlin fitted, I think a 66, but not sure, was a later model with more output than the engines originally fitted to the various Marks of Hurricane.

[109ROAMING]

Me too Terry I've seen both the Hurricane and Buchon with 4 bladed props

my opinion -They look absolutely horible

maybe just because I'm not use to it but they do look hedius

[siznaudin]

I think this is more complicated than any of us realise, with many more factors to be taken into account.
There was one version (Mark) of the Mosquito which was intended for carrier use and which was fitted with 4 blade props. I have seen one version of the "why" for that which suggested that smaller diameter gave better deck clearance and faster engine response (in a wave-off situation). All other Mossies had 3 bladers.
One of the late & great WW2 high altitude fighters, the Ta 152H with LOTS of power got by with a 3 blade prop ("so called "paddle blade").

[gumbyk]

I'm working from helicopter aerodynamics here (and from memory), but the principle should be the same.

More blades (higher disc solidity) gives better low-speed, low altitude performance (i.e. acceleration)

Fewer, longer blades give better high-speed perfmorance, presumably because the tip speed is in a better operating speed range for the higher speed.

I know that they cannot be directly compared (helicopter and prop), but the principles must be similar?

I've been trying to find some simple references, but no luck. The closest I have is this NACA report: http://hdl.handle.net/2060/19930084064

[siznaudin]

That's one fascinating - and h.e.a.v.y report, but great, just the same!
I'll be back in a couple of weeks after I've checked through it!

[siznaudin]

Just remembered something which puzzled me some time back, and the answer to which may well be found in gumbyk's link - I'm just starting on it (wow!)

The Texan/SNJ and CAC Wirraway: same engine (P & W 1340) .. Texan direct drive 2 blade prop, Wirraway geared, 3 blade prop.

[drgondog]

I'm working from helicopter aerodynamics here (and from memory), but the principle should be the same.

More blades (higher disc solidity) gives better low-speed, low altitude performance (i.e. acceleration)

Fewer, longer blades give better high-speed perfmorance, presumably because the tip speed is in a better operating speed range for the higher speed.

I know that they cannot be directly compared (helicopter and prop), but the principles must be similar?

I've been trying to find some simple references, but no luck. The closest I have is this NACA report: http://hdl.handle.net/2060/19930084064

The clasiic rule of thumb is keep the rpm at a level that eliminates Mcr on the blades. That is achieved at higher tip speeds the same way an airfoil works, namely reduce thickness to chord ratio, expand chord quickly (rather than taper) from hub..

[unix_nerd]

Hallo all,

I have got some thoughts to add here:
the best efficiency in terms of thrust per horsepower you've got with few as possible blades - the best is a single bladed prop. I know this sounds strange but there are some NACA test done on this one.
Each additional blade will reduce efficiency by about 1% to 2%.
But there are additional constraints.
The blade tip speed shall not be higher than let's say Mach .8 (0.8*local speed of sound)
An other constraint is the the bending of the blades by the aerodynamic forces.
Mustangs and Corsairs and the like will produce more than 1000kg (2000lb) of thrust along the centerline.
So let's imagine pulling with this force at the prop blades in forward direction. Sure non rotating blades will bend!!!
The rotating blades will have an additional centrifugal force which tries to pull the blades out of the hub.
This force is even higher than the trust (let's say the prop is rotating at 2000rpm).
giving these two forces (a bit simplified) are the main forces on the blade.
OK. Now there comes two other constraints. the cross section shape -which has to be aerodynamically so that that you got thrust at all - a wing cross section therefore.
And not to forget the material. Aluminum and steel can withstand only a certain amount of force per cross section unit(stress) before get damaged or break.
So with all these limitations you have to devide all the power to more blades when you got from 1000hp to 2000hp.

I hope this explanation was good enough - although a bit simplified
Best regards

[GregP]

The Hurrican V had a 4-bladed propeller.