I wonder what determines the number of blades on a prop. I have seen Spitfires with three, four and five blades.
By adding an extra blade wouldn’t have the problem of sluggishness of the “Stuka” when coming out of a dive been fixed ?
Borislav
It all depends on horsepower. A low or mid horsepower producing engine will not improve with addition of prop blades. A high horsepower engine will however. Engine performance is related to two factors. It is refered to as BHP (Basic Horse Power) and SHP (Shaft Horse Power). By adding the number of prop blades to an engine that can’t take it will reduce the SHP resulting in lower performance.
On some aircraft as the BHP increased the makers added more blades to increase SHP resulting in better performance. It sounds more complicated than it is.
Spitfires also had two and six blade propellers, the six blade being contra-rotating. It was a matter of transferring power to the air. More power = more blades.
BTW…I have a WWII vintage Rotol propeller blade off of a Mk IX Spitfire in my living room. It was on MK923 the day she shot down two Me 109 G’s south of Paris (Aug 14th, 1944).
Cheers
Bob S.
As Berny says, there is some aero engineering involved.
Basically a given horsepower translates into so much effort available to create the low pressure area in front of the prop that pulls the plane along. If you use a smaller diameter prop you can add a blade or two (smaller diameter + blade = same effort compared to a larger prop with fewer blades).
If you do this you can reduce drag but the tradeoff is more cabin noise plus a more complex and expensive prop.
Conversely, as has already been said, if you increase horsepower you can either increase the blade diameter or add a blade.
The engine and propeller must be matched so that the required propeller effort equals the available horsepower.
Hey guys;
can’t forget the engines RPM, the blade acts as a flywheel on you cars engine if you want to have the engine run at higher RPM then take some weight off the flywheel, as for the aircrafts engine you want the prop spin at or around 2000 any faster the the prop will strech and fail, so if you load the Hi HP engine with a compairable weight then the engine will only spin at the designed speed thus giving you the proper blade rotation for thrust, by adding an additional blade will increase the thrust + engine/air drag but then you decrease the size & weight of the blades to keep the engine to operate at it’s most effent rate,
God I remembered my most boring class instructors lessons in A&P school,
have fun & relax
cuda
You also have to watch tip speed. A given amount of horsepower needs a set amount of blade area to produce the required thrust, so you can use longer blades, wider blades, or more blades. With longer blades, not only is there a problem with ground clearance, but the tips approach supersonic, which really destroys their efficiency. By using more, but shorter blades, that problem is reduced.
Ashley
You are correct on prop length. A lot goes into the design of the prop for any given aircraft. Look at the P-47. When they went to the new “Paddle Blade” prop it increased the performance of the aircraft. It could then out climb a Spitfire. That advantage didn’t last long with newer Spits just coming out. Pitch, cord ratio, twist ratio and length are just some design factors. It is a fine balance between engine BHP and prop design. See, I wasn’t dozing off in my collage classes.
Hey guys;
also there is another thing that will increase the props effectiveness is that they are governed to increase or retard the pitch thus will increase or retard the performance of the A/C, there many ways to govern the prop, on earlier props they where just a fixed pitch, then there are constant speed props, and there many variable pitch prop and we have the counter-balanced type and we have the oil induced types, good example if you look at an AT-6 prop its a counter-balanced prop (like most personally owned planes) then look at the prop on a B-17 or P-47 they have the oil induced type where oil pressure of the engine is forced into the hub and that will cause the pitch to change, most people think that the engine speed changes with the throttles, that is some what correct after the engine gets to is maximum running speed the more fuel you put in to the engine will change the pitch of the prop, good example flying level at cruise settings push throttle to max and the plane will climb not go faster, the pitch changed and will look for an equalibreum in air pressure and engine speed did not increase, another example Turbo prop engines run at 100% all the time when you move the “throttles” all you’re doing is changing the pitch of the props, and the speed of the props is about 2000 rpm any faster the tip will go super sonic, and the blades do stretch and will fail at faster speeds, as for the Spit’s when Supermarine put 4,5,6 bladed props was to increase the performance of the plane with out changing the prop length & weight, to the more powerful engines they installed, but in the end the plane could only be a few mile per hour faster then originally designed,
Ugly Butt Effective Hawgs
cuda