Fokker 50 Props

[newpic]

Does anyone know why the Fokker 50 has Dowty props which are curved (I think they have a specific name which I cannot recall)? What are the advantages of such a design?

What are the other advantages/disadvantages of having a 6 bladed prop as opposed to a 4 bladed prop (besides the advantage of increased thrust)?

Thanks in advance
Newpic

[Capt Claret]

At a guess, more blades to "absorb" or use the energy put out by the engine, allows a more powerful engine, allows a greater mass to be lifted. And, allows the prop to spin slower for the required thrust, thus keeping tips sub sonic and reducing noise.

[AtoBsafely]

As Capt Claret said,

You need more blades to "absorb" the extra power being provided. The curved tips reduce transonic effects, which reduce efficiency and increase noise.

When the engine is more powerful, you need more "solidity" to the prop disc: either increasing blade area, number of blades or prop speed. Increasing prop speed creates problems with the tip speed and efficiency, and wider blades are heavier and less efficient as well (think aspect ratio of the blade).

If you can, have a look at the different props used on the spitfire from the prototype (~1000hp, two bladed prop) to the latter development trials (~2200hp). I don't think they got past six blades because the hub becomes an engineering/maintenance nightmare. Those were all straight metal props, but now composite materials allow more efficient curved designs.

[Back Seat Driver]

F50 Prop Speed

Newpic, At 100% Np the props turn at 1,200rpm. Cruise power setting is 85% ie. props turning at just 1020rpm. reasons for this are as given above.
(data from FK50 AOM / Power Plant / 1.07.04)

[FougaMagister]

As mentioned above, more prop blades mean greater engine/aerodynamic efficiency. Apart from the Fokker 50 you refer to (Skywest I guess) compared to the 4-bladed RR Dart-powered F27, there are other examples; the 500/600-series ATRs also have 6-blade composite props, as do the Dash8Q400, Saab 2000, ATP, CASA-295 and C-130J. Some, such as Airbus A400M and the latest E-2C/D Hawkeye even have 8-bladed props.

These blades are typically composite material (for weight, corrosion and durability advantages) and sabre-shaped, i.e. curved for better tip efficiency.

Increasing the blade chord was tried some time ago. Check out the earlier marks of Hercules (C-130B, E, H) and the Lockheed Electra/Orion. Not as efficient a design as the 6- or 8-bladed prop.

Cheers

[rigpiggy]

"compared to the 4-bladed RR Tyne-powered F27"

RR Dart's the Tynes are much larger

[Brian Abraham]

more prop blades mean greater engine/aerodynamic efficiency

Quite the reverse. The fewer the number of prop blades the greater the efficiency. The highest theoretical efficiency would be gained with a one bladed prop, but of course a great number of engineering considerations come into play to determine what trade offs will be made to come up with an optimum overall package that represents the final aircraft. The C-130 started life with 3,750 HP and three bladed props, then 4,050 HP and four blade props and finally 4,591 HP and six blade props. In each case it was necessary to increase the solidity by adding blades to absorb the increased power, not to increase efficiency, which in fact would be decreased. Prop diameter was of course limited by the fuselage/engine/engine spacing.