tacx

My Feedback: (1)

I have noticed throughout the forums that 2 blade props out perform 3 or 4 blade props. My question is...If that is the case why did the real warbirds, like the p-40s and 51s use 4 blade props?

TFF

With the amount of horsepower those engines made the props would be 20-25 feet in diamiter if they were 2 blades. But also that big of a prop would be pretty heavy also.

JohnBuckner

My Feedback: (1)

It could be argued that horsepower and torque does not scale down well.

The big 2800's, 3350's and Merlins would be hard put to hit 3000 RPM but our miniture engines are idling at that RPM and to produce any power at all must get up into the 12 to 14,000RPM range however that would be impossible if we simply scaled down the many bladed full scale propellors exactly.

Even when we do cheat with three and four bladed props they must be of reduced diameter and or chord/planform this to the point that they look ridiculous and silly.

It could be argued that a single blade prop is the most efficient of all at the RPM's and airspeeds we operate at past and present:

HighPlains

My Feedback: (1)

Roll Royce Merlin engines did run up to 3000 RPM, but with the gear reduction built into the engine, the props ran at less than half of that RPM.

The Pratt and Whitney could run up to 2700 RPM, but also had various reduction ratios that brought the prop RPM to roughly half as well.

JohnBuckner

My Feedback: (1)

Yes but that was in the upper reachs of their envelope at that time and the point is the direct comparison our modern miniture engines which also operate at RPMs over 14.000 much over in some cases but the point is a basis for direct comparison.

John

airbusdrvr

Two reasons for multi-bladed props in the "real"(full sized) world are: 1)blade tip speeds. As the blade ends approach/exceed supersonic speeds other interesting and noisy issues occur. 2)prop diameters. The distance between the prop mount point and the ground limits the diameter. Therefore, even though less efficient, smaller diameter/more bladed props are required.

airraptor

My Feedback: (66)

Air bus is correct.

a one blade prop is the most efficient prop. two blades are the easiest to make for our model planes.
plus most full scale planes run them as stated to keep the prop tips subsonic (efficient) and because of ground clearence.

take the corsair with its inverted gull wing. that was designed so they could use a shorter landing gear.
If you run a 2 blade prop on these planes you would need around a 18-20" prop. to do this you would need a very long landing gear to have prop clearence. with longer gear comes a long moment arm that would require a much stronger wing and wing box to handle the higher loads impossed by the longer gear. this in turn adds weight to the whole airframe. In a fighter that is not good.

in full scalle some of the 3 and 4 blade composite props are getting more efficient than metal two blade ones in the general aviation.
our model planes use them for to slow the down lines in pattern stuff

jetmech05

I don't buy the less blades is better theory...why? Because for instance the new C-130, the J model, instead of 4 blades they went to 5...ground clearnce is the same......most ultra modern prop aircraft are going with more blades not less....
Oh by the way on the Pratt and Whitney R2800-52W engine you set power by BMEP or MAP not RPM...I remember using 115/145 fuel, with water injection takeoff power was 252 BMEP or 62 inches of MAP which ever came first...RPM was around 2800....In jets like the 737-200 or 727-200...takeoff power is set by EPR not RPM....But GE engines like the 737-300 and newer use RPM..they don't even have an EPR system

airraptor

My Feedback: (66)

Jet mech lol sorry to say but you know enough to get you in trouble.
The C130J has 6 blades (R391 all composite propeller) on the new engines of around 6000 Shaft Horse power.

The reason they went with more blades on the C-130J is because the new engine has more power. yes they are the same length because they didnt change the mounting position of the engines. so you still have the same distance between the prop blades of the engines and the the blades of #2 and #3 engine near the fuselage. so to get more performance of out the plane they did go with more blades that are more efficient than the old metal ones.

there are only a few full scale planes that have props that go super sonic. and those engines you mentioned also do have a RPM limit set on their prop.

Do some research on why planes are designed with multiblade props since the WWII time frame. Look up tip speed of props and it might help you understand better.

If you have more than enough power on our model planes then go ahead and run a 3 or 4 even 7 blade prop on our model planes.

not trying to knock you in anyway.

jetmech05

I haven't been around a 130 in years not since 76...on the F model.....I was mistaken on the number of blades on the J

airbusdrvr

As an above posts alluded to, there is also a power component of this blade diameter/number of blades discussion. When blade diameter is limited, the only way to increase power(engine power) is to increase the number of blades to dispense this additional power(thrust). It may not be optimum but is the only way to increase power/thrust within the diameter restrictions for a particular airplane.

airraptor

My Feedback: (66)

Yes what i was trying to say. The Air Force is testing an 8 blade prop on the older C-130 engines now. they are narrow blades and much more efficient over the old metal props. the old metal blades were very wide to handle the power output of the old engines and very inefficient.

For most of our model planes the two blade prop will give best performance. now if you like the 3 and up blade props then for sure use them. its what makes this hobby what it is. i know for a scale plane a 3 or 4 blade looks best when on the ground but when the engine is spinning you cant tell with human eyes how many blades are there.