3 Blade versus 2 Blade

[bholleran]

I have a couple of questions that I hope someone can answer for me.

Assuming that I am running a Simonini Mini 2 Evo engine.

What configuration will.

1. Give the most thrust - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

2. Be the quietest - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

3. Best Fuel Economy - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

I am guessing that there is going to be a trade off?

Thanks.

Barry

[pete_b]

I have a couple of questions that I hope someone can answer for me.

Assuming that I am running a Simonini Mini 2 Evo engine.

What configuration will.

1. Give the most thrust - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

A three blade running fast ( Wrong reduction ) Over rev the engine use loads of fuel

2. Be the quietest - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

A three blade running slow ( Wrong reduction or too course pitch on the prop) putting more strain on the engine less thrust to take off with

3. Best Fuel Economy - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

As 2 above

It all depends on what you want there is no best prop/reduction ratio for all 3 scenarios.

The best thing for you would be a ground adjustable prop similar to the GSC

I am guessing that there is going to be a trade off?

As above

Thanks.

Barry

[pauldeakin]

What configuration will.

1. Give the most thrust - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

2. Be the quietest - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

3. Best Fuel Economy - A 3 blade or 2 blade and at what reduction ratio \ specific prop etc.

As Pete has shown, there is no simple answer.

Even if you have a standard engine that is producing the correct HP at the correct RPM and a standard reduction and you have a three blade prop and a two blade prop that were specifically designed for that engine and reduction, then it would still not be possible to answer your questions, there are so many design factors to consider that effect thrust, noise and economy from a prop.

Many pilots only consider the pitch, but also the width of the blade the thickness of the profile, the shape of the profile, the shape of the blade (if it is thinner or thicker at the tip) the distribution of the pitch (more pitch on the inside or more pitch at the tip). All these things and others effect thrust, noise and economy.

Typically a three blade prop will be aprox 2/3 the width of the equivalent two blade prop, which often means that whether you have two of three blades is less relevant than the other factors mentioned above.

Paul D

[bholleran]

Ok, mayby I am missing some other assumptions here.

Let assume that both the two and three blade prop are from the same manafucturer, and that they have both been designed for max thrust. Regardless of fuel economy.

1. Which would be quieter?

I am guessing the three blade as it would be producing the same power at lower revs!

Lets assume a different set of 2 and 3 blade props. Both designed to give the same noise level which would give the most the best fuel economy and which would give the best thrust.

I am not sure if it makes the question eeven more complicated.

[pete_b]

Three blade will run slower than a two blade (usually) so a different reduction for two blade.

Most manufacturers have different reduction ratios depending on weather you have a two blade or three blade.

[t_andrews]

I think you'll find that some three blade props are quieter/noisier then two blade props.

You may also find that some three blade props make less thrust then two blade props and verse vica.

As noted, there are so many variables attached to the prop that change, it's more then an art to match prop to engine, then redrive, then application, then economy, then noise - and then modify those parameters in order of YOUR importance.

I remain convinced that the ONLY way to have it all in the eyes and ears of the pilot is to have an in-flight adjustable prop.

Economy is tied directly to rpm of the engine/redrive/prop, but distance economy factors in wing efficiency, prop efficiency for a given trim setting's speed (and prop rpm required) in flight, any lift encountered and how it is handled/ignored, your happiness level for the day etc. etc.

Some prop talk in this thread:

viewtopic.php?f=1&t=1958

The answer to your question is ultimately yours alone, since you make the decision on what levels of each parameter are acceptable. With an inflight adjustable, you tune on the fly for the biggest grin of the moment.

That's my story, and I'm sticking to it (until sufficiently convinced there is a better way, which could happen at any time).

[aquatix]

My thoughts are similar to those above. I had a 3 blade carbon prop on my Walkerjet F200 which seemed perfectly matched to the engine, giving great economy, low noise and tons of power. I tried a wooden 2 blade one which somehow just never seemed right.

The Kobra Simonini Evo came with 3 blade Helix prop, which has been thoroughly tested and matched to the engine to maximise power and economy. I purchased a 2 blade prop from Paul Deakin mainly to keep as a spare, but I've given it about 20 long flights to test in comparison.

It is a Hi-Fly glass fibre 2 piece prop, nicely constructed in a silver weave finish, 5cm shorter but about a third wider and twice as thick (fat) - so I had to purchase longer prop bolts. It is definitely noisier and has slightly less thrust (still way more than necessary though with that powerful engine) as rpm is higher at the same trim speeds. The engine can also rev higher (but still doesn't reach its 8000 rpm peak) and although not measured with scientific accuracy it seems to use a tiny bit more fuel, although the CHT readings are consistently slightly lower, which is a good trade off. I think this is because the pitch is greater close to the hub so produces more airflow over the engine. The extra noise is probably due to the fatter tips and profile, which bludgeons the air rather than cutting through it cleanly.

Overall I prefer the 3 blade prop, despite it giving perhaps a tiny bit more torque steer - although the differences between them are not huge considering the 2 blade prop is about half the price !!