Prop size choice

[Carbon Canary]

The manufacturer of an aircraft offers the option of either a 165cm or a 170cm  3 blade prop……of the same prop type, from the same prop manufacturer.

 

What are the pros and cons of the longer prop vs the shorter one ?

[kasper]

Larger prop = lower blade pitch = better acceleration and climb at the expense of high end speed

 

smaller prop = coarser pitch = slower acceleration and take off and climb but higher top end speed and efficiency at higher speed. 
 

up to you which flight performance end of the envelop you wish to preference . 

Edited May 10, 2023 by kasper

[Carbon Canary]

14 hours ago, kasper said:

Larger prop = lower blade pitch = better acceleration and climb at the expense of high end speed

 

smaller prop = coarser pitch = slower acceleration and take off and climb but higher top end speed and efficiency at higher speed. 
 

up to you which flight performance end of the envelop you wish to preference . 

Thanks Kasper for your insights. 

I have very little understanding of prop design vs objective, but the fundamental effect of a change in prop pitch is clear.

Just to clarify, and I should have included this in my original post - this is for e-props ground adjustable propeller(s).  As you may know, these props are 'skinny'.

 

Are you able to provide a deeper explanation for why blade length (as opposed to pitch) has an effect on climb performance, etc ?.........or is it related to where the greater amount of pitch (AoA) occurs along the length of the blade ? That is, a longer blade has the centre of AoA 'thrust influence' further along the blade, which is spinning faster, etc  or, is it related to a longer prop requiring a lower adjusted pitch angle to achieve the desired WOT rpm ?.....Or something completely different ??

 

I'm expecting the aircraft will spend most of its life in cross country cruise rather than circuit work, so will err toward a prop that delivers best cruise performance.

 

I believe RFGuy is the local agent for e-props, so I may pester him with this question too.

 

Thanks again.

CC

 

 

 

 

[kgwilson]

The larger the prop the closer the tip speed will be to the speed of sound. Direct drive engines have this problem when full power is applied as the prop loses efficiency once the sound barrier is reached. The Lycoming O540 in the C182 with spin the 2 blade prop up to just over the speed of sound at full power hence the loud noise from the prop on takeoff. 

 

Reduction drives on high RPM engines like the 912 allow for slower prop speed at full power and therefore a larger diameter, good for STOL aircraft but less efficient in cruise which is slower than if a smaller diameter prop was installed. The moral is You can't have your lunch & eat it too. Also 3 blades provide better take off & climb performance than 2 but less efficiency in cruise due to higher drag, hence the saying "3 for show 2 for go".

[facthunter]

There's an old saying about props. "Keep your prop as long as possible for as long as possible"  Clearance is often a  limiting factor. Small dia props will have less thrust all other things being equal. Prop tip speed is another limit. Extra noise and loss of efficiency at high Prop RPM.  Nev

[Carbon Canary]

"Reduction drives on high RPM engines like the 912 allow for slower prop speed at full power and therefore a larger diameter, good for STOL aircraft but less efficient in cruise which is slower than if a smaller diameter prop was installed."

 

Yes, this is for a 912 ULS. 

OK, I'm a bit slow.  I still can't get my head around the physics involved of why a longer prop blade (all other things being equal, including pitch) is less efficient in the cruise.

Particularly with Nev stating "Small dia props will have less thrust all other things being equal."  

 

Are you able to provide a 'Prop Design for Dummies' lesson here ?

[kasper]

Well yes the bigger the better is a core rule spoken of ... BUT you are setting a limit of the same number of blades which makes it less different and smaller changes are expected.

 

Overall:

 

The lower the number of blades the fewer leading edges and tips involved and as as result the lower the drag from blades.  On this basis the best prop is a single bladed prop as it has only 1 leading edge and tip producing drag and all thrust (lift) coming from the blade area/pitch that is set to absorb the power applied.

 

When you have a compare of two 3-blade props on the same power engine on an airframe of equal drag you are looking at small changes.

 

A three bladed fixed pitch prop of greater diameter MUST have a lower pitch as it has a greater blade area available without more power so at any rev it has to 'bite' less air as there is more blade absorbing the same power.

A lower pitch blade can be spun up from lower speed more quickly as the excess power allows for that - the increased 'bite' from each increased rpm is less for the same increase in power

 

That is the reason the lower pitch prop will accelerate and climb better than the higher pitch lower span prop.

 

The reason the general spoken rule of bigger is better is that GENERALLY when people want to maximise lower speed performance you add prop blades and reduce span ... if you want to go as fast as possible reduce your number of blades and increase the pitch - in all cases subject to the power requirements ... you may be forced to have more blades to absorb all the power because you run into airframe clearance limits and/or speed of sound issues where at transonic speeds the 'normal' drag laws get screwed up.

 

end of lunch break - Over the hive mind

[Carbon Canary]

40 minutes ago, kasper said:

Well yes the bigger the better is a core rule spoken of ... BUT you are setting a limit of the same number of blades which makes it less different and smaller changes are expected.

 

Overall:

 

The lower the number of blades the fewer leading edges and tips involved and as as result the lower the drag from blades.  On this basis the best prop is a single bladed prop as it has only 1 leading edge and tip producing drag and all thrust (lift) coming from the blade area/pitch that is set to absorb the power applied.

 

When you have a compare of two 3-blade props on the same power engine on an airframe of equal drag you are looking at small changes.

 

A three bladed fixed pitch prop of greater diameter MUST have a lower pitch as it has a greater blade area available without more power so at any rev it has to 'bite' less air as there is more blade absorbing the same power.

A lower pitch blade can be spun up from lower speed more quickly as the excess power allows for that - the increased 'bite' from each increased rpm is less for the same increase in power

 

That is the reason the lower pitch prop will accelerate and climb better than the higher pitch lower span prop.

 

The reason the general spoken rule of bigger is better is that GENERALLY when people want to maximise lower speed performance you add prop blades and reduce span ... if you want to go as fast as possible reduce your number of blades and increase the pitch - in all cases subject to the power requirements ... you may be forced to have more blades to absorb all the power because you run into airframe clearance limits and/or speed of sound issues where at transonic speeds the 'normal' drag laws get screwed up.

 

end of lunch break - Over the hive mind

OK - Thankyou !

So to wrap this up......if I want to go fast, save fuel and don't care about STOL performance, I should choose the shorter of the 3 blade options available.

CC

[skippydiesel]

I don't think anyone mention prop noise - the closer the tips speeds are to hitting the sound barrier,  the nosier (in general) the prop.

 

The Europeans are big on noise pollution - small propeller aircraft manufactures are obliged to take steps to meet the standards - hence most of the newer small aircraft, from Europe, are  noticeably quieter, than their US equivalents.

[facthunter]

You've just proved you don't read my posts Skip. My last post 4 back.  Nev

[skippydiesel]

3 hours ago, facthunter said:

You've just proved you don't read my posts Skip. My last post 4 back.  Nev

Good on yah Nev - missed it - thought you guts were just banking on about acceleration etc

 

Craven appologies!

[kgwilson]

Mentioned even earlier by me in Post 4

[skippydiesel]

19 hours ago, kgwilson said:

Mentioned even earlier by me in Post 4

There you go - double dammed 😭

[spacesailor]

You mean " metres per second per second " .

And , ddi we forget the three & four blade composite prop has more momentum than a single blade prop. Who's engine needs a Bigger flywheel  !.

Just a small point from a ' prop chopper '  who's engine has No flywheel. 

spacesailor

 

[facthunter]

Off hand I can't think of any aircraft engine with a flywheel. The large counterweights in a radial would have a lot of flywheel effect otherwise it's pretty much the prop itself.  Nev

[spacesailor]

Which means ' the lighter that prop ' , the less flywheel effect for the motor .

I tried the single blade prop , by cutting one blade  off the two blade model engines prop!.

And it ran like sheet . LoL

Yes I tried glueing a weight onto the stub , but it threw the weight through the ceiling. 

Mummy 's not happy Jan LoL

Dynamic balance needs two blades methinks,  & I never got to fly that model after the motor shook the stringers off the formers .

Balsa and tissue paper in those days .

spacesailor

 

[Garfly]

On 11/05/2023 at 9:50 AM, Carbon Canary said:

I believe RFGuy is the local agent for e-props, so I may pester him with this question too.

That'd be Mark Kyle.  ("Kyle Communications" on the forum.)

 

[facthunter]

The single bladed prop has to have the blade hinged and also be counterweighted. I don't like the idea nevertheless. More to go wrong even though it has less blades.  You can't get anything much simpler than a 2 bladed wooden prop.  Nev

[Carbon Canary]

2 hours ago, Garfly said:

That'd be Mark Kyle.  ("Kyle Communications" on the forum.)

 

Ooops, yes thanks, I should have searched the forum rather than relying on my dodgy memory.

[Carbon Canary]

To any interested parties, I found this site useful................

 

 

Aircraft Engine and Propeller Sizing | AeroToolbox

[meglin]

Yes, very useful.

And we came to the conclusion that 1.7 m diameter is best for 3-bladed Rotax-912-80 propellers in practice a long time ago, more than 20 years ago.

But I did not see in your reasoning a positive effect of the sweep of the blade ends, as well as the wing sweep. It pushes back the wave crisis and the occurrence of compaction surges.

It was the sweep that made it possible to use a 2 m diameter 2-bladed propeller on the Rotax-912/914 and to provide the Savage aircraft with maximum performance in all flight modes.

It is the sweep that allows the Jabiru engine to have a 1.62 m diameter propeller and provide a significant increase in performance in most flight modes.

[pilot ali]

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[meglin]

The subject of the Bekas airplane is hardly relevant to the topic of propeller diameter selection. We will solve this problem.