Replace 3-blade with 2-blade Scimitar?

[rainman]

The three blade is definitely quieter.  My comparison is based on flying in my plane (three blades) and another 1979 M20K (two blades) on the field.

[Raptor05121]

My ideal setup (ignoring costs) would be a 2-blade MT composite. Best speed, best useful load. I like the looks of my three-blade, but I'm not operating out of short or soft runways, so the increased climb isn't necessary, IMO.

[aviatoreb]

16 hours ago, carusoam said:

1.5) energy use and wear by weight and rotation are complex to measure.  TBO and bearings related to the prop may indicate something here.

3) Prop inertia or fly-wheel effect helps avoid kick-backs after failure to start situations. That small amount of momentum that carries the prop through the compression stroke is helpful here.

5) more blades generally have more flat plate surface area.  This is where gliding distance tests start to shine.  Anyone with a Missile prop knows this one.  Their prop feathers sideways to minimize the flat plate issue with proven spectacular glide ratio results.

Whether it be because generally 4 will be smoother than 3 (more makes for smoother airflow) or it is the mechanical aspects of more compliant material in the MT, the MT prop is very smooth.  This can only be a good thing for engine longevity?  I should think so but no one knows.  And good for panel components too.  And more comfortable for the humans sitting behind the big fan.

Kick back - to tell you the truth I never thought of this when I was buying.  Is this a problem that people are having?  Worrisome prospect.  OTOH, throttle changes cause an almost instant change of rpms as compared to before so it feels.

5) your more blades are generally more flat plate surface.  But don't forget, more blades make for a shorter blade - so smaller diameter, when installed on  fixed horsepower sized engine.  With a 300 (305) hp engine you are pretty much forced to go at least 3 blades since the prop to soak up 300hp as a two blade would be so long it would be dragging on the ground in a mooney.  A 3 blade clears, but a 4 blade gives good ground clearance.  Anyway, the smaller circle of the 4 blade somewhat mitigates your argument of more flat plate surface area, but I don't know how much, and I bet there is still more with a 4 blade, but not so simple to compute.

Edited February 14, 2016 by aviatoreb

[aviatoreb]

2 minutes ago, Raptor05121 said:

My ideal setup (ignoring costs) would be a 2-blade MT composite. Best speed, best useful load. I like the looks of my three-blade, but I'm not operating out of short or soft runways, so the increased climb isn't necessary, IMO.

They do make 2 blades.  But I don't see one on the STC list but maybe they could do a field approval if that is something someone wanted.

[carusoam]

Good Sunday morning Gentlemen,

I think I left out a smoothness argument from the list.

There is a harmonic(?) vibration related to the number of cylinders and the number of blades.  

- A four cylinder engine with a two blade prop is supposed to be smoother than the same four cylinder with a three blader?

This has something to do with where the power stroke of each piston happens compared to where the blade is at the same time.  Prop indexing (when mounting) is required to get this detail correct.  Prop blade AOA vs. airspeed has some effect...

Sounds like a three blader (or more) might be a proper choice for the 252’s six cylinder engine based on smoothness issues.

For Alex's C, a two blade seems to make a lot of sense.

Cody (our resident prop guy), are you reading this and laughing out loud?  Am I even close?

We need a prop oriented fly-in,

-a-

[Hank]

I hear the harmonic argument a lot. My 3-blade was dynamicly balanced when the previous owner installed it. I've not noticed it vibrating a whole lot, and no pax have mentioned it (except one Comanche 250 pilot who said his wife wouldn't fly with him due to the vibration, and he had a 2-blade).

But I, too, will defer to the expert. @Cody Stallings

[aviatoreb]

50 minutes ago, carusoam said:

Good Sunday morning Gentlemen,

I think I left out a smoothness argument from the list.

There is a harmonic(?) vibration related to the number of cylinders and the number of blades.  

- A four cylinder engine with a two blade prop is supposed to be smoother than the same four cylinder with a three blader?

This has something to do with where the power stroke of each piston happens compared to where the blade is at the same time.  Prop indexing (when mounting) is required to get this detail correct.  Prop blade AOA vs. airspeed has some effect...

Sounds like a three blader (or more) might be a proper choice for the 252’s six cylinder engine based on smoothness issues.

For Alex's C, a two blade seems to make a lot of sense.

Cody (our resident prop guy), are you reading this and laughing out loud?  Am I even close?

We need a prop oriented fly-in,

-a-

I don't know - I would guess....that for good harmonic properties, that you would want a number of blades not to be "commensurate" with the number of cylinders ideally.  Like 3 blades in 4 cylinders would be better than 3 blades with 6 cylinders.  But 5 blades in 6 cylinders would cause no harmonics.  That anyway is the mechanical aspect of harmonics.  Maybe it is not an issue anyway? But for aerodyamic noise and vibration resistance as well I would guess more blades is better.  ANd then separate issue, I would think a flexible material like the MT blades would be good for vibration since the blade would flex out the vibrations like a spring rather than stiffly transmit.

I too felt like I had a silky smooth setup with my 3 blade prop on my 6 cylinder whch has been dynamically balanced, but it improved so much when I went to 4 blades it was surprising.

[M20F]

3 hours ago, aviatoreb said:

With a 300 (305) hp engine you are pretty much forced to go at least 3 blades since the prop to soak up 300hp as a two blade would be so long it would be dragging on the ground in a mooney.  

Erik makes a good point which is that thrust, not efficiency makes a plane go.  A lot of these  discussions about X blades fail to account for differing HP, mission, etc. remember what works for Billy may not work for Suzy.   

[DonMuncy]

2 hours ago, carusoam said:

Good Sunday morning Gentlemen,

I think I left out a smoothness argument from the list.

There is a harmonic(?) vibration related to the number of cylinders and the number of blades.  

- A four cylinder engine with a two blade prop is supposed to be smoother than the same four cylinder with a three blader?

This has something to do with where the power stroke of each piston happens compared to where the blade is at the same time.  Prop indexing (when mounting) is required to get this detail correct.  Prop blade AOA vs. airspeed has some effect...

Sounds like a three blader (or more) might be a proper choice for the 252’s six cylinder engine based on smoothness issues.

For Alex's C, a two blade seems to make a lot of sense.

Cody (our resident prop guy), are you reading this and laughing out loud?  Am I even close?

We need a prop oriented fly-in,

-a-

I don't know whether it is luck or not, and I don't have a lot of experience with other combinations, but my K model is substantially quieter and smoother with the MT three blade, as opposed to the McCauley 2 blade. I would buy it again on that basis, without considering the shorter take-off, better climb and lighter weight on the nose. I have not noticed any decrease in cruise speed, but I admit I do not keep up with that very closely.

[carusoam]

From the outside, Erik's four blade sounds very different than expected... I wish there was a way that I could objectively quantify that.  The video of his take-off captures a bit of the difference to the other Mooneys leaving...

Best regards,

-a-

 

[Hank]

1 hour ago, carusoam said:

From the outside, Erik's four blade sounds very different than expected... I wish there was a way that I could objectively quantify that.  The video of his take-off captures a bit of the difference to the other Mooneys leaving...

Best regards,

-a-

 

So where is this video? I looked for it just now, it's not in this thread. But there is a beautiful photo of his new prop!

[IndyTim]

I was just looking for same.

[carusoam]

I'll see if I can find it...

It was at the NJMP fly-in in Dec.

Near the end of the thread, people posted pictures and 201er posted a video.  I think that video is the one to see.

Best regards,

-a-

[carusoam]

Reviewing that video....

the video was taken by Chris in Alan's Cessna.  You can hear Erik's four blader Parked on the ramp.  I'm not sure if anyone captured the take-off...

Best regards,

-a-

[Cody Stallings]

I'm not Laughing Carusoam.

The 3 Blade vibration issue is a real problem on 4 Cyl engines. I have no clue why.

I operate a Hartzell 3 way on a F Model, it's balanced to .02ips an some days it feels like one blade is a foot longer than the others. Strange!!!!

I plan to ask this question this summer at the Hartzell shop training class in Dayton.

As far as 3 ways go in a 6  cyl, I have never really noticed the vibration issues.

I think the 252 would be fine with the 3 way, but it is added drag, takes more power to turn the extra blade an in turn you have a less aggressive blade angle in cruise configuration hints loss of speed.

My Personal opinion: 3 way Propeller is a little to much for a 180-200-210Hp Engine.

I feel like you need to be working with 270-300Hp to really take advantage of a 3 way propeller.

[PTK]

9 hours ago, carusoam said:

Good Sunday morning Gentlemen,

I think I left out a smoothness argument from the list.

There is a harmonic(?) vibration related to the number of cylinders and the number of blades.  

- A four cylinder engine with a two blade prop is supposed to be smoother than the same four cylinder with a three blader?

This has something to do with where the power stroke of each piston happens compared to where the blade is at the same time.  Prop indexing (when mounting) is required to get this detail correct.  Prop blade AOA vs. airspeed has some effect...

Sounds like a three blader (or more) might be a proper choice for the 252’s six cylinder engine based on smoothness issues.

For Alex's C, a two blade seems to make a lot of sense.

Cody (our resident prop guy), are you reading this and laughing out loud?  Am I even close?

We need a prop oriented fly-in,

-a-

Excellent info Anthony. I would only add that metal prop blades due to their almost nonexistent dampening characteristics are extremely susceptible to destructive vibration when they are close to their resonating frequency from a certain engine. The amplitude of the vibrations combined with fatigue of the metal (i.e. aluminum and no endurence limit) are considerations in the testing done when trying to match a certain engine to a certain prop.

[aviatoreb]

I saw a video of my take off that someone took - I thought I saw it on here, but it could be that someone emailed to me - but I cannot find it now.  I do believe it is on here somewhere.

Edited February 15, 2016 by aviatoreb

[Hank]

That would be really neat! Maybe on food the NJMP group will repost it. Or do you have another one?

[Marauder]

That would be really neat! Maybe on food the NJMP group will repost it. Or do you have another one?

Stinky Pant's video show a brief clip at 1:34 of his video. He has music overlaid but you can hear it.

Sent from my iPad using Tapatalk

[Marauder]

Here is bradp's video of Erik's takeoff:

Sent from my iPad using Tapatalk

[Hank]

Looks good and sounds good! Didn't remember the white prop tips, they're really visible here.

[aviatoreb]

7 hours ago, Hank said:

Looks good and sounds good! Didn't remember the white prop tips, they're really visible here.

Thanks - 

It looks to me in that video to agree with something it looks like from inside.  Are the blades curving forward under the "weight" of thrust.  Sort of like a 787 wing in flight.  Vs a rigid aluminum airfoil blade, a compliant blade may bite into the flow differently?

[stevesm20b]

I have the 3 blade Hartzell on my B model. Climbs about 150fpm better than the 2 blade. I true out at 150-152 knots at 8,000 to 10,000ft. I didn't notice any loss of speed with the 3 blade but could have been because the engine was overhauled at the same time. Another thing I noticed was the airplane slows down faster than with the 2 blade when you pull the power back with the prop forward. Which is nice if your high and fast on final. I like the 3 blade, is an advantage when flying into or out of short fields or high altitude fields.

 

[Alan Fox]

As far as balancing , you cant balance out the harmonics without a dampener or counterweight , on 4 cylinder LYCs (without counterweights)  the harmonic energy is transmitted to the blades , about 3 inches from the tip , it is not uncommon for Lycs to shed prop tips....,  If the engine has counterweights it doesn't matter how many blades , the harmonic energy displaces the counterweight and that's that......   I went from a two blade to a three blade in a Beech , 6 cyl Continental counterweighted engine , The difference in Groud roll and climb was SUBSTANTIAL , the difference in top end was negligible....... This is why you see the abundance of three blade upgrades on the 6 cyl Mooneys....   The 3 blade is smoother , but I believe it is because the prop is functioning at a higher  frequency than the 2 blade and is more perception than reality , If you switch from a two blade to a three blade , this will take a while to get used to....Also keep this in mind , Most Hartzell props make it to TBO , most MTs do not make it to tbo , where as an aluminum prop takes a ding , and can be filed out , the composites will tend to delaminate and require overhaul... ( you cant file down the L.E. on a composite )   Cody , I know you are a prop guy , so please correct me if I am incorrect.... 

[aviatoreb]

8 minutes ago, Alan Fox said:

As far as balancing , you cant balance out the harmonics without a dampener or counterweight , on 4 cylinder LYCs (without counterweights)  the harmonic energy is transmitted to the blades , about 3 inches from the tip , it is not uncommon for Lycs to shed prop tips....,  If the engine has counterweights it doesn't matter how many blades , the harmonic energy displaces the counterweight and that's that......   I went from a two blade to a three blade in a Beech , 6 cyl Continental counterweighted engine , The difference in Groud roll and climb was SUBSTANTIAL , the difference in top end was negligible....... This is why you see the abundance of three blade upgrades on the 6 cyl Mooneys....   The 3 blade is smoother , but I believe it is because the prop is functioning at a higher  frequency than the 2 blade and is more perception than reality , If you switch from a two blade to a three blade , this will take a while to get used to....Also keep this in mind , Most Hartzell props make it to TBO , most MTs do not make it to tbo , where as an aluminum prop takes a ding , and can be filed out , the composites will tend to delaminate and require overhaul... ( you cant file down the L.E. on a composite )   Cody , I know you are a prop guy , so please correct me if I am incorrect.... 

I don't doubt what you said about harmonics and a reason why 3 blades are found on Mooneys with 6 cylinders, but in the case of  300hp engine on a Mooney, I would think the 2 blade needed would be so long it would be dragging on the ground almost on taxi.  Does anyone have one?  I don't know.

You see in my MT, I have the newer blade treatment, the deep nickel coating which supposedly is supposed to prevent dings and also delamination you describe.  Fingers cross it is true.