Prop Performance on Mooney's

[D. Snyder]

I am considering the purchase of a late model Mooney M20K 231. I am also considering a four blade prop to replace the existing two blade prop. Has anyone have experience with any of this? It is quite an investment, and I would appreciate some experienced in-put. Thank you!

[N201MKTurbo]

Why do you want the new prop? Is there something wrong with the current prop?

[D. Snyder]

The performance evaluations are not as good as they should be. Along with rate and fuel efficiency they could be much better with something such as a four blade MT, so I am told.

[Alain B]

The 4 blades MT prop will increase the climb rate and shorten the ground roll and save a bit of weight along the way . Also it is much more smoother . As far as ultimate top speed there is not much difference , except at high altitude . 

[Robert C.]

Erik....chime in please

 

[Hank]

Erik has one on his Rocket. @aviatoreb 

[carusoam]

For some reference, check in with Erik...

He put the four blade MT on his Rocket.  Part of a larger resto-Rod transformation project...

1) more blades can weigh more than less blades, the composite construction alters the weight required.

2) more blades have more drag than less blades, the TC birds don't notice the drag as much at altitude.

3) more blades can deliver more thrust Getting off the ground than less blades, The Rocket delivers a lot of hp for this already.

4) The composite construction of the MT gives a noticeably different sound signature than the aluminum blades.

5) in the end, it is a case by case numbers comparison of performance of the combination of prop, engine and airframe...

6) often the prop data gets lumped in with engine changes making a direct comparison challenging.  Many Os went TopProp with the 10% increase of hp.

Hope this gives some guidance on what to look for from a PP point of view.

Best regards,

-a-

[peevee]

My impression is erik's primary motivation was losing weight on the nose and gaining prop clearance.

[AH-1 Cobra Pilot]

There have been a few studies relating longevity of engines by matching the vibrational frequencies of engines to props.  The bottom line is: if you have a 4-cylinder engine, use a two- or four-bladed prop, if you have a 6-cylinder engine, go with the 3-bladed prop.

[Robert C.]

13 minutes ago, Ah-1 Cobra Pilot said:

There have been a few studies relating longevity of engines by matching the vibrational frequencies of engines to props.  The bottom line is: if you have a 4-cylinder engine, use a two- or four-bladed prop, if you have a 6-cylinder engine, go with the 3-bladed prop.

Interesting. Do you happen to have the sources? The 4-bladed prop is on my wish list for upgrades, so this is quite relevant to me.

[Guest]

More blades equal more induced drag.

Clarence

[aviatoreb]

Did someone call?

[peevee]

you better chain that prop to the airframe before someone like me "borrows" it

[aviatoreb]

Anthony correctly represented the topic.

Yes, it was weight and balance that was my primary motivation - actually primarily balance.  The Mooney Rocket has an especially heavy prop - (engine and prop as used in the Cessna 340/414 twins, so a TSIO520NB and the large 3 blade full feathering mcauley - designed in the 1960).  So this prop took about 35 lbs off the nice.  This dramatically changes the balance of airplane from a nose heavy ride to a nicely balanced ride. 

But also all other considerations were in the good direction (except it aint cheap).

-Ground clearance is much better than the Mcauley 3 blade - by quite a lot - and even better than the smaller 3 prop that had originally come with the original and lower powered TSO360 - so better than from factory - that's nice.

-Besides better balance for flight - I was always a bit anxious about so much extra weight on the front gear.

-Its a "smidgeon" faster despite being a 4 blade vs a 3 blade.  While all things being equal, a 3 blade would be faster - all things are not equal - I attribute this to two things.  a) A lighter prop requires less energy to turn and there was a dramatic reduction in the rotating weight, I presume the aerodynamics of this prop are much better as it is a modern scimitar design from a CAD design vs the old school round tipped mcauley designed on a draftsman table.  A smidgeon would be I would say maybe 3kts based on before and after speed runs - although the days were from different seasons - because of a technicality with how the prop change was made.  Long story on that not relevant here - nor even a big deal. 

-It is dramatically smoother.  I think not just because it is 4 blades vs 3 but also because this material is compliant so I presume it is shaping itself to the load as it bites into the air, vs a stiff metal blade.  In fact, while taxing over a bump you can even see the blade defore a tad bit.  It is so smooth that every single person who has flown with me has said, WOW thats smooth.

-It makes a very cool sound.

-And as you would expect, it seems a shorter take off roll (but I never know how to actually measure that without software so I didn't measure the before anyway - but it seems a good bit shorter).  And better climb so it seems - but over time you forget what you used to have - it was a peppy climber before.  I can say I have climbed to FL17 from field level of 475ft in under 13 minutes on a chilly autumn day (but not winter).

-Looks cool!  It has turned into a real ramp-eye-candy and people often seem to ask me about it.

-The only downside - you gotta buy it...anything airplane costs lots of money - and all I'll say is this costs in the same ball park as all the other prop options if you are shopping new.  But contact MT USA for actual pricing.

All this said - Mr Snyder if you have a standard M20K 231, with the TSIO 360 - I wonder if this is even an option as this blade setup is made for the power of a big bore continental such as the TSIO520.

[aviatoreb]

58 minutes ago, Ah-1 Cobra Pilot said:

There have been a few studies relating longevity of engines by matching the vibrational frequencies of engines to props.  The bottom line is: if you have a 4-cylinder engine, use a two- or four-bladed prop, if you have a 6-cylinder engine, go with the 3-bladed prop.

That sounds really plausibly correct - impulses out of phase with the strokes.

But it is hard to believe it would be any kind of problem with this setup - it is so so so very smoooooooth.

My guess is compared to ANY metal prop, no matter how many blades, it is stiffer and heavier, that the engine is happier with a smoother and compliant and lighter carbon fiber wrapped wood prop like the MT. My guess is that a study such as you cite would be swapping blade sets that are identical in build type and material and aerodynamics but different only in blade number.

[KSMooniac]

The 3-blade MT won't disappoint on a stock K...no reason to go for 4 blades. I like the MT on my J. Got it balanced down to 0.01 IPS. 12 pounds lighter than the 2 blade McCauley as well.

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[OR75]

My impression on prop changes

- performance gain seem debatable (and marginal) going from one to another - gain in climb , loss in cruise

- appearance: very subjective but I believe that 3 blades have more ramp appeal than two blades. I am not sure about the 4 blades (again just personal preference)

- noise: looks like composite are quieter. props have different yellow arcs but most are manageable

- ground clearance: any reduction in ground clearance in a Mooney is a plus 

[Raptor05121]

The other day when Hector and I were in formation- my 3 blade needed another 2" MP to keep up with his 2 blade. I will say I had two more warm bodies on board, so the extra weight combined with gauge inaccuracy, I'd say cruise will be about the same.

[KSMooniac]

3-blade scimitar version. They don't have a 2-blade option for us, unfortunately, our I would have taken it. It is also the only prop option with a reduced diameter compared to the OEM prop.

 

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[kortopates]

For the K models in particular, yet the J's  are nearly identical wrt to 2 more considerations I haven't seen discussed above yet, but what have been the con's wrt MT's composite offering to:

• Maintenance wrt to R&R the lower cowling. The 252 lower cowling is already far from easy in getting on and off with the 2 blade. 3 or 4 blades has to appreciably change the process. After much experience I can single handedly  R&R my cowling, although an extra hand is always nice. I wonder how much more difficult it will be with 3 (or 4 blades). The longer body mooney's intended for 3 blades have the lower cowling terminate at a lower height which really helps with this. (4 blades isn't a possibility for me with the TSIO-360)
• Drag impacts to best glide angle and distance. Three bladed props are like an added speed brake when you pull the power back to idle or simulate a engine out for power off landings. Have any of you actually tried to measure the impact. In my K, with 252 prop, I get 2 nm for every 1000' agl. Any idea how 3 blades would effect that? 

Incidentally, Mooney did a lot of prop testing for the 252, they did go with a better performing 2 blade McCauley back in '85 that improved performance over the 231 prop. The factory at least never suggested or used a 3 bladed prop till the long body's. Of course if they had what they have today to work with back in '85 their is no telling.

Edited March 21, 2017 by kortopates

[aviatoreb]

28 minutes ago, kortopates said:

For the K models in particular, yet the J's  are nearly identical wrt to 2 more considerations I haven't seen discussed above yet, but what have been the con's wrt MT's composite offering to:

• Maintenance wrt to R&R the lower cowling. The 252 lower cowling is already far from easy in getting on and off with the 2 blade. 3 or 4 blades has to appreciably change the process. After much experience I can single handedly  R&R my cowling, although an extra hand is always nice. I wonder how much more difficult it will be with 3 (or 4 blades). The longer body mooney's intended for 3 blades have the lower cowling terminate at a lower height which really helps with this. (4 blades isn't a possibility for me with the TSIO-360)
• Drag impacts to best glide angle and distance. Three bladed props are like an added speed brake when you pull the power back to idle or simulate a engine out for power off landings. Have any of you actually tried to measure the impact. In my K, with 252 prop, I get 2 nm for every 1000' agl. Any idea how 3 blades would effect that? 

Incidentally, Mooney did a lot of prop testing for the 252, they did go with a better performing 2 blade McCauley back in '85 that improved performance over the 231 prop. The factory at least never suggested or used a 3 bladed prop till the long body's. Of course if they had what they have today to work with back in '85 their is no telling.

Its true - it is a MAJOR pain getting the lower cowl off with a 3 blade.   I can say it is is already such a major pain with a 3 blade - that with a 4 blade - it is no more painful - essentially the same.

[AH-1 Cobra Pilot]

3 hours ago, M20Doc said:

More blades equal more induced drag.

No.  Induced drag depends on lift produced, regardless of how many blades make it.

[mike20papa]

I believe he means "parasitic drag".  More blades do increase the wetted surface.  Just like the long body vs. the short body Mooney vs the Questair.

The old control line speed guys from the '60's used counter weighted, single blade props - for a reason.

[AH-1 Cobra Pilot]

37 minutes ago, mike20papa said:

More blades do increase the wetted surface. 

Not necessarily.  S=bce. 

[aviatoreb]

48 minutes ago, mike20papa said:

I believe he means "parasitic drag".  More blades do increase the wetted surface.  Just like the long body vs. the short body Mooney vs the Questair.

The old control line speed guys from the '60's used counter weighted, single blade props - for a reason.

Does it mean more wetted surface?  More blades means they use shorter blades.

Also - the above relies on all things being equal as I said, but aerodyanmics of the specific blade design counts too.