Replace 3-blade with 2-blade Scimitar?

[IndyTim]

Since I'm now shopping for a prop, I thought it would be good to re-evaluate 2-blade vs 3-blade.

I understand that a 3-bladed prop, which is on my 252 now, is supposed to be quieter and will climb better than a 2-bladed prop, while giving up a little top end speed, and 15 pounds in useful load.  The 2 bladed is cheaper to buy, cheaper to service, and has the added benefit of not chewing up my cowling paint job every time I change the oil.

My questions - can anyone be more precise about the magnitude of the differences --

-- How much top speed difference?  And does the difference apply at all power settings, ie, a 2-bladed prop is about 2 kts faster across the board?

-- I'm not sure I'm worried about climb.  It's not a jet, but I have never been in a situation where climb was an issue.  How much of a fifference are we talking?

-- as for noise, how much noise?  Enough that it becomes tiresome on a long flight?  Even with noise-canceling headsets?

I'm wary of making the change to a 2-bladed prop - most folks have 3-bladed props on their M20K's now, and there are probably good reasons for that.   But cheaper overhauls, less weight, less propensity to ding the cowling paint, and a couple more knots are all appealing. And  I'm not sure I care about the looks.  I'm also thinking that, since my insurance agent is writing the check, a 2-bladed route might free up funds to be applied elsewhere.

 

[carusoam]

Additional thoughts...

1) Include the aluminum vs composite blades in your comparison.

1.5) thin vs. thick blades, weight vs. re-cut importance...

2) Hartzell vs. Erik's Rocket four blade composite prop.

3) heated vs other ice protection, if you have that in mind...

4) climb to thin air at altitude importance...

5) air resistance at the altitude you intend to fly.

6) harmonic vibrations related to the ratio of blades to cylinders..? 2:4, 3:6

The two blade prop may make more sense hanging around the lower NA altitudes.  The two bladed Long Body was somewhat less effective for the lower powered Eagle.

Best regards,

-a-

[KSMooniac]

Look hard at the MT option...lightest of the choices (important on a K) and near-infinite blade life.

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[M20F]

If you fly high a lot you will miss the third blade.  

[IndyTim]

@carusoam, thanks.  So you're saying the speed differences aren't as significant up high?  I like to fly at 16-17k, sometimes at FL21/22 if the winds are right.  So that would mean I wouldn't see the speed advantage in probably 50% or more of my flying.

I'm still trying to figure out what all the options are for my 252.  I've got Hartzell, Macauley, and MT on my list of manufacturers - any others?

[carusoam]

Tim,

The drag related to having a third blade is considered additional flat plate resistance.  This is more important down low where NA engines are still generating power.  It becomes less important with thin air where you will be conducting the speed runs.

the three bladed props have more drag, but they accelerate and climb better.

For an interesting video, somebody recorded and posted a video of Erik's Rocket in the Take-off run from the last NJMP fly-in...

1 vote for good T/O acceleration

1 vote for good climb

1 vote for how it looks on the plane

1 vote for the weight (use actual manufacturer's weights, not guesses) hubs and blades vary a lot by design.

-1 vote for low altitude increased drag

The 3 blade TopProp came as part of a 310hp STC package that gave performance data for easy comparison.  Buying just a prop may make it difficult to obtain discrete data.  The Prop companies may just have it available.

The TopProp blades come in thick, thin and composite.

The MT comes in composite three and four blades.

Both companies are pretty easy to communicate with.  One is made in America. The other has an office in Florida...

I selected the the thin aluminum TopProp based on my expected type of flying.  I feared the lack of rotational inertia of the composite props, more personal reasons than actual science.

MT's four blade took an extra year to make it to market after I bought my prop.

I am still just a PP, not a mechanic or CFI.

Hope that helps,

-a-

[LANCECASPER]

When Mooney brought back the M20K as the Encore in 1997 the articles in Flying and AOPA that introduced it talked about all the testing that had been done with different props (3 blade, MT, etc) and what all of the tests conformed was the best top speed was the 2 blade from the 252 so that's what they went with. Tom Bowen who is back with Mooney was the one they quoted in the articles.

[FlyWalt]

I'd say the engineers from Mooney have the best data to base their decision on. Which prop came standard on the Encore?

[carusoam]

Lance would you happen to recall how significant the difference was at the top speed?

The scimitar shapes probably came a couple years later, around 2011.

Lance has made a good point.  It is an important decision that will effect three or more things.

T-O run, Climb rate, and Top speed... Some sound as well.

Data for your plane should be available from the vendors. The TopProp is available in two and threeblade formats.

Of course the top speed quoted by the manufacturers is going to have the halo effect of running all out max power, ROP etc...

Distributing power becomes more challenging with more HP.  300hp is being served well with a three blade prop.

How much HP does the 252 have?

Best regards,

-a-

[MB65E]

I would see what the prop shop says about your current prop. Best case is new blades and out the door. I have always thought if you have the HP, then the 3rd blade will help pull you. And face it, it looks great! 

If the hub is shot too, I would look into seeing if the new MT with the carbon semitar blades and nickel leading edge is available. The old style wood/fiberglass blades had issues but the latest MT's I think are AMAZING. They don't have the De-lam problems the older wood core blades did. If you keep the older style out of the rain they are fine, but really sucks when you knock the finish off a new blade. 

Glad to here everyone's ok. Btw.

-Matt

[Hank]

It's definitely and applies-to-oranges comparison, but I have a Hartzell 3-blade on my C. Blade length is the same as the 2-blade models, so there will be no noise reduction as tip speed will be the same. So with the draggy third blade, right now I cruise at 8-10K around 170-172 mph true; book speed is between 160-164 mph at my typical weight. So much for slowing down . . . My only speed mod is the 201-windshield.

Get what you like best. Run the numbers from the vendors, include price and real-world results from others. Ignore Old Wives' Tales and other internet hysteria. The 4-blade on Erik's Rocket really looks good!

[Parker_Woodruff]

Tim,

I have flown the M20K with the 2-blade prop and of course have over 200 hours in N252BH.  The 3 blade is much preferred in my opinion.

I have also flown Don Muncy's M20K with the 3-blade MT and it is impressive (and looks awesome!).  I had previously flown behind his 2-blade factory prop.  Big improvement.

MT 3-blade all the way.

[KSMooniac]

The weight savings with the MT is significant.  Do you have Charlie weights in the tail by chance?  If you take weight off the nose you can likely remove some or all of the weight in the tail, so the weight savings/useful load boost is amplified.

[Marauder]

Wonder if you can install one of these;

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[LANCECASPER]

AOPA August 1997

Out front, the Encore uses the same McCauley two-blade propeller found on the 252 — much to the surprise of the engineering staff. After 3 months of testing four other propeller designs from a total of three manufacturers, the engineers concluded that the original prop delivered the best performance.

When I flew the Encore prototype in March, engineer Bowen was testing the three-blade MT composite propeller from Germany (see " Waypoints: Mooney Gets an Encore," May Pilot). All white, the prop looked great and undoubtedly had the nod from the marketing staff. However, it delivered the worst cruise performance and had the lowest TBO and highest price. A Hartzell two-blade and two other McCauley two-blade variants all turned in respectable performance, but the original McCauley design won the heat.

http://www.aopa.org/News-and-Video/All-News/1997/August/1/Mooney-Encore

 

A similar phenomenon with the Ovation. When the Ovation was introduced in 1994 it had a 3 blade prop. Later when the Ovation 2 was introduced it had a 2 blade since extensive flight testing showed that the 2 blade was 7 knots faster in cruise

 

AOPA JUNE 2003

Let's take the airplane's speed profiles first. Running wide open at 22 inches of manifold pressure and 2,500 rpm, with mixture set at 50 degrees rich of peak (this turns out to be 16 gph), the Ovation2 will turn in a very respectable 190 KTAS at 8,000 feet under standard conditions. This is some seven knots more than the Ovation2's predecessor — the Ovation, built between 1994 and 2000 — thanks to the 2's switch to a more efficient two-blade McCauley propeller.

 

 

Edited February 13, 2016 by LANCECASPER

[aviatoreb]

[KSMooniac]

In MT's defense, they were using a different blade shape back then. The latest style has a scimitar shape and better performance.

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[carusoam]

Great (re)collection of data, Lance!

Both KSMooniac and Aviatoreb are running MTs with composite scimitar blades.

Lance's collection of data sets the bar at 190 kts using 16 gph, NA @ 8,000' and 50°F ROP.

With modern portable technology, anybody can collect valid T/O distance and performance numbers pretty easily without having to be 'Engineer Bowen'.

Wondering what the values are for modern blade designs using a TC'd Powerplant @18k' and the T/O distance @SL. 

I am thinking Tom Bowen is going to get the rare opportunity to collect this data for the new O and Acclaim.  I bet he will be running 2700 rpm when he collects the T/O data and 2550 rpm for cruise data for the NA bird and something similar that is TIT friendly for the TC'd bird.  Then re-do the numbers for LOP.

Long live the 2 vs. 3 vs. 4 blade / ROP vs. LOP discussion!

Unfortunately, it seems there is no easy answer to Tim's question.  But the vendors are pretty easy to contact.

Best regards,

-a-

 

@carusoam

[IndyTim]

  On 2/13/2016 at 5:11 AM, LANCECASPER said:

 

A similar phenomenon with the Ovation. When the Ovation was introduced in 1994 it had a 3 blade prop. Later when the Ovation 2 was introduced it had a 2 blade since extensive flight testing showed that the 2 blade was 7 knots faster in cruise

 

AOPA JUNE 2003

Let's take the airplane's speed profiles first. Running wide open at 22 inches of manifold pressure and 2,500 rpm, with mixture set at 50 degrees rich of peak (this turns out to be 16 gph), the Ovation2 will turn in a very respectable 190 KTAS at 8,000 feet under standard conditions. This is some seven knots more than the Ovation2's predecessor — the Ovation, built between 1994 and 2000 — thanks to the 2's switch to a more efficient two-blade McCauley propeller.

 

 

Expand  

7 knots @ 190 is a significant difference.  Thanks for finding/sharing Lance.

I'll reach out to the vendors, see if they have any recent data points.

I'

[LANCECASPER]

Not that I remember a lot from my physics class other than p=m*v (momentum = mass times velocity). 

A 3 blade takes a greater bite out of the air which is nice for take-off and initial climb, but the additional mass up front which the engine has to turn must pay a penalty somewhere, and it looks like you see it most in cruise when you are set up for greatest efficiency.

[aviatoreb]

Yes turning a mass eats energy.  So all things being equal, one expects 2 blades to be faster than 3, and one expects 3 blades to be faster than 4, but the climb thrust to be better the other way.

In the rocket case, and perhaps other installations, are things are not equal.  For one, the prop that the MT 4 blade replaced was a mcauley old school design, so likely not so aerodynamically efficient vs the highly modern scimitar shape you see in the picture.  For 2, that MT prop is 36lbs lighter than the mcauley it replaces - which is super for w&b reasons, but purely for the discussion here, you are saving the energy required to spin a 36 lbs of heavy mass.

[Andy95W]

The 3-blade also increases frontal area of the airplane.  This is a bit counter-intuitive, since that frontal area provides thrust, but it is there all the same.  This is one of the reasons the third blade is less noticeable at lower speeds (i.e., climb) than in high speed cruise.

Clean, smooth air for the prop blades is an additional factor.  Theoretically, the most efficient propeller is a 1-blade that rotates in the least turbulent air possible.  Big turbo-props use 3 and 4-blade props efficiently because of their slower speed, often 1400-1800 rpm.

At least, that's how it was explained to me years ago.

[aviatoreb]

  On 2/13/2016 at 7:21 PM, N1395W said:

The 3-blade also increases frontal area of the airplane.  This is a bit counter-intuitive, since that frontal area provides thrust, but it is there all the same.  This is one of the reasons the third blade is less noticeable at lower speeds (i.e., climb) than in high speed cruise.

Clean, smooth air for the prop blades is an additional factor.  Theoretically, the most efficient propeller is a 1-blade that rotates in the least turbulent air possible.  Big turbo-props use 3 and 4-blade props efficiently because of their slower speed, often 1400-1800 rpm.

At least, that's how it was explained to me years ago.

Expand  

Agreed - again all things being equal - meaning same blades and same weight.  But two blades will be less heavy than 3 so steal less power, but lighter material on the more blade system makes this nulified.  ANd also if the more blade system is sufficiently better aerodynamic shapes the other argument is nullified too.

[carusoam]

i initially did not follow the physics of the weight of the prop thing...

1) Weight of the prop is slowing things down?   This is somewhat true during acceleration, from 0 to 2700rpm.  It is stretched over the period of time from start-up to take-off.  The bulk of this happens in about three to five seconds as quickly as you push the throttle in...

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.

2) It is definitely a WnB issue.  Dumping Charlie weights would be appreciated by most.  Zipping down the runway would be a tiny bit quicker and could use less runway... In an F=ma kind of way.  Going from zero to 65ias (KIAS or MIAS) depending on your ASI...

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.

4) prop mass and related forces while turning, are going to be large numbers in the thousands of pounds.  Losing a blade in flight is going to be bad for all props.

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.

6) more blades have a tremendous amount of leading edge surface area.  50% more going from two to three blades using the same diameter.  Does this count against prop efficiency? Some of that extra leading edge is traveling at 500 or so mph.

7) more blades has become important as power exceeds 300hp.  Delivering that power to the air requires a longer prop or more blades or a really cool blade design.

8) use of short fields,  2000' and less, are more comfortable with a T/O distance under 1,000'.  Leaving time to for decision making and distance for stopping.  This is a combination MGTW and T/O power delivered through the prop.

Unfortunately, Logic isn't strong enough to provide the answer that mounting up a couple of propellers will.

more thoughts,

-a-

[DonMuncy]

When you talk about mass up front, you have to consider the material. My new composite/wood, MT three blade is a lot lighter than my old McCauley 2 blade aluminum prop.