Prop Performance on Mooney's

[mike20papa]

S=bce? Say what?  Theory is one thing, but performance doesn't prove out.  And ramp appeal - that is not what I call performance!

[Bob_Belville]

24 minutes ago, aviatoreb said:

More blades means they use shorter blades.

Are you sure about that? The prop ground clearance is the same for 3 and 2 blade models for the same model Mooney. 

[Raptor05121]

Yup. I have the same diameter as a 2-blade. 76" IIRC

[kortopates]

I think its a true statement that every 3 bladed aluminium prop for the Mooney's has exactly the same diameter as the two bladed aluminium props; at least that's true of the Hartzell options. I don't think we saw a reduction in prop length till the MT prop's.

I have yet to be fly behind any example of an extra blade on a Mooney, such as a 3 bladed aluminum or 3 bladed MT composite, without seeing very noticeable drag or braking action when you pull power back to idle (since they're non-feathering prop's). But I have experienced nothing smoother than the 3 bladed composite MT - they're amazingly smooth; turbine smooth too me.

 

[Alain B]

Carbon Hartzell and MT prop have a stainless steel or nickel edge to prevent abrasion . The main advantage of an MT prop ( i dont know about Hartzell )  if is you really hit something hard with the prop , blades will shear off instantly , in many case preventing extensive engine damage . I know , i had lived it . 

[KSMooniac]

The thin stainless steel leading edge is far more tolerant of debris IMO than an aluminum prop. If it gets bent/dented badly, it can be replaced and the blade lives on. Nothing to file away and eventually scrap the blade.

As mentioned, I think it would be nearly impossible to bend a crank or internally damage an engine due to prop strike with a composite prop. I hope to never try, though!

Sent from my LG-LS997 using Tapatalk

[Guest]

2 hours ago, Ah-1 Cobra Pilot said:

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

Isn't a propeller an airfoil spinning in a different plane? And still making "lift"

Clarence

[RobertGary1]

8 hours ago, D. Snyder said:

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!

My recommendation is not to put money into such goodies for the first couple of years. The first two years most owners are surprised by something very expensive even with the best prepurchase. There are a ton of things I'd put money into before getting to a prop, not sure what the 231 already has but after unexpected repairs I'd rank things such as  Modern LPV capable GPS, AHRS (non-vacuum) primary attitude, ADS-B, engine monitor before a prop.

-Robert

[aviatoreb]

58 minutes ago, Bob_Belville said:

Are you sure about that? The prop ground clearance is the same for 3 and 2 blade models for the same model Mooney. 

...no not sure.  It is true for my setup.  That I'm sure of.

Mind you in the following - I don't know what I'm talking about - but lets say I'm just saying what I think must be true.  A props blades must create create enough lift to reflect max take off power of the engine.  So I would guess with identical airfoils, then 3 (or 4) blades would be progressively short, all other things being equal.  But other changes (all things not being equal), such as a different kind of airfoil (say round tips vs scimtar) or perhaps wider fatter blades, or whatever else they might do with blades, I would think would counter my idea.

Ok - who here is an aeronautical engineer and knows the truth and who can grade my laymen's understanding of prop design aeronautical engineering.

[aviatoreb]

46 minutes ago, Alain B said:

Carbon Hartzell and MT prop have a stainless steel or nickel edge to prevent abrasion . The main advantage of an MT prop ( i dont know about Hartzell )  if is you really hit something hard with the prop , blades will shear off instantly , in many case preventing extensive engine damage . I know , i had lived it . 

One thing - the leading edges you see on my prop are the upcharge nickel leading edge and they provide greater abrasion protection than the otherwise standard and very very thin stainless steel leading edge.  I read extensively online (across many different kinds of airframes) and many people had abrasion issues of various kinds (esp kicking up gravel) with the stainless steel treatment which was considered too meagerly covering the rest and not one person described such a problem withe nickel treatment you see on mine.  Knock on wood - now that said - let's hope that now I wont be that first one.

Edited March 21, 2017 by aviatoreb

[Hank]

The 3-bladed aluminum air brake mounted on the nose of my C is very smooth. It was installed and balanced in 2002; I rechecked the dynamic balance at annual this January and it read 0.01 ips, so I didn't do anything. It has stayed well balanced because, despite occasional trips to grass fields and many low-use paved fields with debris on the asphalt, I've yet to need anything dressed out of the blades.

As for drag, my lowly C with 3-blade Hartzell and 201 windshield has trued out several times in the 148 -149 KTAS range at altitude. And yes, that big prop (same diameter as 2-blade versions by Hartzell and McCauley) does indeed slow the plane right down. Kinda like hitting  speed brakes, which no one complains about. I routinely pull the throttle to idle 1/8 - 1/4 nm before the pavement, even when coming over trees that require a displaced threshold, and set her down gently with the stall buzzer sounding on the 3rd or 4th stripe. 

And I think the smaller 4-blade MT looks great! Wish I could move my empty CG an inch or two towards the tail . . .

[Seth]

8 hours ago, aviatoreb said:

Did someone call?

I always forget how good it looks until I see you Rocket again!  Love it every time!

 

-Seth

[carusoam]

4 hours ago, aviatoreb said:

 

Ok - who here is an aeronautical engineer and knows the truth and who can grade my laymen's understanding of prop design aeronautical engineering.

Scott from Kansas is our engineer.  

 

Another nice thing about the significant weight removed from the nose is the lower force used to trim the nose up.

Less trim= less drag= Better airspeed!

The only thing that bothered me regarding he light weight props was the lack of flywheel effect.

From engine theory, this was supposed to be very important.  In reality, it doesn't get mentioned anymore...

A well balanced MT on an IO550 has got to be extremely smooth...  (or composite TopProp...)

Speaking of smoothness and harmonic vibrations... harmonics are the things we generally don't feel. But can be disastrous for the engine...  harmonics are also the 3 blade prop on a four cylinder engine topic.  Somebody would have to measure it to report what the combination is capable of... (yellow zones on the tach)

On the other hand the power stroke on the two blade prop on a four cylinder engine is really well known.  It is always on the blade going down...(?)  When it's a three blade prop it is not as easy to describe.  Unless it is a three blade prop on a six cylinder engine.

Homework assignment: Determine when the power stroke occurs for each of the six cylinders, what is the position of the #1 blade? 

I have illustrated the common argument of mismatching blades and cylinders. But have not given any solution...

When the powerstroke is always delivered in the same place, matching the blade always in the same place, the blade is in the same relative wind each time.  

The blade going down vs. blade going up and the attitude of the plane seems to theoretically make a difference...

See if Scott has some insight..?

Best rgerds,

-a-

Edited March 22, 2017 by carusoam

[Bob - S50]

7 hours ago, aviatoreb said:

...no not sure.  It is true for my setup.  That I'm sure of.

Mind you in the following - I don't know what I'm talking about - but lets say I'm just saying what I think must be true.  A props blades must create create enough lift to reflect max take off power of the engine.  So I would guess with identical airfoils, then 3 (or 4) blades would be progressively short, all other things being equal.  But other changes (all things not being equal), such as a different kind of airfoil (say round tips vs scimtar) or perhaps wider fatter blades, or whatever else they might do with blades, I would think would counter my idea.

Ok - who here is an aeronautical engineer and knows the truth and who can grade my laymen's understanding of prop design aeronautical engineering.

On my J, the standard McCauley prop has a 74" diameter.  When I look at the STC for the MT Prop (3 blade - "new" blade), the diameter is 71", so yes, you appear to be correct.

That being the case, the 3 blade would give us another 1.5" of ground clearance.

One thing to keep in mind though, I sort of read an article on prop efficiency by a Dr. David Rogers.  Propeller efficiency is related to airspeed and RPM.  The faster you go, the higher the RPM needed to hit peak efficiency.  The shorter the prop, the higher the RPM to do the same thing.  So a shorter prop flying at a higher speed would want to operate at a higher RPM to perform at its best.  Using formulas in his paper, for my 74" prop, and speeds around 155 +/-, the theoretically best RPM is around 2500.  For a 71" prop at the same speeds, the ideal RPM would be around 2600 RPM.

The STC also lists the weight as 46# and the McCauley 2 blade is 50.5#, so you also gain another 4.5# of useful load.  From the reports I've read on this page, if we ever have to trash our prop and replace it, I'll definitely look into the MT prop.

[ArtVandelay]

Isn't a propeller an airfoil spinning in a different plane? And still making "lift"
Clarence

Which is why in cruise they say you lose a little...more lift, more drag. I don't believe the OWT that cylinders should be divisible by blades.
I believe it's more dependent on horsepower.

[Hank]

Bob, do a weight analysis against a Hartzell 3 blade, it should be a significant weight reduction. 

[Guest]

37 minutes ago, teejayevans said:

Which is why in cruise they say you lose a little...more lift, more drag. I don't believe the OWT that cylinders should be divisible by blades.
I believe it's more dependent on horsepower.

The OWT really messes with my 3 blade MT on my 8 cylinder Lycoming.

Clarence

[Danb]

18 hours ago, aviatoreb said:

Did someone call?

Erik she still looks awesome. You rat!

[AH-1 Cobra Pilot]

16 hours ago, M20Doc said:

Isn't a propeller an airfoil spinning in a different plane? And still making "lift"

A propeller is really several airfoils combined.  Chord, AOA, shape, etc. vary per the distance from the hub.  I am not sure what you are really asking here.
 

[AH-1 Cobra Pilot]

16 hours ago, aviatoreb said:

...no not sure.  It is true for my setup.  That I'm sure of.

Mind you in the following - I don't know what I'm talking about - but lets say I'm just saying what I think must be true.  A props blades must create create enough lift to reflect max take off power of the engine.  So I would guess with identical airfoils, then 3 (or 4) blades would be progressively short, all other things being equal.  But other changes (all things not being equal), such as a different kind of airfoil (say round tips vs scimtar) or perhaps wider fatter blades, or whatever else they might do with blades, I would think would counter my idea.

Ok - who here is an aeronautical engineer and knows the truth and who can grade my laymen's understanding of prop design aeronautical engineering.

I am.

A prop diameter is limited more by Mach number than by number of blades.  For instance, for a 74" diameter prop:
pi*d*omega=v(rotational)     3.14*74in*(12in/1ft)*2700rpm*(1min/60s)=871ft/s (the tip speed of the prop)
Over the airfoil, this number will be a little higher.  There are two main reasons you do not want this number to go sonic: you lose lift, and shock waves can damage your blades.  Thus,prop length is limited, more dependent on engine rpm than anything else.

Engine horsepower is dependent on rpm, (higher is more).  A geared engine uses higher engine rpm to achieve similar prop rpm.  This is why my old P172 used a GO-300.  It had 175hp vs. 150hp for the C-172's standard O-300 engine.  The geared engine turned faster, using more fuel, providing more horsepower, with a lower TBO.  (The airplane also had a constant-speed prop and yielded 8kts and 100lb useful-weight advantages.)