Let's talk propellers.

[testwest]

OK, here is a cruise comparison between a McCauley 212 74" 2-blade prop and a three blade MT MTV-12-B/180-59b at 71", the first one is the original prop for our 77J and the second is the STC'ed composite (wood) three blade for the same model.

Input conditions are 8000 feet, about 155 KTAS, standard day and about 70% power or 140 hp into the prop at 2500 RPM.

McCauley:

And MT:

Some very interesting data here. As expected the 2 blade McCauley is ever so slightly more efficient at cruise, but I would be hard pressed personally to ferret that performance delta out in flight test.

The really neat thing is how much quieter the MT is at cruise, as shown by the noise footprint graphic. The calculation of this footprint is by a modified Gutin noise formula, the original of which dates to 1936 and the modification of the formula from NACA Technical Note 1145, written in 1946 and downloadable here (for the curious): https://apps.dtic.mil/dtic/tr/fulltext/u2/a801118.pdf

I prefer the dirt-farmer approach to interpreting the noise footprint: The orange area is 100-110 db, or about as loud as a Kiss concert near the front row. The smaller the orange area the better, if you are flying.

 

[cliffy]

Does the orange area develop into a "donut" type shape in a 3D presentation?

With the cockpit in the orange area in both presentations is there any perceptible difference in noise level between the two props in the cockpit? 

 

[testwest]

Yes, that is correct. Realize this noise signature is a bit of an approximation and there are all kinds of unsteady inputs that can't really be modeled. I will say the observations of those who have direct experience with both props on the same airplane are accurate, though, especially the assessment of lesser fatigue after a long flight.

[cliffy]

With all the advancements we have in aviation in 120 years we still haven't solved the noise issue both in the cockpit and outside

Someone will sell thousands of SE  airplanes if they can get the interior db down in the low 70s range say like a Cadillac or beamer

[amillet]

A friend had an MT prop on his M20K.  It was starting to delaminate after about 3 years if I remember correctly. Any history on durability of the composite propellers?

[bmcconnaha]

9 hours ago, Oscar Avalle said:

Very interesting discussion, I just replaced my three blade Mccauley with a MT. I am going to fly tomorrow for the first time. I heard so many things, that I am really curious, but I am trying to keep my expectations low. I will report back.

Looking forward to the pirep 

[Tim Jodice]

9 hours ago, testwest said:

OK, here is a cruise comparison between a McCauley 212 74" 2-blade prop and a three blade MT MTV-12-B/180-59b at 71", the first one is the original prop for our 77J and the second is the STC'ed composite (wood) three blade for the same model.

Input conditions are 8000 feet, about 155 KTAS, standard day and about 70% power or 140 hp into the prop at 2500 RPM.

McCauley:

And MT:

Some very interesting data here. As expected the 2 blade McCauley is ever so slightly more efficient at cruise, but I would be hard pressed personally to ferret that performance delta out in flight test.

The really neat thing is how much quieter the MT is at cruise, as shown by the noise footprint graphic. The calculation of this footprint is by a modified Gutin noise formula, the original of which dates to 1936 and the modification of the formula from NACA Technical Note 1145, written in 1946 and downloadable here (for the curious): https://apps.dtic.mil/dtic/tr/fulltext/u2/a801118.pdf

I prefer the dirt-farmer approach to interpreting the noise footprint: The orange area is 100-110 db, or about as loud as a Kiss concert near the front row. The smaller the orange area the better, if you are flying.

 

The more you teach brings up more questions. It takes power to make noise so how much power does a propeller waste making noise?

What is little bit of red close to the center of the two blade?

[carusoam]

17 hours ago, cliffy said:

So what you are saying is that if we have a 500 lb disc and a 5,000 lb disc, once up to an arbitrary speed (say 2700 RPM) that both would take the same HP to maintain that RPM?  (Fluid drag aside)  There's no extra cost in energy expended to keep 5,000 lbs spinning as opposed to 500 lbs? (other than the drag of the spinning object)

Yes.  That is what I believe to be true...

Ideally, An object in motion will stay in motion, until interfered with by an outside object....

Some law of physics... maybe first or second?   
 

Note: there is no mention regarding mass...
 

As for sound profile of the MT prop vs.  any aluminum one on all Mooneys... go to a Mooney fly-in with Erik... and me... there is a huge difference in audible sound given off by the MT vs everything else...

The 2700rpm redline on departure for 310hp has a very impressive, powerful sound... 
 

Best regards,

-a-

 

[cliffy]

7 hours ago, carusoam said:

Yes.  That is what I believe to be true...

Ideally, An object in motion will stay in motion, until interfered with by an outside object....

Some law of physics... maybe first or second?   
 

Note: there is no mention regarding mass...
 

As for sound profile of the MT prop vs.  any aluminum one on all Mooneys... go to a Mooney fly-in with Erik... and me... there is a huge difference in audible sound given off by the MT vs everything else...

The 2700rpm redline on departure for 310hp has a very impressive, powerful sound... 
 

Best regards,

-a-

 

OK lets go down the road a little farther then as I do understand the Law of Motion

If we have a 2 blade prop and a 3 blade prop (assume blade profiles are identical then we would have 50% more induced and parasite drag on the 3 bladder. How much HP would be lost in the additional drag that could be used for thrust OR does the disc density have a factor (also consider similar blade angles and RPMs)

With the disc density I guess we change the blade angles to get the same RPMs  and better climb because of the density? (Less slip?)

[testwest]

OK, for Tim, the power utilized in making noise is encompassed in the amount below 100%, so about 23% of the power going into the prop is not turned into thrust hp but into something else (noise, thermal transfer, radial outflow, radial drag, entropy and so on).

The red area is predicted to be the 125 dB noise footprint from the prop, but the prediction not really useful that close, it is more theoretical (with LOTS of caveats) than practical.

For Cliffy, the blade profiles for a two blade or three blade prop designed for the same power loading and speed range are somewhat different. The "induced drag" of each blade comes from producing half the thrust hp on a two blade or one-third on a three blade. The total rotational drag (among other things) tends to favor more blades at a slower speed and fewer at a higher speed. But as I have said, today's blade profiles are very complex and optimized compared to the WWII era versions, so the performance trade-offs are likely overshadowed by other qualities, like smoothness and noise.

[testwest]

One really fun thing for next year might be to have a Mooney fly-in and visit to Piqua, OH and the Hartzell factory. I have had the tour some years ago, and seeing the history and the current manufacturing effort at Hartzell is really awesome. Their motto is "Built on Honor" and you really get the sense that it is way more than just a slogan when talking to the skilled craftsmen and observing the factory in operation. A true American success story, and highly recommended!

[carusoam]

Cliffy,

This is surely a discussion that is so... complex, and full of variables.

The only thing I have to go on is the MS experience where we compare 2 vs 3 blade experience... where...

For the same 200hp engine...
 

The three blade is best at getting off the ground, and in climb... but the two blade is faster in cruise...

It always gets explained as simply the difference in flat plate type wind resistance...

 

As we approach the engine with around 300hp, the two blader is faster for the same reason... but is really missing the ability to deliver all of the hp during T/O and climb... probably due to a higher AOA?

 

One part of the challenge... we are limited in prop diameter.  So, as hp increases the number of blades inevitably increases...

The cool factor also come into play with Erik’a four blade MT... it is cool to fly in the FLs where the flat plate resistance is greatly reduced... by the thin air... 

 

One real challenge we run into... we don’t get the opportunity to swap props that often...

When I went from the Mac to the Hartzell... it was done at the same time as  a change from 280 to 310hp... completely burying how good the TopProp actually is...

Aside from the fact my prop got mangled while I was out of town... there is an STC in the way of hanging the old prop on the new 310hp engine...

See if @Seth is around... As he has had two different props on his Missile I believe...

@aviatoreb has also had two different prop designs on his Rocket... including the MT-4...
 

When discussing props, it’s always good to invite @Cody Stallings...

Great discussion... Thanks!
 

PP thoughts only, 

Best regards,

-a-

 

[carusoam]

9 minutes ago, testwest said:

One really fun thing for next year might be to have a Mooney fly-in and visit to Piqua, OH and the Hartzell factory. I have had the tour some years ago, and seeing the history and the current manufacturing effort at Hartzell is really awesome. Their motto is "Built on Honor" and you really get the sense that it is way more than just a slogan when talking to the skilled craftsmen and observing the factory in operation. A true American success story, and highly recommended!

That would be really interesting... in the post Covid era...

I’d like to discuss dynamic prop balancing and my spinner diameter with them... at the same time...

Their stock spinner diameter is off by about 1/4” compared to the O’s cowl...leaving a non-aerodynamic step right on the nose... ugh...

Best regards,

-a-

 

[cliffy]

Props have always been interesting to me even as a young kid designing model airplanes (of which I designed and built many of my own).   (Unfortunately I did not go the aero engineer route BUT MY models flew very well :-)

There is an airport museum somewhere "back east" that has a large display of different aircraft propellers including a single blade design! I can't remember which airport but I have been there twice. Brain Fart!

Single blade designs were used to some extent on model aircraft to avoid trailing blade in turbulence and loss of efficiency. Used more in very slow rotation indoor model aircraft than engine powered outdoor airplanes. 

I wonder where multi-blade designs now fit into this scenario? (5-7 or 9 blades) I guess it is quite figured out when one looks at the multiblade designs of high by-pass turbine engines (just big propellers). 

Understanding that blade shapes and angles are "massaged" to get optimum performance from any complete design of a propeller, I guess my question really comes down to a starting point between 2 or 3 blade design in that if we start with a base design of the SAME blade design and build a 2 bladder and 3 bladder I'm going to assume that the 3 bladder would have drag penalties to overcome that have to be made up with design changes just to reach the 2 blade performance starting point. That leaves even more design challenges to show any gain in performance.

Granted with the computational capabilities available today that challenge is greatly reduced than of yesteryear but none the less, still exists. 

Then again, how much of a design gain are we looking at or are we chasing our tail for a microscopic  % of gain? 

More bragging rights than actual performance gain? 

Do we seem to be looking for quantum gains in performance but picking up pennies instead?

$$$$$$ per MPH or 100 FPM?

Point of diminishing returns?   (MT vs Mc or Hart?)

IIRC there is one 2 blade design that we can use on both the 180 and 200 HP airplanes and the only TC change is a 2 degree lower blade angle min setting for the 180 HP. It been a while but that is what I recall. 

Granted doubling the HP and limiting the diameter has HP absorption challenges. Going back to DC-3s they had MANY blade designs. One was called the "tooth pick" prop because of the narrow blades it used. Just look to the        C-130 design evolution and see how the wide blade, square tip prop evolved there. 

As has been mentioned many times- ANYTHING IN AVITAION DESIGN is all ABOUT TRADE OFFS in performance.

There ain't no free lunch!  :-)

 

 

[Oscar Avalle]

On 9/20/2020 at 2:02 AM, bmcconnaha said:

Looking forward to the pirep 

Weather derailed my flight... so next saturday

 

[carusoam]

Cliffy,

When you get that part figured out...

 

We can graduate to the next level...

 

What is this...?  

When would I want to use it?

From the current catalog of Hartzell... composite...

Looks like something from the Grumman canoe catalog...

-a-

[cliffy]

But is it a quantum leap in performance or picking at pennies?

It does look cool though

[Airways]

Looks like the latest Cirrus prop. Should be easy to compare performance of different generations Cirri.

[Tim Jodice]

6 hours ago, Airways said:

Looks like the latest Cirrus prop. Should be easy to compare performance of different generations Cirri.

I couldn't find anything on COPA separating the metal three blade to the composite three blade but they say the three to four blade is purely cosmetic. 

I am guessing that is the case because on a Cirrus even the metal blade is a scimitar current design prop so the only difference is what it is made out of. @FoxMike and myself went one blade up, composite and propellers designed three or four decades later than the original propellers.