NEUFORM Prop Pitch

[NC Bill]

At annual the NEUFORM prop on my CTLS was repitched. It now consistently produces 4700 RPM on T/O and 5250 RPM WOT straight & level  flight.

 

I'm having it re-pitched this week. Any suggestions on what adjustment to make before next test flight?

 

Appreciate your input.

 

 

[FlyingMonkey]

I think it needs to be flatter.  Mine was very similar to yours, then I had it changed to get 5100rpm on takeoff and ~5700 WOT (at about 3000 feet) in level flight at Roger's suggestion and I gained ~7 knots in cruise and ~300fpm in climb. 

[FastEddieB]

That seems like too much pitch.

 

Remember, your engine produces 100hp - but only at 5800 rpm.

 

4700 rpm leaves a lot of takeoff and climb performance off the table.

 

I was unhappy with my Sky Arrow developing slightly less than 5,000 rpm on takeoff and climb.

 

I took 1° out, having been told 1° should equal about 200 rpm, IIRC. That in fact worked out for me - I now see about 5,200 rpm on takeoff. Takeoff and climb performance were noticeably improved.

 

This was all with a Warp Drive prop, but I think the same principle would apply to your prop.

 

As a goof, try to find the hp chart for your engine, and see how much hp you're producing at 4,700 rpm - you might be surprised!

 

Finally, if you're SLSA, be sure that you're free to experiment "outside the book". I did not change my Sky Arrow until I changed it to Experimental.

[FlyingMonkey]

Your cruise speed will be better, your top speed better, your climb will be better and your fuel economy will be better.

 

And you will be thinner and more attractive to women. 

[Anticept]

That goes without saying. Pilots are faster, better climbers, last longer .

You can plug in any other verbs or adjectives you want.

 

But it's us mechanics that makes it all work.

[Znurtdog]

My experience has been that flattening the prop indeed raises the max rpm and improves climb, but has always reduced top speed in the planes I've flown. I understand that the static and cruise rpm is THE important factor in having a happy engine and am not advocating pitching prop for speed only. I just flew an SW that can't get to 5500 rpm unless w/o throttle and descending, and even then, just barely. BUT at 4500' and 5200 rpm, TAS was 125kts at 5.8 gph (ish)  

[Ed Cesnalis]

Your max continuous speed for any given altitude is at WOT and a pitch that will result in 5,500RPM.  Any flatter and you will have to retard the throttle in order to maintain the 5,500 limit.  Any coarser and you will not be able to achieve 5,500.  In either condition you will be unable to achieve maximum continuous power.

 

CTs and other LSA have a history of getting pitched, when new too coarse to realize either best climb or even best continuous speed so in almost all cases the adjustment to a flatter pitch will result in better climb and better speed.

[FastEddieB]

Your max continuous speed for any given altitude is at WOT and a pitch that will result in 5,500RPM.  Any flatter and you will have to retard the throttle in order to maintain the 5,500 limit.  Any coarser and you will not be able to achieve 5,500.  In either condition you will be unable to achieve maximum continuous power.

 .

Well put.

[FlyingMonkey]

Your max continuous speed for any given altitude is at WOT and a pitch that will result in 5,500RPM.  Any flatter and you will have to retard the throttle in order to maintain the 5,500 limit.  Any coarser and you will not be able to achieve 5,500.  In either condition you will be unable to achieve maximum continuous power.

 

CTs and other LSA have a history of getting pitched, when new too coarse to realize either best climb or even best continuous speed so in almost all cases the adjustment to a flatter pitch will result in better climb and better speed.

 

I over-flattened mine a little because, while I most often fly at low altitudes (under 4000ft), when flying longer distances where I would climb higher I wanted to be able to get more rpm so I can maintain higher rpm if desired up around say 7500-8500 feet.

 

I wonder if another reason some LSA come with coarse pitch from the factory is to game the system and meet the LSA speed limit...

[Ed Cesnalis]

I over-flattened mine a little because, while I most often fly at low altitudes (under 4000ft), when flying longer distances where I would climb higher I wanted to be able to get more rpm so I can maintain higher rpm if desired up around say 7500-8500 feet.

 

I wonder if another reason some LSA come with coarse pitch from the factory is to game the system and meet the LSA speed limit...

 

A traditional aircraft engine can be leaned at WOT but the Rotax, carbed or injected goes full rich at WOT.  At your lower altitudes you can only cruise with economy by throttling back to 92% and have the econo mode lean for you or have the needle circuit lean for you, so your extra flat pitch gives you both an economy cruise setting as well as improved climb.  If you were 5,500 @ WOT @ low cruise altitudes not only would you be full rich but you would be cruising at higher than 75% which is not limited by Rotax but it is by others.  Cruising at 90% is doable but maybe not advisable.

 

If you cruise at 10,000' you're more likely to cruise WOT, even though you are full rich the volume of gas/air mixture is reduced enough to keep the rate of fuel burn down.  Always full rich at WOT is good insurance but the high altitude economy does suffer.  I would very much like to lean when cruising at 10,000' or higher.

[ctfarmer]

I am struggling to get a fundamental understanding of the concept of flattening or fining up the pitch to go faster.

 

If we coarsen the pitch we move more air per rev so to move the same amount of air we can rev lower. Accordingly, it we are limited to 5500 rpm max continuously rated speed, how is it possible that we go faster with a finer pitch at 5500 rpm as opposed to the 5500 rpm at a coarser pitch.

 

Roger, to quote your post above.

"If you only set you RPM to max out at 5500 rpm I will always out climb and out speed you in WOT if mine is set for 5600-5700."

 

You have a finer pitch if propped at WOT to 5600-5700 than it propped to 5500rpm WOT. So if in the first case you drop back to 5500 you must be going slower in straight and level flight compared to an identical aircraft set up with a coarser pitched prop also running at 5500. The first instance must move less air by virtue of having less angle of attack.

 

Or is that not what is being said?

 

I can understand pitching for 5800 to get max non continuously rated power on take off.

 

This is aside from fuel usage which may to a degree come down to prop efficiency and how much power is required at any rpm to move so much air ie are you better with greater angle of attack but less rpm or vice versa. Efficiency, i can understand may be variable at various angles of attack. As mentioned as well, it will also come down to where the engine is more efficient.

 

I do not comprehend the idea of going faster with less pitch if we have a 5500 rpm limit. If both were reduced to 5200rpm for example then the coarser pitch will still move more air and you should go faster.???

[Ed Cesnalis]

The coarse pitch moves more air only if the engine can provide the power. If you increase the load to result in a 5200 RPM the Rotax will be capable of less power resulting in less speed.

[Anticept]

As I've said multiple times:

 

The prop has two forces acting on it for thrust. Aerodynamic reaction, and Impulse.

 

A coarser prop means that the prop is going to be acting on impulse force, which means that basically the blade face (the flat side of the prop, which is the side the pilot sees in tractor configurations) slaps the air molecules backwards. This is very wasteful, as the prop imparts a spin on the airstream and some of the force used to propel the prop forward, is also used to push the air backwards.

 

A finer prop uses more of the aerodynamic reaction force (that is, until the angle of attack of the blades is too low to be efficient). This means the prop acts like an airfoil. The air forward of the propeller is at a lower static pressure as the propeller turns, and the air behind the propeller pushes forward on the propeller. Some of the energy is still imparted on the air using the impulse forces so it's not perfect, but you want as much of the aerodynamic reaction force to act on the air as possible, because this means more energy is converted to thrust, and less energy is imparted on the air molecules than with a prop working on the impulse force alone.

 

The most efficient propellers are those which are very large and turn slowly, as there is less turbulent air generated by the prop. Sounds odd especially since we are discussing FASTER turning props, right? Well look at a wind turbine blade. The tips on a turbine blade are moving VERY fast, and the roots are MASSIVE and have a huge twist to them (they work on the same principles as aircraft props, even though they extract energy instead of emit it). Since we have a tiny prop, we don't have the luxury of spinning slowly, as we just don't have as much prop surface area to impart thrust. Therefore, in order to make up for that, we have to turn faster, and design our prop blades around this faster speed.

 

It is the result of the aerodynamic reaction force that the wright brothers prop worked so well. Look at how their prop was shaped. Here's another view too. The wright brothers used a wind tunnel to learn and research propellers during a time where very few leading minds at the time even remotely understood how propellers worked in regards to fluid dynamics. It is the prop, and the cambered wings, that made the wright flyer successful, whereas others were using straight blade props on theirs and were not having much luck. The curvature also meant that the entire propeller was going to be efficient, rather than only one section as a straight blade.

 

Feel free to post any corrects if I am incorrect, but this is basically how I was taught, and it makes pretty good sense. Everything has a "power band," and props are no different.

[Ed Cesnalis]

The first thing I learned from watching guys prepare for the Reno air races is 'a race prop is a climb prop' (flat) not a cruise (coarse) prop.

 

Both climb and speed mean power and that is accessible at high RPM.

[WmInce]

Interesting and excellent discussion.

[FastEddieB]

The first thing I learned from watching guys prepare for the Reno air races is 'a race prop is a climb prop' (flat) not a cruise (coarse) prop.

 

Both climb and speed mean power and that is accessible at high RPM.

 

True to a point.

 

But clearly there's a "sweet spot".

 

Too flat and you have to throttle back to avoid exceeding redline.

 

Just stating the obvious, I know, but that "sweet spot" will vary depending on mission.

 

I tell students a fixed pitch prop is like a car with one forward gear, albeit with a "fluid clutch".

 

Choose a "gear" too low and cruise will suffer - like a car locked into second gear. Redline gets in the way of top speed.

 

Choose a "gear" too high, and acceleration and climb will suffer - like a car locked into top gear. The engine can never attain its full potential (max hp rpm).

 

And, of course, such is the beauty of a constant speed or other in-flight variable pitch prop - you can shift gears!

[WmInce]

True to a point.

But clearly there's a "sweet spot".

Too flat and you have to throttle back to avoid exceeding redline.

Just stating the obvious, I know, but that "sweet spot" will vary depending on mission.

I tell students a fixed pitch prop is like a car with one forward gear, albeit with a "fluid clutch".

Choose a "gear" too low and cruise will suffer - like a car locked into second gear. Redline gets in the way of top speed.

Choose a "gear" too high, and acceleration and climb will suffer - like a car locked into top gear. The engine can never attain its full potential (max hp rpm).

And, of course, such is the beauty of a constant speed or other in-flight variable pitch prop - you can shift gears!

 

Good analogy.

[FlyingMonkey]

I really wish we could use an electric variable prop with 4-5 settings like the CTs in Europe and Canada.

 

But clearly LSA drivers in the USA are way too stupid to work such a control.  Plus it might get us to 121 knots and instant death.   

[Ed Cesnalis]

True to a point.

 

But clearly there's a "sweet spot".

 

Too flat and you have to throttle back to avoid exceeding redline.

 

Just stating the obvious, I know, but that "sweet spot" will vary depending on mission.

 

I tell students a fixed pitch prop is like a car with one forward gear, albeit with a "fluid clutch".

 

Choose a "gear" too low and cruise will suffer - like a car locked into second gear. Redline gets in the way of top speed.

 

Choose a "gear" too high, and acceleration and climb will suffer - like a car locked into top gear. The engine can never attain its full potential (max hp rpm).

 

And, of course, such is the beauty of a constant speed or other in-flight variable pitch prop - you can shift gears!

 

Eddie, time to update your spiel.  When choosing a gear (pitch) the comparison is speed/climb vs economy not cruise vs speed.  

 

In my world top speed and cruise are the same thing and even at full rich the thin air keeps my burn to 5gph.

 

As I replied to Andy most of us are motivated to flatten to get beyond 5,500 so that we can throttle back to 92% or below throttle setting where economy improves.  When you were teaching in planes with a mixture control this was not a concern because you could lean above 92% throttle.

[Anticept]

"If we coarsen the pitch we move more air per rev so to move the same amount of air we can rev lower. Accordingly, it we are limited to 5500 rpm max continuously rated speed, how is it possible that we go faster with a finer pitch at 5500 rpm as opposed to the 5500 rpm at a coarser pitch"[/size]

 

 

But your Rotax 912ULS doesn't have the torque or HP (especially under certain conditions) to turn too course a prop pitch efficiently. You must tune the prop to the engine, not the engine to the prop.

​If you had unlimited power and torque then you could pitch courser, but at some point the prop pitch angle will become too course again and no longer produce thrust. It would be just like stalling a wing at too high a pitch angle. The air flow would depart the surface.

 

​So with our engine we must balance all flight characteristics to accommodate the engine and our specific flight needs like flying with floats that are heavy or flying at very high altitudes.

 

Going too course or too flat just hinders us. We need to find the sweet spot for the engine and for our specific needs.

To add to roger's point: it's not so much that horsepower directly affects the prop efficiency, but rather that more horsepower causes the aircraft to move through the air faster, and therefore needs a coarser prop to remain efficient. It's ALL about prop blade AoA in the end.

[FastEddieB]

Eddie, time to update your spiel.

Nope. I'll keep it as it is for now, and hope it can provide insight to others.

[Ed Cesnalis]

Nope. I'll keep it as it is for now, and hope it can provide insight to others.

 

Your choice,  I see the Rotax as a different animal and the prop pitch question has 2 additional variables that no-one includes in the discussion but me.

 

1. Leaning is limited and no economy power setting is available above 92% throttle.  This is not true in a lycoming or Continental or any traditional aircraft engine with a mixture control.  With my skyhawk I could access all available power by leaning for best power even at throttle settings between 92% and 100%  The additional question posed by the full rich limitation is Do I want access to all available power or will I accept less power in order to bring the needle circuit and its economy into play? (This is a question that comes into play at higher altitudes like above 7,500'.
2. Rotax limits power (redline) via RPM and not actual power settings. Our prop pitch and our cruise altitudes are variables.  You could select a cruise power setting above 75% by limiting cruise altitude and using enough pitch.  With a variable pitch prop an RPM limitation provides a different level of protection than a percentage of power limitation. It would be fun to have the fastest CT cruising the beach but perhaps pitching for this is a bad idea while not prohibited by the 5,500 continuous limitation.

[Anticept]

Your choice, I see the Rotax as a different animal and the prop pitch question has 2 additional variables that no-one includes in the discussion but me.

 

• Leaning is limited and no economy power setting is available above 92% throttle. This is not true in a lycoming or Continental or any traditional aircraft engine with a mixture control.

Edit: apologies for saying it is incorrect. Got ahead of myself on this

 

Both lycoming and continental use enrichment valves in carbureted engines, and economizer (no idea why it's called this, because it dumps MORE fuel) in fuel injected engines, that open above a certain throttle setting in carbureted and fuel injected engines and force a richer mixture.

[Doug G.]

I'm not sure, but I think you both said the same thing from different directions. ?

[Doug G.]

Rotax - no leaning (as in mixture adjustment - except by modifying the needle setting in the carb - not in flight).

- no economizer circuit. That is what I got from both of you.