Prop pitch - myth

[Ed Cesnalis]

I have been hearing this lately and its a myth:

 

"Propellers are made in varying pitch angles, and the angle can be optimized for a specific mission. Generally, a coarse pitch (high angle) lets the propeller take a larger bite out of the air with each revolution, which tends to slow the RPM but increase cruise speed. A fine pitch (low angle) permits higher RPM, generally improving takeoff and climb performance, at the expense of cruise speed."

 

The myth stated simply is that a coarse pitch will increase cruise speed. The fact is a coarse pitch will reduce cruise speed but will gain economy.

 

A fine pitch will not increase climb performance at the expense of cruise speed. That's another way to state the myth. A fine pitch will increase performance in both climb and cruise at the expense of economy, limited by the redline.

 

Fast Eddie says: "It all depends on the engine's torque/HP peaks and the ability to turn that into thrust. As always, a fixed pitch prop is like a car with one gear, and as such, a compromise." He's right that there is a compromise but confused about what the trade off is. The trade off is economy at the expense of speed, not climb performance at the expense of speed.

 

Look at the max engine output line, it tells the story. When you flatten the pitch and lighten the load on the engine more RPM and power is realized. When you coarsen the pitch and increase the load on the engine less RPM and power is realized. "A fine pitch (low angle) permits higher RPM, generally improving takeoff and climb performance, at the expense of cruise speed." Finer pitch always means more power, it can't slow you down, it just costs more gas.

 

[FastEddieB]

Got to go do some weed whacking.

 

Will be formulating a response to post later.

 

As a teaser...

 

"Finer pitch always means more power, it can't slow you down, it just costs more gas."

 

...is demonstrably wrong. And I will so demonstrate later.

[Ed Cesnalis]

Eddie,

 

I have already conceded a couple of times that there is a red line limitation. I posted the power curve that is confined to the motors operating range.

[Ian]

CT I'm struggling with that - Over here we operate the CTLS with a variable pitch propellor - and for the same rpm coarsening the pitch increases speed, when coming back into the circuit prior to landing then moving the prop to a finer pitch decreases airspeed.

 

With the prop in full fine we never see more than about 85-90 knots regardless of rpm - coarsen it then we can get 110+ knots at around 4700 rpm.

[Ed Cesnalis]

Hi Ian,

 

for the same rpm coarsening the pitch increases speed the speed increases because you increase power, if you simply coarsened the pitch you would slow, but if you add throttle to maintain the same RPM at the higher load you will go faster.

 

when coming back into the circuit prior to landing then moving the prop to a finer pitch decreases airspeed. the speed decreases because you reduce power. If you flatten the pitch and want to maintain or increase speed you would have to maintain or increase power.

 

With the prop in full fine we never see more than about 85-90 knots regardless of rpm - coarsen it then we can get 110+ knots at around 4700 rpm. What RPM can you achieve in full fine?

[Jim]

Do any of the prop manufacturers give a chart showing rpm vs thrust at a series of different pitches? Such a document would be very enlightening.

[Ed Cesnalis]

Do any of the prop manufacturers give a chart showing rpm vs thrust at a series of different pitches? Such a document would be very enlightening.

 

Like the bottom line on the graph I posted above?

[Jim]

No. Unless I'm missing something, that's only for one unspecified pitch.

[Ian]

Hi Ian,

 

for the same rpm coarsening the pitch increases speed the speed increases because you increase power, if you simply coarsened the pitch you would slow, but if you add throttle to maintain the same RPM at the higher load you will go faster.

 

when coming back into the circuit prior to landing then moving the prop to a finer pitch decreases airspeed. the speed decreases because you reduce power. If you flatten the pitch and want to maintain or increase speed you would have to maintain or increase power.

 

With the prop in full fine we never see more than about 85-90 knots regardless of rpm - coarsen it then we can get 110+ knots at around 4700 rpm. What RPM can you achieve in full fine?

 

Yes, you're right, there are obviously power adjustments too - BUT - if you merely adjust the pitch AND keep the throttle at the same setting it does have an effect on the speed achieved too.

 

At full fine, static on the ground we see about 5,500 rpm - once airborne it is then possible to overspeed if you don't keep an eye on it.

[FastEddieB]

I just posted this to a similar (identical?) thread on the LightSportTalk Forum:

 

 

Finer pitch always means more power' date=' it can't slow you down, it just costs more gas.

 

[/quote']

 

Reducto ad absurdum...

 

...is there a prop pitch that would result in zero thrust? Might that "slow you down?"

 

On the one hand, keep going finer until you get to 0° and what can you expect? And why?

 

On the other hand, keep going coarser until you get near 30° and what can you expect? And why?

 

Remember that a propeller is nothing but a rotating wing, and you're increasing and decreasing its angle of attack.

 

We like to posit in absolutes. Nature and physics seem largely partial to curves.

 

Which is what you find in the above exercise.

[Ed Cesnalis]

Do any of the prop manufacturers give a chart showing rpm vs thrust at a series of different pitches? Such a document would be very enlightening.

 

I believe the throttle setting is fixed at WOT and the load varies. So I read it as each point has a different pitch, flatter to the right and coarser to the left.

[Jim]

Seems unlikely that you could pitch a prop coarse enough to limit it's WOT rpm to 2500. I don't think that's the way the chart reads.

[Ed Cesnalis]

I just posted this to a similar (identical?) thread on the LightSportTalk Forum:

 

 

 

Reducto ad absurdum...

 

...is there a prop pitch that would result in zero thrust? Might that "slow you down?"

 

On the one hand, keep going finer until you get to 0° and what can you expect? And why?

 

On the other hand, keep going coarser until you get near 30° and what can you expect? And why?

 

Remember that a propeller is nothing but a rotating wing, and you're increasing and decreasing its angle of attack.

 

We like to posit in absolutes. Nature and physics seem largely partial to curves.

 

Which is what you find in the above exercise.

 

 

I keep bringing up the limitations, redine or the motor blowing up will prevent you from using zero degrees. 5,500RPM is our practical limitation. Inability to take off will prevent using 30 degrees.

 

If we can confine the discussion to the operating range of the engine then things make more sense.

 

You claim that you flattened your prop ~1 degree and that it improved your climb but cost you some cruise speed. Look at the power curve and show me where you might have been and where you moved to and then show me how that could have slowed down your cruise and maybe you'll get my point.

 

For example if you were at 5,200 RPM, WOT, your altitude and and you then flattened your pitch so you went to 5,350 RPM, WOT, your altitude the result would be better climb and higher speed at 75% power (cruise.) If your setting was a little flatter or coarser the result would be the same, finer pitch you would get faster climb and faster cruise (75% power.) You could easily be fooled if you didn't adjust your cruise RPM for the new pitch, if you left your cruise RPM the same then you would cruise slower because it would be a lower power setting.

 

I don't see a spot on the power curve where you could have made your climb better but your cruise slower. I say its a myth.

[Ed Cesnalis]

Yes, you're right, there are obviously power adjustments too - BUT - if you merely adjust the pitch AND keep the throttle at the same setting it does have an effect on the speed achieved too.

 

At full fine, static on the ground we see about 5,500 rpm - once airborne it is then possible to overspeed if you don't keep an eye on it.

 

Merely changing the pitch and keeping the same throttle setting is a power adjustment. You need more throttle at the finer pitch just to maintain your power setting.

 

If you were willing to over speed the engine, then the fine setting would be your fastest.

[Ed Cesnalis]

I just posted this to a similar (identical?) thread on the LightSportTalk Forum:...

 

Sorry I moved to this forum because you can't upload images to sportpilottalk.

[chanik]

It is not true in general but for the CT's this is basically true. The main culprit that blows up this argument in general for shallow prop pitch is the tremendous power loss that builds up as the prop tips approach the speed of sound. The power delivered to 'pushing air back' is hugely limited by sound speed. It's the reason we have problems with airport noise complaints: all those idiot Bonanza drivers thrashing the air on takeoff with big props and too shallow pitch, turning props into loudspeakers.

Some numbers: 68" is 0.88m radius prop at 5800RPM/2.43 (gear reduction) is 2400RPM or 40Hz. that is tip speed of 2*PI*0.88m*40 or 240m/Sec in the vertical plane. The lateral speed is perpendicular so can vector add with pythagorean theorum: sqrt of sum of the squares. 120kts is 62m/Sec. Total tip speed would then be 248m/Sec. The loss climbs exponentially, but anything below 0.8Mach, you can ignore it. That is 80% of 340m/Sec at sea level or 270mSec. So the CT prop tips stay comfortably subsonic and quiet. There are other losses that climb when you overspeed the engine too. Really, the efficiency of the 912ULS starts to fall away substantially right where you see the Torque max out, around 5050RPM.

[Ed Cesnalis]

Kurt,

 

Thanks your posts are always interesting. I've watch Jeremy land and tax a couple of times recently and even fly over my house at 3,000'AGL and I was struck by how quiet the CT is.

 

Can you shed some light on the reason for the high noise level in the cockpit? I can guess, you probably know.

[chanik]

The CT cabin is noisy because carbon fiber is ~3x stiffer than Aluminum for the same strength. That coupled with the lightweight structure hanging onto the vibrating engine (the right way to think about it) means noise and especially high frequency sound are poorly damped. You fly inside a high-fidelity loudspeaker. In other planes, a much higher component is transferred through the air and SPL peaks lower

[FastEddieB]

OK, I think I'm starting to get it.

 

Not so much from argument and charts, though those are appreciated and I always keep an open mind.

 

But mainly its been through a ploy called "Moving The Goalposts"

 

In short, we started here:

 

"Finer pitch always means more power, it can't slow you down, it just costs more gas.

 

The "always" got me. That's what made it untrue, since there are times when a finer pitch CAN slow you down, via less thrust available.

 

I think we've now backpedaled to saying, "If one can pitch a prop so that the engine can develop maximum power for any given condition, then it will provide the best performance".

 

I'm close to accepting that, but for two things:

 

1) If you can achieve exactly full RPM in level flight with full throttle, that will be at a different pitch setting than would allow full RPM in a climb.

 

2) Propellers can be more or less efficient at different RPM.

 

Hence, the prop that gives max speed at full throttle/level flight will NOT allow full RPM in the climb.

 

Hence, climb props will call for one pitch for best climb, but a different cruise prop for those seeking best cruise. NOT the same one as you've implied or stated here or elsewhere.

 

I may post a real world example of #2 involving my Cirrus tomorrow.

[Ed Cesnalis]

Eddie,

 

You keyed in on one statement that needs to be in context. I've now stated many times that I'm speaking about what happens within the performance envelope of the engine. Again, you claimed that you flattened the pitch in your light sport and that it cost you some cruise speed. I'm saying that doesn't happen because when you flatten your climb power setting will be faster and your cruise power setting will be faster. ( unless you don't adjust your cruise RPM to the same power, like 75% )

 

In this case you Cirrus is out of light sport range and might have the supersonic prop speeds that our light sports are not in range of.

 

A couple of people just posted that if you flatten your pitch that your cruise will go down, I'm just saying that's not the case in our light sport world.

 

 

It sounds like your experience was typical of what I have seen, you needed a flatter pitch. The CTs that I saw shipped into Carson City couldn't come close to either the 5,500 or 5,800 RPM limitations and Rotax had published a best practices document indicating that the 912 was designed to run at 5,500 RPM. Add the power curve chart and we could see that we were not accessing more than about 90% of the engines rated cruise power.

 

Because we couldn't reach either limitation 5,500 was our first target and turned out to be the best compromise for a fast light sport. The lesson was that the climb / cruise prop compromise in our speed range is speed vs economy not climb vs cruise. Climb and cruise speeds improve together what you give up is economy.

[FastEddieB]

Sometimes I get the idea that the two Ed’s end up bumping heads just because one or both of them is not stating his position clearly.

Let me try to summarize mine...

First, its best to review the ROTAX 912 power output. Max power is 100 hp at 5800 rpm, permissible for takeoff and five minutes max. Max continuous is 92.5 hp at 5500 rpm.

That means, of course, if your plane has 100 hp but you never see 5800 rpm, you never see 100 hp, either. Ever.

I will not consider altitude, but one question I will sometimes ask a new student is, “Up to what altitude do you think this plane can maintain full rated power?” Assume that we’re talking normally aspirated. I get answers all over the board, but most often around 5,000’. Of course, our engines can only produce full rated power at sea level on a standard day - power generally starts dropping off as soon as the wheels leave the ground.

Still, we’ll get best power at 5800 rpm and best continuous at 5500 rpm. Let me focus on the latter, since we don’t want to be limited to five minutes at a time.

Imagine your airport was in a valley, and the most important thing to you was to be able to climb at Vx - as steeply as possible. You would want your prop set so that at full throttle and Vx, the rpm went right to 5500 and stayed there. Let’s say that’s 12º.

Or maybe your main goal is to climb to altitude as rapidly as possible. You find by experimentation that 12.5º gives you 5500 rpm at Vy with full throttle. Great.

But either of these would have to be seen as “climb" settings, and your speed would suffer in cruise. Why? Because you would have to throttle back a lot to keep the rpm under 5500 after leveling off. And a throttled engine clearly doesn’t give you best speed, since it can’t give you best power.

Now let’s say max cruise speed is what you want, and you tweak your prop until full throttle in level flight gives you 5,500 rpm. Let’s say that’s 13.5º. Fantastic - you now are going as fast as you possibly can (continuously) in cruise.

But it should be clear there’s a tradeoff. Now suddenly in the climb the engine is barely turning 5,100 rpm, leaving a lot of horsepower untapped.

I’ll again use the analogy with a car with a manual transmission, except now the shift linkage is broken. You HAVE to get somewhere. You can select any one gear, but that’s it. In a four speed transmission, you’d likely want to choose second or third - first would have the engine turning way too fast once you got going, and fourth would be hell on the clutch and it would take forever to accelerate with the engine going that slowly.

If you select second gear for your trip, its like a climb prop. It gets you going decently, but once up to speed the engine is spinning way too fast.

If you select third gear for the trip, its like a cruise prop. Cruising will be much calmer, but acceleration will suffer.

Anyway, constant speed propellers do give you the best of both worlds, but we’re stuck with either fixed pitch or ground adjustable propellers. And the setting that is best for climbing does not result in maximum speed, and the setting that is best in cruise does not result in the best climb.

Does that help clear things up?

[paul m]

 

Does that help clear things up?

 

Not sure if that helps Ed but it helps me, Eddie. I had lost you guys a ways back.

[Ed Cesnalis]

...Imagine your airport was in a valley, and the most important thing to you was to be able to climb at Vx - as steeply as possible. You would want your prop set so that at full throttle and Vx, the rpm went right to 5500 and stayed there. Let’s say that’s 12º...

 

 

Eddie,

 

I plead guilty to not being clear, I'm not a good communicator It comes from being an 11th grade graduate. Sorry, was never trying to butt heads with you.

 

My typical flight has a difference of 7,000' altitude between destination and departure. On both ends there is an initial climb. You are picking a poor optimization target, not a real world one in order to make your point. ( optimizing your prop for Vx at your departure )

 

On both ends of my flight I will depart flying Vx until I feel that Vy or faster is a better option. The important thing is that I know what Vx is not that my prop pitch will provide 5,500 RPM when I select Vx at wide open throttle. The closest examples to needing that kind of performance would be a CT that either tows gliders or banners and still they are optimizing their props for an altitude/limitation combo like 5,500RPM/7,500'.

 

Most of us make the decision to flatten our props in order to increase performance, either climb or cruise and most of us when doing so realize that we improved speed on both.

 

Perhaps you didn't get why I focus on the power graph. From It you can see at any starting point that you might have been pitched at that a decrease in load ( flattening pitch ) will increase hp at full power and at cruise power. In our slow speed world, increasing climb performance increases cruise speed at the expense of economy. That's according to the Rotax graphs and others and its a different compromise than climb vs cruise speed that we have been hearing.

[GlennM]

I agree with Fast Eddie. Why would there be a variable pitch option for the Flight Design if the fixed pitch affected both climb and cruise. It would just be added weight and complexity for no difference in performance.

[Ed Cesnalis]

There is a big difference in performance that can be realized with the variable pitch. They use the flat settings for climb performance and the coarse settings for economy when they throttle back. For fastest setting they would need the setting that came closest to 5,500RPM at WOT.