Jump to content

NEUFORM Prop Pitch


NC Bill

Recommended Posts

Ed - I think he said WOT in level flight is 5450 with a take off RPM of about 4900. That puts him a little on the coarse side of ideal. 

 

Mine is set to about 5600 WOT with just over 5000 on my initial climb at 60K. My level flight speed with WOT is just a little slower than his at about 122-123 KIAS. But, I don't have -12 flaps. 

 

Of course, all of these indicated speeds when comparing different aircraft is meaningless unless the aircraft are side by side.

Link to comment
Share on other sites

  • Replies 77
  • Created
  • Last Reply

John, your right.

 

We have to know RPM, throttle setting and DA to be able to know the power setting and how the prop is optimized.   5450 is within 50RPM of max continuous setting for 6,000' so it looks close to being optimized for best speed at most efficient altitude, just a little on the coarse side.

 

125+kt is pretty good.

Link to comment
Share on other sites

Our prop pitch is a balancing act since it isn't an in flight adjustable prop in the US. A little too flat and we favor a climb prop (better than too course) and a little too course and that kind of hinders all flight aspects. We just don't have the HP and torque to effectively turn a too course set prop and as we climb higher in altitude it just becomes worse. I have an SW and my prop can get almost 5700 rpm WOT and that puts me over 120 knots and gives me a good climb prop too as I usually fly heavy.

If you set you prop to get around 5600-5700 rpm WOT at your AVERAGE altitude that is a fairly good balance point. Average altitude for owners on the west or east coast at sea level may only be 2K' and some like Charlie Tango's average altitude may be 11K' to 12K' and these different fliers should have different prop pitch to achieve their final goal of 5600-5700 WOT in level flight at their average altitude.

 

Unloading a prop can also help reduce engine temps by not making the engine work so hard.

Link to comment
Share on other sites

Thanks to all. John, your reply summary "WOT in level flight is 5450 with a take off RPM of about 4900" is correct!

 

Throttle setting in the photo attached at 5300 was around 3/4 travel and fuel averaged around the 20.5 to 21l/hr mark.

 

At WOT and 5450, fuel flow was around 23l/hr. Accordingly, that would indicate i was running close to 90% of the available power at that density altitude when using 20.5 to 21l/hrs as shown in the photo.

 

I also note maximum torque for the 912ULS is at 5100 so a prop pitched at WOT to max 5100rpm is the only condition a 912ULS can ever produce its stated maximum torque. A prop pitched to any point away from maximum torque rpm will mean the engine can only ever see something under the max engine torque capability of the engine. Not even running a finer pitched prop at 5800rpm can the torque maximum be reached as it is not there at those rpm's to deliver. 

 

Therefore, propped to any rpm other than 5100/WOT means you are always lower on the torque curve (on either side) and the higher that WOT/rpm point is (and lets for practical purposes say this is up to 5800rpm) will see progressively lower engine output torque at all revs you fly. 

 

In other words, you are gaining power by adding revs but at the same time, at a slower rate, reducing/unloading the engine torque. I can see this would be good for the engine and will reduce exhaust gas temperatures. Hence, Roger clearly one reason for your comments about stressing the engine excessively when propped to lower WOT max rpm settings.

 

I can see a compromise is needed. I do not want to lose any speed at around 90% power setting (given the "full rich above 92% power" posts above) as i am not unhappy with my takeoff performance. I just have to find this for my CT. I will fine the prop by 1 deg and report back in due course.

Link to comment
Share on other sites

If WOT at 5650 is better than at 5450, then why is not WOT at 5750 or 5850 (for less than three minutes) better than 5650? Is there a spot where the improvements drop off with extra RPM at WOT (short of exceeding the manufacturer's limits)?

 

 

No more RPM = more power up to 5,500 and up to 5,800 (WOT).

Link to comment
Share on other sites

Getting too flat gets a better climb, but has diminishing return for speed. Then you want to give it more throttle in cruise to get the same speed as 5600 and you could get there at the expense of fuel and higher cruise rpm. We set our WOTAN rpm to let's say 5600 knowing we will be at less rpm in cruise. This particular pitch setup will give you the best overall speed and fuel economy at that reduced cruise rpm.

 

Too flat or too course is counter productive unless you have special circumstance and need that better climb prop. Too course has nothing, but bad performance all around, stresses the engine and causes increased engine temps.

Link to comment
Share on other sites

You guys are running way over my CTLSi for RPM.  

 

The POH for the CTLS says takeoff is 4800-5000.  I am usually around 4850.

The POH also states that 5500 rpm for max horizontal cruise.  And maximum continuous speed is 5500 rpm. 

The POH also states that the max rpm of 5800 (if one can get there) is not to be run longer than 5 minutes.

 

I am cruising at 9k feet not at WOT around 5300 rpm.  I like to vary the rpm to 5300 max down to 5000 on cruise.   I have not run this plane below 2k feet yet, so I have not seen anything like 5500 rpm.

 

 

I can get 5,500 at 10,000'  My TAS range is from 123-127kts.  

 

I bet you can get a bit more speed, if 9k is your cruising altitude then you probably want to be able to get at least 5,500 @ 9k otherwise some of the hp is not available to you.

Link to comment
Share on other sites

You guys are running way over my CTLSi for RPM.  

 

The POH for the CTLS says takeoff is 4800-5000.  I am usually around 4850.

The POH also states that 5500 rpm for max horizontal cruise.  And maximum continuous speed is 5500 rpm. 

The POH also states that the max rpm of 5800 (if one can get there) is not to be run longer than 5 minutes.

 

I am cruising at 9k feet not at WOT around 5300 rpm.  I like to vary the rpm to 5300 max down to 5000 on cruise.   I have not run this plane below 2k feet yet, so I have not seen anything like 5500 rpm.

These guys are not talking about how the POH says to run the engine, they are talking about how it is set up. In other words they are not saying to run the engine at 5600, but if they went to full throttle and let the engine wind up it would turn 5600. They still pull the throttle back and run the engine below the 5500 rpm max continuous limit.

Link to comment
Share on other sites

No more RPM = more power up to 5,500 and up to 5,800 (WOT).

 

Please correct me if i have read your post incorrectly but it looks like you are saying more rpm = more power.

 

If so, that is demonstrably wrong. Take the prop off and you will run at 5500rpm at very lower throttle setting, low fuel burn and hence low power. If the prop is not offering resistance, power will not be produced so we can safely say the coarser the prop pitch, the more power produced at any rpm.

 

So, fundamentally, an engine pitched to run at 5500/WOT will be producing more power (the output torque will be higher) than one pitched to 5600rpm WOT when both running at the 5500rpm manufacturer specified max continuous limit. Further to that, at long as i can get to above 5100rpm, i have the torque available so to say the engine can't swing a coarse prop is incorrect. It just can't swing it with as much in reserve as one pitched finer.

 

A compromise comes at takeoff for example where both power and torque are lower at 4900/WOT than say 5100/WOT which in the latter case coincides with maximum torque output of a 912ULS. So take off performance will be better for the 5100 case.

 

I just like to make sense of things fundamentally and the case made for the cruise condition that i can pitch above 5500/WOT, throttle back to 5500 and yet go faster is completely counter intuitive (from an engine performance perspective) to my engineering mind.

There is one provisos - what does, in my case, the Neuform prop do with coarser pitch and can it tip it over the edge, so to speak with respect to angle of attack so i start to lose thrust for the same power output.

 

peter

Link to comment
Share on other sites

I was trying to say that per the power curve more power is produced at a higher WOT RPM.  5,800 is close to 100hp and 5,500 is close to 93hp.

 

Max continuous speed/power is at 5500/WOT.  I could see pitching flatter and throttling back not in an attempt to realize max speed/power but in an attempt to find max speed/power with the needle circuit leaning or in the case of the injected engine while in econo-mode ( or whatever its called )

Link to comment
Share on other sites

"Please correct me if i have read your post incorrectly but it looks like you are saying more rpm = more power.'

 

The Rotax engine is better operated and the graphs apply more for an in flight adjustable prop. This is what Rotax would prefer to see on all engines, but that isn't the real world.

 

Of course Rotax numbers are all predicated that you use the right prop, right prop settings and adhere to the proper setup according to Rotax.

Prop pitch too far in either direction will alter all your numbers including the torque and HP that the engine can deliver to the prop.

 

Ed, is correct. Look at the Rotax HP and torque charts. Peak torque is around 5100 rpm IF the engine is propped to get 5800 WOT. This also is predicated on standard atmospheric conditions.  After the torque peaks it starts to decline as rpm increases, but the HP still climbs until their chart ends at 5800 rpm. If you do not have your prop pitch set to get 5800 rpm then those charts are wrong for you.

You don't just need torque to turn the prop, but HP has to follow to develop enough muscle to turn the prop.

In here lies part of the other issue. As we go up in altitude we loose 3% of our engine for every 1K' in altitude above sea level and standard atmospheric conditions. If we are at 10K' we have lost at least 30% of our engine. To help the engine and allow it to function a little better we need a flatter prop since we lost so much torque and HP.

 

Too little prop pitch or too much is counter productive. Balance is the key.

Link to comment
Share on other sites

Roger minor point here, but isn't the engines torque (and horsepower) measured by a dynomometer and does not involve the prop at all? (I wonder if it even involves the gearbox although it may.)

Part of the confusion around this issue has been around the whole torque/horsepower issue. I questioned it at one time because I was thinking in terms of auto racing when you try to shift a bit before peak torque to get best acceleration. This has more in common with racing on Great Salt Lake with a single gear. The gear would be set to get peak torque at a certain distance the car would continue to accelerate beyond that based on maximum horsepower (assuming no mechanical limits are reached).

In addition to this scenario we also have to deal with prop aerodynamics. I believe your method makes the most sense, Roger. (Contrary to what I have said in the past.) :)

Link to comment
Share on other sites

 Ed, is correct. Look at the Rotax HP and torque charts. Peak torque is around 5100 rpm IF the engine is propped to get 5800 WOT. This also is predicated on standard atmospheric conditions.  After the torque peaks it starts to decline as rpm increases, but the HP still climbs until their chart ends at 5800 rpm. If you do not have your prop pitch set to get 5800 rpm then those charts are wrong for you.[/size]

You don't just need torque to turn the prop, but HP has to follow to develop enough muscle to turn the prop.[/size]

 

NO!

 

The propeller has nothing at all to do with the outputs Rotax publishes for each engine.

 

They use a dynamometer. This measures torque output of the engine.

 

How do they measure horsepower on a dynamometer?

 

They don't. They calculate it: HP = (torque X RPM) / 5252.

Link to comment
Share on other sites

Roger, we are going to have to agree to disagree.

The prop attached and its pitch has nothing, absolutely nothing to do with the shape (peak values) of the torque and power curve.

 

The shape of the curve relates to WOT conditions only and resistance turning moment is added to the engine at all rpm's until the peak torque and by calculation, power is obtained for every rpm. It is most normally done at what is called NTP conditions ie normal temperature and pressure which i think also is zero density altitude.

 

What you might be referring to is the instantaneous engine torque and power output which will relate to the turning moment you apply hence related to the prop, pitch and the air conditions and whether you are moving for example. The only points that instaneous output will lie on the actual torque curve will be when you are at full throttle and the rpm is stable in whatever conditions you are flying. At all other times you will be below, and can never be above the torque capability of the engine.

So what this means by the nature of physics, if you are flying at 5500 rpm at less than full throttle, you are not producing as much power or torque as you would with a coarser pitch also at full throttle and 5500rpm.

Take this further, a engine running at NTP with prop pitched finer than 5500/WOT will, at all revs down to at least 5100 (where peak torque is available) produce both less torque and less power than one propped at that point.

 

Does that make sense. Then the compromise comes in. In denser air, lower forward speed, 5500 can't be reached so a finer pitch will give more revs, possibly also more torque if you are under 5100rpm/WOT depending where you lie on the curve and more power resulting better take off or climb performance.

So this allows us another important observation. The peak torque at 5100 at zero density as would be taking off at sea level is key in that takeoff performance under this rpm leaves us under the engines torque peak and down on revs making us well down on power figures. Above 5100 on takeoff will give reducing returns to power as rpm may rise but available torque drops and will also come at a huge cost to top end performance.

 

If the aircraft is trainer, you would likely not need high speed but would love takeoff performance. If you as I do, routinely do 180nm legs or greater, then top end performance is key.

I just can not come to grips with the concept of a finer pitch (above 5500 at WOT/cruise) giving both better takeoff performance and better cruise speed. That by nature would make variable pitch props valueless.

It must also be said that I have no reason nor legal motive to limit performance to 120kts as we have no such limit here so I want top end cruise and economy but at the same time, within reason, operate my engine in a mechanically sympathetic manner.

The fact that on long trips and in turbulence or thermals, I have much greater resistance to overspeeding the engine is part of that equation given the dire consequences of inadvertently going above 5800 for more than a few minutes. Not likely but much less likely with a slightly coarser pitch.

 

I find this interesting reading. Roger, this is almost entirely in line with your suggestions - just drop the WOT by a hundred or so revs.

 

http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop

Link to comment
Share on other sites

Agreed Doug, HP increases at increasing rpm but a a decreasing rate compared to if both rpm and torque are rising.

 

My only point is about where you prop to achieve maximum top speed. That is the part i disagree with in what has been said. Not what compromise you want make to improve takeoff and climb performance to suit your particular application.

 

For that one operating condition, i not accept that you can go faster with a 5500rpm limit, all else being equal, with less pitch, less torque and less power which are both certain outcomes of pitching your prop at anything finer than to see 5500rpm/WOT.

Link to comment
Share on other sites

Starting this engine without a prop will cause an over speed in seconds.

It is in bold black and white in a manual to not do this.

I believe you.

 

But why?

 

Similar engines of all descriptions are routinely started in neutral, and hence with little or no resistance.

 

I can see the engine idling faster with no prop, but why would it overspeed with the idle speed properly set on the carbs? I just can't see enough fuel making it past the carbs at idle to take the engine to redline.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.


×
×
  • Create New...