Prop pitch and RPM

[Roger Lee]

One thing I haven't heard anyone mention, but people complain about and Ed was one of them.

I waited all day.

 

Engine temps.

I thought for sure someone would bring this up. Ed found debris in the oil cooler, but I haven't heard what his summer time temps are now in climb. One thing that reducing prop pitch does is unload the work on the engine which results in cooler oil temps. The engine doesn't have to work as hard to turn the prop. I live in a hot climate. Right now during the winter we are running high 80"sF and touching 90's. In the summer over 100F easily. Many complain of hot temps during climb. I can climb all the way up to whatever during the summer and not see over 235F and usually during normal day temps and climb to 5K I never see over 230F and fly around 215F-218F in cruise during a hot day.

 

Balance is still King.

Balance takes in all considerations not just max rpm.

[ctfarmer]

Roger, I am referring to performance in straight and level flight only. Maybe I read the initial post too simply. I read it to mean what gave best performance in level flight.

 

I am sure we do all know that nothing in the system is linear. But it does not matter what turns the prop. 912, mouse wheel, electric motor. Does not matter. What matters is something turns it at 5500. And that's what happens irrespective of torque curves, linearity, motor condition or anything else.

It is then entirely a matter of the foils angle of attack. More angle of attack - more lift aka thrust.

 

So at 5400 you leave rpm on the table (100rpm )and you are below you best torque as well which is 5500 rpm. So you are now well down on power but you will have slightly increased angle of attack but not the torque to turn it. So yes 5400 is over pitched. No question about that. But there are plenty of conditions that 5500 is not over pitched and certainly some that it is. But likewise, by deduction, the reverse applies and there are conditions that 5600 is under pitched and others where it will be over pitched and a few spot on. Same same. Pick your poison. Just never lower that 5500 wot at the lightest prop loading condition for any part of your flight. That would leave hp and torque on the table for all flight phases.

 

I know I leave lots of hp on the table compared to others in climb and take off. But I am happy with that as trade off for the extra cruise speed.

 

5500 at wot is the magic number for torque and continuous rated power and allows you the most angle of attack the motor can turn in any situation even thought these are widely variable. And this applies no matter what prop or drag it has to contend with and will see max thrust under any set of conditions you want to name.

 

So prop to see 5500 on takeoff, you will see best takeoff performance possible (or 5800 if you wish to run higher for those allowable 5 minutes). And same goes for every other flight condition you fly. The best performance for every situation will be to prop it at 5500 at wot for that particular flight phase (save the intermittent power boost to 5800).

 

As I stated previously, there is no singularity point that somehow all those non linear things come together at one magical point of performance nirvana. That point is not a singularity by curve shape but is set by rotax at precisely the continuous rated rpm point. The only point at any altitude or flight condition where max continuous rated power at wot resides.

[Ed Cesnalis]

ctFarmer,

 

Thanks a lot for your posts they are well stated and on the money.

 

Roger,

 

You frame the argument as though your experience trumps any argument about physics and your good intentions are just trying to help poor Ed out.  This just doesn't fly.  When your experience and physics collide: "We have a problem Houston".  I'm not in need of help and I'm not disadvantaged and confused. I do prefer to see the CT site's advice grounded in reality.

 

 

You will never get past this issue: Retarding the throttle presents and additional obsltrution to the air/fuel mixture resulting in a reduction of manifold pressure (which results in an RPM reduction).  Reducing manifold pressure is a power reduction.

 

This is always true:  If I as a pilot want to reduce speed in my CT I retard my throttle.

[Ed Cesnalis]

ctFarmer,

 

I brought up the wear issue because Roger only considers RPM and not manifold pressure there too.  In that way the 2 issues are similar.

 

There are valve train issues, harmonics and such that would provide wear at lower RPM especially at high power settings but the simpler issue is to equate wear with work not just engine speed.  A 912 spinning at 10,000 RPM but producing 50hp due to the light load might see less wear than I 912 turning 4,000RPM but producing 75hp.

[Aero-Nut]

I do t think there is not any 100% correct answer to this long time debate. If climb is the single most important factor then pitch flat as possible for 5800 during climb but you will suffer in cruise. If you want cruise then pitch for best performance at your given altitude but you suffer on climb. Most people will want to lean one way or another. If you pitch for the middle you will suffer on both ends. Of course an in flight adjustable gives you both which we legally can't have. So decide where you want the best performance and pitch accordingly. If had a very short strip and needed to clear obstacles and needed max climb I would pitch flat. If I cruised long distances frequently and had a 6000 foot runway I would pitch for cruise. Neither apply to me so I am setting it in the middle. My 2 cents.

[FlyingMonkey]

 

Andy, for your argument that your CT had its prop set at 5200 WOT then when changed to 5650, it was faster in cruise and climbed better. Certainly. Have you tried 5500, 5550, 5600 and 5650 and compared each. I also say you can tell those small differences in cruise speed and you don't have to fly with another plane to compare.

I can tell very quickly. The numbers move around a little but in smooth air, i find it easy to see even a knot or two change in the fundamental performance.

 

peter

 

Hey Peter.  Yeah, when my prop was very flat and I coarsened it up to about 5550rpm WOT, I could tell a speed advantage, 3-5 knots IIRC.  However, I also noted the climb dropped off a good bit, maybe 200fpm.  I didn't like that so I took a little pitch back out, and that's where I am now at 5600-5650rpm (I have an analog tach so exact numbers are hard to know) at 3000ft MSL.  That works for me, I still gained a knot or two in cruise and my climb is very good.

 

I split the difference somewhat, but my personal bias is toward climb performance over cruise.  I do fly into some shorter grass fields, and around the mountains in North Georgia, and my thinking is that nobody ever got killed in an airplane because they had too much ability to climb.      

[FlyingMonkey]

 

 

You will never get past this issue: Retarding the throttle presents and additional obsltrution to the air/fuel mixture resulting in a reduction of manifold pressure (which results in an RPM reduction).  Reducing manifold pressure is a power reduction.

 

This is always true:  If I as a pilot want to reduce speed in my CT I retard my throttle.

 

But your style of flying is different from most of us too.  You spend almost all your time flying at 10k MSL, and the difference between your field elevation and cruise altitude is relatively small (3000ft)?

 

On the other hand, I fly mostly at 3000 MSL or less.  HOWEVER, I want the *option* to climb up to 9500ft and cruise when I do longer flights.  If I set my WOT to 5500rpm at 3000ft, I'd gain some speed and be better "optimized for my normal regime.  But I'd have weaker climb, leading to really long climbs up past 7000ft and slower cruise speeds at 9500ft altitude when I got there, since I'd be WOT and probably only making 5300rpm or less.

 

As I am set now, I get 122 knots at my normal cruise altitude of 3000 @ 5500rpm, and when I want to go places I can climb smartly all the way to 10,000 and still make 5500rpm.  So my cruise is higher on longer trips than it would be if I "optimized" to 5500rpm @ 3000 MSL.  Now the downside is that I'm giving up some speed at my normal cruise elevation, but who cares?  When I'm down at 3000ft I'm probably not going more than 100 miles anyway, and 120kt is more than fast enough for me to do that.  In fact, I'll probably be at 5000-5200rpm down there anyway.

 

CT, I think you and I are probably pitched close to the same.  That might be an "optimized" setting for you with 5500 WOT @ 10,000, but you are operating in a smaller altitude range than I am; I want good performance from sea level all the way to 10,000.  I might not be "optimal" at the altitudes I normally fly at, but my performance is very acceptable at any altitude from 0 to 10,000, and I want that flexibility. 

[Ed Cesnalis]

I do t think there is not any 100% correct answer to this long time debate. If climb is the single most important factor then pitch flat as possible for 5800 during climb but you will suffer in cruise. If you want cruise then pitch for best performance at your given altitude but you suffer on climb. Most people will want to lean one way or another. If you pitch for the middle you will suffer on both ends. Of course an in flight adjustable gives you both which we legally can't have. So decide where you want the best performance and pitch accordingly. If had a very short strip and needed to clear obstacles and needed max climb I would pitch flat. If I cruised long distances frequently and had a 6000 foot runway I would pitch for cruise. Neither apply to me so I am setting it in the middle. My 2 cents.

 

Ken,

 

Optimizing for a point between best speed and best climb as you describe is the 'modern era'.  Many/most of us started with props far to coarse for either best speed or best climb.  WOT level flight was likely to produce something like 5,300RPM.

 

I have chosen speed over climb because climb is quite good already and top speed makes more difference to me.

 

There is more than on argument involved in the discussion.  The part that is 100% is a partially closed throttle restricting air-flow will never be max performance.

[Ed Cesnalis]

 

But your style of flying is different from most of us too.  You spend almost all your time flying at 10k MSL, and the difference between your field elevation and cruise altitude is relatively small (3000ft)?

 

On the other hand, I fly mostly at 3000 MSL or less.  HOWEVER, I want the *option* to climb up to 9500ft and cruise when I do longer flights.  If I set my WOT to 5500rpm at 3000ft, I'd gain some speed and be better "optimized for my normal regime.  But I'd have weaker climb, leading to really long climbs up past 7000ft and slower cruise speeds at 9500ft altitude when I got there, since I'd be WOT and probably only making 5300rpm or less.

 

As I am set now, I get 122 knots at my normal cruise altitude of 3000 @ 5500rpm, and when I want to go places I can climb smartly all the way to 10,000 and still make 5500rpm.  So my cruise is higher on longer trips than it would be if I "optimized" to 5500rpm @ 3000 MSL.  Now the downside is that I'm giving up some speed at my normal cruise elevation, but who cares?  When I'm down at 3000ft I'm probably not going more than 100 miles anyway, and 120kt is more than fast enough for me to do that.  In fact, I'll probably be at 5000-5200rpm down there anyway.

 

CT, I think you and I are probably pitched close to the same.  That might be an "optimized" setting for you with 5500 WOT @ 10,000, but you are operating in a smaller altitude range than I am; I want good performance from sea level all the way to 10,000.  I might not be "optimal" at the altitudes I normally fly at, but my performance is very acceptable at any altitude from 0 to 10,000, and I want that flexibility. 

 

Andy,

 

My typical 1 way flight looks like this:

 

 

My range tends to be from below sea level to 13k and I have top performance the way I'm optimized. 

 

I agree we are pitched similarly.  I agree optimizing for speed at 3K isn't important, mostly because that is too much power for cruise anyway.  There is little to gain and much fuel to waste so far better to be optimized for the thinner air where you can better use it.

[FlyingMonkey]

 

 

 

Andy,

 

My typical 1 way flight looks like this:

 

 

 

My range tends to be from below sea level to 13k and I have top performance the way I'm optimized. 

 

I agree we are pitched similarly.  I agree optimizing for speed at 3K isn't important, mostly because that is too much power for cruise anyway.  There is little to gain and much fuel to waste so far better to be optimized for the thinner air where you can better use it.

 

 

Consensus, yay!  

 

Your flight profile looks a lot like what I'm trying to plan for...to be able to cruise high and get max performance for distance cruising when needed, and still do well at the lower levels where I play around.

[Roger Lee]

"You frame the argument as though your experience trumps any argument about physics."

 

In this case it does. My real time experience that is specific to our CTSW, our engine and our prop versus your general graphs that are not specific to any of your use far outweighs in this debate. Your physics are too general and don't speak specifically to YOUR specific situation. And we still don't use MAP to fly by or you wouldn't keep talking about 5500 rpm. It would only be MAP numbers. Map is okay when it's pertinent, 

I talked to two CT owners at the field that have read this debate. They have personally been through all of this and been witness to testing and think you're wrong. Until you have done all we have we aren't in the same ballpark to have a meeting of the minds.  You have only general graphs and limited experience with just a single CT and a Warp prop and only have worked with 5500 rpm. We have not only your graphs, but years worth of testing and research now, always 2-4 CT's side by side for comparison not to mention another hundred LSA that have had prop pitch changes.  Trial and error on many different rpm settings and at least a dozen trials with other props.

 

I had a professor once that told the class it isn't me that needs to slow down it's the rest of you that need to catch up. I took him to heart and became a tester and researcher. It's what I enjoy doing. I redesigned the Hookah dive industry because of my research and designs and threw out 50 years worth of old technology. It only generally applied in the physics end and many never really understood what they were really looking at. The old timers and naysayers said I would kill someone, but these designs lead in the industry and world today. My old company is still online and doing well.

 

Nothing wrong with a graph, BUT you need to do all our same research and publish a graph that is specific to you. Then it will be applicable.

 

​Until then we won't agree and no sense in making this a 20 page debate.

 

I think the horse is getting dehydrated.

[FastEddieB]

"You frame the argument as though your experience trumps any argument about physics."

 

In this case it does.

But...

 

[Anticept]

But...

 

 

Not with that attitude, Scotty.

[Roger Lee]

Trying to apply a generalised physics lesson that isn't specific to your specific issue doesn't work. Too many non specific errors. You have to think outside of the box you are looking in. There are more boxes.

 

Eddie,

 

You forgot that Scotty saved the day more than once and proved others wrong on a regular basis by working outside the box. We all know what we know and we are all smart until the guy comes along and shows you what's in his box this isn't in yours. Then we carry a new box with new smarts.

How many new boxes has NASA carried since the world told them you couldn't fly into space or the moon. There is always someone that has a bigger box than you and it's up to you to be opened minded enough to look, evaluate and try his box of toys before so say it can't be done or it isn't so.

 

Don't you just love philosophy. 

 

How many new boxes have you carried since grade school? 

I'm sure a few. 

 

Kind of like switching wires and then going oops wrong box.

[Anticept]

In the meantime, if you look in Roger's garage, you can see all the boxes he's set on fire, crushed, maimed, soaked, etc ! :P

[Roger Lee]

Hi Corey,

 

You are absolutely right. I have tossed crushed and burned tons of boxes because a taped closed box never learns anything new. My boxes always have the tops cut off and I listen and look in everyone else's box. Everyone has something different in their box no matter what their education.

[Cluemeister]

I'm getting out of here before someone asks for a group hug. 

[S3flyer]

Isn't the argument centered about how each of us define 'optimum'?  Each of us define it differently so the prop pitch will be different as well.  I see 3 main variables that prop pitch affects: cruise, climb and engine temp.  We also are debating between 5500 rpm and 5650 rpm at each of our target altitudes.  5500 maximizes cruise, you lose some climb and the engine will run a bit hotter.  5650 you lose some cruise, gain some climb and lower engine temps.  Weighting the 3 variables the same would mean 5650 is 'best' for you.  Weighting cruise more than the other two would mean 5500 is the way to go.

[FastEddieB]

I'm getting out of here before someone asks for a group hug. 

[Roger Lee]

Okay a group hug. About freak'n time. 

[FlyingMonkey]

Did somebody say group hug??

 

[IrishAl]

I’m not sure I like that ending.

 

This has been an excellent debate.  You guys have rolled your sleeves up for an honest, forthright exchange of views without resorting to vinegar.  It’s entertaining, it’s informative and it’s refreshing – great stuff.

 

But let’s try to establish a concrete conclusion, rather than stopping short for the sake of feeling comfortable again.

 

Sure, lots of this debate has been one person saying the glass is half full and the other saying it’s half empty, but there’s also a real disparity in opinion in it, and both sides can’t be right.

 

It seems to me that the nub of the issue is logical argument versus empirical evidence.

 

Here’s where I think the two sides of the debate can be drawn to a single, assessable point:

 

CT argues that, at 7500’, his best possible performance achievable is with the throttle at WOT and prop pitched for 5500rpm. 

 

However, Roger states that if CT pitches for 5650 at WOT and 7500’, and then throttles back to 5500, he will have increased performance – same speed and less fuel consumption.  

No reduce your prop pitch to get max speed and better flight performance over what you have now. Your max speed will always be at WOT which should be around 5600-5650, but you may cruise at the same speed you are now with reduced throttle and fuel savings and have better climb.

 

CT’s argument is logical - WOT presents the least resistance to inflowing air with a butterfly that’s parallel to the inflow, whereas Roger's advice of 'reducing throttle' involves further restricting an engine that's already restricted by ambient air pressure.

 

My understanding says that CT is correct and Roger is wrong – there simply must be some error in his  trial, because his claim is physically (as in, physics) impossible.

[Tom Baker]

I am going on record and saying I don't agree with Roger on this.

 

First the definition of performance needs to be determined. Is the discussion about speed only or climb and speed combined?

 

If it is speed, then WOT and 5500 rpm would have to give the best speed, because the airplane would be moving farther forward with each rotation of the propeller. The speed of the airplane is always going to be determined by how far it moves forward for each rotation of the propeller, the RPM the engine is turning, and the efficiency of the propeller. With the limit of 5500 RPM for maximum continuous power having the greatest pitch or blade angle to achieve this RPM at WOT will give the greatest speed.

 

I don't think this is happening, but only way this could not be true is if that last little bit of pitch added to hold the RPM at 5500 was causing the performance of the propeller to decrease in efficiency due to the increased blade angle.

[Roger Lee]

You guys have missed the point. It isn't at all about max performance. That has variables with each prop and fuselage design and to worst variable is the pilot.

 

 

You missed all the underlined and capitol references to BALANCED performance.

 

 

 

Plus a CTSW set at 5650 rpm will always out run one set for 5500 rpm. I do it bi weekly. So unless you guys are flying side by side like we do here in AZ you don't have a gear leg to stand on. 

[Ed Cesnalis]

You guys have missed the point. It isn't at all about max performance. That has variables with each prop and fuselage design and to worst variable is the pilot.

 

 

You missed all the underlined and capitol references to BALANCED performance.

 

 

 

Plus a CTSW set at 5650 rpm will always out run one set for 5500 rpm. I do it bi weekly. So unless you guys are flying side by side like we do here in AZ you don't have a gear leg to stand on. 

 

Blurred definitions make it impossible to resolve the argument at hand.

 

A CTSW set at 5650 will always out run one set for 5500 that is true with a stipulation.  The stipulation is that the faster speed is achieved by exceeding the 5500 limitation.  This is a different position than if/when you said the the 5650 setting would result in a faster speed even when throttled back to the 5500 limitation.  Only now do you pick up the problem of top speed not being available with a partially closed throttle.

 

To resolve best speed issues we have to agree on the 5500 limitation being used, otherwise it ceases to be a discussion about best speed.