Prop pitch and RPM

[Ed Cesnalis]

I know Roger's and my memories differ just like I know that I am the one with clear recollection. 

 

The definition has evolved, 10 years ago when we 1st started discussing it the context was:  Climb is already remarkably good so it makes most sense to optimize for max continuous power (5,500).  I wasn't around for a while but the numbers crept up, even you have it to 5,700 and that's valid but it is a different definition.  The difference between the 2 definitions has brought us to the same pitch.  Notice how you can go WOT at a point between 7.5K and the altitude where you were barely getting 5,500.  That's what I have been advocating, optimize for cruise speed at 7.5k or higher and you get the best of both worlds.

 

I'm sure there are many that remain at factory settings or some compromise short of realizing best performance.  I suspect the factory has compromised some but wonder if the are yet flat enough.

[Roger Lee]

"You optimizing for 5500rpm WOT at 10,000 is probably not far off from me optimizing to 5650-5700rpm at 3000ft."

 

Hi Andy,

 

You are correct in thinking 5700 at 3K is close to 5500 at 10K, (might be a few rpm off) You are better balanced though at your average altitude than ED and he is leaving behind some performance only getting 5500 WOT at 10k-13K where he always flies. 

 

Ed,

You would be better off pitching to get 5600-5650 for your high altitude flights and then throttling back to 5500 . 

[Ed Cesnalis]

ED he is leaving behind some performance only getting 5500 WOT at 10k-13K where he always flies. 

 

Ed,

You would be better off pitching to get 5600-5650 for your high altitude flights and then throttling back to 5500 . 

 

Now you've gone off the deep end Roger.      Your telling me I have to reduce my throttle setting to get max speed?  Really?  When I reduce my throttle my plane slows down, it doesn't speed up.

[Roger Lee]

"Now you've gone off the deep end Roger.       You're telling me I have to reduce my throttle setting to get max speed? "        

 

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.

 

You told me I was wrong the first time years ago too and then Woodstock came alive.  I'm right here too.

 

 

Your engine will run better and not be as bogged down. I get to prove this to lots of owners all the time. Always puts a smile on my face and theirs too. I usually like to make a bet because it's a sure thing for me and an easy free lunch.

When you over pitch you loose all good flight performance characteristics. You aren't really over pitched per say, but you are leaving some performance on the table because your engine is working to hard to turn 5500 rpm verses setting the pitch about 1/2 degree less and allowing the engine to work easier and then reduce the throttle back to 5500. You are on the cusp of being over pitched because anything below 5500 WOT in flat and level flight is a loser and flying at your altitudes you need all the extra help you can get since you lose so much HP and torque to turn that prop pitch. The other thing that cost you is running a Warp Drive prop. I found this out during all my testing with all the other props and flights up to Page at high altitudes.

 

 

If I came up to you and set my pitch for 5600-5650 and you left yours at 5500 I would out run you, get better fuel economy and climb better.

 

I'm 100% correct here.

 

Best way to prove me wrong is to do it. 

Look out Woodstock you're about to get even better.

 

p.s.

No one has proven me wrong yet and I'm hungry. 

[Ed Cesnalis]

No one has proven me wrong yet and I'm hungry. 

 

Consider this your lucky day, your dead meat on this one.

 

PS please don't edit your posts, to preserve my amunition

[FlyingMonkey]

Are you going to try Roger's suggestion Ed?  

 

I'm curious about the results now.  

[FastEddieB]

Running at 5500 rpm all the time has more wear attributed to it than running at 5100 rpm.

 

This is one of those things that seems logical and common sense, but one that I think has very little to support it.

 

At any normal engine speed, there should be virtually no metal-to-metal contact. If there was, the engine would seize in very short order All the surfaces are floating on a cushion of oil, and subject to very little wear.

 

If ROTAX allows continuous operation at 5,500 rpm and gives a recommended overhaul time of 2,000 hours at that engine speed, I don't think there's any good reason to think that running it at a lower rpm would necessarily result in longer life. Anecdotally, it seems that many engines run hard at or near redline run fine right up to and beyond overhaul time, and that sometimes "babied" engines don't do nearly as well.

 

All this is assuming you don't have data from ROTAX to support the allegation. If you do, I'd like to see it.

[Roger Lee]

The 2000 hrs. is based on 5500 rpm continuous.  More rpm causes extra work, friction and heat and fuel consumption. These cause extra wear. Will running 5500 rpm all the engine's life get you to TBO, yes.

Will running it down at let's say 5100-5200 rpm all it's life get you to TBO, yes and I have friends with 3K-4K hrs. and have never done anything other than regular maint. and they only run 5100-5200 rpm in cruise.

Does running it at 5500 rpm all it's life cost you fuel with little speed/distance gain, yes. The 2000 hr. TBO is based on 5500 continuous. Being nice to your engine and saving fuel gets you more in the end. So you have to ask yourself do you want instant gratification with extra out of pocket risk or do you want to cruise through TBO for an extra 1K-2K worth of on condition time?

Now ask yourself why does most all run their 912's at 5000-5300 rpm in normal cruise and why we are not all running them at 5500 all it's life.

 

Same thing with your car. Why not run your oil until 25K+, it still lubricates. Why not run your tires until smooth. Why not floor the pedal off the stop sign or stop light. Why not run all belts and hoses until they break (some idiots do).

 

You don't run your car at max all the time so why do it to your plane. 

The answer is we don't want to cause any more wear and tear than we really need and living at maximum all the time rarely gets you any further in the end.. 

 

As this post has been about since the debate it's all about BALANCE and not operating at max all the time.

 

 Just ask Yoda.

 

Being nice to your engine and saving fuel gets you more in the long run. So you have to ask yourself do you want instant gratification, extra fuel cost and possibly extra out of pocket risk or do you want to cruise through TBO for an extra 1K-2K hrs. worth of on condition time?

 

We rarely own things we run at their max 100% of the time and there is a reason for that. Wear and longevity and in this case all that and performance.

 

All this is about being nice to the engine and balance.

 

 

Eddie,

Rotax purposely runs the engine rich at max rpm settings on purpose because of extra heat, wear and detonation avoidance scenarios.

[Roger Lee]

"Consider this your lucky day, your dead meat on this one.

 

PS please don't edit your posts, to preserve my amunition  :)"

 

Been there and done this Ed a hundred times and with a dozen different props on the CT and other CT's. Any results other than what I told you means it wasn't done right.

 

Andy,

Tell Ed what the difference was when you changed pitch? 

Most owners that have pitch issues are worse than Ed and are down around 5200-5300 WOT. Ed's is okay, but just okay and could be better for his high altitude flying.

 

p.s.

Ed,

 

Tested this to over 18'K too. You have a lot of testing to do to catch up. If you try the 1/2 degree pitch decrease on your prop then go the extra mile and try a different prop other than the Warp. You'll have another even bigger surprise. Try a Neuform or Sensenich and turn it to 5650 at your altitude and you'll think you have a new bird.

[Roger Lee]

Here are two real stories,

 

The first year to Page Fly-In I had a Warp and my WOT rpm was only around 5300 set from FD.

I was with ED crossing the Grand Canyon and a few others from the south to the north at 11.5K. We were flying together. ED choose to go ahead and gave it full throttle. He was set up for high altitude flying with more achievable rpm. I could only get 5100 rpm WOT and he could get 5500 WOT and he walked away from me.

 

Second story,

 

I used to have a Warp Drive on my SW. One year going to Page Fly-In a group of us left Payson, AZ which requires a hard straight up climb from the field to get over the Mogollon Rim cliffs. It's a full throttle zero flap climb. I weighed 1470 lbs and more than everyone else and was second to take off. Everyone behind me beat me to the rim and was 1'K higher and I was so low I thought I would have to circle back out of the area and make another run to clear the trees. I did make it by the skin of my teeth so to speak.

 

The next year I put a 3 blade 68" Sensenich prop on and this time pitched for 5650  rpm at 8'K.  This time I weighed the same and took off around #4. I beat everyone to the rim and was almost 1500' above the rim when I got there.

Did prop and rpm make a difference, absolutely.

During my research and testing with props and rpm years ago the Warp performed the poorest in climb and I'm sure it is the blade design being so flat and symmetrical.

 

The Warp does not perform as well in climb as the Neuform or Sensenich and rpm added to that was the difference.

[Roger Lee]

Ed,

 

I'm not purposefully trying to argue with you. I'm only trying to help you and give you the knowledge I gained from many months worth of testing and research. I'm sure you would like a bit more performance. It was a PITA, but I gained a lot of insight and got rid of the BS you hear out in the field.

I ran 5 different prop MFG's and 1 dozen different props. Both short, long, flexible, ridgid, two bladed and three.

[FlyingMonkey]

Roger, are you saying that engines run at 5400-5500rpm don't make it 1k past TBO, but those run at 5000-5200 do?  is that a consistent "this will probably happen", or just a "you will be a bit more likely to have this happen"?  

[FlyingMonkey]

 

Andy,

Tell Ed what the difference was when you changed pitch? 

Most owners that have pitch issues are worse than Ed and are down around 5200-5300 WOT. Ed's is okay, but just okay and could be better for his high altitude flying.

 

 

 

I've said it before, and I'm sure he's seen it, but:

 

When I first got my airplane I was getting 4800rpm on takeoff roll and about 5200rpm WOT at about 3000ft.  The airplane's top speed was 108kt.  After re-pitching the prop the airspeed jumped to 122kt+ WOT and the climb went up about 200-300fpm.  Major difference.  This was pitched for ~5700rpm WOT at  3000ft.  It climbed well all the way up to the Sport Pilot 10k limit.

 

More recently (over last summer), I was thinking with the 5700rpm WOT setting I might be leaving a little cruise speed on the table, so I had a very small amount of pitch put back in.  I gained cruise but lost too much climb, so I had it adjusted again.  I'm now sitting about 5650rpm WOT at 3000ft and I like where it is, though I probably should have just left it at 5700rpm and kept a bit more climb.  Not worth fiddling for the few rpm difference though, and the plane did great out around Page at the higher altitudes there.  IIRC, on a Warpdrive protractor I'm currently adjusted for 22° or 22.5° of pitch at 8" from the tip on a Neuform three blade prop.

[Roger Lee]

"are you saying that engines run at 5400-5500 rpm don't make it 1k past TBO"

 

No I'm not saying that. The TBO is figured on 5500 rpm for the engine life to TBO. This I'm sure also has some liability wiggle room safety factor. I'm not privy to that particular info.

What I am saying is that there can be failures before TBO and you are less likely to experience those failures if you don't press the engine and you are more likely to go past TBO if you don't run it hard.

 

Look at racing engines. Lots of work and teardowns. Look at our cars on the road driven more conservatively and not run at the engine max. There are still engine failures in our everyday cars, but far less than if we pushed them to the max all the time.

 

Kung-Fu master munk says to kung-Fu student munk David Carradine.

Balance Grasshopper, Balance (or something like that  )

 

Master:  Sometimes it is eyes that blind a man.

Student: How can this be?’ 
Master:  Because he has eyes, he does not look. 

[FastEddieB]

Does anything in ROTAX literature actually state or imply increased longevity at lower rpm?

 

If not, I'll just file it away as an informed guess.

 

Albeit from a knowledgeable source.

[Ed Cesnalis]

There is a truism well known among old mountain pilots. It states that when cruising at altitude (above 7.5kDA) a partially closed throttle has the same result as a partially clogged air cleaner. The result either way is lower manifold pressure and that results in reduced power on an engine that is running.

 

Roger is proposing that I am leaving performance on the table cruising at altitude in a wide open throttle condition. The proposal is that more performance could be realized with a partially closed throttle to limit engine speed to 5,500RPM but with less resistance from a yet flatter pitch that would permit overspeeding at altitude if not throttled back.

 

Said more simply, it is being proposed that more performance would be realized with a lower power setting. 

 

It is easy to be fooled when your experience is mostly below 7.5K.  Only 3% of available power is lost for every 1,000' you climb but for the first 7.5K performance only increases even above 10k performance is better than near sea level so again its easy to be fooled when there seems to be no penalty for going higher but the truth is there is a penalty and it begins above 7.5K.

 

From 13,000' to 18,000' the performance penalty is quite clear, consider why.  At 14k the best mp I can see is ambient pressure. If my throttle is wide open only the edge of the throttle plate, the ducting and the air cleaner are reducing pressure, about 1 inch and ram is providing about 1 inch so I can use the ambient number if my throttle isn't partially closed. Its likely to be something like 15" as opposed to say 20"+ you are restricting down to with your partially closed throttle near sea level.

 

So best I can do at altitude is the 5,500RPM limitation and something like 15" of manifold pressure resulting in something like 45hp. So now we do the comparison which involves presenting the angled throttle plate to the already anemic air flow and the result is lower manifold pressure without increase in RPM.  We are already approaching stall speed, the vast majority of our power is providing lift but with a partially closed throttle I'm likely to realize less than 40hp and way up at this test altitude I need more than that to keep from descending.

 

A partially closed throttle is only a negative at high altitude, you never increase performance with a throttle reduction.

 

[Roger Lee]

Hi Eddie,

 

I'm going to use you for a moment.

 

Let's talk motorcycles. When we deal with a Rotax 912 you should think along the lines of a motorcycle engine versus an air cooled, loose tolerance old technology Cont. or Lycoming.

 

I know you have a BMW and it uses Bing carbs.

 

Let's talk wear and tear versus rpm and max power.

Your motorcycle like mine has a red line for rpm. This is max power like a Rotax set for 5800 rpm. The but here is you can only run 5500 continuous and anything over that is only 5 min.

 

So for us the redline for continuous is 5500 rpm in the Rotax. Now your motorcycle has a redline at whatever it may  be. For the sake of this discussion let's put it at 6000 RPM?

 

So you can drive around all day at 5500 rpm?

That would give you great torque and HP. Why don't you drive around at 5500 rpm? Why don't you do that? What are the negatives? Are there any positives to driving all day like that or a long trip? Will your bike engine last as many years or hours versus running at 3000 rpm?

If you believe you should be driving like that all the time then set your Rotax to get 5500 rpm and fly like that for a couple thousand hours.

Same goes for manual transmission cars. Why not drive around up by redline?

 

Both these vehicles will run, but what will be the extra wear and tear. Will it be more likely to give up the ghost sooner for repairs and are you saving any fuel and are you going any faster to all your destinations and is the extra wear worth the it to get there?

 

Now let's add a reduction factor to the motorcycle, like adding or subtracting pitch in our prop. We have a chain and sprocket. If max rpm is 6000 rpm and you put a new smaller primary gear on the engine so the engine can now hit redline much faster, but our top speed is lower. Lots of torque in the lower gears, but we lose top end and everything under that suffers too unless we have a special need for that type of torque. I did this on my Trials bikes for competition.  (Cota 247)

Now let's take the same primary and make it larger. We can go much faster at the top end, but getting there maybe harder and slower because we haven't got the torque and HP to turn that size of primary so it's got great top speed, but sucks down in the lower gears.

So engineer's set this bike up from the factory to get the BEST BALANCED performance from low to high speeds.

 

Setting up a Rotax to only get 5500 rpm WOT is handicapping your performance. The primary sprocket is slightly too big.

 

It's a setup, but answer them truthfully.

[Roger Lee]

Sorry Ed I tried to help, but we aren't getting anywhere.

 

I'm going to have to hang with experience and testing over a general graph not specific to our needs and setups.

I don't need to speculate with graphs. I've done the real thing up to 18'K and with your prop along with others and a CTSW.I've done testing well outside our normal envelope. 

I've done both graphs, real time research, testing and experience. Until you do the real time testing with rpm and props you won't see it if all you rely on is graphs that don't show YOUR SPECIFIC needs at your altitude and prop combination on your engine. Graphs are too general and do not take into consideration your needs and special setup. Rotax publishes graphs for a prop setup for 5800 rpm WOT, an in flight adjustable prop and at much lower altitudes.

[FlyingMonkey]

Another possible way to look at it is that you are not really making full power at 5500rpm.  You make full power at 5800rpm.  If you flatten your pitch a little, aren't you leaving a little power is reserve for climb or other times you might need it?  You are still cruising at 5500rpm, and getting the power made by the engine at that speed.  If you want to fly by MAP, you could re-pitch the prop for higher RPM and then go fly and see what, if any effect the change has on MAP in cruise.

 

I'm not sure what the true and correct answer is, just suggesting some methods for test and verification.  I'd take this on myself, but with analong engines instruments and no MAP, fuel flow, etc, I think any information I provide would be pure seat-of-the-pants feelings and unicorn hugs.

[Ed Cesnalis]

 

Hi ED,You can't change the MP only the rpm setting, so you set it for the best rpm at or above 5500 for that altitude.

 

 

That was just 4 years ago, this conversation goes back 10 years.  My contention that 5500 was the old number is evidenced here.  Prolly this was the time it became 5500 or above, now seems to be 5600-5650.

 

In flight we can change MP and that results in RPM changing too, just move the blue knob.

 

The performance chart above is for a 912ULS published by CPS.

 

------------------------------------------------------------------------

 

Here's my final word.

 

The 2 variables that determine power in your engine are manifold pressure and RPM.  To maximize power simply maximize both.  RPM is easy and 5500 is the max (for reasons you just argued to Eddie) just twist your blades till you get 5500 at WOT at target altitude.  Max manifold pressure is easy too, advance the throttle to the wide open position.  Viola you now have maximum power and with reflex flaps maximum cruise speed.

 

Any plan that doesn't maximize both these numbers results in a lower power setting and a lower cruise speed.

[Cluemeister]

I have enjoyed watching this debate from the sidelines.  

[Roger Lee]

Ed's my friend.

I'm not trying to argue with him just help him realize a tad better performance at his high altitude flights.

[ctfarmer]

From an engineering perspective, i am soundly with CT.

 

We have agreed 5500 is our RPM limit in cruise.

 

A prop pitched to allow you to just reach 5500 at wot must give higher cruise speed than one pitched to allow any higher rpm. Any other outcome does not make sense to me. There is no special singularity point where a prop magically, with pitch change, gets to produce more thrust at a given rpm and/or less power output.

 

No other arguments are relevant about where you want to run to get longer life or whatever else you wish to argue. Those arguments are conjecture to boot but a different subject matter anyway.

 

A prop must give greater thrust if it is flattened to run at 5500 and full throttle. Hence best speed.

 

I flattened mine for 5500 max rpm at full throttle at 7000'. It will happily propel my CTLS to 127kts at full throttle, -12 flaps using about 23.5 litres per hour. Then i back it off to my normal 5300rpm cruise for 20 to 21 litres burn per hour and 120-122kts TAS.

 

I have played and played with this. I have a digital cube and jig to set my prop to within 0.1deg accuracy. I can not get those cruise speed numbers at 5500rpm with a finer prop setting ie higher wot rpm. Nor should i expect to.

 

Climb is an entirely other matter. That is where the compromise comes in. My climb [erformance i am happy with. Yes, i only see 4850 on takeoff at sea level. But how it climbs, i am happy given the cruise advantage.

 

So my compromise is that i want to see that 120kts in cruise at 5300rpm and 20 to 21lph.

 

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

[Roger Lee]

Hi Peter,

 

"A prop pitched to allow you to just reach 5500 at wot must give higher cruise speed than one pitched to allow any higher rpm."

 

Why?

 

Your engine must have enough HP and torque to overcome that pitch setting at any given altitude. My SW is faster than any LS or Sw that is set for 5500 WOT at the same altitude. Proven over hundreds of hours of testing and in flight side by side results. I have also switched prop pitch on the same planes and the one with 5650 will always go faster and climb better than one set for only 5500.

 

"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."

 

 Setting rpm is hard to be exact and rarely can be done. You shoot for a good target range. Each flight may give as much as a 50 rpm difference due to wind alone. No one is accurate enough to fly two different flights at two different times and tell 1-2 knots.

And pilots are the worst evidence on better performance. The worst evidence at a trial is people's testimony. The best evidence is physical. Real live test. Pilots and instruments are not accurate, especially the pilot factor.

 

To be on the same page in this debate the naysayers need to go out and do the same testing a research that was done in Tucson. It will open your eyes. Just saying it isn't that way has no real time testing with different planes, props and pitch settings.

 

The 8 CT's at my field, the CT's from other states, the Gobosh's, Sport Cruisers, and RV-2's, ect.... would agree since they have come here to have their props repitched. They all can't be wrong. It has been proven over the last 8 years at the Page, AZ Fly-In at altitude and different prop settings. I have had to re-pitch props with my Ipad on the ramp because they couldn't keep up or take off above 9500' DA.

 

Climb will indeed be better if the WOT setting on take off nears 5800 rpm. This is what rotax would like to see providing you had an inflight adjustable prop. Since the US doesn't allow these then we balance for climb, cruise and fuel economy.

 

 

There will never be a conclusion here until everyone does their own testing and works hard at removing the human factor. 

[Roger Lee]

This has been fun today since i didn't have anything to do.

 

here is the other thing. We all should know that HP, torque and prop pitch performance are not linear.

If we think 5500 is the best WOT performance with a fixed pitch then someone would think that 5400 is even faster and better and then 5300 rpm. Okay so you say no that is over pitched. (at least I hope you say that last part) So if that is true about 5300 and 5400 what makes you think that 5500 is the best pitch and that by decreasing pitch up to another 150 rpm wouldn't even be better?