Prop pitch and RPM

[FlyingMonkey]

 

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. 

 

Clarification please?  You are saying the airplane set to 5650rpm WOT will beat the one at 5500rpm WOT, even when both are cruising to 5500rpm side by side?

[Roger Lee]

"To resolve best speed issues we have to agree on the 5500 limitation"

 

Here in lies the problem. You have limited yourself. Here are your handicaps. The Warp prop, only 5500 WOT in cruise gives you poor performance at take off. Rotax actually says they want 5200 rpm at take off and 5800 is max performance at take off. So you're 5500 WOT in cruise has handicapped you even more at take off. Flying out of a field that is 7100'+ and at constant altitudes from 10k-13K. Most of us don't do that. I know you may not want to change, but ditching the Warp prop for a Neuform or a Sensenich will make a BIG difference. Before you say no to this remember I had that prop and flew next to many other CT's that had other props.

5650 rpm will always outperform the same plane propped for 5500 WOT. That's a 100% fact. I've won so many bets with that I would be rich if I hadn't only bet lunch. Now you know why I gained weight.   .

​Only getting 5500 WOT in cruise also causes higher oil temps in climb which you also had. You personally had more problems that caused high oil temps, but most can fix theirs with unloading the engine work load.

 

Bottom line is you have to quit thinking what max performance is and think BALANCED performance.

 

p.s.

If you set your prop for 5600 you could run there, but that you won't ever find in writing and I won't tell anyone where I got my info.

 

 

So if you really want max performance like you say you need to get rid of your handicaps. Altitude you're stuck with, but the others are changeable.

[FlyingMonkey]

 

p.s.

If you set your prop for 5600 you could run there, but that you won't ever find in writing and I won't tell anyone where I got my info.

 

 

 

Now wait just a doggone minute!  

 

In another thread you recommended cruising at lower than 5500rpm, IIRC you said 5200rpm.  You strongly hinted that the engine was much more likely to run well past TBO by doing so.  

 

Now you are coyly suggesting 5600rpm might be okay for the engine?  For how long?  I'm sure I could run at 5800rpm all the time too, but I'm guessing not for long!     

[Ed Cesnalis]

 

So if you really want max performance like you say you need to get rid of your handicaps. Altitude you're stuck with, but the others are changeable.

 

I'm not asking for better performance.  I've never seen a CT go faster than mine.  I was trying to discuss where best speed is found and why.

 

Sure 5600 is faster but another change of subject.  I can't afford a new prop and I kind of doubt it would result in any more speed.  Last time out I was true at 123kts,  I've seen as fast as 127, always slower in the summer.  If your fancier props where at 130 or even pushing that I would have some interest but I don't think anyone is seeing that.  I've learned to be content at my current numbers there pretty good.

 

I'm happy where I'm at because I know its the best speed I will get while observing the 5500 limitation.

[Roger Lee]

Andy,

 

"Now wait just a doggone minute!"

 

I knew that would get someone's attention.   

 

You guys keep mixing up sound BEST PRACTICES and BALANCE with MAX performance. Too different animals and several of you have different operational variables. Don't over compare.

ED wants max performance not balanced performance. One is harder on the engine than the other. One gives best all around performance with balanced cruise, climb and fuel.

You have to separate the two.

 

There is an SB from Rotax that says they want to see a minimum of 5200 rpm at take off. Remember what I have posted in other threads. The manual and bulletins are written for three engines and in flight adjustable props. I didn't make it that way I just learned to work within the system and know what the differences were through education. Matter of fact I go back to Rotax school again in March for a week. Instructors aren't let off the hook either. 

Running at high rpms for max will cause more wear, heat and fuel consumption than flying at a reduced throttle setting like the majority of the world does.

This engine was tested at max rpms different from what we fly every day as owners. A liability line in the sand had to be drawn somewhere. 5500-5800 rpm for 5 min. is the line in the sand for MFG liability and buffered owner abuse and use. Just maybe the real numbers are higher. Maybe.

 

Andy,

Keep doing what you're doing with BALANCED operational parameters and don't look to get max all the time. Racers look for max and see what they have for maint.

5500 isn't max, it is continuous, but leaves performance on the table as you have proven to yourself. 

 

Be happy where you are. Your Yoda has you balanced. 

Now you're ready for Yoga. I hear the instructor is a 25 year old blonde. 

[Roger Lee]

ED,

 

"observing the 5500 limitation".

 

You're right it's a limitation.

If you had a Sensenich or Neuform on your plane it would result in better climb and speed. Definitely more climb. I used your very prop in my research on my SW against the others. Remember I have used your prop that is on your plane and you haven't used mine.
If you tried 68" Sensenich or the 66" Neurform you wouldn't go back since you are looking for max performance. Warp is a good choice for off asphalt use, but not as good as it could be and if you always use asphalt strips then others may be a better choice.

 

If you are content then that's all that counts and all is good.

[FlyingMonkey]

 

Be happy where you are. Your Yoda has you balanced. 

Now you're ready for Yoga. I hear the instructor is a 25 year old blonde. 

 

I'm ready for Yoga the same way you're ready for Mr. Universe competition!  

[Roger Lee]

Not so fast Fast Eddie, 

 

You don't get to stir the pot without committing and a few answers. 

Since you believe in 5500 WOT being max power and doesn't cause any more wear than let's say 5100 rpm you didn't answer my question a few post back. 

 

Why don't you run your motorcycle just a couple hundred rpm under redline in all gears to achieve max power? Redline is max power and running under it isn't. Nothing in your manual says not to. Must be a reason? 

 

You don't have to answer any questions except this one and you don't get to dodge it.

[ctfarmer]

We had a group hug possibly too early. The science is sound.

 

You cannot fairly compare this issue using two planes side by side. Who knows what power each engine is developing. One might have the sweetest running engine, run in perfectly and developing more torque. You can only compare one ac setup with different settings.

 

And on motorcycles. Running close to redline in a motorcycle does not equate to max power unless you are at wot. We are talking performance. And yes, in racing where performance is king, that is what happens. The rest of us ride mostly at very low throttle openings. Why is that. Well, clearly, unlike flying, we have speed limits to name one thing,typically far lower than the machines capability. But if you rode a 50cc motorcycle, you might be gearing to get max speed at wot as we can do as standard procedure in aircraft.

And on that 50cc motorcycle,if you wanted to go as fast as possible in that situation, you would wind the throttle on, not back off. And if you ran too high rpm at wot that forced you to back off, you would decrease the sprocket ratio until you match redline and wot. This would give you top speed.

This is a nonsense comparison and a diversion into silliness.

[Roger Lee]

"You cannot fairly compare this issue using two planes side by side. Who knows what power each engine is developing."

 

I know I'm the mechanic, tune them and set the pitch and they are identical planes. I've been working n Rotax engines since the two stroke days and for about 18-20 years. I'm the only Heavy Maint. facility in AZ and I'm one of only a few a Rotax instructors in the US. I was given this task by 2 prop MFG's (but used 5 different prop makes and a dozen props) and had to have as tight a control as possible and document all my findings.  Physical entities like engine, plane and props are very controllable. It's people that inject the variable. It took months for all this testing They are CTSW's and MFG was only a couple months apart. I control the maint., the setup and the flight before take off. They take off at exactly the same time side by side as we do every single flight at my field with my flying buddies. They fly within 50' of each other for testing (take off and cruise). The test is as controlled as I can make it and it's repeatable every time.

​Here still the issue. You guys haven't done this testing and I have and you don't know the parameters and conditions of the testing so you're comments on what maybe should be or trying to logic it out of what should  have happened with some other plane, engine and prop doesn't work or apply.

 

You have to pay your dues and repeat the research and testing.

No one here has this real time testing, but the Tucson guys.

[Roger Lee]

Hi Ct farmer,

Fast Eddie you aren't out of the woods yet.   Tell me why you don't ride your BMW just under redline for all this power?

 

Back to the motorcycle question.

The Rotax is very much more like a motorcycle engine than a Cont. or Lycoming. Rotax uses more old technology versus new technology to make 100 HP in such a small engine.

 

"Running close to redline in a motorcycle does not equate to max power unless you are at wot"  AGREED

"we have speed limits to"  

 

Speed limits have nothing to do with the question and many drive faster than those anyway, but

 

You do not need to ride a motorcycle in top gear and bust a speed limit. Why not run 40 mph  just under redline on your big 1800cc street machine in 2rd gear instead of 5th  and still be under the speed limit?

 

Speed limits and 50cc machines don't apply as we're talking rpm and wear only.

 

My question still stands. You are running a Rotax 300 rpm below redline. Why not run a motorcycle 300 below redline? It makes more power there. Is it not making more power at 5700 rpm with a 6000 rpm redline on the motorcycle versus 3000 rpm?

 

 

If you did this would it not generate more heat, more fuel consumption and more wear?

 

When you answer this explain to me why it differs from a Rotax running at high rpms.

 

 

Before you branch off or make other comments on flying please answer these specific questions.

[FastEddieB]

And on motorcycles...

This is a nonsense comparison and a diversion into silliness.

 

 

Yes.

 

I apologize to Roger for not responding to his motorcycle questions before.

 

The reason was that they seemed to be an attempt to make an analogy between ROTAX aircraft engines and a typical motorcycle engine. This seemed so farfetched as to not really merit consideration. About the only thing they have in common is that they are four stroke internal combustion engines, albeit sharing some basic attributes.

 

I own a couple of BMW K1200RS motorcycles. They spend most of their highway time at about 4,000-4,500 rpm in sixth (top gear). This is with a redline of about 9,000 rpm.

 

I could easily ride down the highway in fifth, fourth, third, or even second gear. Why don't I?

 

Basically, in sixth, the engine is smoother, quieter and gets better gas mileage*. Cruising at 70 mph probably takes no more than 30 or 40 hp, so redline engine speeds would accomplish nothing of value. Those are my primary reasons for cruising in top gear. While there may likely be some difference in engine longevity, that is not on my mind as I shift up the gears into sixth, nor is it a given.

 

And here is a typical hp/torque chart for a modern motorcycle:

 

 

The key to this is that the torque curve, where real life usefulness lies, is pretty darn flat from about 3,500 rpm to about 9,000 rpm, where it falls off gradually. This bears almost no resemblance to a ROTAX, where all the useful continuous power is between about 4,500 rpm and 5,500 rpm.

 

What we do know from ROTAX is that, in theory, the 912 will generally run at least 2,000 hours at 5,500 rpm before needing an overhaul. The engineers seem confident enough in that figure to publish it.

 

I wonder what they would say if you asked them to estimate the longevity of an engine running at only 5,200 rpm? Unless they have actually tested it at that speed, I still hold they would just be guessing, much like Roger is. And if they did test, it would be cool to see a chart correlating engine rpm with expected engine life. But until then, 2,000 hour predicted longevity will have to do across a wide range of engine speeds. Operating it at lower speeds might make it last longer, but it might not.

 

As an aside, Cirrus calls for climbs to be made at 2,700 rpm - the redline. When I flew for Airshares Elite, their SOP called for pulling back the power lever (linked to the prop governor) to 2,500 rpm for the climb. When I asked why, they said "to save the engine". Since they were paying me, I didn't argue, but I think that logic is wrongheaded as well. The first travel of the Cirrus power lever decreases rpm without decreasing MP. That can actually result in higher combustion chamber pressures, and rather than "save" the engine may actually subject it to slightly higher pressures and temperatures.

 

Anyway, before I tell a student or a fellow pilot that a ROTAX flown at 5,200 rpm will last longer and show less wear than one operated at 5,500 rpm, I'd want some data to point to if they asked how I knew that. "It's only common sense" or "Because Roger says so" would not suffice for me.

 

Anyway, I think the questions posed about motorcycle operation have the potential to really derail this thread without adding any meaningful info.

 

 

 

*My BMW usually gets about 42 mpg on the highway. One time in FL I accidentally rode for a couple hours in fourth - the engine is rubber mounted and so smooth I didn't notice it for a long while, in fact until I went to exit the interstate. And guess what? It still got about 42 mpg. I found that surprising, but with a torque curve similar to that shown above, I guess it makes some sense.

[Roger Lee]

"I own a couple of BMW K1200RS motorcycles. They spend most of their highway time at about 4,000-4,500 rpm in sixth (top gear). This is with a redline of about 9,000 rpm.

 

I could easily ride down the highway in fifth, fourth, third, or even second gear. Why don't I?

Basically, in sixth, the engine is smoother, quieter and gets better gas mileage*. Those are my primary reasons for cruising in top gear. While there may likely be some difference in engine longevity, that is not on my mind as I shift up the gears into sixth, nor is it a given."

 

Correct.

 

"While there may likely"

 

That wording is one way to dodge the bullet. You and I both know the answer as to why we don't ride just under max rpm all the time. We have been riding for years.

Wear and tear over time and the increased likelihood of a component failure.

But why not drop down a gear or two and ride it at 8500 rpm all the time?  You and I both know the answer, but it would be nice to hear it.

 

 Forget smoother and quieter. It has nothing to do with performance and wear and that's like speed limits.

 

Your performance chart is similar to the Rotax HP and torque charts so why run a Rotax at higher rpms and not your BMW?. Your HP continues to go up to max rpm, but the torque starts to drop off at some point which is different for each engine.

 

"if they asked how I knew that. "It's only common sense" or "Because Roger says so" would not suffice for me."

You are correct. I can see more schooling in your future since these won't suffice for you. That's a great observation and I'm glad your going back for more classroom education..

Just attending One class level and or just one class will never do it. 

 

"if they asked how I knew that. "It's only common sense" I agree you really have no basis other than hearsay so,

 you need more education in a Rotax class. 

 

​Education, education, education. I go to Rotax school every year, only get my info from top dog at Kodiak and  through invitation only instructor school.

 

My point is you guys are arguing from a point of weakness without the education and continued education.

 

 

 

 

Ctfarmer,

 

"And on motorcycles...

This is a nonsense comparison and a diversion into silliness."

 

No it's not. You guys keep talking about HP, torque, charts as if they only apply to a Rotax engine and its older technology and it's the only one we should run at max.. If what you say holds true then it should apply to other similar engines (like motorcycles) and applications.

This is how little some know about Rotax. It is actually quite close to a motorcycle engine and that is what we tell people in class.  It is nothing like a regular aircraft engine like Cont. or Lycoming. This is why going to Rotax classes and repeating them every so often is very important. Even when you have had the same class you learn new things each time from the same instructor as emphasis is different or from other instructors. Having said that there are certainly some things that are just personal preference.

 

 

Don't forget to HUG!   

[FlyingMonkey]

IMO, ground vehicle (motorcycle, car, truck) engine use is inherently different from aircraft engine use.  Why?

 

Because ground vehicles have contact with the ground.  It is much easier to push against the ground than it is to push against air.  Put a car and an airplane with the same weight and horsepower side-by-side on a runway, and see which one accelerates faster.  Air is thin and makes for a much poorer reaction mass than the fixed Earth, which is much easier to push against.

 

Now, the problem ground vehicles have is rolling drag.  A car with wheels rolling on the ground generates friction.  Wheels to Earth, bearings, axles, differential, etc.  All of this induces drag, which is why for a given horsepower an airplane will usually be faster* than an automobile once it reaches its cruise RPM.  Air alone just doesn't have as much drag on the airplane as all the other drag components mentioned above PLUS air drag, which a car still has.

 

*  Assuming a car with one gear vs. fixed pitch prop, or a car with multiple gears vs. CS prop.

 

What's the point?

 

Well, because of the far superior mass to "push against", the ground vehicle needs little power to maintain speed.  Action/reaction and all, and the "action" is better.  So a car can cruise comfortably as a much lower percentage of maximum power than the airplane, which has to use a much higher percentage of available power to overcome the much thinner, less efficient air as a mass to push against.  

 

Imagine it, why do airplanes need more power up at altitude to maintain power and speed?  The air is thinner, and less air mass means less thrust.  Sure engine efficiency goes down too, but that's just a compounding issue.  Even an electric propeller-driven airplane would eventually reach an altitude where insufficient thrust was produced to keep the airplane flying.  A propeller that produces great thrust on Earth might not even get an airplane moving on the runway on Mars at the same power...the air is simply too thin.

 

So a Rotax 912 engine hooked to a motorcycle could probably easily run at 2500rpm and produce 80 knot speeds, but the same engine on an airplane needs about 4000rpm or more (depending on airplane) to get the same speed.

 

This is my layman's physics concept of why we run airplane engines at relatively high power and motorcycle/car/truck engines...not so much.  Feel free to tell me how wrong I am, I'm used to it.  

[Roger Lee]

"if they asked how I knew that. "It's only common sense" or "Because Roger says so" would not suffice for me."

 

​I can see you might not trust me, but to think you don't believe you have any common sense stuns me.   

 

You left that door wide open on a windy day.   

[Cluemeister]

After reading all the posts, I'm siding with Roger.  

 

Of course, I don't know squat about engines, so that probably hurts his case. 

[FastEddieB]

After reading all the posts, I'm siding with Roger.

 

 

I'd be happy to as well.

 

All I need is some data.

 

I suspect he's right.

 

But I was taught to not automatically assume that lower power settings necessarily result in increased longevity. There are too many anecdotes of engines run hard their entire lives going way beyond TBO, and others of engines that were babied and that still needed work well short of TBO. In fact, I'm pretty sure I was told that operating a ROTAX below 5,000 rpm was not a good idea, allegedly because of increased wear.

 

But the plural of anecdote is not data. If data exists for the ROTAX, I'd like to see it and would file it away on my iPad to support what I would then start telling students.

 

Right now, I'd tell them "Do not abuse your engine. But don't feel you have to baby it. Making it to overhaul is never guaranteed, but I've not been shown that, as long as operated "in the green", rpm by itself is a reliable predictor of engine longevity. Something like 90% of engine wear occurs on startup, and in a properly functioning engine there should be virtually no metal-to-metal contact in normal operations. Which is why rpm and wear are only very loosely correlated, if at all."

[ctfarmer]

This is a now a very different argument to the one originally addressed which was about performance, not life or oil temperature or anything else. But now we are onto longevity i will say this. Using all your logic. Running a taller gear and less rpm/engine speed is what you are clearly advocating. Yes. Exactly what i have been saying too. Gear up, drop the revs for all theses other benefits but to do the same speed with taller gearing you need more open throttle. But you will use less fuel because gas engines are more efficient at higher throttle openings and you also have lower friction losses at lower engine speed. Again totally agree with you. But let me say again - rpm does not represent power.

Power is torque x rpm. In using a higher gear, you increase torque ie throttle opening to compensate for lower rpm and not surprisingly, you get all these other benefits including fuel efficiency and longer life.

 

So, with an ac, more simple physics. A coarser prop pitch will drop your revs for the same airspeed. You compensate to get the same power with more torque which you need to turn a coaser pitch (aka gear in your bike).

[Doug G.]

In racing your shift points come just beyond peak torque so you are accelerating into top torque. The same does not apply to a ground adjustable prop on an airplane, obviously.

[ctfarmer]

Hello Doug

 

We need to dispell the concept that torque does something. Torque, like talk, alone do not result in anything.

 

For a thought experiment, what if you had a motorcycle with peak torque of 70flbs at 7000rpm dropping off to 60 flbs at 12000rpm then 55flbs at redline of 14000rpm? When would you shift?

 

Or further, what happens when you apply 70flbs to a torque wrench but the nut does not turn. Nothing. No effort, no work. Nothing.

 

If you substitute the word power for torque i agree. The ideal time to shift is when the descending power curve for the lower gear crosses the ascending power curve for the higher gear. Power is the key to maximise acceleration. Power is work/time. You want to do lots of work in the shortest time.

[Roger Lee]

You can't blow off torque.

 

Torque and HP are a way to measure some work outputs and apply it to an application. Without enough torque rpm and HP are not enough to efficiently turn a prop. You can have HP and even rpm, but if the gearing is too high and or the prop or item being acted on is too large then it won't have the muscle to turn it efficiently. 

 

Torque

From Wikipedia, the free encyclopedia

 

 

For other uses, see Torque (disambiguation).

Classical mechanics

Second law of motion

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Relationship between force F, torque τ,linear momentum p, and angular momentum L in a system which has rotation constrained in one plane only (forces and moments due to gravityand friction not considered).

Torque, moment, or moment of force (see the terminology below) is the tendency of a force to rotate an object about an axis,^[1] fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object. Mathematically, torque is defined as the cross product of the position vector of the point where the force is applied (distance vector) and the force vector, which tends to produce rotation.

Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.

The symbol for torque is typically , the lowercase Greek letter tau. When it is called moment of force, it is commonly denoted M.

The magnitude of torque depends on three quantities: the force applied, the length of the lever arm^[2] connecting the axis to the point of force application, and the angle between the force vector and the lever arm. In symbols:

 

 

If an engine had extremely low torque then it wouldn't be able to turn a prop at an efficient angle of pitch to utilise its HP and and propel the aircraft properly.

 

You would have a terrible marriage of components.

[Ed Cesnalis]

... Without enough torque rpm and HP are not enough to ...

 

 

Think: torque x rpm = power.  Since torque and rpm determine power the above is confusing.  How about 'without enough power you can't efficiently turn the prop.  If you are lacking power you are therefore lacking rpm or torque or both.  Power is what determines if you can efficiently turn the prop, torque alone can't tell you that.

 

Its helpful to think torque and rpm determine power just like its helpful to think you need both manifold pressure and rpm to determine power setting.

[Roger Lee]

Hi Ed,

Works for me.

[ctfarmer]

Yes so you use a gearbox it you do not have enough torque from the engine to turn an efficient sized prop but you must start with enough rpm to allow the reduction. Simple. So you are back to power as the only relevant matter.

 

Im out. This is silly.

[GravityKnight]

High RPM, load load, vs low RPM high load wear type discussions have been going on as long as I can remember. I have witnessed low speed, high load, engines with noticeably cylinder wall wear due to side loading. Generally more common with long stroke engines. I have also seen spun bearings due to extremely high rpm use over a prolonged period of time. Rods don't particularly love high rpm regardless of load due to the inertia of changing direction so fast. This can be argued so many ways... what it comes down to is the particular engine at hand. Some do better one way, others don't.

 

Considering the case fretting issues if the RPM is too low / load is too high on the earlier 912's I tend to feel the 912 prefers to run at a high-ish rpm. It is a very short stroke engine, so internally the piston speed is not that high. Assuming the oiling system is up to the task, which it seems to be, I prefer to run mine on the finer prop pitch side. In theory the economy will be a little worse when spinning it fast, but I haven't noticed as much of a drop as I would have guessed. And have witnessed poor economy/performance/speed on several 912's with too coarse of prop pitch for the altitude. My RANS is happy cruising around 98-100 knots and I have that at 5450-5500rpm on my 72" KOOL prop to retain 5400+ take off rpm for great climb. With the zipper kit I installed (114hp 11:1 compression), and hacman leaner (I'm at 6600ft ground elevation), I burn ~3.75gph doing this. Most of my flying is spent landing in a field, or climbing up high in the mountains etc. I don't do a lot of cross country so my plane rarely sits at 5500rpm for hours on end.. though I am not afraid to do just that when traveling.

 

Being that most of us don't have an adjustable pitch propeller, we have to pick the happy medium between speed, climb, wear, economy etc. Or we could do what the guy in the hanger across from mine has done, he has a yamaha 120hp motorcyle engine still with the transmission in his zenith ch750. The transmission is functional! You guys using the bike/changing gears analogy above reminded me of this thing. It's ready to fly, but has not been flown yet. He was hoping to recruit me since I have experience in a 750..... not going to jump on that grenade just yet.

 

I guess the point I'm hoping to make is that it all depends on the particular engine, as to which is "better" for the engine.

How about the gear box.... what does it prefer? My assumption is that it doesn't mind higher rpm / lower load operation considering it will experience increased wear if run/idled at too low of an rpm.