Cruising Speed and Best RPM for CTSW

[Jim Meade]

Ok, I really am not trying to be a pain here. What I really want to know is the theory behind choosing max rpm for setting the prop. If it were not for the Rotax limit it looks like the hp would continue to increase beyond 5800. Why is 5800 the magic number if I normally fly at 5000?p The Neuform manual gives a template and says nothing about rpm.

 

You're not being a pain at all. This is an interesting discussion. I do think maybe you're asking two questions. The torque vs. horsepower issue is a long standing one.

 

 

[GlennM]

I am sick at home so I figured I would add a long two cents to the conversation. There appears to be quite a bit of confusion on how the world works:

 

How do you measure horsepower? How did Rotax come up with these curves? I'll be they put them on a dynamometer in a test cell. What does a dynamometer measure? Torque. Only torque. The horsepower figure is calculated: HP=(measured torque X rpm)/5252.

 

Traditionally, there have been "climb" props and "cruise" props with different pitches for different aircraft flight regimes. The benefit of our ground adjustable propellers is that you don't have to buy two propellers for each regime. Adjustable pitch propellers were designed to allow the benefits of each without the need to change propellers. There are limiting factors that preclude the use of one fixed pitch for all flight regimes.

 

The Wright brothers knew that a propeller is just a rotating wing. Too much pitch for a given rpm will result in the blades stalling just like an aircraft wing. However, the engine usually has a say prior to this condition. As load increases, the risk of detonation increases. Our engines do not have knock sensors (microphones that listen for detonation), so the manufacturer provides a limit to the engine rpm and load range that provides enough margin for detonation. On the high speed side, the diameter of the propeller limits the maximum rpm due to the tips going supersonic, which is not very efficient. An AT6 or 210 taking off is a good example. The engine has a say, also. Contrary to forum statements, the engine will not continue to produce more and more power the faster you spin it. There is a limit. As the rpm increases, the time the intake valves are open admitting air is less and less, limiting power output. Therefore, the horsepower drops off eventually. Many times, the maximum rpm limit is set by the design of the internal parts. The faster you spin an engine, the faster the pistons move back and forth and the more load the connecting rods have to take to reverse the piston direction. The engineers at Rotax set all of the limits for the engine based on their analyses of all these limitations. Together with the lawyers, they publish their manuals.

 

Personally, I follow what the manual says. In my experience, I take the explanations given by mechanics, technicians, or nurses for that matter with a grain of salt. As we have seen with the many Service Bulletins and Alerts, if there is a problem with something, Rotax responds quickly. If it were a problem to operate in a given rpm range, there would be something that says to not operate in that rpm range or limit operation there. If I remember correctly, the Cessna 150 with the 0-235 has some sort of rpm limitation, which requires a yellow arc on the tachometer. Sorry for the long post.

[Doug G.]

With this style of prop the angle of thrust changes from the hub to the tip. There is only one adjustment where it will pull close to even along its whole length at any rpm. Any deviation from that means uneven loading along the length of the prop. It is even possible (an extreme case) to have part of the prop, at speed, in opposition to the direction of the movement of the plane. This is what brings all these questions to mind. With a straight prop, you know what a steeper or shallower pitch will do. It is not as obvious, to me, with these props.

Doug

[Doug G.]

I agree there is a limit. When I look at the graphs, 5800 is not the limit. It is, I am sure a good limit for engine longevity. But remember, it is only allowed for 5 minutes. When you look at the fuel consumption curve there is a slight leaning of the engine at the high rpms which may be a part of the reason for the limit.

The three blade prop not only provides ground clearance, but gives a smaller diameter that keeps the tip speeds down. (And makes it quieter.)

(Speaking of which, where would you measure the pitch on this prop?)

I am going to explore this further when I get back home and do some angle measurements on my prop - should be interesting.

Doug

[GlennM]

I thought most propellers were made with varying angles to balance the thrust over more of the blade due to the different velocities as you go from hub to tip. Is there something different with the Neuform that I haven't noticed?

[Ed Cesnalis]

The concept of operating "10 inches over square" means little to a US CT driver for 2 reasons.

1. We have ground adjustable props so manifold pressure is not very useful to us.
2. We have geared engines and the "square" numbers don't apply

However: the concept of operating over square is [roughly speaking] what we are arguing about. If pilot flying a constant speed prop configuration follows the over square rule settings he will not present a high load at a low power setting.

 

If a CT pilot has his prop pitched to achieve 5,500 to 5,800 RPM he will not be presenting a high load at a low power setting even at WOT.

 

Pilots do fly 12" over square and argue that with aggressive leaning it is the only efficient way to cruise which may be true but at the same time they fly within limits and flatten their props when they apply full power.

[Roger Lee]

The Rotax numbers are based on a couple of things. For the end user it is based on 5800 rpm. Second some numbers are from a dyno and some are theoretical based engineering numbers. For the end user all those numbers are nice to look at, but your setup with a ground adjustable prop is based on rpm. You don't really have control over anything, but prop pitch. For the end user as far as Rotax is concerned HP is maxed at 5800 rpm. There are other numbers, but you won't learn about those unless you ask in a Rotax class and it's usually the Heavy Maint. class. I tried to tell people what some of the real numbers were, which aren't published anywhere, but that didn't work out so well. Some wanted it in writing and that isn't going to happen from Rotax. I can't imagine what people would do to their engines.

[Roger Lee]

Doug,

Several prop MFG's have a different spot to measure, but 8" from the tip is close for most and different props have different pitch to achieve the same rpm as another, but they are usually close. You really only care what the final rpm is since that is all you have control over. Measure back 8" from the tip if you want to check to see if all blades are equal or if you want to change the pitch. Most props will fall within a certain pitch degree range because to little pitch doesn't generate any lift off the blades and too much pitch is like a wing being stalled.

[Roger Lee]

You guys sound like you are in two different ball fields. One is about props and engine's in general which may apply outside our CT or LSA world, but several of those items don't apply to the CT or LSA run by a Rotax engine and a ground adjustable prop.

The other is just in the CT ballpark, which we then don't have control over certain aspects of other props and engine setups.

One thing that would make a difference in anyone's perspective would be to take a class from Eric Tucker (Rotax) at the Heavy maint level. You will learn things that aren't in the manual, not in writing and give info you can't get any other way. Everyone from the old timer to the reasonably new guy comes out of that class and say they never really understood until they got out of that class how much they didn't know. The problem is the Heavy Maint. class is limited to who can go.

 

 

 

[Doug G.]

That is true, the issue here is the ground adjustable feature which is made to allow fine tuning of the prop to give you the best results taking into account rotation, pitch, and relative wind, the effect of which increases as we fly faster. There is only one setting where the prop gives something close to even loading along the entire blade. This is where the propeller is most efficient. Anything that varies from that will reduce the total thrust produced - at any rpm.

The system proposed is to adjust the prop to allow max rpm to be 5800. It sounds good, but I cannot figure out what the reasoning is. I understand in a car top speed is associated with max hp and acceleration is based on max torque, but in our planes we are looking for max thrust for any given rpm which I see as something different than simply maxing out engine rpm.

 

For example if we had flat props and could only get 5800 rpm by bringing the pitch angle to 1 degree, would that be the best setting? We would still get max rpm and max hp from the engine, but we would not get off the ground - probably wouldn't even be able to taxi. ---max rpm, max hp, little thrust, no efficiency! So then if we turn our props to get 5800 rpm - is that the best thrust for the prop, the most efficient angle? Unlikely, we have only used the prop as a governor to limit rpm. (I would prefer to use the throttle to do that.)

 

On our props changing the angle from the point of most efficiency simply means that somewhere along the prop we will have an airfoil that is striking the air at a less than optimal angle producing uneven loading along the prop (the very thing it was designed to avoid), and reducing thrust throughout the entire rpm range.

[Doug G.]

Roger, I am not much on the Rotax secret society, and if Rotax is keeping something like how to set your propeller a secret they are damaging their reputation.

I am currently with Brian Carpenter whom I would guess you would agree knows a lot about both Rotax, and propellers. Brian has a way of demystifying this stuff, and providing the theory behind it.

I would add that Rotax does not give instructions on how to set your prop, Neuform does, and they don't say anything about rpm, because it makes no difference if the right prop is used and the prop is set correctly.

 

I don't care about mysteries and magic numbers, just tell me why it works. What's the theory? I am a reasonably intelligent person I am pretty certain I will comprehend it.

[Ed Cesnalis]

dggrant ,

 

The system proposed is to adjust the prop to allow max rpm to be 5800. It sounds good, but I cannot figure out what the reasoning is

The reasoning is simple and I've told you before. Lets break it into 3 conditions. 1) 5,800 max. 2) greater than 5,800 max. 3) less than 5,800 max.

1. 5,800 max permits access to up to 98hp
2. greater than 5,800 limits access to power. Such a set up will not permit you to realize 98hp in climb or 92 hp in cruise. The limitations will increase with altitude. Example: if I am cruising over Yosemite National Park at 13,500 and my prop was pitched to achieve greater than 5,800 then I would have to throttle back to 5,500 and I would have 40 some hp and I would not be able to maintain altitude. I have a 100hp engine and I need all the power that it can produce.
3. less than 5,800 or less than 5,500 limits access to power and as the pitch gets coarser it presents too big a load to open your throttle fully.

Summary: The reasoning to set for 5,800 or 5,500 max is to permit access to all the power that the Rotax can deliver without operating beyond permissible RPM. Secondary reasons are fuel economy and avoiding vibration.

 

Question for you; if as you say there is only one correct setting for you prop then why would it be adjustable?

 

For example if we had flat props and could only get 5800 rpm by bringing the pitch angle to 1 degree, would that be the best setting? We would still get max rpm and max hp from the engine, but we would not get off the ground - probably wouldn't even be able to taxi. ---max rpm, max hp, little thrust, no efficiency! So then if we turn our props to get 5800 rpm - is that the best thrust for the prop, the most efficient angle? Unlikely, we have only used the prop as a governor to limit rpm. (I would prefer to use the throttle to do that.)

If you have to use your throttle to limit to 5,800 as you would prefer than you would have to have a pitch that is too flat and you would not have access to full power without redlining the engine. Case #1 above. If we had flat props they would not be air foils and they would not produce much lift so that isn't a useful analogy.

 

On our props changing the angle from the point of most efficiency simply means that somewhere along the prop we will have an airfoil that is striking the air at a less than optimal angle producing uneven loading along the prop (the very thing it was designed to avoid), and reducing thrust throughout the entire rpm range.

Why do most propeller driven aircraft have adjustable [constant speed] props? Why are are props adjustable if only one setting makes sense?

 

I would add that Rotax does not give instructions on how to set your prop, Neuform does,

 

Rotax does not give pitch instructions because their engines can turn a variety of props.

 

and they don't say anything about rpm, because it makes no difference if the right prop is used and the prop is set correctly.

 

It makes all the difference in the world, the pitch determines the load and the power available from the engine.

[Roger Lee]

No secrets , mysteries or magic numbers. If people want to know more about Rotax, FD or any MFG they need to go to a MFG sponsored school. There are things you may never know unless you attend a school. Not everything is in writing and writings don't keep up with current developments. There are things you just can't get out of a book. There just aren't enough pages to talk about 20+ years of Rotax time or experience and 12+ years of FD time. These manuals are written for world use and not just very specific US use and or regs. so then you may not be able to separate the difference without the school knowledge. I have been to all these schools and know first hand how enlightening they are, especially when you think you already know the answers. It can be humbling.

It would be hard for someone who has never attended a Rotax school to debate well with someone who is a Rotax IRC with years of experience and several schools behind them.

 

If you could get everything out of a manual then there would be no reason to take Brian's school. There are things there that need to be taught and shown even if some things are in the manual and it's the same for any complex subject.

Quote:

"I am currently with Brian Carpenter whom I would guess you would agree knows a lot about both Rotax, and propellers. Brian has a way of demystifying this stuff, and providing the theory behind it."

No secret numbers here either just better education.

 

This sentence just proved my point. You are learning things that aren't well understood by someone who has never gone to a school.

No different with the Rotax or CT. Without the schooling people tend to debate or argue without all the facts or demystified knowledge.

I try to tell people the facts or theory, but many still debate without the full MFG knowledge base.

 

I attended Brian's class back in 2008 so I know how much I learned.

[Ed Cesnalis]

This is well written and says what I am trying to say in response to dggrant http://www.rotax-owner.com/rotax-blog/item/9-understanding-the-ground-adjustable-prop

[Jim Meade]

This is well written and says what I am trying to say in response to dggrant http://www.rotax-own...adjustable-prop

 

Who is the author and what are the author's credentials? It is posted on a site that claims to be authorized by Rotax but doesn't provide any proof and is operated by persons who do not give their names. Aviation is a life-risking business. I would not go to a doctor who refused to give his name and operated in a hospital that did not tell who was responsible for it's functions. Why does Rotax-Owner.com and RFSC not post who owns and operates them? Who/what is MFG? Aviation is rife with pseudo-authorities who do not give their credentials. We see them in every airport lounge, every aviation meeting. I'm with dggrant - it is nicest to ask a question and get an answer that is verifiable by reference to authoritative sources or is reproducible.

 

 

[Jim Meade]

Roger, I am not much on the Rotax secret society, and if Rotax is keeping something like how to set your propeller a secret they are damaging their reputation.

I am currently with Brian Carpenter whom I would guess you would agree knows a lot about both Rotax, and propellers. Brian has a way of demystifying this stuff, and providing the theory behind it.

I would add that Rotax does not give instructions on how to set your prop, Neuform does, and they don't say anything about rpm, because it makes no difference if the right prop is used and the prop is set correctly.

 

I don't care about mysteries and magic numbers, just tell me why it works. What's the theory? I am a reasonably intelligent person I am pretty certain I will comprehend it.

 

"Pay no attention to that man behind the curtain"

[chanik]

This has turned into one heck of a thread; wonder if Bobby is still following it all. I admit I tend towards Jim Meade's philosophy insofar as I hate unreferenced data and take it all, not with a grain, but more like a big bag of rocksalt. GlennM is exactly correct how engines are characterized but I would add that the test engineer will vary the load with the dynamometer just as if they had a variable pitch prop. So it may not be immediately obvious, but JUST referring to the ROTAX graphs (post 63), you can see that Rotax runs the engine at wide open throttle as low as 5500RPM (as seen from manifold pressure) and then they leave it WOT as they lower the dyno torque to get up to 5800RPM. That is a direct observation from ROTAX test data. Notwithstanding any other opinions or claims to the contrary, I feel very confident pitching a prop to load the engine as low as 5500 at sea level WOT since ROTOX tests their engine to that. That would be optimal for fuel economy, sacrificing climb somewhat.

 

One more observation purely from the curves: the sweetspot is really 5100RPM, (although not OK to set that for WOT). After 5100, where the torque curve plateaus, there is some more power but not much for the added fuel consumption. from 5100 to 5500, the power increases just 4.5% while fuel consumption goes up 24%. Fine for takeoff or emergencies but awful for cruise economy. As someone who has designed ignitions systems, it is worth noting that engine efficiency always improves as the engine runs close to the edge of detonation. Just like aircraft efficiency goes up as you push the CG to it's rearmost point, it's both optimal and dangerous to run close to the edge.

[Roger Lee]

Why post questions and have the experts answer the question then tear the expert and his advise apart especially if the questioner has far less experience. If you're never going to believe the experts then why not do the research yourself. Why not independently verify the information instead of ripping the experienced helpers.

[Doug G.]

To try to simplify - if you had a wing with a twist in it you could only get max lift at one place on that wing, the rest of it would not produce that much lift. Our propellers are wings with a relative wind, they have a twist because the relative wind varies depending how far from the hub you are. There is only one angle that the propeller is most efficient - just like the wing, the speed does not matter.

I know this is not the only factor, but it is a major one.

[Roger Lee]

Hi Ed,

I have heard through the grapevine that the Safety Officer wrote that article.

[Roger Lee]

You guys can call Steve at Sensenich propellers in Florida. He did his Phd on props. He is a wealth of info. I worked with him when I tested those props for Sensenich 2&3 blades against Warp Drive and Neuform 2&3 blade props.

[chanik]

I'm not trying to tear anyone up here, especially since I think almost everyone is trying to be helpful. But the nature of 2nd and 3rd hand information often drifts from 'just the facts'. 5800 WOT seems a decent compromise rule of thumb I think, but it doesn't speak directly to the tested engine limits and only Rotax has thoroughly done that. I don't mind when someone asks me for more backup information. For instance, I'll post about regulations but always make sure to read and link to the FARs and ACs etc. Noone should just take my word for it just because I claim to have alot of experience with reglatory language. I find posts most useful when the facts are clearly referenced and separated from the opinions of both the author and other authors. Just my opinion.

 

On a lighter note, here are some very expert opinions from the past:

"The bomb will never go off. I speak as an expert in explosives". --Admiral William Leahy, US Atomic Bomb Project

 

"The concept is interesting and well-formed, but in order to earn better than a 'C' the idea must be feasible". --A Yale University management professor in response to Fred Smith's paper proposing reliable overnight delivery service. [smith went on to found FEDEX]

 

"I think there is a world market for maybe five computers". --Thomas Watson, chairman of IBM, 1943

 

"Heavier-than-air flying machines are impossible". --Lord Kelvin, president, Royal Society, 1895

 

"Airplanes are interesting toys but of no military value". --Marechal Ferdinand Foch, Professor of Strategy, Ecole Superieure de Guerre, France

[Ed Cesnalis]

...the nature of 2nd and 3rd hand information often drifts from 'just the facts'. 5800 WOT seems a decent compromise rule of thumb I think, but it doesn't speak directly to the tested engine limits and only Rotax has thoroughly done that...

 

In this particular thread common sense is all that is needed yet references are demanded. Rotax has revealed what info they have regarding authors but some here act as though perfectly valid recommendations backed up by things like logic, experience and schooling have no value because of Rotax's lack of transparency yet admit that only Rotax has the research.

 

I can't tell you who authored the article but I can tell you that prior to seeing it I had arrived at all the same conclusions based on simple logic. The logic goes like this:

• If
  • you have a 100hp engine
    • and you want to realize that power
    • and you need to comply with red lines.

  [*]Then

  • you must present a load that will permit it

It is the same as doing the dyno test, a load had to be presented that resulted in ~100hp at redline.

 

There is some similar logic about avoiding RPM ranges known to cause wear.

 

All we had to see was the published power / torque curve and published redline(s) and we had all the info needed to know where a prop needs to be pitched.

[FredG]

It is reasonable to ask for the source of recommendations about how to operate a widely used and commercially available aircraft engine.

 

If the source is expert opinion, then fine. If the source is a Rotax manual, that's fine too. My observation is that some list users (and I'm in this category) want a distinction between these types of information. There are plenty of very technically savy users of this list. In my experience, when they question conventional wisdom, they often have a reason to do so. In my work, skepticism is valued and leads to a more complete and useful base of evidence.

 

Thanks.

[Duane Jefts]

It is reasonable to ask for the source of recommendations about how to operate a widely used and commercially available aircraft engine.

 

If the source is expert opinion, then fine. If the source is a Rotax manual, that's fine too. My observation is that some list users (and I'm in this category) want a distinction between these types of information. There are plenty of very technically savy users of this list. In my experience, when they question conventional wisdom, they often have a reason to do so. In my work, skepticism is valued and leads to a more complete and useful base of evidence.

 

Thanks.

 

 

Ditto: