Cruising Speed and Best RPM for CTSW

[Doug G.]

CT, I think I am approaching this from a homebuilder's standpoint, and since we already have a propeller chosen it probably makes no difference. We also do no have a peak horsepower, just an end to the graph. I have never seen a dyno graph that looks like this one, have you? It tells me that Rotax, in this higher compression version of the 80 hp engine limits the hp with the prop rather than letting it go to actual peak. I suspect the main reason for this is reliability.

 

In another post Roger says the yellow bands on our tachs don't mean much. He didn't explain the comment, but an engine that is capable of a higher rpm than it runs at could explain the comment.

 

I will quit there. I did not intend for this to go on so long.

Doug

[chanik]

So a little story. For the first two years of it's life my CTSW was pitched for 5200 WOT, as set from the factory. Roger Lee flagged that as a problem when I took the plane to him for annual. Now if I were to decide the issue based on perceived expertise, I would have sided with the team of engineers at FD who ought to know better than some LSRM likely semi-delirious from working in 120deg heat. (I don't know how those south AZ types manage it.) But some quick research showed that Roger was right, FD wrong. My point is that helping people navigate and interpret complex information and can be very valuable but for that to be true it needs to be possible to 'drill down' to source material. Opinions stated as hard facts, claims to vague authority, or needing vast training, and just mainly failing to provide evidence or references make such pronouncements dubious and often serve to muddy the waters. That's why I don't like articles like the one at ROAN even if their observations are basically accurate. The other point is I really like to be able to see the whole picture, including the 'corners of the box' and performance tradeoffs. The "just do X" sort of expert advice also leaves me unsatisfied even when X is a decent compromise.

[Ed Cesnalis]

CT, I think I am approaching this from a homebuilder's standpoint, and since we already have a propeller chosen it probably makes no difference. We also do no have a peak horsepower, just an end to the graph. I have never seen a dyno graph that looks like this one, have you? It tells me that Rotax, in this higher compression version of the 80 hp engine limits the hp with the prop rather than letting it go to actual peak. I suspect the main reason for this is reliability.

 

In another post Roger says the yellow bands on our tachs don't mean much. He didn't explain the comment, but an engine that is capable of a higher rpm than it runs at could explain the comment.

 

I will quit there. I did not intend for this to go on so long.

Doug

 

http://libserv5.tut.ac.za:7780/pls/eres/wpg_docload.download_file?p_filename=F971169504/Barnard.pdf

 

Doug,

 

I have seen similar torque curves there are some in the link. You are correct the peak hp would be higher if the graph didn't stop at redline. And the redline 5,800 is an artificial number with some margin built in.

 

 

 

 

 

[Roger Lee]

Hi Ed,

You and I have been friends for a while and this isn't directed at you, it's just a general observation. I always like yours and my debates. They are from solid positions with the goal to help each other. It's the resident experts getting frustrated when they try to help and the debate falls from the subject towards them personally. They won't post their frustration publicly, but I get them in PM's.

 

The real peak numbers were given on the forum, but people choose not to believe them. Instead they attacked the author. There isnt any secret info out there on Rotax. It is all in the manuals or in the Rotax school. Instead of attacking our resident experts go out and independently verify the info. All the avenues to do this are there. It may take you $10K+ for schools, years of experience, but that is what your resident experts bring to the table. How can an owner that has had just one Rotax or CT for less than a year compete with that when they attack the author and don't getherein own information. If you really want to be up on everything about Rotax you will have to take all the classes and then work on 12+ engines per year for about 3 years to see enough and gain enough knowledge and insite. When you go to a class whether its the LSRM class with Rainbow or a Rotax class you may retain about 15-20 % of all that is taught. So you need time in the field to bring that all together and enough time so see everything. To see everything may take 5 years. Depending on who teaches the school may depend on how deep and complete the info goes. The LSA. Community is missing a lot of information from manuals or documents and the only way to bridge that gap is to take a Rotax or FD school. People can complain about that, but it's the way it is and it's that way for the GA world too.

Our resident experts have spent a lot of years and thousands of dollars to get there so you may want to give them a break and believe they may have a pretty good handle on what's going on especially when more than one concurs.

They offer their advise for nothing more than the good feeling that they helped someone who ask a question. There is no other reward. On other forums good knowledgable people have been pushed away and those things should never happen.

[GlennM]

I don't post my comments to attack anyone in particular, I post them when I have doubts about the accuracy of information to show there might be another reason other than the one stated. In particular, I have a hard time with reasons being so and so says so or it is whispered in a school, but not in the manual. Maybe this results from most of my career being in testing and failure analysis. I have always asked why when something does not seem right. I agree with many that the advice given is worth taking, but something inside me always has to verify, since (as was stated earlier) your life or wallet are on the line. I have been to schools on products that my company produces and I have found errors in the reasons behind statements made during training. I can only imagine the engineers at Rotax rolling their eyes at some of the statements made on forums like this.

[WmInce]

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.

 

Roger,

 

Of those three props that you referenced above, how would you rank them? Which one is your favorite for the CTLS and why?

 

Thank you in advance for your respected opinion.

 

 

 

wmince

[chanik]

http://libserv5.tut.ac.za:7780/pls/eres/wpg_docload.download_file?p_filename=F971169504/Barnard.pdf

 

Doug,

 

I have seen similar torque curves there are some in the link. You are correct the peak hp would be higher if the graph didn't stop at redline. And the redline 5,800 is an artificial number with some margin built in.

 

That's a great article. The main thing limiting engine RPM in any IC engine is the valve train, and in particluar springs. To go faster you need stiffer springs which means more force on the rods and cams, more problems with valve bounce (which can burn exhaust valves), more cam spot heating at higher speeds with uneven oil flow which tends to boil it causing foaming, on and on. Until the late 1950s it was a hard ~5K RPM practical limit as stiff enough springs could not even be made that wouldn't quickly fail. (Ducati developed a springless Desmodromic system because of this http://en.wikipedia.org/wiki/Desmodromic). Then you have airflow problems and higher vacuum losses with your intake and higher losses from frame loss (engine friction) so it is all a losing battle even if you can design it to not fail. When you were done, you would have maybe a 7000RPM less reliable engine that, extrapolating the ROTAX dyno curves to approximate losses, would give you maybe 8% more HP resulting in 4% more cruise speed (goes as squareroot of HP) assuming you had a small enough diameter prop and pitched it to take the higher speed. So very much a losing battle

[Roger Lee]

Hi Wmince,

I'm happy to give you all the details, but may be too much to write and give you a chance to ask questions. I think I posted something about it on another thread. Give me a call and I can tell all you want to know.

I'm in Sedona, AZ today for an air show. You can call me on my cell or wait until I get home tomorrow after 12 noon. My numbers are listed below.

[FredG]

Kurt, valve bounce (and float if the springs are not sufficiently strong), are clearly consequences of high RPM engine operation. However, I thought another limitation on maximum reciprocating internal combustion engine speed was piston speed (my impression from observing the development of high performance motorcycle engines over the past 30 years and informal reading on the subject). Limitations on piston speed led to the development of over square engine designs (i.e., cylinder bore > piston stroke). At a given engine displacement, increasing the bore diameter and shortening the stroke reduces piston speed (and hence inertial forces on the crankshaft, connecting rod, and piston) at all RPMs (and allows for higher RPM without sacrificing reliability, at least regarding the crank, rod, and piston).

 

Also, two stroke engines have no valve train, as you know, and are limited only by inertial forces created by the reciprocating piston and con rod.

 

Interesting stuff. In the modern era, liter class motorcycle engines redline at over 10,000 rpm and produce over 100 hp. Of course, they are quite over square and have overhead cams with 4 valve heads, reducing the reciprocating mass of the valve train. Why don't aviation engines have 4 valve heads? More efficient use of the area available for the valves.

[chanik]

Kurt, valve bounce (and float if the springs are not sufficiently strong), are clearly consequences of high RPM engine operation. However, I thought another limitation on maximum reciprocating internal combustion engine speed was piston speed (my impression from observing the development of high performance motorcycle engines over the past 30 years and informal reading on the subject). Limitations on piston speed led to the development of over square engine designs (i.e., cylinder bore > piston stroke). At a given engine displacement, increasing the bore diameter and shortening the stroke reduces piston speed (and hence inertial forces on the crankshaft, connecting rod, and piston) at all RPMs (and allows for higher RPM without sacrificing reliability, at least regarding the crank, rod, and piston).

 

Another very valid limit but ROTAX is just a very short 2.4" stroke so the valve thing bites first. If you use mean piston speed of stroke (in feet)*2*RPM you need to stay below 1mile/minute for a racing sort of engine (meaning doesn't last long) and more like 4000fpm if you want it to last awhile. There is tons of talk about this subject on hot-rodder forums. here's one http://forums.corvetteforum.com/engine-mods/1324568-safe-maximum-piston-speed.html

Solve for RPM and ROTAX hits 4000 at 10,000 RPM. So it's just that your valve train is blown to pieces before you hit this problem.

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[Doug G.]

At the risk of another post -- lol!