Cruising Speed and Best RPM for CTSW

[Bobby CAU]

 

Dear fellow professional in CT,

 

1. what is the maximal crushing speed for CT?

2. what is the BEST rpm using during cruising?

3. if im using rpm 4600 for cruising, is that ok with the engine?

 

Looking forward of your response ...

have a great weekend !!

 

regards,

Bobby CAU

[coppercity]

Hi Bobby,

 

I cruise at 5200-5400 rpm and achieve cruise speeds of 115-122 knots true airspeed but burning 5-5.6 gallons per hour. That is based on a fixed pitch propellor, not the variable pitch available in some countries.

 

4600 is not good if your using 100ll fuel, ok if your using unleaded fuel and wish to get a better fuel economy but lower cruise speed.

[Bobby CAU]

 

Thanks Eric for the quick response.

 

is that ok cruising at 5200 to 5400 to your destination?.

may i know how long your did cruising with that rpm?.

 

im use to be cruising at 4000 and gets about 85 knots but i would try to increase the speed so that will be shorter the distance.

please advise me if you have any idea ok.

 

regards,

Bobby CAU

[Roger Lee]

Hi Bobby,

 

Hope this helps.

 

The Rotax 912 series engine was not designed to run under 5000 rpm. It was specially designed to run in the mid 5000's. Running in the 4000's will cause excessive vibration (no you can't feel it) to the engine and more heat. The jetting in the carbs is set up to run in the 5000's. Running in the 4000's with 100LL will absolutely cause more leading and faster. If you run a steady diet of 100LL run the engine around 5300-5400 in cruise to help keep the lead blown out. We are not talking about 4000 rpm to land or slow to look at something. That is intermittent. We are talking the most used or sustained throttle position that is important to the engine.

Set your WOT to be around 5500-5700 for a ground adjustable prop (closer to 5600 is better). Use a cruise rpm of 5100-5400 rpm. Your Rotax 912 can really run at 5500 rpm all day 24/7 if you wanted. That said I see no reason to run at 5500 24/7 just because it burns more fuel, you don't go that much faster than 5200 or so and there is a little more wear at the higher rpms if it is a 24/7 run. My 2006 CTSW cruises around 135 mph at 5100-5200 depending on the days conditions. The 6 CT's at my field I have set up are about the same and yes they can go faster with more throttle in cruise. (i.e.5500 rpm) Can a CT fly faster at 5700 rpm WOT compared to 5500 rpm, yes. IF you set the prop to get 5700 rpm at your average altitude then it will run better at the higher altitudes because it will be able to get more rpm up high verses too much pitch causing it to bog down some at the higher altitudes due to loss of HP and torque to turn the prop at that particular pitch setting. The rule of thumb is a 3% loss of HP for every 1K ft.

 

I have a tech article over on the Rotax-Owner.com forum under blogs about prop pitch and rpm.

 

I write the tech articles for Rotax on that site and this will help you understand the correlation between engine and prop setup.

 

 

I guess I will share a few more tidbits about the Rotax that you won't find in print.

In the manual it says to only run 5500-5800 rpm for up to 5 minutes. In truth it could run 5800 all day. It could actually hit 6200 rpm and not cause any damage. The engine probably won't float a valve until around 7000 rpm. Rotax set the 5500-5800 rpm and time limit as a liability safety margin. So now you can see that it was designed for higher rpms and running way down in the 4000's isn't great. That's almost like running a 3 speed manual shift car in 3rd. gear at 15 mph. Not quite that bad, but it makes my point.

[CT4ME]

I do most my cruising at 5200 or 5400, for hours, getting an honest 118-120kts. 5200 is a bit less noisy, and only gives up a knot or two... also maybe saving a few tenths GPH.

Tim

[Jim Meade]

 

In the manual it says to only run 5500-5800 rpm for up to 5 minutes. In truth it could run 5800 all day. It could actually hit 6200 rpm and not cause any damage. The engine probably won't float a valve until around 7000 rpm. Rotax set the 5500-5800 rpm and time limit as a liability safety margin

 

What is your reference for this?

 

 

 

[Roger Lee]

Lots of Rotax schools taught by Rotax and having a friend in Rotax doesn't hurt. You learn a lot of things in a Rotax school that is not in print for a couple of reasons. You learn even more after you have the first two classes and many years experience and then go to the Rotax Heavy Maint class. Things are discussed there that are not always taught in the other two schools or out for the public. This is why it is advisable to attend a Rotax school and not just do the minimum to just get by the regs for doing authorized maint. You should still follow the Rotax 5 min. time limit for over 5500 rpm because that is what is in print from Rotax, but it isn't something to worry about like many do. It puts a really nice safety margin in the system for the owner.I also have a network of other Rotax Heavy Maint. rated mechanics around the country that are very active with the engines which can help keep ahead of a mechanic with no school or a lot of experience. If someone had attend this last Feb. Heavy maint. school this rpm topic was one of the things we talked about. I was there for all 9 days in Feb. and helped with the classes. Re-doing classes are a great way to keep up on new procedures and information not to mention doing maint. that you rarely get to do.

[Jim]

Anyone have a number for best endurance? I've found that the 912 just sips at the fuel between 4500-4800 and gets me home at 85 knots.

[Roger Lee]

Best endurance would be an rpm that will maintain altitude without any loss and this may be down around 4000 rpm. Speeds will be down around 50 knots. I have a friend that has been up in the CT for over 9 hours on a tank.

The less rpm does use less fuel because of less work, but it causes internal vibration within the engine you can't feel. Most have heard of harmonic vibration rpms for engines to stay away from and this is one for the Rotax. Rotax school teaches to stay away from prolonged cruise rpms less than 5000 rpm because the engineers at Rotax designed it for above 5000 for continuous run.

[Doug G.]

Around 4600 is what the performance stats say gives the best fuel burn - 65% rpm - gives me 85 kts or so. 75% @ about 5000 gives something over 100 kts (and a little higher fuel consumption, of course). I usually cruise around 5000 rpm.

[Doug G.]

I should add that so far it has been all on mogas.

[Jim]

Best endurance would be an rpm that will maintain altitude without any loss and this may be down around 4000 rpm. Speeds will be down around 50 knots. I have a friend that has been up in the CT for over 9 hours on a tank.

The less rpm does use less fuel because of less work, but it causes internal vibration within the engine you can't feel. Most have heard of harmonic vibration rpms for engines to stay away from and this is one for the Rotax. Rotax school teaches to stay away from prolonged cruise rpms less than 5000 rpm because the engineers at Rotax designed it for above 5000 for continuous run.

 

Thousands of 912's putter around the pattern all day doing pattern work at 3500-4000. I'd imagine they still will see 1500 hours with no problems. That said, I really enjoy flying low and slow at 4500-4800. I always tweek the rpm for smoothest running and I doubt that it's any problem.

[FlyingMonkey]

Okay, we've discussed best endurance...what about best range? 9 hours at 55kt probably isn't it. I'm guessing something like 4800rpm around 95-100kt?

[Roger Lee]

We really aren't even talking about pattern work in the thread, but sustained cruise rpm.

Pattern work is still WOT on take offs and very little time spent at low rpm settings in the 4000 range compared to cruise rpms and it is spent at varying rpms. Being a throttle jockey in the pattern is normal and different than cruise. Time spent in the varying rpms in the pattern is small compared to the total time at cruise rpms. It may cost you a gearbox inspection a tooth on a gear ect... and sooner than you need or outlined in the manual or an internal issue. There have been a few valve head separations , gearbox rebuilds and other issues that may have been directly linked to the excessive vibration. It does happen. Why do you think Rotax teaches it this way in school. Isn't because they dislike the 4000 numbers. Intermittent use in this range is perfectly acceptable and normal, it's the people that run in long extended periods in this range for their cruise which is the longest engine logged time next to idle over the life of the engine. When things go bad from an extend issue everyone thinks there is some kind of advanced warning and that all that needs to be done is to stop doing it r to fix it for a few dollars. I can tell you from first hand experience it will come swift, unexpected and will be expensive. The last guy I know that had an engine issue cost him $8K.

[CT4ME]

Here's something from the Rotax Owners website...

tim

[Roger Lee]

Thanks Tim,

 

I hadn't seen that for a long time and forgot it even existed.

A few here wanted to see something in print so we're set.

[Jim Meade]

Who wrote that? Why doesn't Rotax put more of this kind of information out in published form? I'm very suspicious of information put out in schools or training sessions that is not backed by the company. What's the deal, do we have rogue engineers giving us the "real truth"?

[Doug G.]

Interesting Tim, especially with the FD promotion of range and fuel consumption stats.

As long as we are talking about speeds and running too slow - I have an issue with taxiing. If I keep the idle at the recommended 2000+ I end up using the brakes a lot. Even at idle I need to slow it down. I should probably drag one foot!

It sounds like I am making a choice between damaging the brakes vs. damaging the engine.

[sandpiper]

You won't hurt the brakes.

[Doug G.]

Do I need to taxi at 2000+ rpm? Is idle too slow?

[Jim]

I taxi at 2000 rpm. Any lower and I can hear the gearbox growl. New brake pads are about $70, way less than a gearbox rebuild.

[Roger Lee]

You can idle or taxi at less than 2000 rpm. If you're just sitting and or warming up it's good and smoother for the engine to be at or above 2000 rpm. If you need to taxi at 1750rpm that's okay because you won't be there too long and hopefully you'll be warmed up a ways.

[Bobby CAU]

 

dear all folks,

THANKS for the all great answer and this is become interesting topics to talk about.

 

but, im even more confusing now since in ROTAX manual book totally says something different, im attach the manual.

 

so, should i cruising at 5100 as mentioned or below as ROTAX suggested.

and i dont know why ROTAX confusing us with their statement either in class or in manual.

 

as i said, all the time i flew with 4000 to 4200 rpm and gives me 85-90 knots (this is fast too me) but then if i cruising at 5100, it will be even more faster right?!.

stupid question is, how to slow down the plane if just in case we just need to get below 80 knot?.

reason may be for sight seeing ...

 

regards,

Bobby CAU

[CT4ME]

I think we're splitting hairs... all this just means the Rotax is fully capable, and even likes, running at 5,400 all day. Common sense may tell you that running it less is "taking it easy" on the engine, but this may not be true. That doesn't mean you can't "take it easy", it's just not benefiting the engine like you think. Sure, you'll get better GPH, less noise, and a nice leisurely flight... it's just not helping the engine the way you'd think.

Tim

[Roger Lee]

Howdy Bobby,

 

How's your weather over there?

 

I agree with Tim, don't split the hairs too much.

Constant speed prop is only slightly different than our ground adjustable prop. We aren't allowed those here in the US on the CT or LSA. Why do you think going faster than 85 knots is too fast? We all fly here at 105-120 knots and we sight see. If you need to slow down just pull the throttle back. The speed dies reasonably quick especially if you pull the nose up slightly.

Remember what I said about the Rotax manuals. They are written for the world users and all its different aircraft and configurations and are guide lines for the most part. Unfortunately there are some hard rules and some guide lines and the manuals are written for all three engines, 912 & the 914 engines and it's hard sometimes if you don't know the difference to separate them out in the manual. Manuals get translated form one language to another and sometimes that doesn't come across clear, other times the manuals lag behind new teachings and that could go on for a couple of years. Then there are the Service Bulletins, Alerts and Instructions that everyone should read and abide by.

 

We sight see here all the time at 5100-5300 rpm at that particular speed and so do many other larger aircraft.

 

The bottom line is when you are just cruising stay above 5000, but by all means if you want to slow down to 4200 for a few minutes that won't hurt anything.

Don't get confused here. Varying or intermittent throttle below 5000 is perfectly okay for the short term, absolutely no issues. When you are cruising which usually implies longer extended throttle time then be over 5000. When you want to sight see pull the throttle back and slow down and and enjoy the sights. The whole concept here is to be kind to your engine over the 1500-2000 hrs.

 

Rotax like any company with an engine can not write down every circumstance a user may use their engine or even how it may be installed. So they have to draw the line in the sand some place. That means some users will fall below that line and some above that line. It's just a guide. This is why school is so important to know the difference between a guide line and a hard rule.

 

Go enjoy the sights, as population grows we are loosing them.