Cruise rpm

[RIVERFOX]

What do most of you with 80hp 912 use for a standard max cruise RPM?

[cap01]

with the pitch i have set in the prop , im a little shy at takeoff , 4900 rpm . at cruise i usually 5200-5300 burning 4.3-4.5 gph . i dont have the taper tips but will probably take the prop off this winter and get it shaved . so ill see what difference that will make ?

[DesertFox4]

5200 - 5500. 5500 when extra cooling was needed on a hot desert day.

[HighWing]

Standard max cruise on the 912 UL is 5500 rpm. I know a lot of guys that shy away from 5500 and some who will cruise higher. I flew at least above 5000 as I have heard that the gear box takes more load at lower rpms. 5500 max cruise is spelled out in the Operators Manual, but I would often be flying with friends and we would throttle back to the slowest airplane. When solo and wanting to get somewhere, 5500 was the norm.

Lowell

[cap01]

yep lowell , ive read that your not doing the engine any favors by running her slow . shes made to run all the time at 5500 .

[Av8r3400]

At my current pitch (tapered Warp 3-blade, 912UL 80 hp) I cruse at:

~5400 for 105 mph @ 3.25 GPH (Verified on my trip to S-n-F this spring)
~5200 for 100 mph
~4800 for 90-95 mph
~4500 for 80-85 mph

[cap01]

larry , curious as to what you get for a static rpm ? my rpms and speeds arent too far off from you but im burning a bunch more gas .

[kitfox2009]

I have just recently installed a 70 inch IVO IFA with Patriot blades on my Model 5 Vixen 80 HP 912UL. At 5500 RPM it is indicating 118 MPH!!! I only have 11 hours on it so I am not sure if this is accurate or not. Anyway it is a huge improvement to my old prop.
I have not installed the limiting washers so I have to be careful on takeoff not to exceed 5800.
Cheers

Don

[Av8r3400]

larry , curious as to what you get for a static rpm ? my rpms and speeds arent too far off from you but im burning a bunch more gas .

On take off I can get about 5200 rpm. I can redline the tach (just barely) in straight and level flight (110 mph plus).

(My crappy Matco brakes won't allow a full power run up! :mad:)

[RIVERFOX]

Larry, my rpm and speeds are just about the same but my take off rpm is only 4900. Do you have separate filters at each carb or do you have the older carb heat with the long tubes running from thealum. box up front?

[Av8r3400]

I removed the carb heat setup in favor of the dual filters. My filters (K&N) are about 6" in diameter by 2" thick.

[sdemeyer]

I'm close to Ava8tor3400.

5200 = 105 MPH (ias) 3.5 GPH @1500ft
5000 = 102 MPH (ias) 3.2 GPH @1500ft

I generally cruise around 5000 rpm. I have a Warp standard three blade and 80HP 912ul.


Scott

[Newkid]

I think you can cruise at whatever RPM your heart desires just so long as it's above 5000 RPM. We used to have a little IV Speedster with a 912 UL and a Powerfin 3 blade, and we used to cruise at 5700 to 5800 RPM all day long. Full bore strait and level was about 6100 RPM. We have a friend that did the same thing in his Rans S-7 except he had a UL with high compression pistons. His motor now has 1600 hrs. on it and is still running strong. Both planes burned right around 4 gph.

[RIVERFOX]

Hey guys thanks for all the input. This time of year I do a lot of air to air photography of "birds of prey" along the St. Croix River and I spend much time at 4000 rpm and 40 IAS. One service center said stay above 4900 rpm and the other said 4000-4200 rpm was fine and no problem at all. Don't know who to believe. All my temps for coolant, EGT and oil are fine. I see no reason why those rpm's would be harmful. Can anyone give me a good reason why those slower rpm's would be detrimental?

[DesertFox4]

riverfox, years ago I was told to stay out of the 4300 to 4800 rpm range for continuous operation in the 912 series engines. You are below that rpm so from what I've understood you should be ok. You should be getting some incredibly low gals. per hour readings at 4000 rpms.

[HighWing]

My take on the low RPMs has very little to do with the engine and all to do with the torsional vibration strain on the gear box. But who knows?

[DesertFox4]

Lowell, that's my understanding too that the restrictions are for the gearbox at those rpms.

[RIVERFOX]

4200 rpm got me 1.87gph on my last check.
4600 rpm got me 2.31gph. In fact I used that power setting for take off and did one hour, landed and checked as soon as I landed. Sure beat my Citabra fuel consumption.

[Av8r3400]

Per the seminars by both LEAF and Lockwood:

5200 rpm minimum at full throttle for take off. Less than that leads to excessive manifold pressure. Caused by too course pitch in the prop.

Engine redline is 5800 rpm for 5 minutes maximum.

Maximum continuous RPM is 5500 rpm.

I've never heard of problems with low RPM settings.



This is how I operate my engine. If you want to do otherwise, that's up to you.

[Papa Beach]

The Rotax Operator's Manual for 912 series engines (Page 10.8, section 10.1.2.1) recommends running the engines at the following cruise power settings:
max cruise of 5500, 75% at 5000, 65% at 4800 and 55% at 4300. I'm not saying it's the bible but it's the only source I've got.

[JetPilot]

There seems to be a wives tale going around about not cruising below 5000 RPM which I beleive to be FALSE :mad: I cruise my Rotax 912-S at 4200 RPM all the time, it reduces noise and fuel usage is much lower, and the engine seems very happy with almost 300 hours of this. I have read the Rotax manuals looking for limitations, and there are limitations on cruise RPM. There are RPM limitations listed, such as not idling below 1800 RPM, and 5500 maximum continuous RPM, so obviously thy did not just forget this issue, but there is NOTHING saying not to cruise below 5000 RPM. So it seems some guys have just " Invented ", or heard from someone else thier own limitation and pass it on as fact. Ever seen the AirCam twin engine Plane ??? That plane has two 912-S on them and they cruise them at very low RPM using about 3 1/2 gallons per hour per side as normal cruise procedure, Lockwood sells that plane...

My guess is that someone very correctly said not to have a high throttle setting and then pitch the prop to below 5000 RPM cruise, that would be lugging the engine, which would be very bad. Some simple minds probably just understood " Dont cruise below 5000 RPM " and spread it around, which is why there I think there is so much misunderstanding about this among the Rotax community. You should NEVER prop your engine to the point where it is lugging at low RPM and high throttle, This is very bad... If you have the prop pitch set correctly, and you pull the power back to a fuel saving, less stressful less than 5000 RPM cruise, I do not see a problem.

Rotax centers do not always get it right either, you may get a different stroy from every guy you talk to, guys that work at Rotax Centers could very well be subject to the same type of misconceptions as everyone else. Until I hear it DIRECTLY from the Rotax engineers, and they have had 20 years to put this limitation into the manual if it were important, I will continue to cruise below 5000 RPM and a low throttle setting.

The above is just what I have found out based on hearing the same garbage and researching it as much as possible over the years. So the above is just my OPINION. Just as guys that try to add thier own limitations to the Rotax manual are merely stating opinion, which I beleive to be with good intentions, but misguided.

Mike

[HighWing]

I love these discussions especially the part about any differing opinions being garbage. Well, this "simple mind" got his caution about cruising below 5000 rpm from Eric Tucker who used to be a regular at the Kitfox factory fly-ins. This was a long time ago for the guys only having 300 hours on their engines and in a time when we got to rub shoulders from time to time with the experts. Maybe things have changed, but for me, until I see or hear something specifically negating what we were told then, I plan on sticking with that advice. To each his own,though, it's only money.

Lowell

[JetPilot]

Well, this "simple mind" got his caution about cruising below 5000 rpm from Eric Tucker who used to be a regular at the Kitfox factory fly-ins.

Lowell

The fact that Rotax puts NO SUCH LIMITATION in the manual is pretty good evidence.

Rotax has a warning about low RPM idling.
A limitation on maximum RPM.
A limitation on Continuous RPM.

The LACK of any limitation by Rotax on cruising below 5000 RPM tells me a lot, there is no limitation. This is just an example of the kind of way down the chain type information that spreads misconceptions. Lowell, did you ever do the excercise where you get a bunch of people in a circle, and start out with one person telling the next a simple fact, and on down the line. By the time it gets to the last person there is none of the truth left, what you get is total garbage. So I will take it straight from Rotax, not made up limitation some guy that said something at a fly in, and got passed down the line etc. etc....

So we are supposed to assume, that Eric Tucker got his information direct from a good source like the Rotax Factory ( I doubt it ) Or that that he did not misunderstand it. And that you totally understood what this guy was saying, and did not misunderstand it, etc. etc. Throwing names around does not produce any evidence on this subject at all. My guess is that somewhere along the line, some simple mind misunderstood the bad idea of running high throttle settings while lugging the engine down to less than 5000 RPM in cruise with an overpitched prop.

So we are all supposed to beleive 5 th hand information at best, and ignore the very obvious lack of any such limitation in the Rotax manual, even when Rotax went to the trouble to publish sevearl types of RPM limitations ( Rotax obviously did not just forget to put RPM limitations in the maunal ).

You have zero facts, and zero logic, you just quote some 5th hand information to support your post. This is the type of information I would NEVER listen do. I operate my Rotax 912 on facts, and good information according to the Rotax manual and their service bullitens. I do not operate my engine based on 5th hand wivestails as Lowell would have us do.

Mike

[JetPilot]

The AirCam kit is sold by Lockwood Aviation, a Rotax service center. It is standard practice to cruise the Rotax 912-S on these planes at around 4000 RPM. Lockwood, Rotax, nor anyone else have given any warnings or limitations about cruising with the Rotax 912-S below 5000 RPM on these planes... Why ??? Because there is no limitation against it.

Just another nail in the coffin of the made up 5000 RPM Cruise limitation of the 912-S engine...

http://www.clamarfloats.com/downloads/0709_feature.pdf

http://www.kitplanes.com/issues/pdfs/0806-2230.pdf

Above are a couple of examples of the 4000 ish cruise RPM's used in these planes, there are many more available if you do your research...

Lowell, I will give you a good peice of advice, do not beleive everything you hear at arishows ;)


Mike

[84KF]

"I cruise my Rotax 912-S at 4200 RPM all the time, it reduces noise and fuel usage is much lower, and the engine seems very happy ...."


Same here...912UL w/ Warp Dr. 4200-4800, and she just purrs right along happy and smooth. Sure, I may not be tooling along at 100mph+, but I get where I want to go eventually, and I don't feel like I beat the heck out of the engine gettin' there.

[HighWing]

"So we are supposed to assume, that Eric Tucker got his information direct from a good source like the Rotax Factory ( I doubt it ) Or that that he did not misunderstand it."

Do I have to be the one who tells our resident expert who Eric Tucker is?

[JetPilot]

"So we are supposed to assume, that Eric Tucker got his information direct from a good source like the Rotax Factory ( I doubt it ) Or that that he did not misunderstand it."

Do I have to be the one who tells our resident expert who Eric Tucker is?

I know who Eric Tucker is, and he does NOT work for the Rotax Factory. Like I say, you can get as many different opinions from Rotax Centers as you like. The ONLY thing that counts is what Rotax says. Rotax is very specific and publishes the care and limitations of thier engines, so I just do not buy it when you want to " Invent " a new limitation.

I have not seen Eric Tucker post here, or write anything on this issue. Even if he did, it would just be another opinion until something came from Rotax. He is Human and capable of error, or even simple miscommunication to the point that Lowell misunderstoood something :eek:

I genreally discount people that claim to speak for God. I also have a hard time lisening to a guy that claims to speak for Eric Tucker on this issue.

Mike

[JetPilot]

Since this is becomming a discussion, I did my research on this. First, after a pretty good search, there are ZERO publications limiting cruise of 912 engines below 5000 RPM. Second, there are several covering many RPM's, INCLUDING crusing below 5000 RPM. Here it is straight from the Rotax operators manual:

http://www.rotax-aircraft-engines.com/pdf/dokus/d04670.pdf

Page 5-3 Revision April 01 2010

" RUN THE ENGINE ACCORDANCE WITH THE FOLLOWING TABLE "

They list RPM's of

5800 RPM
5500 RPM
5000 RPM
4800 RPM
4300 RPM

The graphs on the previous pages cover the full RPM range from 2500 to 5800 RPM.

There is a service letter Rotax SL-912-016 that warns against " HIGH ENGINE LOAD WITH LOW RPM " and a couple other things.

http://www.rotax-aircraft-engines.com/pdf/dokus/d04670.pdf

http://www.rotax-aircraft-engines.com/pdf/dokus/d04645.pdf

Everything in these recent Rotax publications agrees with what I said in my first post.

Mike

[av8rps]

Below are a couple pictures of my flight last weekend to Osh SPB (80 miles from my house). One shows my panel which shows my cruise power setting and my IAS and GPS groundspeed.

It was a near calm wind day and was just beautiful flying. My Kitfox is a Model IV-1200 on Aerocet amphib floats. It weighs 776 lbs empty, and that day I had 20 gallons of fuel, 15 lbs of baggage, and my 185 lbs. It powered by a 81 hp 912ul and I use an IVO in flight adjustable 3 blade 68" ultralight prop.

The IVO IFA is used primarily to give me improved out of the water takeoff performance by allowing a full 5800 rpm on takeoff. Fwiw, during straight and level flight if I adjust the prop for maximum pitch, I get 5400-5500 rpm with wide open throttle.

If you look at the panel picture you will see I have my engine set to a cruise setting of 4800 rpm, with a couple of inches of throttle left. At these settings I am seeing 100 mph IAS while showing 103 mph groundspeed (in very calm wind at 2000 ft msl).

If I am willing to run the throttle right against the panel and adjust my prop to max pitch (5400-5500 rpm), I will see my top speed of 118-124 mph tas depending on load and temps. And I've verified these numbers many times - they're real. And remember - this is on floats. l'd anticipate a 5-7 mph increase in speed if I replaced the floats with standard wheel gear. Besides the 175 lbs of weight I would lose, I'd see a serious amount of drag reduction (both from the floats and the AOA).

Now some may think the only way to get good speeds like mine is if you have an IFA prop. But after playing with my prop for hundreds of hours now, I found that if I adjusted my prop to simulate a fixed pitch, setting it to give me 5400 rpm with wide open throttle straight and level, that I would still have an acceptable climb rate and takeoff. Granted, my little amphib gets off the water much better if I can get max rpm on takeoff, but even if I don't change the pitch for takeoff it is quite acceptable. Even with fixed pitch setting it provides better performance than the average amphib. So if I had to fly a Kitfox as an LSA, I wouldn't be heartbroken to not have the IFA part of the IVO. My kitfox would still fly just great.

On the subject of low rpm...I fly my Kitfox regularly in the low 4000 rpm range when I am just loitering around the neighborhood. From memory I recall my cruise speed to be around 80 mph at 4200 rpm. There again, I can tweak my prop for best operation (so as not to lug the engine or to run it too lightly loaded - fwiw I do have a manifold pressure gauge).

I've even flown my Kitfox (on floats) as low as 3400 rpm, indicating 63 mph with partial flaps. Fuel burn is right around 1 gallon per hour at those settings, and as long as the airplane is light and it's not overly warm outside, it will fly all day long at that speed and rpm setting and will have no trouble maintaining altitude. If I ever find myself nearly out of fuel those are the settings I will use to get me to my fuel stop. 63 mpg isn't bad for an airplane, much less an amphibious floatplane.

I do believe the IFA is very advantageous if you like to operate regularly at low rpms like I do, as you can make sure you are not lugging the engine at those low power settings. But overall I feel the 912ul can handle lower rpms, you just have to get to know what speeds your particular engine runs best at. And that's more of a feeling than it is a specific rpm or speed imho.

Paul S
Central Wisconsin

[DesertFox4]

Good stuff Paul. Love your little amphib. Sorry I didn't get over to see you when I was at Osh. My only regret. Next time for sure.

[av8rps]

Thanks for the compliment. Yeah, I love my little amphib too. The more I fly it the more impressed I am with it.

Hopefully we can meet up next year at OSH.

[kitfox2009]

Great information Paul

Could you tell me what manifold pressures you find are acceptable both at high and low engine speeds. I have the same engine/prop combination on a Vixen but have not installed a manifold pressure gauge yet. Hopefully it will be in tomorrow`s mail.
I do 118/120 mph at mid 5000`s on wheels and 1400 gross.

Thanks for the info.

Don

[av8rps]

In reply to Dons question about manifold pressure guages

I'm sorry to have to tell you, but for the most part the vacuum manifold pressure gauge is useless. Mine will show 25" when the engine is loaded too light (prop pitch too flat) and only 27" when lugging overpitched at high throtttle settings. The range is so narrow that it is hard to tell a lot about how things are set.

So I think to use one effectively a digital one with more accurate range readings would work better. But maybe there's something wrong with mine? Anyhow, it is still nice to have an idea of how you are generally set, but it sure doesn't work as effectively as the manifold pressure gauge on my lake amphib - that one works very well to help maximize the performance of the lycoming 0-360 and the hartzell constant speed prop. It will show manifold pressure readings from 15" to 30", so the readings make it quit obvious how things are set.

All that said, in my opinion the best way to fly these IVO IFA's is to just pay attention to what your engine is telling you. You should be able to feel when the engine is lugging too much, and you can also tell when the prop is too flat. In reading some of the other posters rpm numbers and fuel burn, and knowing how my engine reacts to a variety of rpm and prop pitch settings, it is apparent there are a lot of guys flying around with props that are not pitched for maximum efficiency.

More on the discussion about using lower rpms - I simply follow Rotax's information that says 5000 rpm is a 75% power setting, 4800 represents 65%, 5500 represents 100% cruise power, and 5800 is your max rpm for limited periods such as takeoff and max climbs. And that you should NEVER run a 912 more than 5800. (if you do Rotax highly recommends tearing it down to inspect for damage). All that said, I can't imagine Rotax supporting policies that would indicate you can only run their 912 at 75% or higher power settings. If that is true, it would amke their venerable 912 engine sale-proof when compared to other aircraft engine choices. Who would buy an engine with such limited operational requirements? Sure, a lot of aircraft have specific operational limitations, but to have an engine that shouldn't be operated at less than 75% power just doesn't sound right, or make sense to me. So however all that discussion got going, I'm sure somewhere along the line something was misunderstood or overstated. If this really bothers us (it doesn't bother me whatsoever fwiw), then should seek the advice of a qualified expert from Rotax. But since it doesn't bother me at all, I'm just going to keep all I've learned in this thread in the back of my mind should I ever need it. While I think it is unlikely that I will ever need it, I'm old enough to admit that I'm still always learning, and therefore sometimes wrong.:)

Last subject - C'mon, we all just need to get along;:(
Now I don't mean to stir the pot here, but we should all be able to speak on any subject while still being perceived as at least being civil to eachother. Face it, us aviators are a pretty small crowd, and getting smaller by the day. So we need to stick together and not let little issues divide us. So lets keep the discussions going while keeping them productive and civil. And when we disagree with each other, remember there is almost always something to be learned from other peoples opinions and experiences.

Personally, I like people that challenge my thinking. And I hope they can appreciate when I challenge theirs. That's how we learn from eachother. And it's also half the fun of this forum. But let's not ruin the fun for our moderator by making him have to scold any of us - he's here to have fun too. I really appreciate that our moderators are willing to do all the work associated with this forum, so we don't have to. I'm sure the job is thankless enough, let's not make it any harder for them.

So let's all just agree that regardless of our opinions, we will always remain buddies, and even though we may disagreee we will always treat one another with mutual respect...

(There's my two cents worth)

Paul S
Central Wisconsin

[av8rps]

Don,

One last thought on the IVO IFA prop settings;

If it were me rather than to focus on the manifold pressure gauge, I would rather install one of the navman fuel meters. That way you could watch your fuel burn rate, and that probably is one of the best ways to know how efficient your engine is operating at any given prop setting. I have one purchased for my kitfox but probably won't get around to installing it until next year. But I look forward to using it as I think it will be the cat's meow...

Paul S
Central Wisconsin

[kitfox2009]

G`day Paul

Thanks for the info and heads-up on the MP gauge. I plan on playing around with it and will keep the forum posted on results. Also appreciate your suggestion on the fuel flow meter. What type are you planning on installing?
On another note, as a relatively new member of the KF community, I find this forum to be a wealth of information, especially on topics such as this whereby folks have the opportunity to disclose their ACTUAL operating performances and results. This is excellent material for us all!
Let`s just have fun with it as well as LEARN from each other.

Don

[jrthomas]

Maybe I'm a little dense but I didn't find any problems with JetPilots remarks. Tone of ones voice is impossible to get across in these posts and I think sometimes we read into a remark a tone that was not intended. I've read my Rotax manual from cover to cover, over and over and I believe Jet Pilot gave good, solid facts that the Rotax manual backs up. Jet Pilot may have been a little blunt and straight forward but I don't believe any offence was intended. I almost got a little rattled when I read the post about the 5800 rpm cruise and the 6100 rpm full throttle. I fully expected to read some (are you crazy?) responces. I thought it myself but selfcontrol prevailed. Anyway, Jet Pilot, keep the posts coming. I, for one, understand where you're coming from and if quoting directly from the book offends someone, oh well!