Desired rpm on takeoff/initial climb? - ROTAX 912ULS

[FastEddieB]

I had a nice chat with Roger Lee yesterday, covering all sorts of things (thanks, Roger!)

He asked me something I'm embarrassed to say I didn't have an exact answer for: "What is your RPM on takeoff?"

Its embarrassing because its something I glance at on every takeoff roll, but I couldn't remember exactly what it was - I think I guessed about 5,200 or 5,300 rpm.

I just looked at the two videos I posted and managed to get glimpses of the tach right before liftoff:



...and in the initial climb:



Not great images (captured from a screen shot of a video), but they clearly show about 5,000 rpm on both takeoff and initial climb.

Both are with full throttle at about 2,000' msl.

In my prior post on climbing to 13,500', you will also note that rpm hovers around 5,000 rpm in the climb.

This seems problematical - the ROTAX makes its max hp at 5,800 rpm, and max continuous at 5,500 rpm - if I'm never seeing numbers at least close to those, I'm also never seeing the power my engine is capable of.

My Sky Arrow has a 3-bladed Warp Drive prop, and its set as it was when I bought the plane. The POH says takeoff rpm should be between 5,100 and 5,400 rpm, so I'm not WAY out of that range. The POH also has the factory prop pitch listed, and the plane came with a protractor device for measuring prop blade angle. I plan on checking my prop against those in the next few days.

It seems to me (and Roger) that my blades need a bit less twist. He's a believer that faster-turning is better (within limits, of course).

I'm curious as to what others are seeing on their ROTAX's on takeoff and climb.

That said, when adjusting the pitch, how much change should I go for initially - 1º? 2º? or some other number. I wouldn't mind seeing about 5,400 rpm on the takeoff roll.

Thanks in advance!

PS - I realize the rpm loss could be an engine issue rather than a prop pitch issue, but the engine seems to be developing good power. I also realize the tach itself could be off, so I'll check that as well if I can find a device to do so locally.

[rfane]

Eddie,

In a climb attitude, the RPM is going to drop. Reason being is the prop is more heavily loaded in a climb attitude. As you climb higher, rpm will also decrease as engine performance decreases with the thinner air.

If you get 5,400 rpm on the takeoff roll, be very careful that you don't blow through redline when you level off.

I have my prop pitched so that I can see 5,500 rpm WOT at 6,500 - 7,500 feet, in level attitude. There's lot's of high hills and mountains out this way, so I am usually up in the 5,500 - 7,500 area for cruise, and 9,500 + if I am going on a trip. This still gives me 5,300 rpm WOT at 11,500.

I see 5,000 - 5,050 rpm on takeoff roll, and it will stay there during my climb out, until I bring up the flaps, decreasing pitch. It then increases to 5,100 - 5,150 while I am cruise climbing. If I level off at anything under 6,000 feet, I will exceed 5,500 rpm, and will need to throttle back.

I personally haven't seen a Rotax exceed 5,100 rpm on the takeoff roll.

[CharlieTango]

there are a few considerations, 1st what is your objective?

cruising at 5,500? ( this used to be what roan recommended. saying the 912 was designed to run at this rpm its entire life)

cruising at highest available speed? here you need 5,500 and wot

realizing best climb? need to see 5,800 rpm (any flatter would be pointless but elevation does change the max)

5,200+ for take off? (this is better then "lugging the motor")

complying with the manufacturer's requirement? ( this may be too coarse for best performance and engine health)

1 setting that works year round? (need some comprimise here)

best fuel economy? (might be false economy)

in general lsa climb well and optimizing for climb makes less sense then optimizing for cruise. elevation and seasonal conditions play a big part.

my prefered pitch allows me at least 5,500 rpm @ 10,000' da any time of year. if i lived in florida i might like it a bit coarser.

1 degree of pitch gives you ~200rpm change (i think)

[CharlieTango]

...I personally haven't seen a Rotax exceed 5,100 rpm on the takeoff roll.

Au contrair, mon ami (roger) you have seen it. of course you weren't in my cockpit you were behind me in your ct trying to keep up

[roger lee]

Hi Eddie,

Nice talking to you the other day. Don't worry so much on the take off rpm to make this adjustment since we have a ground adjustable prop we care more about the WOT setting and that will take care of the take off rpm on its own. This is a balancing act to find the best prop pitch for each person's aircraft since we all have different plane profiles and home or cruise altitudes. If you are turning 5200 rpm WOT flat a level then try taking out 1.75 degrees. 2 degrees will get you between 5500-5600 and that's where you really would like to be. If you are closer to 5600 WOT then you will have better performance up high and on take off. We as users don't usually ever run at WOT for our normal cruise and we don't usually run over 5500 for 5 minutes either. If you go to let's say 5700 on WOT in cruise then even in a climb at full throttle it would never be over 5500 in the take off climb. We would have a good climb prop, but loose a little in our WOT cruise because we would be a little flat. I get 5600 rpm WOT on my plane in cruise at 4K'MSL and it is probably close to that even at 6K-7K. This gives me about 5050rpm on take off with zero flaps and right at 4950-5000 rpm with a 15 degree flap take off. I have very good top end cruise, climb and fuel economy. I cruise at 5000-5200 depending on altitude, weather and time. 5100 is my normal. You can not dial this in to be exactly on any number or it will drive you crazy. close is good. Your rpm will change slightly with altitude and seasonal weather.

[FastEddieB]

Thanks for all the feedback.

As background, the majority of my logged hours are with fixed-pitch props.

But, most of my recent experience (pre-Sky Arrow) has been with a constant speed prop.

And I think that biased me a little into over-reacting. Here's why:

My previous plane was rated at 315hp @ 2,700 rpm. Of course, with a constant speed prop, its easy to get that 2,700 rpm and the associated 315 hp for takeoff on a standard day at sea level. In fact, owners complain if their governors are set wrong and they're only seeing 2,650 rpm on takeoff. Why? Because they're not ever developing full power.

I understand the compromises of a fixed pitch prop (or, in this case, ground-adjustable). The way it seems our props are generally set, we have NEVER seen 100 hp from our engines - look at the rpm/power charts and its obvious at 5,000 rpm (on takeoff) there's significantly less than 100 hp available.

Anyway, I was kinda primed to see 5,000 rpm on takeoff as abnormally slow. I see from the responses here its not that unusual. But, remember, my POH does give 5,100 to 5,400 as the numbers to look for on takeoff roll - numbers I'm not quite seeing, at least at 2,000'.

And Roger's first question to me was, in fact, what my WOT engine speed was in cruise. Looking at my Check Flight log after my last annual, it looks like WOT at 5,500' resulted in about 5,300-5,400 rpm. On my recent trip to 13,500', WOT resulted in about 5,200 rpm.

At this point, I still think I'll try dialing out a tiny bit of pitch. It sure seems that it would be nice to be able to pull max continuous power (5,500 rpm) in level flight, even if one chose not to ever actually fly at that power setting. I'm thinking try 1º less propeller pitch and see if that really does result in a 200 rpm gain.

Thoughts?

PS -In reality most of my flights are relatively short - I've only been on a handful of cross countries (twice to Sebring, once to Myrtle Beach, that sort of thing). This spring I hope to do a few longer ones, to Sun 'n' Fun, and maybe visiting friends in the Midwest and Pennsylvania. And I'd love to head to points really west at some point!

[FastEddieB]

I went out to the hangar yesterday.

1) I see now how the "protractor thingie" works.



2) The current pitch was exactly what the Sky Arrow POH calls for (23º @ 11" from the tip, IIRC)

3) "Dialing out" 1º of pitch should be easy, but...

4) I loosened the 4 "clamping bolts" on one blade, but could not rotate the blade. Looking at the situation, I went ahead and loosened the bolts on the other blades as well - still no joy.

I was attempting it by hand - I didn't want to get carried away.

Questions:

a) Do the hub mounting bolts also have to be loosened? It seems like they would hold some pressure on the blades, but none of the instructions mention loosening them.



b) I was thinking about making a tool out of wood (with appropriate padding) for twisting the blades - should that be necessary?

Thanks in advance...

[comperini]

a) Do the hub mounting bolts also have to be loosened? It seems like they would hold some pressure on the blades, but none of the instructions mention loosening them.

Yes. all bolts must be loosened.

b) I was thinking about making a tool out of wood (with appropriate padding) for twisting the blades - should that be necessary?

Kind of a "lever" thingy? Yes, that works great. Once loosened you can wiggle (twist) the blades, but its easy to over correct, or under correct. A lever is not needed to rotate the blades, but it does allow you a little more control over the process.

[FastEddieB]

Yes. all bolts must be loosened.

Thanks - sorry for the delayed reply.

Loosening the hub bolts as well would not be such a huge deal, but I didn't feel like doing all the re-safety-wiring involved at the time. Most of the instructions I've seen only mention the clamping bolts, but maybe they assume the prop is not yet mounted.

Now that I know the prop is properly set to Sky Arrow specs, I think I'll leave it alone for now. Probably futz with 1º less pitch at some point n the future if I feel motivated.

Thanks again!