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Need advice - in flight adjustable prop

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7 months 2 weeks ago #17919 by John_in_Aus
Need advice - in flight adjustable prop was created by John_in_Aus
Hello All,

I am new to the forums and not long ago I purchased a 2006 Sportstar that has the 100HP 912 motor. It also came factory fitted with an inflight adjustable prop, and I am looking for some advice as to how to get the best out of it. So far, those I have spoken to have given the following suggestion.

In the cruise, increase the pitch and throttle to get RPM around 5200 and MP at 27-28. Does that sound about right?

I have some questions...

1) MP - when is too much?

2) I have the SN of my engine (5647190) How do I know if it has the new crank case that was apparently changed in 2006?

Any advice you can offer is most valuable.

Cheers
John

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7 months 2 weeks ago #17922 by Roger Lee
Replied by Roger Lee on topic Need advice - in flight adjustable prop
Do you have an in flight adjustable prop or ground adjustable? What is you wide open throttle (WOT) in cruise?
A new crankcase replacement would have been very expensive and it's doubtful someone did this for just a few more hours toward TBO.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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7 months 2 weeks ago #17924 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Hi Roger,

Bu the crank case I mean - I have an engine number beyond that of when the crank cases needed replacing. My SN is 5647190

My prop is In Flight Adjustable. WOT would push the RPM to 5500+ - on full fine of course.

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7 months 2 weeks ago - 7 months 2 weeks ago #17928 by Roger Lee
Replied by Roger Lee on topic Need advice - in flight adjustable prop
When you say beyond the serial numbers I take that as newer. Your 5647190 engine is newer than the ones affected in the SB.

Some of what I'm about to say is personal preference and others may chime in with a different opinion and all may be correct. There is a range of rpms in cruise you can fly and you'll find the best for each flight with some time in the air. (Ground adjustable props are set differently since they can't be altered in flight)
Some may set the prop to run at 5500 rpm WOT and just fly there all the time. Some will set the prop to get 5500+ and then throttle back to between 5100-5400. You don't want it set to get less than 5500 and fly there at full throttle all the time. There are zero redeeming qualities about over pitching below 5500 rpm. You lose fuel economy, speed, higher engine temps and climb all suffer.
i.e. Don't set it to only get 5100 at WOT and just sit there for all your cruising. Too much stress on the engine. You may find running a high rpm like 5200-5300 in cruise with the prop being able to achieve 5500 WOT gives you the best mileage vs fuel economy. I tend to like 5600-5650 WOT (I'm only ground adjustable) then cruise down around 5100-5300 depending on what my mission is for that flight. If I'm just flying around the pumpkin patch sightseeing or am I going somewhere that has some distance.

So a lot of what you do will be just personal preference, the type of aircraft you have and the mission for that flight.
Go out a do some flights and see what works best for you.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last edit: 7 months 2 weeks ago by Roger Lee.
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7 months 2 weeks ago #17934 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Thanks Roger,

Great info. I will do some pottering about and let you know the result.

Question... MP - What is too high? I can't seem to find anything in the manual about max MP etc, and I don't want to over stress this engine.

Cheers
John

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7 months 2 weeks ago #17935 by Bill Hertzel
Replied by Bill Hertzel on topic Need advice - in flight adjustable prop
The 912ULS Operators Manual , Page 5-6 Shows...

Performance Data for Variable Pitch Propeller.
Max.Power ... 27.5
5500 rpm ... 27"
5000 rpm ... 26"
4800 rpm ... 26" (25"?)
4300 rpm ... 24"

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7 months 2 weeks ago #17936 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Thanks Bill,

I have that info - But I am looking for max! I am sure I can get those MPs above that if I over pitch etc - Hence I am looking for how high is too high - if that makes sense.

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7 months 2 weeks ago #17937 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Also Roger - If I can ask....

I believe my engine has a TBO of 1500 hours correct?

But I am told that its that or 15 years - Can you please advise? My bird is 11 years old, so only 4 years away from the deadline, and I just would like to know my options.

Cheers
John

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7 months 2 weeks ago - 7 months 2 weeks ago #17938 by Bill Hertzel
Replied by Bill Hertzel on topic Need advice - in flight adjustable prop
I would take those numbers to be Max!

Ex. At 4300rpm do you really want more than 24"?
That would be over-stressing the engine.

At 5000rpm any MP over 26" is too high! That is only 1.5" from WOT Max!
This implies that at 5000 Feet where the ambient pressure is about 25", you can go Wide Open Throttle and increase the Prop Pitch until the engine slows down to not less than 5000 rpm and still be good.
Last edit: 7 months 2 weeks ago by Bill Hertzel.

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7 months 2 weeks ago #17939 by Bill Hertzel
Replied by Bill Hertzel on topic Need advice - in flight adjustable prop
If you Aircraft is registered Light Sport, The Manufacturer makes the TBO rules.
If it is Experimental, you can fly it until it falls off!

For Non-Certified Engines, Rotax TBO times are Recommendations.
At least that is how I understand it.

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7 months 2 weeks ago #17940 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Thanks Bill,

Yep. My bird is manufactured by Evektor Registered RAA (Recreational Aviation Australia) - but I would have thought the TBO for the engine would be set by ROTAX??

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7 months 2 weeks ago #17941 by Roger Lee
Replied by Roger Lee on topic Need advice - in flight adjustable prop
Hi John,
You can go "On Condition" inspections past TBO here in the US. Other countries may have different rules and I don't know what they are.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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7 months 2 weeks ago #17943 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Thanks Roger,

I am very grateful for your input.

So Max MP - Is Bill correct above? Should I not go above those marked in the manual?

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7 months 2 weeks ago #17944 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Actually Roger, I have just been away educating myself on what MP actually is, as I was mistaken, and a pity that it's not really covered in our curriculum for a Rec pilot.

Now that I understand what it is, and what it's actually happening inside the engine, I am now not in fear of going too high on MP, knowing that it actually can't. :)

Thanks heaps Roger, and I will report back with some numbers soon.

Cheers
John

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7 months 1 week ago #17963 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
UPDATE.

SO... Took the bird up today and did some playing with my adjustable prop. Had some great results. I got her settled in the cruise and coursed the prop up as far as possible while keeping the RPM at around 5300. I actually had to back off, as it was a little lumpy and she was pushing 115kt. I still had more prop and more throttle to go. not WOT, so she could really boogie!!

Got it settled and was cruising along with a Ground Speed of 118kts. MP was about 25 from memory (will check the video) and that was about 3000ft up.

Pretty pleased with the result. She really pumped along nicely. :)

Will have a bit more of play next time I am up - hopefully some smoother air.

Cheers
John

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7 months 1 week ago #17983 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
@Roger Lee,

Do you have any good resources on understanding how the MP work in these beasts. I am trying to get a really good understanding of how it wall works and the "what not to do's" if you know what I mean.

I have in my mind basically not not let the RPM get too low while the prop is course. So anything below 5200 with the prop coursed up, is bad right? So am I right in thinking, if I keep the RPM above 5200 and below 5500, and keep adjust the prop until it's settled nicely - is about right? It's the MP that I need to really understand.

Cheers
John

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7 months 1 week ago #17984 by Bill Hertzel
Replied by Bill Hertzel on topic Need advice - in flight adjustable prop
There are numerous sites on the web that discuss constant speed propeller operation.
Your variable pitch prop is a constant speed prop except that the governor is You!

Most discussions assume a direct drive prop so you will need to divivd engine speed by 2.43 to get the Rotax Propeller RPMs .
Rotax propellers turn slower than Lycoming and Continental engines.
5800rpm = 2386 Prop speed... 5000rpm = 2057 Prop speed.

- - -
This assumes the limit stops have been properly set.
You will Take Off with Wide Open Throttle(WOT) and the prop set to Fully Fine (Minimum Pitch).
Assuming you now wish to throttle back to cruise power of 5000rpm @ 26"map.
Reduce the throttle to 5000 rpm.
You may be seeing 24" of MAP.
Increase the Pitch until the MAP comes up to 26"
You do not have constant speed governor so the RPMs will drop a little (~4800rpm?)
Increases the throttle to return to 5000rpm.
The MAP will increase due to the throttle opening.
Adjust the Pitch to correct the MAP. This also effects the RPM.
Continue to alternate the Throttle and Pitch settings to get to the target.

With experience, you will learn to overcorrect one setting so that when you adjust the other they both converge.
Ex. You may find that if you initially reduce the throttle to 5150rpm when you increase the pitch to set 26"MAP the RPMs drop right to 5000 in just one try.

Important! When reducing throttle to land be sure to reset the Prop to the "Fine" Position.
You do not want the prop set to "Course" in the event of a Go-Around.

You can fly around all day in the "Fine" position.
It is like driving in low gear. You won't get anywhere fast but it won't do any harm.

If you do not understand how MAP is controlling the power let us know.
It will be a long answer. ;)

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7 months 1 week ago #17985 by John_in_Aus
Replied by John_in_Aus on topic Need advice - in flight adjustable prop
Thanks Bill. Great info there.

Yeah - I understand that the gauge is displaying the difference in pressure between ambient and that which is behind the throttle plate. But I am not sure how that correlates, and what it actually means - if you know what I mean.

I feel I should know this, but it was just simply not in the material that was delivered in my training.

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7 months 1 week ago #17986 by PeterKl
Replied by PeterKl on topic Need advice - in flight adjustable prop
Hi John,
if I may recommend to have a look at SI912-016-R1 you'll find that it pretty much covers your question. It is heard that this SI was released after endless discussions on the envelope one needs to stay in when fiddling about with revs and MAP. Perhaps you manage to somehow overlook the exclusions that are numerously included and identify the real important ones to stick to the. Example, I tend to keep revs up in the range of 5.000-5.200 and cruise at approx. 200 km/h by manually setting the pitch. A friend of mine cruises at the same speed but pitches the (identical) prop so his engine only revs at, sit down, 4.300. He has no probs with temps, pressures or anything else and saves like 20% gas. The chart in the SI mentioned seems to cover that as OK.
If I find a way to upload the chart, I will do that. However, it's in the SI anyway.
Regards
Peter

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7 months 1 week ago - 7 months 1 week ago #17988 by Bill Hertzel
Replied by Bill Hertzel on topic Need advice - in flight adjustable prop
I believe that would be the Service Letter SL912-016 Pertaining to Manifold Pressures.
Not Service Instruction SI912-016 Pertaining to Fluids.
Last edit: 7 months 1 week ago by Bill Hertzel.

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