Prop pitch -again

[opticsguy]

I got my CT in December of 06, and flew it as it was delivered that winter. This is a pic from one of my first x-country flights down to the TX coast.

 

 

In case you can't read it, that's 118 IAS, 126 true, 5400 MSL, 24 in manifold pressure, at 5100 RPM! IIRC, I couldn't get it past 5200 RPM at full throttle. Needless to say, once the TX summers came around I had to take over a degree of pitch out of the prop.

 

My question to the Canadians and others with variable pitch. What RPM would you cruise at and how fast?

[Jacques]

again...yes..

 

5200 WOT is too much pitch on the prop

 

as Roger wrote so many times you should have 5500-5600 WOT level flight at your ''usual'' cruise altitude

 

have a look at this Rotax Service Letter

 

http://legacy.rotaxo.../sl-912-016.pdf

 

specially at chapter 3.1.2)

 

I don't have 'yet' an inflight ajustable prop so I can't comment on performance

 

maybe C-ICEY or Jos Jonkers will chime in

[Roger Lee]

For top cruise speeds at our normal altitudes we are only a few MPH off, but the in flight adjustable gets a big advantage in take off rpm and climb and can adjust better at high altitude.

[C ICEY]

I-CEY a 2003 CT2k with Kaspar prop with hydraulic variable pitch. 308kg EW.

"performance" is relative to the task at hand.

 

During the takeoff roll, I immediately have to coarsen pitch one notch so I don't ever-rev (+5800) As I accelerate to 'roll', I put in one more notch for my climb at 80 knots. In my first few minutes of climb I will juggle airspeed and prop pitch to maintain 5500-5600 rpm with WOT, as it give me the best climb and the quickest temp rise to boil off the water in the oil before I throttle back. (note that the hydraulic lever goes in notches that sometimes require a higher or lower airspeed to optimize that RPM so climb might be 75 or 85 depending on which notch) I then coarsen the pitch enough to give me 5400-5500 for cruise climbs. Typical Cruise, again depending on the flight goal and altitude, is to throttle back and pitch for 5300+-. depending on altitude and load, this will give me a decent cruise. Down low (3000'ASL) WOT pitched for 5450 gives me an honest 120 knots. Sometimes, its nice to fine pitch and throttle as far back as to maintain 5000 and watch the scenery. fuel economy without injection ;-) If I have a sense of urgency, whatever the altitude, it is WOT and pitch as close to <5500 and go.

 

all that said, I am on and off the throttle and prop constantly because that's the way the terrain goes: climb and drop. Anything much above the terrain is boring. the CT is an awesome platform to explore BIG mountains.

[Roger Lee]

Hi Icey,

 

Having that prop pitched so you can only get 5200 or less rpm and running it WOT there is what cracks pre mid 2006 cases. Even if you have a newer case that should tell you how much stress over propping a small engine causes. If it were me I would pitch for WOT above 5500 rpm and then cut back to 5000. This won't over stress the case and other parts.

[opticsguy]

no matter what pitch I had, 5300 RPM used over 6 gal/hour and 5150 (my normal cruise RPM) would give me 5 or so. Airspeed didn't seem to matter.

 

Have never really cruised even close to WOT, and I usually throttle back to 28 psi MP once I'm 500 ft AGL.

[Ed Cesnalis]

no matter what pitch I had, 5300 RPM used over 6 gal/hour...

 

You tried only coarse pitches. My 5300 RPM WOT altitude is 12,000' where there isn't enough air to burn over 6 and if I fly at low altitudes the 5300 RPM throttle setting is too low to use over 6.

 

Your range of settings might work well for you but would be too coarse for me.

[C ICEY]

Hi Icey,

 

Having that prop pitched so you can only get 5200 or less rpm and running it WOT there is what cracks pre mid 2006 cases. Even if you have a newer case that should tell you how much stress over propping a small engine causes. If it were me I would pitch for WOT above 5500 rpm and then cut back to 5000. This won't over stress the case and other parts.

 

I think you missed the part that at WOT I've got 5450. Then, everything else is "throttled back"

[opticsguy]

I had the mechanic add "just a touch" of extra pitch at my annual and I finally got to fly in smooth air yesterday. Climb didn't seem too bad on a 60 degree day, but at 3000 ft I was getting 120 indicated (124 true) at 4900 RPM. I didn't try for WOT, but I doubt I could have gotten over 5150. I'm thinking a solid degree of pitch should be taken out. More maybe?

[Roger Lee]

Hi Scott,

 

If all you can get is 5150 WOT then you are way over pitched. This is what was cracking engine cases with the mid 2006 or earlier engines. You should be up around 5550-5650 WOT at your average altitude for a good all around performance. 5150 WOT will cost you fuel, up to 1.5 gph, it will cost you roll out distance, climb and cruise. It will cost you spin up time to save yourself during a bad landing when you want rpm back in a hurry. At 5150 I would flatten the pitch about 2.5 degrees and then fly it and see where you are. Prop pitch is not just about top speed, but all around performance and your engine health. This kind of over pitching is detrimental to your engine. I have re-pitched over 100 props in the last several years on LSA alone and this is where everyone has their props pitched to get best all around performance.

 

There are special considerations at times, but not for the average flier.

[Ed Cesnalis]

The CTLSi cruises most efficiently and at best engine performance at 4800 rpm. At 9,500 over NV the average TAS is 110kts. Why go faster? The burn rate is 3.3 gph.

 

Best performance at 9,500 is not 110kts even my lowly carburated CT can do 125kts TAS at that altitude.

 

Why go faster?

[Roger Lee]

Hi CTLSi,

 

You shouldn't be cruising below 5000 with the FI and actually should be higher with that engine. It may get good fuel economy there, but wasn't designed to run there for extended periods for its life time.

[Roger Lee]

Food for thought,

 

Just because some engines can operate at different rpm or parameters doesn't mean they were designed by the MFG to run there. They must have had a good reason to set those parameters. Unless someone has a specific engineering degree and has worked with the MFG in a specific engine's design it is usually a good idea to heed their warnings and operational parameters. In this case Rotax / Bombardier has been building engines since the 1920's and the 912 engine since late 1989. Their experience, engineering expertise and 33+ years with the 912 trumps most peoples knowledge.

[Ed Cesnalis]

Roger,

 

Let me challenge that thinking. 1st we don't know how the prop is pitched and how the throttle is set when he is cruising at 4,800. If the throttle setting is low enough (and if 3.2gph is to be believed it would indicate a very low setting) there would be no downside. He may be at 55% power and in that case where is the harm? If his throttle setting is high enough he would be running over square and this is what the parameters are trying to avoid.

 

Fixed pitch props take mp, and over square considerations off the table at least until its time to optimize your pitch so the mfg presents a simpler picture based on RPM alone but the underlying assumptions change when the pitch changes.

[Roger Lee]

I believe CTLSi's prop was set from the factory correctly and at least 5500+ WOT, but that may not translate out well if he flys high all the time because FD doesn't set them for a high altitude cruise. Just like your prop wasn't in the beginning. Tweaking the prop for specific flying characteristics falls to the owner.

 

When I give answers or an opinion, unless told otherwise I consider everything to be as it should with the original setup. After some further reply's we may find that the correct setup isn't as it should be or it was altered. Then the answer would certainly change. Unfortunately there is no way to know who has correct setups for every situation and who doesn't so until informed otherwise I assume the person has the correct settings or setup and is experiencing a problem. I find many times a mechanic may change something with no idea what the correct parameters should be and sometimes without the owners knowledge. If they later come back and we find out it isn't setup correctly, altered or there is additional info we didn't have in the first post then answers and solutions can certainly change.

That's the drawback of forums and diagnosing problems is that sometimes certain assumptions need to be made and lacking any further or detailed info from a post lends itself to sketchy answers or even incorrect ones. Sometimes with my answers I have to give the most common problematic fixes with only the info available.

If everyone was at the same level of training and or experience then some assumptions might be canceled out.

All in all I think we al do a good job here of figuring out problems, giving fixs and helping other forum members.

 

In CTLSi's case I assume he has the proper prop pitch from the get go.

[Ed Cesnalis]

...

In CTLSi's case I assume he has the proper prop pitch from the get go.

 

Ok Roger, in t hat case I will disagree with you.

 

If he has proper prop pitch then it is flat enough to avoid a high throttle setting with a low RPM and the motor is protected at all throttle settings. I know FD used to deliver planes with pitches so coarse that 5,500 (let alone 5,800) couldn't be achieved.

 

The exception would be an RPM range with bad harmonics, does that remain a concern? Was the longstanding concern with vibrations at lower settings ever validated?

 

This is one of those undocumented issues, the old ROAN doc of best practices says the 912s were designed to run their whole lives at 5,500 yet the 912iS is albout economy and needs to be throttled back to get into its economy mode. Interesting if the advice is to compromise the design intent to comply with an undocumented RPM range.

[Roger Lee]

Quote:

"The exception would be an RPM range with bad harmonics, does that remain a concern? Was the longstanding concern with vibrations at lower settings ever validated? "

 

Yes they are still a concern and that was the basis for my answer. His engine runs no different than ours as far as the gearbox, rpms and vibration concerns. He is just fuel injected. He does run a little smoother than us. His starts and stops aren't any different. He still has a 10.8:1 compression.

[Ed Cesnalis]

 

Yes they are still a concern and that was the basis for my answer. His engine runs no different than ours as far as the gearbox, rpms and vibration concerns. He is just fuel injected. He does run a little smoother than us. His starts and stops aren't any different. He still has a 10.8:1 compression.

 

Ah,

 

Interesting that the undocumented range to avoid remains after all these years.

 

Assuming there is a need to keep cruise RPM high for the sake of vibration and at the same time a need to keep the throttle below 92%, even at altitude to cruise in economy mode there is a 'squeeze' ( not to high and not too low) on cruise throttle settings. If the pitch isn't optimum the range that fits between high enough and low enough can be small or non-existent at least at some altitudes.

 

Cecil, what RPM do you get at wide open throttle and what altitude?

[Roger Lee]

Hi CTLSi,

 

The A&P is wrong (just like he was using a Tempest oil filter) and should know better because he went to the FI class in Sept., unless you picked up your plane prior to that and then he still should have known better. A good rpm to be at if your average altitude is 8500-9500 would be a WOT pitch setting of 5500+ and then cruise at 5100-5500 rpm. 5000 rpm cruise is okay, but on the low side especially if you burn 100LL and not good if you get less than 5400 at WOT. A nice rpm cruise setting is 5100-5400 with an achievable WOT of 5550-5650. The whole idea behind our prop pitch is to reduce engine stress and vibration first, performance is second.

 

Ed's actual pitch setup is different from 99% of us because of his normal field elevation and normal cruise altitude, but the principals are the same.

[Roger Lee]

There is a range for rpm settings and there are times for special setups like when flying floats (higher rpms are better and needed at times), but there should be no arguments to the upper and lower limits if you have gone to Rotax schools and ask the right questions. Rotax does have areas of rpms they prefer and since I didn't Mfg the engine I'll have to go with that.

[Doug G.]

Is it possible that the 3.2 gal is incorrect? Either exaggerated, or the Dynon is set wrong?

[Ed Cesnalis]

Roger,

 

I believe this range exists because of the old ROAN best practices document and because of your reports.

 

... Rotax does have areas of rpms they prefer and since I didn't Mfg the engine I'll have to go with that.

 

Rotax also publishes info that shows lower RPM ranges are permissible most go with that. I have always cruised at 5,500 so you can see I have chosen to believe Rotax over Rotax, hmmmmm

[Roger Lee]

No he's probably correct. When at Page we had a new CTLSi flying with us. When he and I were at 5100 rpm I was burning around 4.7 and the CTLSi was at 3.6

[Ed Cesnalis]

Is it possible that the 3.2 gal is incorrect? Either exaggerated, or the Dynon is set wrong?

 

Max burn at 9,500 seems to be ~5gph and when you give up 15kts by throttling back on that engine the savings should be pretty big. 3.2 sounds too good to be true for 110kts.

[Tom Baker]

I thought the CTLS's because they are longer, were shipped with the propellers removed to make room in the shipping crate. Maybe they just do that when there are 3 in the crate instead of 2. If they are shipped removed the pitch would not be set at the factory.