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

[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.

[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?

[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?

[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.

[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.

[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?

[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

[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.

[Ed Cesnalis]

CT, I have been crusing at 8500 most of the time. The last time i opened it all the way up, the tach went into the yellow so i backed it off, it can run at 5500 at about 90% throttle it seems. Remember, like you, there is little moisture right now, so density altitude is only about a K higher on most days.

 

I am going out today for some cross-wind landings. I will try a wide open throttle today.

 

You are permitted 5,800 for 5 min. Sounds as though you are really flat, too flat for my carbed 912. If I had your FI I would consider flatter than I currently run so that I could take more advantage of the economy you have available.

 

Thanks

[Ed Cesnalis]

So you also think I should cruise at 5500 and not lower? I would probably be at a 4.5 gph but be booking at around 120kt true (given no cross or headwind).

 

I think you need to consider prop pitch, RPM and throttle setting. 5,500 seems to be the best RPM but its somewhat of a Rotax secret. You want to cruise below 92% throttle to be in eonco mode and you want your pitch to permit both at the same time.

 

5,500 @ 9,500 @ 91% throttle would be a great setting but it would limit your top speed and permit you to overspeed at highest throttle settings.

5,500 @ 9,500 @ WOT would give you top performance for altitude but limit economy.

5,200 @ 9,500 @ 91% would be focused on economy cruising at altitude.

 

See what your starting point is ( current WOT RPM ) and then decide what your target is ( speed vs economy ) and see how far off or on your pitch is.

 

BTW winds only effect your ground speed not your true air speed or your indicated speed.

[Ed Cesnalis]

I didn't say he needs 5,500 all the time, even when he asked I said he should learn his pitch and I gave 3 options as examples.

 

His needs due to where he lives are similar to mine. I bet he simply needs to open his throttle a bit and burn a little more fuel and go a little faster but who knows till you check WOT RPM?

 

rpm range between 5100-5500 for cruise rpm provided you can get 5500-5650 WOT. These rpms will also put you down in your economy cruise rpms below 92%

 

There is some overlap there, if you can get 5,500 and cruise at 5,500 you are 100% not 92%

[FlyingMonkey]

My engine usually will not idle below 1900 when cold, but once warm will idle at 1700-1800. 1800 is the top of the yellow on my tach, and it's dead accurate for chatter, any rpm below that and the gearbox noise is audible. If I start to hear chatter I just add a little throttle to idle above the yellow on the ground. It sounds like CTLSi's engine may be set with a little too much RPM for idle, unless the gearbox or something else is different on an LSi. OTOH, I have found that a little higher idle can give a little cushion on landing that can make thing smoother, especially when just learning to land the thing. When I had my idle reset lower my landings immediately got worse until I adjusted to the change.